| Because that lithium ion power batterys (LIPBs) possess some outstanding advantages, such as large energy density, high working voltage, environment friendly, LIPBs have a broad application prospect in the field of large power tools, electric vehicles, energy storage, and so on. As an important factor influencing the performance of LIPBs, cathode materials that have high energy density, long life and excellent safety propeties will undoubtedly become the worldwidely research focus. Due to its poor thermal stability performance, LiCoO2 cathode material, which keeps ahead in portable electronics products, is difficult to achieve the security demanding for power battery, thus, it is highly required to develop a substitute for LiCoO2 cathode material. Spinel LiMn2O4 (LMO) cathode materials possess three-dimensional channel for lithium ions, a high voltage platform (about 3.9 V vs. Li+/Li), resource-rich and low cost, environment friendly, as well as good safety, thus, LMO is an ideal candidate for LIPBs cathode materials. As the increase of the demand for battery capacity, power, safety and environment friendly, LMO has become one of the major cathode materials for LIPBs. However, the low reversible cycle capacity (110-120 mAh/g) and fast fading capacity under high temperature (55 ℃), greatly limited the industrial development of LMO. Therefore, for industrial applications of LiMn2O4, there still a lot of work need to improve. In this dissertation, we analyzed LMO cathode materials firstly, and found that the modified LMO cathode material has better stability. Then, mixing with nickel cathode material NCA could improve the energy density of the battery. Through a series of optimizing methods, the high temperature performance of LMO/NCA composited battery was improved. Finally, the batteries fabricated by the meliorative design were with higher energy density, better low temperature performance and rate cycle, lower cost than LiFePO4 batteries. The main results of this dissertation are as follows:1. LMO and NCA materials were analyzed by particle size distribution (DLS), BET, ICP, XRD and other methods, and found that LMO, functionalized with overdose lithium or doped with other elements, possesses more stable crystal structure. The increase of Mn valence would suppress the Jahn-Teller effect. The lattice constant of charge state reduced 1.5% to the discharge state. Decreasing the specific surface area would avoid the dissolution of Mn. Compared the processing performance, discharge platform, security performance and other aspects with pure LMO battery, LMO/NCA battery is with increasede packing density and suppressed dissolution of Mn, however, the thermal stability would reduce. The comprehensive comparison indicated that the suitable ratio was LMO:NCA= 80:20. Compared the performance of pure LMO and LMO/NCA batteries, the weight/volume energy density of LMO/NCA battery increased more than 10%, the thickness expansion under 60℃ storage reduced, the capacity retention rate increased about 7%, and the capacity retention rate at 60℃1 C 500-cycles increased by 3%. While the performance of low temperature and rate cycle were similar.2. To optimize the performance of LMO/NCA battery, various anode materials, separators, electrolytes, and conditions were tested. Among natural graphites with different morphology and particle size, the spherical graphite was significantly better than the flake graphite for high temperature (55℃) cycle performance. The influence of separator materials (PP and PE) on the performance of high temperature cycle was not obvious. The commercial electrolyte would effectively suppress the thickness expansion during storage. The self-made electrolyte with C additives could significantly improve the high temperature storage and cycle performance of the battery, and the low temperature and rate cycle performance were also improved. By using the battery clamps, it could effectively suppress the thickness expansion of the batteries, and increase the capacity retention rate and recovery rate. The temperature inflection point of battery storage performance was at about 70℃, while voltage inflection point was at about 4.10 V. We designed LMO/NCA battery with the silimar specification of LiFePO4 materials, and found that the performance of energy density, low temperature rate, high temperature storage and cycle characteristics were improved, the safety was eligible, and the cost was lower. |
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