Molybdenum polysulfide electrode with high capacity for all-solid-state sodium battery

All-solid-state sodium secondary batteries have attracted significant attention as an alternative for post‑lithium-ion batteries. However, to obtain all-solid-state sodium secondary batteries with high-energy density, it is necessary to develop a high-capacity positive electrode material. In this study, two types of amorphous (a-)MoSx (x ≃ 6) active electrode materials were prepared using a mechanochemical (MC) process and through the thermal decomposition (TD) of (NH4)2Mo2S12·2H2O. Herein, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) analyses were conducted, and the results revealed significantly different local structures of a-MoSx (MC) and a-MoSx (TD). The all-solid-state sodium cells with a-MoSx (MC) and a-MoSx (TD) possessed high reversible capacities of 510 and 690 mAh g−1, as well as high-capacity retention rates of 80% and 92% for the 2nd to 30th cycles, respectively. XPS analysis of the discharge–charge products further suggested that dissociation and formation of disulfide bonds occurred reversibly during the discharge–charge reaction. This study shows that a-MoSx (MC) and a-MoSx (TD) are promising active electrode materials for all-solid-state sodium batteries.