High-Performance Aqueous Zinc-Ion Battery Based on Layered H2 V3 O8 Nanowire Cathode

Synergistic Manipulation of Zn2+ Ion Flux and Desolvation Effect Enabled by Anodic Growth of a 3D ZnF2 Matrix for Long-Lifespan and Dendrite-Free Zn Metal Anodes

Aqueous rechargeable Zn metal batteries have attracted widespread attention due to the intrinsic high volumetric capacity, low cost, and high safety. However, the low Coulombic efficiency and limited lifespan of Zn metal anodes resulting from uncontrollable growth of Zn dendrites impede their practical application. In this work, a 3D interconnected ZnF₂ matrix is designed on the surface of Zn foil (Zn@ZnF₂ ) through a simple and fast anodic growth method, serving as a multifunctional protective layer. The as-fabricated Zn@ZnF₂ electrode can not only redistribute the Zn²⁺ ion flux, but also reduce the desolvation active energy significantly, leading to stable and facile Zn deposition kinetics. The results reveal that the Zn@ZnF₂ electrode can effectively inhibit dendrites growth, restrain the hydrogen evolution reactions, and endow excellent plating/stripping reversibility. Accordingly, the Zn@ZnF₂ electrode exhibits a long cycle life of over 800 h at 1 mA cm^-2 with a capacity of 1.0 mAh cm^-2 in a symmetrical cell test, the feasibility of which is also convincing in Zn@ZnF₂ //MnO₂ and Zn@ZnF₂ //V₂ O₅ full batteries. Importantly, a hybrid zinc-ion capacitor of the Zn@ZnF₂ //AC can work at an ultrahigh current density of ≈60 mA cm^-2 for up to 5000 cycles with a high capacity retention of 92.8%.

In Situ Electrochemical Transformation Reaction of Ammonium-Anchored Heptavanadate Cathode for Long-Life Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (ZIBs) are promising portable and large-scale grid energy storage devices, as they are safe and economical. However, developing suitable ZIB cathode materials with excellent cycling performance characteristics remains a challenging task. Here, ammonium heptavanadate (NH₄)₂V₇O₁₆·3.2H₂O (NHVO) nanosquares with mixed-valence V⁵⁺/V⁴⁺ as a cathode are developed for high-performance ZIBs. The layered NHVO shows a capacity of 362 mA h g^-1 at 0.05 A g^-1, with a high energy density of 263.5 W h kg^-1. It exhibits an initial specific capacity of 250.7 mA h g^-1 at a current density of 4 A g^-1 and retains 255 mA h g^-1 capacity after 1000 charge/discharge cycles. The V₇O₁₆-based cathode was demonstrated with a phase transition to the V₂O₅-based cathode upon initial cycling. Moreover, the in situ generated V₂O₅-based cathodes show excellent electrochemical properties, which provide a different perspective on the electrochemical reaction of cathode materials for aqueous ZIBs.

Unravelling V6O13 Diffusion Pathways via CO2 Modification for High-Performance Zinc Ion Battery Cathode

Vanadium-based oxide is widely investigated as a zinc ion battery (ZIB) cathode due to its ability to react reversibly with Zn²⁺. Despite its successful demonstration, modification with simple molecules has shown some promise in enhancing the performance of ZIBs. Thus, this presents an immense opportunity to explore simple molecules that can dramatically improve the electrochemical performance of electrodes. Thus, the effect of CO₂ modification is studied in this work by decomposing oxalic acid within a hydrated V₆O₁₃ framework. Based on the collective results, the presence of CO₂ drastically lowers the relative energy of Zn²⁺ diffusion through the pathways by forming weak electrostatic interactions between O_CO2 and Zn²⁺. This leads to an enlarged diffusion contribution, which consequently results in enhanced stability and better rate performance. The as-synthesized CO₂-V₆O₁₃ electrode delivers one of the highest specific capacities reported for vanadium-based oxides of ca. 471 mAh g^-1. Furthermore, an excellent cyclic stability of 80% capacity retention after 4000 cycles at 2 A g^-1 is recorded for CO₂-V₆O₁₃, which suggests the importance of simple molecules in the material framework toward the enhancement of ZIB cathode performance.

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.

A new tunnel-type V4O9 cathode for high power density aqueous zinc ion batteries

A mixed-valence vanadium oxide V4O9 with a tunnel structure was synthesized as a cathode material for aqueous zinc ion batteries. A Zn//V4O9 battery displays high specific capacity and excellent rate performance.

Advanced Filter Membrane Separator for Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries with low cost and inherent safety are considered to be the next-generation energy storage device. However, they suffer from poor cycling stability and low coulombic efficiency caused by the serious zinc dendrites during the cycling. In this work, a porous water-based filter membrane is first proposed as separator due to its good toughness and uniform pore distribution. The results demonstrate that the symmetrical cell using a filter membrane can cycle over 2600 h with a low voltage hysteresis of 47 mV. Moreover, an aqueous Zn//NaV₃ O₈ ·1.5H₂ O cell based on the filter membrane is constructed, which demonstrates a high capacity retention of 83.8% after 5000 cycles at 5 A g^-1 . The mechanism research results reveal that the excellent dendrites inhibiting the ability of the filter membrane should be attributed to its uniform pore distribution rather than its composition. This work proposes a filter membrane separator and reveals the great influence of separator on the zinc stripping/plating process, which will shed light on the development of high-performance aqueous zinc-ion batteries.

