Thermal Conductivity in Aged Li-Ion Cells under Various Compression Conditions and State-of-Charge

Thermal conductivity (TC) is a parameter, which significantly influences the spatial temper-ature gradients of lithium ion batteries in operative or abuse conditions. It affects the dissipation ofthe generated heat by the cell during normal operation or during thermal runaway propagation fromone cell to the next after an external short circuit. Hence, the thermal conductivity is a parameterof great importance, which concurs to assess the safety of a Li-ion battery. In this work, an alreadyvalidated, non-destructive measurement procedure was adopted for the determination of the evolu-tion of the through-plane thermal conductivity of 41 Ah commercially available Li-ion pouch cells(LiNiMnCoO2-LiMn2O4/Graphite) as function of battery lifetime and state of charge (SOC). Resultsshow a negative parabolic behaviour of the thermal conductivity over the battery SOC-range. Inaddition, an average decrease of TC in thickness direction of around 4% and 23% was measured forcells cycled at 60◦C with and without compression, respectively. It was shown that pretension forceduring cycling reduces battery degradation and thus minimises the effect of ageing on the thermalparameter deterioration. Nevertheless, this study highlights the need of adjustment of the batterypack cooling system due to the deterioration of thermal conductivity after certain battery lifetimewith the aim of reducing the risk of battery overheating after certain product life.