基于中子衍射技术研究锂离子电池高镍（≥0.9）正极材料的结构特征及元素掺杂优化机制

The high-nickel layered ternary cathode materials with high capacity, low cost, long cycle life and so on, have been paid more attention in the lithium-ion battery industry under the higher capacity and longer life requirements for power batteries. With the increase of nickel content, the structure stability of high-nickel layered ternary cathode materials will decline, which could lead to the loss of capacity and the deterioration of safety. One of the key factors closely related to these problems is the structural characteristics of the materials. The layered framework of the nickel-cobalt-manganese ternary materials is mainly composed of Ni, Co, Mn, and O elements, and the relationship between structural features and material properties for higher nickel (Ni ≥ 0.9) materials still needs to be deeply investigated. X-ray diffraction can’t distinguish Ni, Co and Mn elements in the system because their coherent scattering lengths are close. The neutron diffraction has unique advantages for the accurate analysis of Ni, Co, Mn and light elements like F for the neutron coherent scatter length difference is relatively large. This project intends to apply this technique as the main means combined with X-ray diffraction and other analytical methods, to study the crystal structure characteristics of the high-nickel layered materials and the optimization mechanism of light elements doping. The influence of anti-sites and the layer transformation of the transition metals in the crystal structure on the deintercalation of lithium, and the relationship between the composition, structure and the material stability as well as the electrochemical performances are analyzed. The high throughput calculation coupled with neutron diffraction is used to study the screening optimization of the light elements replacing oxygen. The mechanism of anion doping to improve the structural stability for high nickel layered cathode materials is also studied. The thermal failure induced by the structural change of the material is analyzed in-situ by an neutron diffraction combined with thermal analysis. The influencing factors on the optimization of the high nickel material structural characteristics and comprehensive performances are determined.

锂离子电池三元正极材料随着镍含量增高，稳定性降低，进而引起容量衰减、安全性下降等问题。与这些问题密切相关的关键因素之一是材料的组成结构及变化。三元材料的层状框架主要由Ni、Co、Mn、O元素组成，对于更高镍（Ni≥0.9）体系材料，尚有待于深入理解组成结构特征与材料性能的关系。中子衍射技术对于精确解析Ni、Co、Mn及F等轻元素具有独特的优势，本项目拟采用该技术作为主要手段，结合X射线衍射及其它分析手段，开展高镍（≥0.9）材料结构特征及轻元素掺杂的优化作用机制研究。研究晶体中反位、过渡金属层变化等结构特征及对脱嵌锂特性的影响，解析其组成结构与稳定性及电化学性能的关系；通过中子衍射技术耦合高通量计算，研究替代氧的轻元素的筛选优化，提出阴离子掺杂提高结构稳定性的作用机制；利用中子衍射-热分析联用技术原位研究材料结构特征与热失效的关系。确定该体系材料结构特征及综合性能优化的影响因素。

新能源汽车的快速推广与发展对于提供动力能量的锂离子电池性能提出了更高的要求。制约锂离子电池能量密度及安全性等的主要瓶颈是正极材料，是目前行业内的关注热点及致力于突破的关键所在。本项目聚焦于高镍三元正极材料体系，特别是超高镍（Ni≥0.9）材料，结合中子衍射技术，深入解析组成结构与材料性能的关系，在晶体成核、生长、阴阳离子掺杂调控结构、热稳定性等方面开展工作。. 研究了材料结晶、成核的生长规律，优化调控超高镍单晶材料的研制，提出了一种新型多步式的制备方法；研究了高镍层状正极材料晶体生长规律及对阳离子有序度的影响，解析微结构演变机制并建立构效关系；研制的高镍单晶LiNi0.9Co0.05Mn0.05O2正极材料在1C下的初始放电比容量达到197.6mAh g−1，在4.6V高电压1C下的放电比容量为206.1mAh g−1，在10C的高倍率下，仍然有157.2mAh g−1的比容量，表现出优异的电化学性能。. 通过第一性原理计算进行高通量筛选，确定了有利于高镍正极材料优化的阴阳离子掺杂元素Nb、F，利用中子衍射技术准确解析元素占位，结合DFT计算对其充放电过程中的结构演变等进行解析，明确Nb占据的为过渡金属3b位置，Nb掺杂有利于促进Li+/Ni2+混排效应，抑制相转变，一定程度上稳定了材料的结构，尤其在高压充放电条件下。在1C下首周放电比容量高达195.1mAh g-1，容量保持率为86%，5C下首周放电比容量为185.8mAh g-1。优化调控F掺杂量，强化了过渡金属氧八面体的结构稳定性，通过形成稳定的含氟CEI抑制锂离子在界面的传输，改善了材料的循环稳定性。在2.7-4.5V、0.1C倍率下，样品的放电比容量高达235.1mAh g-1，表现出超高的放电比容量性能，循环100周后，放电比容量为185.8mAh g-1，容量保持率为97.9%，且在10C倍率下，仍具有131.1mAh g-1的放电比容量。元素掺杂及形貌优化也提高了高镍材料的热稳定性和高温循环性能。. 这些工作的开展及取得的成果为高镍NCM正极材料特别是更高镍体系的设计及调控，提供了基础支持和有效的策略。