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

Reversible Deposition/Dissolution of Double Hydroxides to Modulate Electrolyte pH Enabling High-Performance Aqueous Zinc-Ion Batteries

Vanadium oxide has been extensively studied as a host of zinc ion intercalation but still suffers from low conductivity, dissolution, and byproduct accumulation during cycling. Here, we hydrothermally synthesize the VO2@MXene Ti3C2 (MV) composite and find that in the MV//3 M Zn(CF3SO3)2//Zn system, the double hydroxide Zn12(CF3SO3)9(OH)15·nH2O (ZCOH) uniformly covers VO2 during the charging process and dissolves reversibly during the discharge process. In situ X-ray diffraction of the MV combined with in situ pH measurements reveals that ZCOH acts as a pH buffer during cycling, which is beneficial to the cycling stability of batteries. And the theoretical calculation indicates that the decomposition energy required by ZCOH on the MV surface is lower than that on pure VO2, which is more conducive to ZCOH dissolution. The coin battery exhibits high-rate performance of 65.1% capacity retention at a current density of 15 A g-1 (compared to 0.6 A g-1) and a long cycling life of 20,000 cycles with a capacity retention of 80.7%. For a 22.4 mA h soft-packaged battery, its capacity remains at 72.1% after 2000 cycles. This work demonstrates the active role of ZCOH in the electrochemical process of VO2 and provides a new perspective for exploiting this mechanism to develop high-performance aqueous zinc-ion battery vanadium oxide cathode materials.

Interfacial Chemistry in Aqueous Lithium-Ion Batteries: A Case Study of V2O5 in Dilute Aqueous Electrolytes

Aqueous lithium-ion batteries (ALIBs) are promising for large-scale energy storage systems because of the cost-effective, intrinsically safe, and environmentally friendly properties of aqueous electrolytes. Practical application is however impeded by interfacial side-reactions and the narrow electrochemical stability window (ESW) of aqueous electrolytes. Even though higher electrolyte salt concentrations (e.g., water-in-salt electrolyte) enhance performance by widening the ESW, the nature and extent of side-reaction processes are debated and more fundamental understanding thereof is needed. Herein, the interfacial chemistry of one of the most popular electrode materials, V2O5, for aqueous batteries is systematically explored by a unique set of operando analytical techniques. By monitoring electrode/electrolyte interphase deposition, electrolyte pH, and gas evolution, the highly dynamic formation/dissolution of V2O5/V2O4, Li2CO3 and LiF during dis-/charge is demonstrated and shown to be coupled with electrolyte decomposition and conductive carbon oxidation, regardless of electrolyte salt concentration. The study provides deeper understanding of interfacial chemistry of active materials under variable proton activity in aqueous electrolytes, hence guiding the design of more effective electrode/electrolyte interfaces for ALIBs and beyond.

Unveiling the Mysteries: Acetonitrile's Dance with Weakly-Solvating Electrolytes in Shaping Gas Evolution and Electrochemical Performance of Zinc-ion Batteries

Aqueous Zn-metal battery (AZMB) is a promising candidate for future large-scale energy storage with commendable capacity, exceptional safety characteristics, and low cost. Acetonitrile (AN) has been widely used as an effective electrolyte constituent to improve AZMBs' performance. However, its functioning mechanisms remain unclear. In this study, we unveiled the critical roles of AN in AZMBs via comparative in situ electrochemical, gaseous, and morphological analyses. Despite its limited ability to solvate Zn ions, AN-modulated Zn-ion solvation sheath with increased anions and decreased water achieves a weakly-solvating electrolyte. As a result, the Zn||Zn cell with AN addition exhibited 63 times longer cycle life than cell without AN and achieved a 4 Ah cm-2 accumulated capacity with no H2 generation. In V2O5||Zn cells, for the first time, AN suppressing CO2 generation, elevating CO2-initiation voltage from 2→2.44 V (H2: 2.43→2.55 V) was discovered. AN-impeded transit and Zn-side deposition of dissolved vanadium ions, known as "crosstalk," ameliorated inhomogeneous Zn deposition and dendritic Zn growth. At last, we demonstrated an AN-enabled high-areal-capacity AZMB (3.3 mAh cm-2) using high-mass-loading V2O5 cathode (26 mg cm-2). This study shed light on the strategy of constructing fast-desolvation electrolytes and offered insights for future electrolyte accommodation for high-voltage AZMB cathodes.

Understanding the Super-Theoretical Capacity Behavior of VO2 in Aqueous Zn Batteries

VO2 material, as a promising intercalation host, is widely investigated not only in aqueous lithium-ion batteries, but also in aqueous zinc-ion batteries (AZIBs) owing to its stable tunnel-like framework and multivalence of vanadium. Different from lithium-ion storage, VO2 can provide higher capacity when storing zinc ions, even exceeding its theoretical capacity (323 mAh g-1), but the specific reason for this unconventional performance in AZIBs is still unclear. The present study proposes a catalytic oxygen evolution reaction (OER) coupled with an interface oxidation mechanism of VO2 during the initial charging to a high voltage. This coupling induces a phase transformation of VO2 into a high oxidation state of V5O12∙6H2O, enabling a nearly two-electron reaction and providing additional zinc storage sites to achieve super-theoretical capacity. Furthermore, it is demonstrated that these vanadium oxide cathodes (V2O3, VO2, and V2O5) will all undergo phase change after the first charge or short cycle. Notably, water molecules participate in the final formation of layered vanadium-based hydrate, highlighting their crucial role as "pillars" for stabilizing the structure. This work significantly enhances the understanding of vanadium-based oxide cathodes.

