Domain size of phase-separated naxcoo2 as investigated by x-ray microdiffraction

Abstract

O3-NaCoO2 is a promising cathode material for sodium ion secondary batteries (SIBs). NaxCoO2 shows phase separation (PS) into the O3 and O′ 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 NaxCoO2 at x = 0.97 and ∼1. We found that r (≈400 nm) of the O′ 3 domain is comparable to the particle size d (=331 ± 87 nm) in the as-grown O3-NaCoO2 film. This observation suggests that individual particles of NaxCoO2 are single phase to minimize the strain at the O3–O′ 3 phase boundary.