Highly Reversible Anionic Redox in Tunnel-Type Oxides Contributes to Enhanced Capacity as Cathode Material for Sodium-Ion Batteries

Tunnel-type Na 0.44 MnO 2 has emerged as a promising cathode material for sodium-ion batteries (SIBs) owing to its exceptional structural stability. However, its practical application is significantly constrained by the intrinsically limited movable sodium content in the pristine structure, yielding a modest initial charge capacity of merely 55 mAh g –1 . While anionic redox reactions provide a viable pathway for capacity enhancement, which have been exclusively studied in layered oxides to date. In this work, we demonstrate the first successful implementation of highly reversible anionic redox activity in tunnel-type oxides through Ti-substituted Na 0.55 Mn 1– x Ti x O 2 ( x = 0.22, 0.33, 0.39). The optimized Na 0.55 Mn 0.61 Ti 0.39 O 2 cathode delivers an enhanced first charge capacity of 91.6 mAh g –1, featuring a stable plateau around 4.1 V, which is contributed by oxygen redox activity, as verified by soft X-ray absorption spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Combined with scanning transmission electron microscopy (STEM) images, it is revealed that oxygen oxidation occurs concomitantly with Na + extraction from hexagonal channels. Remarkably, in situ X-ray diffraction and STEM characterizations indicate the exceptional structural integrity of the tunnel framework throughout electrochemical cycling, demonstrating its resilience against anionic redox-induced degradation. In addition, the final Na 0.55 Mn 0.61 Ti 0.39 O 2 |hard carbon full cells could deliver an impressive energy density of 190 Wh kg –1 with good cycling stability and minimal voltage hysteresis. This work explores a fresh perspective for designing high-performance SIB cathodes through synergistic cationic–anionic redox chemistry in stable tunnel-type oxides.