Pom-Pom Flower-like Morphology of δ-MnO₂ with Superior Electrochemical Performances for Rechargeable Aqueous Zinc Ion Batteries

Abstract

A rechargeable aqueous zinc-ion battery is an encouraging alternative for grid-scale energy storage applications, owing to its advantages of high safety, low cost, and environmental benignity. Since MnO₂ is found to be one of the most efficient intercalation cathode materials for ZIBs, the layered type δ-MnO₂ polymorph exhibits reversible intercalation/de-intercalation of Zn²⁺ ions with a high capacity. Unfortunately, the δ-MnO₂ cathode suffers from poor cyclability, low-rate capability, and structural degradation during charge–discharge cycles. Therefore, δ-MnO₂ with Pom-Pom Flower-like morphology have been synthesized using a facile hydrothermal method. The unique morphology of δ-MnO₂ provides a high surface area with numerous reaction sites, leading to excellent electrochemical performance. The obtained results revealed that the δ-MnO₂ electrode retained ~99% of its initial capacity even after 250 cycles, which can be ascribed to the reversible Zn²⁺ insertion/de-insertion from the current unique morphology of the layered δ-MnO₂ nanostructure. In addition, the electrochemical and structural investigation also indicates a two-step co-insertion of H⁺ and Zn²⁺ ions into the interlayer of δ-MnO₂ during the discharge process. Thus, the superior electrochemical performances of the δ-MnO₂ cathode paves a way for the high capacity and a long lifespan of zinc-ion batteries.

1. Introduction

Rechargeable lithium-ion batteries (LIBs) were invented in 1994, which led to the revolution in the electronic world [1,2]. LIBs then became the leading device in portable electronics, mobiles, and electric vehicles [3]. However, the researchers have now switched their attention in exploring the alternatives of LIBs due to the scarcity of lithium and thereby, its increasing price [4]. Among all the recent battery research based on multivalent ions (such as aluminium [5,6], zinc [7,8,9], magnesium [10,11,12], etc.), the aqueous zinc-ion battery exhibits successful results towards Zn-ion intercalation/deintercalation owing to its non-toxicity, low cost, high theoretical capacity (~819 mAh g⁻¹ and 5855 mAh cm⁻³), favorable low redox potential (−0.78 V vs. SHE), and higher zinc-ion diffusion rates [13,14]. Furthermore, aqueous ZIBs do not even require an expensive inert-gas glove box for the assembly of cells as they can be integrated in air, resulting in the ease in manufacturing procedure [15,16]. On the other hand, the high stability of metallic zinc with good chemical stability in water makes Zn a favorable anode for ZIBs. Furthermore, zinc-containing solutions such as ZnSO₄ or Zn(NO₃)₂ act as electrolytes for ZIBs. However, developing an efficient cathode for aqueous ZIBs has become an attractive goal now for researchers to commercialize ZIB [4]. Recently, various cathode materials, such as vanadates [17], Prussian blue analogous [18,19], polymers, NASICON-type phases (Na₃V₂(PO₄)₃) [20], and MnO₂ [21], have been investigated so far to build potential aqueous ZIBs. However, the NASICON-type structures and Prussian blue analogous offer a deplorable capacity [21,22]. Mantia et al. used copper hexacyanoferrate as a cathode with an aqueous electrolyte for zinc-ion batteries. [23] However, the material shows less capacity of only ~55 mAh g⁻¹ at 100th cycle. On the other hand, Alshareef et al. reported a layered Mg_xV₂O₅·nH₂O cathode for aqueous ZIBs, which delivered a high capacity of ~264 mAh g⁻¹ after 1000 cycles at 1000 mA g⁻¹ [24]. However, the toxic nature of vanadium-based oxides limits their application as cathode materials for ZIBs. The major hurdle to investigate a suitable cathode material is the high electrostatic repulsion that exists between divalent zinc ions and the high-charge density of zinc ions that leads to the lethargic transport kinetics, which severely hinders the development of aqueous ZIBs [15]. Recently, MnO₂ is found to be an excellent cathode among the majority of the transition metal oxides due to its high theoretical capacity, less toxicity, rich natural abundance, low cost, and moderate potential window [4,25]. There are diverse crystal structures of MnO₂ named α, β, γ, δ, and λ, in which the first three phases (α, β, γ) exhibit 1D tunnel-like structures and the latter two (δ, λ) belong to a 2D layered structure and a 3D spinel structure, respectively [26,27]. As per the literature reports, it is found that all the polymorphs can host Zn²⁺ ions for aqueous ZIBs. However, the studies conducted so far revealed that the tunnel structure of MnO₂ shows serious phase transition during charge and discharge [28,29,30]. During the cycling process, the phase transition of α-MnO₂ to δ-MnO₂ and β-MnO₂ to the layer-type phase impacts the electrode structure and is believed to be the main reason for the capacity fade [26,31]. On the other hand, the layered δ polymorph of MnO₂ exhibits minimal structural change during Zn²⁺ ion insertion/extraction with a high surface area, resulting in the reduced diffusion paths for Zn²⁺ ions [27]. In addition, the layered δ-MnO₂ structure is also energetically and kinetically favorable and stable with a low proton adsorption energy, which inhibits phase transition during cycling [25,32]. Moreover, since δ-MnO₂ exhibits excellent capacity with high-rate performances, it is found to be a viable choice for high-energy density applications such as portable electronics and electrical vehicles. Furthermore, its cost-effectiveness also makes it considerable for large-scale industrial applications where cost is a concern [27,32]. Unfortunately, the MnO₂ cathodes generally suffered from low electrical conductivity, serious structural degradation, and the dissolution of Mn²⁺ ions, leading to the fast capacity decay and subservient rate capability.

