| The energy densigy of lithium ion batteries mostly depends on the cathode materials. However, the capacities of the mostly used cathode materials are all below 200mAh g-1, which seriously restricts the large-scale application of lithium ion batteries in field of electric automobile. Therefore, developing novel cathode materials with high capacity for lithium ion batteries becomes a hot research topic. In this thesis, on the basis of general review of current research and development of cathode materials for lithium ion batteries, layered-lithium-rich composites Li[LixNiyMnzCo1-x-y-z]O2 with high discharge capacity and high voltage plateau were taken as the research object. The influences of synthesis methods on the composition, structure, and electrochemical performances of Li[LixNiyMnzCo1-x-y-z]O2 were systematically studied.Composites Li[LixNi0.34-xMn0.47Co0.19]O2 (0.18≤x≤0.21) were prepared by a solid state reaction method assisted with co-precipitating. The effect of transition-metal salts (i.e., acetate, sulfate, chloride, and nitrate of the transition-metal salts) on the eleetrochemical performance of Li[LixNi0.34-xMn0.47Co0.19]O2 (0.18≤x≤0.21) cathode material were investigated. The sample prepared from nitrate exhibites the best electrochemical performance. The first discharge capacity at the current density of 20 mA g-1 can reach up to 295.6 mAh g-1, and no obvious capacity fading was observed after cycling 30 cycles at different rates. The improved electrochemical performance of sample prepared using nitrate of transition-metal salts was attributed to the increased Li2MnO3 phase.Three composite samples with the same chemical composition Li[Li0.19Ni0.14Co0.14Mn0.53]O2 were prepared by high-temperature solid-state reactions but with different cooling methods (i.e., quenching in liquid nitrogen and in air, as well as naturally cooling in furnace). It was found that cooling methods have no apparent impacts on the phase compositions and particls size, but improved the integrity of layered structure, decreasd the mixing degree of cations, improved the stability of SEI film, and decreased the polarity in charge-discharge process. Meanwhile, quenching in liquid nitrogen improved the electrochemical activity of Li2MnO3 component in layered-type lithium-rich composite. As a consequence, the sample quenched in liquid nitrogen was found to show a best electrochemical performance, as represented by an initial discharge capacity of 285.6 mAh g-1 at a current density of 20 mA g-1. When the current density increased up to 500 mA g-1, the initial discharge capacity was as high as 153.6 mAh g-1, showing a capacity retention of 86% even after 30 cycles. Such an optimum electrochemical performance has been ascribed to the best layered structure and improved activation of the component Li2MnO3 in the composite.We systematicly investigated the crystal structure, morphology, conductivity and electrochemical characteristics of Li[Li1.04Mn0.47Ni0.25Co0.14]O2 composition synthesized with various lithium precurors, i.e. lithium acetate, lithium hydrate, lithium carbonate, and lithium nitrate. The sample synthesized with lithium carbonate has better hexagonal ordering, higher activity of MnO2 formed after the first charger and the smallest surface layer resistance and charge transfer resistance, which contributed to its highest capacity and best rate capability with initial discharge capacity of 279.4 mAh g-1 at a current density of 20 mA g’1 and 187.2 mAh g"1 at a current density of 500 mA g-1, respectively. Gompartively, the smaller surface area of sample using LiOH·H2O as staring material exhibited a stable charge-transfer resistance, and an excellent cycle performance, which gives to its best cycling stability with capacity retention rate of still as high as 92% after 30 cycles when the current density is increased to 500 mA g-1.A novel room-temperature solid-state method with oxalic acid and nitrates as raw material has also been explored to prepare lithium-rich layered manganese oxides Li1.13Ni0.25Mn0.50Co0.12O2. It was found that the composite has the best electrochemical performance when it was prepared at 350℃ for 3 h, then at 900℃ for 16 h, and quenching in air. |
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