In Operando Synchrotron Studies of NH4+ Preintercalated V2O5·nH2O Nanobelts as the Cathode Material for Aqueous Rechargeable Zinc Batteries

NH₄⁺ preintercalated V₂O₅·nH₂O nanobelts with a large interlayer distance of 10.9 Å were prepared by the hydrothermal method. The material showed a large specific capacity of 391 mA·h·g^-1 at the 500 mA·g^-1 current density in aqueous rechargeable zinc batteries. In operando synchrotron X-ray diffraction demonstrated that the material experienced reversible solid-solution reaction and two-phase transition during charge-discharge cycling, accompanied by the reversible formation/decomposition of a ZnSO₄Zn₃(OH)₆·5H₂O byproduct. In operando X-ray absorption spectroscopy confirmed the reversible reduction/oxidation of V, together with small changes in the VO₆ local structure. The formation of byproduct was attributed to the dehydration of [Zn(H₂O)₆]²⁺, which concurrently improved the desolvation of [Zn(H₂O)₆]²⁺ into Zn²⁺. Bond valence sum map analysis and electrochemical impedance spectroscopy demonstrated that the byproduct improved the charge transfer kinetics of the electrode. Cyclic voltammetry and galvanostatic intermittent titration technique showed that the electrode reaction was dominated by ionic intercalation where the discharge capacity in the voltage window of 1.4-0.85 V was attributed to the intercalation of [Zn(H₂O)₆]²⁺, followed by the intercalation of Zn²⁺ at 0.85-0.4 V.

Operando pH Measurements Decipher H+/Zn2+ Intercalation Chemistry in High-Performance Aqueous Zn/δ-V2O5 Batteries

Vanadium oxides have been recognized to be among the most promising positive electrode materials for aqueous zinc metal batteries (AZMBs). However, their underlying intercalation mechanisms are still vigorously debated. To shed light on the intercalation mechanisms, high-performance δ-V₂O₅ is investigated as a model compound. Its structural and electrochemical behaviors in the designed cells with three different electrolytes, i.e., 3 m Zn(CF₃SO₃)₂/water, 0.01 M H₂SO₄/water, and 1 M Zn(CF₃SO₃)₂/acetonitrile, demonstrate that the conventional structural and elemental characterization methods cannot adequately clarify the separate roles of H⁺ and Zn²⁺ intercalations in the Zn(CF₃SO₃)₂/water electrolyte. Thus, an operando pH determination method is developed and used toward Zn/δ-V₂O₅ AZMBs. This method indicates the intercalation of both H⁺ and Zn²⁺ into δ-V₂O₅ and uncovers an unusual H⁺/Zn²⁺-exchange intercalation-deintercalation mechanism. Density functional theory calculations further reveal that the H⁺/Zn²⁺ intercalation chemistry is a consequence of the variation of the electrochemical potential of Zn²⁺ and H⁺ during the electrochemical intercalation/release.

Latest Advances in High-Voltage and High-Energy-Density Aqueous Rechargeable Batteries

Abstract

Aqueous rechargeable batteries (ARBs) have become a lively research theme due to their advantages of low cost, safety, environmental friendliness, and easy manufacturing. However, since its inception, the aqueous solution energy storage system has always faced some problems, which hinders its development, such as the narrow electrochemical stability window of water, poor percolation of electrode materials, and low energy density. In recent years, to overcome the shortcomings of the aqueous solution-based energy storage system, some very pioneering work has been done, which also provides a great inspiration for further research and development of future high-performance aqueous energy storage systems. In this paper, the latest advances in various ARBs with high voltage and high energy density are reviewed. These include aqueous rechargeable lithium, sodium, potassium, ammonium, zinc, magnesium, calcium, and aluminum batteries. Further challenges are pointed out.

Graphic Abstract

Aqueous can be better in terms of safety, friendliness, and energy density.

Progress of Organic Electrodes in Aqueous Electrolyte for Energy Storage and Conversion

Aqueous batteries using inorganic compounds as electrode materials are considered a promising solution for grid-scale energy storage, while wide application is limited by the short life and/or high cost of electrodes. Organics with carbonyl groups are being investigated as the alternative to inorganic electrode materials because they offer the advantages of tunable structures, renewability, and they are environmentally benign. Furthermore, the wide internal space of such organic materials enables flexible storage of various charged ions (for example, H⁺ , Li⁺ , Na⁺ , K⁺ , Zn²⁺ , Mg²⁺ , and Ca²⁺ , and so on). We offer a comprehensive overview of the progress of organics containing carbonyls for energy storage and conversion in aqueous electrolytes, including applications in aqueous batteries as solid-state electrodes, in flow batteries as soluble redox species, and in water electrolysis as redox buffer electrodes. The advantages of organic electrodes are summarized, with a discussion of the challenges remaining for their practical application.

