In Situ Fracture Behavior of Single Crystal LiNi0.8Mn0.1Co0.1O2 (NMC811)

The mechanical properties of the key battery material NMC811 are determined by in situ compression. Individual single crystal particles withstand up to 14 GPa mean pressure, with highest loads to fracture perpendicular to the (0001)_NMC basal planes. In real-time, complex 3D fracture paths and particle rotation occur to compensate for lack of activation of 3D dislocation slip in the layered battery structure.

Abstract

Single crystal particle morphologies have become highly desirable for next generation cathode materials, removing grain boundary fracture and thereby reducing the surface area exposed to electrolyte. The intrinsic mechanical behavior of single crystal layered oxides, however, is poorly understood. Here, faceted single crystal LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) particles are compressed in situ in a scanning electron microscope (SEM), to determine mechanical deformation mechanisms as a function of crystallographic orientation. In situ, the dynamical deformation sequence observed is initial cracking at the compression zone, followed by accelerated transparticle crack propagation and concurrent (0001) slip band formation. The greatest loads and contact pressure at fracture, non-basal cracking, and activation of multiple basal slip systems in larger (>3 μm) particles, occur for compression normal to the (0001) layered structure. Loading on {012} preferentially activates basal fracture and slip at lower loads. Regardless of particle orientation, non-basal slip systems are not observed, and non-basal cracking and particle rotation occur during compression to compensate for this inability to activate dislocations in 3-dimensions. Crystallographic dependent mechanical behaviour of single crystal NMC811 means that particle texture in cathodes should be monitored, and sources of localised surface stress in cathodes, e. g. particle-to-particle asperity contacts during electrode manufacture, should be minimised.

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