Zinc-ion batteries built on water-based electrolytes featuring compelling price-points, competitive performance, and enhanced safety represent advanced energy storage chemistry as a promising alternative to current lithium-ion battery systems. Attempts to develop rechargeable aqueous zinc-ion batteries (ZIBs) can be traced to as early as the 1980s; however, since 2015, the research activity in this field has surged throughout the world. Despite the achievements made in exploring electrode materials so far, significant challenges remain at the material level and even on the whole aqueous ZIBs system, leading to the failure of ZIBs to meet commercial requirements. This review aims to discuss how to pave the way for developing aqueous ZIBs. The current research efforts related to aqueous ZIBs electrode materials and electrolytes are summarized, including an analysis of the problems encountered in both cathode/anode materials and electrolyte optimization. Some concerns and feasible solutions for achieving practical aqueous ZIBs are discussed in detail. We would like to point out that merely improving the electrode materials is not enough; synergistic optimization strategies toward the whole battery system are also deeply needed. Finally, some perspectives are provided on the subsequent optimization design for further research efforts in the aqueous ZIB field.

Issues and opportunities facing aqueous zinc-ion batteries

Rechargeable zinc-ion batteries (ZIBs) are promising for large scale energy storage and portable electronic applications due to their low cost, material abundance, high safety, acceptable energy density and environmental friendliness. This tutorial review presents an introduction to the fundamentals, challenges, recent advances and prospects related to ZIBs. Firstly, the intrinsic chemical properties, challenges and strategies of metallic zinc anodes are underscored. Then, the multiple types of cathode materials are classified and comparatively discussed in terms of their structural and electrochemical properties, issues and remedies. Specific attention is paid to the mechanistic understanding and structural transformation of cathode materials based on Zn ion-(de)intercalation chemistry. After that, the widely investigated electrolytes are elaborated by discussing their effect on Zn plating/stripping behaviours, reaction kinetics, electrode/electrolyte interface chemistries, and cell performances. Finally, the remaining challenges and future perspectives are outlined for the development of ZIBs.

Materials chemistry for rechargeable zinc-ion batteries

Aqueous zinc-ion batteries have rapidly developed recently as promising energy storage devices in large-scale energy storage systems owing to their low cost and high safety. Research on suppressing zinc dendrite growth has meanwhile attracted widespread attention to improve the lifespan and reversibility of batteries. Herein, design methods for dendrite-free zinc anodes and their internal mechanisms are reviewed from the perspective of optimizing the host-zinc interface and the zinc-electrolyte interface. Furthermore, a design strategy is proposed to homogenize zinc deposition by regulating the interfacial electric field and ion distribution during zinc nucleation and growth. This Minireview can offer potential directions for the rational design of dendrite-free zinc anodes employed in aqueous zinc-ion batteries.

Interfacial design of dendrite‐free zinc anodes for aqueous zinc‐ion batteries

Rechargeable zinc metal batteries (RZMBs) offer a compelling complement to existing lithium ion and emerging lithium metal batteries for meeting the increasing energy storage demands of the future. Multiple recent reports have suggested that optimized electrolytes resolve a century-old challenge for RZMBs by achieving extremely reversible zinc plating/stripping with Coulombic efficiencies (CEs) approaching 100%. However, the disparity among published testing methods and conditions severely convolutes electrolyte performance comparisons. The lack of rigorous and standardized protocols is rapidly becoming an impediment to ongoing research and commercialization thrusts. This Perspective examines recent efforts to improve the reversibility of the zinc metal anode in terms of key parameters, including CE protocols, plating morphology, dendrite formation and long-term stability. Then we suggest the most appropriate standard protocols for future CE determination. Finally, we envision future strategies to improve zinc/electrolyte stability so that research efforts can be better aligned towards realistic performance targets for RZMB commercialization. Zinc metal batteries (ZMBs) provide a promising alternative to lithium metal batteries but share the formidable challenges in reversibility. The authors discuss the key performance metrics of ZMBs and propose a protocol to assess the true reversibility of zinc metal anodes.

Realizing high zinc reversibility in rechargeable batteries

Zn metal has been regarded as the most promising anode for aqueous batteries due to its high capacity, low cost, and environmental benignity. Zn anode still suffers, however, from low Coulombic efficiency due to the side reactions and dendrite growth in slightly acidic electrolytes. Here, the Zn plating/stripping mechanism is thoroughly investigated in 1 m ZnSO4 electrolyte, demonstrating that the poor performance of Zn metal in mild electrolyte should be ascribed to the formation of a porous by-product (Zn4SO4(OH)6·xH2O) layer and serious dendrite growth. To suppress the side reactions and dendrite growth, a highly viscoelastic polyvinyl butyral film, functioning as an artificial solid/electrolyte interphase (SEI), is homogeneously deposited on the Zn surface via a simple spin-coating strategy. This dense artificial SEI film not only effectively blocks water from the Zn surface but also guides the uniform stripping/plating of Zn ions underneath the film due to its good adhesion, hydrophilicity, ionic conductivity, and mechanical strength. Consequently, this side-reaction-free and dendrite-free Zn electrode exhibits high cycling stability and enhanced Coulombic efficiency, which also contributes to enhancement of the full-cell performance when it is coupled with MnO2 and LiFePO4 cathodes.

Designing Dendrite-Free Zinc Anodes for Advanced Aqueous Zinc Batteries

A ZnCl2 water-in-salt electrolyte for a reversible Zn metal anode.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/cey2.70

The electrolyte comprising more robust water and superhalides transforms Zn-metal anode reversibly and dendrite-free

Although the utilization of mildly acidic electrolyte can enhance the stability and reversibility of Zn anode, it is still challenging to achieve long cycling life for aqueous Zn batteries at low currents, due to the structural instability of cathode materials during the charge/discharge processes. Herein, we report a series of polypyrenes with different linking patterns and electronic structures as Cl−-hosting organic cathodes for aqueous Zn dual-ion batteries (AZDIBs). The comparative study demonstrates that the electronic structures are pivotal to the redox activity of the polypyrenes, which can be tuned by altering the linking patterns on the pyrene unit. Owing to the high surface area, the highly delocalized HOMO distribution, the high HOMO level and the narrow band gap, the polymer CLPy with 1,3,6,8-linking pattern delivers a much higher capacity of 180 mA h g−1 than the two linear counterparts (24 mA h g−1 for LPy-1 and 44 mA h g−1 for LPy-2). Impressively, CLPy exhibits ultra-stable cyclabilities with the capacity retentions of 97.4% after 800 cycles at 50 mA g−1 and 96.4% after 38 000 cycles at 3 A g−1. CLPy also shows a low self-discharge rate with around 90% capacity retention after resting for 28 days. The excellent electrochemical performance demonstrates that CLPy can be a promising cathode material for high-performance AZDIBs.

Aigerim Daniyar

Papers

A ZnCl2 water-in-salt electrolyte for a reversible Zn metal anode.

The electrolyte comprising more robust water and superhalides transforms Zn-metal anode reversibly and dendrite-free

Tailoring the linking patterns of polypyrene cathodes for high-performance aqueous Zn dual-ion batteries