Revealing how silicon oxide accelerates calendar ageing of commercial 21700 nickel-rich lithium-ion cells

This study investigates the calendar ageing behaviour of NMC811//SiOx-graphite 21700 cylindrical cells (LGM50), which were held at varying states of charge (SoC), for up to 70 weeks at 4 different temperatures. Throughout the ageing study, capacity and internal resistance were monitored, and significant capacity loss over time, with greater loss at high SoC and temperature, were observed. A combination of electrochemical testing and post-mortem characterisation techniques point to the anode, more specifically, the silicon-oxide particles, as the driver of capacity fade through unstable solid-electrolyte interface dynamics, leading to the gradual loss of lithium inventory. Despite the greatest capacity loss being at ~80% SoC and the apparent improvement in capacity retention at higher SoCs, i.e. the “spoon-shape effect”, the degradation is most pronounced at 100% SoC, driven by the instability of the silicon-oxide SEI. This is demonstrated by post-calendar ageing cycling of the cells, where those aged at 100% SoC degraded at a faster rate than 80% SoC.