Recent Advancements in Chalcogenides for Electrochemical Energy Storage Applications

Abstract

Energy storage has become increasingly important as a study area in recent decades. A growing number of academics are focusing their attention on developing and researching innovative materials for use in energy storage systems to promote sustainable development goals. This is due to the finite supply of traditional energy sources, such as oil, coal, and natural gas, and escalating regional tensions. Because of these issues, sustainable renewable energy sources have been touted as an alternative to nonrenewable fuels. Deployment of renewable energy sources requires efficient and reliable energy storage devices due to their intermittent nature. High-performance electrochemical energy storage technologies with high power and energy densities are heralded to be the next-generation storage devices. Transition metal chalcogenides (TMCs) have sparked interest among electrode materials because of their intriguing electrochemical properties. Researchers have revealed a variety of modifications to improve their electrochemical performance in energy storage. However, a stronger link between the type of change and the resulting electrochemical performance is still desired. This review examines the synthesis of chalcogenides for electrochemical energy storage devices, their limitations, and the importance of the modification method, followed by a detailed discussion of several modification procedures and how they have helped to improve their electrochemical performance. We also discussed chalcogenides and their composites in batteries and supercapacitors applications. Furthermore, this review discusses the subject’s current challenges as well as potential future opportunities.

1. Introduction

Energy storage has become an increasingly important research field. More and more researchers are focusing their attention on creating and researching novel materials for use in energy storage systems due to the promotion of sustainable development goals since traditional energy sources such as oil, coal, and natural gas have finite supplies. The extraction and processing of these conventional nonrenewable energy sources have disastrous consequences for the environment. It is well established that coal gasification contributes significantly to global warming through CO₂ emissions into the atmosphere. Furthermore, these conventional nonrenewable energy sources are further constrained by the depletion of reservoirs, geopolitical conflicts, and volatile oil prices. Provided the challenges mentioned above, sustainable renewable energy sources have been hailed as a model for reducing our dependency on nonrenewable energy sources. However, renewable energy generation is intermittent; hence, the deployment of renewable energy sources necessitates the use of efficient and reliable energy storage devices. We can reduce our dependency on oil and coal while also reducing environmental degradation by deploying clean and renewable energy technologies.

Material science and engineering have the potential to revolutionize energy supply and storage through the development of novel materials. Modern energy storage systems require the creation of new materials with improved power and energy storage capabilities and higher efficiency and safety levels. One of the most significant advances in material science and engineering has been the application of electrochemical energy storage devices to store energy electrochemically [1,2]. Electrochemical energy storage (EES) technologies include capacitors, supercapacitors, batteries, and fuel cells; these have found widespread utilization in hybrid/all-electric vehicles, modern portable electronic gadgets, and commercial/residential applications. Each of these electrochemical storage technologies has its unique charge storage mechanism. Figure 1 depicts a typical device configuration of the (a) fuel cell, (b) battery, and (c) supercapacitor, showing the anodes, cathode, electrolyte, and separator, which form the main parts.

The Ragone plot (Figure 2) compares the various electrochemical storage devices in terms of specific power density and specific energy. Capacitors stores electrical energy in an electric field; they exhibit higher specific power at lower specific energy. Supercapacitors are electrochemical storage devices that store energy at the electrolyte–electrolyte interface. Energy storage mechanisms involve electrochemical double-layer activity and/or redox activity on the electrode surface. They exhibit higher specific energy compared to capacitors with overlapping performance for the specific power. Battery stores energy via electrochemical reactions, exhibiting high energy density but suffers from low power densities. A fuel cell is an electrochemical device that harvests electricity directly from chemical energy, using hydrogen and oxygen as fuel. When compared to other electrochemical energy storage systems, they have the highest energy densities. For electrochemical storage systems to be effective, the electrode material and electrolyte medium must have exceptional ionic and electronic transport characteristics. Metal chalcogenides are widely used as electrode material in electrochemical energy storage applications such as lithium-ion batteries, sodium-ion batteries, and supercapacitors. This is due to its extraordinary features, which include high reversible capacity, a significant number of electroactive sites, thermal stability, and a stable structure enabling cyclability [3,4,5,6]. Furthermore, the deeper faradic reaction of chalcogenides allows for greater capacity; it is inexpensive and can even be considered a green material.

The term chalcogenides refer to materials that contain one or more chalcogen elements, such as sulfides, selenides, or tellurides. Chalcogenides are binary compounds containing at least one chalcogen anion and another electropositive element; dichalcogenides contain two chalcogens. The unique properties of transition metal chalcogenides have been explored in all areas of material sciences and engineering. Transition metal chalcogenides (MX or MX2) have been attracting an increasing amount of research attention as electrode materials for electrochemical energy storage devices because of their high power density and stability, especially for supercapacitors. M denotes transition metals such as Ni, Co, Mn, Cu, Zn, Mo, or W, while X denotes chalcogens (such as S, Se, and Te). Metal sulfides, metal selenides, and metal tellurides, for example, have proven remarkable high-power density and cyclic stability and, as a result, have been used in supercapacitor applications. The crystalline structure, morphology, synthesis approach, and chemical composition all influence the electrochemical performance of the electrode of an electrochemical energy storage system. Layer and spinel structures are the most reported crystal structures in electrochemical energy storage applications.

Several published works have suggested advances in the use of metal chalcogenides as a viable material for enhanced electrochemical storage systems in terms of energy storage mechanisms, structure optimization, and stabilization. Others have argued that to better understand and facilitate the creation of innovative materials for electrochemical applications, it is necessary to combine both computational and experimental studies [8]. Different synthesis methods were employed in a report on cobalt sulfide to achieve different morphologies and chemical phases that have been used in electrochemical energy storage applications such as supercapacitors and lithium-ion batteries [5]. Furthermore, cobalt sulfide films have demonstrated amazing potential in electrochemical energy storage applications [9], suggesting that various nanostructured materials may be developed depending on the synthesis approach. As a result, Section 3 discusses a few different synthesis techniques desirable for electrode materials for electrochemical energy application. Despite the progress, a lot of issues remain to be resolved. Renewable electrochemical energy storage technologies necessitate investigating and improving high-efficiency and ecologically friendly electrode materials.

In this review, the latest advances in synthesis methods, design concepts, and electrochemical and mechanical performance of energy storage devices are covered with a specific focus on batteries and supercapacitors. We begin by discussing electrochemical energy storage systems based on chalcogenides, followed by a detailed description of various selected metal chalcogenide synthesis processes. We will pay particular attention to the role of chalcogenides in various battery and supercapacitor technologies. Finally, future research objectives and perspectives on chalcogenides for flexible electronics are highlighted and reviewed.

2. Types of Electrochemical Energy Storage Devices

2.1. Batteries

An electrochemical battery energy storage solution is required for our sustainable future. For decades, rechargeable batteries have been transforming the battery industry. These rechargeable batteries, including Li-ion, Pb–acid, Ni metal-hydride, and Ni–Cd batteries, dominate the global market. Lithium-ion batteries are currently in the industry. Li-ion batteries are made up of four major components: a cathode, an anode, an electrolyte, and a separator. This is depicted in Figure 3.

While graphite is often used for the anode electrode, lithium metal oxides or phosphates, such as LiCoO₂ and LiFePO₄, are commonly used for the cathode electrode. In order to maintain charge balance, electrons migrate from the anode to the cathode during discharge, which implies Li-ions flow through the electrolyte and cross over the separator. On the other hand, the charging process is triggered by a charging power source, which causes the Li ions to migrate in the opposite direction [10]. As previously mentioned, one of the most important components of electrochemical Li-ion batteries is the electrode materials, which significantly impact battery performance. Therefore, it is important to review the role of chalcogenides in the synthesis and design of appropriate electrode material for high-performance Li-ion batteries. A closely related lithium element is sodium (group 1 element). Hence sodium-ion batteries have been investigated as an alternative sustainable energy storage solution. Because Na-ion has a larger atomic radius compared to Li-ion, they are expected to migrate at a slower rate [11]. However, reports demonstrated that the migration barrier could be shortened for a layered structured system [12,13,14].

Figure 4 illustrates the structure of Na-ion batteries showing the materials for the four major components: a cathode, an anode, an electrolyte, and binders. Investigating the potential of Na-ion batteries as an alternative electrochemical energy storage device is critical in the development of sustainable, innovative storage systems.

While investigating the electrochemical performance and environmental friendliness of several new electrode materials, most Na-ion batteries and, to a large extent, Li-ion batteries focused on transition metal sulfides-based materials such as cobalt sulfides, iron sulfides, nickel sulfides, and tin sulfides. The storage of Na-ions in metal sulfide materials depends on the transition metal elements that are present in the metal sulfide materials. Developing new electrode materials with specific structures necessitates a thorough understanding of Na-ion transport properties in various electrode materials. The importance of understanding the role of chalcogenides in the synthesis and design of acceptable electrode materials for high-performance Na-ion batteries cannot be overstated.

As with so many other battery energy storage solutions for our sustainable future, there is a continuing desire for new technologies with higher storage capacities and fewer demands on critical minerals. Metal–Sulfur batteries have recently received significant attention [16]. In theory, lithium–sulfur batteries, for example, could store five times as much energy as lithium-ion batteries. Similar to other electrochemical batteries, lithium–sulfur batteries are made of two electrodes (lithium anode and sulfur cathode), a separator, and an electrolyte (either solid or liquid). The fundamental charging and discharging process is depicted in Figure 5, and it is governed by the equation $\mathrm{S}+2\mathrm{Li}\to {\mathrm{Li}}_2\mathrm{S}$. From Figure 5, lithium metal is oxidized to Li⁺ at the anode, and sulfur is reduced to lithium sulfide (Li₂S) at the cathode during the discharge process. Lithium–sulfur batteries have several drawbacks, the most significant of which are their short cycle life, low utilization of active materials, early failure due to electrical shorting, and anode rechargeability. Another considerable drawback that has received more attention is the suppression of the polysulfide shuttle in lithium–sulfur batteries. According to a study, the effects of polysulfide shuttle can be mediated by including polysulfide absorbant and mediators as part of the cathode material [17]. Several strategies have been used to suppress polysulfide shuttle effects, including the use of electrolyte additives at the anode, the use of porous layers to impede diffusion, the use of a selective electrolyte separator, and the encapsulation of polysulfides at the cathode [18,19].

The ideal material for electrochemical energy storage must exhibit superior electrochemical properties. The electrochemical performance of electrode materials is mostly determined by the composition and structure/morphology of the materials. There has been a lot of interest in electrocatalysts of chalcogenides because of their intrinsic electrochemical activities [20,21]. Transition metal dichalcogenide (molybdenum disulfide) was utilized as a catalyst in a study to convert polysulfides into a sulfur cathode because of its inherent sulfur deficiencies. Nanocomposites of MoS_2−x nanoflakes were created on thin films of reduced graphene oxide utilizing co-dispersion and filtration processes. The steps in the preparation of MoS_2−x/rGO are depicted in Figure 6. Surface sulfur deficiency at the cathode had a favorable effect on the kinetics of polysulfide shuttling. As a result, the nanocomposite sulfur cathode with only a small amount of MoS_2−x/rGO (4%) exhibited a high-rate capability and a long cycle life. This emphasizes the importance of designing cathode material composition and structure/morphology to improve the electrochemical performance of emerging technologies [22].

Metal–Air batteries have also demonstrated enormous potential in the field of electrochemical energy storage systems. A conventional metal–air battery consists of a metal electrode, an air-breathing cathode, and a lithium salt-containing electrolyte. The anode of metal–air batteries can be made using low-cost, environmentally friendly elements, such as Fe, Na, Al, and Zn. Metal–Air batteries are both safer and offer comparable energy density. Figure 7 shows the theoretical energy density and specific energy of commonly published metal–air batteries.

In a conventional metal–air battery structure (Figure 8), metal is converted to ions at the anode, while oxygen is transformed into hydroxide ions at the cathode. The utilization of air (ambient oxygen) has the extra benefit of lowering costs and simplifying cell construction. Metal–Air batteries such as Li–air battery [24,25], Na–air battery [26,27], Al–air battery [28,29], Mg–air battery [30,31], Zn–air battery [32,33], Fe–air battery [34,35,36]. Sn–air battery has widely been published, and the electrochemical performance and prospects of improved specific power and specific energy are well established. Indeed, the prospects of using metal chalcogenides as a possible anodic material offer a green and sustainable solution. In this vain, Lyu et al. [37] used a hybrid CoS₂/rGO cathode catalyst to fabricate Li–O₂ batteries. The electrodes exhibited high-rate performance. Furthermore, cobalt sulfide nanoparticles coated on graphene nanosheets have been shown to produce Zn–air batteries with outstanding cycling stability and high rechargeability [38].

2.2. Supercapacitors

Electrochemical energy storage systems such as supercapacitors offer tremendous opportunities for clean energy storage. They comparatively offer high power density, long cycle life, and fast charging. It is expected that next-generation storage devices will be dominated by electrochemical storage systems. The development of electrochemical energy storage systems with superior energy requires novel electrode materials, and these electrode materials are critical to the electrochemical performance of the storage device. In this section, we will explore supercapacitors that are based on chalcogenides electrodes. A manufactured supercapacitor can be classified as symmetric or asymmetric based on the configuration of the electrodes. The anodic and cathodic electrode materials in symmetric supercapacitors are identical, whereas the anodic and cathodic electrode materials in asymmetric supercapacitors are distinct. Asymmetric supercapacitors have a significantly greater potential range. This has made it more appealing. When it comes to supercapacitors, the electrode is the most critical component. For a supercapacitor to have improved electrochemical performance, the materials that are utilized to construct the electrodes must be of high performance. Scientists are attempting to develop low-cost electrode materials that have a high specific surface area, good electronic conductivity, and are stable.

The electrochemical activity of TMCs significantly influences the effectiveness of electrodes used for supercapacitor electrodes. The electrochemical activity of the electrode material regulates electrochemical performance, such as energy density and power density. Many factors influence supercapacitor electrochemical performance, including nanostructure orientation and bond mechanisms. Even though several innovative materials (for example, metal oxides) have demonstrated considerable potential as supercapacitor electrodes, several of them have also demonstrated significant limitations [40,41].

