Energy Storage Materials and Devices

This review article attempts to provide a comprehensive review of the recent progress in cerium oxide (CeO₂)-based resistive random-access memories (RRAMs). CeO₂ is considered the most promising candidate because of its multiple oxidation states (Ce³⁺ and Ce⁴⁺), remarkable resistive-switching (RS) uniformity in DC mode, gradual resistance transition, cycling endurance, long data-retention period, and utilization of the RS mechanism as a dielectric layer, thereby exhibiting potential for neuromorphic computing. In this context, a detailed study of the filamentary mechanisms and their types is required. Accordingly, extensive studies on unipolar, bipolar, and threshold memristive behaviors are reviewed in this work. Furthermore, electrode-based (both symmetric and asymmetric) engineering is focused for the memristor’s structures such as single-layer, bilayer (as an oxygen barrier layer), and doped switching-layer-based memristors have been proved to be unique CeO₂-based synaptic devices. Hence, neuromorphic applications comprising spike-based learning processes, potentiation and depression characteristics, potentiation motion and synaptic weight decay process, short-term plasticity, and long-term plasticity are intensively studied. More recently, because learning based on Pavlov’s dog experiment has been adopted as an advanced synoptic study, it is one of the primary topics of this review. Finally, CeO₂-based memristors are considered promising compared to previously reported memristors for advanced synaptic study in the future, particularly by utilizing high-dielectric-constant oxide memristors. Full article

A growing number of people are interested in using silver nanowires (AgNWs) as potential transparent and conductive materials. The production of high-performance and high-throughput AgNWs was successfully optimized in this work using a one-step, straightforward, and reproducible modified polyol approach. The factors influencing the morphology of the silver nanowires have undergone extensive research in order to determine the best-optimized approach for producing AgNWs. The best AgNW morphology, with a length of more than 50 m and a diameter of less than 35 nm (aspect ratio is higher than 1700), was discovered to be produced by a mixture of 44 mM AgNO₃, 134 mM polyvinylpyrrolidone (PVP) (Mo.Wt 40,000), and 2.4 mM KCl at 160 °C with a stirring rate of 100 rpm. With our improved approach, the overall reaction time was cut from almost an hour with the conventional polyol method to a few minutes. Scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultraviolet (UV) spectroscopy were used to characterize AgNWs. The resultant AgNWs’ dispersion was cleaned using a centrifuge multiple times before being deposited on glass and PET substrates at room temperature. In comparison to commercial, delicate, and pricey indium-doped tin oxide (ITO) substrates, the coated samples displayed exceptionally good sheet resistance of 17.05/sq and optical haze lower than 2.5%. Conclusions: Using a simple one-step modified polyol approach, we were able to produce reproducible thin sheets of AgNWs that made excellent, flexible transparent electrodes. Full article

A highly polar perovskite SrTiO₃ (STO) layer is considered as one of the promising artificial protective layers for the Zn metal anode of aqueous zinc-ion batteries (AZIBs). Although it has been reported that oxygen vacancies tend to promote Zn(II) ion migration in the STO layer and thereby effectively suppress Zn dendrite growth, there is still a lack of a basic understanding of the quantitative effects of oxygen vacancies on the diffusion characteristics of Zn(II) ions. In this regard, we comprehensively studied the structural features of charge imbalances caused by oxygen vacancies and how these charge imbalances affect the diffusion dynamics of Zn(II) ions by utilizing density functional theory and molecular dynamics simulations. It was found that the charge imbalances are typically localized close to vacancy sites and those Ti atoms that are closest to them, whereas differential charge densities close to Sr atoms are essentially non-existent. We also demonstrated that there is virtually no difference in structural stability between the different locations of oxygen vacancies by analyzing the electronic total energies of STO crystals with the different vacancy locations. As a result, although the structural aspects of charge distribution strongly rely on the relative vacancy locations within the STO crystal, Zn(II) diffusion characteristics stay almost consistent with changing vacancy locations. No preference for vacancy locations causes isotropic Zn(II) ion transport inside the STO layer, which subsequently inhibits the formation of Zn dendrites. Due to the promoted dynamics of Zn(II) ions induced by charge imbalance near the oxygen vacancies, the Zn(II) ion diffusivity in the STO layer monotonously increases with the increasing vacancy concentration ranging from 0% to 16%. However, the growth rate of Zn(II) ion diffusivity tends to slow down at relatively high vacancy concentrations as the imbalance points become saturated across the entire STO domain. The atomic-level understanding of the characteristics of Zn(II) ion diffusion demonstrated in this study is expected to contribute to developing new long-life anode systems for AZIBs. Full article

