Effect of Lithiation upon the Shear Strength of NMC811 Single Crystals

An experimental protocol is developed to measure the shear strength of NMC811 single crystals within the cathode of a lithium-ion cell. The cathode is placed upon a set of thick metallic substrates that possess a wide range of indentation hardness. For each choice of substrate, the top surface of the cathode is indented by a Vickers indenter to a sufficient depth that the cathode layer is subjected to an approximately spatially uniform compressive normal traction equal to the hardness of the substrate. The sensitivity of plastic flow and fracture of the single crystals to substrate hardness is determined by observation of the particles in the indented top surface of the cathode using a scanning electron microscope. It is found that the shear strength of fully lithiated NMC811 single crystals along their basal plane is 86 +/- 12 MPa, and decreases to 39 +/- 5 MPa upon cell charging (delithiation of the cathode). This implies that particle slip and fracture will occur under mild mechanical loading, for example by calendering during manufacture and by electrical cycling of the compacted cathode. The indentation protocol developed here has application to a wide range of single crystal cathode materials.