核壳结构的富锂相层状固溶体锂离子电池材料的设计、制备及其性能改善机理研究

It is a hot research topic in the field of positive materials for Li-ion battery to prepare high capacity Li-rich layered solid soltion materials and improve their cycleability, safety, power ability and so on. In this project, a novel structure with multi-layers core-shell is put forward by us in place of the early reported core-shell and subsequent concentration-gradient core-shell structure, which can not only overcome such drawbacks of high capacity Li-rich layered materials as low cycleability, safety and power capability, but also help to clarify working mechanism of core-shell structure in core-shell structured materials. Firstly，synthesis of multi-layers core-shell structured Li-rich layered materials via coprecipitation method is explored in detail. Subsequnently, the effects of core-shell structure on elctrochemcial reaction mechanism, thermal stability of Li-rich layered materials as well as their structural stabilty during cycling are investigated, the impacts of sintering temperature and time on formation and development of core-shell structure are studied and recognized, the conductivity property of Li+ and electron in core-shell structured materials and interface between electrode and electrolyte is attempted to understand, and then the performance improvement mechanism of core-shell structure is revealed. Finally, the multi-layers core-shell structured Li-rich layered materials with high capacity as well the improved cycleability, safety and power ability can be obtained by optimizing synthesis method, designing core and shell materials, and controlling difference in compositions bewteen different layers of core-shell structure. This project can enrich the conception of core-shell, provide new stategies for designing materials in Li-ion battery, and establish theoretical and experimental base for application of Li-rich layered materials.

高容量富锂相层状固溶体材料的制备及其循环性能、安全性、倍率性能等问题一直是锂离子电池正极材料的研究热点之一。本项目创新性提出新型多级核壳结构概念，替代早期发展的简单核壳结构和随后的浓度梯度核壳结构，来解决高容量与循环性、安全性、倍率性等性能不兼顾的问题，同时也致力于澄清核壳结构材料的工作机理问题。首先努力探索共沉淀路线制备多级核壳结构材料的方法和工艺；在此基础上，调查核壳结构对材料的充放电机理、循环结构稳定性、热稳定性的影响，认识烧结温度及时间对核壳结构形成发展效应，理解核壳结构中及电极/电解质界面的锂离子/电子传导特性，进而揭示核壳结构材料的性能提高机理。最后，采用优化的合成工艺、核材料、壳材料及成分梯度，获得高容量与循环性、安全性、倍率性等性能兼顾的多级核壳结构富锂相层状材料。本课题的成功开展将丰富核壳结构概念，为锂离子电池材料设计提供新思路，为富锂相层状材料应用提供理论和实验基础。

针对下一代高比能量密度锂离子电池（>300wh/kg）用正极材料，从能量密度上看目前层状和层状富锂相材料是仅有选择。但如何克服高容量同时带来的循环稳定性和安全特性问题，是我们急需解决的难题。通过共沉淀路线制备完美核壳结构材料备显示有可能解决上述难题。根据项目申请内容，本项目具体做了如下三大方面的研究：.1）核壳结构富锂相材料的核与壳材料的探寻研究：在前期基础上，深入研究Li1.2Ni0.2Mn0.6O2-Li1.2Co0.4Mn0.4O2-Li1.2Ni0.4Mn0.4O2固溶体材料（LiNiO2-LiCoO2-LiMnO2-Li2MnO3四元四面体立体相图中用椭圆圈起的部分，Li1.2(Co0.4Mn0.4)1-x-y(Ni0.4Mn0.4)x(Ni0.2Mn0.6)yO2 (0<=x+y<=1)），探索不同的合成路线、不同的原材料、组分、锂含量、共沉淀路线工艺以及随后的烧结路线对材料性能的影响，寻找合适的核与壳材料，这是我们设计核壳结构富锂相材料关键基础；.2）核壳结构材料制备条件及提高机理研究：富锂相相材料制备、结构和电化学反应机制较为复杂，这里我们首先选取了非富锂相NCM作为研究替代对象，研究核壳结构的设计、制备及性能提高的可能机理，这里我们选取了NCM523、NCM333和NCM712作为具体研究对象，研究结果为制备优异的核壳结构富锂相材料打下坚实的基础；.3）核壳结构富锂相材料制备研究：在1）和 2）的研究基础上，选定了一些高镍高钴富锂相材料作为核材料（高容量，结构不稳定），高锰富锂相材料作为壳材料，通过控制烧结温度和时间等条件，初步研究了它们的制备和电性能，结果发现循环性能得到了提升。本项目将核壳结构的设计理念引入到层状NCM和富锂相固溶体材料中，通过特殊结构设计(核壳、多级核壳、核—浓度梯度层—壳、全浓度梯度)，克服了层状NCM和富锂相材料一些缺点，显著提高了层状NCM和富锂相材料的循环性能、热稳定性等，同时揭示了这些特殊结构材料性能提高机制，为设计锂离子电池材料提供了新思路，推动了层状NCM和富锂相固溶体材料的研究。