Effect of Electrochemical Charging on Elastoplastic Properties and Fracture Toughness of Li[subscript X]CoO[subscript 2]

Abstract

Mechanical degradation of lithium-ion battery (LIB) electrodes has been correlated with capacity fade and impedance growth over repeated charging and discharging. Knowledge of how the mechanical properties of materials used in LIBs are affected by electrochemical lithiation and delithiation could provide insight into design choices that mitigate mechanical damage and extend device lifetime. Here, we measured Young's modulus E, hardness H, and fracture toughness K[subscript Ic] via instrumented nanoindentation of the prototypical intercalation cathode, Li[subscript X]CoO[subscript 2], after varying durations of electrochemical charging. After a single charge cycle, E and H decreased by up to 60%, while K[subscript Ic] decreased by up to 70%. Microstructural characterization using optical microscopy, Raman spectroscopy, X-ray diffraction, and further nanoindentation showed that this degradation in K[subscript Ic] was attributable to Li depletion at the material surface and was also correlated with extensive microfracture at grain boundaries. These results indicate that K[subscript Ic] reduction and irreversible microstructural damage occur during the first cycle of lithium deintercalation from polycrystalline aggregates of Li[subscript X]CoO[subscript 2], potentially facilitating further crack growth over repeated cycling. Such marked reduction in K[subscript Ic] over a single charge cycle also yields important implications for the design of electrochemical shock-resistant cathode materials.