A number of commercial LIB cells used in EVs now contain silicon in the graphitic anodes in an effort to increase energy densities. However, silicon is known to age at faster rates than graphite during battery cycling, in part due to the large volumetric expansion of silicon during lithiation.
Here we present results from a cycle-ageing study of commercial 21700 cylindrical cells which utilise Si-Gr anodes (LG M50T). We cycled cells at three different temperatures of 10°C, 25°C, and 40°C, and over two SoC ranges of 0-30% and 0-100% SoC. The resultant data were analysed using conventional methods of capacity fade and resistance increase, as well as incremental capacity analysis (ICA) and OCV-fitting. The OCV-fitting method allowed us to quantitatively track degradation modes of loss of active material (LAM) and loss of lithium inventory (LLI) during ageing. Crucially, the method used here allowed us to separately quantify the rates of degradation for silicon and graphite within the anode.
Our results show that up to 80% of the silicon capacity is lost during the first 2 kA.h of charge throughput. The temperature and SoC dependence of the observed degradation then enables us to determine the best operating conditions for limiting degradation and extending cell lifetime. The increased rate of silicon degradation relative to graphite also raises questions on the overall benefits of including silicon in the anodes of commercial cells, with a clear trade-off between energy density and lifetime.