Investigation of voltage and expansion hysteresis of Si-alloy-C/NMC622 pouch cells using dilatometry

Silicon is a promising candidate to replace graphite as the anode active material for lithium-ion cells due to its high specific capacity. However, the material undergoes large volume changes upon lithiation causing mechanical stress and accelerated capacity fade when used in cells. To overcome these problems, knowledge about the expansion behaviour of silicon-based cells is vital. In this study, stacked pouch cells with a Si-alloy/graphite composite anode have been investigated by means of dilatometry. Experiments have been conducted with a specifically developed measurement set-up to determine the cell expansion under well-defined mechanical pressure. Upon full charge, the cells show a reversible thickness change of approx. 3.3 % and a significant hysteresis behaviour for both the cell voltage and the thickness change. The cell expansion shows an irregularity, and the maximum cell thickness is observed at about 85 % state of charge during discharge and not when the cell is fully charged. The hysteresis is further assessed by additional electrical measurements on stacked pouch cells and single-layer cells combined with operando dilatometry. The results indicate that the expansion irregularity during discharge is the result of cathode expansion, since the Si-alloy/graphite anode does not show significant contraction in this region.