LiNixMnyCozO2 (NMC) is the current choice of cathode for high-performance Li-ion batteries. The structural and mechanical stability of NMC plays a vital role in determining the electrochemical performance of batteries. However, the dynamic mechanical properties of NMC during Li reactions are widely unknown because of the microscopic heterogeneity of composite electrodes as well as the challenge of mechanical measurement for air-sensitive battery materials. We employ instrumented nanoindentation in an inert environment to measure the elastic modulus, hardness, and interfacial fracture strength of NMC of a hierarchical meatball structure as a function of the state of charge and cycle number. The mechanical properties significantly depend on the lithiation state and degrade as the electrochemical cycles proceed. The results are further compared with the properties of bulk NMC pellets. We perform first-principles theoretical modeling to understand the evolution of the elastic property of NMC on the basis of the electronic structure. This work presents the first time systematic mechanical measurement of NMC electrodes which characterizes damage accumulation in battery materials over cycles. (c) 2017 The Electrochemical Society. All rights reserved.