Ultralow-strain Ti substituted Mn-vacancy layered oxides with enhanced stability for sodium-ion batteries

Anionic redox reaction (ARR) in layered manganese-based oxide cathodes has been considered as an effective strategy to improve the energy density of sodium-ion batteries. Mn-vacancy layered oxides deliver a high ARR-related capacity with small voltage hysteresis, however, they are limited by rapid capacity degradation and poor rate capability, which arise from inferior structure changes due to repeated redox of lattice oxygen. Herein, redox-inactive Ti4+ is introduced to substitute partial Mn4+ to form Na2Ti0.5Mn2.5O7 (Na4/7[□1/7Ti1/7Mn5/7]O2, □ for Mn vacancies), which can effectively restrain unfavorable interlayer gliding of Na2Mn3O7 at high charge voltages, as reflected by an ultralow-strain volume variation of 0.11%. There is no irreversible O2 evolution observed in Na2Ti0.5Mn2.5O7 upon charging, which stabilizes the lattice oxygen and ensures the overall structural stability. It exhibits increased capacity retention of 79.1% after 60 cycles in Na2Ti0.5Mn2.5O7 (17.1% in Na2Mn3O7) and good rate capability (92.1 mAh g−1 at 0.5 A g−1). This investigation provides new insights into designing high-performance cathode materials with reversible ARR and structural stability for SIBs.