Aqueous Rechargeable Zn-Iodine Batteries: Issues, Strategies and Perspectives.

The static aqueous rechargeable Zn-Iodine batteries (ARZiBs) have been studied extensively because of their low-cost, high-safety, moderate voltage output, and other unique merits. Nonetheless, the poor electrical conductivity and thermodynamic instability of the iodine cathode, the complicated conversion mechanism, and the severe interfacial reactions at the Zn anode side induce their low operability and unsatisfactory cycling stability. This review first clarifies the typical configuration of ARZiBs with a focus on the energy storage mechanism and uncovers the issues of the ARZiBs from a fundamental point of view. After that, it categorizes the recent optimization strategies into cathode fabrication, electrolyte modulation, and separator/anode modification; and summarizes and highlights the achieved progress of these strategies in advanced ARZiBs. Given that the ARZiBs are still at an early stage, the future research outlook is provided, which hopefully may guide the rational design of advanced ARZiBs.

Mechanisms of adsorption and diffusion of Na on a VSe2 monolayer with engineering-induced vacancies.

Although research on vacancy engineering of anode materials has sufficiently advanced to obtain heightened battery capacity, the effect on the diffusion barrier underlying the mechanism remains to be elucidated. Herein, we investigated the effect of vacancy engineering on Na adsorption and diffusion on a vanadium diselenide (VSe2) monolayer using first-principles calculations to reveal the underlying physics behind the performance optimization of anode materials in a sodium-ion battery. The results demonstrate that the structure of the substrate is responsible for the difference between the adsorption energy and diffusion barrier that resulted from cation and anion vacancies. As there is an absent Se atom (VSe) on the surface layer of the substrate, diffusion of Na on the surface could become pressurized with a high diffusion barrier up to 0.33 eV and a high adsorption energy (-1.92 eV) to capture additional Na atoms. However, because the V layer is sandwiched between two Se layers, there is less interaction with Na, and the adsorption energy and diffusion barrier are -1.58 and 0.13 eV, respectively, when a V atom is nonexistent (VV). Moreover, the defective VSe2 increased the battery capacity, with little impact on open-circuit voltage. In this work, we analyzed the effect of vacancy engineering on VSe2 monolayer material, which provides theoretical clues for the design of efficient sodium-ion batteries with heightened capacity.

In Situ Defect Induction in Close-Packed Lattice Plane for the Efficient Zinc Ion Storage.

In situ electrochemical activation brings unexpected electrochemical performance improvements to electrode materials, but the mechanism behind it still needs further study. Herein, an electrochemically in situ defect induction in close-packed lattice plane of vanadium nitride oxide (VNx Oy ) in aqueous zinc-ion battery is reported. It is verified by theoretical calculation and experiment that the original compact structure is not suitable for the insert of Zn2+ ion, while a highly active one after the initial electrochemical activization accompanied by the in situ defect induction in close-packed lattice plane of VNx Oy exhibits efficient zinc ion storage. As expected, activated VNx Oy can achieve very high reversible capacity of 231.4 mA h g-1 at 1 A g-1 and cycle stability upto 6000 cycles at 10 A g-1 with a capacity retention of 94.3%. This work proposes a new insight for understanding the electrochemically in situ transformation to obtain highly active cathode materials for the aqueous zinc-ion batteries.

Nb2O5 microstructures: a high-performance anode for lithium ion batteries.

We report the synthesis of three-dimensional (3D) urchin-like Nb2O5 microstructures by a facile hydrothermal approach with subsequent annealing treatment. As anode materials for lithium-ion batteries, the 3D urchin-like Nb2O5 microstructures exhibit superior electrochemical performance with excellent rate capability as well as long-term cycling stability. The electrode delivers high capacity of 131 mA h g-1 after 1000 cycles at a high current density of 1 A g-1. The excellent electrochemical performance suggests the 3D urchin-like Nb2O5 microstructures may be a promising anode candidate for high-power lithium ion batteries.

