层状正极材料失效机制的电子显微学研究

层状氧化物具有独特的锂离子通道结构和较高的电压窗口，在二次离子电池上是至关重要的正极材料。但在高电压下，不可逆晶体结构的剧烈演变会引起电化学性能衰减。本文利用扫描透射电子显微镜(STEM)对锂/钠离子电池的层状氧化物（P2型锰基正极材料和钴酸锂）的微观结构变化进行了电子显微学分析。通过研究分析层状氧化物的晶体结构演变，揭示了材料的晶体结构、价态变化与其电化学性能之间的构效关系。

本文分析了脱钠后的Na_2/3Ni_xCo_1/3-xMn_2/3O₂（x= 0,1/6,1/3）的原子分辨率 STEM 图像以揭示钴和镍掺杂对层状氧化物结构完整性的影响。脱钠后钴的掺杂能减轻晶格应力并保留层状结构，且只发生局部 P2-O2 相变。在掺杂镍后层状氧化物中发现了各种类型的缺陷和相变。此外阳离子混排导致的尖晶石和岩盐相的形成是不可逆容量损失的关键证据。不同过渡金属的掺杂产生的协同作用对材料结构本身的影响较为复杂，因此对三元层状氧化物正极材料在成分上进行优化从而使相变产生的影响最小化。

面对目前层状正极材料失效机制的不确定性，与裂纹产生相关的退化机制仍有待进一步阐释。本文利用聚焦离子束（FIB）系统和像差校正 STEM 研究了高压下不同电流倍率下LiCoO₂的失效机制。从不同循环倍率下LiCoO₂ 颗粒的横截面原子分辨率图像中发现，低倍率下的微裂纹周围出现了岩盐相和尖晶石相，而高倍率下的微裂纹周围没有明显的相变现象。快速的高倍率循环可以导致更大的裂纹，从而进一步导致晶内断裂。基于高分辨率 STEM图像和EELS分析，揭示了LiCoO₂ 颗粒从层状结构到尖晶石，最后到岩盐相的详细相变路径。

本论文对锂/钠离子电池层状正极材料在高电压下的失效机制进行了阐释，通过电子显微学分析对其裂纹起因有了新的认知；并发现层状氧化物的不同结构演化，分析与电化学性能之间的构效关系。这对研究人员在高电压下的正极材料研发提供理论基础。

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