水系锌二次电池MnO_(2)正极的晶体结构、反应机理及其改性策略

Crystal Structures,Reaction Mechanisms,and Optimization Strategies of MnO_(2) Cathode for Aqueous Rechargeable Zinc Batteries

水系锌二次电池凭借其安全性高、环境友好、成本低廉、能量密度较高等诸多优势,有望应用于下一代大规模储能系统。电池的发展依赖于电极材料,二氧化锰由于其高丰度、低成本、毒性小等优势,在水系锌二次电池领域得到广泛应用。本文将从二氧化锰的晶体结构、反应机理及电化学性能出发,对其在水系锌二次电池中的研究进展进行系统综述。特别地,针对其容量低、循环稳定性差等问题,本文从储能机理(包括嵌入-脱嵌机制和溶解-沉积机制)角度出发,总结相对应的优化策略,为先进水系锌锰二次电池的设计开发提供参考。

Because of the advantages of high safety,environment-friendliness,affordability,and ease of processing,aqueous rechargeable zinc batteries(ARZBs)are promising candidates for nextgeneration large-scale energy storage systems.In recent years,various cathode materials based on vanadium/manganese/cobalt oxides,Prussian blue analogs,and organic compounds have been reported.Among them,manganese dioxide(MnO_(2))is widely used in ARZBs due to their outstanding advantages of low toxicity,eco-friendliness,and high capacity(616 mAh·gbased on two-electron transfer).However,the diversity of the crystal structures of MnO_(2)and the unpredictability of the electrochemical reaction make it difficult to investigate the specific internal storage mechanism,which impedes further development of the optimal modification strategies.To date,the main recognized energy storage mechanisms are(de)intercalation and dissolution-deposition mechanisms.In the traditional(de)intercalation mechanism,the predominant issues related to MnO_(2)during the cycling process include Mn dissolution,irreversible phase transformation,structural collapse,and sluggish ion diffusion kinetics.On the other hand,the detailed reaction path for the dissolution-deposition mechanism,which was developed in recent years,remains controversial.In addition,the incomplete dissolution-deposition of MnO_(2)and the highly acidic environment inevitably leads to corrosion and hydrogen evolution of the zinc anode,as well as low Coulombic efficiency.Accordingly,optimization strategies for different reaction mechanisms have been proposed to make zinc-manganese batteries more competitive.For the(de)intercalation mechanism,modification of composite materials and nanostructure optimization strategies can be adopted to inhibit the dissolution of MnO_(2)and increase the number of highly active reaction sites,thus enhancing the electrochemical performance.Moreover,the guest pre-intercalation strategy can help optimize the crystal structure of MnO_(2),preventing the collapse of the internal structure during cycling.Besides,defect engineering and element doping strategies focus on regulating the distribution of the electronic structure for affecting the properties of MnO_(2),resulting in lowering the energy barrier of zinc insertion.For the dissolution-deposition mechanism,the introduction of a neutral acetate and a halide mediator can effectively facilitate the dissolution-deposition of MnO_(2).Meanwhile,metal element catalysis can accelerate the reaction kinetics of the MnO_(2)dissolution-deposition,so that high-rate performance can be achieved.Furthermore,the decoupling battery system can separate the cathodic and anodic electrolytes to restrain the hydrogen and oxygen evolution reactions and enhance the potential difference.The flow battery system can effectively eliminate the influence of concentration polarization and stabilize the ion concentration in the electrolytes,thus leading to a large capacity(>100 mA h).Undoubtedly,MnO_(2)as a high-capacity,high-voltage cathode material has broad development prospects for ARZBs.Here,we systematically summarize the crystal structures and reaction mechanisms of MnO_(2).We also discuss the optimization strategies toward advanced MnO_(2)cathode materials for resolving the highlighted issues in zinc-manganese batteries,which are expected to provide research directions for the design and development of high-performance ARZBs.