Abstract
Layered oxide NaxMO2 (M: transition metal) is a promising cathode material for sodium-ion secondary battery. Crystal structure of O3- and P2-type NaxMO2 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 NaxMO2 were reproduced. We further evaluated the thermal expansion coefficients (αa and αc) along a- and c-axis at 300 K. The anisotropy of the thermal expansion was quantitatively reproduced without adjustable parameters for O3-type NaxMO2. Deviations of z from the model for P2-type NaxMO2 are ascribed to Na vacancies characteristic to the structure.
Highlights
Sodium-ion-secondary battery (SIB) stores electrochemical energy through Na+ intercalation/deintercalation process
The BAC stacking in O3 forms NaO6 octahedron, and the by the MO2 layers as BAC (BAB) stacking in P2 forms NaO6 triangular prism
Constant] and z well reproduced the experimental values for O3-type NaxMO2
Summary
Sodium-ion-secondary battery (SIB) stores electrochemical energy through Na+ intercalation/deintercalation process. Layered oxide NaxMO2 (M: transition metal) is a typical cathode material for SIBs1–3. Crystal structure of this material is categorized into two typical structures: O3 and P2 types[4]. In the P2-type NaMO2, the sodium and oxygen sheets stack as BAB resulting in the prismatic Na site. An isostructural O3-type LiMO2 is widely used as a cathode material in lithium-ion-secondary battery (LIB)[68,69]. This family is studied as a thermoelectric material[70] and a cathode material of solid oxide fuel cell (SOFC)[71]. By introducing T-linear expansion of the hard sphere, the anisotropy of the thermal expansion was quantitatively reproduced without adjustable parameter for O3-type NaxMO2
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