富锂锰基材料表面异质层的构建及其储锂特性

Design and synthesis of cathode material for lithium ion batteries (LIBs) with high rate capability and long cycle life is one of the key issues to accelerate the development of LIBs with high property. Based on the relationship between the surface structure of the material and the electrochemical performances, this project is focused on the re-construction of the surface layer. A new interface is established on the pristine surface of Li-rich manganese based material using hydro/solvothermal solution or molten salt containing the component of Li-rich material itself as the ‘second phase’. Then, the structure of the interface will be re-constructed under the set condition and a new reasonable surface which is called the surface heterosphere is obtained to improve the rate capability and cycle stability of Li-rich manganese cathode material. Then, the problem of lattice mismatches is well avoided, when traditional surface coating using materials with different lattices is applied, such as oxides, carbon, fluoride and so on. The relevance between the experiment condition and secondary reconstruction is further studied. And the rule between the factors of interface reaction (ionic exchange degree, migration rate) and structure of the heterosphere (thickness, ionic order, environment of elements, structure of the defect) is also revealed. Thus, under the condition of maintaining high energy density of the main phase, ionic doping reconfiguration, vacancy formation and crystal orientation change in the surface heterosphere are efficiently formed to improve the surface electronic conductivity, enhance the electrode kinetics processes and restrict the surface phase transformation. It offers new experimental basic and theoretical principle for the development of lithium ion batteries with high energy density, high power density and long cycle life.

设计合成具有高倍率性能和长循环寿命的锂离子电池正极材料，是加速高性能锂电池发展的核心问题之一。本项目拟从富锂锰基材料表面结构和电化学性能的关联性出发，利用含有材料自身成分的“第二相”（水热/溶剂热液、融熔盐）与材料原始表面接触形成新的界面，通过界面二次重构来构建新的表面层——表面异质层，从而避免了传统异相晶体结构（氧化物、碳、氟化物等）包覆过程中晶格失配的问题。系统研究二次重构工艺参数与表面异质层结构的关系，揭示离子交换程度、迁移速率等界面反应因素与异质层厚度、有序度、元素化学环境及缺陷结构之间的规律。使材料在保持体相高能量密度的同时，借助表面异质层中离子掺杂混排、空位及晶面取向转变的有效形成，提高材料表面电子电导、加速电极动力学过程及抑制表面层相变的发生，为高能量密度、高功率密度、长循环寿命的富锂锰基锂电池正极材料的研究提供新的实验依据和理论基础。

随着能源问题、环境问题的日益突出及科技的快速发展，人们对锂离子电池的能量密度、功率密度和寿命提出了更高的要求。正极材料容量的提升将直接决定整个电池的能量密度。因而，高容量富锂锰基锂离子电池正极材料的开发研究对于高性能锂离子电池的发展具有十分重要的意义。然而富锂锰基材料自身电子电导率低以及循环过程中输出电压和容量的衰退成为其实际应用的主要瓶颈。本项目首先采用快速微波法制备了富锂锰基正极材料。然后以低温熔融盐浸渍法成功实现了富镧、铈、铝的表面异质层的构建，得到了具有表面异质层的富锂锰基锂离子电池正极材料。电化学性能测试表明，具有表面异质层结构的电极材料，其高倍率性能及循环稳定性都得到了极大的提升。电池在0.02、0.1、0.2、0.4、1和2 A g−1的电流密度下，可分别实现280.7、244.7、218.7、196.3、162.8和132.6 mA h g−1的比容量。同进，在经过100次循环后（1 A g−1的电流密度下），容量扔可达138.8 mA h g−1。超高电流下（5 A g−1），500次循环后，仍能保持59.3 mA h g−1的容量，容量保持率都在80%以上。电极动力学研究结果表明，由于表面异质层的构建，可以在充放电过程中有效保护富锂锰基正极材料免受电解液的腐蚀，阻止材料结构由表面向内部破坏，显著降低了电池的电荷转移阻抗和极化电阻，提高了锂离子传递速率，使得材料不仅具有优异的倍率性能，同时在循环过程中保持良好的稳定性。研究还发现，这种表面异质层的形成，避免了传统异相晶体结构包覆过程中晶格失配问题及包覆层在反复充放电过程中瓦解脱落的问题，使得正极材料的使用寿命大幅提高。综上所述，本项目的成果有效解决富锂锰基材料循环过程中表面结构退化，输出电压和容量衰退的问题，为高能量密度、高功率密度、长循环寿命的锂离子电池的生产与开发提供新的实验依据和理论基础。