In this study, electrical models for cylindrical/pouch-type lithium Li-ion batteries and supercapacitors were investigated, and the impedance spectra characteristics were studied. Cylindrical Li-ion batteries use Ni, Co, and Al as the main materials, while pouch-type Li-ion batteries use Ni, Co, and Mn as the main materials. Herein, 2600–3600 mAh 18650-type cylindrical Li-ion batteries, 5000 mAh 21700-type cylindrical Li-ion batteries, 37–50.5 Ah pouch-type Li-ion batteries, and a 2.7 V, 600 F supercapacitor are compared and analyzed. For a cylindrical Li-ion battery, the $R_S$ value of a battery with a protection device (circular thermal disc cap) is in the range of 14–38 mΩ. For the 18650-type cylindrical Li-ion battery with a protection device, the $R_S$ value of the battery is between 48 and 105 mΩ, and the protection device increases the $R_S$ value by at least 33 mΩ. A good Li-ion battery exhibits $R_S$. Moreover, it has small overall $R_P$ and $C_P$ values. For the 21700-type cylindrical Li-ion battery with a protection device, the $R_S$ value of the battery is 25 mΩ. For the pouch-type Li-ion battery, the $R_S$ value of the battery is between 0.86 and 1.04 mΩ. For the supercapacitor, the $R_S$ value of the battery is between 0.4779 and 0.5737 mΩ. A cylindrical Li-ion battery exhibits a semicircular shape in the impedance spectrum, due to the oxidation and reduction reactions of Li ions, and the impedance increases with a slope of 45° in the complex plane, due to the $Z_W$ generated by Li ion diffusion. However, for a pouch-type Li-ion battery, the impedance spectrum exhibits a part of the semicircular shape, due to the oxidation and reduction reactions of Li ions, and the $Z_W$ generated by Li ion diffusion does not appear. In a supercapacitor, the oxidation and reduction reactions of ions do not appear at all, and the $Z_W$ generated by Li ion diffusion does not occur. Full article

Batteries based on organic electrolytes have been raising safety concerns due to some associated fire/explosion accidents caused by the unusual combination of highly flammable organic electrolytes and high energy electrodes. Nonflammable aqueous batteries are a good alternative to the current energy storage systems. However, what makes aqueous batteries safe and viable turns out to be their main weakness, since water molecules are prone to decomposition because of a narrow electrochemical stability window (ESW). In this perspective we introduce aqueous batteries and then discuss the state-of-the-art of water-in-salt (WIS) electrolytes for aqueous energy storage systems. The main strategies to improve ESW are reviewed, including: (i) the use of fluorinated salts to make a solid electrolyte interphase (SEI); (ii) the use of cost-effective and highly soluble salts to reduce water activity through super concentration; and (iii) the use of hybrid electrolytes combining the advantages of both aqueous and non-aqueous phases. Then, we discuss different battery chemistries operated with different WIS electrolytes. Finally, we highlight the challenges and future technological perspectives for practical aqueous energy storage systems, including applications in stationary storage/grid, power backup, portable electronics, and automotive sectors. Full article

Surface coating approaches for silicon (Si) have demonstrated potential for use as anodes in lithium-ion batteries (LIBs) to address the large volume change and low conductivity of Si. However, the practical application of these approaches remains a challenge because they do not effectively accommodate the pulverization of Si during cycling or require complex processes. Herein, Si-embedded titanium oxynitride (Si-TiON) was proposed and successfully fabricated using a spray-drying process. TiON can be uniformly coated on the Si surface via self-assembly, which can enhance the Si utilization and electrode stability. This is because TiON exhibits high mechanical strength and electrical conductivity, allowing it to act as a rigid and electrically conductive matrix. As a result, the Si-TiON electrodes delivered an initial reversible capacity of 1663 mA h g⁻¹ with remarkably enhanced capacity retention and rate performance. Full article

The increasing demand for high energy storage devices calls for concurrently enhanced dielectric constants and reduced dielectric losses of polymer dielectrics. In this work, we rationally design dielectric composites comprising aligned 2D nanofillers of reduced graphene oxide (rGO) and boron nitride nanosheets (BNNS) in a polyvinylidene fluoride (PVDF) matrix through a novel press-and-fold technique. Both nanofillers play different yet complementary roles: while rGO is designed to enhance the dielectric constant through charge accumulation at the interfaces with polymer, BNNS suppress the dielectric loss by preventing the mobility of free electrons. The microlaminate containing eight layers each of rGO/PVDF and BNNS/PVDF films exhibits remarkable dielectric performance with a dielectric constant of 147 and an ultralow dielectric loss of 0.075, due to the synergistic effect arising from the alternatingly electrically conductive and insulating films. Consequently, a maximum energy density of 3.5 J/cm³—about 18 times the bilayer composite counterpart—is realized. The high thermal conductivities of both nanofillers and their alignment endow the microlaminate with an excellent in-plane thermal conductivity of 6.53 Wm⁻¹K⁻¹, potentially useful for multifunctional applications. This work offers a simple but effective approach to fabricating a composite for high dielectric energy storage using two different 2D nanofillers. Full article

Graphite felts act as electrodes in VRFBs thanks to their properties such as chemical strength and electrical conductivity or 3D-structure. However, there are significant drawbacks to be overcome, such as their low wettability, sluggish kinetic reversibility and electroactivity towards faradaic processes related to vanadium electroactive species. As a consequence, it is key to alter the fibres to enhance their electrochemical performance during battery operation. Most of the previously reported modifications have been focused on incorporating surface oxygenated functional groups, even though the role of those groups on the electrocatalytic activity towards vanadium faradaic processes is still not clear. Aiming to gain knowledge on this issue, this work investigates the influence of electro-oxidation and electro-reduction treatments, performed in different acidic media (H₂SO₄ or HNO₃ solutions), on their subsequent electrochemical response towards VO²⁺/VO₂⁺ and V³⁺/V²⁺ faradaic processes. The chemical and electrochemical properties of the modified felts were analyzed to understand two key parameters that affect the vanadium reaction catalysis: the depth and oxidation degree of the fibres. A treatment with HNO₃, a strong oxidizing agent, leads to a deep oxidation of the fibre and the development of a high density of oxygenated functional groups, mainly C=O, which hinder the redox reactions of vanadium, especially for the faradaic reactions from the catholyte. Full article