Recent Progress on Zinc-Ion Rechargeable Batteries

Porous layered ZnV2O4@C synthesized based on a bimetallic MOF as a stable cathode material for aqueous zinc ion batteries

Vanadium-based oxides are considered potential cathode materials for aqueous zinc ion batteries (AZIBs) due to their distinctive layered (or tunnel) structure suitable for zinc ion storage. However, the structural instability and sluggish kinetics of vanadium-based oxides have limited their capacity and cycling stability for large-scale applications. To overcome these shortcomings, here a porous vanadium-based oxide doped with zinc ions and carbon with the molecular formula ZnV2O4@C (ZVO@C) as the cathode material is synthesized by the pyrolysis of a bimetallic MOF precursor containing Zn/V. This electrode demonstrates a remarkable specific capacity of 425 mA h g-1 at 0.5 A g-1 and excellent cycling stability with about 97% capacity retention after 1000 cycles at 10 A g-1. The excellent electrochemical performance of ZVO@C can be attributed to more reaction active sites and the faster reaction kinetics for zinc ion diffusion and storage brought about by the porous layered spinel-type tunnel structure with high surface area and massive mesoporosity, as well as the enhanced electron transport efficiency and more stable channel structure achieved from the doped conductive carbon. The reaction mechanism and structural evolution of the ZVO@C electrode are analyzed using X-ray diffraction and X-ray photoelectron spectroscopy, revealing the formation of a new phase of ZnxV2O5·nH2O during the first charge, which participates in reversible cycling together with ZVO@C during the charging and discharging processes. Moreover, the energy storage mechanism is revealed, in which zinc ions and hydrogen ions jointly participate in intercalation and extraction. The present study demonstrates that constructing composite vanadium-based oxides based on bimetallic organic frameworks as precursor templates is an effective strategy for the development of high-performance cathode materials for AZIBs.

Transition metal dichalcogenide-based materials for rechargeable aluminum-ion batteries: A mini-review

Rechargeable aluminum-ion batteries (AIBs) have emerged as a promising candidate for energy storage applications and have been extensively investigated over the past few years. Due to their high theoretical capacity, nature of abundance, and high safety, AIBs can be considered an alternative to lithium-ion batteries. However, the electrochemical performance of AIBs for large-scale applications is still limited due to the poor selection of cathode materials. Transition metal dichalcogenides (TMDs) have been regarded as appropriate cathode materials for AIBs due to their wide layer spacing, large surface area, and distinct physiochemical characteristics. This mini-review provides a succinct summary of recent research progress on TMD-based cathode materials in non-aqueous AIBs. The latest developments in the benefits of utilizing 3D-printed electrodes for AIBs are also explored.

Stable Zinc Anode Facilitated by Regenerated Silk Fibroin-modified Hydrogel Protective Layer

Inherent dendrite growth and side reactions of zinc anode caused by its unstable interface in aqueous electrolytes severely limit the practical applications of zinc-ion batteries (ZIBs). To overcome these challenges, a protective layer for Zn anode inspired by cytomembrane structure is developed with PVA as framework and silk fibroin gel suspension (SFs) as modifier. This PVA/SFs gel-like layer exerts similar to the solid electrolyte interphase, optimizing the anode-electrolyte interface and Zn2+ solvation structure. Through interface improvement, controlled Zn2+ migration/diffusion, and desolvation, this buffer layer effectively inhibits dendrite growth and side reactions. The additional SFs provide functional improvement and better interaction with PVA by abundant functional groups, achieving a robust and durable Zn anode with high reversibility. Thus, the PVA/SFs@Zn symmetric cell exhibits an ultra-long lifespan of 3150 h compared to bare Zn (182 h) at 1.0 mAh cm-2-1.0 mAh cm-2, and excellent reversibility with an average Coulombic efficiency of 99.04% under a large plating capacity for 800 cycles. Moreover, the PVA/SFs@Zn||PANI/CC full cells maintain over 20 000 cycles with over 80% capacity retention under harsh conditions at 5 and 10 A g-1. This SF-modified protective layer for Zn anode suggests a promising strategy for reliable and high-performance ZIBs.

Construction of Selective Ion Transport Polymer at Anode-Electrolyte Interface for Stable Aqueous Zinc-Ion Batteries

Rampant dendrite formation and serious adverse parasitic reactions induced by migration of dissolved V/Mn cathode ions on Zn anode have hampered the high performance of aqueous zinc-ion batteries (AZIBs). Inspired by the coordination chemistry between functional groups of polymer and electrolyte ions, a freestanding layer consisting of dopamine-functionalized polypyrrole (DA-PPy) nanowires served as a selective ion transport layer at the anode-electrolyte interface to address these two issues, which could simultaneously avoid polarization caused by the introduction of an additional interface. On the one hand, the DA-PPy layer displays excellent zinc ion and charge transfer ability, as well as provides chemical homochanneling for zinc ions at the interface, which endow the DA-PPy layer with properties as a chemical guider and physical barrier for dendrite inhibition. On the other hand, the DA-PPy layer can trap excess transition metal ions fleeing from the cathodes, thus serving as a chemical barrier, preventing the formation of Vx+/Mnx+-passivation on the surface of the zinc anode. Consequently, the AZIBs based on V2O5 and MnO2 cathodes involving the DA-PPy functional layer show a great improvement in the capacity retention.

