Synthesis, Electronic Structure, and Redox Chemistry of Li2MnP2S6, a Candidate High-Voltage Cathode Material

While significant efforts have been made to harness the large capacity of sulfide-based cathodes, there has been limited focus on increasing their voltage. Here, by a novel iodide-assisted synthesis route, we successfully synthesized lithium metal thiophosphates Li₂MP₂S₆ (M = Mn, Fe, and Co), of which Li₂MnP₂S₆ is a new compound. Electrochemical extraction of Li from Li₂FeP₂S₆ and Li₂MnP₂S₆ was performed at ∼3 V, significantly higher than other sulfide-based cathodes. Despite the similar voltages, these two materials were found to operate by very different redox mechanisms. Density functional theory calculations and X-ray absorption spectroscopy show that while Li₂FeP₂S₆ exhibits mostly traditional cationic redox, Li₂MnP₂S₆ redox involves significant participation of anionic redox. Our analysis of Li₂MnP₂S₆ is also used to contextualize recent work on other Li-rich thiophosphate cathodes. This work introduces a new synthetic route to access sulfide-based materials and sheds insights into the high-voltage redox mechanism in thiophosphate-based cathodes.