Low Temperature Li Plating and Corrosion Safety Hazard in Li-Ion Batteries

The Li(Ni 0.5 Mn 0.3 Co 0.2 )/Li x C 6 (NMC532/graphite) Li-ion battery chemistry has proven to be reliable in electric vehicle battery applications due to its high capacity, good energy density, and high cyclability. 1 Under normal operating conditions (at slightly above room temperature), the long-term battery stability and response to charge/discharge is very well understood. However, there are other possible applications for Li-ion batteries where they would be forced to operate under extreme cold such as high elevation/cold climate electric vehicles, satallites, electric aircraft, unmanned underwater vehicles. At these ultra-low temperatures, the degradation mechanisms are less understood and less predictable by current modeling approaches. What is known is that destructive events 2 like thermal runaway can occur even at very low temperatures (-30 o C, which is a rated temperature for Li-ion batteries). They are caused by Li plating events where stratified mossy-like Li 0 deposition occurs around highly stressed regions (e.g. edges, regions of high curvature) while deposition is more uniform around the electrode center. Morphological studies by SEM reveal considerable fusing (i.e. indistinguishable boundaries or complete collapse of particle-particle interfaces) of graphite particles, which leads to drasitic morphological changes to the graphite electrode structure. Moreover, severe gassing and displacement of anode material can lead to significant warping of the electrode material. Low temperature volumetric expansion and contraction can cause warping of the electrode in ripple-type formation. In addition, XPS and Raman spectroscopy revealed formation of lithium carbides from the corrosion of Li x C 6 -Li 0 on the peak of the ripple-type pattern and absent at the trough. The reasoning for this anisotropic distribution of Li is attributed to the preference for electron transfer at regions of high stress (e.g. edges, peaks, and regions of high curvature). In this study, a non-thermal runaway overpressurization of the cell was found to occur after a 50Ah NMC532/graphite cell was cycled at room temperature after repetitive low temperature experiments. In addition, a parallel cell was stored at room temperature (i.e. no movement, no current, no temperature) for 2 weeks before it spontaneously went into catastrophic thermal runaway. The presentation will provide a thorough study of the 50 Ah large format NMC532 /graphite cells 3 and will elucidate low temperature degradation processes that occur, leading to thermal runaway. References 1. Harlow, J. E., Ma, X., Li, J., Logan, E., Liu, Y., Zhang, N., Ma, L., Glazier, S. L., Cormier, M. M. E., Genovese, M., Buteau, S., Cameron, A., Stark, J. E. & Dahn, J. R. J. Electrochem. Soc. 166 , A3031–A3044 (2019). 2. Gao, S., Lu, L., Ouyang, M., Duan, Y., Zhu, X., Xu, C., Ng, B., Kamyab, N., White, R. E. & Coman, P. T. J. Electrochem. Soc. 166 , A2065–A2073 (2019). 3. Ng, B., Coman, P. T., Mustain, W. E. & White, R. E. J. Power Sources 445 , 227296 (2020).