3D Hierarchical Sunflower-Shaped MoS2 /SnO2 Photocathodes for Photo-Rechargeable Zinc Ion Batteries

Photo-rechargeable zinc-ion batteries (PRZIBs) have attracted much attention in the field of energy storage due to their high safety and dexterity compared with currently integrated lithium-ion batteries and solar cells. However, challenges remain toward their practical applications, originating from the unsatisfactory structural design of photocathodes, which results in low photoelectric conversion efficiency (PCE). Herein, a flexible MoS2 /SnO2 -based photocathode is developed via constructing a sunflower-shaped light-trapping nanostructure with 3D hierarchical and self-supporting properties, enabled by the hierarchical embellishment of MoS2 nanosheets and SnO2 quantum dots on carbon cloth (MoS2 /SnO2 QDs@CC). This structural design provides a favorable pathway for the effective separation of photogenerated electron-hole pairs and the efficient storage of Zn2+ on photocathodes. Consequently, the PRZIB assembled with MoS2 /SnO2 QDs@CC delivers a desirable capacity of 366 mAh g-1 under a light intensity of 100 mW cm-2 , and achieves an ultra-high PCE of 2.7% at a current density of 0.125 mA cm-2 . In practice, an integrated battery system consisting of four series-connected quasi-solid-state PRZIBs is successfully applied as a wearable wristband of smartwatches, which opens a new door for the application of PRZIBs in next-generation flexible energy storage devices.

Enabling Wasted A4 Papers as a Promising Carbon Source to Construct Partially Graphitic Hierarchical Porous Carbon for High-Performance Aqueous Zn-Ion Storage

Considering the superiorities of abundance, easy collection, low cost, and nearly constant composition, the wasted A4 papers are deemed as a recyclable and scalable carbon source to fabricate functional carbon materials for Zn-ion hybrid supercapacitors (ZIHSCs), which integrate the supercapacitors' high-power output and batteries' high energy density. Herein, the wasted A4 papers are efficiently converted into an advanced carbon material owning a hierarchical porous structure with a high surface area and interconnected multiscale channels, a graphitic structure, and a good level of N/O codoping. By taking advantage of these features, an express electron/ion transfer pathway, a large accessible surface interface, and a robust architecture are achieved for swift kinetics, numerous active sites, and excellent steadiness to afford a charming Zn2+ storage capability for the aqueous coin-type ZIHSC device (a high capacity of 244 mAh g-1 at 0.1 A g-1 with a capacity conservation of 116.4 mAh g-1 even amplifying the current density by 200 times, a supreme energy density of 190.4 Wh kg-1, a supreme power output of 18 kW kg-1, and an eminent durability of 93.8% over 10,000 cycles at 10 A g-1). Excitingly, the quasi-solid ZIHSC device also bespeaks an enjoyable capacity of 211.7 mAh g-1, a high energy density of 159.3 Wh kg-1, good mechanical flexibility, and a low self-discharge rate. This work puts forward a simple and scalable strategy to enable the wasted A4 paper as a competitive carbon source to construct advanced cathode material for Zn2+ storage.

Molecular Connectors Boosting the Performance of MoS2 Cathodes in Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) are promising energy storage systems due to high energy density, low-cost, and abundant availability of zinc as a raw material. However, the greatest challenge in ZIBs research is lack of suitable cathode materials that can reversibly intercalate Zn2+ ions. 2D layered materials, especially MoS2 -based, attract tremendous interest due to large surface area and ability to intercalate/deintercalate ions. Unfortunately, pristine MoS2 obtained by traditional protocols such as chemical exfoliation or hydrothermal/solvothermal methods exhibits limited electronic conductivity and poor chemical stability upon charge/discharge cycling. Here, a novel molecular strategy to boost the electrochemical performance of MoS2 cathode materials for aqueous ZIBs is reported. The use of dithiolated conjugated molecular pillars, that is, 4,4'-biphenyldithiols, enables to heal defects and crosslink the MoS2 nanosheets, yielding covalently bridged networks (MoS2 -SH2) with improved ionic and electronic conductivity and electrochemical performance. In particular, MoS2 -SH2 electrodes display high specific capacity of 271.3 mAh g-1 at 0.1 A g-1 , high energy density of 279 Wh kg-1 , and high power density of 12.3 kW kg-1 . With its outstanding rate capability (capacity of 148.1 mAh g-1 at 10 A g-1 ) and stability (capacity of 179 mAh g-1 after 1000 cycles), MoS2 -SH2 electrodes outperform other MoS2 -based electrodes in ZIBs.

Layered porous Mn0.18V2O5@C with manganese and carbon provided by a metal-organic framework precursor as a cathode material for aqueous zinc-ion batteries

At present, vanadium-based cathodes for aqueous zinc-ion batteries (AZIBs) are limited by their slow reaction kinetics, poor electrical conductivity, and low capacity retention. To overcome these problems, here, we design a layered porous Mn0.18V2O5@C as the cathode material for AZIBs using a manganese-containing metal-organic framework as a template through a simple solvothermal method. Such an electrode delivers an excellent specific capacity (380 mA h g-1 at 0.1 A g-1) accompanied by superior cycling stability (about 85% capacity retention for 2000 cycles at 6 A g-1). The excellent electrochemical performance of Mn0.18V2O5@C is ascribed to the improved interface activity including smooth zinc ion transport, abundant ion reaction active sites and accelerated charge transfer resulting from the coordination of the porous structure, doped conductive carbon, and the stable channel structure derived from the pillar effect of doping manganese ions, preventing a premature collapse of the electrode structure. It is also revealed by structural evolution analysis that the residual zinc ions also combine with the original Mn0.18V2O5 to form a ZnxMnyV2O5 phase, which serves as an additional structural pillar and in the meantime, also participates in the following cycles. These favorable electrochemical results suggest that Mn0.18V2O5@C is a suitable cathode material for AZIBs.

