Thermal Expansion in Layered Na x MO2

Layered oxide Na x MO 2 (M: transition metal) is a promising cathode material for sodium-ion secondary battery. Crystal structure of O3- and P2-type Na x MO 2 with various M against temperature ( T ) was systematically investigated by synchrotron x-ray diffraction mainly focusing on the T -dependences of a - and c -axis lattice constants ( a and c ) and z coordinate ( z ) of oxygen. Using a hard-sphere model with minimum Madelung energy, we confirmed that c / a and z values in O3-type Na x MO 2 were reproduced. We further evaluated the thermal expansion coefficients ( α a and α c ) along a - and c -axis at 300 K. The anisotropy of the thermal expansion was quantitatively reproduced without adjustable parameters for O3-type Na x MO 2 . Deviations of z from the model for P2-type Na x MO 2 are ascribed to Na vacancies characteristic to the structure.