Therefore, it is essential to investigate novel electrode materials that exhibit superior electrochemical performance. A possible approach in this regard is the employment of chalcogenides, which operate based on a pseudocapacitive (fast reversible redox mechanism) or hybrid (combination of charge accumulation and fast reversible redox mechanism) charge storage mechanism to store and release electrochemical energy. Chalcogenides, especially metal sulfides, metal selenides, metal tellurides, etc., have been widely applied in supercapacitor applications due to their excellent electrochemical activity. Despite this, there are a few drawbacks to using chalcogenides as an electrode material, including low conductivity, poor cyclability, a low surface area, and aggregation problems [42,43,44]. Several synthesis approaches have been used to create electrodes with high surface area by utilizing nanostructured features such as nanowires, nanorods, and nanobelts. However, to attain improved electronic conductivity and stability, the use of different active materials must be investigated.

In a reported attempt to improve the electrochemical activity by improving the surface area, chalcogenides based supercapacitors, ternary metal chalcogenides hybrid supercapacitors (MX; M = Zn–Co–Ni; X = S, Se, Te) were fabricated [45]. On nickel-based substrates, a well-organized nanostructured needlelike feature that looks similar to sea urchins was fabricated. The sample had a lot of voids and exhibited improved specific capacitance and stability. By using selenization, tellurization, and sulfurization of precursors, the morphology of the sample could be altered to affect the specific capacitance. Some chalcogenides-based electrode material exhibits inherently high electronic conductivity [46,47]. For example, MoS₂ has a lower electronic conductivity compared to WS₂; to tackle this, researchers have employed different synthesis methods to fabricate various nanostructured electrodes for supercapacitor applications.

Furthermore, other nanocomposites have been investigated to improve electronic conductivity [48,49,50,51]. Teli et al. produced an interconnected crumpled-nanoflake structured MoS₂ on Ni foam using a one-step electrodeposition method [46]. The fabricated electrodes exhibited diffusion-controlled or capacitive-controlled charge storage kinetics and excellent coulombic efficiency. When employed as an electrode material for supercapacitor applications, WS₂/α-NiMoO₄ composite electrodes with nano/micro cavity-like hierarchical architecture exhibited high specific areas with increased pore size, superior electrical conductivity, and improved stability [52]. Cobalt sulfide (CoS) has generated much research attention because they are low-cost and possess a weak Co–S bond that exhibits theoretically high capacitance. As a result of these properties, transition metal sulfides for use in supercapacitor applications have been developed. For example, Zhang’s research group fabricated CoS nanowires on layered Ti₃C₂T_x using the solvothermal method [52]. With the Ti₃C₂T_x layer providing large interlayer spacing, improved conductivity, and chemical diversity on the surface, the samples exhibited remarkably improved electrochemical performance. In another research, 2D hierarchical MoS₂ lamella inserted in the CoS₂ flake was fabricated. The 2D MoS₂ multilayers provided support preventing interlayer expansion and structural collapse. The electrochemical performance and cyclic stability of these chalcogenides can be improved with secondary support [53]. Other typical chalcogenides [54,55,56,57,58,59,60,61,62] also demonstrated great potential for supercapacitor applications. However, before delving into their performance in-depth, a brief overview of their synthesis methodologies is required, which is covered in the next section.

3. Chalcogenides for Electrochemical Energy Storage Devices

3.1. Synthesis of Chalcogenides

The synthesis of chalcogenide-based compounds to attain desirable characteristics for electrochemical applications is covered in this section. Chalcogenide-based compounds come in a wide range of structural orientation and bond mechanisms, ranging from binary to multinary compound systems. The synthesis techniques used in preparing chalcogenide-based nanostructures dictate structural orientation, stoichiometry tuning, surface modification, and bond mechanisms. As a result, novel cutting-edge fabrication and processing methods and a fundamental understanding of the relationship between the structure and properties of chalcogenide-based compounds have been the focus of scientists over the past decades. Scientists have developed numerous strategies for preparing nanostructured chalcogenide compounds. The methodologies for creating modified chalcogenide-based nanostructures can be broadly classified into three types: one-pot synthesis, in situ synthesis, and ex situ synthesis (Figure 9).

The one-pot synthesis systems are exceptionally easy to set up and manage, it involves sequential chemical reactions in a single reactor. It is quite efficient and less time-consuming, and it may entail the synthesis of complex structures in a single reaction step. It is also suitable for industrial processing. It is always necessary to use a substrate during in situ synthesis because it supports the growth or nucleation of reactants. This approach allows for the creation of nanostructures in a variety of shapes, including planar and non-planar ones. The term “ex-situ synthesis approach” refers to the technique of synthesizing the reacting components separately and then combining them through an intermediary procedure after they have been created. It takes longer and consumes more energy to do ex situ synthesis than it would otherwise. This is due to the additional intermediary step. Nonetheless, the uniqueness of each of these synthesis techniques requires specialized equipment and expertise in manipulating the procedure to attain the desired structures.

In order to make nanostructured materials, one or a combination of these synthesis methods can be used alone or in conjunction with the others. In a representative approach, a novel route for the preparation of a two-phase film of 4Ni(OH)₂·NiOOH@Co₉S₈ on Ni foam was developed through a chemical bath followed by hydrothermal electrodeposition. This was carried out by a one-step synthesis process in an autoclave. The reported improved electrochemical performance was explained based on the synergistic effect of NiCo–OH and CoS providing fast ion and electron conductions [63]. In a similar report, Cu₂MoS₄ nanostructures were prepared using a one-pot hydrothermal route. The I-phase Cu₂MoS₄ electrode exhibited exceptional pseudocapacitive charge-storage via ion intercalation/de-intercalation [64].

MoS₂ NPs rolled r-Graphene/mesoporous MnOx nanofibers composite were prepared by a two-step template-free method [65]. By applying a two-stage approach, the morphology of the electrodes was tuned to a flaky pasty and nanofiber morphology. The electrodes exhibited improved thermal charge/discharge cycle stability [65]. The synthesis procedure has a direct impact on the electrode’s electrochemical performance, as demonstrated in the described work. A one-pot hydrothermal method approach for making cobalt–magnesium selenates electrode was employed as an advanced electrode for electrochemical energy storage [66]. By changing growth time and temperature, nanosheet arrays and nanowire morphology were observed. Figure 10 illustrates the synthesis procedure used and the observed morphology.

Various nanostructures of NiCoSe₂, NiCo₂S₄, and NiCo₂O₄ were obtained using the hydrothermal synthesis method. Electrode fabricated using these materials can be used for both electrochemical sensor and supercapacitor applications. They reported improved electrochemical kinetics and excellent specific capacitance [67]. Many other authors have reported the synthesis of various chalcogenides nanostructures via numerous methods for electrochemical energy storage applications.

Table 1. Chalcogenides for supercapacitors.

Material	Synthesis	Potential Window (V)	Electrolyte (M)	Scan Rate (mV/s)	Specific Capacitance (F/g/C/cm2/mF cm−2/Cg−1)	Current Density (A/g/mA/cm2/mAcm−2)	% Retention Capacity/Cycles	Energy Density (Wh/kg mWh/cm3/μWhcm−2)	Power Density (W/kg/mW/cm3/mWcm−2)	Ref.
NiS/rGO hybrid	Microwave-assisted hydrothermal	0.46	2 M KOH	5–50	1188.9	5	68.1	7.1	1836	[68]
NiS nanostructures	Microwave-hydrothermal	0.46	2 M KOH	100	119	1	100/2000	16.5	250	[69]
CuFeS2 nanomaterial	Solvothermal	0.55	6 M KOH	1–50	589	4	82/2000	-	-	[70]
NiSe2@rGO nanocomposite	Hydrothermal	1.7	2 M KOH	2-50	467		93/6500	34.3	-	[71]
MoSSe/rGO composite	Solvothermal	0.35	3 M KOH	5–50	373	1	89.5/4600	-	-	[72]
MoS2/Ni3S2	Hydrothermal	0.58	1 M KOH	10–100	1.033	1	62.5/10,000	35.93	1064.76	[73]
NiSe2@Fe3Se4	chemical bath deposition	1.7	6 M KOH	2–50	147	5	92/10,000	52	398	[74]
NiGa2S4 nanosheets	Hydrothermal	1.1	6 M KOH	10–100	488	0.5	109/20,000	40	5.5	[75]
CF@CoS	tube furnace	0.7	2 M KOH	10–100	3576.0	5.0	80/4000	149.4	4.3	[76]
copper sulfide	Chemical reaction	0.8	2 M KOH	1–50	1173	2	99/1500	301.4	1400	[77]
LaDySn2O7-SnSe nanocomposite	Hydrothermal	0.2–0.8	0.1 M KOH	10–100	1780	0.3	127/2000	67.46	6157.411	[78]
Zinc cobalt sulfide nano hybrid	hydrothermal	1.8 V	1MTEABF4acetonitrile (ACN)	10	785	2	106/10,000	49.6	1799.6	[79]
NiCo2Se4/RGO (NCSG)	two-step hydrothermal	0.4	6 M KOH	30	1776	2	93.5/5000	66.2	1500	[80]
MoS2/graphene oxide/meso-MnO2 nanocomposite	two stage ultra-sonication	-	1 M H2SO4 and KI	1–500	980	0.5	-	-	-	[65]
MoS2@3DG//NPC	solution-based process	0.0–4.0	1 M LiPF6	0.5	88.3	1.0	78/2000	156	197	[81]
MoS2-C-RGO//PDPC	hydrothermal	0.01–3.0	1 M LiPF6	0.1	759	2	80/10,000	188	200	[82]
MoS2/CP//AC	ball-milling and pyrolysis	1.5–4.5	1 M LiPF6	10	530	1	79.61/1000	87.74	200	[83]
(P-MoS2/PANI/rGO HNSs)//rGO	hydrothermal	0.3–1.1	1 M Li2SO4	5–20	221.9	5	93.5/30,000	56	0.45	[84]
MoS2 @C//AC	hydrothermal	0.0–4.5	1 M LiPF6	1–20	1083.4	1	72.12/3000	119.68	11.25	[85]
MoS2-C//PDPC	hydrothermal	0.4–3.0	1 M NaClO4	5	200	2	77.3/10,000	111.4	12000	[86]
TiO2/MoS2 @NC//AC	Solid phase reaction	2.0–4.5	1 M NaClO4	0.2	66.3	2	70/3000	148	200	[87]
VSe2/rGO//AC	hydrothermal	1–4	1 M NaClO4	0.5	365	1	68/1000	106	125	[88]
TiS2 @Cpvp//AC	in situ conversion	2.0–4.0	1 M NaPF6	0.2–1.2	340	2	87/5000	101.7	200	[89]
MoS2/CoS2-RGO//Na3V2(PO4)3/C	facile hydrothermal	0.01–4.5	1 M NaClO4	0.1	596.3	2	92.3/1000	152.98	562.5	[90]
CNF@VS2//CNF@GS	Hydrothermal	0.01–3.0	1 M NaClO4	0.5	52.3	5	82/4500	166	0.4	[91]
MoS2/SnS2-RGO//CC	-	0.01–3.00	1 M NaClO4	-	550.3	2	100/3000	113.9	561.25	[92]
MoSe2/TiO2−x/graphene//AC	hydrothermal	0.5–3.5	1M NaPF6	-	415.2	2	88/3000	109	100	[93]
MoSe2/C//AC	hydrothermal	0.01–3.0	0.8 M KPF6	0.1	320	1	81.58/1000	169	106	[94]
WS2@NCNs//NCHS	Hydrothermal	2.6–4.2	0.8 M KPF6	0.1	44	2	92.2/2000	103.4	235	[95]
MoS2/graphene	L-cysteine-assisted hydrothermal process	−0.1–0 (vs. SCE)	1 M Na2SO4	20	243	1	92.7/1000	73.5	19.8	[96]
MoS2/CA (Carbon Aerogel)	Hydrothermal	−0.8–0.2 (vs. Hg/HgO)	1 M Na2SO4	2–50	260	1	92.4/1500	-	-	[97]
MoS2/graphene aerogel	Hydrothermal	-	1 M Na2SO4	-	268	0.5	93/1000	-	-	[98]
1T-2H MoS2/GA (Graphene Aerogel)	Hydrothermal	−0.8–0.4	1 M H2SO4	-	416	-	-	-	-	[99]
NiCo2S4/CS (Carbon Sponge)	Direct carbonization	0–0.9	3 M KOH	5–20	1093	0.5	90/500	-	-	[100]
CoS2/rGO	Solvothermal	−0.3–0.65	6 M KOH	5	331	0.5	97/2000	30	4750	[101]
CuS/rGO	Solvothermal	−0.4–0.6	6 M KOH	50	2317.8	1	96.2/1200	-	-	[102]
CuS/rGO	Solvothermal	0–0.5	6 M KOH	10	906	1	89/5000	105.5	2.5	[103]
CuS/rGO	Solvothermal	−1–−0.1	2 M KOH	5–100	368.3	1	88.4/1000	-	-	[104]
CuS/rGO	Hydrothermal	0–0.5	3 M KOH	5–80	1604	2	97/5000	21	315	[105]
NiCo2.1Se3.3 NSs/3D G/NF	CVD + Hydrothermal	0–0.6	6 M KOH	3–20	742.4	1	83.8/1000	-	-	[106]
NiCo2Se4-rGO	Hydrothermal	−0.2–0.5	1 M KOH	10–80	2038.55	1	90/1000	67.01	903.61	[107]
NiCo2Se4/rGO	Hydrothermal	−1–0	6 M KOH	30	1776	2	93.5/5000	66.2	1500	[80]
NiCo2Se4/rGO	Hydrothermal	−0.1–0.4	6 M KOH	30	139	2	87.27/5000	37.83	1433.55	[108]
CoNi2S4/rGO	One step pyrolysis	0–1.6	6 M KOH	50	-	-	93.7/8000	54.8	798	[109]
(Cu2NiSnS4)/rGO	Hydrothermal	0–0.6	6 M KOH	-	16.38	5	89.2/2000	5.68	98.8	[110]
MoS2/MWCNT	Hydrothermal	−0.8–0.2	1 M Na2SO4	2–50	452.7	1	95.8/1000	-	-	[111]
MoS2@CNTs/Ni core	PLD	−1–0	1 M Na2SO4	200	512	1	100/2000	25.5	44.2	[112]
CuS/MWCNTs	Hydrothermal	−0.4–0.6	6 M KOH	-	2831	1	90/600	-	-	[113]
CuS/CNT	PVP assisted refluxing	0–0.6	2 M KOH	20	2204	10	89/10,000	-	-	[114]
CuS/CNT	Hydrothermal	−0.2–0.6 (vs. Hg/HgO)	2 M KOH	1–50	566.4	1	94.5/5000	50.4	-	[115]
NiS/CNT	CBD + SILAR	−0.2–0.6	1 M Na2S, 2 M S, and 0.1 M KCl in methanol: water (7:3)	100	398.16	1	98.39/1000	35.39	145.45	[116]
CNT/CoS	CBD + SILAR	−0.2–0.6	1 M Na2S, 2 M S, and 0.1 M KCl in methanol: water (7:3)	100	288.43	1	95.3/1000	25.63	145.45	[116]
CNT/CuS	CBD + SILAR	−0.2–0.6	1 M Na2S, 2 M S, and 0.1 M KCl in methanol:water (7:3)	100	176.2	1	78.57/1000	15.66	145.45	[116]
CNT/PbS	CBD + SILAR	−0.2–0.6	1 M Na2S, 2 M S, and 0.1 M KCl in methanol:water (7:3)	100	101.94	1	69.49/1000	9.06	145.45	[116]
NiS2/MWCNTs	Solid state method	−0.2–0.5	2 M KOH	2–20	2054.28	2	99.99/10,000	-	-	[117]
Ni3S2/CNT	Electrodeposition + ion exchange	−0.1–0.6	3 M KOH	30	1643	1	91.5/2000	-	-	[118]
VS2/MWCNT	SILAR	−0.6–0.2	2 M KCl	100	182	2	95.9/1000	42	2.8	[119]
TiS2/CNTs	Atomic layer deposition/sulfurization	−1.5–1.5	21 m LiTFSI/5% PVA	1–50	195	1	95/10,000	60.9	1250	[120]
NiS/graphene/CNT	Hydrothermal	−0.1–0.35	6 M KOH	100	2377	1	68/1000	14	16	[121]
MoS2@rGO-CNT	Micropatterning + pyrolysis	0–1	H2SO4/PVA gel	-	13.7	1	96.6/10,000	5.6	-	[122]
Hollow MoS2/Carbon	Sonogashira coupling	−0.5–0.5	0.5 M Na2SO4	-	418	0.5	-	-	-	[123]
MnS/MoS2/Carbon	Calcination+ sulphuration	0–1.55	2 M KOH	-	1162	0.5	81/5000	31	388.3	[124]
CoSe2/N-doped Carbon	Selenylation	0–0.4	6 M KOH		120.2	1	92/10,000	40.9	980	[125]
MoS2/NiCo2S4@Carbon	Self-templating	0–0.5	6 M KOH	10	250	2	90.1/10,000	36.46	73.75	[126]
CoTe/MOF	Hydrothermal	0–0.5	2 M KOH	5–100	297.27	2	83.33/10,000	43.84	738.88	[127]
Ni3S2/AC	Electrodeposition	0–1.4	PVA-KOH gel	10	2797.43	1	83.47/10,000	55.32	1053.71	[128]
ZnS/CuS/AC	SILAD	0–1.2	PVA-KOH gel	-	1925	4	79/2000	-	-	[129]
MoS2/GA	Hydrothermal	−0.3–0.7	1 M Na2SO4	50	268	0.5	93/1000	-	-	[130]