Surface Engineering for Ultrathin Metal Anodes Enabling High-Performance Zn-Ion Batteries

Zn-ion batteries with low cost and high safety have been regarded as a promising energy storage technology for grid storage. It is well-known that the metal anode surface orientation is vital to its reversibility. Herein, we demonstrate a facile route to control the Zn metal anode surface orientation through electrodeposition with electrolyte additives. An ultrathin (101)-inclined Zn metal anode (down to 2 μm) is obtained by adding a small amount of dimethyl sulfoxide (DMSO) in the ZnSO₄ aqueous electrolyte. Scanning electron microscopy indicates the formation of flat terrace-like surfaces, while in situ optical observations demonstrate the reversible plating and stripping. DFT calculations reveal that the large reconstruction of the Zn-(101) surface with DMSO and H₂O adsorption to lower the interface energy is the main driving force for surface preference. Raman, XPS, and ToF-SIMS characterizations are performed to unveil the surface SEI components. Exceptional electrochemical performance is demonstrated for the (101)-inclined Zn metal anode in a half cell, which could cycle for 200 h with a low overpotential (<50 mV). The Zn||V₂O full cells are assembled, showing much better cycle performance for the 5 μm (101)-inclined Zn metal anode as compared to the commercialized 10 μm Zn metal foil, with a maximum specific capacity of 359 mAh/g and >170 mAh/g after over 300 cycles. We hope this study will spur further interest in the control of surface crystallographic orientation for a stable ultrathin Zn metal anode.

Electrochemically stable tunnel-type α-MnO2-based cathode materials for rechargeable aqueous zinc-ion batteries

The purpose of this study is the synthesis of α-MnO₂-based cathode materials for rechargeable aqueous zinc ion batteries by hydrothermal method using KMnO₄ and MnSO₄ as starting materials. The aim is to improve the understanding of Zn²⁺ insertion/de-insertion mechanisms. The as-prepared solid compounds were characterized by spectroscopy and microscopy techniques. X-ray diffraction showed that the hydrothermal reaction forms α-MnO₂ and Ce⁴⁺-inserted MnO₂ structures. Raman spectroscopy confirmed the formation of α-MnO₂ with hexagonal MnO₂ and Ce-MnO₂ structures. Scanning electron microscopy (SEM) confirmed the formation of nanostructured MnO₂ (nanofibers) and Ce-MnO₂ (nanorods). The electrochemical performance of MnO₂ was evaluated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) tests in half-cells. CV results showed the reversible insertion/de-insertion of Zn²⁺ ions in MnO₂ and Ce-MnO₂. GCD cycling tests of MnO₂ and Ce-MnO₂ at 2500 mA/g demonstrated an impressive electrochemical performance, excellent cycling stability throughout 500 cycles, and high rate capability. The excellent electrochemical performance and the good cycling stability of MnO₂ and Ce-MnO₂ nanostructures by simple method makes them promising cathode materials for aqueous rechargeable zinc-ion batteries.

Etching-Induced Surface Reconstruction of NiMoO4 for Oxygen Evolution Reaction

Rational reconstruction of oxygen evolution reaction (OER) pre-catalysts and performance index of OER catalysts are crucial but still challenging for universal water electrolysis. Herein, we develop a double-cation etching strategy to tailor the electronic structure of NiMoO4, where the prepared NiMoO4 nanorods etched by H2O2 reconstruct their surface with abundant cation deficiencies and lattice distortion. Calculation results reveal that the double cation deficiencies can make the upshift of d-band center for Ni atoms and the active sites with better oxygen adsorption capacity. As a result, the optimized sample (NMO-30M) possesses an overpotential of 260 mV at 10 mA cm-2 and excellent long-term durability of 162 h. Importantly, in situ Raman test reveals the rapid formation of high-oxidation-state transition metal hydroxide species, which can further help to improve the catalytic activity of NiMoO4 in OER. This work highlights the influence of surface remodification and shed some light on activating catalysts.

Anchoring I3- via Charge-Transfer Interaction by a Coordination Supramolecular Network Cathode for a High-Performance Aqueous Dual-Ion Battery

Iodine is considered to have broad application prospects in the field of electrochemical energy storage. However, the high solubility of I₃^- severely hampers its practical application, and the lack of research on the anchoring mechanism of I₃^- has seriously hindered the development of advanced cathode materials for iodine batteries. Herein, based on the molecular orbital theory, we studied the charge-transfer interaction between the acceptor of I₃^- with a σ* empty antibonding orbital and the donor of pyrimidine nitrogen with lone-pair electrons, which is proved by the results of UV-vis absorption spectroscopy, Raman spectroscopy, and density functional theory (DFT) calculations. The prepared dual-ion battery (DIB) exhibits a high voltage platform of 1.2 V, a remarkable discharge-specific capacity of up to 207 mAh g^-1, and an energy density of 233 Wh kg^-1 at a current density of 5 A g^-1, as well as outstanding cycle stability (operating stably for 5000 cycles) with a high Coulombic efficiency of 97%, demonstrating excellent electrochemical performance and a promising prospect in stationary energy storage.

