Effect of Mechanical Stress Factors on Large Format Li-Ion Cell Thermal Runaway Characteristics

Abstract

Expansion and contraction of Li-ion cell electrodes during cycling is known to exert mechanical stress and strain within cell components. During cell charge, Li-ion intercalation into the graphite anode structure increases anode swelling and thickness. Upon cell discharge, Li-ions de-intercalate from the carbon anode structure causing cell-level relaxation and a proportional reduction in cell swelling. Li-ion cell swelling has been shown to be a function of cell discharge rate, state-of-charge (SoC), and age. The impact of mechanical stress-strain swelling on Li-ion cells creates high contact pressure between electrode layers, degrades conductive carbon particle contact resistance, and irreversibly deforms the cell case structure. Mechanical stress loading also causes fatigue within internal cell components leading to an increase in cell impedance and capacity loss which may reduce cell cycle life performance. Li-ion battery (LIB) manufacturers have long recognized that the effects of cell-stack level stress fatigue on battery module architectures must be considered in LIB packaging designs requiring long cycle life performance. In LIB designs utilizing prismatic or elliptical-cylindrical cell geometries, cells are commonly packaged into modules by restraining the module stack by compression loading. In this manner, the adverse effects of cell stack deformation during charge-discharge cycling can be mitigated. In space applications, compression loading of cell modules also enables LIB’s to survive launch vehicle shock and vibration environments at high SoC’s. More importantly, mechanical abuse of LIB’s can result in an energetic catastrophic failure event. At high SoC’s, the hazards of a LIB safety incident may include venting, smoke, and fire indicative of catastrophic thermal runaway. Recently, the NASA has investigated the severity and consequences of catastrophic thermal runaway of large format Li-ion cell-based LIBs for human space flight applications. Thermal runaway safety testing of certain space-qualified Li-ion cell designs has shown that there is a correlation between applied cell compression forces and thermal runaway severity. This work focuses on the relationship between compression force and SoC on large format Li-ion cells qualified for human space flight applications. Empirical results from simulated internal short circuit trigger testing designed to induce catastrophic thermal runaway will be discussed.