Boosting Zn-Ion Storage Performance of Bronze-Type VO2 via Ni-Mediated Electronic Structure Engineering

Aqueous rechargeable zinc-ion batteries are emerging as attractive alternatives for post-lithium-ion batteries. However, their electrochemical performances are restricted by the narrow working window of materials in aqueous electrolytes. Herein, a Ni-mediated VO₂-B nanobelt [(Ni)VO₂] has been designed to optimize the intrinsic electronic structure of VO₂-B and thus achieve much more enhanced zinc-ion storage. Specifically, the Zn/(Ni)VO₂ battery yields a good rate capability (182.0 mA h g^-1 at 5 A g^-1) with a superior cycling stability (130.6 mA h g^-1 at 10 A g^-1 after 2000 cycles). Experimental and theoretical methods reveal that the introduction of Ni²⁺ in the VO₂ tunnel structure can effectively provide high surface reactivity and improve the intrinsic electronic configurations, thus resulting in good kinetics. Furthermore, H⁺ and Zn²⁺ cointercalation processes are determined via in situ X-ray diffraction and supported by ex situ characterizations. Additionally, quasi-solid-state Zn/(Ni)VO₂ soft-packaged batteries are assembled and provide flexibility in battery design for practical applications. The results provide insights into the interrelationships between the intrinsic electronic structure of the cathode and the overall electrochemical performance.

Graphene-like Vanadium Oxygen Hydrate (VOH) Nanosheets Intercalated and Exfoliated by Polyaniline (PANI) for Aqueous Zinc-Ion Batteries (ZIBs)

A new approach is employed to boost the electrochemical kinetics and stability of vanadium oxygen hydrate (VOH, V₂O₅·nH₂O) employed for aqueous zinc-ion battery (ZIB) cathodes. The methodology is based on electrically conductive polyaniline (PANI) intercalated-exfoliated VOH, achieved by preintercalation of an aniline monomer and its in situ polymerization within the oxide interlayers. The resulting graphene-like PANI-VOH nanosheets possess a greatly boosted reaction-controlled contribution to the total charge storage capacity, resulting in more material undergoing the reversible V⁵⁺ to V³⁺ redox reaction. The PANI-VOH electrode obtains an impressive capacity of 323 mAh g^-1 at 1 A g^-1, and state-of-the-art cycling stability at 80% capacity retention after 800 cycles. Because of the facile redox kinetics, the PANI-VOH ZIB obtains uniquely promising specific energy-specific power combinations: an energy of 216 Wh kg^-1 is achieved at 252 W kg^-1, while 150 Wh kg^-1 is achieved at 3900 W kg^-1. Electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) analyses indicate that with PANI-VOH nanosheets, there is a simultaneous decrease in the charge transfer resistance and a boost in the diffusion coefficient of Zn²⁺ (by a factor of 10-100) vs the VOH baseline. The strategy of employing PANI for combined intercalation-exfoliation may provide a broadly applicable approach for improving the performance in a range of oxide-based energy storage materials.

Building High Rate Capability and Ultrastable Dendrite-Free Organic Anode for Rechargeable Aqueous Zinc Batteries

Aqueous zinc-ion batteries (ZIBs) are an alternative energy storage system for large-scale grid applications compared with lithium-ion batteries, when the low cost, safety, and durability are taken into consideration. However, the reliability of the battery systems always suffers from the serious challenge of the large Zn dendrite formation and "dead Zn," thus bringing out the inferior cycling stability, and even cell shorting. Herein, a dendrite-free organic anode, perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) polymerized on the surface of reduced graphene oxide (PTCDI/rGO) utilized in ZIBs is reported. Moreover, the theoretical calculations prove the reason for the low redox potential. Due to the protons and zinc ions coparticipant phase transfer mechanism and the high charge transfer capability, the PTCDI/rGO electrode provides superior rate capability (121 mA h g-1 at 5000 mA g-1, retaining the 95% capacity of that compared with 50 mA g-1) and a long cycling life span (96% capacity retention after 1500 cycles at 3000 mA g-1). In addition, the proton coparticipation energy storage mechanism of active materials is elucidated by various ex-situ methods.

Freestanding Potassium Vanadate/Carbon Nanotube Films for Ultralong-Life Aqueous Zinc-Ion Batteries

Among various energy storage devices, aqueous zinc-ion batteries (ZIBs) have captured great attention due to their high safety and low cost. One of the most promising cathodes of aqueous ZIBs is layered vanadium-based compounds. However, they often suffer from the capacity decaying during cycling. Herein, we prepared KV₃O₈·0.75H₂O (KVO) and further incorporated it into a single-walled carbon nanotube (SWCNT) network, achieving freestanding KVO/SWCNT composite films. The KVO/SWCNT cathodes exhibit a Zn²⁺/H⁺ insertion/extraction mechanism, resulting in fast kinetics of ion transfer. In addition, the KVO/SWCNT composite films possess a segregated network structure, which offers the fast kinetics of electron transfer and guarantees an intimate contact between KVO and SWCNTs during cycling. As a result, the resultant batteries deliver a high capacity of 379 mAh g^-1, excellent rate capability, and an ultralong cycle life up to 10,000 cycles with a high capacity retention of 91%. In addition, owing to the high conductivity and flexibility of KVO/SWCNT films, flexible soft-packaged ZIBs based on KVO/SWCNT film cathodes were assembled and displayed stable electrochemical performance at different bending states.