Averting H+-Mediated Charge Storage Chemistry Stabilizes the High Output Voltage of LiMn2O4-Based Aqueous Battery

H+ co-intercalation chemistry of the cathode is perceived to have damaging consequences on the low-rate and long-term cycling of aqueous zinc batteries, which is a critical hindrance to their promise for stationary storage applications. Herein, the thermodynamically competitive H+ storage chemistry of an attractive high-voltage cathode LiMn2O4 is revealed by employing operando and ex-situ analytical techniques together with density functional theory-based calculations. The H+ electrochemistry leads to the previously unforeseen voltage decay with cycling, impacting the available energy density, particularly at lower currents. Based on an in-depth investigation of the effect of the Li+ to Zn2+ ratio in the electrolyte on the charge storage mechanism, a purely aqueous and low-salt concentration electrolyte with a tuned Li+/Zn2+ ratio is introduced to subdue the H+-mediated charge storage kinetically, resulting in a stable voltage output and improved cycling stability at both low and high cathode loadings. Synchrotron X-ray diffraction analysis reveals that repeated H+ intercalation triggers an irreversible phase transformation leading to voltage decay, which is averted by shutting down H+ storage. These findings unveiling the origin and impact of the deleterious H+-storage, coupled with the practical strategy for its inhibition, will inspire further work toward this under-explored realm of aqueous battery chemistry.

Cobalt ion doping and morphology tailoring enable superior zinc-ion storage in sodium vanadate nanoflowers

Layered sodium vanadium materials have aroused increasing interest owing to their open layered structures and high theoretical capacity. Nevertheless, the strong electrostatic interactions between vanadium oxide layers and intercalated Zn2+ and the weak electronic conductivity severely limit their further development. Here, we design a series of cobalt ion-doped sodium vanadium electrode materials with nanoflower-like morphologies. Due to the open interlayer space and improved electron transfer enabled by cobalt ion preintercalation and sufficient contact area between the electrode and electrolyte provided by the three-dimensional (3D) flower-like morphology, the cobalt ion-doped sodium vanadate (CNVO-2) cathode exhibits excellent electrochemical performance, including an exceptional specific capacity (411 mA h g-1 at 0.5 A g-1) and ultrahigh structural stability (90.4 % capacity retention after 3000 cycles at 10 A g-1), outperforming many advanced ZIBs cathode materials. In addition, through various ex situ characterization techniques, an ionic exchange and multiple ion cointercalation mechanism is first revealed in sodium vanadate cathode material.

Initiating a High-Rate and Stable Aqueous Air Battery by Using Organic N-Heterocycle Anode

Alkaline metal-air batteries are advantageous in high voltage, low cost, and high safety. However, metal anodes are heavily eroded in strong alkaline electrolytes, causing serious side reactions including dendrite growth, passivation, and hydrogen evolution. To address this limitation, we successfully synthesized an organic N-heterocycle compound (NHCC) to serve as an alternative anode. This compound not only exhibits remarkable stability but also possesses a low redox potential (-1.04 V vs. Hg/HgO) in alkaline environments. To effectively complement the low redox potential of the NHCC anode, we designed a dual-salt highly concentrated electrolyte (4.0 M KOH+10.0 M KCF3 SO3 ). This electrolyte expands the electrochemical stability window to 2.3 V through the robust interaction between the O atom in H2 O molecule with the K+ of KCF3 SO3 (H-O⋅⋅⋅KCF3 SO3 ). We further demonstrated the K+ uptaken/extraction storage mechanism of NHCC anodes. Consequently, the alkaline aqueous NHCC anode-air batteries delivers a high battery voltage of 1.6 V, high-rate performance (101.9 mAh g-1 at 100 A g-1 ) and long cycle ability (30,000 cycles). Our work offers a molecular engineering strategy for superior organic anode materials and develops a novel double superconcentrated conductive salt electrolyte for the construction of high-rate, long-cycle alkaline aqueous organic anode-air batteries.

Trifunctional Rb+-Intercalation Enhancing the Electrochemical Cyclability of Ammonium Vanadate Cathode for Aqueous Zinc Ion Batteries

Rechargeable aqueous zinc-ion batteries (AZIBs) have been highly desired due to their low cost, intrinsic safety, environmental friendliness, and great potential in large-scale power storage systems. However, their practical applications are impeded by unstable long-term electrochemical performances induced by microstructure degradation of the cathode material, hydrogen evolution reaction in the electrolyte, and dendritic growth on the zinc anode upon cycling. In this work, rubidium cations (Rb+) are introduced to synthesize an Rb+-preintercalated NH4V4O10 (NVO-Rb) composite. The contribution of Rb+ ions as pillars in V-O interlayers to facilitating Zn2+ storage is investigated first, and then the influences of partial Rb+ ions from the NVO-Rb cathode on the aqueous electrolyte and zinc anode are specially inspected from different viewpoints. Based on a series of characterization results, it is comprehensively elucidated that the partial Rb+ ions into the electrolyte suppress the generation of byproducts on the cathode and regulate the dendrite growth on the zinc anode, thus effectively promoting the long-term electrochemical performances of NVO-based AZIBs. The assembled Zn∥Zn(CF3SO3)2∥NVO-Rb cell can exhibit a high specific capacity and optimized Zn2+ diffusion kinetics, especially an improved electrochemical cyclability with a capacity retention of 87.6% at 5 A g-1 over 10000 cycles. This study enlightens the multiple roles of cation-preintercalation in the layered structure material and provides a feasible strategy for the development of high-performance aqueous batteries.

