Domain Size of Phase-Separated NaxCoO2 as Investigated by X-Ray Microdiffraction

Resumen

O3-NaCoO ${}_2$ is a promising cathode material for sodium ion secondary batteries (SIBs). Na ${}_x$ CoO ${}_2$ shows phase separation (PS) into the O3 and O ${}^{\text{'}}$ 3 phases in the Na concentration range of 0.89 $?x?$ 0.99. In order to estimate the domain size (r) in the two-phase region, we performed X-ray microdiffraction (XRMD) of thin films of Na ${}_x$ CoO ${}_2$ at x = 0.97 and ~1. We found that r (?400 nm) of the O ${}^{\text{'}}$ 3 domain is comparable to the particle size d (=331 ± 87 nm) in the as-grown O3-NaCoO ${}_2$ film. This observation suggests that individual particles of Na ${}_x$ CoO ${}_2$ are single phase to minimize the strain at the O3?O ${}^{\text{'}}$ 3 phase boundary.