Template-free synthesis of highly porous V2O5 cuboids with enhanced performance for lithium ion batteries.

Highly porous hierarchical V2O5 cuboids have been synthesized by a template-free PVP-assisted polyxol method and the formation mechanism is studied. The cuboids are assembled from numerous mesoporous nanoplates and the preferred orientation of each single nanoplate exposes the 〈110〉 facets, facilitating lithium-ion diffusion by offering a prior channel. This material exhibits a high capacity of 143 mA h g(-1), high rate capacity of 10 C and long life cycling performance up to 1000 cycles. The excellent electrochemical performance of V2O5 cuboid electrodes is due to its unique porous cuboid morphology and optimized structural stability upon cycling. This research provides an effective route to the construction of complex porous architectures assembled from nanocrystals through a surfactant-assisted synthesis method.

Low-current-density stability of vanadium-based cathodes for aqueous zinc-ion batteries.

Vanadium-based cathodes have received widespread attention in the field of aqueous zinc-ion batteries, presenting a promising prospect for stationary energy storage applications. However, the rapid capacity decay at low current densities has hampered their development. In particular, capacity stability at low current densities is a requisite in numerous practical applications, typically encompassing peak load regulation of the electricity grid, household energy storage systems, and uninterrupted power supplies. Despite possessing notably high specific capacities, vanadium-based materials exhibit severe instability at low current densities. Moreover, the issue of stabilizing electrode reactions at these densities for vanadium-based materials has been explored insufficiently in existing research. This review aims to investigate the matter of stability in vanadium-based materials at low current densities by concentrating on the mechanisms of capacity fading and optimization strategies. It proposes a comprehensive approach that includes electrolyte optimization, electrode modulation, and electrochemical operational conditions. Finally, we presented several crucial prospects for advancing the practical development of vanadium-based aqueous zinc-ion batteries.

Conversion-type anode chemistry with interfacial compatibility toward Ah-level near-neutral high-voltage zinc ion batteries.

High-voltage aqueous zinc ion batteries (AZIBs) with a high-safety near-neutral electrolyte is of great significance for practical sustainable application; however, they suffer from anode and electrode/electrolyte interfacial incompatibility. Herein, a conversion-type anode chemistry with a low anodic potential, which is guided by the Gibbs free energy change of conversion reaction, was designed for high-voltage near-neutral AZIBs. A reversible conversion reaction between ZnC2O4·2H2O particles and three-dimensional Zn metal networks well-matched in CH3COOLi-based electrolyte was revealed. This mechanism can be universally validated in the battery systems with sodium or iodine ions. More importantly, a cathodic crowded micellar electrolyte with a water confinement effect was proposed in which lies the core for the stability and reversibility of the cathode under an operating platform voltage beyond 2.0 V, obtaining a capacity retention of 95% after 100 cycles. Remarkably, the scientific and technological challenges from the coin cell to Ah-scale battery, sluggish kinetics of the solid-solid electrode reaction, capacity excitation under high loading of active material, and preparation complexities associated with large-area quasi-solid electrolytes, were explored, successfully achieving an 88% capacity retention under high loading of more than 20 mg cm-2 and particularly a practical 1.1 Ah-level pouch cell. This work provides a path for designing low-cost, eco-friendly and high-voltage aqueous batteries.

Reversible Oxygen Redox Chemistry in High-Entropy P2-Type Manganese-Based Cathodes via Self-Regulating Mechanism.

