Synthesis And Properties Of Ion Doped Li₃V₂?PO₄?₃ Nanomaterials For Lithium Ion Batteries

The lithium-ion batteries have attracted more and more attention because of a greener environment,convenience and small self discharge.In the past decade,the cathode materials of lithium ion batteries have achieved great progress.Cathode materials affect the electrochemical performance of lithium ion batteries.However,current commercial cathode materials,such as LiCoO₂,LiMn₂O₄ and LiFePO₄,have failed to meet the demands of high power density and high energy density.Thus,exploiting excellent materials is one of the important directions of lithium ion battery.Compared with traditional cathode materials,monoclinic Li₃V₂?PO₄?₃ with excellent electrochemical performances,including good cycling stability,high theory capacity,low cost,security and environmental friendliness,is recognized as an ideal cathode material for lithium ion batteries.In this paper,polyanion compound of Li₃V₂?PO₄?₃ is used as the research object and the research aims at improving the cycling stability and rate performance.?1?A series of Li_3-xK_x V₂?PO₄?₃?x=0.00?0.02?0.04 and 0.06?have been fabricated by sol-gel method.Oxalic acid and sucrose are used as chelating agent and carbon source,respectively.Influence of K⁺doping on the battery was studied.The results of XRD and EDS indicate that K⁺have been successfully doped and the cell volume of Li₃V₂?PO₄?₃were enlarged.The large cell volume is beneficial to the diffusion of Li-ion,accelerating chemical reaction kinetics.The electrochemical research results showed that,after 200 cycles,the retention rate o f capacity Li_3-x-x K_x V₂?PO₄?₃?x=0.04?possesses the highest capacity retention rate of 99.31%.When the current density increases from 0.1 C to 2 C,the discharge capacities are131.4 mAh g^-1 and 128.3 mAh g^-1,respectively.The relative small fading capacity indicates the K⁺doping could improve the rate performance.?2?A series of Li₃V_2-x-x Co_x?PO₄?₃?x=0.00?0.02?0.04 and 0.06?have been fabricated by sol-gel method.Oxalic acid and phenolic resin are used as chelating agent and carbon source,respectively.Influence of Co ion doping on electrochemical performance was studied.The result of XRD indicates that the doped Co²⁺does not change nonoclinic structure of Li₃V₂?PO₄?₃,but slightly changes the cell parameter and enlarges channel diffusion of lithium ion,resulting in the biggest Li⁺diffusion coeffecient.The electrochemical test indicates that capacity and retention rate of Li₃V_2-x-x Co_x?PO₄?₃?x=0.04?are 107.4 mAh g^-11 and 97.3%,respectively,after 200cycles.The decaying degree of low at different current densites the discharge capacity of Li₃V_2-x-x Co_x?PO₄?₃?x=0.04?is all larger.The doped Co²⁺could reduce the dissolution of vanadium,which results in good reversibility and rate performance.?3?A series nanoplates of Li_3-xK_x V_2-x-x Co_x?PO₄?₃ have been fabricated by sol-gel method.Oxalic acid and sucrose are used as chelating agent and carbon source,respectively.F127 is used as soft template.Influence of double ion(K⁺and Co²⁺)doping on electrochemical performance was studied.The result of XRD indic ates that the doped Co²⁺and K⁺do not have change nonoclinic structure of Li₃V₂?PO₄?₃,but slightly change the cell parameter and enlarge cell volume.The electrochemical results indicate the doping could decrease the capacity fading and increase the capacity retention and rate performance.The reason is that the double ion doping can reduce the charge transfer resistance and increase the diffusion coefficient of lithium ion.