Analogous Confinement Effect Enables High Stability and High Capacity Ammonium Storage in Polyaniline@Poly(o-fluoroaniline)@Carbon Layer

Rechargeable aqueous ammonium ion (NH4 + ) batteries have attracted much attention due to the unique properties of NH4 + . Polyaniline (PA) with outstanding conductivity is a potential cathode material, but it can be oxidized to pernigraniline (PG) rapidly, resulting in its poor stability. In this study, polyaniline@poly(o-fluoroaniline)@carbon layer (PA@POFA@C) is prepared for excellent and durable NH4 + storage. PA@POFA@C exhibits a high capacity of 208 mAh g-1 at 0.2 A g-1 and maintains 126 mAh g-1 at 10 A g-1 . More importantly, an excellent capacity retention rate of 88.24% is achieved after 2000 cycles with ≈100% coulombic efficiency. Spectroscopy studies suggest analogous confinement effect can effectively limit the escape of hydrogen in imine group, and form the hydrogen-restricted region between the PA and POFA layer which can provide H+ for the complete reduction of PG. Meanwhile, the hydrophobic effect of POFA effectively restrains the hydrolysis of PG. Interestingly, the introduction of C layer improves the hydrophilicity of electrode and shortens the activation process, serving as the outermost protective layer of the electrode. Finally, PA@POFA@C achieves desirable electrochemical performances with analogous confinement effect. This research provides ideas for the preparation of advanced polymer electrodes for aqueous NH4 + batteries.

In Situ Molecular Engineering Strategy to Construct Hierarchical MoS2 Double-Layer Nanotubes for Ultralong Lifespan "Rocking-Chair" Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc ion batteries (AZIBs) have gained considerable attention owing to their low cost and high safety, but dendrite growth, low plating/stripping efficiency, surface passivation, and self-erosion of the Zn metal anode are hindering their application. Herein, a one-step in situ molecular engineering strategy for the simultaneous construction of hierarchical MoS2 double-layer nanotubes (MoS2-DLTs) with expanded layer-spacing, oxygen doping, structural defects, and an abundant 1T-phase is proposed, which are designed as an intercalation-type anode for "rocking-chair" AZIBs, avoiding the Zn anode issues and therefore displaying a long cycling life. Benefiting from the structural optimization and molecular engineering, the Zn2+ diffusion efficiency and interface reaction kinetics of MoS2-DLTs are enhanced. When coupled with a homemade ZnMn2O4 cathode, the assembled MoS2-DLTs//ZnMn2O4 full battery exhibited impressive cycling stability with a capacity retention of 86.6% over 10 000 cycles under 1 A g-1anode, outperforming most of the reported "rocking-chair" AZIBs. The Zn2+/H+ cointercalation mechanism of MoS2-DLTs is investigated by synchrotron in situ powder X-ray diffraction and multiple ex situ characterizations. This research demonstrates the feasibility of MoS2 for Zn-storage anodes that can be used to construct reliable aqueous full batteries.

Advances of Zn Metal-Free "Rocking-Chair"-Type Zinc Ion Batteries: Recent Developments and Future Perspectives

Aqueous zinc ion battery (AZIBs) has attracted the attention of many researchers because of its safety, economy, environmental protection, and high ionic conductivity of electrolytes. However, the battery greatly suffers from zinc dendrite produced by zinc metal anode leading to poor cycle life and even unsafe problems, which limit its further development for various important applications. It is known that the success of the commercialization of lithium-ion batteries (LIBs) is mainly due to replacement of lithium metal anode with graphite, which avoids the formation of Li dendrite. Therefore, it is an important step to develop aqueous zinc ion anode to replace conventional zinc metal one with zinc-metal free anode material. In this review, the working principle and development prospect of "rocking-chair" AZIBs are introduced. The research progress of different types of zinc metal-free anode materials and cathode materials in "rocking-chair" AZIBs is reviewed. Finally, the limitations and challenges of the Zn metal-free "rocking-chair" AZIBs as well as solutions are deeply discussed, aiming to provide new strategies for the development of advanced zinc-ion batteries.

Unraveling the Mechanism of Cooperative Redox Chemistry in High-Efficient Zn2+ Storage of Vanadium Oxide Cathode

The inferior capacity and cyclic durability of V2 O5 caused by inadequate active sites and sluggish kinetics are the main problems to encumber the widespread industrial applications of vanadium-zinc batteries (VZBs). Herein, a cooperative redox chemistry (CRC) as "electron carrier" is proposed to facilitate the electron-transfer by capturing/providing electrons for the redox of V2 O5 . The increased oxygen vacancies in V2 O5 provoked in situ by CRC offers numerous Zn2+ storage sites and ion-diffusion paths and reduces the electrostatic interactions between vanadium-based cathode and intercalated Zn2+ , which enhance Zn2+ storage capability and structural stability. The feasibility of this strategy is fully verified by some CRCs. Noticeably, VZB with [Fe(CN)6 ]3- /[Fe(CN)6 ]4- as CRC displays conspicuous specific capacity (433.3 mAh g-1 ), ≈100% coulombic efficiency and superb cyclability (≈3500 cycles without capacity attenuation). Also, the mechanism and selection criteria of CRC are specifically unraveled in this work, which provides insightful perspectives for the development of high-efficiency energy-storage devices.