(chalcogenides for supercapacitors),

Table 2. Chalcogenides for batteries.

Material	Synthesis	Voltage Range	Cycling Performance (mAh g−1/No. of Cycles	Maximum Rate Capability (mAh g−1/)	Current Density (A/g/mA g−1)	Ref.
MoS2@N-doped carbon nanowall @carbon cloth	Solvothermal	0.01–3	619.2/100	235	2	[131]
MoS2/carbon fiber @MoS2@C	Electrospinning	0.01–2.5	332.6/1000	233.6	10	[132]
MoS2@CF	Vacuum infiltration	0.05–3	240/500	171	5	[133]
MoS2/single-wall carbon nanotubes	Liquid Phase Exfoliation	0.1–3	390/100	192	20	[134]
N-doped hollow MoS2/C nanospheres	Hydrothermal	0.01–3	128/5000	242	5	[135]
MoS2/graphene	Ball milling/exfoliation	0.01–2.7	421/250	201	50	[136]
MoS2/S-doped graphene	Hydrothermal	0.005–3	309/1000	264	5	[137]
Fe3O4@MoS2 graphite paper	Modified hydrothermal	0.01–3	388/300	231	3.2	[138]
1T MoS2/graphene tube	Solvothermal	0.01–3	313/200	175	2	[139]
MoS2/cotton-derived carbon fibers	hydrothermal	0.01–3	323.1/150	355.6	2	[140]
NBT/C@MoS2 NFs	electrospinning	0.01–3.0	448.2/600	2000	200	[141]
N-doped amorphous micron-sized carbon ribbons @MoS2	Hydrothermal	0.01–3	305/300	302	2	[142]
Hollow flower-like MoS2@C-RGO	one-pot hydrothermal	0.01–3	637/50	467	1	[143]
amorphous MoS3@carbon nanotube	Facile acid precipitation method	0.05–2.8	565/100	2350	20	[144]
MoSe2/N, P-rGO	solvothermal	0.01–3	378/1000	216	15	[145]
MoSe2@MWCNT	one-step hydrothermal	0.01–3	459/90	385	2	[146]
VS4-rGO	in situ hydrothermal	0.01–3	540/50	123	20	[147]
VS4/rGO	hydrothermal	0.01–2.2	241/50	219.9	0.5	[148]
Co9S8@C nanospheres	facile hydrothermal	0.01–3	305/1000	297	5	[149]
CoS2@ multichannel carbon nanofibers	facile solvothermal	0.4–2.9	315.7/1000	201.9	10	[150]
CoS2eMWCNT	simple hydrothermal	1–2.9	568/100	550.5	0.8	[151]
Sb2S3 nanorods/C	facile hydrothermal	0–2	570/100	337	2	[152]
Sb2S3/S-doped graphene sheets	ultrasonication	0.01–2.5	524.4/900	591.6	5	[153]
ZnSeSb2S3@C	sulfurization reaction	0.01–1.8	630/120	390.6	0.8	[154]
Sb2Se3/C rods	self-assembly reaction	0.01–2.5	485.2/100	311.5	2	[155]
Sb2Se3 nanowire-based membrane	Hydrothermal/vacuum filtration	0.01–2	296/50	153	1.6	[156]
SnS/3D N-doped graphene	Facile, self-assembly method	0.01–2.5	509.9/1000	404.8	6	[157]
NiS2@CoS2@N-doped carbon coreshell nanocubes	co-precipitation method	0.01–3	600/250	560	5	[158]
SnS2/EDA-RGO	Hydrothermal	0.01–3	480/1000	250	11.2	[159]
MnS@CNF film	Electrospinning/thermal treatment	0.01–3	220/200	107	0.5	[160]

(chalcogenides for batteries), and

Table 3. Chalcogenides for fuel cells.

Material	Synthesis	Electrolyte	Current Density (mA cm−2/Ag−1)	Potential (mV/V)	Ref.
β-NiS	Facile Hydrothermal	1 M NaOH	40	598	[161]
Ni3S2@NF	Hydrothermal	1 M NaOH	275	531	[162]
Ni–Co9S8	Hydrothermal	1 M KOH	75	478	[163]
NiCo2S4/C	Pyrolysis/hydrothermal	1 M KOH	100	180	[164]
NiS@C2-550	Hydrothermal	1 M KOH	360	~577	[165]
MoS2/Ni3S2/NiFe-LDH/NF	two-step hydrothermal	1 M KOH	280	~548	[166]
NiSe2	Hydrothermal	1 M KOH NG	105	502	[167]
CoSe2	one-step selenylation	1.0 M KOH	10	330	[168]
Ni–Co–S	electrodeposition	1.0 M KOH	100	363	[169]
(CoMn)Se2	selenation	1.0 M KOH	10	270	[170]
NiCoSe2	electrodeposition	1.0 M KOH	10	256	[171]
NiFeCoSex	electrodeposition	1.0 M KOH	10	150	[172]
NiCoFe-PS	hydrothermal electrodeposition	1.0 M KOH	10	195	[173]
CoOSeP/Co foil	thermal selenization	1.0 M KOH	10	155	[174]
CoP/CN@MoS2	solvothermal	1.0 M KOH	10	289	[175]
Co9S8/Co3O4	hydrothermal-sulfurization method	1.0 M KOH	20	260	[176]
Co3O4/MoS2	liquid-phase deposition	1.0 M KOH	20	230	[177]
Ni3S2/Co(OH)2	facile hydrothermal	1.0 M KOH	10	257	[178]
Co9S8/NiCo LDH	two-step hydrothermal	1.0 M KOH	30	278	[179]
NiFe/Co9S8	chemical bath deposition	1.0 M KOH	10	219	[180]
CoCO3/CoSe	two-step hydrothermal	1.0 M KOH	10	255	[181]
CoO/CoSe2	Hydrothermal	1.0 M KOH	10	510	[182]
CoMoNiSeNF	one-pot hydrothermal	1.0 M KOH	10	405	[183]
Co9S8	hydrothermal	1.0 M KOH	10	1.66	[184]
p-CoSe2/CC	electrodeposition	1.0 M KOH	10	1.62	[185]
Co0.85Se	liquid-phase chemical conversion	1.0 M KOH	10	1.60	[186]
Co0.75Ni0.25Se/NF	thermal conversion	1.0 M KOH	10	1.60	[187]
CoMoNiSeNF	hydrothermal	1.0 M KOH	10	1.45	[183]
CoeO/CoeSe/Cu	electrodeposition	1.0 M KOH	10	1.65	[188]
Co9S8/NiCo LDH/NF	hydrothermal	1.0 M KOH	10	1.63	[179]

(chalcogenides for fuel cells) show a summary of various chalcogenide materials synthesis route and electrochemical performance parameters.

3.2. Applications of Chalcogenides in Batteries

Rechargeable batteries, such as metal-ion batteries (MIBs), metal–sulfur batteries (MSBs), and metal–air batteries (MABs), have received a lot of press recently because of their improved efficiency, cheaper cost, and lower pollution. However, these energy storage devices’ electrochemical performance (power density, energy density, charge–discharge rate, cycle performance, etc.) still needs to be improved. As a result, much research has shown that electrode materials have a large impact on electrochemical performance, implying that finding suitable materials is a key strategy for developing an effective energy storage device. Metal chalcogenides (MCs) have received a lot of interest because of their diverse structural kinds, abundant supply, low cost, high capacity, and incredible electrical conductivity [189,190,191]. Furthermore, as shown in Figure 11a, research on chalcogenides as electrode materials in rechargeable batteries has increased rapidly in the last decade, with MIB research taking the lead. Figure 11b shows the proportions and instances of such electrodes in various rechargeable batteries.

3.2.1. Applications of Chalcogenides in Metal-Ion Batteries

Metal-ion batteries are key enablers in the present transition from fossil fuels to renewable energy for a better society, with cleverly developed materials serving as the technology driver. Despite this, they have significant scalability and durability issues. A fundamental question is how to design appealing structural frameworks for enhanced electrodes and electrolytes for the next generation of batteries using the most efficient materials. Metal chalcogenides have received a lot of interest as a potential electrode for attaining the finest characteristics [192,193,194]. The most prevalent MIBs are listed in

Table 4. Electrochemical performance of some common MIBs in literature.

