Iron Vanadate & Vanadium Hexacyanoferrate As Cathode For Zinc-iIon Batteries

Recently,with the development of science and technology,we pay more and more attention to the development of new energy resources.However,these energy sources are limited by geographical environment,climatic conditions and other factors,so an energy storage system is required to carry out reliable and stable power supply.Although lithium-ion batteries have the advantages of high energy density,high voltage platform,no memory effect,long cycle life and so on,the further application of lithium-ion batteries is restricted by problems such as the rising cost caused by limited lithium resources and safety risks caused by the toxic and flammable electrolytes.As a result,rechargeable multivalent metal-ion batteries have attracted attention from researchers as a potential alternative to lithium-ion batteries,especially in large-scale energy storage systems.Among them,rechargeable zinc-ion batteries（ZIBs）have attracted most extensive attention.Because Zn metal is very stable and naturally abundant,it can be directly used as the anode.Moreover,the comparatively low electrochemical potential and high density endow the Zn anode with intrinsically high volumetric energy density.At present,the cathode materials for zinc ion batteries are mainly manganese-based,vanadium-based,Prussian blue analogue and some other cathode materials.In recent years,vanadium-based materials have been widely tried to be used as the cathode of zinc-ion batteries.It has multiple valence states and can realize multi-electron transfer when used as the cathode material,so that the battery can obtain a higher capacity.At the same time,it has a variety of crystal morphology and can be changed with the change of the oxidation state of vanadium.On the other hand,the open frame structures of Prussian blue analogues allow simple and reversible ion intercalation/deintercalation while maintaining their own structural stability,but are often constrained by lower specific capacity and voltage platforms.In this paper,we tried to prepare two kinds of cathode materials for zinc-ion batteries,and carried out characterization tests on them.Then,we used them as cathode materials to assemble zinc ion batteries for electrochemical performance test.The specific work is as follows:（1）we synthesized amorphous Fe VO₄ with a hydrangea-like structure through a facile hydrothermal method,in which ultrathin Fe VO₄ nanosheets like flower petals were self-assembled into Fe VO₄ spheres.It is generally accepted that the ternary vanadium oxide Fe VO₄possesses higher electronic conductivity than the binary vanadium oxide V₂O₅.In addition,the bionic design can not only enlarge the contact area between the electrolyte and the cathode,but also effectively retain the integrity of the structure during cycling.Moreover,the amorphous crystalline structure may provide more free volume for Zn²⁺insertion.As a result,the asprepared Fe VO₄ delivers outstanding long-term cycling stability with a stable capacity of 70 m A h g^-1 after 2500 cycles at a high current density of 5 A g^-1,corresponding to above95%capacity retention.This research illustrates that the hydrangea-like amorphous Fe VO₄spheres show attractive potential for use in high-rate ZIBs.At-20°C,the battery can still produce a very high discharge capacity of about 80 m A h g^-1 at 0.1 A g^-1,which makes it possible for the material to be used in practical applications.Meanwhile,ex-situ Raman spectroscopy,XPS and other methods were used to study the zinc storage mechanism of amorphous Fe VO₄ and the mechanism of Zn²⁺insertion/extraction was confirmed.（2）V_1.5Fe（CN）₆ material was prepared by co-precipitation method and used as cathode material for zinc-ion battery.The experimental results show that the voltage platform is about1.73V with good cycling stability.The specific capacity of about 135 m Ah g^-1 can be obtained at the current density of 0.2A g^-1,and about 77.8%of the capacity can be retained after 50cycles.In addition,the material also shows excellent rate performance.The cell can obtain a specific capacity of about 72 m Ah g^-1 at A high current density cycle of 5 A g^-1,and still has A specific capacity of 132 m Ah g^-1 at A current density of 0.2 A g^-1.Then,we also studied the electrochemical reaction kinetics by CV and GITT.It is shown that it is mainly controlled by pseudocapacitance and its ion diffusion coefficients is between-8.5 and-10.