水系锌离子电池钒基正极材料储能机制、存在的问题及其改性策略

Energy Storage Mechanism,Issue and Modification Strategies of Vanadium-Based Cathode Materials for Aqueous Zinc Ion Batteries

中性或弱酸性体系下的水系锌离子电池(AZIBs)因高安全、低成本及高能量密度等特性成为近年来研究的热点。其中,备受关注的钒基化合物具有比容量高、结构灵活多样等优点在AZIBs领域展现出了广阔的市场应用前景。主要总结了钒基材料的4种反应机制并叙述了钒基正极材料在AZIBs中的研究进展,在AZIBs中,Zn^(2+)有着较大的离子半径,随着循环的进行Zn^(2+)不断嵌入/脱出,引起材料结构的变化,从而导致活性物质从导电集流体上脱落,严重影响电池的循环寿命;钒基材料本身的导电性能较差,不利于电子的转移;钒基材料在AZIBs中的电压窗口比较窄。针对这些问题,主要从离子和分子预嵌、表面修饰和复合材料制备、缺陷设计及金属离子掺杂、自支撑电极结构设计、电解液优化等5个方面进行了总结,并对未来AZIBs钒基正极材料的研究方向进行了总结与展望。

With the development of science and technology and the consumption of nonrenewable resources,the maximum use of re⁃newable resources has become particularly important.However,solar energy,wind energy,tidal energy and other renewable energy have discontinuities in time and space,and large energy storage devices are needed to convert this energy into electrical energy.Aque⁃ous zinc ion batteries(AZIBs)in neutral or mild acid systems have become a research hotspot in recent years due to its high safety,low cost,and high energy density.It has been widely studied by studied by scholars due to the high zinc storage capacity of vanadiumbased compounds in AZIBs system,on account of its various valence states,such as+2,+3,+4 and+5.According to the literature,vanadium-based compounds existed many crystal structures such as layers(V2O5),tunnel(VO2,V6O13),and it showed a broad mar⁃ket application prospect in AZIBs field because of its high specific capacity and flexible structure.The four reaction mechanisms of va⁃nadium-based materials were summarized in this article included Zn^(2+)insertion/extraction;the ions and molecules of co-intercalation/deintercalation;chemical conversion reaction;displacement/intercalation reaction.In AZIBs,Zn^(2+)had a large ion radius,with the cycle goes on,Zn^(2+)was constantly embedded/detached led to changes in material structure,then the active material falling off from the conductive current collector,seriously hindered the cycle life of the battery.The conductivity of vanadium-based materials was poor,which was not good for electrons transfer;and the voltage window of vanadium-based materials in AZIBs was narrow.With consider⁃ation of the above problems,it was summarized from the following five aspects of ion and molecular pre-intercalation,surface modifica⁃tion and composite material preparation,defect design,metal ion doping,the structure design of self-supporting electrode,electrolyte optimization and so on.The introduction of cations,anions or water molecules between vanadium oxide layers could support the col⁃umn to achieve the effect of stable structure and expanding the spacing of layers.A small amount of metal ions and molecules could pro⁃vide more active sites for Zn^(2+)insertion and effectively reduce electrostatic repulsion.Layered vanadate had a monoclinic layered struc⁃ture,and the layer was occupied by M-ions(metal ions)and connected by ion bond to form column layer,which had strong structural stability and high zinc storage capacity.The charge shielding effect of H2O in crystal could obviously reduce the effective charge of em⁃bedded Zn^(2+),thus improving the specific capacity and ratio performance of the material.Due to the poor conductivity of vanadiumbased materials,the dissolution of vanadium in aqueous solution and the insertion of large radius ions in the circulation process,the researchers adopted carbon(C),graphene oxide(GO),reduced graphene oxide(rGO),carbon nanotubes(CNT),single wall car⁃bon nanotubes(SWCNT),and the composite or coating of conductive active substances such as polyaniline(PAIN)to improve their property.The material had the characteristics of good conductivity,stable structure,and excellent electrochemical performance.Under the condition of carbon-based material composite or coating,it could reduce the falling off active materials of electrode,the elec⁃trode flexibility and conductivity improvement,and could effectively avoid collapse of structure in the process of Zn^(2+)embedding/strip⁃ping.The defects such as cation or anion vacancy were introduced into the lattice,which could inhibit phase transition and increase more active sites for Zn^(2+)storage.Moreover,the strong ion lattice interaction between active cathode materials such as vanadium oxides and Zn^(2+)resulted in the limitation of charge discharge and cycle stability at high rate.The preparation of anoxic vanadium oxide cath⁃ode could promote the reaction kinetics and increase the capacity of Zn^(2+),and make the diffusion path of Zn^(2+)highly reversible.The traditional coated electrode would cause agglomeration,unstable structure,and no flexibility,which would cause the material to fall off and dissolve.The synthesis of selfformed materials could maintain the original morphology of the material,and it was beneficial to the close contact between the material and electrolyte and the diffusion path of Zn^(2+)was short,the full growth on the conductive substrate could increase the conductivity of the material.This article described the existing problems and optimization strategies of AZIBs vanadium-based cathode materials,and summarized and prospected the research direction of AZIBs in the future.