Accessing polyanionic redox in high voltage Li-rich thiophosphates

In the search for novel cathodes for lithium-ion batteries, Li-rich sulphides are attracting increasing interest due to the possibility of multielectron redox chemistry. However, despite the success of polyoxyanion cathodes such as LiFePO4, their thiophosphate counterparts are largely unexplored, and the understanding of their redox processes remains incomplete. Here, we investigate the Li-rich thiophosphate Li2FeP2S6 which exhibits the highest voltage (3V) for a sulphide, but shows limited lithium extraction due to phase instability at high states of charge. Isostructural transition metal™ substitutions are explored by ab initio simulations accelerated by machine learning techniques, identifying a previously unknown Mn-substituted compound, Li2Fe0.8Mn0.2P2S6, which exhibits higher capacity than the pure Fe system. HAXPES and molecular dynamics simulations indicate that Mn incorporation activates S-S dimerization and polyanionic redox beyond the TM redox capacity with no evidence of TM migration.

This study widens the possibilities to explore new thiophosphate structures as lithium-ion cathode materials.