State-of-Charge Dependence of Mechanical Response of Lithium-Ion Batteries: A Result of Internal Stress

Effect of state-of-charge on the mechanical response of lithium-ion batteries subjected to mechanical abuse loading are studied. Indentation tests are performed on a cylindrical cell and a pouch cell at different states-of-charge. The test results evidence that the mechanical behavior of the cylindrical cell is significantly state-of-charge dependent, while for the pouch cell it is almost state-of-charge independent. To clarify the cause of such a difference in state-of-charge dependence, two control tests are designed and executed. Based on the control test results, we conclude that the internal stress to resist the charging-induced volume expansion of jellyroll is the major reason for state-of-charge dependence of the mechanical behavior. We also conclude that the difference in state-of-charge dependence between the two types of cells is closely associated with the casing stiffness (stiff metallic casing vs. soft pouch) and the construction method of jellyroll (winding versus stacking), which together create different conditions of resisting the volume expansion and retaining the internal stress. A supplementary test is carried out on the pouch cells under a normal cyclic charging-discharging condition to measure the volume expansion and the reaction force of the unconstrained and constrained pouch cell respectively. (c) 2018 The Electrochemical Society.