The irreversible oxygen-redox reactions in the high-voltage region of sodium-layered cathode materials lead to poor capacity retention and structural instability during cycling, presenting a significant challenge in the development of high-energy-density sodium-ion batteries. This work introduces a high-entropy design for layered Na0.67Li0.1Co0.1Cu0.1Ni0.1Ti0.1Mn0.5O2 (Mn-HEO) cathode with a self-regulating mechanism to extend specific capacity and energy density. The oxygen redox reaction was activated during the initial charging process, accompanied by the self-regulation of active elements, enhancing the ionic bonds to form a vacancy wall near the TM vacancies and thus preventing the migration of transition metal elements. Systematic in situ/ex situ characterizations and theoretical calculations comprehensively support the understanding of the self-regulation mechanism of Mn-HEO. As a result, the Mn-HEO cathode exhibits a stable structure during cycling. It demonstrates almost zero strain within a wide voltage range of 2.0-4.5 V with a remarkable specific capacity (177 mAh g-1 at 0.05 C) and excellent long-term cycling stability (87.6% capacity retention after 200 cycles at 2 C). This work opens a new pathway for enhancing the stability of oxygen-redox chemistry and revealing a mechanism of crystal structure evolution for high-energy-density layered oxides.

Proton Self-Doped Polyaniline with High Electrochemical Activity for Aqueous Zinc-Ion Batteries.

Aqueous zinc-ion batteries are promising energy storage devices due to their low cost, good ionic conductivity, and high safety. Conductive polyaniline is a promising cathode because of its redox activity, but because the neutral electrolyte protonates only weakly, it displays limited electrochemical activity. A polyaniline cathode is developed with proton self-doping from manganese metal-organic frameworks (Mn-MOFs) that alleviates the deprotonation and electrochemical activity concerns arising during the charge/discharge process. The MOFs carboxyl group provides protons to prevent deprotonation and allows the polyaniline to reach a high zinc storage redox activity. The proton self-doped polyaniline cathode has a superior specific capacity (273 mAh g-1 at 0.5 A g-1 ), a high rate property (154 mAh g-1 at 20 A g-1 ), and excellent cyclability retention (87% over 4000 cycles at 15 A g-1 ). This research provides fresh insight into the development of innovative polymers as cathode materials for high-performance AZIBs.

Constructing Ionic Self-Concentrated Electrolyte via Introducing Montmorillonite Toward High-Performance Aqueous Zn-MnO2 Batteries.

Aqueous zinc metal batteries are regarded as one of the most promising alternatives to lithium-ion batteries for large-scale energy storage due to the abundant zinc resources, high safety, and low cost. Herein, an ionic self-concentrated electrolyte (ISCE) is proposed to enable uniform Zn deposition and reversible reaction of MnO2 cathode. Benefitting from the compatibility of ISCE with electrodes and its adsorption on the electrode surface for guidance, the Zn/Zn symmetrical batteries exhibit the long-life cycle stability with more than 5000 and 1500 h at 0.2 and 5 mA cm-2 , respectively. The Zn/MnO2 battery also exhibits a high capacity of 351 mA h g-1 at 0.1 A g-1 and can enable a stability over 2000 cycles at 1 A g-1 . This work provides a new insight into electrolyte design for stable aqueous Zn-MnO2 battery.

Reconstructing interfacial manganese deposition for durable aqueous zinc-manganese batteries.

Low-cost, high-safety, and broad-prospect aqueous zinc-manganese batteries (ZMBs) are limited by complex interfacial reactions. The solid-liquid interfacial state of the cathode dominates the Mn dissolution/deposition process of aqueous ZMBs, especially the important influence on the mass and charge transfer behavior of Zn2+ and Mn2+. We proposed a quasi-eutectic electrolyte (QEE) that would stabilize the reversible behavior of interfacial deposition and favorable interfacial reaction kinetic of manganese-based cathodes in a long cycle process by optimizing mass and charge transfer. We emphasize that the initial interfacial reaction energy barrier is not the main factor affecting cycling performance, and the good reaction kinetics induced by interfacial deposition during the cycling process is more conducive to the stable cycling of the battery, which has been confirmed by theoretical analysis, quartz crystal microbalance with dissipation monitoring, depth etching X-ray photon-electron spectroscopy, etc. As a result, the QEE electrolyte maintained a stable specific capacity of 250 mAh g-1 at 0.5 A g-1 after 350 cycles in zinc-manganese batteries. The energy density retention rate of the ZMB with QEE increased by 174% compared to that of conventional aqueous electrolyte. Furthermore, the multi-stacked soft-pack battery with a cathodic mass load of 54.4 mg maintained a stable specific capacity of 200 mAh g-1 for 100 cycles, demonstrating its commercial potential. This work proves the feasibility of adapting lean-water QEE to the stable aqueous ZMBs.

