高镍型富锂单晶正极材料的构建及结构稳定性研究

The discharge capacity of lithium rich cathode material is more than 250 mAh/g, and it has been considered as one of the most important candidate cathode materials for the next generation of high energy density lithium ion batteries. Improve the structure and thermal stability, inhibit the formation of spinel phase and slow down the voltage drop during Li+ insertion/extraction process have been the key scientific issues and challenges of Li-rich layered cathode materials, which need to be solved urgently. By reducing the Li-rich phase amount to decrease the degree of structural phase transition, and at the same time increased the nickel content to compensate the capacity loss caused by reducing the Li-rich phase. By control the precursor preparation process and Li salt used amount to introduce nickel to Li slabs which can form a support structure to stabilize the layer structure of lithium deficient state. Then, doping La and Ce of strong metal oxide bond energy into the transition metal layers to establish a more stable hexagonal crystal system and inhibit the oxygen release. Nano scale fast ionic conductor LiLaO2 and Li3PO4 were deposited on the surface of the material during the preparation process to stabilize the lattice oxygen and improve the thermal stability. Finally, develop the single crystal particle preparation technology to improve the compaction density and safety performance. The aim of this project is to design and prepare a Li-rich material with high nickel content. Through the above means to build Lithium rich single crystal cathode material with high nickel content of a stable structure and high safety performance.

富锂正极材料的放电容量超过250mAh/g，是发展下一代高能量密度锂离子电池的重要候选正极材料之一。提高富锂材料在脱嵌锂过程中的结构稳定性和热稳定性，一直是研究者们关注并迫切希望解决的关键科学问题。本项目通过降低富锂相组分，减小因其活化所导致的结构相变，同时提高镍含量构建高镍型富锂材料，以弥补因减少富锂相所造成的容量损失；通过调控前驱体制备工艺和锂盐的用量，预先在锂层引入适量镍形成支撑，稳定脱锂态的层状结构；在过渡金属层引入具有强金属氧化键能的La、Ce以改善八面体结构的稳定性，并抑制氧析出；在材料表面原位沉积纳米级快离子导体LiLaO2和Li3PO4包覆层，稳定材料表面的晶格氧，提高热稳定性；改变二次颗粒团聚形貌，开发微米级单晶一次颗粒制备技术，提高材料的压实密度和安全性能。通过以上手段构建结构稳定性好、安全性能高的高镍型富锂单晶正极材料。

富锂材料中虽然具有比较高的放电容量，但其在充放电过程中由于Li2MnO3组分的活化和与电解液之间的副反应导致材料在循环过程中结构不稳定、放电容量和电压衰减较快。本项目主要通过对材料优化组分、构建异质结构、表面改性和单晶化研究进行性能改善。通过调控富锂材料中Li2MnO3组分并适当增大Ni含量制备出兼具高容量和高循环寿命的Li1.167Mn0.383Ni0.4Co0.05O2富锂材料，虽然其放电容量相对于Li1.2Mn0.54Ni0.13Co0.13O2有所降低（从274.0mAh g-1降到239.1mAh g-1），但其结构稳定性增强，1.0C下的容量保持率从79.3%提高到94.7%，材料结构衰减得到抑制。在材料八面体结构3a和3b位引入具有强金属氧化键能的 La3+、Ce4+以提高材料结构稳定性，La和Ce掺杂后材料循环容量保持率分别提高到86.7%和98.2%，可见Ce4+更有利于提高材料稳定性。这是由于非电化学活性的金属离子能支撑材料结构防止塌陷，同时其较大的离子半径拓宽锂离子脱嵌通道，同时改变镍和锰综合价态以改善材料结构稳定性，并引入尖晶石-层状异质结构促进锂离子脱嵌。对碳酸盐前驱体进行H2O2和KMnO4预处理研究，结果发现可以在材料中构建层状-尖晶石异质结构，并提高Mn+4含量以提高材料的结构稳定性。制备出微纳级单晶颗粒对富锂材料性能改善非常显著，经单晶化后高镍富锂材料在1.0C下循环100周的容量保持率从87.6%（团聚颗粒）增大到90.5%；单晶Li1.2Mn0.54Ni0.13Co0.13O2材料在1.0C下循环100周的容量保持率从73.4%（团聚颗粒）提高到89.0%。在富锂表面原位沉积超薄快离子包覆层Li3PO4和LiLaO2以减小材料与电解液之间的副反应，提高材料结构稳定性。对单晶高镍富锂材料包覆Li3PO4和LiLaO2其循环100周的容量保持率可达91.3 %和95.9%。单晶化工艺不仅可以促进材料结构有序度提高，还能避免团聚颗粒在循环过程中开裂所造成的副反应加剧和结构衰减。综上，本项目所研究的制备和改性手段可有效提高材料结构稳定性，能够促进富锂材料的实际应用。