Large format Li-ion cells were used to study the mechanical responses of single cells of thickness 6.5 mmand stacks of three cells under compressive loading. Various sequences of increasingdepth indentations were carried out using a 1.0 inch (25.4 mm) diameter steel ball with steel plate as a rigid support surface. The indentation depths were between 0.025" and 0.250" with main indentation increments tests of 0.025" steps. Increment steps of 0.100" and 0.005" were used to pinpoint the onset of internal-short that occurred between 0.245" and 0.250". The indented cells were disassembled and inspected for internal damage. Load vs. timecurves were compared with the developed computer models. Separator thinning leading to the short circuit was simulated using both isotropic and anisotropic mechanical properties. Our study show that separators behave differently when tested as a single layer vs. a stack in a typical pouch cell. The collective responses of the multiple layersmust be taken into account in failure analysis. A model that resolves the details of the individual internal cell components was able to simulate the internal deformation of the large format cells and the onset of failure assumed to coincide with the onset of internal short circuit.

Internal short circuit of large-format Li-ion pouch cells induced by mechanical abuse was simulated using a modified mechanical pinch test. A torsion force was added manually at ~40% maximum compressive loading force during the pinch test. The cell was twisted about 5 degrees to the side by horizontally pulling a wire attached to the anode tab. The combined torsioncompression force created small failure at the separator yet allowed testing of fully charged large format Li-ion cells without triggering thermal runaway. Two types of commercial cells were tested using 4-6 cells at each state-of-charge (SOC). Commercially available 18 Ahr LiFePO 4 (LFP) and 25 Ahr Li(NiMnCo) 1/3 O 2 (NMC) cells were tested, and a thermal runaway risk (TRR) score system was used to evaluate the safety of the cells under the same testing conditions. The aim was to provide the cell manufacturers and end users with a tool to compare different designs and safety features.

A certification process for batteries is described for NASA space vehicles and U.S. Navy requirements for systems that support human-rated systems or manned support platforms. Due to the nature of the catastrophic hazards induced by bad cell and/or battery designs, it is imperative to design stringent controls to prevent such hazards from resulting in a loss of crew, platform or mission. Testing using the relevant design configuration and environment is crucial to obtaining a safe battery for use in a human-rated environment.

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