Electrochemically Induced Structural and Morphological Evolutions in Nickel Vanadium Oxide Hydrate Nanobelts Enabling Fast Transport Kinetics for High-Performance Zinc Storage

Suitable intercalation cathodes and fundamental insights into the Zn-ion storage mechanism are the crucial factors for the booming development of aqueous zinc-ion batteries. Herein, a novel nickel vanadium oxide hydrate (Ni_0.25V₂O₅·0.88H₂O) is synthesized and investigated as a high-performance electrode material, which delivers a reversible capacity of 418 mA h g^-1 with 155 mA h g^-1 retained at 20 A g^-1 and a high capacity of 293 mA h g^-1 in long-term cycling at 10 A g^-1 with 77% retention after 10,000 cycles. More importantly, multistep phase transition and chemical-state change during intercalation/deintercalation of hydrated Zn²⁺ are illustrated in detail via in situ/ex situ analytical techniques to unveil the Zn²⁺ storage mechanism of the hydrated and layered vanadium oxide bronze. Furthermore, morphological development from nanobelts to hierarchical structures during rapid ion insertion and extraction is demonstrated and a self-hierarchical process is correspondingly proposed. The unique evolutions of structure and morphology, together with consequent fast Zn²⁺ transport kinetics, are of significance to the outstanding zinc storage capacity, which would enlighten the mechanism exploration of the aqueous rechargeable batteries and push development of vanadium-based cathode materials.

An Overview and Future Perspectives of Rechargeable Zinc Batteries

Aqueous rechargeable zinc-based batteries have sparked a lot of enthusiasm in the energy storage field recently due to their inherent safety, low cost, and environmental friendliness. Although remarkable progress has been made in the exploration of performance so far, there are still many challenges such as low working voltage and dissolution of electrode materials at the material and system level. Herein, the central tenet is to establish a systematic summary for the construction and mechanism of different aqueous zinc-based batteries. Details for three major zinc-based battery systems, including alkaline rechargeable Zn-based batteries (ARZBs), aqueous Zn ion batteries (AZIBs), and dual-ion hybrid Zn batteries (DHZBs) are given. First, the electrode materials and energy storage mechanism of the three types of zinc-based batteries are discussed to provide universal guidance for these batteries. Then, the electrode behavior of zinc anodes and strategies to deal with problems such as dendrite and passivation are recommended. Finally, some challenge-oriented solutions are provided to facilitate the next development of zinc-based batteries. Combining the characteristics of zinc-based batteries with good use of concepts and ideas from other disciplines will surely pave the way for its commercialization.

Boosting Zinc-Ion Storage Capability by Effectively Suppressing Vanadium Dissolution Based on Robust Layered Barium Vanadate

Vanadium-based compounds with an open framework structure have become the subject of much recent investigation into aqueous zinc-ion batteries (AZIBs) due to high specific capacity. However, there are some issues with vanadium dissolution from a cathode framework as well as the generation of byproducts during discharge that should not be ignored, which could cause severe capacity deterioration and inadequate cycle life. Herein, we report several barium vanadate nanobelt cathodes constructed of two sorts of architectures, i.e., Ba_1.2V₆O₁₆·3H₂O and BaV₆O₁₆·3H₂O (V₃O₈-type) and Ba_xV₂O₅·nH₂O (V₂O₅-type), which are controllably synthesized by tuning the amount of barium precursor. Benefiting from the robust architecture, layered Ba_xV₃O₈-type nanobelts (Ba_1.2V₆O₁₆·3H₂O) exhibit superior rate capability and long-term cyclability owing to fast zinc-ion kinetics, enabled by efficiently suppressing cathode dissolution as well as greatly eliminating the generation of byproduct Zn₄SO₄(OH)₆·xH₂O, which provides a reasonable strategy to engineer cathode materials with robust architectures to improve the electrochemical performance of AZIBs.

Interfacial Engineering Coupled Valence Tuning of MoO3 Cathode for High-Capacity and High-Rate Fiber-Shaped Zinc-Ion Batteries

Aqueous Zn-ion batteries (ZIBs) have garnered the researchers' spotlight owing to its high safety, cost effectiveness, and high theoretical capacity of Zn anode. However, the availability of cathode materials for Zn ions storage is limited. With unique layered structure along the [010] direction, α-MoO₃ holds great promise as a cathode material for ZIBs, but its intrinsically poor conductivity severely restricts the capacity and rate capability. To circumvent this issue, an efficient surface engineering strategy is proposed to significantly improve the electric conductivity, Zn ion diffusion rate, and cycling stability of the MoO₃ cathode for ZIBs, thus drastically promoting its electrochemical properties. With the synergetic effect of Al₂ O₃ coating and phosphating process, the constructed Zn//P-MoO_{3- x} @Al₂ O₃ battery delivers impressive capacity of 257.7 mAh g^-1 at 1 A g^-1 and superior rate capability (57% capacity retention at 20 A g^-1 ), dramatically surpassing the pristine Zn//MoO₃ battery (115.8 mAh g^-1 ; 19.7%). More importantly, capitalized on polyvinyl alcohol gel electrolyte, an admirable capacity (19.2 mAh cm^-3 ) as well as favorable energy density (14.4 mWh cm^-3 ; 240 Wh kg^-1 ) are both achieved by the fiber-shaped quasi-solid-state ZIB. This work may be a great motivation for further research on molybdenum or other layered structure materials for high-performance ZIBs.

Unlocking the Door of Boosting Biodirected Structures for High-Performance VN x O y /C by Controlling the Reproduction Mode

Diverse reproduction modes of bio-organisms open new intriguing opportunities for biochemistry-enabled materials. Herein, a new strategy is developed to explore biodirected structures for functional materials via controlling the reproduction mode. Yeast with sexual or asexual reproduction mode are employed in this work. They result in two different biodirected structures, from bowl-like hollow hemisphere to "bubble-in-sphere" (BIS) structure, for the VN x O y /C composites. Benefitting from the hierarchical structure, nanoscale particles and conductive biomass-derived carbon base, both VN x O y /C biocomposites achieve high power/energy density, good reliability, and excellent long-term cycling stability in aqueous Zn-ion batteries. Deep investigations further reveal that different biodirected structures greatly influence the electrochemical properties of biocomposites. The bowl-like structures with thin shells and folded double layers achieve larger surface area and more active sites, which ensure their faster kinetics and better high rate capability. The BIS structures with a more compact assembly and higher stack capability are favorable to the better energy storage. Therefore, this work not only introduces a new clue to boost biodirected structures for functional materials, but also propels the development of Zn-ion batteries in diverse applications.

