Thermal Expansion in Layered Na MO.

Layered oxide Na MO (M: transition metal) is a promising cathode material for sodium-ion secondary battery. Crystal structure of O3- and P2-type Na MO 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 MO were reproduced. We further evaluated the thermal expansion coefficients (α and α ) along a- and c-axis at 300 K. The anisotropy of the thermal expansion was quantitatively reproduced without adjustable parameters for O3-type Na MO. Deviations of z from the model for P2-type Na MO are ascribed to Na vacancies characteristic to the structure.