Therefore, among all the numerous efforts to enhance the electrochemical performances of the MnO₂ cathode, morphological optimization continuously attempted to resolve these issues. Though the considerable signs of progress have been noticed toward the fabrication of MnO₂ polymorphs mainly using the carbon materials (such as graphene, carbon nanotubes, and N-doped carbons), the direct growth of MnO₂ cathodes on current collectors, binder-free MnO₂ cathodes, etc. remains a big challenge to synthesize a sustainable morphology-based MnO₂ cathode with excellent electrochemical performances using a low-cost synthesis method without even using any carbon source. Hence, in the present work, the synthesis of a pom-pom flower-like architecture of δ-MnO₂ by a facile hydrothermal synthesis method using KMnO₄ and MnSO₄ as additives is being reported for the first time. The obtained morphology of δ-MnO₂ exhibits a high surface area with excellent structural stability and provides a pathway for ion transport, which can improve the overall conductivity of the cathode. As expected, the pom-pom shaped flower-like morphology of δ-MnO₂ as a cathode offers excellent rate capability, high capacity, and long-term cyclability; thus, providing a new pathway for developing high-performance aqueous ZIBs.

2. Experimental

2.1. Material Synthesis

A δ-MnO₂ cathode with a pom-pom shaped flower-like morphology was successfully synthesized using the facile hydrothermal synthesis method. Initially, 10 mmol of KMnO₄ (Emplura, 99%, India) and 1.7 mmol of MnSO₄.H₂O (Alfa Aesar, 99%, India) were dissolved separately in 50 mL and 20 mL of deionized water (DI), respectively. Thereafter, both solutions were mixed and kept for stirring. After vigorous stirring for 30 min, the obtained homogeneous solution was transferred to a Teflon-lined stainless-steel autoclave and heated up to 150 °C for 14 h. Afterward, the resultant solid product was centrifuged and washed with DI and ethanol several times. The sample was then dried overnight at 75 °C. At last, the as-prepared sample was put for calcination at 360 °C for 2 h to obtain the pure sample of δ-MnO₂.

