RESUMEN
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 °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 °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 NaxCrO2 as a positive electrode material for safe rechargeable sodium batteries is also discussed.
RESUMEN
New electrode materials of layered oxides, Na2/3Ni1/3Mn2/3-xTixO2 (0 ≤ x ≤ 2/3), are successfully synthesized, and their electrochemical performance is examined in aprotic Na cells. A Na//Na2/3Ni1/3Mn1/2Ti1/6O2 cell delivers 127 mA h g(-1) of reversible capacity and the average voltage reaches 3.7 V at first discharge with good capacity retention.
