Thermal Stability of Na_xCrO₂ for Rechargeable Sodium Batteries; Studies by High-Temperature Synchrotron X-ray...

Thermal stability and phase transition processes of NaCrO₂ and Na_0.5CrO₂ 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 °C. In contrast, for the desodiated phase, in-plane distorted P3-type layered oxide (P'3 Na_0.5CrO₂), 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 °C. On further heating, phase segregation to two P3 layered metastable phases, which have different interlayer distances (17.0 and 13.5 Å, presumably sodium-rich and sodium-free P3 phases, respectively) are observed on heating to 287-477 °C, but oxygen loss is not observed. Oxygen loss is observed at temperatures only above 500 °C, resulting in the formation of corundum-type Cr₂O₃ and O3 NaCrO₂ as thermodynamically stable phases. From these results, possibility of Na_xCrO₂ as a positive electrode material for safe rechargeable sodium batteries is also discussed.