On the accuracy and simplifications of battery models using in situ measurements of Lithium concentration in operational cells

In an effort to evaluate the accuracy of various battery models we use neutron imaging which is a nondestructive in situ measurement technique that has been successfully used to track the migration of lithium between the anode and cathode electrode layers during battery operation. In this work we extend the previous results, acquired during steady state conditions, by studying the transient behavior of the lithium concentration distribution across the electrode during charging and discharging. The thermal neutron beam-line at the National Institute for Standards and Technology (NIST) Center for Neutron Research was used to measure the Lithium concentration in an operating Lithium Iron Phosphate (LFP) pouch cell battery with typical commercial electrodes. A stroboscopic imaging technique was developed to generate images with longer effective exposure time, increasing the signal to noise ratio and enabling measurement of changes in lithium concentration during high power transients. The measurement of the solid phase lithium concentration distribution across the electrode for high rate (7.5C / 3C) short duration (20-40s) pulses are compared with the simulated distributions from a commercial battery simulation software package using the same experimental conditions.