The topographic origin of chemomechanical failure in high-nickel cathodes revealed by operando dark-field X-ray microscopy

Mechanical degradation in high-nickel cathodes, driven by anisotropic volume changes, remains a primary bottleneck for the lifetime of high-energy density batteries.Yet, the nanoscale origins of strain and defect initiation within individual primary particles remain unclear.Here, in the first application of operando dark-field X-ray microscopy (DFXM) to a battery system, we directly observe a dislocation wall nucleating from a pre-existing topographical flaw on the surface of a single NMC 811 primary particle.We show that failure is not stochastic, but deterministically driven by surface topography; the flaw creates a local stress concentration of #100 MPa, exceeding the material's yield strength, while the bulk particle remains elastic.This observation establishes a direct physical link between primary particle surface morphology and the onset of irreversible plasticity.These findings prescribe a clear design rule: beyond bulk chemical stability, engineering particles with pristine, defect-free surfaces is essential for mitigating strain localisation and extending battery lifetime.