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

Abstract

Volume expansion of lithium-ion cells is a well-known ageing phenomenon, in which the outer and inner geometry of the cell is subject to deformation due to physicochemical reactions during ageing and operation, resulting in corresponding degradation effects such as loss of capacity and higher internal resistance in the cell. In a cell module the external deformation of the cells poses a major challenge to the mechanical design due to the resulting swelling forces.In this work, compression force and thickness measurements were performed on large prismatic lithium-ion cell with various compression magnitudes to quantify the influence of mechanical stress on state of health of the cells. The impact of the bracing is shown for lithium-ion cells cycled with 1C constant current constant voltage charge and 1C constant current discharge up to 500 cycles with 100% depth of discharge and at every 50 cycles, the capacity of the cells was determined, and electrochemical impedance spectroscopy measurements were conducted. The cells were cycled until the threshold of 80 % state of health was reached. In-situ thickness measurements showed cell expansion during charging and contraction during discharging due to lithiation as well as de-lithiation additionally the measurements showed an irreversible thickness change in the cell due to ageing phenomenon. To quantify the increase in internal resistance a distribution of relaxation time (DRT) analysis was performed on the electrochemical impedance spectroscopy (EIS) data.