Study On The Preparation And Electrochemical Performances Of Ion-doped Li₃V₂（PO₄）₃

With the increasing demand for energy, many countries have reached a consensus to looking for a high efficient, clean, economic, and security energy. Lithium-ion batteries is considered to be one of the most promising new energy due to its high energy density, and Li3V2(PO4)3 as cathode material for lithium-ion batteries has attracted much attention for researchers because of its high theoretical capacity, high discharge voltage, stable crystal structure and good cycleability. However, the demonstration of high discharge capacity and good rate capability of Li3V2(PO4)3 is restricted owing to its relatively low electronic conductivity. Based on this situation, we have synthesized Li3V2(PO4)3/C composites, Li3V2(PO4)3-xSx/C, Li3V2(PO4)3-xIx/C, and Li3V2-xAlx(PO4)3-yFy/C composites modified by ion-doping via a sol-gel method, and further performend physical characterization and examination of electrochemical properties for these as-prepared materials. The main contents and results of this study are as follows:Firstly, the series of Li3V2(PO4)3/C composites calcined at different temperatures were synthesized by a sol-gel method. Among these composites, Li3V2(PO4)3/C composite calcined at 800 oC for 8 h displays the optimal electrochemical performances. The initial discharge capacity is 169.65 mAh g–1 at the current rate of 30 mA g–1 in the voltage range of 3.0~4.8 V, and the discharge capacity of 131.72 mAh g–1 remains after 100 cycles. Even if the current density is increased to 120 mA g-1, the composite still delivers the the initial discharge capacity 156.04 mAh g–1 and the discharge capacity of 123.5 mAh g–1 after 100 cycles, indicating that this composite synthesized by this method exhibits good rate capability and cycleability.Secondly, the series of Li3V2(PO4)3-xSx/C composites with different amounts of ion doping were synthesized by a sol-gel method. Among these composites, Li3V2(PO4)2.95S0.05/C composite demonstates the optimal electrochemical performances. The initial discharge capacity is 180.02 mAh g–1 at the current rate of 30 mA g–1 in the voltage range of 3.0~4.8 V, and the discharge capacity of 143.45 mAh g–1 remains after 100 cycles. When the current density is increased to 120 mA g-1, the Li3V2(PO4)2.95S0.05/C composite delivers the the 1st discharge capacity 167.16 mAh g–1 and the 100 th discharge capacity of 132 mAh g–1, indicating that the the initial discharge capacity and rate capability of S2--doping Li3V2(PO4)3/C composite have been improved.Thirdly, the series of Li3V2(PO4)3-xIx/C composites with different amounts of ion doping were synthesized by a sol-gel method. Among these composites, Li3V2(PO4)2.96I0.04/C composite manifests the optimal electrochemical performances. The initial discharge capacity was 178.47 mAh g–1 at the current rate of 30 mA g–1 in the voltage range of 3.0~4.8 V, and the discharge capacity of 136.65 mAh g–1 remains after 100 cycles. When the current density is increased to 120 mA g-1, the Li3V2(PO4)2.96I0.04/C composite delivers the the 1st discharge capacity 166.49 mAh g–1 and the 100 th discharge capacity of 127.09 m Ah g–1, indicating that the the initial discharge capacity and rate capability of I--doping Li3V2(PO4)3/C composite have been improved.Lastly, the series of Li3V2-xAlx(PO4)3-yFy/C composites with different amounts of ion co-doping were synthesized by a sol-gel method. Among these composites, Li3V1.93Al0.07(PO4)2.99F0.01/C composite proposes the optimal electrochemical performances. The initial discharge capacity was 175.62 mAh g–1 at the current rate of 30 mA g–1 in the voltage range of 3.0~4.8 V, and the discharge capacity of 139.33 mAh g–1 remains after 100 cycles, When the current density is increased to 120 mA g-1, the Li3V1.93Al0.07(PO4)2.99F0.01/C composite delivers the the 1st discharge capacity 158.03 mAh g–1 and the 100 th discharge capacity of 124.61 mAh g–1, which is slightly higher than that of the pristine Li3V2(PO4)3/C composite. The results suggest that the the initial discharge capacity and rate capability of Al3+-F--co-doping Li3V2(PO4)3/C composite have been improved to a certain extent.