Structural Stabilization of Cation-Disordered Rock-Salt Cathode Materials: Coupling between a High-Ratio Inactive Ti 4+ Cation and a Mn 2+ /Mn 4+ Two-Electron Redox Pair.

Cation-disordered rock-salt cathode materials are featured by their extraordinarily high specific capacities in lithium-ion batteries primarily contributed by anion redox reactions. Unfortunately, anion redox reactions can trigger oxygen release in this class of materials, leading to fast capacity fading and major safety concern. Despite the capability of absorbing structural distortions, high-ratio d 0 transition-metal cations are considered to be unfavorable in design of a new cation-disordered rock-salt structure because of their electrochemically inactive nature. Herein, we report a new cation-disordered rock-salt compound of Li 1.2 Ti 0.6 Mn 0.2 O 2 with the stoichiometry of Ti 4+ as high as 0.6. The capacity reducing effect by the low-ratio active transition-metal center can be balanced by using a Mn 2+ /Mn 4+ two-electron redox couple. The strengthened networks of strong Ti-O bonds greatly retard the oxygen release and improve the structural stability of cation-disordered rock-salt cathode materials. As expected, Li 1.2 Ti 0.6 Mn 0.2 O 2 delivers significantly improved electrochemical performances and thermal stability compared to the low-ratio Ti 4+ counterpart of Li 1.2 Ti 0.4 Mn 0.4 O 2 . Theoretical simulations further reveal that the improved electrochemical performances of Li 1.2 Ti 0.6 Mn 0.2 O 2 are attributed to its lower Li + diffusion energy barrier and enhanced unhybridized O 2p states compared to Li 1.2 Ti 0.4 Mn 0.4 O 2 . This concept might be helpful for the improvement of structural stability and electrochemical performances of other cation-disordered rock-salt metal oxide cathode materials.