Tailoring the redox-active transition metal content to enhance cycling stability in cation-disordered rock-salt oxides

Abstract

Lithium-excess cation-disordered rock-salt oxides (DRXs) are investigated intensively as cathode materials for future lithium-ion batteries combining cationic and anionic redox reactions. However, the lattice oxygen redox can cause severe oxygen release resulting in rapid capacity fading. Here, we investigate a series of xLi2TiO3-(1 - x)LiMnO2 (0 ≤ x ≤1) materials and find that only Li1.2Mn0.4Ti0.4O2 (x = 0.4) and Li1.1Mn0.7Ti0.2O2 (x= 0.2) can form phase-pure DRXs, which both deliver high capacity (> 250 mAh g−1). The newly discovered Li1.1Mn0.7Ti0.2O2 DRX exhibits remarkably high capacity retention of 84.4% after 20 cycles compared to only 60.8% for Li1.2Mn0.4Ti0.4O2. Our result indicates that the irreversible oxygen loss is reduced by raising the Mn content. Theoretical calculations further reveal that increasing the redox-active Mn content from Li1.2Mn0.4Ti0.4O2 to Li1.1Mn0.7Ti0.2O2 causes the orbitals near the Fermi level to change from O 2p non-bonding (LiOLi unhybridized orbitals) to (MnO)* antibonding bands, exhibiting a high OO aggregation barrier, preventing O2 release and resulting in sustained capacity retention. Hence, these new findings demonstrate that regulating oxygen redox by tailoring the redox-active transition metal content is an effective strategy to enhance the cycling stability of DRXs.