Thermal transport in lithium-ion batteries: The effect of degradation

In this work, we report thermal conductivities of pristine and degraded electrodes, from commercial lithium -ion batteries. The thermal conductivities were measured with and without electrolyte solvent and at different compaction pressures. The effect of degradation on thermal transport, both internal and external, is assessed. In addition, the overall cell cooling efficiency is reported as a function of the state-of-health, and full cell thermal conductivities are estimated. A reduction of the electrode thermal conductivity of up to 65% is found for degraded material. The reduction appears to be the most extreme for dry graphite anodes. Both mechanical clamping of the cells during cycling, and cold temperatures, appear to mitigate the reduction in thermal conductivity. The cell cooling efficiency is found to decrease by 50% at a state-of-health of 70-75%. Decreased wetting, due to a reduction in the amount of electrolyte and gassing, is believed to be responsible for the discrepancy between cell cooling efficiency and thermal conductivity of the electrodes. The main cause of a reduction in full cell thermal conductivity was found to be due to a reduction in anode thermal conductivity and reduction of electrolyte solvent.