Atomic Pinning of Trace Additives Induces Interfacial Solvation for Highly Reversible Zn Metal Anodes

Aqueous Zn metal batteries are considered promising energy storage devices due to their high energy density and low cost. Unfortunately, such great potential is at present obscured by two clouds called dendrite growth and parasitic reactions. Herein, trace amounts of sodium cyclamate (CYC-Na) are introduced as an electrolyte additive, and accordingly, an atomic-pinning-induced interfacial solvation mechanism is proposed to summarize the effect of trace addition. Specifically, coadsorption of -NH- and -SO3 groups overcomes the ring-flipping effect and pins the CYC anion near the Zn anode surface in parallel, which significantly modifies the Zn2+ solvation sheath at the interface. This process homogenizes the surface Zn2+ flux and reduces the H2O and SO42- content on the surface, thus eliminating byproducts and leveling Zn deposition. Cells with trace CYC-Na cycle stably for 3650 h and still cycle for 330 h at high depths of discharge of 56.9%. This work dispels the clouds for efficient trace additives for AZMBs.

An Electrochemical Perspective of Aqueous Zinc Metal Anode

Based on the attributes of nonflammability, environmental benignity, and cost-effectiveness of aqueous electrolytes, as well as the favorable compatibility of zinc metal with them, aqueous zinc ions batteries (AZIBs) become the leading energy storage candidate to meet the requirements of safety and low cost. Yet, aqueous electrolytes, acting as a double-edged sword, also play a negative role by directly or indirectly causing various parasitic reactions at the zinc anode side. These reactions include hydrogen evolution reaction, passivation, and dendrites, resulting in poor Coulombic efficiency and short lifespan of AZIBs. A comprehensive review of aqueous electrolytes chemistry, zinc chemistry, mechanism and chemistry of parasitic reactions, and their relationship is lacking. Moreover, the understanding of strategies for suppressing parasitic reactions from an electrochemical perspective is not profound enough. In this review, firstly, the chemistry of electrolytes, zinc anodes, and parasitic reactions and their relationship in AZIBs are deeply disclosed. Subsequently, the strategies for suppressing parasitic reactions from the perspective of enhancing the inherent thermodynamic stability of electrolytes and anodes, and lowering the dynamics of parasitic reactions at Zn/electrolyte interfaces are reviewed. Lastly, the perspectives on the future development direction of aqueous electrolytes, zinc anodes, and Zn/electrolyte interfaces are presented.

Oxygen vacancy enriched Na1.19V8O20·4.42H2O nanosheets for fast and stable Zn-ion batteries

Here, a facile hydrothermal method is employed to prepare an oxygen vacancy enriched sodium-ion intercalation Na1.19V8O20·4.42H2O nanosheet cathode with large interlayer spacing, fast reaction kinetics, and stable structure for superior zinc-ion batteries (ZIBs). The assembled ZIB exhibits a high specific capacity and excellent structural stability without capacity decay over 2000 cycles. Moreover, the multiple ion co-intercalation mechanism and partial phase transition mechanism are elucidated based on ex situ characterization techniques.

Enabling selective zinc-ion intercalation by a eutectic electrolyte for practical anodeless zinc batteries

Two major challenges hinder the advance of aqueous zinc metal batteries for sustainable stationary storage: (1) achieving predominant Zn-ion (de)intercalation at the oxide cathode by suppressing adventitious proton co-intercalation and dissolution, and (2) simultaneously overcoming Zn dendrite growth at the anode that triggers parasitic electrolyte reactions. Here, we reveal the competition between Zn²⁺ vs proton intercalation chemistry of a typical oxide cathode using ex-situ/operando techniques, and alleviate side reactions by developing a cost-effective and non-flammable hybrid eutectic electrolyte. A fully hydrated Zn²⁺ solvation structure facilitates fast charge transfer at the solid/electrolyte interface, enabling dendrite-free Zn plating/stripping with a remarkably high average coulombic efficiency of 99.8% at commercially relevant areal capacities of 4 mAh cm^-2 and function up to 1600 h at 8 mAh cm^-2. By concurrently stabilizing Zn redox at both electrodes, we achieve a new benchmark in Zn-ion battery performance of 4 mAh cm^-2 anode-free cells that retain 85% capacity over 100 cycles at 25 °C. Using this eutectic-design electrolyte, Zn | |Iodine full cells are further realized with 86% capacity retention over 2500 cycles. The approach represents a new avenue for long-duration energy storage.

Construction of TiO2/TiOF2 heterojunction as a cathode material for high-performance Mg2+/Li+ hybrid-ion batteries

Anatase TiO₂ has attracted significant interest as a cathode material for Mg-ion batteries or Mg²⁺/Li⁺ hybrid-ion batteries. However, owing to the semiconductor property and slower Mg²⁺ diffusion kinetics it still suffers from poor electrochemical performance. Herein, a TiO₂/TiOF₂ heterojunction consisting of in situ formed TiO₂ sheets and TiOF₂ rods, was prepared by adjusting the amount of HF in the hydrothermal process, and used as cathode of Mg²⁺/Li⁺ hybrid-ion battery. The TiO₂/TiOF₂ heterojunction prepared by adding 2 mL HF (TiO₂/TiOF₂-2) exhibits high electrochemical performance, with a high initial discharge capacity (378 mAh/g at 50 mA/g), an outstanding rate performance (128.8 mAh/g at 2000 mA/g), and good cycle stability (capacity retention of 54 % after 500 cycles), which is much superior to that of Pure TiO₂ and Pure TiOF₂. The reactions of Li⁺ intercalation/detercalation in the TiO₂/TiOF₂ heterojunction are revealed by investigating the evolution of the hybrids during different electrochemical states. Moreover, theoretical calculations prove that the Li⁺ formation energy in the TiO₂/TiOF₂ heterostructure is much lower than that of TiO₂ and TiOF₂, demonstrating that the heterostructure plays a crucial role in the enhanced electrochemical performance. This work provides a novel method to design cathode materials with high performance by constructing heterostructure.

