Dilatometry study on current rate, temperature, and pressure influence on the expansion behaviour of Si-alloy/graphite - NMC622 pouch cells

Silicon as anode material gained increased attention over the last years due to its high specific capacity and abundance. The main challenge remains the large volume change upon (de-)lithiation with the consequence of high mechanical stresses and fast degradation. A thorough understanding of the expansion behaviour is crucial for a further improvement silicon-based materials. Therefore, this article provides dilatometry data for stacked pouch cells with a Si-alloy/graphite anode and a NMC622 cathode. The focus is placed on the influence of the applied pressure, the temperature, and the current rate on the cell thickness change. Upon high-rate charging, a significantly increased cell expansion has been observed. The effect is more pronounced at low temperatures and high applied pressures and is assigned to the appearance of lithium plating. The increased cell expansion is accompanied by an excess in measured charging capacity. This effect is further investigated by conducting modified C-rate tests on fresh and aged cells. The presence of plated lithium is confirmed by a post-mortem analysis. A theory based on the appearance of soft short-circuits caused by dendrites is provided as an explanation attempt for the observed effects.