Mapping Lattice Distortions in LiNi_0.5Mn_1.5O₄ Cathode Materials

Spinel LiNi0.5Mn1.5O4 (LNMO) can adopt two crystallographic structures: an ordered P4332 structure and a disordered Fd3̅m structure. The disordered phase is associated with the reduction of a small amount of Mn4+ to Mn3+. LNMO single-crystals likely contain local regions of both ordered and disordered regions, which ensemble-averaged characterizations fail to distinguish. Herein, we employ high-spatial-resolution synchrotron X-ray nanodiffraction techniques to identify lattice distortions and structural defects in LNMO samples with octahedral and plate-like morphologies containing ∼6% and ∼22% of Mn3+, respectively. Differences in properties between the two particles give rise to different distributions of lattice variations, which may indicate differences in phase distributions. Bragg coherent diffraction is also used to observe phase heterogeneities in single grains. Lattice distortions and structural defects could shut down or open up local diffusion pathways for lithium ions, making lithium ion diffusion more complicated and potentially more tortuous than that in a perfect LNMO lattice.