Solid Electrolyte Interface in Zn-Based Battery Systems

Due to its high theoretical capacity (820 mAh g^-1), low standard electrode potential (- 0.76 V vs. SHE), excellent stability in aqueous solutions, low cost, environmental friendliness and intrinsically high safety, zinc (Zn)-based batteries have attracted much attention in developing new energy storage devices. In Zn battery system, the battery performance is significantly affected by the solid electrolyte interface (SEI), which is controlled by electrode and electrolyte, and attracts dendrite growth, electrochemical stability window range, metallic Zn anode corrosion and passivation, and electrolyte mutations. Therefore, the design of SEI is decisive for the overall performance of Zn battery systems. This paper summarizes the formation mechanism, the types and characteristics, and the characterization techniques associated with SEI. Meanwhile, we analyze the influence of SEI on battery performance, and put forward the design strategies of SEI. Finally, the future research of SEI in Zn battery system is prospected to seize the nature of SEI, improve the battery performance and promote the large-scale application.

Electrolyte Salts and Additives Regulation Enables High Performance Aqueous Zinc Ion Batteries: A Mini Review

Aqueous zinc ion batteries (ZIBs) are regarded as one of the most ideally suited candidates for large-scale energy storage applications owning to their obvious advantages, that is, low cost, high safety, high ionic conductivity, abundant raw material resources, and eco-friendliness. Much effort has been devoted to the exploration of cathode materials design, cathode storage mechanisms, anode protection as well as failure mechanisms, while inadequate attentions are paid on the performance enhancement through modifying the electrolyte salts and additives. Herein, to fulfill a comprehensive aqueous ZIBs research database, a range of recently published electrolyte salts and additives research is reviewed and discussed. Furthermore, the remaining challenges and future directions of electrolytes in aqueous ZIBs are also suggested, which can provide insights to push ZIBs' commercialization.

Proton Storage in Metallic H1.75MoO3 Nanobelts through the Grotthuss Mechanism

The proton, as the cationic form of the lightest element-H, is regarded as most ideal charge carrier in "rocking chair" batteries. However, current research on proton batteries is still at its infancy, and they usually deliver low capacity and suffer from severe acidic corrosion. Herein, electrochemically activated metallic H_1.75MoO₃ nanobelts are developed as a stable electrode for proton storage. The electrochemically pre-intercalated protons not only bond directly with the terminal O3 site via strong O-H bonds but also interact with the oxygens within the adjacent layers through hydrogen bonding, forming a hydrogen-bonding network in H_1.75MoO₃ nanobelts and enabling a diffusion-free Grotthuss mechanism as a result of its ultralow activation energy of ∼0.02 eV. To the best of our knowledge, this is the first reported inorganic electrode exhibiting Grotthuss mechanism-based proton storage. Additionally, the proton intercalation into MoO₃ with formation of H_1.75MoO₃ induces strong Jahn-Teller electron-phonon coupling, rendering a metallic state. As a consequence, the H_1.75MoO₃ shows an outstanding fast charging performance and maintains a capacity of 111 mAh/g at 2500 C, largely outperforming the state-of-art battery electrodes. More importantly, a symmetric proton ion full cell based on H_1.75MoO₃ was assembled and delivered an energy density of 14.7 Wh/kg at an ultrahigh power density of 12.7 kW/kg, which outperforms those of fast charging supercapacitors and lead-acid batteries.

Monodispersed flower-like MXene@VO2 clusters for aqueous zinc ion batteries with superior rate performance

Monoclinic B phase VO₂ with a distinctive tunnel structure is regarded as a viable cathode material for use in aqueous zinc ion batteries (AZIBs). However, the low electron conductivity and poor rate performance prevent it from being used further. Herein, we report 3D flower-like MXene nanosheets loaded with the VO₂ cluster (MXene@VO₂) synthesized via a one-step hydrothermal process, where MXene nanosheets were spontaneously stacked as a skeleton for the growth of VO₂ nanobelts. The synergistic effect between MXene nanosheets with high electronic conductivity and VO₂ nanobelts with a unique tunnel structure benefitted the electron and Zn²⁺ transport; the 3D hybrid structure with a high specific surface area provided an increased contact area with the electrolyte and a shortened distance of the Zn²⁺ transfer path. As a result, this material exhibits a promising Zn²⁺ storage behavior with a superior rate capability (363.2 mA h g^-1 at 0.2C and 169.1 mA h g^-1 at 50C) and outstanding long-cycling performance (206.6 mA h g^-1 and 76% capacity retention over 5000 cycles at 20C). In addition, a self-charging battery could be prepared by using oxygen in air to oxidize vanadium oxide with lower valence states. Our prepared MXene@VO₂ composite with a synergistic effect has been proved to be a promising cathode for AZIBs, offering a progressive paradigm for the development of AZIBs.

Synthesis and Electrochemical Performance of the Orthorhombic V2O5·nH2O Nanorods as Cathodes for Aqueous Zinc Batteries

Aqueous zinc-ion batteries offer the greatest promise as an alternative technology for low-cost and high-safety energy storage. However, the development of high-performance cathode materials and their compatibility with aqueous electrolytes are major obstacles to their practical applications. Herein, we report the synthesis of orthorhombic V₂O₅·nH₂O nanorods as cathodes for aqueous zinc batteries. As a result, the electrode delivers a reversible capacity as high as 320 mAh g⁻¹ at 1.0 A g⁻¹ and long-term cycling stability in a wide window of 0.2 to 1.8 V using a mild ZnSO₄ aqueous electrolyte. The superior performance can be attributed to the improved stability of materials, inhibited electrolyte decomposition and facilitated charge transfer kinetics of such materials for aqueous zinc storage. Furthermore, a full cell using microsized Zn powder as an anode within capacity-balancing design exhibits high capacity and stable cycling performance, proving the feasibility of these materials for practical application.

