Influence of reversible swelling and preload force on the failure behavior of a lithium-ion pouch cell tested under realistic boundary conditions

Safety of lithium-ion batteries plays an important role in the context of advancing electrification for vehicles. Pouch cells suffer from low structural strength and are often constrained within a battery module to guarantee mechanical integrity. The effect of constraints and SOC-dependent changes on the mechanical abuse behavior was not sufficiently investigated. A total number of 36 pouch cells were indented with a flat-end cylinder under different boundary conditions until mechanical failure and thermal runaway occurred. The pouch cells were constrained at 30 % SOC with a preload force of 0, 300 or 4000 N and charged to 0 %, 30 %, 60 % or 100 % SOC before indentation. The maximum indentation force, corresponding indentation, initial stiffness and failure behavior indicated a dependency on the preload force. The stiffness at greater indentation was similar for all boundary conditions indicating a pre-compression and flattening of unevenness. Internal stress within the separator resulted in earlier short circuit and mechanical failure for increasing preload force. The mechanical constraint led to increased gas pressure during thermal runaway. The results in this publication give rise to an additional consideration of preload force and boundary conditions imposed by a battery module in abuse testing and simulation approaches in the future.