Improved electrochemical properties in the Li3Fe2(PO4)3 by titanium and vanadium doping

The nominal Li_3-xFe_2-xTi_x(PO₄)_2.55(VO₄)_0.45 (x=0~0.3) compounds were synthesized by a sol-gel process. Different from the single VO₄^3- doping, further introduction of Ti⁴⁺ ion was easy to result in precipitation of a little secondary phases, besides the main NASICON products. The simultaneous substitution of Ti⁴⁺ and VO₄^3- for Fe³⁺ and PO₄^3-, respectively, in the Li₃Fe₂(PO₄)₃ resulted in a net improvement in the rate capability and cycling performance, as compared with the single Ti⁴⁺ or VO₄^3--substituted compound. The sample with x=0.2 presented a high initial discharging capacity of 125.4 mAh/g at the rate of 0.5C, about 25% higher than the Ti⁴⁺-substituted Li_2.8Fe_1.8Ti_0.2(PO₄)₃, and 102.6 mAh/g after 60 cycles at 2C, about 12% higher than the single VO₄^3--substituted one. The high rate performance between 0.5C to 10C suggested that this sample had a good stability and reversibility. These results proved that the combination of the Ti⁴⁺ substitution for Fe³⁺ with the VO₄^3- substitution for PO₄^3- was a promising method of improving electrochemical performance of the studied Li₃Fe₂(PO₄)₃ cathode material.