Structural Insight into the Abnormal Capacity of a Co-Substituted Tunnel-Type Na_0.44MnO₂ Cathode for Sodium-Ion Batteries

Tunnel-type (T-type) Na0.44MnO2 (NMO) is a promising cathode material for sodium-ion batteries (SIBs) owing to its high rate performance and cycling stability compared to manganese-based layered oxides. However, the low specific capacity still restricts its practical applications. Herein, a Co-doped T-type NMO is synthesized through a facile solid-state reaction method and utilized as a cathode material for SIBs. A T-type Na0.44Mn0.9925Co0.0075O2 (NMO-3) electrode can deliver a high reversible capacity of 138 mAh g–1 at 0.1C, a superior rate capability (133, 130, 121, 106, and 93 mAh g–1 at 0.5, 1, 2, 5, and 10C, respectively), and excellent cycling stability (85.2% at 10C after 500 cycles). The substitution of Co3+ by Mn3+ leads to the enlargement of small and S-shaped tunnel spaces, which facilitates the insertion/deinsertion of Na+ into/from NMO-3 and greatly enhances its rate capability and cycling stability. Moreover, the reduced energy barriers for Na+ diffusion in small tunnels make the inactive Na+ easier to be deintercalated, which should be responsible for its high specific capacity that exceeds the theoretical capacity of T-type NMO.