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日本語

The increasing demand for large-scale energy storage has stimulated research on developing all-solid-state sodium (Na) ion batteries using low-cost and abundantly available Na resources. The development of solid electrolytes that exhibit high ionic conductivity at room temperature is essential for the practical application of all-solid-state Na-ion secondary batteries. In order to meet this challenge, interest both in Japan and overseas has been focused on Na₃SbS₄ as the most promising candidate amongst the various solid electrolytes due to its high conductivity of 1 mS cm^-1 or higher at room temperature. However, in order to achieve the high conductivity, post processing is required through ball milling, and achieving high ion conductivity through a simpler synthetic process has been notably problematic.

In response to this challenge, the research team lead by the Doctoral student Hirotada Gamo used a liquid-phase synthesis method suitable for mass production to develop a Cl-substituted Na₃SbS₄ solid electrolyte. By partially substituting S in the Na₃SbS₄ with Cl, the ionic conductivity at room temperature was enhanced by a factor of three (0.9 mS cm^-1) compared to the sample without substitution (0.3 mS cm^-1). In order to elucidate the effect of the structural change caused by the Cl substitution on the ionic conduction properties, the ionic conduction path was visualized by precise analysis of the crystal structure.

As a result, it was demonstrated that the partial substitution of Cl for S in Na₃SbS₄ results in the formation of a crystal structure framework in which Na ions are loosely bound to S (or Cl), i.e., there is a weak electrostatic interaction between Na and S (or Cl), which promotes ion diffusion, especially in the crystallographic c-axis direction. The increase in ionic conductivity by Cl substitution is attributed to the formation of a crystal structure with a three-dimensional ion diffusion pathway.

Additionally, the team discovered that the Cl-substituted Na₃SbS₄ solids exhibit better stability against the Na anode than the non-Cl substituted samples.. This improvement in electrochemical stability leads to a reduction in the interfacial resistance between the anode and the solid electrolyte, demonstrating that Cl substitution in large amounts is effective in improving the stability against the anode..

The research team has pointed the way towards future research by discovering important design guidelines for developing an ideal solid electrolyte with desirable characteristics such as high ionic conductivity and superior electrochemical stability. They believe that the solid electrolyte developed in this research could be combined with liquid-phase coating technology to realise all-solid-state Na-ion batteries with a high storage capacity and stable cycling.