Thermal conductivity inside prismatic lithium-ion cells with dependencies on temperature and external compression pressure

Significant temperature increases and gradients inside lithium-ion battery cells are highly likely during extreme fast charging and discharging, with adverse effect on aging and safety. To assess the temperature inside the cell, the thermal conductivity of the electrode-separator composite must be determined precisely. Furthermore, the dependency of the thermal conductivity on battery states like the cell temperature or composite layer pressure has to be considered. In this work, these dependencies are investigated for a large-format lithium-ion cell with a flat-wound jelly roll and prismatic aluminum hardcase with a Nickel Manganese Cobalt (NMC) cathode and graphite anode. A precise thermal conductivity test bench with defined compression boundaries for the battery cell is constructed in compliance with the guarded heater principle. Validation tests with a stainless steel reference material prove a high accuracy and robustness against changes in compression pressure. In addition, a novel method to obtain the thermal conductivity of the aluminum cell-case is introduced. According to the measurement result, the thermal conductivity increases by 11.9% when the compression pressure rises by 37.1 kPa, which is explained by thermal contact resistances. Compared to the absolute value at 20 ^∘C, the thermal conductivity decreases with increasing temperature by more than -1% per K.