Manipulating Deposition Behavior by Polymer Hydrogel Electrolyte Enables Dendrite-Free Zinc Anode for Zinc-Ion Hybrid Capacitors

Rechargeable aqueous zinc-ion hybrid capacitors (ZHCs) have aroused unprecedented attention because of their high safety, cost effectiveness, and environmental compatibility. However, the intractable issues of dendrite growth and side reactions at the electrode-electrolyte interface deteriorate durability and reversibility of Zn anodes, deeply encumbering the large-scale application of ZHCs. Concerning these obstacles, a negatively charged carboxylated chitosan-intensified hydrogel electrolyte (CGPPHE) with cross-linked networks is reported to stabilize Zn anodes. Beyond possessing good mechanical characteristics, the CGPPHE with polar groups can reduce the desolvation energy barrier of hydrated Zn²⁺ , constrain the 2D Zn²⁺ diffusion, and uniformize electric field and Zn²⁺ flux distributions, assuring dendrite-free Zn deposition with high plating-stripping efficiency. Concurrently, the hydrophilic CGPPHE alleviates harmful hydrogen evolution and corrosion by abating water activity. Accordingly, Zn|CGPPHE|Zn and Zn|CGPPHE|Cu cells exhibit an extended life exceeding 350 h (1600 mAh cm^-2 cumulative capacity under 20 mA cm^-2 ) and a high average coulombic efficiency of 98.2%, respectively. The resultant flexible ZHCs with CGPPHE and template-regulated carbon cathode present perfect properties in capacity retention (97.7% over 10 000 cycles), energy density (91.8 Wh kg^-1 ), and good mechanical adaptability. This study provides insight into developing novel hydrogel electrolytes toward highly rechargeable and stable ZHCs.

Manipulating OH- -Mediated Anode-Cathode Cross-Communication Toward Long-Life Aqueous Zinc-Vanadium Batteries

The anode-cathode interplay is an important but rarely considered factor that initiates the degradation of aqueous zinc ion batteries (AZIBs). Herein, to address the limited cyclability issue of V-based AZIBs, Al₂ (SO₄ )₃ is proposed as decent electrolyte additive to manipulate OH^- -mediated cross-communication between Zn anode and NaV₃ O₈  ⋅ 1.5H₂ O (NVO) cathode. The hydrolysis of Al³⁺ creates a pH≈0.9 strong acidic environment, which unexpectedly prolongs the anode lifespan from 200 to 1000 h. Such impressive improvement is assigned to the alleviation of interfacial OH^- accumulation by Al³⁺ adsorption and solid electrolyte interphase formation. Accordingly, the strongly acidified electrolyte, associated with the sedated crossover of anodic OH^- toward NVO, remarkably mitigate its undesired dissolution and phase transition. The interrupted OH^- -mediated communication between the two electrodes endows Zn||NVO batteries with superb cycling stability, at both low and high scan rates.

Electrolyte Engineering Enables High Performance Zinc-Ion Batteries

Zinc-ion batteries (ZIBs) feature high safety, low cost, environmental-friendliness, and promising electrochemical performance, and are therefore regarded as a potential technology to be applied in large-scale energy storage devices. However, ZIBs still face some critical challenges and bottlenecks. The electrolyte is an essential component of batteries and its properties affect the mass transport, energy storage mechanisms, reaction kinetics, and side reactions of ZIBs. The adjustment of electrolyte formulas usually has direct and obvious impacts on the overall output and performance. In this review, advanced electrolyte strategies are overviewed for optimizing the compatibility between cathode materials and electrolytes, inhibiting anode corrosion and dendrite growth, extending electrochemical stability windows, enabling wearable applications, and enhancing temperature tolerance. The underlying scientific mechanisms, electrolyte design principles, and recent progress are presented to provide a better understanding and inspiration to readers. In addition, a comprehensive perspective about electrolyte design and engineering for ZIBs is included.

Operando Monitoring of Local pH Value Changes at the Carbon Electrode Surface in Neutral Sulfate-Based Aqueous Electrochemical Capacitors

The operando monitoring of pH during the charging and discharging of an electrochemical capacitor in an aqueous neutral salt solution is presented. Proper knowledge of transient and limiting pH values allows for a better understanding of the phenomena that take place during capacitor operation. It also enables the proper assignment of the reaction potentials responsible for water decomposition. It is shown that the pH inside the capacitor is strongly potential-dependent and different for individual electrodes; therefore, the values of the evolution potentials of hydrogen and oxygen cannot be precisely calculated based only on the initial pH of the electrolyte. The operando measurements indicate that the pH at the positive electrode reaches 4, while at the negative electrode, it is 8.5, which in theory could shift the theoretical operating voltage well beyond 1.23 V. On the other hand, high voltage cannot be easily maintained since the electrolyte of both electrode vicinities is subjected to mixing. Operando gas monitoring measurements show that the evolution of electrolysis byproducts occurs even below the theoretical decomposition voltage. These reactions are important in maintaining a voltage-advantaged pH difference within the cell. At the same time, the electrochemical quartz crystal microbalance (EQCM) measurements indicated that the ions governing the pH (OH^-) that initially accumulated in the vicinity of the positive electrode enter the carbon porosity, losing their pH-governing abilities. pH fluctuations in the cell are important and play a vital role in the description of its performance during the cyclability at a given voltage. This is especially noticeable in cell floating at 1.3 V, where the pH difference between electrodes is the highest (6 units). The increase of the electrode separation distance acts similarly to the introduction of a semipermeable membrane toward the increase of the capacitor cycle life. During floating at 1.6 V, where the pH difference is not as high anymore (4 units), the influence of separation in terms of electrode stability, although present, is less notable.

