Thermal Expansion in Layered NaxMO2

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Abstract

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

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