Enriching Oxygen Vacancy Defects via Ag-O-Mn Bonds for Enhanced Diffusion Kinetics of δ-MnO2 in Zinc-Ion Batteries

Aqueous zinc-ion batteries (ZIBs) have received great attention due to their environmental friendliness and high safety. However, cathode materials with slow diffusion dynamics and dissolution in aqueous electrolytes hindered their further application. To address these issues, in this work, a MnO₂-2 cathode doped with 1.12 wt % Ag was prepared, and after 1000 cycles of charge/discharge at 1 A·g^-1, the capacity remained at 114 mA·h·g^-1 (only 57.7 mA·h·g^-1 for pristine MnO₂). Cyclic voltammetry (CV), the galvanostatic intermittent titration technique (GITT), the electrochemical quartz crystal microbalance (EQCM) method, and density functional theory (DFT) calculation on pristine δ-MnO₂ and MnO₂-2 also proved the superior performance of MnO₂-2. More investigation disclosed that its superior performance is attributed to the improved diffusion kinetics of the cathode brought by the enriched oxygen vacancy defects due to the formation of Ag-O-Mn bonds. Meanwhile, the kinetic mechanism of the Zn/MnO₂-2 cell can be described as a reversible process of the dissolution/precipitation of the ZHS phase and consequent insertion/extraction of Zn²⁺ and H₃O⁺. Herein, the primary issues of ZIB cathode materials have been addressed and solved to a certain extent. More importantly, such a modification in the design of the advanced manganese-based aqueous ZIB cathode materials can provide further insight and facilitate the development and application of this large-scale energy storage system in the near future.

Understanding the Solvation-Dependent Properties of Cyclic Ether Multivalent Electrolytes Using High-Field NMR and Quantum Chemistry

Efforts to expand the technological capability of batteries have generated increased interest in divalent cationic systems. Electrolytes used for these electrochemical applications often incorporate cyclic ethers as electrolyte solvents; however, the detailed solvation environments within such systems are not well-understood. To foster insights into the solvation structures of such electrolytes, Ca(TFSI)2 and Zn(TFSI)2 dissolved in tetrahydrofuran (THF) and 2-methyl-tetrahydrofuran were investigated through multi-nuclear magnetic resonance spectroscopy (17O, 43Ca, and 67Zn NMR) combined with quantum chemistry modeling of NMR chemical shifts. NMR provides spectroscopic fingerprints that readily couple with quantum chemistry to identify a set of most probable solvation structures based on the best agreement between the theoretically predicted and experimentally measured values of chemical shifts. The multi-nuclear approach significantly enhances confidence that the correct solvation structures are identified due to the required simultaneous agreement between theory and experiment for multiple nuclear spins. Furthermore, quantum chemistry modeling provides a comparison of the solvation cluster formation energetics, allowing further refinement of the preferred solvation structures. It is shown that a range of solvation structures coexist in most of these electrolytes, with significant molecular motion and dynamic exchange among the structures. This level of solvation diversity correlates with the solubility of the electrolyte, with Zn(TFSI)2/THF exhibiting the lowest degree of each. Comparisons of analogous Ca2+ and Zn2+ solvation structures reveal a significant cation size effect that is manifested in significantly reduced cation-solvent bond lengths and thus stronger solvent bonding for Zn2+ relative to Ca2+. The strength of this bonding is further reduced by methylation of the cyclic ether ring. Solvation shells containing anions are energetically preferred in all the studied electrolytes, leading to significant quantities of contact ion pairs and consequently neutrally charged clusters. It is likely that the transport and interfacial de-solvation/re-solvation properties of these electrolytes are directed by these anion interactions. These insights into the detailed solvation structures, cation size, and solvent effects, including the molecular dynamics, are fundamentally important for the rational design of electrolytes in multivalent battery electrolyte systems.

Enhanced Electrochemical Performance of Zn/VOx Batteries by a Carbon-Encapsulation Strategy

Aqueous zinc ion batteries show tremendous potential in emerging energy storage devices. However, it is challenging to explore the desired cathode materials that match well with the Zn anode. In this work, we report two kinds of carbon-encapsulated VO_x microspheres grown by controlling the calcination temperature. The assembled Zn/VO₂@C-0.5 batteries deliver a high specific capacity and reversible rate performance. They can still maintain 260 mA h g^-1 at 5 A g^-1 after 1000 cycles. In addition, the cells possess an energy density of 280 W h kg^-1 at a power density of 140 W kg^-1. The soft pack devices also show favorable mechanical stability and durable cycle ability. The excellent zinc ion storage capacity can be attributed to the large tunnel structure of VO₂ materials and the high conductivity of amorphous carbon.

Recent Advances in Electrolytes for "Beyond Aqueous" Zinc-Ion Batteries

With the growing demands for large-scale energy storage, Zn-ion batteries (ZIBs) with distinct advantages, including resource abundance, low-cost, high-safety, and acceptable energy density, are considered as potential substitutes for Li-ion batteries. Although numerous efforts are devoted to design and develop high performance cathodes and aqueous electrolytes for ZIBs, many challenges, such as hydrogen evolution reaction, water evaporation, and liquid leakage, have greatly hindered the development of aqueous ZIBs. Developing "beyond aqueous" electrolytes can be able to avoid these issues due to the absence of water, which are beneficial for the achieving of highly efficient ZIBs. In this review, the recent development of the "beyond aqueous" electrolytes, including conventional organic electrolytes, ionic liquid, all-solid-state, quasi-solid-state electrolytes, and deep eutectic electrolytes are presented. The critical issues and the corresponding strategies of the designing of "beyond aqueous" electrolytes for ZIBs are also summarized.

