Effects of Stack Stress on Lithium-Ion Cell Degradation

Lithium-ion battery cells typically operate under compressive mechanical stress over the entirety of their service lives. This compressive stress is initially applied as a manufacturing stack pressure to maintain intimate contact between the cell components. Here we present results from long term cycling studies of lithium-ion cells in which stack stress is monitored during cycling. These results show that stack stress in lithium-ion cells is a dynamic quantity that fluctuates during charge/discharge due to charging strains and irreversibly increases with cycling due to permanent electrode expansion. Furthermore, we show that higher levels of compressive mechanical stress lead to accelerated rates of capacity fade through lithium consumption by side reactions. We explain the observed coupling between compressive stress and chemical degradation through local transport inhomogeneity that arises from localized separator deformation caused by compressive stress. A post mortem analysis of separators harvested from the aged cells confirms the occurrence of localized pore closure. Experiments of cells constructed with pre-deformed separators show accelerated rates of capacity fade compared to cells constructed with pristine separators, supporting the hypothesis of separator deformation leading to accelerated capacity fade.