Research On The Preparation And Modification Of LiMn₂O₄/C Composite Cathode Materials

The spinel LiMn2O4 cathode material is a kind of the promising cathode material for rechargeable lithium ion battery due to its advantages of abundant manganese resources, low cost, non-toxic, environment friendly, high discharge specific capacity, high energy density and high working voltage. Caused by Jahn-Teller effect, dissolution of Mn3+ and decomposition of electrolyte, the fading of capacity and deterioration of cyclic performance of LiMn2O4 cathode material occur in the process of charging and discharging. To solve the above-mentioned problems, the electrochemical property of LiMn2O4 was optimized by surface modification and ion doping in this thesis. The structure, morphology and electrochemical property of materials were investigated by SEM, XRD and electrochemical tests.(1) The pure spinel LiMn2O4 cathode material was prepared by solid-state reaction with Li2CO3 and electrolytic MnO2 as raw materials. The result indicated that sample presented single spinel structure, good spinel morphology and high discharge specific capacity of 123.4mAh/g. But after 20 cycles, the capacity retention rate was only 77%, giving a rapid fading.(2) LiMn2O4/C composite materials were fabricated with carbon nanotubes and high temperature pyrolysis carbon as carbon source and as-prepared LiMn2O4 via ultrasonic liquid dispersion method with solid phase sintering to improve rate and cycle performance. The effects of annealing temperature and time and carbon coated amounts on the structure, morphology and electrochemical performance of materials were investigated. The result showed that among LiMn2O4/CNTs composite cathode materials synthesized by carbon nanotubes as carbon source, the sample coated with CNTs of amount of 5% presented the best electrochemical performance, which gave discharge specific capacity of 120.1mAh/g at 0.5C rate. After 20 cycles, capacity retention rate kept 96%. At 2C rate, it also still maintained good cycle stability. In addition, the sample coated with CNTs of amount of 5% also displayed better cycle stability. LiMn2O4/C composite cathode materials synthesized by high temperature pyrolysis carbon as carbon source also behaved excellent rate discharge property. The optimal coating amount was 3%.(3) The LiBxMn2-xO4(x=0,0.01,0.02,0.05) cathode materials were synthesized by sol-gel method. The effect of sinterring temperature and time and different Boron doping amounts on the structure, morphology and electrochemical performance of materials was investigated. The result indicated that the sample doped 1% amount of Boron gave better electrochemical reversibility after different discharge rate. Thus it can be seen that 1% amount of Boron doping can stabilize the spinel structure, which improved the electrochemical reversibility of materials. The EIS showed the Jahn-Teller effect and charge transfer impedance can be decreased by Boron doping. The CV test also showed that the Boron doping can stabilize structure of spinel, which kept better electrochemical property.