Mapping Competitive Reduction Upon Charging in LiNi_{0.8}Co_{0.15}Al_{0.05}O₂ Primary Particles

Side reactions involving surface reduction play a critical role in the failure of LiNi0.8Co0.15Al0.05O2 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 1.5nm 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.涉及表面还原的副反应是锂镍钴铝氧化物（LiNi₀.₈Co₀.₁₅Al₀.₀₅O₂）作为锂离子电池（Li-ion battery）正极材料时无法达到理论容量的关键诱因。尽管其宏观层面的影响已被广泛认知，但背后的纳米尺度机制尚未完全阐明。本研究将X射线光谱学（X-ray spectroscopy）与多种X射线显微成像模式相结合，在多个荷电状态下实现了镍氧化程度的空间分辨，并揭示了仅通过整体测量所无法察觉的异质性；通过采用形貌可控的颗粒样品，我们实现了该材料的高分辨成像，进而发现单个一次颗粒内部存在镍氧化态的梯度分布。在高荷电状态下，这类梯度分布显示出潜在氧缺位区域，其向颗粒内部渗透的深度远超此前的观测结果。此外，借助表面敏感X射线联用扫描隧道显微镜（scanning tunneling microscopy）技术，我们可在材料表面直接检测氧化态：材料的第一原子层主要以二价镍（Ni(II)）形式存在，而在颗粒内部1.5nm处则已出现二价镍与三价镍（Ni(III)）/四价镍（Ni(IV)）的混合价态。上述研究结果揭示了不可逆表面转变与最终决定器件功能的体相反应之间的微妙相互作用，这将优化表面钝化策略——而该策略正是突破当前性能局限的关键所在。