Material	Structure	Preparation Method	Rate Performance [Capacity (mAh g−1) @ Current Density (Ag−1)]	Cycle Performance [Capacity (mAh g−1) @ Current Density (Ag−1) @ Cycles]	Ref.
LIB anode
α-MnSe/CNF	Microspheres	Selenization	[email protected]	[email protected]@100	[195]
α-MnSe/C	Microspheres	Selenization	[email protected]	784.82@1@100	[195]
Co9S8@CN	Nanoparticles	Direct annealing	618.1@ 0.1	406.5@1@100	[196]
SnS2@g-C3N4	Porous nanosheets	Sintering	[email protected]	[email protected]@600	[197]
ZnS-QD@NC	Polymorph structures	Pyro-synthesis technique	[email protected]	620@1@500	[198]
CoSe/Co–NCNS	Layered structure	Selenization + annealing	[email protected]	640@1@500	[199]
Sb2Se3/C	Helical nanoparticles	Selenization	[email protected]	[email protected]@1000	[200]
SnSe2/graphene	Heterostructure nanosheets	Hydrothermal	[email protected]	[email protected]@1500	[201]
SnS/TiS2/GO	Misfit-layered heterostructure	Solvent-free hand-grinding	[email protected]	[email protected]@1000	[202]
g-C3N4@WS2	Hierarchical	Solvothermal	[email protected]	[email protected]@1000	[203]
BiSbTe3/N-rGO	Nanocomposite	Solvothermal	[email protected]	[email protected]@80	[204]
In2Te3–TiO2–C	Hierarchical layered	High-energy ball-milling	~450@10	[email protected]@500	[205]
SIB anode
NiSe2@BCNNTs	3D nanotubes	Pyrolysis	[email protected]	382.4@2@2000	[206]
SnS2/CNT@rGO	Hierarchical powdery	Solvothermal	[email protected]	301@1@1000	[207]
MoSe2/CN	3D flower-like	Hydrothermal	[email protected]	328.7@1@500	[208]
FeS2@TiO2	Core-shell structure	Hydrothermal	222.2@10	[email protected]@600	[209]
PNBH-VS4	Hollow microspheres	Hydrothermal	[email protected]	[email protected]@250	[210]
MoS2/CNF	Nanoflower	Hydrothermal	[email protected]	314@1@300	[211]
CoS2	Aggregated nanoparticles	Facile-sulfuration process	[email protected]	500@2@500	[212]
Cu2Se	Micro-sized monoclinic	Selenization	[email protected]	~260@1@1000	[213]
MC-NCNF/MoSe2	Multi-channeled	Electrospinning+heat-treatment	285@10	[email protected]@300	[214]
SnS2/EPC	Graphene-like	Hydrothermal	[email protected]	340@2@450	[215]
NiSe2@N-TCF/CNTs	Nanofibers	Electrospinning + pyrolysis	[email protected]	[email protected] @1000	[216]
FeS2@N/CNTs@rGO	Urchin-like hierarchitecture	Hydrothermal	[email protected]	570@2@>750	[217]
NSPCFS@CoS2	Nanoparticles	Sulfuration	540.7@4	546.3@1@2095	[218]
Fe7Ni3S11/CN	Hollow-spheres	One-pot hydrothermal + post-annealing	[email protected]	477@2@900	[219]
MoS2@RGO	Few-layer nano-roses	One-step solvothermal	[email protected]	[email protected]@500	[220]
WS2/Ni3S2@NC	Layered-heterostructure	Solvothermal + annealing	[email protected]	[email protected]@80	[221]
PIB Anode
CoSSe–C	Porous-shell nanospheres	Sulfo-selenization + calcination	[email protected]	[email protected]@3000	[222]
MoS2/MXene	Layered nanosheets	Hydrothermal	[email protected]	[email protected]@50	[223]
ZnSe@N-PCNF	Ultrafine nanocrystals	Electrospinning + thermal-treatment	[email protected]	[email protected]@1000	[224]
FeSe2/NC	Nanosheets	Pyrolysis + annealing + carbonizing	203@10	301@1@250	[225]
Co0.85Se@NC	Mesoporous structure	Annealing + selenidation	[email protected]	114.7@1@250	[226]
ZnSeNP@NHC	Hollow polyhedron	Pyrolysis + selenization	[email protected]	[email protected]@1200	[227]
WSe2/N,P-C-2	Ultrathin nanosheets	One-pot calcination	[email protected]	333 @0.1@100	[228]
NPCP@MoSe2	Hollow-interlayered nanosheets	Solvothermal	[email protected]	[email protected]@800	[229]
MoSe2/N–C	Ultra-thin nanosheets	Selenization + calcination	[email protected]	[email protected]@100	[230]
Co0.85Se/G	Hollow structure	Hydrothermal	[email protected]	[email protected]@200	[231]
CoSe2@NCF	Flexible core/sheath	Solvothermal	[email protected]	[email protected]@200	[232]
AIB cathode
CoSe@C	3D nanoparticles	Pyrolysis + annealing	427@1	62.4@5@100	[233]
NC@ZnSe	Dispersed porous	Pyrolysis + selenization	[email protected]	[email protected]@250	[234]
WS2@NCNFs	Layered nanoplates	Electrospinning + annealing	[email protected]	[email protected]@100	[235]
MoSe2@C	Sheet nanocomposite	Hydrothermal	[email protected]	[email protected]@3000	[236]
Ni0.6Co0.4S@MXene@NC	Layered sheet	Sulfurization + carbonization	481.2@ 0.4	125.2@1@300	[237]
VS4	Nanowire clusters	Amine ions-assisted method	[email protected]	[email protected]@120	[238]
NiSe2@GO	3D sponge	Selenidation + annealing	[email protected]	~164@1@250	[239]
Bi2Te3/Sb2Te3	Nanoflakes	Solvothermal	230@1	203@1@300	[240]
Fe-NiSe	Nanoflake	Hydrothermal	304@1	197@[email protected]	[241]
MIB cathode
1T-VSe2@rGO	Thin-layered nanoparticles	Hydrothermal	[email protected]	[email protected]@500	[242]
Cu7.2S4	Nanotubes	Microwave-induced selective-etching	[email protected]	59.1@1@1600	[243]
ZIB cathode
1T-WS2	Nanosheet	Hydrothermal	[email protected]	[email protected]@1000	[244]
TiS2	Layered	Hydrothermal	76@1	~53.2@1@500	[245]
HMIB cathode
Cu2S@C/MLIB	Monodisperse	Sulfurization + carbonization	[email protected]	[email protected]@50	[246]
MoS2/MLIB	Flower-like architectures	Solvothermal	[email protected]	[email protected]@65	[247]
VS4/MLIB	Nanodendrites	Solution-phase approach	[email protected]	110@1@1500	[248]
FeS-CNFs/MLIB	1D network	Electrospinning + thermal treatment	[email protected]	[email protected]@800	[249]
MoS2&MoSe2/MLIB	Nanosheets	Exfoliation processing	[email protected]&[email protected]	[email protected]@10&[email protected]@3	[250]
Co3S4-F/MLIB	Particle-like	Solvothermal	[email protected]	[email protected]@100	[251]
VS2/MLIB	Nanosheets	One-pot solvothermal	[email protected]	[email protected]@200	[252]
TiS2/SMIB	Layered structure	As-received	200@1	67.5@20@3000	[253]

, along with their method of preparation, structure, and electrochemical properties.

Li-Ion

LIBs are a good energy storage technology because of their high performance (capacity, cycling performance, rate capability, and others). However, existing commercially available graphite electrodes have low energy due to their small surface area, deterioration, large volume expansion, low theoretical capacity (372 mAh/g, based on un-lithiated graphite) [254], and low power due to sluggish kinetics [255]. Because of their excellent electroconductivity, rich redox chemistry, multiple valence states, various crystalline morphologies, and high specific capacity, MCs and relevant composites have attracted increasing attention as electrode materials for LIBs in recent years [256,257]. Nanostructures of chalcogenides, such as nanorods, nanowires, nanofibers, nanoflakes, nanocages, flower-like, and hollow spheres of chalcogenides, have all been successfully manufactured and tested as electrode materials in LIBs to date [196,197,199,204,205]. Many studies have recently demonstrated that doping with cations or heteroatoms can improve the lithium storage kinetics of MC electrodes by producing edge/surface defects and local charge polarization, both of which are significant in boosting charge and electron transport [199]. N-doped carbonaceous materials, for example, have substantial chemisorption for poly-sulfides, which can improve cycle performance, while S-doping could improve conductivity due to its more effective reduction and stronger electron donor ability [258].

A self-assembled approach was used to successfully manufacture V₃S₄ nanosheets anchored on an N, S co-coped graphene (VS/NSG) aerogel (Figure 12a) [259]. Decomposition, sulfuration, and N, S co-doping happened during the heat treatment process. The electrochemical performance was significantly improved by the dominant pseudocapacitive contribution (57.78% at 1 mV/s), including rate capacity of 480 and 330 mAh/g at 5 and 10 A/g, respectively (Figure 12b) and stable cyclic performance of 692 mAh/g after 400 cycles at 2 A/g. Because of the conductive network that had been built, this hybrid structure (Figure 12c,d) not only had good electron conductivity but also had a significant ability to buffer volume growth. Apart from cation doping, the introduction of carbon-based composites of various dimensions, such as 1D, 2D, and 3D nanostructures, can also provide a viable way to design target materials with multifunctional or enhanced properties due to their excellent electronic conductivity and large surface area. For example, Xu et al. [203] described the solvothermal synthesis of a hierarchical g-C₃N₄@WS₂ composite. The g-C₃N₄@WS₂ composite, as synthesized, had a high discharge capacity of 1136.1 mAh/g at 0.1 C and excellent cycling stability of 433.8 mAh/g after 1000 cycles. MC having hierarchical porous architectures have a large number of channels for Li⁺ transport, resulting in increased rate capacity [203,205]. Because of their high porosity, crystallinity, inherent modularity, and huge specific surface area, MOFs have recently been widely employed as precursors to manufacture certain derivatives of MCs, which preserve their unique shape and so display outstanding rate capacity.

Na-Ion

SIBs have emerged as one of the most promising alternatives for energy storage due to their high theoretical specific energy density (760 Wh/kg), long cycling life, low cost, low redox potential, and excellent energy storage mechanism [258,260,261]. However, there are significant obstacles that continue to stymie SIB development. Na’s huge atomic weight and low standard electrochemical potential may result in explosions, corrosion, and high-power consumption. Second, during cycling, Na metal prefers to accumulate in a dendritic structure and endures a large volume change. Furthermore, high reactivity may lead to electrolyte degradation and battery breaking and reforming, resulting in poor cycle performance [262,263,264]. Furthermore, because Na⁺ has a larger radius (1.02) than Li⁺ (0.76), it is more difficult to intercalate/de-intercalate in the crystal lattice, causing severe mechanical stress/strain on the electrodes, resulting in excessive material capacity degradation [265,266]. Searching for appealing materials such as anode/cathode and appropriate electrolytes is the major technique for improving SIB performance. Due to their excellent theoretical gravimetric, thermodynamic stability, remarkable redox reversibility, and high theoretical capacity, conversion reaction-based materials, such as metal oxides/sulfides/selenides, specifically transition MCs, have recently been developed as electrode materials for SIBs. Nanostructure design, doping, and compounding with other materials, similar to LIBs, can provide improved performance because hollow structures can accommodate volume changes during sodiation/desodiation while also providing more surface area and well-defined cavities, which can improve SIB stability [192,267,268].

To combat these challenges, Chen et al. [219] used a simple one-pot hydrothermal approach with post-annealing to create a bimetallic solid solution of Fe₇Ni₃S₁₁ nanoparticles confined with nitrogen-doped carbon (CN). Used as an anode in SIBs, the material had a reversible specific capacity of 567 mAh/g at 0.2 A/g and maintained a capacity of 477 mAh/g after 900 cycles at 2 A/g with a capacity retention rate of 90%. Furthermore, Y. Chen et al. [269] created a heterojunction bimetallic sulfide nanosheet SnS₂/FeS₂/rGO composite by introducing FeS₂ to grow synchronously with SnS₂ on rGO in situ (Figure 12e) as an electrode in SIB and produced a capacity of 768.3 mAh/g at 0.1 A/g and 541.2 mAh/g at 1 A/g (see Figure 12f). Chen et al. [206] also used a simple pyrolysis process to create a novel self-supporting NiSe₂ composite enclosed in boron carbonitride (BCN) nanotubes (NiSe₂@BCNNTs). The as-made NiSe₂@BCNNTs anode demonstrated a remarkable reversible Na⁺ storage capacity of 787 mAh/g at a current density of 100 mA/g and improved long-term cycling stability of 382.4 mAh/g at 2 A/g after 2000 cycles.

K-Ion

Potassium has a lower reduction potential than lithium and sodium, allowing potassium-ion batteries (KIBs) to operate at greater potentials and hence provide more energy density. However, the development of KIBs has been hampered by the lack of adequate electrode materials, which affect cell capacity and probable overload [270]. Because of their high theoretical capacities and low cost, metal chalcogenide materials have attracted a lot of attention for usage as anodes in KIBs [271,272,273].

Yang et al. [274] used a direct vacuum filtration and subsequent reduction process to make flexible Sb₂Se₃ nanorods supported by holey rGO composite membranes (Sb₂Se₃ @h-rGO), where the pre-electrostatic interaction of Sb³⁺ and O²⁻ pushed the formation of Sb–O–C bonds between Sb₂Se₃ nanorods and h-rGO. Free-standing Sb₂Se₃ @h-rGO electrodes showed outstanding cycling stability when used as anodes for KIBs, with a high capacity of 382.8 mAh/g at 100 mA/g after 500 cycles. In order to compensate for the significant volume change of nanocrystals throughout the potassiation and depotassiation processes, hollow carbon nanospheres containing iron telluride nanocrystals (FeTe₂-C) are manufactured via easy infiltration and a one-step tellurization procedure (Figure 12g) [275]. The synergistic impact of the heterointerfaced FeTe_1.1 and metalloid Te produced after one cycle, as well as the yolkshell architecture with uniformly distributed nanocrystals contained in a carbon shell, result in excellent electrochemical characteristics. Even at a high current density of 10 A/g, the FeTe₂-C electrode exhibits outstanding long-term cycle performance (171 mAh/g during the 500th cycle at a high current density of 0.5 A/g) and great rate capability (126 mAh/g). Park et al. [222] recently published a new strategy for the synthesis of cobalt sulfoselenide solid solution embedded in the porous shells of hollow carbon nanospheres, in which a porous hollow carbon nanosphere was used as a co-infiltrating matrix to create electrode material with high conductivity and robust stability. CoSSe-C was an outstanding anode for KIBs, with a cycle life of 286 mAh/g for 3000 cycles at 1.5 A/g and a rate capability of 185 mAh/g at 5.0 A/g.

Mg-Ion

MIBs are widely used as a safe, low-cost system with a high charge density, theoretical capabilities, and lower reduction potential (Mg metal can reach 3833 mAh/cm³) (−2.37 V vs. SHE). Above all, unlike lithium-metal and sodium-metal batteries, magnesium metal batteries can successfully avoid dendrite formation during the cycle, making them safer [276,277,278]. The rechargeable MIB electrode, on the other hand, has slow kinetics and poor cycle stability due to its high charge density, slow solid-state diffusion rate, and great polarization ability of Mg²⁺ [279,280]. Furthermore, the low specific capacity of the classic carbonate electrolyte is due to a lack of proper active sites for Mg²⁺ intercalation in the lattice, as well as magnesium’s ability to rapidly form a dense passivation film. The strong contact between Mg²⁺ and the lattice of the cathode material prevents Mg²⁺ from becoming embedded in the cathode material, lowering the Mg²⁺ diffusion rate.

Exploring innovative and effective materials such as cathodes to achieve high voltage, big capacity, and a long stable cycle is critical for improving MIB electrochemical performance [281]. Cu_7.2S₄ [243], VS₂ [282], and MoSe₂ [283] are considered promising cathode materials for magnesium batteries because they have more active sites, expandible spacing leading to increased active sites and increased diffusion rate, and improve the mobility and desolvation of Mg²⁺ at the electrolyte/electrode interface [284]. Gao et al. [242] employed a simple hydrothermal process to create 1T-VSe₂ nanoparticles that cooperated with reduced graphene oxide (1T-VSe₂@rGO) composite in an APC electrolyte environment, and it was used as the cathode material of the second-generation rechargeable MIB. The 1T-VSe₂@rGO composite cathode material demonstrated outstanding reversible capacity (235.5 mAh/g at 50 mA/g) and long cycle life (62.7% of the initial capacitance is preserved at 50 mA/g after 500 cycles). A 46% initial capacitance rate capability was recorded, equating to a current density of 1000 mA/g.

