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 were 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 near edge structure spectroscopy show that while Li2FeP2S6 exhibits traditional cationic redox, Li2MnP2S6 redox involves participation and rehybridization of coupled Mn–S and S–S states. 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.