Visualization of the Heterogeneous Shrinkage of Electrode Sheets during Charging in Lithium-Ion Batteries

Understanding the volume changes in lithium-ion batteries (LIBs) during charge and discharge cycles is crucial for improving battery lifespan and safety. These changes primarily originate from deformations in the electrode sheets, which are expected to be heterogeneous. However, existing methods often lacked spatial resolution and thus are insufficient in mapping this nonuniformity. This study introduced an optical method for mapping thickness changes in electrode sheets through image sharpness evaluation, revealing significant spatial heterogeneity in the expansion and shrinkage of electrode sheets under the operational conditions. The thickness variations of lithium iron phosphate (LFP) and lithium nickel cobalt manganese oxide (NCM) cathodes during cycling were quantitatively investigated. Results demonstrated that the electrode thickness decreased during delithiation and increased during lithiation, corresponding to changes in the crystal structure. Additional factors, such as internal stress accumulated during the manufacturing process, were also identified and analyzed. Results indicate that thickness nonuniformity is influenced by both the state of charge of particles and the evolution of internal stress during cycling. The proposed image-based method offered spatial resolution on the expansion and shrinkage of electrode sheet without interference from gas generation, contributing to a deeper understanding of capacity degradation and the development of strategies to extend the LIB lifespan.