Al-Ion

Rechargeable aluminum-ion batteries (RAIBs) have emerged as one of the most promising multivalent metal-ion battery types, owing to their low flammability, low cost, and high volumetric energy density (8040 mAh/cm³) [285]. Despite extensive efforts, RAIBs’ electrochemical performance was nevertheless hampered by many difficulties, including low discharge voltage, poor cycling stability, and electrode disintegration. Alternative cathode materials capable of generating higher energy densities have been the subject of recent research. Transition metal chalcogenides have caught the attention of researchers because of their weak binding strengths between host frameworks, which could lead to more favorable kinetics for aluminum storage, such as high reversible specific capacities of more than 250 mAh/g [191,233,235,241].

Figure 12. (a) A schematic representation of the VS/NSG electrode preparation method. (b) the rate capability of the VS/NSG electrode. (c) SEM pictures of VS/NSG as prepared. (d) In the designated white box of VS/NSG, TEM picture, HRTEM image, and FFT patterns. Adapted from reference [259]. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license. (e) The SnS₂/FeS₂/rGO composite’s synthesis method. (f) The SnS₂/FeS₂/rGO composite’s cycling performance at 0.1, 0.5, and 1 Ag⁻¹. Adapted with permission [269]. Copyright (2021) American Chemical Society. (g) The hollow carbon nanosphere containing the iron telluride nanocrystals and its formation mechanism. Adapted with permission [275]. Copyright (2020) John Wiley and Sons. (h) A schematic illustration of the CoSe₂-NC, CoSe₂-NPC, CoSe₂-NPCS, and CoSe₂-NCS synthetic stages, respectively. Adapted with permission [286]. Copyright (2021) American Chemical Society. (i) A Zn/TiS₂ aqueous battery schematic example. (j) Performance at 1000 mA/g in cyclic mode. Adapted with permission [245]. Copyright (2020) American Chemical Society.

Figure 12. (a) A schematic representation of the VS/NSG electrode preparation method. (b) the rate capability of the VS/NSG electrode. (c) SEM pictures of VS/NSG as prepared. (d) In the designated white box of VS/NSG, TEM picture, HRTEM image, and FFT patterns. Adapted from reference [259]. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license. (e) The SnS₂/FeS₂/rGO composite’s synthesis method. (f) The SnS₂/FeS₂/rGO composite’s cycling performance at 0.1, 0.5, and 1 Ag⁻¹. Adapted with permission [269]. Copyright (2021) American Chemical Society. (g) The hollow carbon nanosphere containing the iron telluride nanocrystals and its formation mechanism. Adapted with permission [275]. Copyright (2020) John Wiley and Sons. (h) A schematic illustration of the CoSe₂-NC, CoSe₂-NPC, CoSe₂-NPCS, and CoSe₂-NCS synthetic stages, respectively. Adapted with permission [286]. Copyright (2021) American Chemical Society. (i) A Zn/TiS₂ aqueous battery schematic example. (j) Performance at 1000 mA/g in cyclic mode. Adapted with permission [245]. Copyright (2020) American Chemical Society.

RAB’s cathodes containing ultrafine CoSe₂ particles embedded in nitrogen-doped porous carbon nanosheet (NPCS)/MXene hybrid materials were developed using a spatial isolation technique to solve the associated problems of limited structural stability and poor reaction kinetics [286]. As shown in Figure 12h, the two-dimensional NPCS structures were created via self-assembly of metal frameworks on MXene surfaces. This synthetic technique allowed the researchers to manipulate the particle size of active materials even at high pyrolysis temperatures, allowing them to investigate the impact of size on electrochemical performance. The CoSe₂-NPCS electrode delivered a high discharge capacity, outstanding rate capability, and long-term cycling stability, according to spectroscopic examination. Zhang et al. [239] described a simple method for making a novel cathode material for RAIBs out of a standard transition metal chalcogenide NiSe₂ with a 3D sponge-structure and graphene oxide (GO) nanosheets decoration (3D NiSe₂ sponges@carbon composite). After 250 cycles, the product had a high capacity of 281 mAh/g at 500 mA/g and a better cycling performance of 164 mAh/g at 1000 mA/g.

Zn-Ion (ZIB)

Because of their non-toxicity, natural abundance, low cost, good air stability, high theoretical specific capacity (820 mAh/g), low redox potential (0.76 V vs. SHE), high energy density, and good reversibility in mild aqueous electrolyte, rechargeable zinc ion batteries (ZIBs) are considered one of the most promising. TMD has been examined as a suitable cathode material for reversible storage of zinc ions to improve the performance of ZIBs due to their unique layered architectures [287,288].

Tang et al. [244] revealed for the first time that 1T-WS₂ nanosheets could be a viable cathode choice for rechargeable ZIBs. The as-synthesized WS₂-200 has reversible discharge capacities of 233.26, 206.25, and 179.99 mAh/g at current densities of 50, 100, and 200 mA/g, respectively, thanks to the large interlayer spacing and high conductivity of 1T phase. WS₂-200 has an initial discharge capacity of 85.11 mAh/g even at a high current density of 1000 mA/g. In the work of Wang et al. [245], layered TiS₂ was effectively demonstrated as a lasting host material for neutral aqueous ZIBs with a comparatively low charge voltage of 0.4 V. As shown schematically in Figure 12i, TiS₂ undergoes a reversible (de)intercalation process in conjunction with the valence change of the titanium element during cycling. In a Zn/TiS₂ cell, the TiS₂ material provided a stable capacity of 120 mAh/g at 100 mA/g, a high-rate capability of 76 mAh/g at 1000 mA/g, and strong cycle performance (70% capacity retention after 500 cycles at 1000 mA/g (Figure 12j)). Ex situ X-ray diffraction analysis revealed that the superior electrochemical capabilities are due to the good maintenance of TiS₂ during cycling, in which the Zn²⁺ cations can be easily inserted/extracted into/from the layered structure with only minor structural change.

Hybrid Metal Ion

Mg–Li batteries hybrid (MLIBs), which feature lithiation/delithiation at the cathode and magnesiation/demagnesiation at the anode, have received a lot of attention because they combine the benefits of the Mg anode and Li intercalation cathode with quick kinetics and high reversibility. In Mg–Li batteries, a number of chalcogenides have been effectively produced, including VS₄ [248], VS₂ [252], CuS [289,290], Cu₂S [246], Cu₂Se [291], FeS [249] and MoS₂ [247].

Cai et al. [251] first built an Mg–Li hybrid battery with a spinel Co₃S₄ cathode and an Mg anode. With a discharge voltage platform of 1.10 V vs. Mg²⁺/Mg and a high energy density of 439.5 Wh/kg at 0.1 A/g, the particle-like Co₃S₄-F provides an initial high discharge capacity of 779.8 mAh/g and retains 399.5 mAh/g in the 100th cycle. The synergistic effect of hierarchical Co₃S₄-F micro/nano particles and the APC-LiCl hybrid electrolyte resulted in the remarkable spinel Co₃S₄-F electrochemical performance during cycling. During the discharge process, the conversion reaction of Co₃S₄-F to metallic Co and sulfide Li₂S contributes to the ultrahigh capacity of the Mg–Li hybrid battery. On the other hand, due to the vast reserves of Na and Mg in the earth’s crust, the Na⁺/Mg²⁺ hybrid-ion battery (NMIB) has been provided some thought. A metallic Mg anode, a TiS₂ generated titanium sulfide cathode, and a 1.0 M NaBH₄ + 0.1 M Mg(BH₄)₂/diglyme hybrid electrolyte were used to create a highly reversible Na⁺/Mg²⁺ hybrid-ion battery [253]. With a large discharge capacity (200 mAh/g at 1 C rate), high-rate capability (75 mAh/g at 20 C rate), and long cycle life, the battery demonstrated outstanding electrochemical performance (90% capacity retention after 3000 cycles).

3.2.2. Applications of Chalcogenides in Metal–Sulfur Batteries

Li–S Batteries

Because of the vast reserves, low cost, and environmental friendliness of the S cathode material, as well as the high theoretical energy density (2567 Wh/kg) and specific capacity (1675 mAh/g), LSBs have become a promising contender for next-generation energy storage devices. However, ordinary LSBs, which include the S cathode, metallic Li anode, separator, and electrolyte, still have many flaws that make them unsuitable for practical applications [292,293]. There are several issues with the S cathode, including low conductivity of S and its discharge products (Li₂S), dissolution of lithium polysulfides followed by severe shuttle effects, and severe volume change of the S electrode on lithiation, all of which result in poorer utilization of S, self-discharging, lower Coulombic efficiency, and weaker cycle stability [294,295]. Various approaches have been reported to solve these difficulties, including combining sulfur with a range of conductive materials, optimizing the organic electrolyte, and integrating multifunctional separators or interlayers. Meanwhile, the most effective way is to investigate acceptable electrode materials with high conductivity and stability, as well as new electrolytes [296,297].

Because of their low Li/Li⁺ lithiation voltages, significant chemical interaction with sulfur-containing species, and good conductivity, TMC materials have proven to be promising. Furthermore, nanostructure design and compounding with other materials can improve the electrode’s performance. The unique nanostructure (nanotubes, core-shell, porous, yolk-shell, and hollow morphologies) can provide various active adsorption sites to fix S particles and polysulfide, allowing the stability to be maintained even further. The use of conductive materials on the cathode, such as metal oxides/sulfides, carbon materials, and conductive polymers, can improve electroconductivity, provide more active sites, prevent polysulfide dissolution, and lower interface resistance [297,298]. The electrochemical performance of composites is proved to be dependent on S dispersion, and MS materials can operate as S carriers for LSBs due to their superior conductivity, which can provide strong chemical adsorptions with polysulfides and prevent the shuttle process [299].

Jin et al. [300], for example, realized a strong sulfur limitation to eliminate polysulfide shuttling in the nanostructure of carbon-coated metal sulfides instead of pure sulfur cathodes. Figure 13a displays nanoparticles with a carbon shell and an iron disulfide core (FeS₂@C) that were made using the plasma evaporation process and then sulfurized. During the lithiation reaction, the FeS bonds in FeS₂ chemically bind the sulfur atoms, while the carbon shell physically limits sulfur loss within the enclosed structure. After 300 cycles, the FeS₂@C cathode delivers a steady specific capacity of 862 mAh/g due to the outstanding dual constraint of S (Figure 13b). Furthermore, by etching and subsequently selenizing layered double hydroxide (Figure 13c–e), Xu et al. [301] created CoSe₂ porous hollow flowers (CoSe₂-PHF), which combined the strong catalytic activity of transition metal compounds with the high electrical conductivity of selenium. The CoSe₂-PHF produced may effectively speed up the catalytic conversion of LiPSs, accelerate electron transport, and increase active sulfur usage during the charge–discharge process. As a result, the LSBs with CoSe₂- PHF/S-based cathodes had a reversible specific capacity of 955.8 mAh/g at 0.1 C and 766 mAh/g at 0.5 C, as well as a remarkably low-capacity decay rate of 0.070% per cycle within 400 cycles at 1 C (Figure 13f,g). The specific capacity of 542.9 mAh/g may be maintained even at a high rate of 3 C.

Na-S Batteries

Because of the natural abundance and low toxicity of sodium and sulfur, as well as a two-electron transfer per sulfur atom, which delivers six times the energy density of conventional Li-ion batteries, sodium–sulfur batteries (SSB) have emerged as an appealing choice for large-scale energy storage applications [302,303,304,305]. The development of SSB is hampered by the dissolution of intermediate sodium polysulfides. Short-chain sulfur (S₂₄) loading is one promising strategy for avoiding this problem, as it may prevent the formation of soluble polysulfides during the sodiation process. In addition to the creation of traditional carbon hosts, the incorporation of sulfiphilic sites into composite hosts has garnered some interest. It has long been known that polar metal or sulfide materials, such as Fe/Ni/Cu clusters, Co, and NiS₂, can provide strong polar-polar interactions for polysulfide immobilization, preventing polysulfide diffusion via a chemical catalytic effect and thus speeding up conversion kinetics from polysulfides to Na sulfides [296,306,307,308,309].

Using the synergistic catalytic effect of CoS₂ and the suitable hole size, F. Xiao et al. [309] reported a new approach for generating short-chain sulfur in bigger pores (>0.5 nm) (Figure 14a–d). They estimated that CoS₂ could act as a catalyst to weaken the SS bond in the S₈ ring structure, allowing for the synthesis of short-chain sulfur molecules based on density functional theory simulations. This prevents the formation of soluble polysulfides and provides the composite materials used in SSB with better electrochemical characteristics. Figure 14e,f demonstrates the optimized CoS₂/N-doped carbon/S electrode’s electrochemical performance, which includes high reversible specific capacities of 488 mAh/g after 100 cycles at 0.1 A/g and 403 mAh/g after 1000 cycles at 1 A/g, as well as improved rate performance of 262 mAh/g at 5.0 A/g.

Mg–S Batteries

The development of rechargeable Mg–S batteries is becoming a more appealing choice in recent times than current Li–S batteries owing to the low cost. Developing high-energy-density Mg–S batteries, however, is fraught with difficulties. To begin with, sulfur’s inherent electrical and ionic insulating properties necessitate the use of a large number of conductive additives throughout the electrode preparation process. During cycling, magnesium polysulfide (MgPS) intermediates and solid MgS deposition and decomposition are involved, which is a more complex multi-electron solid-liquid-solid conversion than in Li–S batteries [310]. The shuttling phenomenon happens when soluble polysulfides are produced, leading to capacity loss, Coulombic efficiency decline, and anode corrosion. On the other hand, the bivalent Mg ion’s large ionic size and slow diffusion capabilities will further impede the reaction kinetics [311]. In reversible Mg–S batteries, then, to promote cathode reaction kinetics and achieve fast Mg ion conduction and diffusion is critical [312,313]. To tackle this challenge, Zhao et al. [314] presented a Co₃S₄@MXene heterostructure as a sulfur host for reversible Mg–S batteries. The chemical interaction between the decorated Co₃S₄ nanocrystals host and polysulfide intermediates could well absorb and catalyze the polysulfide conversion, thus improving the electrochemical redox kinetics, according to XPS measurements and DFT calculations. Meanwhile, the MXene matrix has the potential to dramatically enhance Mg ion transport dynamics. As a result, the produced Mg–S batteries using Co₃S₄@MXene-S as the cathode material were able to display high sulfur utilization with a specific capacity of 1220 mAh/g and a capacity of 528 mAh/g after 100 cycles, as well as a reasonable rate performance even at 2 C. The innovative cathode design for reversible high-energy Mg–S batteries was revealed in this study.

