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 Li₂Mn_0.902P₂S₆ is a new compound. Electrochemical cycling revealed Li₂FeP₂S₆ and Li₂MnP₂S₆ can both be cycled at an equilibrium voltage of ~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 show that while Li₂FeP₂S₆ exhibits traditional cationic redox, Li₂MnP₂S₆ involves participation and rehybridization of coupled Mn-S and S-S states. This work reinforces the promise of high-voltage sulfide-based cathodes for Li-ion batteries with the potential for significant capacity by combining both cationic (transition metal) and anionic (sulfur) redox.