Design And Electrochemical Properties Of Vanadium-based Cathode Materials For Aqueous Zinc Ion Batteries

The increasing demand for energy in industrial society and the continuous consumption of traditional energy lead to the aggravation of global environmental problems.Therefore,human society needs more green,safe and low-cost energy storage technology.In terms of safety,zinc ion batteries are much higher than lithium-ion batteries.In addition,the preparation process of zinc ion battery is simple and the cost is lower.Therefore,zinc ion battery has become one of the most promising candidates for large-scale energy storage technology.Compared with other cathode materials for zinc ion batteries,vanadium-based materials have higher specific capacity.Aiming at the capacity attenuation of vanadium-based cathode materials in the cycle process,researchers put forward strategies such as cation pre-intercalation,pre-embedding of structural water molecules,introduction of defects and so on.However,vanadium-based materials still have some problems,such as slow reaction kinetics and complex electrochemical reaction mechanism.Therefore,finding suitable vanadium-based cathode materials have become one of the key research directions in the field of zinc ion battery materials.Based on this,this paper explores the vanadium oxide cathode material system based on ammonium ion pre-embedding.The main research contents and innovations are as follows:1.Synthesis,electrochemical performance test and electrochemical reaction mechanism of high-performance vanadium based aqueous zinc ion battery cathode material(NH4)2V7O16·3.2H2O.(NH4)2V7O16·3.2H2O with nano block morphology was successfully synthesized by hydrothermal method.V7O16 based material was applied to the cathode field of aqueous zinc ion battery for the first time,and showed excellent electrochemical properties.When the current density is 4 A g-1,it shows an initial capacity of 250.7 mA h g-1,and the capacity can still be maintained at 255 mA h g-1 after 1000 cycles.The charge/discharge mechanism of(NH4)2V7O16·3.2H2O cathode is studied by XRD and XPS.It is successfully proved that(NH4)2V7O16 will undergo phase transition to V2O5 based material in the initial cycle,and the V2O5 based cathode material generated by in-situ electrochemical conversion shows excellent electrochemical properties.It further enriches the energy storage mechanism of zinc ion battery.2.On the basis of(NH4)2V7O16 based cathode material,ion doping and oxygen defects are further introduced.The electrochemical properties of(NH4)2V7O16·3.2H2O are successfully improved by Mn2+ doping and nitriding treatment.Different contents of manganese ions were doped into the interlayer of V7O16 by hydrothermal method,and the best doping amount of manganese ions was determined.When the ratio of Mn2+to NH4+is 1:2,the most excellent(NH4)2V7O16·3.2H2O is obtained.When the current density is 4 A g-1,the battery shows a specific capacity of 325 mA h g-1 and 297 mA h g-1 after 1000 cycles.Compared with the(NH4)2V7O16 cathode,the specific capacity was significantly improved.The analysis shows that manganese ion doping can reduce the charge transfer resistance and improve the conductivity.The electrochemical performance of(NH4,Mn)2V7O16 cathode was further improved by defect engineering technology.NH3 was used to nitride(NH4,Mn)2V7O16 at different temperatures,and the optimum nitriding temperature was determined to be 200℃.The initial specific capacity of the battery was increased to 334 mA h g-1,and the specific capacity after 1000 cycles was 345 mA h g-1.XPS characterization showed that oxygen defects were successfully introduced after nitriding.The introduction of oxygen defect weakens the electrostatic interaction between zinc ion and vanadium oxide layer,which helps to accelerate the migration rate of zinc ion.In addition,the nitrided cathode material has higher conductivity,higher zinc ion diffusion coefficient and higher capacitance contribution.In situ XRD technology was used to explore the charging and discharging mechanism of nitrided(NH4,Mn)2V 7O16.It was found that there was still a phase transformation process to V2O5 based material,which provided a reference for improving the performance of vanadium based aqueous zinc ion battery cathode material.