Impact of Electrochemical Charging on Fracture Toughness and Elastoplastic Properties of LiCoO2

Capacity fade and impedance growth of lithium ion battery (LIB) electrodes have been correlated with mechanical degradation of active particles through extended cycling.  The design of fracture-resistant electrodes requires understanding of material mechanical properties such as elastic modulus E, hardness H, and fracture toughness K_Ic over the entire range of compositions through which the active materials are cycled.  In this work, we measured E, H, and K_Ic of the prototypical layered material Li_XCoO₂ using instrumented nanoindentation, varying the composition, X, through varying durations of galvanostatic charging.  We found that within a single charge cycle, E and H decreased by up to 60%, while K_Ic decreased by up to 70% compared to the fully-lithiated state. X-ray diffraction, Raman spectroscopy, and optical and electron microscopy demonstrated that the drop in mechanical properties was correlated with extensive grain boundary fracture and attributable to Li depletion at the sample surfaces.  Such differences in mechanical properties of Li-storage materials can impact the critical length scales (either grain size or particle size) that mark the boundary between fracture-resistant and fracture-prone electrode designs.  This work demonstrates that when fracture toughness is used in battery design, it cannot be assumed to be constant over the course of charge cycling. This evidence of electrochemical-history-dependent fracture toughness is particularly important to the design of Li-storage compounds that undergo significant chemical expansion or phase changes during electrochemical cycling.