Nanoimaging of solid solution domains during phase transitions inside LiNi0.5Mn1.5O4

Lithiation dynamics in many battery cathode materials remain poorly understood, because of the difficulty in differentiating inter and intra-particle heterogeneity. In this work, operando X-ray nanodiffraction microscopy is used to directly image the structural evolution inside Li1-xMn1.5Ni0.5O4 single crystals during electrochemical delithiation. Metastable domains of solid-solution intermediates are detected, but are not associated with the reaction front between the lithiated and delithiated phases as predicted by current theory. Unusually persistent strain gradients inside the single crystals suggest that the shape and size of solid solution domains is instead templated by lattice defects, which guide the entire (de)lithation process. Morphology, strain distributions, and tilt boundaries reveal that the (Ni2+/Ni3+) and (Ni3+/Ni4+) phase transitions proceed through different mechanisms, offering solutions for reducing local strain and structural degradation in high voltage spinel active materials.