Cu/Co/CoS2 embedded in S,N doped carbon as highly-efficient oxygen reduction and evolution electrocatalyst for rechargeable zinc-air batteries

In order to improve the retarded oxygen reduction and evolution reaction (ORR/OER) in rechargeable metal-air cells in electrochemical energy conversion systems, constructing multiphase nanostructured catalysts is an alternative strategy, where...

Synthesis, characterisation, and feasibility studies on the use of vanadium tellurate(vi) as a cathode material for aqueous rechargeable Zn-ionbatteries

Aqueous rechargeable zinc-ion batteries (AZIBs) have drawn enormous attention in stationary applications due to their high safety and low cost. However, the search for new positive electrode materials with satisfactory electrochemical performance for practical applications remains a challenge. In this work, we report a comprehensive study on the use of the vanadium tellurate (NH₄)₄{(VO₂)₂[Te₂O₈(OH)₂]}·2H₂O, which is tested for the first time as a cathode material in AZIBs.

(NH₄)₄{(VO₂)₂[Te₂O₈(OH)₂]}·2H₂O is tested as a cathode in an aqueous Zn-ion battery for the first time, showing a discharge capacity of 283 mA h g⁻¹ in half-cells and excellent capacity retention (91%) in concentration cells after 20 cycles.

Interfacial Engineering Strategy for High-Performance Zn Metal Anodes

Due to their high safety and low cost, rechargeable aqueous Zn-ion batteries (RAZIBs) have been receiving increased attention and are expected to be the next generation of energy storage systems. However, metal Zn anodes exhibit a limited-service life and inferior reversibility owing to the issues of Zn dendrites and side reactions, which severely hinder the further development of RAZIBs. Researchers have attempted to design high-performance Zn anodes by interfacial engineering, including surface modification and the addition of electrolyte additives, to stabilize Zn anodes. The purpose is to achieve uniform Zn nucleation and flat Zn deposition by regulating the deposition behavior of Zn ions, which effectively improves the cycling stability of the Zn anode. This review comprehensively summarizes the reaction mechanisms of interfacial modification for inhibiting the growth of Zn dendrites and the occurrence of side reactions. In addition, the research progress of interfacial engineering strategies for RAZIBs is summarized and classified. Finally, prospects and suggestions are provided for the design of highly reversible Zn anodes.

Towards the application of carboxymethyl chitin/ionic liquid blend as polymer electrolyte membrane for aqueous batteries

Carboxymethyl chitin (CMChit) has the potential to be used as a solid polymer electrolyte (SPE) based on its ionic conductivity value of the order of 10^-6 S·cm^-1 in self-standing membranes. In controlled humidity of 65RH%, carboxymethyl chitin based membrane blended with 1-Butyl-3-methylimidazolium acetate (BMIM[Ac]) ionic liquid (IL) (40 wt%) showed a threshold value of ionic conductivity in the order of 10^-4 S·cm^-1 and electrochemical stability was up to 2.93 V. The effects of the relative humidity and ionic liquid weight fraction on the ionic conductivity and structural changes were investigated in detail. Furthermore, the X-ray diffraction (XRD) diffractogram indicated a clear reduction of crystallinity of the CMChit. The Field-emission scanning electron microscopy (FESEM) observation of the cross-sections confirmed the homogeneity of the prepared blend. This electrolyte was tested in symmetric cells based on Zn//SPE//Zn and showed good reversibility and potential for application in proton-conducting batteries.

Harnessing the Volume Expansion of MoS3 Anode by Structure Engineering to Achieve High Performance Beyond Lithium-Based Rechargeable Batteries

Beyond-lithium-ion storage devices are promising alternatives to lithium-ion storage devices for low-cost and large-scale applications. Nowadays, the most of high-capacity electrodes are crystal materials. However, these crystal materials with intrinsic anisotropy feature generally suffer from lattice strain and structure pulverization during the electrochemical process. Herein, a 2D heterostructure of amorphous molybdenum sulfide (MoS₃ ) on reduced graphene surface (denoted as MoS₃ -on-rGO), which exhibits low strain and fast reaction kinetics for beyond-lithium-ions (Na⁺ , K⁺ , Zn²⁺ ) storage is demonstrated. Benefiting from the low volume expansion and small sodiation strain of the MoS₃ -on-rGO, it displays ultralong cycling performance of 40 000 cycles at 10 A g^-1 for sodium-ion batteries. Furthermore, the as-constructed 2D heterostructure also delivers superior electrochemical performance when used in Na⁺ full batteries, solid-state sodium batteries, K⁺ batteries, Zn²⁺ batteries and hybrid supercapacitors, demonstrating its excellent application prospect.

Recent Advances on Spinel Zinc Manganate Cathode Materials for Zinc-Ion Batteries

Zinc metal is abundant in nature, non-toxic, harmless, and cheap. Zinc-ion batteries (ZIBs) have also emerged as the times require, which has attracted scholars' research interest. In the zinc-ion batteries, the cathode material is indispensable. Manganese oxides are widely used in electrode materials because of their various valence states (+2, +3, +4, +7). ZnMn₂ O₄ (ZMO) is a mixed metal oxide with a spinel structure similar to LiMn₂ O₄ . Due to the synergistic effect of Zn and Mn, it has the advantages of high theoretical capacity. In recent years, researchers have gradually applied ZnMn₂ O₄ to zinc ion batteries. In order to obtain high-energy-density zinc ion batteries, it is also very important to match electrolytes with a wide operating voltage window and develop a highly reversible anode. In the first instance, we investigate the research progress of spinel ZnMn₂ O₄ as a reliable candidate material for zinc ion batteries. Later on, we review the optimization and modification measures of anode and electrolyte to improve the electrochemical properties of spinel ZnMn₂ O₄ . On this basis, we propose the reasonable research direction and development prospects for this material. It is hoped that there will be a help to researchers in this field.