Reversible Multielectron Redox Chemistry in a NASICON-Type Cathode toward High-Energy-Density and Long-Life Sodium-Ion Full Batteries.

Na-superionic-conductor (NASICON)-type cathodes (e.g., Na3 V2 (PO4 )3 ) have attracted extensive attention due to their open and robust framework, fast Na+ mobility, and superior thermal stability. To commercialize sodium-ion batteries (SIBs), higher energy density and lower cost requirements are urgently needed for NASICON-type cathodes. Herein, Na3.5 V1.5 Fe0.5 (PO4 )3 (NVFP) is designed by an Fe-substitution strategy, which not only reduces the exorbitant cost of vanadium, but also realizes high-voltage multielectron reactions. The NVFP cathode can deliver extraordinary capacity (148.2 mAh g-1 ), and decent cycling durability up to 84% after 10 000 cycles at 100 C. In situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy characterizations reveal reversible structural evolution and redox processes (Fe2+ /Fe3+ , V3+ /V4+ , and V4+ /V5+ ) during electrochemical reactions. The low ionic-migration energy barrier and ideal Na+ -diffusion kinetics are elucidated by density functional theory calculations. Combined with electron paramagnetic resonance spectroscopy, Fe with unpaired electrons in the 3d orbital is inseparable from the higher-valence redox activation. More competitively, coupling with a hard carbon (HC) anode, HC//NVFP full cells demonstrate high-rate capability and long-duration cycling lifespan (3000 stable cycles at 50 C), along with material-level energy density up to 304 Wh kg-1 . The present work can provide new perspectives to accelerate the commercialization of SIBs.

Recent Progress on Phosphate Cathode Materials for Aqueous Zinc-Ion Batteries.

Rechargeable zinc-ion batteries (ZIBs) are attractive for large-scale energy storage due to their superiority in resources, safety, and environmental friendliness. However, the lack of suitable ZIBs cathode materials limits their practical applications. In consideration of the excellent electrochemical performance of phosphate materials in monovalent ion (Li+ , Na+ ) batteries, they were also employed as ZIBs cathode materials recently and performed well with high potential. But they also suffer from low capacity and poor conductivity, and the energy storage mechanism is not clear yet. This Review provides a state-of-the art overview on the developments of phosphate cathode materials in ZIBs, including NASICON-type phosphates, fluorophosphates, olivine-structured, layered-structured, and novel-structured phosphate materials mainly. This study presents the reaction mechanism and electrochemical performance of phosphate cathode materials in aqueous ZIBs, and future research directions are discussed, which are intended to provide guidance for exploring high-potential cathode materials for ZIBs.

Balanced Interfacial Ion Concentration and Migration Steric Hindrance Promoting High-Efficiency Deposition/Dissolution Battery Chemistry.

