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, electrode expansion is quantified from in situ neutron images taken during cycling of pouch cells with lithium iron phosphate positive and graphite negative electrodes. Apart from confirming the overall expansion as a function of state of charge and the correlation with graphite transitions that have been observed in previous dilatometer experiments we show the spatial distribution of the expansion along the individual electrodes of the pouch cell. The experiments were performed on two cells with different electrode areas during low and high c-rate operation. The measurements show how charging straightened the cell layers that were slightly curved by handling of the pouch cell during setup of the experiment. Subsequent high charging rate, that exceeded the suggested operating voltage limits, was shown to have a strong influence on the observed expansion. Specifically, during high-rate cycling, the battery showed a much larger and irreversible expansion of around 1.5% which was correlated with a 4% loss in capacity over 21 cycles.