Mechanical Properties of Cycled Single Crystal LiNi0.8Mn0.1Co0.1O2 (NMC811) Particles

Single crystal (SC) particle morphologies are attracting significant attention as an alternative to polycrystalline (PC) secondary particles within battery cathodes, to circumvent the degradation paths associated with weak grain boundaries. In the pristine state, the key cathode material LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) exhibits anisotropic mechanical behaviour due to its trigonal crystal lattice. Here the mechanical properties of cycled SC NMC811 particles are evaluated in real time using in situ compression in a scanning electron microscope (SEM), as a function of both particle orientation, and electrochemical charge‐discharge rate. After 100 cycles, the SC NMC811 particles retain their external morphology, however their non‐basal and basal plane fracture strengths systematically decrease as a function of increasing charge rate C/10→2 C, consistent with accelerated lattice degradation. For all charge rates, the cycled and discharged NMC811 single crystal particles retain the crystallographic dependence of their strength and deformation mechanisms, with cycled SC particles strongest for compression normal to the (0001) layered structure. The accelerated mechanical softening of cycled NMC811 SC particles at higher C‐rates occurs in parallel with degradation of the electrochemical performance of the NMC811 single crystals, and indicates a higher risk of fracture‐related degradation processes with fast‐charging regimes.