An interfacial coating with high corrosion resistance based on halloysite nanotubes for anode protection of zinc-ion batteries

Aqueous zinc-ion batteries are recognized as one of the most potential neutral aqueous batteries because of the high energy density, high specific capacity, low cost, and low pollution. However, the applications of zinc-ion batteries are seriously limited by the capacity fading, easy-corrosion, side reaction, and hydrogen evolution. Herein, we report a uniform halloysite nanotubes (HNTs) coating which can guide Zn²⁺ ions stripping/plating on the HNTs/Zn interfaces and protect the Zn anode. The HNTs coating significantly suppresses the corrosion of Zn anode and effectively reduces the hydrogen evolution and the formation of by-product. Furthermore, the HNTs-Zn anode exhibits lower resistance than bare Zn. Compared with the bare Zn anode batteries, HNTs-Zn/MnO₂ batteries exhibit good capacity retention and can increase the discharge capacity to 79% at 3 C after 400 cycles. The novel design of interfacial coating based on halloysite nanotubes through electrophoretic deposition method provides a new way to fabricate economic and stable aqueous zinc-ion batteries.

A carbon-coated spinel zinc cobaltate doped with manganese and nickel as a cathode material for aqueous zinc-ion batteries

Here, a new amorphous material composed of carbon-coated zinc cobaltate doped with manganese and nickel ZNMC@C (ZnNi_0.5Mn_0.5CoO₄@C) with a spinel structure is proposed as a cathode material for use in aqueous zinc-ion batteries. This cathode material exhibits a high charge/discharge capacity with an initial capacity of about 160 mA h g^-1 and its capacity retention rate remains at 60% after 500 cycles at 0.2 A g^-1, which is higher than that of some reported spinel cathode materials. This superior electrochemical performance can be ascribed to the synergistic effect of the co-doping of manganese and nickel, which produces reversible multivalence redox transition activity (Co⁴⁺/Co³⁺, Ni⁴⁺/Ni³⁺/Ni²⁺, and Mn⁴⁺/Mn³⁺) that facilitates the insertion and migration of zinc ions and the existence of an outer amorphous carbon coating that effectively inhibits the dissolution of the cathode structure and stabilizes the cathode structure. In addition, the cycling mechanism of ZNMC@C was analyzed in detail through electrochemical measurements of the different cycling stages, including the kinetic behavior based on cyclic voltammetry and electrochemical impedance spectroscopic analysis and the reaction mechanism from X-ray photoelectron spectroscopy, ex situ X-ray diffractometry and ex situ scanning electron microscopy analysis. These research results suggest that the ZNMC@C composite material could be a competitive cathode material for Abs (aqueous rechargeable batteries).

Aqueous Rechargeable Zn-ion Batteries: Strategies for Improving the Energy Storage Performance

The growing demand for the renewable energy storage technologies stimulated the quest for efficient energy storage devices. In recent years, the rechargeable aqueous zinc-based battery technologies are emerging as a compelling alternative to the lithium-based batteries owing to safety, eco-friendliness, and cost-effectiveness. Among the zinc-based energy devices, rechargeable zinc-ion batteries (ZIBs) are drawing considerable attention. However, they are plagued with several issues, including cathode dissolution, dendrite formation, etc.. Despite several efforts in the recent past, ZIBs are still in their infant stages and have yet to reach the stage of large-scale production. Finding stable Zn²⁺ intercalation cathode material with high operating voltage and long cycling stability as well as dendrite-free Zn anode is the main challenge in the development of efficient zinc-ion storage devices. This Review discusses the various strategies, in terms of the engineering of cathode, anode and electrolyte, adopted for improving the charge storage performance of ZIBs and highlights the recent ZIB technological innovations. A brief account on the history of zinc-based devices and various cathode materials tested for ZIB fabrication in the last five years are also included. The main focus of this Review is to provide a detailed account on the rational engineering of the electrodes, electrolytes, and separators for improving the charge storage performance with a future perspective to achieving high energy density and long cycling stability and large-scale production for practical application.

Toward a High-Performance Aqueous Zinc Ion Battery: Potassium Vanadate Nanobelts and Carbon Enhanced Zinc Foil

Aqueous rechargeable zinc ion batteries are promising candidates for grid-scale applications owing to their low cost and high safety. However, they are plagued by the lack of suitable cathode and anode materials. Herein, we report on potassium vanadate (KVO) nanobelts as a promising cathode for an aqueous zinc ion battery, which shows a high discharge capacity of 461 mA h g^-1 at 0.2 A g^-1 and exhibits a capacity retention of 96.2% over 4000 cycles at 10 A g^-1. Furthermore, to enhance the energy efficiency in an aqueous zinc ion battery, a facile and effective method on the anode is demonstrated. The energy efficiency increases from 47.5% for Zn//KVO coupled with the zinc foil anode to 66.5% for AB-Zn//KVO coupled with an acetylene black film improved zinc foil anode at 10 A g^-1. The remarkable electrochemical performance makes AB-Zn//KVO a strong candidate for a high-performance aqueous zinc ion battery.

