High-Energy Lateral Mapping (HELM) Studies of Inhomogeneity and Failure Mechanisms in NMC622/Li Pouch Cells

While
it is expected that inhomogeneity negatively affects battery
performance, a quantitative understanding of the influence of inhomogeneity
has remained elusive due to the difficulty of measuring it in a precise
and rapid manner. Here, the ability of high-energy synchrotron X-rays
to effectively probe the inhomogeneity in battery cathode films is
demonstrated both for fundamental studies of single-layer cathode
films and for improving manufacturing processes for industrially relevant
multilayer stacks. High-energy lateral mapping studies were performed
for very high energy density batteries (∼300 Wh/kg) made from
NMC622 cathodes and Li metal anodes, where NMC622 denotes Li­(Ni0.6Mn0.2Co0.2)­O2. It was first
demonstrated for a multilayer pouch cell (7 layers, ∼3 mm thick)
that both local and long-range variations in the NMC loading can be
precisely quantified, allowing the quality of the coating process
to be assessed. Next, it was shown that for a single cathode layer
extracted from a pouch cell battery cycled to failure that local variations
in the cathode state-of-charge (SOC) can be mapped with a sensitivity
of about 0.1%. In this manner it was possible to identify three hot
spots in which the local performance was much worse than for the rest
of the cell as well as to gain insights into the specific failure
mechanisms affecting both these local regions and the cell as a whole.

尽管普遍认为不均匀性（inhomogeneity）会对电池性能产生负面影响，但由于难以实现精准且快速的测量，学界对其影响的定量理解始终尚未清晰。
本研究验证了高能同步辐射X射线（high-energy synchrotron X-rays）可有效表征电池正极薄膜的不均匀性，其应用场景既涵盖单层正极薄膜的基础研究，也可用于优化工业级多层叠片的制备工艺。
研究针对由NMC622正极（NMC622即Li(Ni₀.₆Mn₀.₂Co₀.₂)O₂）与金属锂负极构成的超高能量密度电池（约300 Wh/kg）开展了高能侧向扫描表征。
首先针对7层、厚度约3 mm的多层软包电池（pouch cell），研究实现了对NMC负载量局部与长程变化的精准定量，由此可评估涂布工艺的质量。
随后，针对从循环至失效的软包电池中提取的单层正极片，研究实现了正极荷电状态（state-of-charge, SOC）局部变化的成像表征，灵敏度可达约0.1%。
借此，研究得以识别出三处局部性能远低于电池整体水平的热点区域，并针对影响这些局部区域乃至整个电池的具体失效机制获得了深入认知。