Plastic deformation of LiNi0.5Mn1.5O4 single crystals due to domain orientation dynamics

The core principles and nanoscale mechanisms of ion deintercalation in battery cathode materials remain poorly understood, in particular, the relationship between crystallographic defects (dislocations, small angle grain boundaries, vacancies, etc.) and microscopic features of Li deintercalation. Here, we used operando scanning X-ray diffraction microscopy (SXDM) to investigate the local strain and lattice tilt inhomogeneities inside Li1−xMn1.5Ni0.5O4 cathode crystals (diameter from 1 to 2 µm) during electrochemical delithiation and lithiation. The technique has been combined to operando multi crystal X-ray diffraction (MCXD) to differentiate between inter- and intra-particle heterogeneity in the sample. Operando SXDM revealed three distinct domains within the crystal that displayed metastable angular rotations of the lattice during both of the phase transitions. These rotations, reaching up to 0.4°, likely arise due to a dynamic lattice mismatch between phases with different unit cell parameters coexisting within the particle. The persistent location of tilt boundaries implies the presence of inherent structural defects locally facilitating the defect formation. Residual misorientations were observed in the particle even after the full discharge suggesting an irreversible change of the lattice structure. Tilt boundaries, affecting ionic conductivity, may impede the rate capability and lead to localized strain, stress, and potential capacity fade due to cracking. However, the observed self-healing angular lattice reorganization could enable the coexistence of two phases in a single crystal. Understanding this phenomenon can optimize cathode material microstructure.