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.