A Multifunctional Anti-Proton Electrolyte for High-Rate and Super-Stable Aqueous Zn-Vanadium Oxide Battery

Large volumetric expansion of cathode hosts and sluggish transport kinetics in the cathode-electrolyte interface, as well as dendrite growth and hydrogen evolution at Zn anode side are considered as the system problems that cause the electrochemical failure of aqueous Zn-vanadium oxide battery. In this work, a multifunctional anti-proton electrolyte was proposed to synchronously solve all those issues. Theoretical and experimental studies confirm that PEG 400 additive can regulate the Zn²⁺ solvation structure and inhibit the ionization of free water molecules of the electrolyte. Then, smaller lattice expansion of vanadium oxide hosts and less associated by-product formation can be realized by using such electrolyte. Besides, such electrolyte is also beneficial to guide the uniform Zn deposition and suppress the side reaction of hydrogen evolution. Owing to the integrated synergetic modification, a high-rate and ultrastable aqueous Zn-V₂O₃/C battery can be constructed, which can remain a specific capacity of 222.8 mAh g^-1 after 6000 cycles at 5 A g^-1, and 121.8 mAh g^-1 even after 18,000 cycles at 20 A g^-1, respectively. Such "all-in-one" solution based on the electrolyte design provides a new strategy for developing high-performance aqueous Zn-ion battery.

Sodium Pre-Intercalated Carbon/V2 O5 Constructed by Sustainable Sodium Lignosulfonate for Stable Cathodes in Zinc-Ion Batteries: A Comprehensive Study

The aqueous zinc-ion battery (AZIB) has been widely investigated in recent years because it has the advantages of being green, safe, and made from abundant raw materials. It is necessary to continue to study how to prepare cathode materials with excellent performance and high cycling stability for future commercialization. In this work, a strategy was proposed that uses sustainable sodium lignosulfonate as both carbon and sodium sources to obtain a sodium pre-intercalated vanadium oxide/carbon (VO/LSC) composite as the cathode of AZIB. The carbon matrix could improve the electronic conductivity of vanadium oxide, while the sodium lignosulfonate could provide sodium ions pre-intercalated into the layered vanadium oxide simultaneously. Through this strategy, vanadium-based cathode materials with high stability and excellent rate capability were obtained. The VO/LSC cathode delivered high capacities of 350 and 112.8 mAh g^-1 at 0.1 and 4.0 A g^-1 , respectively. Zinc sulfate and zinc trifluoromethyl sulfonate were selected as electrolytes, and the influence of electrolytes on the performance of VO/LSC was analyzed. The oxygen in the environment was used to oxidize the low-priced vanadium oxide to achieve a self-charging AZIB. This paper provides a valuable strategy for the design of vanadium-based cathode material for AZIB, which can broaden the research and application of AZIB.

Achieving high-rate and durable aqueous rechargeable Zn-Ion batteries by enhancing the successive electrochemical conversion reactions

The mild electrolyte working environment of rechargeable aqueous Zn-ion batteries (AZIBs) features its promising characteristic and potential application for large-scale energy storage system. However, the poor cycling stability significantly hinders the broad application of AZIBs due to the complex electrochemical conversion reactions during charge-discharge process. Herein, we propose a strategy to improve the electrochemical performance of AZIB by enhancing the successive electrochemical conversion reactions. With a rational design of electrode, an even homogeneous electric field can be achieved in the cathode side, resulting to significantly enhanced efficiency of successive electrochemical conversion reactions. Charge storage mechanism studies reveal that the reversibility behaviors of byproducts alkaline zinc sulfate (ZHS) can dramatically determine the H⁺/Zn²⁺ de/intercalation process, and a high reversibility characteristic ensures the facilitated electrochemical kinetics. As expected, the resultant AZIB possesses outstanding electrochemical performance with a high specific capacity of 425.08 mAh⋅g^-1 at 0.1 A⋅g^-1, an excellent rate capacity of about 60% (246.6 mAh⋅g^-1 at 1 A⋅g^-1) and superior cycling stability of 93.7% after 3000 cycles (at 3 A⋅g^-1). This effective strategy and thinking proposed here may open a new avenue for the development of high-performing AZIBs.

Structure evolution and energy storage mechanism of Zn3V3O8 spinel in aqueous zinc batteries

Spinel-type materials are promising for the cathodes in rechargeable aqueous zinc batteries. Herein, Zn₃V₃O₈ is synthesized via a simple solid-state reaction method. By tuning the Zn(CF₃SO₃)₂ concentration in electrolytes and the cell voltage ranges, improved electrochemical performance of Zn₃V₃O₈ can be achieved. The optimized test conditions give rise to progressive structure evolution from bulk to nano-crystalline spinel, which leads to capacity activation in the first few cycles and stable cycling performance afterward. Furthermore, the energy storage mechanism in this nano-crystalline spinel is interpreted as the co-intercalation of zinc ions and protons with some water. This work provides a new viewpoint of the structure evolution and correlated energy storage mechanism in spinel-type host materials, which would benefit the design and development of next-generation batteries.

