多晶储锂正极材料的循环失效微观机理

Li-ion Batteries, characteristic of high energy density, high voltage, low leakage current and no memory effects, are a versatile solution to a variety of emerging applications such as portable electronics, pure electric or hybrid electric vehicles and stationary storage systems. However, Li-ion batteries experience significant capacity fading owing to electrochemical shock occurred to polycrystalline positive electrodes. Inspired by this consideration, this proposed research intends to answer the fundamental question related to the long-term cyclability of polycrystalline positive electrode materials: how the microstructure of the composite interfaces evolves irreversibly and what is the corresponding capacity fading mechanism? In this research, in-depth study on microstructural evolution of the composite interfaces during charge-discharge cycling will be carried out via in-situ TEM and nanoindetation techniques. The objectives of this proposed research are to establish the relationship between physicochemical feature evolution in polycrystalline positive electrode materials and corresponding electrochemical response during charge-discharge cycles, and to understand the capacity fading mechanism of electrode materials on atomic scale. It is anticipated that the experimental results can establish a firm basis to optimize both synthesis parameters and material properties of positive electrode materials, and in turn, to fully exploit the potential of Li-ion batteries.

锂离子电池由于其能量/功率密度高、工作电压高、自放电小、无记忆效应等技术特性，在便携式电子设备、电动工具、汽车动力电源、可再生能源的静态存储等重大技术领域有着广阔的应用前景。但是，其循环寿命主要受制于多晶正极材料由于电化学冲击效应所导致的循环失效。本项目拟就多晶正极材料微结构演化与循环失效的微纳尺度力学机制这一关键科学问题，结合电化学性能测试、原位透射电镜研究与纳米力学测试分析，主要研究多晶储锂正极材料在不同的电化学循环状态（尤其是在高倍率充放电状态下），多相界面微结构演化及循环失效的微观机制，筛选出材料类型-显微组织-循环状态的优化判据，为获取长循环寿命锂离子正极材料提供理性设计原则。

随着电子设备向小型化、轻薄化、智能化、多功能化发展，以及面对动力电池轻型化、远续航、长寿命的迫切需求，对锂离子电池的能量密度和循环寿命提出了更高的技术要求。正极材料的电化学性能决定了锂离子电池的能量密度和循环寿命。正极材料在高电压循环时，晶体结构转变，库仑效率低、循环电压／容量剧烈衰减、倍率性能低下，使其性能难以得到进一步提高。因此，设计与合成高结构稳定的锂离子正极材料，研究其在高电压充放电时的结构演化机制，将为高比容量长寿命型锂离子电池的研究奠定基础。. 本项目立足于高电压正极材料，通过调控其形貌、微结构实现性能调控，并采用XRD、HRTEM、STEM、XPS等材料表征手段对在电化学循环前后的正极材料结构微观缺陷进行了表征。首先，采用原位碳热还原、梯度掺杂等表面结构调控技术制备了多种具有特定表面结构的高电压富锂层状正极材料，实现富锂层状结构正极材料的放电比容量、快速充放电能力及循环稳定性得到了明显地改善；其次，针对高比容量硅酸盐基锂离子正极材料的离子／电子导电性差和循环稳定性差的本征缺陷，采用溶胶凝胶和可控水热法，设计合成三维有序分级多孔和可控晶面暴露的Li2FeSiO4/C复合正极，实现了高比容量、优异的倍率性能及长效循环稳定性；最后，针对液态电解液的电压窗口窄和安全性差的问题，设计合成了几种固态复合聚合物电解质，结合上述高电压正极材料探索了固态锂离子电池技术的可行性。