Influence from Mechanical Stress on State of Health of Large Prismatic Lithium-Ion-Cells under Various Temperatures

Abstract

The expansion of lithium-ion cells is an aging phenomenon that causes deformation of the cell’s external and internal geometry due to physicochemical reactions during aging and operation. This deformation leads to degradation effects such as capacity loss and increased internal resistance in the cell. In a cell module, expansion of the cells presents a challenge to the mechanical design due to resulting swelling forces. This work presents expansion measurements performed on large prismatic lithium-ion cells cycled at 1 C for up to 1000 cycles at different ambient temperatures and constant compression forces to evaluate the impact of mechanical stress on cell health. Intermediate tests were conducted every 50 cycles to determine cell capacity and perform electrochemical impedance spectroscopy measurements. Thickness measurements showed cell expansion during charging and contraction during discharging due to lithiation and de-lithiation. Additionally, an irreversible change in cell thickness occurred due to aging. Electrochemical impedance spectroscopy data were analyzed using distribution of relaxation time analysis to quantify the increase in internal resistance. The results suggest that compression force has a negligible impact on cells cycled at high temperature. However, at lower temperatures, higher compression force resulted in more rapid aging compared to lower compression force.