In-Situ Nanoindentation Measurement of Local Mechanical Behavior of a Li-Ion Battery Cathode in Liquid Electrolyte

The state-of-the-art cathode materials experience severe structural and mechanical degradation over lithiation cycles albeit their small deformation. It has been a great challenge to characterize the mechanical behavior of composite electrodes in-situ and in real-time because of their environmental sensitivity and intricate microscopic heterogeneity. We use nanoindentation to measure the in-situ mechanical behavior of individual phases in a cathode composite electrode LiNixMnyCozO2. We focus on the understanding of the mechanical properties of the constituents in dry and wet conditions. We evaluate the influence of electrolyte soaking on the elastic modulus, hardness, and volume change of the conductive matrix with different degrees of porosity. More interestingly, we measure the modulus, hardness, and fracture strength of agglomerated active particles and sintered pellets, and compare their mechanical properties in the dry and liquid environment. We show that the electrolyte enhances the fracture strength of NMC agglomerated particles. The increase in interfacial strength may be a result of the additional capillary force between primary particles. Results offer mechanistic understanding of the complex behavior of composite electrodes and will feed chemomechanical models on Li-ion batteries.