Aqueous zinc ion batteries: focus on zinc metal anodes

Despite the prevalence of lithium ion batteries in modern technology, the search for alternative electrochemical systems to complement the global battery portfolio is an ongoing effort. The search has resulted in numerous candidates, among which mildly acidic aqueous zinc ion batteries have recently garnered significant academic interest, mostly due to their inherent safety. As the anode is often fixed as zinc metal in these systems, most studies address the absence of a suitable cathode for reaction with zinc ions. This has led to aggressive research into viable intercalation cathodes, some of which have shown impressive results. However, many investigations often overlook the implications of the zinc metal anode, when in fact the anode is key to determining the energy density of the entire cell. In this regard, we aim to shed light on the importance of the zinc metal anode. This perspective offers a brief discussion of zinc electrochemistry in mildly acidic aqueous environments, along with an overview of recent efforts to improve the performance of zinc metal to extract key lessons for future research initiatives. Furthermore, we discuss the energy density ramifications of the zinc anode with respect to its weight and reversibility through simple calculations for numerous influential reports in the field. Finally, we offer some perspectives on the importance of optimizing zinc anodes as well as a future direction for developing high-performance aqueous zinc ion batteries.

Mechanistic Investigation of a Hybrid Zn/V2 O5 Rechargeable Battery with a Binary Li+ /Zn2+ Aqueous Electrolyte

Low-cost, easily processable, and environmentally friendly rechargeable aqueous zinc batteries have great potential for large-scale energy storage, which justifies their receiving extensive attention in recent years. An original concept based on the use of a binary Li⁺ /Zn²⁺ aqueous electrolyte is described herein for the case of the Zn/V₂ O₅ system. In this hybrid, the positive side involves mainly the Li⁺ insertion/deinsertion reaction of V₂ O₅ , whereas the negative electrode operates according to zinc dissolution-deposition cycles. The Zn//3 mol L^-1 Li₂ SO₄ -4 mol L^-1 ZnSO_4/ //V₂ O₅ cell worked in the narrow voltage range of 1.6-0.8 V with capacities of approximately 136-125 mA h g^-1 at rates of C/20-C/5, respectively. At 1 C, the capacity of 80 mA h g^-1 was outstandingly stable for more than 300 cycles with a capacity retention of 100 %. A detailed structural study by XRD and Raman spectroscopy allowed the peculiar response of the V₂ O₅ layered host lattice on discharge-charge and cycling to be unraveled. Strong similarities with the well-known structural changes reported in nonaqueous lithiated electrolytes were highlighted, although the emergence of the usual distorted δ-LiV₂ O₅ phase was not detected on discharge to 0.8 V. The pristine host structure was restored and maintained during cycling with mitigated structural changes leading to high capacity retention. The present electrochemical and structural findings reveal a reaction mechanism mainly based on Li⁺ intercalation, but co-intercalation of a few Zn²⁺ ions between the oxide layers cannot be completely dismissed. The presence of zinc cations between the oxide layers is thought to relieve the structural stress induced in V₂ O₅ under operation, and this resulted in a limited volume expansion of 4 %. This fundamental investigation of a reaction mechanism operating in an environmentally friendly aqueous medium has not been reported before.

One-step synthesis of MnOx/PPy nanocomposite as a high-performance cathode for a rechargeable zinc-ion battery and insight into its energy storage mechanism

Rechargeable aqueous zinc-ion batteries (ZIBs) have attracted significant attention in the energy storage field. Manganese-based materials are the most promising cathode materials for ZIBs but they suffer from low electronic conductivity. Herein, a high-performance cathode for ZIBs based on nanocomposites consisting of mixed-valence manganese dioxide (Mn III and IV) and polypyrrole (MnO_x/PPy) is prepared through an efficient one-step organic/inorganic interface redox reaction. The role of polypyrrole (PPy) in the MnO_x/PPy cathode is elaborated. It not only provides an effective conductive network for MnO_x but also contributes to the capacity of the composite. By optimizing the amount of PPy, the MnO_x/PPy composite with 12 wt% PPy exhibits the highest capacity. As a result, the corresponding Zn-MnO_x/PPy battery delivers a high capacity (302.0 mA h g^-1 at 0.15 A g^-1), excellent rate performance (159.9 mA h g^-1 at 3 A g^-1) and superior cycling stability. Furthermore, the results of ex situ characterization analysis reveal that H⁺ and Zn²⁺ insertion/extraction both occur in MnO_x/PPy particles during the discharging/charging process, while only Zn²⁺ insertion/extraction occurs in the PPy electrode. This work develops an efficient one-step synthesis method for large scale production of manganese-based materials/conducting polymers as the cathode for ZIB application, and provides an insight into its energy storage mechanism.