2.2. Material Characterisation

To check the crystallinity and single phase of the calcined sample, XRD was collected using a Shimadzu X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å, scanning angle (2θ) = 5–90°). The transmittance property was carried out by Fourier-transform-infrared spectroscopy (FT-IR) in the range of 400–4000 cm⁻¹ using a Nicolet iS50 model of FT-IR. Raman spectroscopy (a Renishaw Raman microscope, 514 nm laser) was also used to study the phase composition of the calcined sample. The surface morphology, particles size, and elemental distribution in the sample were carried out by field emission-scanning electron microscopy (FE-SEM), field emission-transmission electron microscopy (FE-TEM, Philips TecnaiF20 at 200 KeV), and energy dispersive spectroscopy (EDS) mapping, respectively. Furthermore, the chemical state of the ions in the sample was examined by XPS (ThermoVG Scientific Instrument), using Al Kα as the X-ray source. The surface area and pore size distribution of the sample were measured by Brunauer Emmett Teller (BET) using nitrogen adsorption/desorption isotherm (Micromeritics Instrument Corp. Gemini-V).

2.3. Electrochemical Measurements

To scrutinize the electrochemical performances, the working electrode was fabricated by mixing the active materials, Ketjan black and teflonated acetylene black. The obtained paste was coated onto a stainless-steel mesh, which was then dried under vacuum at 120 °C, followed by cold pressing using a hydraulic press machine. The 2032-type coin cells were assembled in an open-air environment using glass fiber soaked with 2M ZnSO₄ solution as the electrolyte between the as-prepared cathode and zinc-foil anode. The electrochemical measurements were performed using WBCS 3000 (WonATech). A cyclic voltammetry (CV) test was carried out using an AUTOLAB PGSTAT302N potentiostat between 0.8 V to 1.9 V vs. Zn/Zn²⁺ at different scan rates.

3. Results and Discussion

3.1. Structural Identification and Morphological Studies

Figure 1 depicts the Rietveld refinement of calcined δ-MnO₂ using a monoclinic structure with the C2/m space group. The unit cell parameters were found to be a = 5.14, b = 2.84, and c = 7.14 Å with α = γ = 90°, β = 100.76°. The refinement was carried out using the general structure analysis system (GSAS)—II programs of Robert B. Von Dreele and Brian H. Toby [33]. The refinement parameters consisted of the scale factor and 36 background terms in a linear interpolation function and unit cell dimensions. Peak fitting, including symmetric corrections and micro-strain anisotropic broadening terms, were considered while carrying out the refinement. It is evident that all the reflections were very well accounted for during the fitting, indicating the formation of pure δ-MnO₂ with the perfect match between the observed and calculated profile. The crystal structure is also provided in the inset of Figure 1.

FT-IR and Raman spectroscopy are also carried out to further confirm the crystal structure and bonding of δ-MnO₂. As shown in Figure 2a, the broad and weak absorption bands located at ~3334 cm⁻¹ and ~1620 cm⁻¹ represent the stretching and bending frequency of the O-H group of the interlayered water molecules, respectively [27,34]. Furthermore, the absorption bands detected around ~1110 and ~1043 cm⁻¹ can be ascribed to the O-H bending vibration combined with the Mn atom [34]. On the other hand, the absorption peak found at ~468 cm⁻¹ is the distinctive peak of birnessite MnO₂, as it represents the Mn-O stretching mode of the octahedral layers of the layered MnO₂ structure [27,34,35]. Raman spectroscopy was carried out between 200–1500 cm⁻¹ to further confirm the structure of δ-MnO₂, as shown in Figure 2b. The three typical Raman bands can be clearly observed at ~478, ~561, and ~635 cm⁻¹, which are, in fact, the vibrational bands of δ-MnO₂. More precisely, the absorption peak obtained at ~635 cm⁻¹ mainly corresponds to the symmetric stretching vibration of [MnO₆] octahedron, whereas the peaks that appeared at ~478 and ~561 cm⁻¹ referred to the stretching vibration of Mn-O in the basal plane of MnO₂ sheets [14,36].

