Accessing polyanionic redox in high voltage Li-rich thiophosphate-type materials

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 poorly understood. Here, we investigate Li2FeP2S6, a Li-rich thiophosphate-type material that exhibits the highest known voltage (3 V) for a sulfide electrode in a solid-state configuration. Through isostructural transition-metal substitutions, we identify a Mn-substituted compound, Li2Fe0.8Mn0.2P2S6, with higher capacity than the parent Fe system while maintaining high voltage. Hard X-ray photoelectron spectroscopy and ab initio molecular dynamics simulations indicate that Mn substitution increases capacity beyond the formal transition-metal redox limit by activating P2S6 polyanionic redox, involving interlayer S–S bond formation with no evidence of Fe or Mn cation migration. This demonstration of polyanionic redox in a thiophosphate-type material highlights the opportunity to explore alternative Li-rich thiophosphate structures as high-capacity lithium-ion cathodes.