Side reactions involving surface reduction play a critical role in the failure of LiNi_0.8Co_0.15Al_0.05O₂ to reach its theoretical capacity as a cathode material for Li-ion batteries. While macroscopic consequences are known, the underlying nanoscopic mechanisms are not fully elucidated. By coupling X-ray spectroscopy with several X-ray microscopy modalities, we have spatially resolved the extent of Ni oxidation at several states of charge and uncovered heterogeneity that is hidden when considering ensemble measurements alone. The use of morphologically controlled particles enabled high-resolution imaging of these materials, uncovering gradients of Ni oxidation states within individual primary particles. At high states of charge, these gradients revealed regions of possible oxygen deficiency extending deeper into the particle than previously observed. Surface-sensitive X-ray coupled scanning tunneling microscopy allows oxidation states to be measured at the material’s surface, showing predominantly Ni^II in the first atomic layer and mixtures of Ni^II with Ni^III/Ni^IV already appearing 1.5 nm into the particle. These results reveal the subtle interplay between irreversible surface transformations and the bulk reactions that ultimately define function, which will refine strategies of surface passivation that are key to overcoming current performance limitations.

中文翻译：

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在LiNi _0.8 Co _0.15 Al _0.05 O ₂一次粒子中充电时的映射竞争性还原

涉及表面还原的副反应在LiNi _0.8 Co _0.15 Al _0.05 O ₂的失效中起关键作用达到其理论上作为锂离子电池正极材料的能力。虽然宏观的后果是已知的，但潜在的纳米机制尚未完全阐明。通过将X射线光谱学与几种X射线显微镜技术相结合，我们在空间上解决了几种电荷状态下的Ni氧化程度和未发现的异质性（仅考虑整体测量时会被隐藏）。形态控制颗粒的使用可以对这些材料进行高分辨率成像，从而揭示单个初级颗粒内Ni氧化态的梯度。在高电荷状态下，这些梯度揭示出可能的缺氧区域比以前观察到的更深地延伸到颗粒中。^II在第一原子层和Ni的混合物^II用Ni ^III /镍^IV已经出现1.5纳米到颗粒。这些结果揭示了不可逆的表面转变与最终定义功能的本体反应之间的微妙相互作用，这将完善表面钝化的策略，这些策略是克服当前性能限制的关键。