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. © 2024 American Chemical Society.

Y.-T.C, M.M, and C.J.B. gratefully acknowledge support from the University of Minnesota in the form of new faculty start-up. The authors acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing resources that contributed to the research results reported within this paper. This work used Bridges-2 at Pittsburgh Supercomputing Center through allocation MAT230036 from the Advanced Cyberinfrastructure Coordination Ecosystem: Services & Support (ACCESS) program, which is supported by National Science Foundation grants #2138259, #2138286, #2138307, #2137603, and #2138296. This research was partially supported by KAKENHI Grant Number JP20KK0124(AM) and JST PRESTO (Grant Numbers JPMJPR21Q8(AM) and JPMJPR21Q9(TM)).

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