Nanoscale Detection of Intermediate Solid Solutions in Equilibrated LixFePO4 Microcrystals.

Redox-driven phase transformations in solids determine the performance of lithium-ion batteries, crucial in the technological transition from fossil fuels. Couplings between chemistry and strain define reversibility and fatigue of an electrode. The accurate definition of all phases in the transformation, their energetics, and nanoscale location within a particle produces fundamental understanding of these couplings needed to design materials with ultimate performance. Here we demonstrate that scanning X-ray diffraction microscopy (SXDM) extends our ability to image battery processes in single particles. In LiFePO4 crystals equilibrated after delithiation, SXDM revealed the existence of domains of miscibility between LiFePO4 and Li0.6FePO4. These solid solutions are conventionally thought to be metastable, and were previously undetected by spectromicroscopy. The observation provides experimental verification of predictions that the LiFePO4-FePO4 phase diagram can be altered by coherency strain under certain interfacial orientations. It enriches our understanding of the interaction between diffusion, chemistry, and mechanics in solid state transformations.