Oxygen Defects in β-MnO2 Enabling High-Performance Rechargeable Aqueous Zinc/Manganese Dioxide Battery

Rechargeable aqueous Zn/manganese dioxide (Zn/MnO2) batteries are attractive energy storage technology owing to their merits of low cost, high safety, and environmental friendliness. However, the β-MnO2 cathode is still plagued by the sluggish ion insertion kinetics due to the relatively narrow tunneled pathway. Furthermore, the energy storage mechanism is under debate as well. Here, β-MnO2 cathode with enhanced ion insertion kinetics is introduced by the efficient oxygen defect engineering strategy. Density functional theory computations show that the β-MnO2 host structure is more likely for H+ insertion rather than Zn2+, and the introduction of oxygen defects will facilitate the insertion of H+ into β-MnO2. This theoretical conjecture is confirmed by the capacity of 302 mA h g-1 and capacity retention of 94% after 300 cycles in the assembled aqueous Zn/β-MnO2 cell. These results highlight the potentials of defect engineering as a strategy of improving the electrochemical performance of β-MnO2 in aqueous rechargeable batteries.

Ultrafast Synthesis of Calcium Vanadate for Superior Aqueous Calcium-Ion Battery

Recently, multivalent aqueous calcium-ion batteries (CIBs) have attracted considerable attention as a possible alternative to Li-ion batteries. However, traditional Ca-ion storage materials show either limited rate capabilities and poor cycle life or insufficient specific capacity. Here, we tackle these limitations by exploring materials having a large interlayer distance to achieve decent specific capacities and one-dimensional architecture with adequate Ca-ion passages that enable rapid reversible (de)intercalation processes. In this work, we report the high-yield, rapid, and low-cost synthesis of 1D metal oxides MV₃O₈ (M = Li, K), CaV₂O₆, and CaV₆O₁₆·7H₂O (CVO) via a molten salt method. Firstly, using 1D CVO as electrode materials, we show high capacity 205 mA h g⁻¹, long cycle life (>97% capacity retention after 200 cycles at 3.0 C), and high-rate performance (117 mA h g⁻¹ at 12 C) for Ca-ion (de)intercalation. This work represents a step forward for the development of the molten salt method to synthesize nanomaterials and to help pave the way for the future growth of Ca-ion batteries.

Microstructural Engineering of Cathode Materials for Advanced Zinc-Ion Aqueous Batteries

Zinc-ion batteries (ZIBs) have attracted intensive attention due to the low cost, high safety, and abundant resources. However, up to date, challenges still exist in searching for cathode materials with high working potential, excellent electrochemical activity, and good structural stability. To address these challenges, microstructure engineering has been widely investigated to modulate the physical properties of cathode materials, and thus boosts the electrochemical performances of ZIBs. Here, the recent research efforts on the microstructural engineering of various ZIB cathode materials are mainly focused upon, including composition and crystal structure selection, crystal defect engineering, interlayer engineering, and morphology design. The dependency of cathode performance on aqueous electrolyte for ZIB is further discussed. Finally, future perspectives and challenges on microstructure engineering of cathode materials for ZIBs are provided. It is aimed to provide a deep understanding of the microstructure engineering effect on Zn2+ storage performance.

A High Capacity Bilayer Cathode for Aqueous Zn-Ion Batteries

Aqueous Zn-ion batteries (ZIBs) are promising candidates for grid-scale energy storage because they are intrinsically safe, cost competitive, and energy intense. However, the development of ZIBs is currently challenged by the performance of cathode materials. Herein, we report on Ca_0.67V₈O₂₀·3.5H₂O (CaVO) nanobelts as a type of ZIB cathode with a discharge capacity of 466 mAh g^-1 (equivalent to an energy density of 345.6 Wh kg^-1) at 0.1 A g^-1 and a capacity retention rate of 100%, 95%, and 74% at 5.0 A g^-1 for 500, 1000, and 2000 cycles, respectively. Through a combined theoretical and experimental study, we reveal that the outstanding energy and power performances of CaVO are deeply rooted in its Zn²⁺-transport friendly, bilayer ρ-type V₂O₅ structure, and the structure-derived reversibility in single-phase Zn²⁺-intercalation/deintercalation process. We also uncover that Ca²⁺ as a structural stabilizer in CaVO undergoes a fast, performance-harmless ion-exchange with Zn²⁺ in the electrolyte and the entire Zn²⁺-intercalation/deintercalation process is accompanied by a counter migration of solvent water. Last, we show that a successful synthesis of CaVO depends critically on pH value of the precursor solution and the structural stability of CaVO is controlled by the co-presence of Ca²⁺/Zn²⁺ and structural water.

Binder-Free Centimeter-Long V2O5 Nanofibers on Carbon Cloth as Cathode Material for Zinc-Ion Batteries

Recently, rechargeable aqueous zinc-ion batteries (AZBs) have attracted extensive interest due to their safety, abundance, low cost, and low toxicity. However, aqueous electrolytes require a polymeric binder to prevent dissolution of the active material in addition to its binding properties. This study highlights binder-free, centimeter long, single-crystal, V2O5 nanofibers (BCS-VONF) on carbon cloth, as the cathode material for AZBs synthesized via a simple one-step hydrothermal process. BCS-VONF in 3.0 M Zn(OTf)2 exhibit promising electrochemical performance with excellent capacity retention. Even in the absence of a binder, BCS-VONF were found to be very stable in 3.0 M Zn(OTf)2. They will not yield to the dissolution and detachment of the active material on the current collector. The novel strategy described in this study is an essential step for the development of BCS-VONF on carbon cloth, as a promising cathode material for AZBs.