XPS is further employed to emphasize the surface chemistry and oxidation state of the ions of the fabricated δ-MnO₂ sample. Figure 3a shows the survey spectrum with a clear display of the elemental signal of only Mn, O, and C, confirming again the high purity of the sample. Furthermore, the high-resolution Mn 2p spectra shown in Figure 3b exhibits two peaks positioned at ~653.2 eV and ~641.3 eV with the splitting energy difference of ~11.8 eV between the two-spin doublet, indicating the +4 oxidation state of Mn in δ-MnO₂ [25,37]. Figure 3c shows the high-resolution XPS spectra of O 1s with the two deconvoluted peaks at ~529.2 eV and ~531.1 eV, which mainly belong to the tetravalent Mn-O-Mn bond and hydrated trivalent Mn-OH bond, respectively [25,38].

The surface morphology and detailed microstructural analysis of the calcined δ-MnO₂ sample are further characterized by FE-SEM and FE-TEM analysis. The FE-SEM image demonstrates pom-pom flower-like morphology with a diameter of nearly ~1 µm, as shown in Figure 4a. The inset of Figure 4a shows the original pom-pom flower image, as collected from Google, indicating a clear match with our current morphology. It is believed that the obtained morphology is mainly composed of primary nanosheets, which are assembled to form a pom-pom shaped flower like morphology, thereby reducing the total surface energy [25]. As can also be seen, the pom-pom flower-like morphology consists of micropores, which may provide a large contact area between the electrolyte and cathode, resulting in the fast-ion transfer during the charge/discharge processes [39]. The detailed morphological analysis is further illuminated by TEM images, as shown in Figure 4b,c, which are taken at low and high magnifications, respectively. FE-TEM images are perfectly analogous to the FE-SEM observation, demonstrating the pom-pom flower-like morphology of the calcined δ-MnO₂ sample. It is evident that the pom-pom shaped flower-like morphology is composed by the assembly of nanosheets, thus leading to the porous architecture. Moreover, the HR-TEM image (Figure 4d) exhibits a clear lattice fringe with a spacing of 0.342 nm, which corresponds to the (002) plane of δ-MnO₂. Energy dispersive spectra (EDS) were also employed to confirm the pure phase formation of calcined δ-MnO₂. The elemental mapping affirms the presence of a homogenous distribution of Mn (Mn-K: red) and O (O-K: green) without the presence of any other impurities, as shown in Figure 4e,f, respectively.

The specific surface area and pore-size distribution of the pom-pom flower-like morphology of δ-MnO₂ were further investigated by the Brunauer-Emmett-Tellar (BET) measurement, using a nitrogen adsorption/desorption isotherm. Figure 5 displays a type IV isotherm with an H3 hysteresis loop, illustrating the existence of mesoporosity in the sample. The inset of Figure 5 shows a Barrett-Joyner-Halenda (BJH) adsorption pore-size distribution curve with an average pore diameter of ~9.8 nm and a total pore volume of ~0.0585 cm³g⁻¹. The specific surface area of the δ-MnO₂ sample is found to be ~21.085 m²g⁻¹. The existence of a porous morphology within a pom-pom shape flower-like morphology of δ-MnO₂ offers high surface area along with a reduced diffusion distance for Zn²⁺ ions, which eventually improved the overall conductivity of the cathode and enhanced the electrochemical performances. It is also believed that the porosity reduces the risk of mechanical degradation and improves the overall lifespan of the battery [40,41].

3.2. Electrochemical Performances

Since δ-MnO₂ acts as an excellent host for zinc-ion insertion due to its layered structure, it gives better electrochemical results. As represented in Figure 6a, the electrochemical performance of pom-pom shaped flowerlike δ-MnO₂ was first determined by cyclic voltammetry (CV) at varied scan rates ranging from 0.1 mV s⁻¹ to 2 mV s⁻¹, which exhibits similar characteristics. The two cathodic peaks were observed at ~1.2 V and ~1.3 V, which can be ascribed to the intercalation of Zn²⁺ and H⁺ ions into MnO₂ and the reduction of Mn⁴⁺ into its lower oxidation state, respectively. In contrast, the two anodic peaks located at ~1.5 V and ~1.6 V are due to the de-intercalation of Zn²⁺ and H⁺ ions from MnO₂ and the oxidation of Mn³⁺ to Mn⁴⁺, respectively [37]. Thus, it can be concluded that it is a two-step reaction of Zn²⁺ ion insertion followed by the formation of spinel-type ZnMn₂O₄ [37]. The detailed reaction mechanism is discussed in the later sections. The peak shifting can be clearly noticed with the increase in the scan rates, which indicates the high reversibility and excellent cycling stability during the polarization period [37]. The capacitive current is also quantitively calculated using CV data at different scan rates by applying the following equation:

$$i=a{\nu }^b$$

(1)

where i is the peak-specific current, ν is the scan rate, and a and b are constants. Equation (1) can be expressed in the logarithm form as shown in Equation (2):

$$\log i=b log\nu +\log a$$

(2)

