Activation of Anionic Redox for Stoichiometric and Li-Excess Metal Sulfides through Structural Disordering: Joint...

Extensive research efforts have been dedicated to Li-excess compounds with anionic redox reaction as potential high-capacity positive electrode materials for Li-ion battery applications. The origin of activation on anionic redox is still under debate, and a unified understanding is necessary, especially for sulfide-based compounds without conductive d electrons. Herein, joint experimental and theoretical study is conducted for Li-excess and stoichiometric compounds with different crystal structures, cation-disordered rocksalt, and cation-ordered layered structures. In contrast to the understanding of Li-excess oxides, sulfide-based compounds with ordered layered structures are electrochemically less active compared with the materials with disordered structure. Theoretical study reveals that a unique local structure for a sulfide ion coordinated by 6 Li ions, SLi₆ configuration, is formed, which can be found only for the disordered structure and not for the layered structure. The unique local structure triggers electron delocalization for sulfide 3p orbitals, leading to superior electronic conductivity, as experimentally evidenced, and thus anionic redox is successfully activated. Furthermore, such nonuniform local structures lead to easier structural distortion and efficient S-S dimerization, and even trimerization (S₃^2-), upon delithiation. Although sulfide-based compounds as battery electrode materials suffer from dissolution after oxidation, this practical problem is effectively mitigated by the use of highly concentrated electrolyte solutions with fewer free solvent molecules, leading to superior reversibility for anionic redox. The insights derived can guide the development of high-energy electrode materials with anionic redox with or without transition metal ions possessing conductive d electrons.