Cation-disordered rock-salt cathode materials are featured by their extraordinarily high specific capacities in lithiumion 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 transitionmetal 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.
Ni-rich LiNixCoyMn1-x-yO2 (0.5 < x < 1) cathode materials have attracted a great deal of interests due to its high energy density and low cost. However, they are subject...
Owing to the capacity boost from anion redox activities, cation-disordered rock-salt oxides are considered as potential candidates for the next-generation of high energy density Li-ion cathode materials. Unfortunately, the anion redox process that affords ultra-high specific capacity often triggers irreversible O 2 release, which brings about structural degradation and rapid capacity decay. In this study, we present a partial chlorine (Cl) substitution strategy to synthesize a new cation-disordered rock-salt compound of Li 1.225 Ti 0.45 Mn 0.325 O 1.9 Cl 0.1 and investigate the impact of Cl substitution on the oxygen redox process and the structural stability of cation-disordered rock-salt cathodes. We find that partial replacement of O 2− by Cl − expands the cell volume and promotes anion redox reaction reversibility, thus increasing the Li + ion diffusion rate and suppressing irreversible lattice oxygen loss. As a result, the Li 1.225 Ti 0.45 Mn 0.325 O 1.9 Cl 0.1 cathode exhibits significantly improved cycling durability at high current densities, compared with the pristine Li 1.225 Ti 0.45 Mn 0.325 O 2 cathode. This work demonstrates the promising feasibility of the Cl substitution process for advanced cation-disordered rock-salt cathode materials.
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