Synthesis,\nElectronic Structure, and Redox Chemistry\nof Li<sub>2</sub>MnP<sub>2</sub>S<sub>6</sub>, a Candidate High-Voltage\nCathode Material

While significant efforts have been made to harness the\nlarge capacity\nof sulfide-based cathodes, there has been limited focus on increasing\ntheir voltage. Here, by a novel iodide-assisted synthesis route, we\nsuccessfully synthesized lithium metal thiophosphates Li<sub>2</sub><i>M</i>P<sub>2</sub>S<sub>6</sub> (<i>M</i> =\nMn, Fe, and Co), of which Li<sub>2</sub>MnP<sub>2</sub>S<sub>6</sub> is a new compound. Electrochemical extraction of Li from Li<sub>2</sub>FeP<sub>2</sub>S<sub>6</sub> and Li<sub>2</sub>MnP<sub>2</sub>S<sub>6</sub> was performed at ∼3 V, significantly higher\nthan other sulfide-based cathodes. Despite the similar voltages, these\ntwo materials were found to operate by very different redox mechanisms.\nDensity functional theory calculations and X-ray absorption spectroscopy\nshow that while Li<sub>2</sub>FeP<sub>2</sub>S<sub>6</sub> exhibits\nmostly traditional cationic redox, Li<sub>2</sub>MnP<sub>2</sub>S<sub>6</sub> redox involves significant participation of anionic redox.\nOur analysis of Li<sub>2</sub>MnP<sub>2</sub>S<sub>6</sub> is also\nused to contextualize recent work on other Li-rich thiophosphate cathodes.\nThis work introduces a new synthetic route to access sulfide-based\nmaterials and sheds insights into the high-voltage redox mechanism\nin thiophosphate-based cathodes.