The energy density and lifetime of lithium-ion cells for automotive applications have both substantially improved in recent years. This enables electric vehicles in both the consumer and commercial sectors that have greater range, better reliability, and longer service lives [1]. These factors all contribute to the widespread adoption of electric vehicles. However, inhomogeneities exist within cells that contribute to uneven internal degradation and, when severe, can trigger rapid failure. These inhomogeneities can be related to manufacturing tolerances or distributions of component parameters [2], as well as gradients in internal or external conditions. As such, the reality of heterogeneous aging is a complex, multivariate problem to solve.

In this latest work, we cycle automotive-grade prismatic cells for several thousand cycles (until 10-30 % capacity fade) and harvest components for post mortem aging characterization and experimental parameter identification. The constituent electrodes are a Ni-rich layered oxide (approximately LiNi­_0.76Mn_0.15Co_0.09O₂) and graphite. In addition to microscopy and electrochemical testing on harvested electrodes, we quantify local lithium plating on graphite with our recently developed method utilizing ex-situ, ⁷Li nuclear magnetic resonance spectroscopy [3]. We believe this to be the first report of spatially-resolved lithium plating quantification in commercial lithium-ion cells. Clear patterns emerge (see Figure 1), reflecting the geometry of the cell and showing reproducible heterogeneity that cannot be attributed solely to defects in manufacture. Several tests even confirm local lithium plating and unusually rapid degradation at comparatively mild cycle conditions (slow charge/discharge, low state-of-charge, and room temperature).

The wound jellyroll design in contemporary prismatic cells enables very high packing efficiency and energy density, but includes inherently weak points from where lithium plating and internal stresses [4] can propagate. In some cases, cells achieve satisfactory lifetimes without accelerated aging. In others, heterogeneities cause extreme local degradation that triggers end-of-life even though other regions may remain relatively intact. We explore the causes of these patterns and question how such damage can occur, even under cycling conditions usually considered safe.

References
[1] P. Svens et al., IEEE Trans. Transp. Electrif., (2022), doi:10.1109/TTE.2022.3158838.
[2] D. Beck et al., Energies, 14 (2022), 3276, doi:10.3390/en14113276.
[3] Y. Fang et al., manuscript submitted, (2022).
[4] P. Gupta and P. Gudmundson, J. Power Sources, 511 (2021), 230465, doi:10.1016/j.jpowsour.2021.230465.