Lithium plating is a key degradation mechanism which results from operating in cold temperatures (or high C-rates) and can ultimately contribute to catastrophic failure of lithium-ion batteries via short circuit. Predicting the onset of Li plating via non-invasive methods could allow for BMS’s to modify cell operating conditions to minimise cell degradation and extend cell lifetime. In order to detect Li plating reliably, the scientific community need to understand more about reversible and irreversible Li plating and how they affect one another which this work aims to address through analysing the relaxation voltage response after individual vs. successive charge pulses.

Examining the behaviour of lithium-ion batteries at extreme cold temperatures exaggerates the visual evidence for lithium plating. A two-plateau feature occurs in the voltage during cell relaxation following a constant-current charge pulse (of sufficiently high C-rate) and consists of an upper voltage plateau before the cell relaxes to its open circuit voltage (lower voltage plateau). A technique called differential voltage relaxation analysis (DVRA) was used to determine the duration of the upper plateau or ‘time period’ of Lithium stripping. Recently Mei et al. [1] explained the upper plateau is due to Li plating dominating, the mixed potential region marks the transition where intercalation begins to compete with and eventually overtakes Li plating resulting in the lower plateau which is dominated by chemical intercalation [1].

This work focuses on identifying features in the voltage profile indicative of Li plating on a commercial 21700 cylindrical cell, namely the Molicel P45B. This cell is particularly suitable for low temperature studies, as it is rated to being discharged down to -40°C. For different temperatures, we compare cell voltage features in individual pulses at different SOC’s and C-rates to those in successive pulses, for cells at beginning of life and during degradation. In addition to examining the ‘time period’ as an indicator of reversible plating, degradation mode analysis (DMA) is used to determine irreversible plating. The impact of cell state of health and heat generation on the interpretation of Li plating/stripping results from the DVRA method is studied on an instrumented cell with a thermocouple inserted into the cell core is used to identify any thermal gradients between surface and core temperatures.

References:
[1] W. Mei et al., ‘Unveiling voltage evolution during Li plating-relaxation-Li stripping cycling of lithium-ion batteries’, Energy Storage Materials, vol. 66, p. 103193, Feb. 2024, doi: 10.1016/j.ensm.2024.103193.