Study Of The Electrochemical Performance Of Mg-Cl And Mg-Br Co-doping On Li₃V₂（PO₄）₃/C

Due to the advantages,lithium vanadium phosphate（Li₃V₂（PO₄）₃）has become a promising lithium ion cathode material.However,low conductivities are still barriers for applications of Li₃V₂（PO₄）₃（LVP）,and one of the effective methods to improve the performance of LVP materials is ion doping.Few researches related to co-doping,especially cation and anion co-doping,are reported.In this paper,Li₃Mg_0.2V_1.8(PO_4-xCl_x)₃/C and Li₃Mg_0.2V_1.8(PO_4-xBr_x)₃/C were prepared by co-doping LVP materials with Mg-Cl and Mg-Br,respectively.The purpose is enhancing the conductivity so that the electrochemical performance of LVP material can be improved.In order to investigate the effect of Mg-Cl or Mg-Br doping,the structure and electrochemical properties of the Li₃Mg_0.2V_1.8(PO_4-xCl_x)₃/C（3x=0.06,0.12,0.18 and 0.24）and Li₃Mg_0.2V_1.8(PO_4-xBr_x)₃/C（3x=0.06,0.12,0.18 and 0.24）materials are studied systematically.The LVP/C materials were prepared by different carbon atoms ratios（3.5,4.5 and5.5）and studied the electrochemical performance（LVP/C-3.5,LVP/C-4.5 and LVP/C-5.5）.The LVP/C was synthesized by 5.5 ratio of carbon atoms,which has the better electrochemical properties.The capacity retention rate is 90.7%at 0.5 C after 100 cycles.Even at 2 C or 5 C,the capacity can reach 104.9 mAh g^-1or 103.6 mAh g^-1.LVP/C materials co-doped with 5.5 carbon atoms ratio and different Mg-Cl ratios were synthesized by carbothermal reduction.XRD and NMR analyze the effect of Mg-Cl elements on LVP structure.What’s more,the four-probe conductivity test and GITT indicate the influence of co-doping on the electrical conductivity.The conductivity and lithium ion diffusion coefficient of Li₃Mg_0.2V_1.8(PO_4-xCl_x)₃/C（3x=0.12）are the largest,so that its cycle and rate performance are the excellent.In the voltage of3.0 V-4.3 V（vs.Li⁺/Li）,the capacity of LVMPCl-0.12/C is 124.0 mAh g^-1after 100 cycles at 0.5 C.In addition,the capacity reaches 121.0 mAh g^-1at 1 C.Therefore,the optimal material is Li₃Mg_0.2V_1.8(PO_4-xCl_x)₃/C（3x=0.12）.After research of Mg-Cl co-doping,different Mg-Br ratios were synthesized by carbothermal reduction.The XRD shows that the Mg-Br elements do not change the crystal structure of LVP.Additionally,EDS and XPS demonstrate successfully that Mg-Br elements are co-doped into LVP/C materials.The cycle and rate performance of Li₃Mg_0.2V_1.8(PO_4-xBr_x)₃/C（3x=0.18）material are better.In the voltage range of3.0 V-4.3 V（vs.Li⁺/Li）,the capacity can reach 122.7 mAh g^-1after 100 cycles at 0.5 C.Even at 1 C,the capacity of Li₃Mg_0.2V_1.8(PO_4-xBr_x)₃/C（3x=0.18）still reaches 122.7 m Ah g^-1.Thus,the optimal material Li₃Mg_0.2V_1.8(PO_4-xBr_x)₃/C（3x=0.18）.