Li-Ion polymer cells thermal property changes as a function of cycle-life

• 45 °C cycling impacts on thermal conductivity (K) of LIP cells were studied.
• Extended cycling at 45 °C reduces K-values of all LIP cells.
• K-values thought the thickness of the cell reduces more than along the length/width.
• In-plane K-values are dominated by the high K-values of current collectors.
• Current 4.35 V LIP cells have higher K-values than the past designed 4.2 V cells.

The impact of elevated temperature charge–discharge cycling on thermal conductivity (K-value) of Lithium Ion Polymer (LIP) cells of various chemistries from three different manufacturers was investigated. These included high voltage (Graphite/LiCoO2:3.0–4.35 V), wide voltage (Si:C/LiCoO2:2.7–4.35 V) and conventional (Graphite/LiCoO2:3.0–4.2 V) chemistries. Investigation results show limited variability within the in-plane and through-plane K-values for the fresh cells with graphite-based anodes from all three suppliers. After 500 cycles at 45 °C, in-plane and through-plane K-values of the high voltage cells reduced less vs. those for the wide voltage cells. Such results suggest that high temperature cycling could have a greater impact on thermal properties of Si:C cells than on the LIP cells with graphite (Gr) anode cells we tested. This difference is due to the excess swelling of Si:C-anode based cells vs. Gr-anode cells during cycling, especially at elevated temperatures. Thermal modeling is used to evaluate the impact of K-value changes, due to cycles at 45 °C, on the cells internal heat propagation under internal short circuit condition that leads to localized meltdown of the separator.