富锂锰基正极材料的实际放电比容量优于市场上常见的钴酸锂、磷酸铁锂和三元正极材料，可达250 mAh/g以上，同时其锰元素含量相对较高，减少了对价格相对高昂的钴元素的使用，可以降低成本，减少对环境的破坏，因此具备发展前景，是下一代锂离子电池正极材料最有前途的候选材料之一。但富锂锰基正极材料还存在一些不足，如首圈库伦效率低、倍率性能差、电压衰减等，限制了其推广应用，需要探索更佳的制备方法和工艺，通过改性手段优化其性能。本文选取无钴富锂锰基正极材料Li1.2Ni0.2Mn0.6O2作为研究对象，顺应无钴化趋势，以降低成本、保护环境。探究了实验室条件下共沉淀-高温固相法制备原始样品Li1.2Ni0.2Mn0.6O2的工艺，对其中影响较大的高温固相反应温度对材料的影响进行了分析，确定了最佳反应温度为900℃。为了进一步改善原始样品电化学性能，对上述材料进行La元素掺杂改性，制备Li1.2Ni0.2Mn0.6-xLaxO2(x=0.002, 0.005, 0.01)，探究掺杂量的影响，确定了最佳掺杂量为x=0.005。微量La掺杂可以提升材料的性能，首圈放电比容量可达217.9 mAh/g，较未掺杂样品提升22.6%，首圈能量密度达794.1 Wh/kg，较未掺杂样品提升22.0%，0.2 C下循环100圈的容量保持率为93.0%，同时具有更高的锂离子扩散系数。经分析，La元素的掺入未发挥扩大层间距的作用，但在样品颗粒表面形成了一层岩盐相薄膜，有利于Li+的传输，同时也减少了电解液的侵蚀。此外La-O键键能大于Mn-O键，也能够减少氧的不可逆释放，减少过渡金属离子迁移，从而提升材料的电化学性能。此外，本文对熔盐法制备的工艺也进行了探究，通过熔盐法可以降低反应温度，材料的电化学性能也得到了提升，首圈库伦效率最高可提升至96.24%，首圈放电比容量最高可提升至257.7 mAh/g。实验确定了熔盐法最佳反应温度和熔盐用量，前驱体与熔盐物质的量的比为1:2，反应温度为800℃时制备的样品具有最佳的综合性能，与高温固相法合成的材料相比，其粒径更小、比表面积更大，有利于锂离子的传输，同时其兼具良好机械强度，电池容量、首圈库伦效率、倍率性能都得到了提升。
The actual discharge specific capacity of lithium-rich manganese-based cathode material material is better than that of lithium cobalt acid, lithium iron phosphate and terpolymer cathode material which are common on the market. It can reach more than 250 mAh/g. At the same time, the manganese content of lithium-rich manganese-based cathode material is relatively high. The less use of expensive cobalt element can reduce the cost and reduce the damage to the environment, so it has the development prospect and becomes one of the most promising candidates for cathode materials of the next generation lithium-ion battery. However, its low initial coulomb efficiency, poor rate performance and voltage decay limit the application of it. It is necessary to explore better preparation methods and processes and optimize its performance through modification. In this thesis, cobalt-free lithium-rich manganese-based cathode material Li1.2Ni0.2Mn0.6O2 is selected as the research object in order to conform to the trend of cobalt-free cathode, reduce costs and protect the environment. The preparation process of basic material Li1.2Ni0.2Mn0.6O2 by coprecipitation-high temperature solid phase method under laboratory conditions is investigated. The influence of reaction temperature on the material is analyzed. The best reaction temperature is determined to be 900℃. In order to further improve the performance of the basic materials, the above material is doped with La element to prepare Li1.2Ni0.2Mn0.6-xLaxO2(x=0.002, 0.005, 0.01). The influence of doping amounts on the material is explored. It is proved that a small amout of La doping can effectively improve the electrochemical performance, and the best doping amount is determined to be x=0.005. The discharge capacity of the first circle is 217.9 mAh/g, which is 22.6% higher than that of the undoped sample. The energy density of the first circle is 794.1 Wh/kg, which is 22.0% higher than that of the undoped sample. The capacity retention rate of 100 cycles at 0.2 C is 93.0%. And the lithium-ion diffusion coefficient is higher than that of the undoped sample. According to the analysis, the addition of La element does not expand the layer separation. However, a layer of rock salt film forms on the surface of the sample particles. It is beneficial to the transmission of lithium ion. The erosion from electrolyte is reduced as well. In addition, La-O bond energy is greater than Mn-O bond, which can also reduce the irreversible release of oxygen and reduce the migration of transition metal ions. So the performance of the material is improved.In addition, the process of preparing material by molten salt method is also explored. The reaction temperature can be reduced by molten salt method. The electrochemical performance of the materials has been improved. The maximum coulomb efficiency of the first circle can be increased to 96.24%. And the maximum specific discharge capacity of the first circle can be increased to 257.7 mAh/g. The best reaction temperature and amount of molten salt were determined by the molten salt method. The sample prepared at 800℃ with the ratio of precursor to molten salt substance of 1:2 has the best comprehensive performance. Compared with the pristine material, the sample has smaller particle size and larger specific surface area, which promotes the transmission of lithium ions. At the same time, it also has good mechanical strength, improved battery capacity, initial coulombic efficiency and rate performance.