It is found that the material shows supercapacitor and battery behavior if the value of b is close to 1 and 0.5, respectively. The value of b can be calculated by plotting a linear graph between log i and log ν, where b is the slope of the graph, as displayed in Figure 6b. Since the graph clearly shows a good linear relationship, it indicates the diffusion-controlled characteristic during the charge storage process [37]. The calculated value of b for the anodic and cathodic processes are ~0.558 and ~0.668, respectively. Since the average value of b is nearly 0.613, which is close to 0.5, it determines the diffusion-controlled mechanism (battery behavior) for the δ-MnO₂ cathode. Furthermore, the diffusion/surface-confined contribution can be evaluated using the following equations:

$$\begin{gathered} i\left(V\right)=k_{1}v+k_2{v}^{1/2} \mathrm{or} \\ \frac{i\left(V\right)}{v^{\frac{1}{2}}}=k_1{v}^{1/2}+k_2 \end{gathered}$$

(3)

where i(V) is the total current at a constant potential; and k₁ν and k₂ν^1/2 represent the separate contribution of surface capacitive effects and diffusion-controlled insertion, respectively. The value of k₁ and k₂ can be obtained from the slope and intercept of a linear plot of i(V)/v^1/2 versus v^1/2. Though the current contribution from the surface capacitive effect (k₁v) and diffusion-controlled insertion processes (k₂v^1/2) are also calculated, the diffusion-controlled contribution is near ~90% at 0.1 mV s⁻¹, as shown in the bar graph of Figure 6c. However, this ratio keeps decreasing as the scan rate increases and eventually reaches the final value of ~70% for the diffusion-controlled mechanism at 2 mV s⁻¹. Thus, it can be suggested that this process takes less time for electrolyte ions to diffuse and react with internal active materials [37]. Figure 6d and e represent the galvanostatic charge/discharge curves of the pom-pom shape flower-like mesoporous δ-MnO₂ cathode at a constant current density of 200 mA g⁻¹ within the voltage window of 0.8 V to 1.9 V for the initial 3 cycles and 10th to 20th cycles, respectively. Upon the first discharge, the potential of the cell sharply decreases until 0.8 V vs. Zn²⁺/Zn with a specific capacity of ~120 mAh g⁻¹, while on the subsequent charge, the potential of the cell smoothly increases up to 1.9 V with the clear plateau at 1.6 V and specific capacity of ~141 mAh g⁻¹. Furthermore, the discharge/charge capacity for the 2nd and 3rd cycles is found to be ~202/189 mAh g⁻¹ and ~220/221 mAh g⁻¹, respectively, with a significant enhancement of ~100% into the coulombic efficiency. More importantly, it can be also observed from Figure 6e that the pom-pom shape flower-like δ-MnO₂ exhibits high reversibility in the successive cycles with nearly the same discharge/charge capacity at constant discharge and charge potential plateaus at 1.3 V and 1.5 V, respectively. The electrode delivered the specific capacity of ~299.9/298.9 mAh g⁻¹ for the 20th cycle with almost 100% coulombic efficiency. Since the layered δ-MnO₂ host has a large interlayer distance including this novel porous morphology, it is believed that the insertion/de-insertion of Zn²⁺ ions is highly favorable and accelerated; and thus, a decent capacity is achieved. More importantly, the plateaus are in accordance with the CV results [24,38]. The significant difference observed between the first and successive discharge and charge cycles can be accredited to the inappropriate wetting of the electrode and the gradual activation of the electrode material [27]. In addition, the electrode’s high surface area along with the mesoporosity may also help to accommodate the absolute volume expansion/contraction that occurred during the charge and discharge of Zn²⁺ ions. Moreover, Figure 6e exhibits the perfect overlapping between the discharge and charge cycles with nearly similar capacity after the initial three cycles, indicating the excellent reversibility of the electrode material.

