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.