Study On Preparation And Modification Of Spinel Lithium Manganese

Recently, spinel LiMn2O4 has been recognized as one of most promising cathode materials for lithium ion batteries because of its high discharge voltage plateau, low cost, environmental friendliness and facilitation of synthesis. However, the poor cyclability of LiMn2O4, especially at elevated temperature (55℃), has impeded its industrialization. In this dissertation, pristine, Al-doped and surface modified spinel LiMn2O4 cathode materials were prepared by solid-state method convenient to large-scale production, and the effects of reaction conditions, the content of dopant and surface coating materials on the structure and electrochemical performance of LiMn2O4 were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscope (EIS), cyclic voltammetry (CV) and the charge-discharge tests. The preparation of pristine, Al-doped and surface modified LiMn2O4 was optimized.Spinel LiMn2O4 was synthesized by two-step high temperature solid-state method including the processes of precalcination and calcination and the raw materials are LiOH-Li2COutectic molten salt and MnO2. The influences of preparation conditions on the structure, morphology and electrochemcial performance of LiMn2O4 were investigated. The results show that optimum molar ratio of Li to Mn is 1.12:2 and the optimum temperatures for precalcination and calcination are 600 and 750℃, respectively. LiMn2O4 prepared at optimum conditions displays higher capacity and better cycle performance and exhibits high capacity of 133 mAhg-1 at 0.2C and retention of 87% after 30 cycles.Based on the optimal preparation conditions for LiMn2O4, LiAlxMn2-xO4 (x 0.05,0.1,0.15) cathode materials were prepared by solid-state method using Al2O3 as Aluminium sources and the effect of doped-Al content on the structure and electrochemical performance of LiAlxMn2-xO4 was studied. The doping of Al does not change the spinel structure and the capacity of Al-doped LiMn2O4 decreases compared with the pristine one. LiAlxMn2-xO4 exhibits much better cycle performance than that of pristine LiMn2O4 and LiAl0.1Mn1.9O4 shows best cycle performance among all LiAlxMn2-xO4 samples. LiAl0.1Mn1.9O4 presents initial capacity of 117 mAh·g-1 at 0.5C at room temperature and remains capacity of 106 mAh·g-1 (retention of 94%) after 30 cycles, while LiMn2O4 exhibits initial capacity of 123 mAh·g-1 and capacity retention of 86% at 0.5C after 30 cycles. LiAl0.1Mn1.9O4 displays capacity of 116 mAh·g-1 and retention of 65% at 0.2C at 55℃, while LiMn2O4 shows capacity of 65 mAh·g-1 after 30 cycles and retention of 50% under the same conditions.The surface of LiMn2O4 was modified by fumed SiO2 and the effects of the SiO2 content on the structure and electrochemical performance of LiMn2O4 were investigated. The results show that the addition of SiO2 does not alter the spinel structure. The SiO2 coated LiMn2O4 exhibits much better cycle performance and somewhat lower discharge capacity than that of LiMn2O4. The LiMn2O4 with 4wt.% coated SiO2 shows best electrochemical performance among all SiO2 coated LiMn2O4 materials and displays initial discharge capacity of 130 mAh·g-1 at 0.2C at room temperature and retention of 95% after 30 cycles, and retention rate of 70% after 30 cycles at 0.2C at 55℃.