To further examine the rate capability, the pom-pom flower-like δ-MnO₂ electrode was cycled at various current densities ranging between 200 mA g⁻¹ to 1600 mA g⁻¹, as displayed in Figure 6f. Upon cycling for 10 cycles at each of the current densities of 200, 400, 800, and 1600 mA g⁻¹, the electrode material delivered charge capacities of ~290.9, ~230.5, ~186.4, and ~154.3 mAh g⁻¹, respectively. The decrease in capacity with the increase of current density is a general phenomenon, as ions do not attain sufficient time to insert/de-insert at high current rates. However, it is worth noticing that the capacity fading at a high current density is considerably less as compared to the increment in the current density from 400 to 1600 mA g⁻¹. Therefore, the obtained high-rate performances can be attributed to the current unique morphology with mesoporosity, which may have offered more active sites for electrochemical reactions and numerous diffusion channels for Zn²⁺ insertion/de-insertions [40]. Furthermore, when the current density again returned to all the current densities, the recovery of the specific capacity was near ~82%, indicating the high structural stability of the pom-pom flower-like δ-MnO₂ electrode.

To further evaluate the structural stability of the pom-pom flower-like δ-MnO₂ electrode, the cycling performance was tested again at 400 mA g⁻¹ for the initial 15 cycles, followed by 1000 mA g⁻¹ until 250 cycles, as shown in Figure 6g. After the activation of the electrode at 400 mA g⁻¹, the electrode shows stable and distinguished cycling performances at a high current density of 1000 mA g⁻¹, without any significant capacity decay until 250 cycles. The specific capacity is found to be ~238 mAh g⁻¹ for the 15th cycle at 400 mA g⁻¹, which remained ~166 mAh g⁻¹ after 250 cycles with nearly 100% coulombic efficiency, again indicating the high durability of the electrode material. As per the comparative

Table 1. A comparison between the specific capacities of the current pom-pom flower-like morphology of δ-MnO₂ electrode and previously reported δ-MnO₂ electrode based on various morphologies.

Material	Morphology	Specific Capacity/Cycle Number/Current Densities	Ref.
δ-MnO2	Nanosheets	133 mAh g−1 after 100 cycles at 100 mAg−1	[27]
δ-MnO2/graphite	Nanoflower	114 mAh g−1 after 100 cycles at 400 mAg−1	[39]
δ-MnO2	Submicrospheres	96 mAh g−1 after 100 cycles at 100 mAg−1	[14]
Ni doped δ-MnO2	Random particles	16t mAh g−1 after 110 cycles at 100 mAg−1	[42]
δ-MnO2	Floret-like particles	60 mAh g−1 after 125 cycles at C/25 C-rate	[13]
δ-MnO2	Flower-like	70 mAh g−1 after 100 cycles at 100 mAg−1	[43]
δ-MnO2	Flake-like particles	97 mAh g−1 after 50 cycles at 100 mAg−1	[44]
δ-MnO2/CNT	Flower-like	96 mAh g−1 after 100 cycles at 400 mAg−1	[45]
KMO-NNT *	Nanowire	148 mAh g−1 after 400 cycles at 1.623C	[46]
δ-MnO2	Nanoflake	112 mAh g−1 after 100 cycles at 83 mAg−1	[47]
δ-MnO2	Flower-like	80 mAh g−1 after 70 cycles at 200 mAg−1	[48]
δ-MnO2	Pom-pom shaped flower	166 mAh g−1 after 250 cycles at 1000 mAg−1	Our work

* KMO-NNT: K⁺-inserted δ-MnO₂ nanowires.

, which includes various morphologies, and their corresponding discharge capacity with the number of cycles and current densities of the reported δ-MnO₂ electrode, the obtained pom-pom flower-like δ-MnO₂ electrode in the present work delivered superior electrochemical performances among all the reported data.