Hierarchical Porous Metallic V2O3@C for Advanced Aqueous Zinc-Ion Batteries

Aqueous Zn-ion batteries (ZIBs) are a potential electrochemical energy storage device because of their highly intrinsic safety, low cost, and large capacity. However, it is still in the primary stage because of the limited selection of cathode materials with high rate and long-life cycling stability. In addition, the energy storage mechanisms of ZIBs have not been well established. In this work, we report the synthesis of porous V₂O₃@C materials with high conductivity and further illustrate its application as the intercalation cathode for aqueous zinc-ion batteries. The unique channel and appropriate pore size distribution of corundum-type V₂O₃ are beneficial to the rapid zinc ion intercalation and removal, leading to a high rate capability. Also, the carbon framework structure achieves a high cyclic stability. The porous V₂O₃@C cathode delivers high capacities of 350 mA h g^-1 at 100 mA g^-1, an excellent rate capability (250 mA h g^-1 at 2 A g^-1), and an impressive long-life cycling stability with 90% capacity retention over 4000 cycles at 5 A g^-1. The storage mechanism of zinc ions in the Zn/V₂O₃ system was studied by various analytical methods and first-principles calculation.

Cathode Interfacial Layer Formation via in Situ Electrochemically Charging in Aqueous Zinc-Ion Battery

The issue of material dissolution is common in aqueous batteries, leading to serious performance deterioration. However, it is difficult to be solved so far. In this study, a single component cathode solid electrolyte interface (SEI) layer (CaSO₄·2H₂O) is observed via in situ electrochemically charging process, as demonstrated in a Ca₂MnO₄ cathode for an aqueous zinc-ion battery. Density functional theory calculation confirms its electronic insulation and ionic conductor properties, indicating that it is an appropriate SEI film. The material dissolution seems to be effectively suppressed by the presence of the SEI layer on the cathode side. Meanwhile, this in situ formed interface layer is advantageous for lowering impedance, ameliorating interface, and reducing activation energy. As a result, significantly superior rate performance and cycle stability are exhibited. The observation of a protective SEI layer in an aqueous system may provide an insight into the development of high stability aqueous batteries.

Vanadium Pentoxide Nanosheets in-Situ Spaced with Acetylene Black as Cathodes for High-Performance Zinc-Ion Batteries

The 2D materials of ultrathin nanosheets possess plentiful active sites, effectively enhancing the electrochemical kinetics of various electrode materials. However, 2D materials usually suffer from the aggregation issue due to the strong van der Waals force of the individual nanosheets, leading to irreversible stacking and decreasing capacity. In this work, we develop a universal method to in-situ space the nanosheet cathodes for the electrodes of ZIBs. As a proof of concept, the V₂O₅ nanosheets with in-situ AB spacer were obtained, which were used as the cathode of aqueous ZIBs. Owing to plentiful active sites usable for electrolyte/electrode interfaces of the spaced V₂O₅, the ion diffusion kinetics of the electrodes would be enhanced. In contrast with the stacked V₂O₅, the spaced V₂O₅ delivers an exceptional specific capacity (452 mAh g^-1), superior rate capability, and stable cycle life (the capacity retention of 92% after 5000 cycles). Moreover, the resultant ZIBs assembled with spaced V₂O₅ show an impressive energy/power density (158 Wh kg^-1 at 19.3 kW kg^-1). These superior electrochemical properties make the method of in-situ adding AB spacer hold promising potential to obtain advanced nanosheet electrodes for energy storage systems.

Defect Promoted Capacity and Durability of N-MnO2- x Branch Arrays via Low-Temperature NH3 Treatment for Advanced Aqueous Zinc Ion Batteries

Defect engineering (doping and vacancy) has emerged as a positive strategy to boost the intrinsic electrochemical reactivity and structural stability of MnO₂ -based cathodes of rechargeable aqueous zinc ion batteries (RAZIBs). Currently, there is no report on the nonmetal element doped MnO₂ cathode with concomitant oxygen vacancies, because of its low thermal stability with easy phase transformation from MnO₂ to Mn₃ O₄ (≥300 °C). Herein, for the first time, novel N-doped MnO_{2- x} (N-MnO_{2- x} ) branch arrays with abundant oxygen vacancies fabricated by a facile low-temperature (200 °C) NH₃ treatment technology are reported. Meanwhile, to further enhance the high-rate capability, highly conductive TiC/C nanorods are used as the core support for a N-MnO_{2- x} branch, forming high-quality N-MnO_{2- x} @TiC/C core/branch arrays. The introduced N dopants and oxygen vacancies in MnO₂ are demonstrated by synchrotron radiation technology. By virtue of an integrated conductive framework, enhanced electron density, and increased surface capacitive contribution, the designed N-MnO_{2- x} @TiC/C arrays are endowed with faster reaction kinetics, higher capacity (285 mAh g^-1 at 0.2 A g^-1 ) and better long-term cycles (85.7% retention after 1000 cycles at 1 A g^-1 ) than other MnO₂ -based counterparts (55.6%). The low-temperature defect engineering sheds light on construction of advanced cathodes for aqueous RAZIBs.

