Mechanical degradation of lithium-ion battery (LIB) electrodes has been correlated with capacity fade and impedance gr.owth over repeated charging and discharging. Knowledge of how the mechanical properties of materials used in LlEs 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-Ic via instrumented nanoindentation of the prototypical intercalation cathode, LixCoO(2), after varying durations of electrochemical charging. After a single charge cycle, E and H decreased by.up to 60%, while K-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-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-IC reduction and irreversible microstructural damage occur during the first cycle of lithium deintercalation from polycrystalline aggregates of LixCoO(2), potentially facilitating further crack growth over repeated cycling. Such marked reduction in K-Ic over a single charge cycle also yields important implications for the design of electrochemical shock-resistant cathode materials. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected]. All rights reserved.