3.2.3. Applications of Chalcogenides in Metal–Air Batteries

Because of its high energy density and environmental friendliness, rechargeable metal–air batteries have recently piqued researchers’ curiosity. Metal–Air batteries, among the several types of batteries, have received a lot of attention in recent years due to their impressive specific energies. Metal–Air batteries, with an abundant supply of oxygen from the atmosphere, may theoretically be lighter, cheaper, and last longer than traditional batteries.

Li–Air

Due to its high theoretical energy density, rechargeable Li–O₂ batteries (LOBs) have gained increasing interest and therefore been recognized as one of the potential prospects for electrochemical power sources [315,316]. Unfortunately, several technological challenges remain, including poor capacity, short cycle life, and low coulombic efficiency, which are mostly caused by the slow reaction kinetics and side reactions of the O₂ cathode catalyst [317,318,319]. As a result, developing effective cathode catalyst materials is critical. By using extremely active catalysts, researchers are currently working to improve the efficiency and selectivity of the process that takes place in the O₂ cathode. Although Pt, Ru, Pd, Au, and their oxides have good catalytic properties, their high costs prevent them from being used in large-scale commercial applications. Particularly, transition metal chalcogenides and their composites have therefore gained popularity for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) [320,321,322,323,324] due to their low cost, higher conductivity, and high intrinsic reactivities.

CoS₂ nanocages produced from ZIF-67 and interconnected carbon nanotubes (CoS₂/CNTs) were recently developed and employed in the manufacturing of air electrodes for LiO₂ batteries (Figure 15a–d) [325]. The reversible capacity was 2398.9 mAh/g, which is 90% of the discharge capacity. When the cycling capacity was limited to 500 mAh/g, the battery maintained cycling stability for 52 cycles while keeping the cutoff potential above 2.0 V. CNTs are used as channels for fast electron transfer, which compensates for CoS₂‘s weak conductivity. Additionally, the porous CoS₂ nanocages may provide abundant active sites for an ORR and an OER, facilitating O₂ diffusion and electrolyte infiltration. Li et al. [326] also described a method for causing CoSe₂ to transition from the cubic to the orthorhombic phase by doping it with phosphorus (Figure 15e). Because phosphorus has a lower electronegativity than selenium, it is useful for altering the number of d electrons on the Co cation and so contributes significantly to structural phase transition and electrocatalytic activity. As a result, the LiO₂ battery with the phosphorus-doped orthogonal phase CoSe₂ (o-CoSe₂|P) electrode shows outstanding rate capability and cyclability (Figure 15f).

Zn–Air

Because of its low cost, great safety, and ultra-high theoretical energy density (1086 Wh/kg), zinc–air batteries (ZABs) are one of the most promising clean energy technologies for meeting the next generation green energy demand [327,328]. Nonetheless, the sluggish kinetics of the processes involved in these devices, particularly the hydrogen evolution reaction (HER), OER, and ORR [329,330], significantly limit the devices’ work efficiency. As a result, low-cost catalysts with high activity and endurance are critical for the development of effective rechargeable zinc–air batteries. The most effective electrocatalysts for these processes are currently noble metal Pt, Ir, and Ru-based catalysts and alloys, which are expensive [331,332]. Chalcogenide-based catalysts [333] have piqued interest as cost-effective, high-efficiency, diverse composition, customizable electronic structure, unique redox characteristics, robust durability, and many active site alternatives for a variety of catalytic reactions. Transition metal sulfides (TMSs), for example, have drawn a lot of interest because of their high conductivity, ease of synthesis, and high activity [334].

Liu et al. [335], for example, used a metal-organic framework as a precursor to creating a CoS₂@MoS₂@NiS₂ nano polyhedron with a double-shelled structure that exhibited high catalytic activity for HER, OER, and ORR. Low overpotentials of 156 and 200 mV were attained for HER and OER, respectively, with a current density of 10 mA/cm², an ORR half-wave potential of 0.80 V, and a power density of 80.28 mW/cm² in Zn–air battery. Hybrid materials made of bimetallic (Co and Fe) sulfide (Co₈FeS₈) nanoparticles supported on N, S-doped carbon microparticles can serve as dual-function oxygen electrocatalysts, according to Chen and coworkers [336]. The materials are made by pyrolyzing silica-protected Fe- and Co-modified S-containing polyphthalocyanine at various temperatures (Figure 16a–e). The materials were provided with the names (Fe,Co)SPPc-T-sp, where T stands for pyrolysis temperature, and sp stands for silica protection. Among them, the material synthesized at 900 °C, denoted (Fe,Co)SPPc-900- sp, exhibits excellent electrocatalytic activities in an alkaline solution for both the ORR and the OER, with a half-wave potential (E_1/2) of 0.830 V vs. RHE and a current density of 10 mA/cm² at an overpotential (η₁₀)of 353 mV. Figure 16f shows an alkaline Zn–air battery with (Fe,Co)SPPc-900- sp as an air electrode that had a high peak power density of 158.6 mW/cm² (Figure 16g) and excellent charge–discharge cycling stability (for over 50 h), demonstrating superior catalytic performance over the corresponding battery with Pt/C and RuO₂ electrocatalysts (Figure 16h).

Wang et al. [337] described a simple method for making a new MnS_xO_2x/MnCo₂S₄ heterostructured electrocatalyst. Controlling the S–O atomic ratio on the surface of MnS_xO_2x fine-tuned the density and configuration of the active centers. With an overpotential of 367 mV @ 50 mA/cm² in 1.0 M KOH, the optimized MnS_0.10O_1.90/MnCo₂S₄ sample had outstanding oxygen evolution reaction performance. More remarkably, the overpotential at 10 mA/cm² in 0.2 M phosphate buffer solution was just 414 mV, with cycle efficiency declining by only 3.8% at 10 mA/cm² after 140 h. Li et al. [338] created a 3D ordered macroporous N-doped trimetal sulfides (3DOM N-Co_0.8Fe_0.1Ni_0.1S_x) bifunctional catalyst. The porous 3DOM structure provided several routes for bulk movement and catalytic active site exposure. This 3DOM catalyst demonstrates high bifunctional catalytic activity, with an ORR half-wave potential of 0.80 V (versus RHE) and low OER overpotential of 370 mV at a current density of 10 mA/cm² due to the synergistic action of trimetallic sulfide and N-doped carbon layer. At 10 mA/cm², the zinc–air battery with 3DOM N-Co_0.8Fe_0.1Ni_0.1S_x has a stable cyclability and a lifetime of over 200 h.

Al–Air

Many appealing features of an alkaline electrolyte aluminum–air (Al–air) battery include abundant resources, nontoxicity, zero-emission, easy recycling of Al(OH)₃ by-product, theoretical specific energy of 8135 Wh/kg, and fast replacement of Al electrode, all of which are ideal for electric vehicles [339]. However, due to the sluggish kinetics of the ORR and/or OER for the cathodic process, numerous major difficulties have been met for Al–air batteries, including reaction instability and high polarization. The development of highly efficient, low-cost, and long-lasting electrocatalysts or air electrodes for both alkaline and non-aqueous conditions is a vital component in putting this technology into practice [340].

In that regard, Cao et al. [341] used a one-step electrospinning method combined with a pyrolytic process to make graphitic layers covered with Co₉S₈ nanoparticles trapped inside N,S dual-doped hierarchical porous carbon nanofibers. The obtained Co₉S₈@G/NS-PCNFs exhibited ORR activity (E_onset = 0.93 V vs. RHE, E_1/2 = 0.82 V vs. RHE) comparable to those of state-of-the-art Pt/C (E_onset = 0.98 V vs. RHE, E_1/2 = 0.84 V vs. RHE) in alkaline electrolyte Al–air batteries. Specific capacity of 2812 mAh/g at a current density of 35 mA/cm², a high open-circuit voltage of 1.74 V, and a peak power density of 79.04 mW/cm² were achieved, demonstrating good electrochemical capabilities.

3.3. Applications of Chalcogenides in Supercapacitors

Supercapacitors (SCs), because of their high power density, long cycling life, quick charging-discharging rate, and environmental safety, are thought to be one of the most popular technologies for energy storage. However, charge accumulation is limited on the electrode surface, which limits the energy density of SCs in practical engineering applications. As a result, various research projects have been launched to obtain high energy density while maintaining its power density and cycling stability. The innovation of hybrid supercapacitors (HSCs), which consists of negative and positive electrodes with charge storage operations based on capacitive and faradaic processes, respectively, has emerged in recent decades. Nonetheless, selecting appropriate negative and positive electroactive material that can work well in a variety of potential window ranges in the same electrolyte is a major problem in the production of HSCs. One of the most significant factors for the design architecture and manufacture of the negative and positive electrodes for HSCs is the above-mentioned component. Carbon-based materials were formerly thought to be a potential negative electroactive material for HSCs because of their extended cycle life, large specific surface area (SSA), good chemical stability, strong electronic conductivity, and enhanced porosity. Various kinds of carbon nanostructural architectures, such as graphene, activated carbon (AC), and carbon nanotubes (CNTs), have been employed as negative electrode materials in the recent past for aqueous alkaline-based HSCs.

Due to the abundance of carbon-based resources on the planet, ease of processing, and environmental friendliness, AC is a viable option. Metal chalcogenides which have faradaic and battery-like characteristics, on the other hand, have been widely used as the positive electrode material in HSCs in recent years. Because metal chalcogenides have superb reversible redox mechanisms on the bulk or surface of the electrode materials, they have a higher energy density as compared to the various commercialized carbon-based electrode materials [342]. In addition, the abundant redox-active sites, variable oxidation states, decreased bandgap, and significant electrical conductivity of transition metal chalcogenides (TMCs) are thought to be favorable battery types and SC electrode material [343]. They also allow for fast ion transport kinetic as a result of decreased diffusion path and low resistance at the electrode/electrolyte interface. This helps to increase the electrochemical properties in terms of rate capability, high capacity, and long cycle stability. The extended metallic characteristic of chalcogenides, as opposed to metal oxides/hydroxides, is largely responsible for their superior properties. This section focuses on various applications of chalcogenides in pseudocapacitors and HSCs.

3.3.1. Applications of Chalcogenides in Pseudocapacitors

Because electrochemical capacitors, particularly pseudocapacitors, have greater specific capacitances than EDLCs, they are promising for energy storage systems. Among various nanomaterial electrodes, such as carbon, transition metal oxides and/or hydroxides, and conducting polymers, TMCs (e.g., sulfides, tellurides, selenides, etc.) have been identified as one class of promising electrode materials for pseudocapacitors, due to their rich electroactive sites and electrical conductivity.

Metal Tellurides

Research on transition metal tellurides (TMTes) for electrochemical energy storage is still in its infancy. However, Te has a higher electrical conductivity, lower electronegativity, and a bigger atomic size than that of S and Se, allowing for more electrolyte ions to be accommodated with better diffusion kinetics. Tellurides are more distinctive among the chalcogenides because their chemical and physical properties are intermediate between metal and non-metal, resulting in products that have good electrochemical properties as well as high conductivity. Kumbhar et al. [344] described the pseudocapacitive performance of samarium telluride (Sm₂Te₃) thin films obtained via single-stage chemical synthesis. In a LiClO₄-propylene carbonate electrolyte, cyclic voltammetry indicated a specific capacitance of 207 F/g as well as a power density of 14.18 kW/kg. In recent years, the production of multilayer transition metal dichalcogenide nanocrystals with good energy conversion and storage capabilities has been thoroughly described. However, due to Te’s metallic nature, chemical synthesis of homogeneous, well-defined MoTe₂ nanoarchitectures remains a problem, particularly for achieving metasTable 1 T’-MoTe₂. For this reason, Liu et al. [345] synthesized ultrathin 1T’-MoTe₂ nanosheets using a colloidal chemical approach. Changing the Mo precursors and reaction environment can influence the morphologies of 1T’-MoTe₂, with CO playing a key role in determining the characteristic of the nanosheet. The few-layer 1T’-MoTe₂ nanosheets, after optimization, were employed as an efficient SC electrode, with a specific capacitance of 1393 F/g at a current density of 1 A/g. Using a simple one-step solvothermal technique with no surfactants, highly disseminated CoTe electrode material was effectively generated [346]. A solvothermal approach employing ethylene glycol as a solvent produced a regular nanowire-based CoTe, as opposed to the traditional hydrothermally generated irregularly-shaped CoTe. At a current density of 1 A/g, the produced CoTe nanowire electrode delivered a very high specific capacitance (643.6 F/g) and outstanding cyclic stability, even after 5000 cycles at a high current density (5 A/g), demonstrating exceptional rate performance.

Metal Selenides

In the electrochemical energy-storage industry, highly conductive metal selenides are gaining traction as viable electrode materials in pseudocapacitors. However, phase-pure-bimetallic selenides are difficult to come by, and their charge-storage methods remain a mystery. Metal selenides have proven to be a new family of faradic electrode materials that have been used in supercapacitors recently. In comparison to metal sulfides, no significant study on metal selenides has been documented in the TMC class of electrode materials. Selenium is the group VIA closest neighbor, with the same valence electrons and oxidation state as sulfur. As a result, the electrochemical and chemical properties of sulfides and selenides are nearly identical. Because of their superior electronic conductivity, superb redox chemistry, and better mechanical and thermal stability, metal selenides can be useful in supercapacitors [347]. Due to the high electrical conductivities, superior life cycle, pseudocapacitive behaviors, large SSA, and cost efficiency, silver nanostructured materials and their compounds, as well as silver-doped materials, have been employed as pseudocapacitor electrode materials. A well-controlled deposition of a pseudocapacitive thin-film nanostructured silver selenide (Ag₂Se) on a stainless steel substrate was prepared recently by using a simple, low-cost, room temperature, and scalable successive ionic layer adsorption and reaction technique and employed as an electrode material for supercapacitors [348]. After 2000 charge–discharge cycles, the Ag₂Se electrode retains 85.10% of its capacitive performance when used in a pseudocapacitor. This performance could be attributed to the surface-induced capacitive and diffusion-controlled reactions at the electrode/electrolyte interface.