The solid-liquid transition reaction lays the foundation of electrochemical energy storage systems with high capacity, but realizing high efficiency remains a challenge. Herein, in terms of thermodynamics and dynamics, this work demonstrates the significant role of both interfacial H+ concentration and Mn2+ migration steric hindrance for the high-efficiency deposition/dissolution chemistry of zinc-manganese batteries. Specially, the introduction of formate anions can buffer the generated interfacial H+ to stabilize interfacial potential according to the Nernst equation, which stimulates high capacity. Compared with acetate and propionate anions, the formate anion also provides high adsorption density on the cathode surface to shield the electrostatic repulsion due to the small spatial hindrance. Particularly for the solvated Mn2+ , the formate-anion-induced lower energy barrier of the rate-determining step during the step-by-step desolvation process results in lower polarization and higher electrochemical reversibility. In situ tests and theoretical calculations verify that the electrolyte with formate anions achieve a good balance between ion concentration and ion-migration steric hindrance. It exhibits both the high energy density of 531.26 W h kg-1  and long cycle life of more than 300 cycles without obvious decay.

Interfacial thermodynamics-inspired electrolyte strategy to regulate output voltage and energy density of battery chemistry.

The electrochemical behaviors of battery chemistry, especially the operating voltage, are greatly affected by the complex electrode/electrolyte interface, but the corresponding basis understanding is still largely unclear. Herein, the concept of regulating electrode potential by interface thermodynamics is proposed, which guides the improvement of the energy density of Zn-MnO2 battery. A cationic electrolyte strategy is adopted to adjust the charge density of electrical double layer, as well as entropy change caused by desolvation, thus, achieving an output voltage of 1.6 V (vs. Zn2+/Zn) and a capacity of 400 mAh g-1. The detailed energy storage behaviors are also analyzed in terms of crystal field and energy level splitting. Furthermore, the electrolyte optimization benefits the efficient operation of Zn-MnO2 battery by enabling a high energy density of 532 Wh kg-1 based on the mass of cathode and a long cyclic life of more than 500 cycles. This work provides a path for designing high-energy-density aqueous battery via electrolyte strategy, which is expected to be extended to other battery systems.

Building Ultra-Stable and Low-Polarization Composite Zn Anode Interface via Hydrated Polyzwitterionic Electrolyte Construction.

Aqueous zinc metal batteries are noted for their cost-effectiveness, safety and environmental friendliness. However, the water-induced notorious issues such as continuous electrolyte decomposition and uneven Zn electrochemical deposition remarkably restrict the development of the long-life zinc metal batteries. In this study, zwitterionic sulfobetaine is introduced to copolymerize with acrylamide in zinc perchlorate (Zn(ClO4)2) solution. The designed gel framework with hydrophilic and charged groups can firmly anchor water molecules and construct ion migration channels to accelerate ion transport. The in situ generated hybrid interface, which is composed of the organic functionalized outer layer and inorganic Cl- containing inner layer, can synergically lower the mass transfer overpotential, reduce water-related side reactions and lead to uniform Zn deposition. Such a novel electrolyte configuration enables Zn//Zn cells with an ultra-long cycling life of over 3000 h and a low polarization potential (~ 0.03 V) and Zn//Cu cells with high Coulombic efficiency of 99.18% for 1000 cycles. Full cells matched with MnO2 cathodes delivered laudable cycling stability and impressive shelving ability. Besides, the flexible quasi-solid-state batteries which are equipped with the anti-vandalism ability (such as cutting, hammering and soaking) can successfully power the LED simultaneously. Such a safe, processable and durable hydrogel promises significant application potential for long-life flexible electronic devices.

Organic-Inorganic Hybrid Cathode with Dual Energy-Storage Mechanism for Ultrahigh-Rate and Ultralong-Life Aqueous Zinc-Ion Batteries.

