Improvement Effect Of The Doping On The Electrochemical Performance Of Li_1+xV₃O₈

Layered structure Li_1+xV₃O₈ is a potential cathode material used in lithium-ion batteries next generation. It has high theoretical capacity, low price and other advantage. However, the electrochemical performance of Li_1+xV₃O₈ remarkably depends on the synthesis methods. The material synthesized by a high temperature solid-state reaction has low discharge capacity and poor cycle performance. In this dissertation, we improved the electrochemical performance of Li_1+xV₃O₈ by a doping method and by a low temperature solid-state reaction. The main study focused on below three topics.Firstly, the four cations with different valences, such as B³⁺, Bi³⁺, Ce⁴⁺ and Co²⁺, were employed to dope Li_1+xV₃O₈. The results showed that the sample with a theoretical composition of Li_1.2V_2.9Co_0.1O₈ prepared at 500℃ has a capacity of 213mAh/g at the 1st cycle and 174 mAh/g at the 40th cycle. Co²⁺ doped improve the discharge plateau of Li_1+xV₃O₈.Secondly, a series of samples doped with Co²⁺ and F^- were prepared by solid-state reaction at different temperatures. The sample with theoretical composition of Li_1.2V_2.99 Co_0.01O_7.98F_0.02 prepared at 400℃ showed better electrochemical performance than other samples do. Li_1.2V_2.99Co_0.01O_7.98F_0.02 exhibited a discharge capacity of 233 mAh/g at the 1st cycle, and 192mAh/g at the 40th cycle, improved the discharge capacity and cycle performance.Thirdly, the molar ratio of lithium, vanadium and fluorion in Li_1+xV₃O_8-yF_y was optimization at low synthesis temperature. It can be found that when x=0.3 or 0.35 in Li_1+xV₃O_8-yF_y, a single phase of the fluorion doped lithium vanadium oxides can be prepared at 480℃. The charge-discharge results showed that the sample with a theoretical composition of Li_1.35V₃O_7.98F_0.02 has the best electrochemical performance. It exhibited a discharge capacity of 280 mAh/g at the 1st cycle, and an improved plateau. Doping fluorion can also improve the thermal stability of Li_1+xV₃O_8-yF_y.