High-Power and Ultralong-Life Aqueous Zinc-Ion Hybrid Capacitors Based on Pseudocapacitive Charge Storage

Rechargeable aqueous zinc-ion hybrid capacitors and zinc-ion batteries are promising safe energy storage systems. In this study, amorphous RuO2·H2O for the first time was employed to achieve fast and ultralong-life Zn2+ storage based on a pseudocapacitive storage mechanism. In the RuO2·H2O||Zn zinc-ion hybrid capacitors with Zn(CF3SO3)2 aqueous electrolyte, the RuO2·H2O cathode can reversibly store Zn2+ in a voltage window of 0.4-1.6 V (vs. Zn/Zn2+), delivering a high discharge capacity of 122 mAh g-1. In particular, the zinc-ion hybrid capacitors can be rapidly charged/discharged within 36 s with a very high power density of 16.74 kW kg-1 and a high energy density of 82 Wh kg-1. Besides, the zinc-ion hybrid capacitors demonstrate an ultralong cycle life (over 10,000 charge/discharge cycles). The kinetic analysis elucidates that the ultrafast Zn2+ storage in the RuO2·H2O cathode originates from redox pseudocapacitive reactions. This work could greatly facilitate the development of high-power and safe electrochemical energy storage.

Preparation and electrochemical performance of VO2(A) hollow spheres as a cathode for aqueous zinc ion batteries

Rechargeable aqueous zinc ion batteries (ZIBs), owing to their low-cost zinc metal, high safety and nontoxic aqueous electrolyte, have the potential to accelerate the development of large-scale energy storage applications. However, the desired development is significantly restricted by cathode materials, which are hampered by the intense charge repulsion of bivalent Zn2+. Herein, the as-prepared VO2(A) hollow spheres via a feasible hydrothermal reaction exhibit superior zinc ion storage performance, large reversible capacity of 357 mA h g-1 at 0.1 A g-1, high rate capability of 165 mA h g-1 at 10 A g-1 and good cycling stability with a capacity retention of 76% over 500 cycles at 5 A g-1. Our study not only provides the possibility of the practical application of ZIBs, but also brings a new prospect of designing high-performance cathode materials.

Annealing induced a well-ordered single crystal δ-MnO2 and its electrochemical performance in zinc-ion battery

Herein, the formation and electrochemical performance of a novel binder-free turbostratic stacked/ well-ordered stacked δ-MnO₂-carbon fiber composite cathodes in deep eutectic solvent (DES) based zinc-ion battery (ZIB) is reported. Results of morphological, elemental, and structural analyses revealed directly grown and interconnected δ-MnO₂ crumpled nanosheets on a carbon fiber substrate. Moreover, an improvement via a simple annealing strategy in the stacking, surface area and conductivity of the δ-MnO₂ sheets was observed. Annealing induces the rearrangement of δ-MnO₂ sheets resulting in the transformation from turbostratic stacking to a well-ordered stacking of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\delta }$$\end{document}δ-MnO₂ sheets, as indicated by the selected area electron diffraction (SAED) hexagonal single crystal pattern. Besides, the formation of the well-ordered stacking of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\delta }$$\end{document}δ-MnO₂ sheets exhibited improved electrochemical performance and cyclability, as cathode material for ZIB. The novel strategy described in this study is an essential step for the development of binder-free δ-MnO₂-C fiber composite with a well-ordered stacking of δ-MnO₂ sheets. This study also demonstrated comparable electrochemical performance between the turbostratic \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\delta }$$\end{document}δ-MnO₂ sheets and the well-ordered stacked δ-MnO₂ sheets.

Recent Progress on Zinc-Ion Rechargeable Batteries

The increasing demands for environmentally friendly grid-scale electric energy storage devices with high energy density and low cost have stimulated the rapid development of various energy storage systems, due to the environmental pollution and energy crisis caused by traditional energy storage technologies. As one of the new and most promising alternative energy storage technologies, zinc-ion rechargeable batteries have recently received much attention owing to their high abundance of zinc in natural resources, intrinsic safety, and cost effectiveness, when compared with the popular, but unsafe and expensive lithium-ion batteries. In particular, the use of mild aqueous electrolytes in zinc-ion batteries (ZIBs) demonstrates high potential for portable electronic applications and large-scale energy storage systems. Moreover, the development of superior electrolyte operating at either high temperature or subzero condition is crucial for practical applications of ZIBs in harsh environments, such as aerospace, airplanes, or submarines. However, there are still many existing challenges that need to be resolved. This paper presents a timely review on recent progresses and challenges in various cathode materials and electrolytes (aqueous, organic, and solid-state electrolytes) in ZIBs. Design and synthesis of zinc-based anode materials and separators are also briefly discussed.

Ultralong cycle stability of aqueous zinc-ion batteries with zinc vanadium oxide cathodes

Rechargeable aqueous zinc-ion batteries are promising candidates for large-scale energy storage but are plagued by the lack of cathode materials with both excellent rate capability and adequate cycle life span. We overcome this barrier by designing a novel hierarchically porous structure of Zn-vanadium oxide material. This Zn_0.3V₂O₅·1.5H₂O cathode delivers a high specific capacity of 426 mA·h g^-1 at 0.2 A g^-1 and exhibits an unprecedented superlong-term cyclic stability with a capacity retention of 96% over 20,000 cycles at 10 A g^-1. Its electrochemical mechanism is elucidated. The lattice contraction induced by zinc intercalation and the expansion caused by hydronium intercalation cancel each other and allow the lattice to remain constant during charge/discharge, favoring cyclic stability. The hierarchically porous structure provides abundant contact with electrolyte, shortens ion diffusion path, and provides cushion for relieving strain generated during electrochemical processes, facilitating both fast kinetics and long-term stability.