A solvothermal approach was adopted to fabricate monodispersed hollow NiCoSe₂ (H-NiCoSe₂) for SC application, as shown schematically in Figure 17a [349]. The synthesized composite material displayed hollow sub-microsphere structures (Figure 17b), which contributed to enhanced ionic mobility. After about 100 CV cycles, it showed a competitive pseudocapacitance of 750 F/g at a current density of 3 A/g as compared to the other electrode materials (Figure 17c). The inherent energy-storage process of the as-synthesized H-NiCoSe₂ is convincingly revealed by the systematic physicochemical or electrochemical properties of the device, which accounted for the superb pseudocapacitance after cycling. An array of Co_0.85Se hollow nanowires (HNW) was also used to make an SC electrode, which was made utilizing wet chemical hydrothermal selenization of originally formed nanowires of cobalt hydroxyl carbonate on conductive CFP (carbon fiber paper) (Figure 17d) [350]. Scanning electron microscopy revealed the dense self-organized structures of the Co_0.85Se HNWs, which helped in facilitating ionic transport, as shown in Figure 17e. The as-synthesized electrode showed a strong pseudocapacitive characteristic (Figure 17f), as well as high capacitance retention and endurance.

Metal Sulfides

Due to the benefits of high theoretical capacitance, transition metal sulfides (TMSs) are projected to meet the high energy storage requirements of SCs. Simultaneously, they have a good electronic conductivity, which is mostly due to the lower electronegativity of sulfur compared to oxygen. Many researchers have developed a range of sulfides with various phase and metal structures, such as NiS, NiS₂, Ni₃S₂, Co₉S₈, MoS₂, and CoS, etc., in recent years. The introduction of vacancies/defects engineering, for example, can greatly increase the performance of TMSs. TMSs have been proposed as potential electrode materials for electrochemical pseudocapacitors. However, they are usually made of mixed composites or commercially stoichiometric TMSs (for example, NiCo₂S₄ and Ni₂CoS₄). The composites materials offer to enhance electrochemical performance. Several strategies have been used to improve the electrochemical performance of bi-metallic sulfides based on the above concerns. Through the sulfuration of NiCo-layer double hydroxide (NiCo-LDH) as a precursor on 3D graphene aerogel, a solid-solution sulfide composite electrode material (Ni_0.7Co_0.3)S₂/3DGA was prepared, as shown in Figure 17i [351]. The as-synthesized electrode delivered a capacitance of 2165 F/g at 1 A/g and retained about 78.5% of the initial capacitance after 1000 cycles for pseudocapacitors, according to the electrochemical studies shown in Figure 17g,h. These findings suggest that engineering LDH precursors can be used to generate a variety of TMSs for energy storage due to their ease of ionic transport.

Recently, an easy and direct-one-step chemical bath deposition technique was used to create binder-free nickel sulfide (NiS) nanostructures on commercial Ni-foam [352]. The electrochemical properties of the device were studied for pseudocapacitor performance. The porous architectures aided fast ionic/charge movement in the electrodes, which is advantageous for the development of electrochemical SCs. The as-synthesized device delivered a high pseudocapacitance (1442 F/g at 1.1 A/g), as well as good rate capability and a high specific energy and power density. As a result, the produced NiS electrodes are a suitable candidate for energy storage, as they provide faster ion/electron transport, a large SSA, higher electrical conductivity, and strong structural stability. For high-performance pseudocapacitors, Hong et al. [353] fabricated a binder-free electrode consisting of nanowire arrays of Co₃O₄@Co-NiS core/shell with varying Ni/Co ratios. The as-fabricated electrode provided the structural properties required for efficient active material use. This hierarchical electrode delivered a high specific capacitance of 1844 F/g at 5 mA/cm² due to the superiority of the well-ordered structure, making it a potential material to be used in electrochemical energy storage. Liu et al. [62] have recently synthesized a metal sulfide porous composite material with exceptional pseudocapacitance qualities. FeCo-MIL-88 was first synthesized using a solvothermal technique. The FeCo-MIL-88 was then treated at high temperature with thiourea, nickel chloride, and sodium sulfide to produce Fe_xCo_yS_z-MIL-88@NimSn. Finally, Fe_.92Co_.08S@NiS/NiO composite was formed by calcining Fe_xCo_yS_z-MIL-88@NimSn in an N₂ atmosphere. The lamellar architecture created a huge gap for efficient electron transmission, and the composite exhibited notable electrochemical capabilities. This featured a high specific capacitance of 1868 F/g at 1 A/g and a high level of cycling stability even after 5000 cycles.

3.3.2. Applications of Chalcogenides in Hybrid Capacitors

In order to overcome the limitations of rechargeable batteries and conventional SCs, transition metal hybrid capacitors have been advanced. They are mostly used in high-efficient energy storage devices due to their excellent electrical conductivity and electrochemical activity. The negative electrode of the hybrid capacitor is made up of SC materials (carbon-based materials), while the positive electrodes are made up of battery-type or pseudo-capacitor materials. In comparison to conventional SCs and batteries, the resulting hybrid capacitor has a high energy and power density as well as good cycling stability. Nanomaterials, for example, cobalt and nickel-based transition metal hydroxides, sulfides, oxides, phosphates, tellurides, nitrides, etc., have been well researched as superb electrode materials in hybrid capacitors, storing energy in aqueous electrolytes via characteristic reversible redox actions.

Metal Sulfides

Scientists are particularly interested in nickel sulfide nanoparticles (NiS) with hierarchical structures because of their numerous uses in domains such as catalysis, energy, materials, etc. NiS hierarchical structures have appealing features such as a low density and large SSA, which are ideal for electrocatalytic and energy storage applications. NiS is also highly stable, with better redox behavior, and has lower dissolution in alkaline solutions rich in hydroxide ions. As a result, NiS and its various composites are used in battery-type hybrid capacitors as a positive electrode [354]. Nesakumar and co [355] used thiourea and aqueous nickel chloride mixture as an electrolyte to create hierarchical NiS on nickel foam (NiS@Ni) by adopting a simple and easy cathodic electrodeposition technique. Common electroanalytical procedures (cyclic voltammetry, CV and galvanostatic charge–discharge, GCD) were used to examine the positive electrode performance of the NiS@Ni in aqueous KOH. In aqueous KOH, the CV and GCD curves of NiS@Ni revealed distinct reversible redox peaks, confirming the Faradaic battery-type features. The NiS@Ni exhibited an excellent specific capacitance of 1109 C/g at a scan rate of 5 mV/s and 2 A/g, which could be employed as a potential battery-type hybrid-capacitor positive electrode. In the same way, Kaliaraj and Ramadoss [356] also utilized RF magnetron co-sputtering to create a thin film of nickel–zinc sulfide nanocomposite (Ni–ZnS) on a stainless steel substrate, which was employed as the cathode material in an HSC. When compared to the thin film of the ZnS electrode, the electrochemical analysis demonstrated that the Ni–ZnS thin film nanocomposite electrode delivered a greater specific capacitance. The findings show that doping with Ni has a significant impact on the electrochemical performance of ZnS and that the as-synthesized material can be employed as a possible positive electrode for HSCs.

The structure of vanadium sulfide (VS₂), which is likewise a family of the transition metal dichalcogenides (TMDs), can be expressed as vanadium metal sandwiched between two sulfur layers, which in turn forms a tri-layer of S–V-S glued together using weak Van der Waals forces. The structure allows guest ions to be inserted without altering it, allowing for fast and efficient Faradic operations. It is believed that carbon addition to VS₂ improves the storage performance. With this knowledge, Meyer et al. [357] adopted a hydrothermal synthesis and subsequent optimization of carbon-supported vanadium disulfide nanocomposites that could be used as SC electrodes. The enhanced electronic conductivity of multi-walled CNTs, combined with vanadium disulfide’s mild electrocatalytic capacity, generated a synergy that improved effective redox transfer of charges, resulting in increased capacitance. The optimum electrode’s minimum resistance to charge transfer and peak to peak voltage difference indicated considerably enhanced charge transfer, which was due to the increased electronic conductivity bolstered by the carbon content. Their findings indicate that vanadium disulfide electrodes with carbon support could be utilized in SCs in the future.

Molybdenum disulfide (MoS₂) nanostructure is one of the metal sulfides that have been studied for SC applications in recent years. Because of structural breakdown, a considerable change in volume during cycling, nanostructure aggregation, and poor conductivity, MoS₂ nanostructures have low-rate efficiency and cyclability. In addition, MoS₂’s low cycle stability remains a problem, restricting its broad use in energy storage devices; as a result, several ways are frequently offered to address these concerns, including MoS₂ modification with appropriate atom doping. Nitrogen and carbon doping has proven to be effective in addressing these shortcomings. Fayed and co-researchers used a simple one-step hydrothermal approach to co-doped carbon and nitrogen into MoS₂ nanoflakes [358]. The as-synthesized material, when used as an electrode in SC, recorded a specific power and specific energy of 912 W/kg and 45 Wh/kg, respectively, with retention of about 90% after 3000 cycles. All of these findings demonstrated the material’s utility in energy storage applications as a result of heteroatom doping. Recently, vacuum freeze-drying, cross-linking mechanism, heat treatment, and hydrothermal technique were employed by Liu et al. [359] to fabricate a variety of nickel–cobalt–bimetallic sulfide doped with graphite carbon (NCBS/C) nanohybrids. By altering the composition, the electrochemical characteristics of the NCBS/C nanohybrids electrode materials were studied. When used in an asymmetric SC, the NCBS/C nanohybrid and AC as the positive and the negative electrode respectively displayed high electrochemical performance. The transition metal ions cross-linking was well disseminated in the carbon source matrix, which accounted for these performances.

To manufacture ternary arrays of Ni–Co–S linked-nanosheet on conductive carbon substrates as HSC electrodes, Chen et al. [360] adopted a simple one-step electrodeposition process, which resulted in outstanding energy storage capabilities. The asymmetric SC fabricated by the nanosheet arrays of the ternary sulfide (positive electrode) and porous graphene film (negative electrode) (Figure 18a) demonstrated enhanced electrochemical performance for real-life energy storage applications (Figure 18d,e). This was attributed to the nanosheets’ mesoporous nature and open framework of the 3D nanostructures, as shown in Figure 18b,c. The device also displayed long-term cycling stability even up to 50,000 cycles, as shown in Figure 18f.

Metal Selenides

Transition metal selenides (TMSes) and their composite have been reported recently as electrode material in battery-type SCs, which have high theoretical capacitance. Because of their possible use in various energy storage devices, they have received a lot of interest owing to their great electrochemical activities and high inherent electrical conductivity. However, the reported TMSe’s low energy densities, which are due to their poor mobility of electrolytic ions and tiny active surface area, severely limit their real-life application. For its use in HSC, a simple electrodeposition approach was used to develop Zn–Co–Se on diverse conductive substrates (Figure 19a) [361]. With a maximum operating potential of 1.6 V, the constructed HSC (Zn-Co-Se@Graphene-Ink) (Figure 19c) as the outer and core electrode produced an ultrahigh energy density (approximately 16.97 Wh/kg) and a power density of 539.63 W/kg as shown in Figure 19b. A series of two asymmetric SC devices were used to light up light-emitting diode (LED) bulbs with high intensity for real-time applications, which was successful. As a result, the research offers a novel approach to creating TMSes for energy storage and conversion. Because of the slow diffusion-controlled electrochemical mechanisms in aqueous alkaline electrolytes, nickel chalcogenides as electrode materials frequently have poor rate capabilities and cycle stability. Recently, Co ions were used to manufacture nanotubes of Ni_xCo_1−xSe₂ [362]. Co ions served as both electronic and morphological modulators. Furthermore, by adjusting the Co²⁺ input amount, the electrochemical kinetics of the device was adjusted utilizing a conventional two-step hydrothermal technique, which delivered a high specific capacitance (1157 F/g at 1 A/g). These performances could be attributed to the synergistic impact that combines the complementary benefits of metal Co and Ni ions, resulting in a large increase in specific capacitance and cycling performance.

A unique, simple approach was also used to fabricate nanosheet networks of porous ultrathin nickel selenide (NiSe NNs) on nickel foam, as shown in Figure 19d [363]. The removal of Mn produced NNs with a highly porous architecture. The ultra-thin nanosheets and superabundant pores of the device aided in the creation of electroactive sites resulting in fast ionic reactions, as shown in Figure 19e. This novel technique for developing new electrode materials for use in HSCs delivered an enhanced electrochemical performance (Figure 19f,g) and provides new paths toward commercializing HSCs using TMSe electrodes. Structural architecture and heteroatom doping are two popular means for improving supercapacitor electrochemical performance. Using a facile two-step hydrothermal process with a SiO₂ template, Qu et al. [364] effectively synthesized a novel tremella-like Ni–Co selenide with ultra-thin and porous nanosheets. The selenization process lowered the resistance and increased the device’s conductivity. The assembled HSC delivered an unusual energy density (53.7 Wh/kg at a power density of 822.5 W/kg), which is higher than the majority of Ni–Co selenide. These features demonstrate the material’s enormous potential in practical SC applications.

Metal Tellurides

Metal tellurides, despite being less studied in SC applications, have distinct advantages over other chalcogenide elements. Tellurium (Te) is part of the same O family as other elements with similar chemical properties. It does, however, have some unique physical and chemical features. Te atoms are known to be bonded together in a hexagonal lattice by weak Van der Waals forces to form 1D architectures. Because they provide a large SSA, a robust inter-penetrating channel, and a proficient electron/ion transport pathway, metal tellurides have recently attracted new interest due to their substantial applications in the domains of energy storage, sensing, and thermoelectric devices. For example, Bhol et al. [365] reported the production of Co-decorated Te nanotubes (NTs) employing TeNTs as a template (Figure 20a), as well as the evaluation of their electrochemical performance in HSCs. To begin, they synthesized 1D TeNTs. They further constructed an asymmetric SC with AC as the cathode material. At a current density of 2 A/g, the fabricated device (CoTe-2/AC) demonstrated a high specific capacitance (147 F/g) in a 4 M KOH electrolyte. Furthermore, the CoTe-2/AC assembly attained an exceptional energy density of 51.1 Wh/kg (at a power density of 2294 W/kg), confirming the as-prepared Co-decorated TeNTs to be an effective electrode material (Figure 20b). The device also exhibited good stability during the cycling operations even after 4000 cycles, as shown in Figure 20c. The material’s exceptional electrochemical performance is due to its 1D nanostructure with good conductivity. The hydrothermal technique was also used in preparing nickel–cobalt telluride nanorods (NiCoTe-NRs) in one pot utilizing ascorbic acid and cetyltrimethylammonium bromide as reducing agents (Figure 20d) [366]. After modification, the synthesized composite material (NCT-1 NR) was employed as a supercapacitor electrode. A hybrid asymmetric supercapacitor device based on NCT-1 (positive electrode) and orange-peel-derived AC (negative electrode) delivered a high energy density of 43 Wh/kg and a power density of 905 W/kg, as well as a 91% cyclic stability after 25,000 cycles (Figure 20e,f). In a follow-up work by Bhol and his group [54], cobalt–iron (Co–Fe) adorned TeNTs were synthesized using a wet chemical approach. Co–Fe adorned TeNTs together with TeNTs in the backbone had a 1D structure that provided improved conductivity and displayed enhanced electrochemical performance. It was then paired with the AC electrode to optimize the energy density performance. In the 4M KOH electrolyte, the asymmetric arrangement delivered a specific capacitance of 179.2 F/g (at 0.9 A/g). This possible discovery demonstrates TeNTs’ usefulness as a template for bimetallic tellurides synthesis with unique geometries. For energy storage applications, the synergistic impact of several metals and anisotropic shape is advantageous.

