Na2S–NaI solid solution as positive electrode in all-solid-state Na/S batteries

All-solid-state sodium-sulfur (Na/S) batteries are promising next-generation batteries with high safety and high energy density. Sodium sulfide (Na2S) has application as active material in positive electrodes owing to its advantages such as low cost, low toxicity, and a large theoretical capacity. However, the electronic and sodium ion conductivities of Na2S are significantly low, and ascertaining the entire contribution of the active materials to cell capacities is challenging. Therefore, facilitating an electronic and ionic conduction path in the positive electrode is essential. In this study, Na2S was mixed with sodium iodide (NaI) via a mechanochemical process and was used as an active material in an all-solid-state Na/S battery. Consequently, a Na2S–NaI solid solution was formed, and the ionic conductivity of Na2S–NaI increased by five orders of magnitude compared to that of Na2S. Na2S–NaI showed large charge–discharge capacities and cycle capability. In particular, 90Na2S·10NaI showed a large capacity and high cycle efficiency, and 94% of Na2S acted as an active material. The result of this study leads to the development of all-solid-state Na/S batteries with high capacities, high rates, and high cycles.