Thermal Expansion in Layered Na _x MO₂

Area of Science:

• Materials Science
• Electrochemistry
• Solid-State Chemistry

Background:

• Layered oxide Na_xMO₂ (M: transition metal) is a key cathode material for sodium-ion secondary batteries.
• Understanding crystal structure and thermal expansion is crucial for optimizing battery performance.

Purpose of the Study:

• To systematically investigate the crystal structure of O3- and P2-type Na_xMO₂ as a function of temperature.
• To analyze the temperature dependence of lattice constants and atomic coordinates.
• To evaluate and understand the anisotropic thermal expansion behavior.

Main Methods:

• Synchrotron X-ray diffraction was employed to study crystal structures.
• Analysis focused on temperature-dependent lattice parameters (a, c) and oxygen coordinate (z).
• A hard-sphere model with minimum Madelung energy was used for theoretical validation.

Main Results:

• The c/a ratio and z coordinate for O3-type Na_xMO₂ were successfully reproduced using the hard-sphere model.
• Thermal expansion coefficients (α_a, α_c) at 300 K were evaluated, quantitatively reproducing the anisotropy for O3-type materials.
• Deviations in z for P2-type Na_xMO₂ were attributed to inherent sodium vacancies.

Conclusions:

• The hard-sphere model effectively explains the structural and thermal expansion properties of O3-type Na_xMO₂.
• The study provides quantitative insights into the anisotropic thermal expansion of these cathode materials.
• Sodium vacancies significantly influence the structure of P2-type Na_xMO₂.