Expansion of Lithium Ion Pouch Cell Batteries: Observations from Neutron Imaging

The expansion of battery material during lithium intercalation is a concern for the cycle life and performance of lithium ion batteries. In this paper, the electrode expansion is quantified from in situ neutron images taken during cycling of pouch cells with lithium iron phosphate positive and graphite negative electrodes. The impact of various charging cycles and rates on electrode expansion was investigated. At low C-rates the measured battery thickness was a function of State of Charge SOC and independent of a small external clamping force. A 0.3 increase in total thickness was observed when charging the battery from 0 to 30 SOC, up to the stage 4-3 transition associated with the first voltage plateau. As the battery was charged beyond 30 SOC the expansion rate was reduced and then continued again after 70 SOC. The observed 0.5 expansion is attributed to a 1.7 swelling of the carbon anode however the expansion of the negative electrode could be larger if the iron phosphate electrode also contracted during battery charging. This result is consistent with previous dilatometer experiments on lithium ion batteries with graphite materials which show up to 4 increase in electrode thickness during charging of carbon. The charging rate was shown to have a strong influence on the observed expansion. During high-rate cycling 5C charge and discharge over a voltage range larger than the advisable 3.6V high voltage limit, the battery showed a much larger and irreversible expansion of around 1.5 which was correlated with a 4 loss in capacity over the duration of 21 cycles.