<i>In Situ</i> Photoacoustic Monitoring of Thermomechanical Changes in Graphite Anodes during Cryogenic Thermal Cycling

Commercial Li-ion batteries have been optimized to operate in temperate environments. While moderately high or low temperatures are known to reduce battery performance and safety, the effect of passive exposure to more extreme low temperatures remains largely unexplored. In this work, the effect of thermally cycling a Li-ion battery at a controlled rate between room temperature and cryogenic levels (83 K) was characterized using <i>in situ</i> transient grating spectroscopy. Our results show that the acoustic pulses generated by transient grating spectroscopy travel within the porous composite graphite electrode and their time-of-flight is sensitive to changes in state of charge as well as temperature. At room temperature, an increase in time-of-flight was observed when the state of charge of the composite graphite electrode was increased which is attributed to the volume expansion of the electrode. During controlled-rate cooling, a decrease in time-of-flight was observed for cells at different states of charge that is primarily ascribed to an increase in the effective Young's modulus of the porous composite graphite electrode. This claim was validated with variable-temperature, synchrotron X-ray diffraction on <i>ex situ</i> graphite electrode samples at different states of charge where minimal thermal volume contraction (<1%) of the graphite active material at different degrees of lithiation was observed during cooling to cryogenic temperatures. Upon subsequent controlled-rate warming, time-of-flight values for cells at different states of charge returned to their original values, which suggests that passive exposure to extreme low temperatures induces reversible thermomechanical changes.