Preparation And Electrochemical Properties Of Li₃V₂?PO₄?₃/LiMPO₄ Composite Cathode Materials For Lithium Ion Batteries

Li₃V₂?PO₄?₃ with three-dimensional Li diffusion channel is an appealing cathode for lithium-ion battery due to the high specific capacity and potential.In this paper,the best prepared conditions of Li₃V₂?PO₄?₃in the low cost carbothermal reduction are studied.To solve the inner defect of electronic conductivity of Li₃V₂?PO₄?₃ material,other ion-doping and preparation of composites Li₃V₂?PO₄?₃/Li MPO₄(M=Fe_0.8Mn_0.2?Fe_0.5Mn_0.5)are studied.The crystal structure and morphology of Li₃V₂?PO₄?₃are characterized by SEM and XRD refinement,respectively.The way of galvanostatic charge/discharge test,EIS and CV are conducted to explore the electrochemical properties.Li₃V₂?PO₄?₃ cathode materials are prepared by carbon thermal reduction.The factors of carbon coating content?molar ratio of carbon/vanadium=1:1,2:1,3:1,4:1?,calcination temperature?700?,750?,800??and calcination time?8h,12h,16h,20h?on the performance of Li₃V₂?PO₄?₃ are investigated,respectively.The coating carbon content?the molar ratio of carbon/vanadium=2:1?,the calcination temperature?750??and the time?20h?are the optimum preparation conditions.When the cell charges/discharges at the rate of 0.1,0.2,0.5 and 1C in the potential range of 2.5-4.5V,the discharge specific capacity of Li₃V₂?PO₄?₃ is 161.2,120.3,99.8 and 90.6m Ahg^-1,respectively.The capacity retention is 83.1%after charge/discharg back to 0.1C.After Na doping,high rate performance and cyclic stability of Li_2.95Na_0.05V₂?PO₄?₃ are improved,and the discharge specific capacity is 131.5,124.1,109and 95.7m Ahg^-1 under the rate of 0.1,0.2,0.5 and 1C,respectively.However,the electrochemical properties of Li₃V₂?PO₄?₃get bad after Ti doping.The composites of x Li₃V₂?PO₄?₃-y Li Fe_0.8Mn_0.2PO₄?x:y=1:0,3:1,1:1,1:2,0:1?are synthesised via carbon thermal reduction.The cathode material of x Li₃V₂?PO₄?₃-capacity ratention at the high rate.Li₃V₂?PO₄?₃-2Li Fe_0.8Mn_0.2PO₄ display the best rate performance and discharge specific capacity,at the rate of 0.1,0.2,0.5,1,2C and 5C,the specific discharge capacity is 158,150.7,136.2,129.8,120.5 and 106.2 m Ahg^-1,respectively.After recycling at the 0.1C,the discharge capacity of 40^th cycle is still up to 154.2m Ahg^-1.The capacity rentention of Li₃V₂?PO₄?₃-2Li Fe_0.8Mn_0.2PO₄is 95.4%,93.4%and 81%(the initial capacity is 111.2m Ahg^-1)at the 150^th,280^th and 330^th cycle,respectively.From the calculation results of Randles-Sevcik equation,the lithium diffusion coefficient of Li₃V₂?PO₄?₃-2Li Fe_0.8Mn_0.2PO₄is in the order of magnitude range of 10^-7-10^-9 cm² s^-1.The composites of x Li₃V₂?PO₄?₃-y Li Fe_0.5Mn_0.5PO₄?x:y=4:1,3:1,2:1,1:1,1:2,1:3,1:4?are synthesised via carbon thermal reduction.At the rate of 0.1,0.2 and 0.5,Li₃V₂?PO₄?₃-4Li Fe_0.5Mn_0.5PO₄ releases the optimal specific capacity which is 155.9,140.7 and106.8m Ahg^-1,respectively.It still maintains the best capacity of 137.3 m Ahg^-1 and the capacity rentention of 88.1%when charge/discharge back to 0.1C.However,the2Li₃V₂?PO₄?₃-Li Fe_0.5Mn_0.5PO₄ exhibits the highest discharge capacity(91 m Ahg^-1)at the rate of 1C,and the capacity keeps in a stable value(96 m Ahg^-1).Therefore,2Li₃V₂?PO₄?₃-Li Fe_0.5Mn_0.5PO₄expresses the excellent performance at the high rate,Li₃V₂?PO₄?₃-4Li Fe_0.5Mn_0.5PO₄shows the best performance at the low rate.