Recent Development of Mn-based Oxides as Zinc-Ion Battery Cathode

Manganese-based oxide is arguably one of the most well-studied cathode materials for zinc-ion battery (ZIB) due to its wide oxidation states, cost-effectiveness, and matured synthesis process. As a result, there are numerous reports that show significant strides in the progress of Mn-based oxides as ZIB cathode. However, ironically, due to the sheer number of Mn-based oxides that have been published in recent years, there remain certain contemplations with regards to the electrochemical performance of each type of Mn-based oxides and their performance comparison among various Mn polymorphs and oxidation states. Thus, to provide a clearer indication of the development of Mn-based oxides, the recent progress in Mn-based oxides as ZIB cathode was summarized systematically in this Review. More specifically, (1) the classification of Mn-based oxides based on the oxidation states (i. e., MnO₂ , Mn₃ O₄ , Mn₂ O₃ , and MnO), (2) their respective polymorphs (i. e., α-MnO₂ and δ-MnO₂ ) as ZIB cathode, (3) the modification strategies commonly employed to enhance the performance, and (4) the effects of these modification strategies on the performance enhancement were reviewed. Lastly, perspectives and outlook of Mn-based oxides as ZIB cathode were discussed at the end of this Review.

Interface Engineering via Ti3C2Tx MXene Electrolyte Additive toward Dendrite-Free Zinc Deposition

Zinc metal batteries have been considered as a promising candidate for next-generation batteries due to their high safety and low cost. However, their practical applications are severely hampered by the poor cyclability that caused by the undesired dendrite growth of metallic Zn. Herein, Ti₃C₂T_x MXene was first used as electrolyte additive to facilitate the uniform Zn deposition by controlling the nucleation and growth process of Zn. Such MXene additives can not only be absorbed on Zn foil to induce uniform initial Zn deposition via providing abundant zincophilic-O groups and subsequently participate in the formation of robust solid-electrolyte interface film, but also accelerate ion transportation by reducing the Zn²⁺ concentration gradient at the electrode/electrolyte interface. Consequently, MXene-containing electrolyte realizes dendrite-free Zn plating/striping with high Coulombic efficiency (99.7%) and superior reversibility (stably up to 1180 cycles). When applied in full cell, the Zn-V₂O₅ cell also delivers significantly improved cycling performances. This work provides a facile yet effective method for developing reversible zinc metal batteries.

Inorganic Colloidal Electrolyte for Highly Robust Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) is a promising electrical energy storage candidate due to its eco-friendliness, low cost, and intrinsic safety, but on the cathode the element dissolution and the formation of irreversible products, and on the anode the growth of dendrite as well as irreversible products hinder its practical application. Herein, we propose a new type of the inorganic highly concentrated colloidal electrolytes (HCCE) for ZIBs promoting simultaneous robust protection of both cathode/anode leading to an effective suppression of element dissolution, dendrite, and irreversible products growth. The new HCCE has high Zn²⁺ ion transference number (0.64) endowed by the limitation of SO₄^2-, the competitive ion conductivity (1.1 × 10^-2 S cm^-1) and Zn²⁺ ion diffusion enabled by the uniform pore distribution (3.6 nm) and the limited free water. The Zn/HCCE/α-MnO₂ cells exhibit high durability under both high and low current densities, which is almost 100% capacity retention at 200 mA g^-1 after 400 cycles (290 mAh g^-1) and 89% capacity retention under 500 mA g^-1 after 1000 cycles (212 mAh g^-1). Considering material sustainability and batteries' high performances, the colloidal electrolyte may provide a feasible substitute beyond the liquid and all-solid-state electrolyte of ZIBs.

Electrolyte Concentration Regulation Boosting Zinc Storage Stability of High-Capacity K0.486V2O5 Cathode for Bendable Quasi-Solid-State Zinc Ion Batteries

Vanadium-based cathodes have attracted great interest in aqueous zinc ion batteries (AZIBs) due to their large capacities, good rate performance and facile synthesis in large scale. However, their practical application is greatly hampered by vanadium dissolution issue in conventional dilute electrolytes. Herein, taking a new potassium vanadate K_0.486V₂O₅ (KVO) cathode with large interlayer spacing (~ 0.95 nm) and high capacity as an example, we propose that the cycle life of vanadates can be greatly upgraded in AZIBs by regulating the concentration of ZnCl₂ electrolyte, but with no need to approach "water-in-salt" threshold. With the optimized moderate concentration of 15 m ZnCl₂ electrolyte, the KVO exhibits the best cycling stability with ~ 95.02% capacity retention after 1400 cycles. We further design a novel sodium carboxymethyl cellulose (CMC)-moderate concentration ZnCl₂ gel electrolyte with high ionic conductivity of 10.08 mS cm^-1 for the first time and assemble a quasi-solid-state AZIB. This device is bendable with remarkable energy density (268.2 Wh kg^-1), excellent stability (97.35% after 2800 cycles), low self-discharge rate, and good environmental (temperature, pressure) suitability, and is capable of powering small electronics. The device also exhibits good electrochemical performance with high KVO mass loading (5 and 10 mg cm^-2). Our work sheds light on the feasibility of using moderately concentrated electrolyte to address the stability issue of aqueous soluble electrode materials.

Simulation of dendritic growth of a zinc anode in a zinc–nickel single flow battery using the phase field-lattice Boltzmann method

The mechanism of zinc dendrite formation was explored to obtain high-safety zinc nickel single liquid batteries.