Synthesis and Characterization of (1-x)Li2FeSiO4·xLi3PO4/C as a Cathode Material for Lithium Ion Batteries

(1- x )Li 2 FeSiO 4 · x Li 3 PO 4 /C, i.e. Li 2+ x Fe 1- x P x Si 1- x O 4 /C, are proposed as a novel cathode material for lithium ion batteries. (1- x )Li 2 FeSiO 4 · x Li 3 PO 4 /C composites are prepared by a citric acid assisted sol-gel method. A complete solid solution can be formed for any value of x if the calcination temperature is high enough. However, the coexistence of Li 2 FeSiO 4 -rich and Li 3 PO 4 -rich phases is confirmed when the calcination temperature is 700°C and x is higher than 0.1 and lower than 0.9. The discharge capacity declines with an increase in x when the current density is low, which is due to the reduction in the number of Fe 2+ /Fe 3+ redox couples. When the applied current density is high enough (1 C or higher), Li 2.05 Fe 0.95 P 0.05 Si 0.95 O 4 has the highest discharge capacity and capacity retention because of its best crystallinity, lowest charge transfer resistance and highest lithium ion diffusion coefficient. Lower calcination temperature leads to better pore size distribution, higher carbon content and smaller particle size, resulting in better electrochemical performance. The carbon network wraps and connects the Li 2.05 Fe 0.95 P 0.05 Si 0.95 O 4 particles when the calcination temperature is 700°C, and it has a hierarchically porous structure. These features are of great benefit to its electrochemical performance.