The exploitation of cathode materials with high capacity as well as high operating voltage is extremely important for the development of aqueous zinc-ion batteries (ZIBs). Yet, the classical high-capacity materials (e.g., vanadium-based materials) provide a low discharge voltage, while organic cathodes with high operating voltage generally suffer from a low capacity. In this work, organic (ethylenediamine)-inorganic (vanadium oxide) hybrid cathodes, that is, EDA-VO, with a dual energy-storage mechanism, are designed for ultrahigh-rate and ultralong-life ZIBs. The embedded ethylenediamine (EDA) can not only increase the layer spacing of the vanadium oxide, with improved mobility of Zn ions in the V-O layered structure, but also act as a bidentate chelating ligand participating in the storage of Zn ions. This hybrid provides a high specific capacity (382.6 mA h g-1 at 0.5 A g-1 ), elevated voltage (0.82 V) and excellent long-term cycle stability (over 10 000 cycles at 5 A g-1 ). Assistant density functional theory (DFT) calculations indicate the cathode has remarkable electronic conductivity, with an ultralow diffusion barrier of 0.78 eV for an optimal Zn-ion diffusion path in the EDA-VO. This interesting idea of building organic-inorganic hybrid cathode materials with a dual energy-storage mechanism opens a new research direction toward high-energy secondary batteries.

Quasi-solid-state Zn-air batteries with an atomically dispersed cobalt electrocatalyst and organohydrogel electrolyte.

Quasi-solid-state Zn-air batteries are usually limited to relatively low-rate ability (<10 mA cm-2), which is caused in part by sluggish oxygen electrocatalysis and unstable electrochemical interfaces. Here we present a high-rate and robust quasi-solid-state Zn-air battery enabled by atomically dispersed cobalt sites anchored on wrinkled nitrogen doped graphene as the air cathode and a polyacrylamide organohydrogel electrolyte with its hydrogen-bond network modified by the addition of dimethyl sulfoxide. This design enables a cycling current density of 100 mA cm-2 over 50 h at 25 °C. A low-temperature cycling stability of over 300 h (at 0.5 mA cm-2) with over 90% capacity retention at -60 °C and a broad temperature adaptability (-60 to 60 °C) are also demonstrated.

Copper-Stabilized P'2-Type Layered Manganese Oxide Cathodes for High-Performance Sodium-Ion Batteries.

Layered sodium manganese oxides are promising low-cost and high-capacity cathode materials for commercialization of sodium-ion batteries (SIBs). P'2-type Na0.67MnO2 with an orthorhombic structure has been considered as a significant candidate for SIBs. However, the Jahn-Teller distortion and undesired phase transitions will lead to poor structural stability and unsatisfactory cycling performance. Herein, a systematic investigation on partially copper-doped P'2-type Na0.67CuxMn1-xO2 (x = 0, 0.05, 0.1, and 0.2) series as cathodes for SIBs reveals the relationship between doping concentrations and Na storage properties. With proper copper content, P'2 Na0.67Cu0.1Mn0.9O2 exhibits a suppressed Jahn-Teller effect as well as relatively less phase transitions, which can deliver a high specific capacity of 222.7 mA h g-1 at 10 mA g-1 within 1.5-4.2 V, with a capacity retention of 76% at 1 A g-1 after 300 cycles. The electrochemical mechanism is systematically investigated via in situ X-ray diffraction observations and density functional theory calculations, which provide fundamental guidelines for developing high-performance cathodes for SIBs.

MOF-derived porous carbon inlaid with MnO2 nanoparticles as stable aqueous Zn-ion battery cathodes.

Cathodes derived from metal-organic framework materials offer unique advantages in terms of improved structural reversibility and electron conduction efficiency. Nevertheless, the capacity contribution of cathodes based on the carbon framework system has not been clearly discussed or is controversial in aqueous batteries. In this essay, we have uncovered the capacity contribution arising from the adsorption of anions/cations onto the carbon surface by examining the bonds of the carbon and the details of unsteady voltage in the CV/GITT during the discharge. Benefiting from the synergistic contribution of the double-layer capacitance and pseudocapacitance, Zn/C-MnO2 exhibits excellent long-cycling stability and fast kinetics. To the best of our knowledge, this is the first report on the ion adsorption-based double layer effect in aqueous zinc ion batteries. Such a capacity contribution mechanism, and a renewed knowledge of the discharge mechanism, will contribute to the development of high-performance aqueous zinc ion batteries.