Design of a Scalable Dendritic Copper@Ni2+, Zn2+ Cation-Substituted Cobalt Carbonate Hydroxide Electrode for Efficient Energy Storage

Design and fabrication of novel electrode materials with excellent specific capacitance and cycle stability are urgent for advanced energy storage devices, and the combinability of multiple modification methods is still insufficient. Herein, Ni²⁺, Zn²⁺ double-cation-substitution Co carbonate hydroxide (NiZnCo-CH) nanosheets arrays were established on 3D copper with controllable morphology (3DCu@NiZnCo-CH). The self-standing scalable dendritic copper offers a large surface area and promotes fast electron transport. The 3DCu@NiZnCo-CH electrode shows a markedly improved electrochemical performance with a high specific capacity of ∼1008 C g^-1 at 1 A g^-1 (3.2, 2.83, and 1.26 times larger than Co-CH, ZnCo-CH, and NiCo-CH, respectively) and outstanding rate capability (828.8 C g^-1 at 20 A g^-1) due to its compositional and structural advantages. Density functional theory (DFT) calculation results illustrate that cation doping adjusts the adsorption process and optimizes the charge transfer kinetics. Moreover, an aqueous hybrid supercapacitor based on 3DCu@NiZnCo-CH and rGO demonstrates a high energy density of 42.29 Wh kg^-1 at a power density of 376.37 W kg^-1, along with superior cycling performance (retained 86.7% of the initial specific capacitance after 10,000 cycles). Impressively, these optimized 3DCu@NiZnCo-CH//rGO devices with ionic liquid can be operated stably in a large potential range of 4 V with greatly enhanced energy density and power capability (110.12 Wh kg^-1 at a power density of 71.69 W kg^-1). These findings may shed some light on the rational design of transition-metal compounds with tunable architectures by multiple modification methods for efficient energy storage.

Aqueous MnV2 O6 -Zn Battery with High Operating Voltage and Energy Density

Aqueous Zn ion batteries (AZIBs), featuring low cost, long-term cycling stability, and superior safety are promising for applications in advanced energy storage devices. However, they still suffer from unsatisfactory energy density and operating voltage, which are closely related to cathode materials used. Herein, the use of monoclinic MnV₂ O₆ (MVO) is reported, which can be activated for high-capacity Zn ions storage by electrochemically oxidizing part of the Mn²⁺ to Mn³⁺ or Mn⁴⁺ while the remaining Mn²⁺ ions act as binders/pillars to hold the layer structure of MVO and maintain its integrity during charging/discharging process. Moreover, after introducing carbon nanotubes (CNT), the MVO:CNT composite not only provides robust 3D Zn-ion diffusion channels but also shows enhanced structural integrity. As a result, a MVO:CNT cathode delivers a high midpoint voltage (1.38 V after 3000 cycles at 2 A g^-1 ) and a high energy density of 597.9 W h kg^-1 . Moreover, DFT analyses clearly illustrate stepwise Zn ion insertion into the MnV₂ O₆ lattice_, and ex-situ analyses results further verify the highly structural reversibility of the MnV₂ O₆ cathode upon extended cycling, demonstrating the good potential of MnV₂ O₆ for the establishment of viable aqueous Zn ion battery systems.

An In Situ Artificial Cathode Electrolyte Interphase Strategy for Suppressing Cathode Dissolution in Aqueous Zinc Ion Batteries

Rechargeable aqueous zinc ion batteries have attracted increasing attention as a new energy storage system because of the high ionic conductivity and safe aqueous electrolyte. The spontaneous vanadium dissolution in aqueous electrolytes is one major problem because the water with serious polarity would corrode the crystal structure of vanadium-based cathodes. Here, an in situ artificial cathode electrolyte interphase (CEI) strategy is proposed to kinetically suppress the vanadium dissolution in aqueous zinc ion batteries. The strontium ion is introduced into vanadium oxide layers as a sacrifice guest, which would directly precipitate upon getting out from the vanadium-based cathode to in situ from a CEI coating layer on the surface. This strategy is proven with the help of various technologies, and the remarkable ability of the CEI layer to suppress cathode dissolution is evaluated by multiple electrochemical and chemical methods. As a result, the cathode after CEI conversion exhibits the best recharge capacity retention after open circuit voltage rest for 3 days in comparison with other cathodes. This work reports a general strategy to construct the electrode-electrolyte interface for suppressing vanadium-based cathodes dissolution in aqueous electrolytes and beyond.

Selenium Defect Boosted Electrochemical Performance of Binder-Free VSe2 Nanosheets for Aqueous Zinc-Ion Batteries

As a typical transition-metal dichalcogenides, vanadium diselenide (VSe₂) is a promising electrode material for aqueous zinc-ion batteries due to its metallic characteristics and excellent electronic conductivity. In this work, we propose a strategy of hydrothermal reduction synthesis of stainless-steel (SS)-supported VSe₂ nanosheets with defect (VSe_2-x-SS), thereby further improving the conductivity and activity of VSe_2-x-SS. Density functional theory calculations confirmed that Se defect can adjust the adsorption energy of Zn²⁺ ions. This means that the adsorption/desorption process of Zn²⁺ ions on VSe_2-x-SS is more reversible than that on pure SS-supported VSe₂ (VSe₂-SS). As a result, the Zn//VSe_2-x-SS battery showed more excellent electrochemical performance than Zn//VSe₂-SS. The VSe_2-x-SS electrode shows a good specific capacity of 265.2 mA h g^-1 (0.2 A g^-1 after 150 cycles), satisfactory rate performance, and impressive cyclic stability. In addition, we also have explored the energy-storage mechanism of Zn²⁺ ions in this VSe_2-x-SS electrode material. This study provides an effective strategy for the rational design of electrode materials for electrochemical energy-storage devices.

