Accessing polyanionic redox in high voltage Li-rich thiophosphates

In the search for novel positive electrode materials for lithium-ion cells, Li-rich sulfides are attracting increasing interest. Despite the success of polyoxyanion-based cathodes such as LiFePO4, their thiophosphate counterparts have remained largely unexplored. Here, the Li-rich thiophosphate Li2FeP2S6, which exhibits the highest known voltage (3 V) for a sulfide electrode, is investigated in a solid-state configuration. Through examination of isostructural transition-metal substitutions, we identify a novel Mn-substituted compound, Li2Fe0.8Mn0.2P2S6, with higher capacity than the parent Fe system while maintaining the high voltage. Hard X-ray Photoelectron Spectroscopy and ab initio molecular dynamics simulations indicate that Mn substitution activates P2S6 polyanionic redox involving interlayer S--S bond formation with no evidence of Fe or Mn cation migration, and increases capacity beyond the formal transition-metal redox limit. This demonstration of polyanionic redox in a thiophosphate material highlights the opportunity to explore alternative Li-rich thiophosphate structures as high-capacity lithium-ion cathodes.