3.4. Chalcogenides for Flexible Devices

3.4.1. Flexible Supercapacitors

Due to rapid technological advancements, wearable and portable devices have increasingly become a part of our daily life. As a result, the design of wearable energy storage gadgets is increasingly focusing on the creation of flexible wearable technologies. In wearable devices, a flexible SC is essential. Flexible electrodes, being a crucial component of flexible SC, have an impact on the capacitor’s flexibility, stability, and performance. The electrospinning technique is used to create nanofiber materials. They are appropriate for usage as flexible electrodes due to their high flexibility. They also have a lot of potential as the next generation of wearable SCs. Carbon-based materials for SCs, on the other hand, cannot compete with conventional batteries due to their poor energy density, which limits their applicability. Transition metal chalcogenides such as metal sulfides, metal selenides, metal tellurides, etc., have shown high electrochemical performance for flexible devices. As a result, TMCs with specific morphologies, such as hollow and linear, must be prepared. TMCs in flexible SCs require qualities such as high power density, long cycling stability, and exceptional mechanical integrity. The electrolyte and separator used in flexible SCs must also have outstanding mechanical qualities, as well as the properties of traditional capacitor electrolytes and separators. The shape of electrode materials is crucial for fabricating flexible SCs. The materials must be able to bear mechanical stress as well as have the qualities that high-performance energy storage technologies require.

For example, through a hydrothermal reaction, nanowire arrays of binary sulfides Ni₃Co₆S₈ (NCS) were produced directly on a nitrogen-doped carbon foam (NCF), which was used as a binder-free electrode for an all-solid-state SC (Figure 21a) [367]. Nanosheet’s unique needle-like structures (Figure 21b–e) efficiently improved the active material consumption efficiency and enhanced the electrochemical performance. The NCS–NCF electrode delivered a high-specific capacitance of 243 mAh/g, as well as good rate capability and outstanding cycling stability, as shown in Figure 21f,g. To construct an asymmetric all-solid-state SC, the NCS–NCF was used as a positive electrode and the NCF electrode as a negative electrode. The fabricated device was used in a real-life application to light an LED lamp, which showed an outstanding performance over time (Figure 21h), indicating its potential as an electrode in flexible SCs. A composite material made up of copper sulfide, zinc sulfide, and porous carbon (PC) was also prepared by Zhai et al. [129], which demonstrated an outstanding SC electrode performance. The combination of copper sulfide/zinc sulfide with high pseudo-capacitance and PC material with superb double-layer capacitance produced exceptional electrochemical performances as a flexible SC electrode. The fabricated composite electrode material, when used in symmetric flexible SC, delivered a high energy density, 0.39 Wh/cm² at 4.32 W/cm² power density. As a result, the copper sulfide/zinc sulfide/PC cotton nanocomposites developed served as an energy storage device with high energy density, which is fascinating and crucial for increasing the flexible SC practical applications.

Layered metal chalcogenide is a promising new material for high-performance flexible SC electrodes. However, due to poor conductivity caused by the low mobility of carriers, SC performance based on layered TMCs cannot be improved further. To help address this, Mu and co-researchers [368] manufactured flexible interdigital all-solid-state micro-SCs (MSCs) with high mobility of carriers using layered III-VI InSe metal chalcogenides. The MSCs exhibited exceptional performance, including long-term cycling stability and enhanced mechanical flexibility after several bending cycles. The MSCs also delivered a high energy density and an extraordinary power density, which could be attributed to the unique structural layers of the material. Because the electronic structures of ultra-thin 2D crystals are so closely linked to their qualities, several efforts have been made to adjust their electronic structures to satisfy the high requirements for the fabrication of next-generation smart and portable electronics. For the first time, Hu et al. [369] showed that hydrogen inclusion could alter the conductive behavior of a layered ternary chalcogenide of Cu₂WS₄ (Figure 22a) from semiconducting to metallic, accompanied by a large increase in electrical conductivity. The synthesized material showed nanosized free-standing sheets with a single-crystalline structure as shown in Figure 22b–d. Furthermore, the metallicity of Cu₂WS₄ with hydrogen inclusion was stable and could withstand the high-temperature treatment. With a capacitance of 583.3 F/cm³ at a current density of 0.31 A/cm³ (Figure 22e) the constructed all-solid-state flexible SC based on the hydrogenated-Cu₂WS₄ nanosheet film demonstrated remarkable electrochemical capabilities with excellent capacitance retention after 3000 cycles (Figure 22f).

To increase the sulfide conductivity and transfer rate of ions, an in situ carbon coating technique was employed to embed MXene loaded with sulfide in carbon nanofibers (CNFs) via electrospinning [370]. As a CNF, polyacrylonitrile (PAN) with a good rate of carbon conversion was utilized, and as a sacrificial pore-forming agent, polyvinylpyrrolidone (PVP) possessing a low carbon conversion rate was used. A thermally-induced phase separation procedure was used to create PAN-PVP-based porous CNFs (PCNF) with high meso- or macropore structures. In situ, nanoparticles of FeCo₂S₄ and Ti₃C₂T_x MXene with ultra-thin structures were fixed uniformly in PCNF, and a flexible hybrid film (FeCo₂S₄/MXene/PCNF) was created as the SC electrode material. The device produced a continuous pathway for fast electrolyte transport. As a result, the optimized, flexible hybrid membrane film exhibited outstanding cycling stability, effectively addressing the issues of weak cyclic stability and sulfide conductivity.

3.4.2. Flexible Batteries

Various flexible electronic gadgets such as skin-like sensors, roll-up displays, flexible watches, etc., are currently being developed at a rapid pace. These devices require flexible storage systems, e.g., flexible batteries. As a result, efficient electrocatalysts must be developed to optimize the cathodic reactions of the storage devices at the same time. Nonetheless, the slow electrochemical kinetics of the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) severely limit their energy exploitation and power generation. For HER, ORR, and OER, noble metals such as IrO₂, Pt, and RuO₂ have been utilized as extremely efficient standard catalysts. Their high cost and paucity, on the other hand, expressly limit their application to industrial areas on a massive scale. Therefore, designing highly stable, active, cost-effective, versatile catalysts to improve OER, ORR, and HER activities for scientific purposes is critical. Transition metals chalcogenides and related compounds such as sulfide, oxides, tellurides, and selenide-based catalysts for flexible battery storage are emerging as a class of cost-effective prospective alternatives to the conventional noble metal catalysts. For example, Harish et al. [371] recently created Fe–Sn oxy-selenide (Fe_xSn_1−xOSe) through a simple and easy hydrothermal and subsequent selenization procedure with enriched oxygen vacancies. The Fe_0.33Sn_0.67OSe air-cathode-based flexible zinc–air battery (FZAB) delivered a high-power density of 153.96 mW/cm² and an ultralong 400-h cycle life, demonstrating its prospects in flexible energy storage.

Exploration of efficient, cost-effective, and corrosion-resistant oxygen electrocatalysts aids in the advancement of ZABs. A unique ternary spinel CoIn₂Se₄ nanomaterial was created utilizing a simple and environmentally acceptable process, with In³⁺ and Co²⁺ filling the octahedral and tetrahedral positions of the crystalline structure, respectively, as shown in Figure 23a [372]. The SEM image in Figure 23b showed the composite’s nanosheets with equal distribution of the various elements. The fabricated composite material (CoIn₂Se₄) was employed as the air cathode for a solid FZAB because of its lower charge resistance and higher active site exposure. The system outperformed the catalyst made of Pt@Ir/C, with a much higher specific capacity of 733 mAh/g_Zn and outstanding flexibility, as well as a long cycle life of over 400 cycles (Figure 23c). The CoIn₂Se₄-based FZABs were connected in series and successfully powered LED screens for more than 10 h as displayed in Figure 23d,e. Nitrogen-doped carbon (NC) materials and TMSs have both been reported as effective ORR and OER electrocatalysts. An in situ generated 3D-array-structured catalyst constituted of nanoneedles of Co_1−xS enwrapped by NC was used to study the electrochemical performance of FZABs [373]. When cycling at 5 mA/cm², the FZAB with the catalyst delivered a significantly higher peak power density (242 mW/cm²) and a high round-trip efficiency of 64%. A macro-porous architecture of hydrangea-shaped Co_0.85Se electrocatalyst was also synthesized by Feng and colleagues [374]. Certain defects (S-doped) were inserted into the crystal structure to increase the electronic conductivity of Co_0.85Se (Figure 23f). Due to the synergistic effect of Se and S, the S-doped Co_0.85Se displayed an extraordinary electrocatalytic activity when used as a Li–S battery cathode on flexible carbon cloth (Co_0.85SeS@CC). The device’s performance (Figure 23h,i) can be attributed to the porous and open networks (Figure 23g) which allowed electrolyte penetration and accommodated sulfur volume fluctuation during the charge–discharge operation.

The use of self-supported electrodes with oxygen catalysts grown directly on the electrode support provides a viable alternative for improving ZAB performance. Yin et al. [375] first used a simple technique to make a self-supporting air electrode for ZABs with Mn doping and grown NiS on nickel foam (Mn–Ni₃S₂/NF). After a simple hydrophobic treatment, the optimized Mn–Ni₃S₂/NF electrode could serve as the flexible quasi-solid-state ZAB’s air cathode, which delivered a high maximum power density of 75.8 mW/cm², superb charge–discharge property, and enhanced stability after 100 cycles. Hybrid Zn batteries (HZBs), which mix Zn-ion (ZIBs) and ZABs, are primarily concerned with the development of high-performance cathodes. As a trifunctional electrode material for HZB, a freestanding hollow fiber joined by heterostructures of S, N-co-doped CNTs (SNCNT) contained Ni₃S₂ nanoparticles was proposed recently [376]. The porous and conductive networks of the device allowed for quick electron/ion routes and a large number of active spots, resulting in higher electrochemical performance. Furthermore, flexible HZB devices fabricated based on the Ni₃S₂@SNCNT flexible cathode achieved outstanding performance and adaptability in a variety of working environments. Even during abrupt changes in the working environment, they have a good unbroken working ability.

4. Conclusions and Future Remark

TMCs are ideal materials for electrochemical energy storage applications because of their unique chemical, structural, electrical, and electrochemical properties. As a result, this review opens with a brief overview of the importance of energy, energy storage devices, and their synthesis based on metal chalcogenides over traditional carbon-based electroactive materials in terms of improved energy and power densities. Due to their intrinsic fragile architecture and defect-assisted constrained charge transports, TMCs perform poorly as standalone electrode materials. This has prompted researchers to experiment with various organic/inorganic materials to improve the electrochemical performance of pristine TMCs. Fabrication of ternary and quaternary TMCs, composite manufacturing with carbon or conducting polymers, and heteroatom doping using several in situ, one-pot, and ex situ synthesis methods are all examples of modifications that have been studied in recent years. By changing TMCs with the various organic or inorganic materials, the inherent constraints of analogous materials will be compensated while maintaining or improving the performance of old systems, and new capabilities will be realized that are not possible with conventional materials. We have also included a list of many well-known synthesis techniques for creating modified and nanostructured TMCs. Furthermore, we have focused on the critical role of various modifying agents, such as carbon-based allotropes, heteroatoms, other metal-based composites, etc., in significantly influencing the pristine features of electroactive TMCs. How these agents aid in optimizing the electrochemical performance of TMCs by optimizing their rate of redox reaction kinetics and charge transport processes has also been discussed. Special attention was paid to utilizing various TMCs (such as selenides, tellurides, and sulfides) and their composites as electrode materials in solid-state batteries, supercapacitors, and their flexible components in real-life applications.

Although great efforts have been made in recent years to improve TMCs’ electrochemical performance to realize state-of-the-art applicable energy storage devices, there are still several key problems that need to be addressed. Prospects in this particular path will be defined by how we address such issues and overcome considerable limitations, some of which have been listed as follows:

(i).

Although several studies on modified TMCs have been published, their architecture-dependent feature modification has not been thoroughly examined. TMCs, for example, can be made in a variety of morphologies, including hierarchical, core-shell, and surface ornamentations.

(ii).

Advanced characterization methods such as in situ spectroscopy and imaging are appealing to the research industry because they provide real-time information about the reaction methods involved. These techniques can be used to establish a relationship between the material’s composition, structure, characteristics, and electrochemical performance. Furthermore, these techniques would aid in determining the underlying electrochemical reactions as well as monitoring the structural changes in the material during applications.

(iii).

Doping various heteroatoms such as oxygen, nitrogen, phosphorous, etc., into electroactive materials has been shown to improve their capacitive performance. Although this is relatively prevalent in carbon-based electroactive materials, it is quite rare in TMCs. Heteroatoms have been discovered to improve the electroactive materials’ electronic conductivity, which could aid rate performance in electrochemical energy storage applications. Furthermore, because of their affinity for various ionic species, these heteroatoms can act as anchoring points for ions within the electrolytes, thus increasing charge storage capabilities dramatically. For high-performance storage applications, heteroatom-doped TMC-based hybrid electroactive materials will be of great interest.