Act in contravention: a non-planar coupled electrode design utilizing "tip effect" for ultra-high areal capacity, long cycle life zinc-based batteries

Aqueous zinc-based batteries (ZBBs) have great potential as commercial energy storage devices. However, the poor cycling stability of zinc anode under high areal capacity limits their further application. Herein, a coupled non-planar electrode design achieved by the tailored flat-top pyramid carbon felt (TCF) is proposed for ZBBs, which can effectively increase the zinc deposition sites, adjust the deposition morphology, optimize the current and electrolyte flow velocity distribution and provide necessary space for zinc plating. Interestingly, by utilizing "tip effect", the coupled TCFs enable precise control of the zinc dendrite growth position, effectively reducing the risk of short circuit. Based on such coupled TCFs, zinc-iodine flow batteries can deliver an ultra-high areal capacity of 240 mAh cm^-2 and a superb cycling stability over 300 cycles (areal capacity of 160 mAh cm^-2) at a high current density of 40 mA cm^-2. Therefore, we provide an effective strategy for high areal capacity zinc anode design, which may promote the development of high energy density and long cycle life ZBBs.

Chemically assembling chromium vanadate into an urchin-like porous rich matrix with ultrathin nanosheets for rapid Zn2+ storage

Aqueous zinc ion battery (AZIB) is a promising battery system developed in recent years, which has the advantages of safety, environmental protection and low price. However, it is still a puzzle to develop and improve cathode materials with satisfactory performance. In this paper, the chromium vanadate (CrVO₃) electrode material was reported for the first time. The obtained CrVO₃ have mesoporous structure (the mesopore sizes: 2-50 nm), excellent conductivity, high surface area (129.3 m² g^-1) and uniform thickness of 2 nm, which provides a short path for rapid transfer of zinc ions, a large surface area for high pseudocapacitance, and sufficient voids to mitigate volume expansion. Given these structural advantages, the CrVO₃ cathode delivers high capacities of 188.8 and 112.8 mAh g^-1 at 0.5 and 4 A g^-1 and excellent long cycle stability, respectively. More importantly, the Zn//CrVO₃ battery provided an energy density of 231.9 Wh kg^-1 at a power density of 100.4 W kg^-1. Meanwhile, insight into the formation mechanism and Zn²⁺ storage mechanism by ex situ methods. The results show that the porous CrVO₃ is a promising cathode material for AZIBs, which provide a valuable idea for the design of porous vanadate with significantly enhanced performances in electrochemical energy storage.

Direct Proof of the Reversible Dissolution/Deposition of Mn2+/Mn4+ for Mild-Acid Zn-MnO2 Batteries with Porous Carbon Interlayers

Mild-acid Zn-MnO2 batteries have been considered a promising alternative to Li-ion batteries for large scale energy storage systems because of their high safety. There have been remarkable improvements in the electrochemical performance of Zn-MnO2 batteries, although the reaction mechanism of the MnO2 cathode is not fully understood and still remains controversial. Herein, the reversible dissolution/deposition (Mn2+/Mn4+) mechanism of the MnO2 cathode through a 2e- reaction is directly evidenced using solution-based analyses, including electron spin resonance spectroscopy and the designed electrochemical experiments. Solid MnO2 (Mn4+) is reduced into Mn2+ (aq) dissolved in the electrolyte during discharge. Mn2+ ions are then deposited on the cathode surface in the form of the mixture of the poorly crystalline Zn-containing MnO2 compounds through two-step reactions during charge. Moreover, the failure mechanism of mild-acid Zn-MnO2 batteries is elucidated in terms of the loss of electrochemically active Mn2+. In this regard, a porous carbon interlayer is introduced to entrap the dissolved Mn2+ ions. The carbon interlayer suppresses the loss of Mn2+ during cycling, resulting in the excellent electrochemical performance of pouch-type Zn-MnO2 cells, such as negligible capacity fading over 100 cycles. These findings provide fundamental insights into strategies to improve the electrochemical performance of aqueous Zn-MnO2 batteries.

High-Capacity Layered Magnesium Vanadate with Concentrated Gel Electrolyte toward High-Performance and Wide-Temperature Zinc-Ion Battery

Aqueous zinc-ion batteries (ZIBs) have emerged as the most promising alternative energy storage system, but the development of a suitable cathode and the issues of Zn anodes have remained challenging. Herein, an effective strategy of high-capacity layered Mg_0.1V₂O₅·H₂O (MgVO) nanobelts together with a concentrated 3 M Zn(CF₃SO₃)₂ polyacrylamide gel electrolyte was proposed to achieve a durable and practical ZIB system. By adopting the designed concentrated gel electrolyte which not only inherits the high-voltage window and wide operating temperature of the concentrated electrolyte but also addresses the Zn dendrite formation problem, the prepared cathode exhibits an ultrahigh capacity of 470 mAh g^-1 and a high rate capability of 345 mAh g^-1 at 5.0 A g^-1, and the assembled quasi-solid-state ZIBs achieve 95% capacity retention over 3000 cycles as well as a wide operating temperature from -30 to 80 °C, demonstrating a promising prospect for large-scale energy storage. In situ X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis (TGA) investigations also demonstrate a complex reaction mechanism for this cathode involving the (de)insertion of Zn²⁺, H⁺, and water molecules during cycling. The water molecules will reinsert into the interlayer and act as "pillars" to stabilize the host structure when Zn²⁺ is fully extracted.