Since the different reaction mechanisms, such as H⁺/Zn²⁺ insertion, conversion reaction, and deposition-dissolution reactions, have been reported so far for ZIBs, ex situ XRD was conducted after the 200th cycles to confirm the storage mechanism and structural stability of the pom-pom shape flower-like δ-MnO₂ electrode; and the obtained result is shown in Figure 7. As can be seen, the diffraction peaks that appeared at ~25.7° and ~34.2° mainly belong to MnOOH (JCPDS No. 41-1379), demonstrating the intercalation of H⁺. On the other hand, the reflections located at ~36.9°, ~59.8°, and ~68.6° can be well indexed with the spinel ZnMn₂O₄ (JCPDS No. 24-1133), which also confirmed the insertion of Zn²⁺. Furthermore, since the H⁺ ion becomes consumed during the discharge, it indicates the formation of MnOOH, which increases the concentration of OH^- in the electrolyte around electrode surfaces [25]. Therefore, the existence of OH⁻ tends to participate in the formation of Zn₄SO₄(OH)₆·5H₂O, which is also found in the ex-situ XRD pattern. Hence, it is believed that the following reactions take place at the cathode and anode:

Cathode:

$${\mathrm{MnO}}_2+{ \mathrm{H}}^{+}+{ \mathrm{e}}^{-} \to \mathrm{MnOOH}$$

(4)

$$2{\mathrm{MnO}}_2+{\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-} \to {\mathrm{ZnMn}}_2{\mathrm{O}}_4$$

(5)

$$4{\mathrm{Zn}}^{2+}+{\mathrm{SO}}_4^{2-}+6{\mathrm{OH}}^{-}+2{\mathrm{e}}^{-}+5{\mathrm{H}}_2\mathrm{O} \to { \mathrm{Zn}}_4{\mathrm{SO}}_4{\left(\mathrm{OH}\right)}_6.5{\mathrm{H}}_2\mathrm{O}$$

(6)

Anode:

$$\mathrm{Zn}\to { \mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}$$

(7)

Thus, both H⁺ and Zn²⁺ ions intercalate into the layered δ-MnO₂ during discharge and act as a support pillar to the layered structure, followed by the deposition of Zn₄SO₄(OH)₆·5H₂O (ZSH). To confirm the formation of ZSH, ex situ SEM was also carried out after 200 cycles in the discharged state, which is given in the inset of Figure 7. The obtained image is almost like the reported ZSH data, which further approves the H⁺ and Zn²⁺ intercalation mechanism and the existence of ZSH in the δ-MnO₂ sample [25]. Based on the above observation, it can be concluded that the layered structure is gradually transformed into a spinel phase upon long-range cycling. Hence, the obtained pom-pom flower-like morphology of the δ-MnO₂ electrode provides a high reversible capacity and excellent cycling stability under the abovementioned reaction mechanisms.

4. Conclusions

In summary, a novel pom-pom flower-like mesoporous δ-MnO₂ cathode was successfully synthesized using a hydrothermal synthesis method, followed by annealing at 360 °C for 2 h under air atmosphere. The mesoporous nature shortened the diffusion distance of Zn²⁺ ions along with accommodation of the volume changes that occurred during Zn²⁺ ions insertion/de-insertion, and thereby, offered better electrochemical results. Since the obtained δ-MnO₂ electrode delivered a specific capacity of ~166 mAh g⁻¹ after 250 cycles at a high current density of 1000 mAg⁻¹, it indicates the high suitability of the δ-MnO₂ cathode for aqueous zinc-ion batteries. Furthermore, the ex-situ study confirmed the two-step insertion of H⁺ and Zn²⁺ ions accompanied by the precipitation/dissolution of Zn₄SO₄(OH)₆·5H₂O (ZSH) into a pom-pom shape flower-like δ-MnO₂ cathode during discharge/charge processes. It is believed that the present work offers a novel synthesis route to achieve a δ-MnO₂ cathode for the high capacity and long lifespan of zinc-ion batteries.