面对越来越大的资源压力和环境危机，发展可再生的清洁能源已成为人类社会的共识，其中，可充电锂电池具有能量密度高，便携，安全，环保的优点，已成为当前最普及的便携储能设备。而它的性能在很大程度上取决于组成材料，特别是正极材料。Ni-Co-Mn三元系材料（NCM）具有优异的电化学性能，是当前最具应用潜力的锂离子电池正极材料之一。其中，富镍的NCM材料具有更高的能量密度，但是，随着镍含量的增加，NCM材料也面临着结构稳定性下降、循环稳定性与倍率性能变差的问题。因此，当前NCM材料面临的主要挑战是在不牺牲稳定性和倍率性能的情况下实现电池的高容量。本论文首先根据目前NCM体系在理论计算研究中存在的问题，对计算方法进行了探索：探究了不同势能泛函，包括PBE +U、PBE +opt-PBE-vdW及SCAN泛函对NCM体系的电子结构性质、点阵性质、充放电性质等计算结果的影响，证实了PBE+opt-PBE-vdW方法是能够正确预测NCM材料性质及行为的计算方法；并通过团簇展开方法，拟合了NCM体系的脱锂结构与能量的预测模型，从而将确定大体系的NCM结构的稳定脱锂构型所需的计算量减少至约原本的千分之一。在优化计算方法，解决NCM体系在电子结构、脱锂构型的计算研究上的问题后，本论文重点围绕NCM材料的多元掺杂改性展开，使用第一性原理计算方法，根据NCM材料的三种典型掺杂位点：锂位点，过渡金属（TM）位点及氧位点，选择Na、Ti、F元素，进行单一元素掺杂及双元素组合共掺杂的性能对比研究，探究掺杂对正极材料各方面性质的影响，包括掺杂稳定性、结构稳定性、充放电电压、锂离子扩散能力、缺陷形成的难易程度，发现Na掺杂具有支柱离子的典型作用，能够抑制点阵变形，但它最合适的掺杂策略可能为单一掺杂；Ti掺杂能够有效抑制Li/Ni反位缺陷与O空位缺陷的形成，同时在共掺杂中具有较好的协同效果；F掺杂在与阳离子共掺杂时，能够表现出远超单一掺杂时的效果；而共掺杂的协同效应来源于掺杂原子对对局域环境的复杂的共同作用。本论文在对这一系列掺杂策略进行系统的、针对性研究的基础上，了解了不同掺杂元素对之间的协同效应及背后的机理，解答了多组分掺杂策略中目前仍然存在的疑问，并提出Ti-F共掺杂作为NCM811材料掺杂改性的最佳策略。
The development of renewable clean energy has become the consensus of human society to meet the surging energy needs. Rechargeable lithium batteries show the advantage of high energy density, safety and environmental friendliness, which is believed to be the most popular portable energy storage devices. The performance of lithium batteries depends on its components, especially the cathodes. Ni-Co-Mn ternary cathodes (NCM) show excellent electrochemical properties and are currently one of the most promising cathode materials for lithium-ion batteries. In particular, Ni-rich NCMs exhibit higher capacity, but also show lower cycle stability with the Ni content increasing. Therefore, the prior challenge for NCM cathodes is to achieve high capacity without sacrificing stability and rate capability. Considering the problems existing in the theoretical study of NCM system, this paper firstly explored the influence of calculation methods and functionals on the calculated electrochemical properties of NCM, including PBE + U, PBE + opt-PBE-vdW and SCAN functional. It is confirmed that PBE + opt-PBE-vdW can correctly predict the properties and behavior of NCM materials. Further, based on cluster expansion method, the paper constructed a structure-energy fitting model to predict ground state configuration of delithiated NCM, which reduces the calculation loads to about one thousandth.After optimizing the calculation methods, the paper carried a systematic study on the doping modification of NCM system, especially the synergistic effect of multi-component doping. To address the typical modification function according to doping sites, Na, Ti and F are selected as the representative elements for Li, TM and O-site doping as well as low valence cation, high valence cation and anion doping respectively. The single doping cases and their combined co-doping cases were investigated and analyzed, revealing the role of each dopant according to doping sites and the collaborative effects of doping pairs. The paper reveals the effects of doping on various properties of cathode materials, including doping stability, structural stability, charge-discharge voltage, Li diffusion ability and defect formation, assessing the effects of different doping strategies on the electrochemical performance of Ni-rich NCM. The results are as follows: Na-doping acts as typical pillar ion and inhibits lattice distortion, however, the most suitable doping strategy for Na may be single-doping. Ti-doping can inhibit the formation of defects effectively and exhibits quite good synergetic effect in co-doping. F-doping shows much better performance in cation-F co-doping cases. The synergetic effects of co-doping originate in the combined interaction of doping pair on the local environment. Based on the systematic research on typical-site doping and their combined co-doping, this paper reveals the synergetic effects of co-doping and the mechanism, answering the existed questions in multi-component doping studies. In conclusion, the paper proposes Ti-F co-doping as the best doping strategy for NCM811 cathodes.
