Thermal Stability of NaxCrO2 for Rechargeable Sodium Batteries; Studies by High-Temperature Synchrotron X-ray Diffraction

Thermal stability and phase transition processes of NaCrO2 and Na0.5CrO2 are carefully examined by high-temperature synchrotron X-ray diffraction method. O3-type NaCrO2 shows anisotropic thermal expansion on heating, which is a common character as layered materials, without phase transition in the temperature range of 27-527 degrees C. In contrast, for the desodiated phase, in-plane distorted P3-type layered oxide (P'3 Na0.5CrO2), phase transition occurs in the following order. Monoclinic distortion associated with Na/vacancy ordering is gradually lost on heating, and its symmetry increases and changes to a rhombohedral lattice at 207 degrees C. On further heating, phase segregation to two P3 layered metastable phases, which have different interlayer distances (17.0 and 13.5 angstrom, presumably sodium-rich and sodium-free P3 phases, respectively) are observed on heating to 287-477 degrees C, but oxygen loss is not observed. Oxygen loss is observed at temperatures only above 500 degrees C, resulting in the formation of corundum-type Cr2O3 and O-3 NaCrO2 as thermodynamically stable phases. From these results, possibility of NaxCrO2 as a positive electrode material for safe rechargeable sodium batteries is also discussed.