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

Tunnel-type (T-type) NaMnO (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 NaMnCoO (NMO-3) electrode can deliver a high reversible capacity of 138 mAh g at 0.1C, a superior rate capability (133, 130, 121, 106, and 93 mAh g at 0.5, 1, 2, 5, and 10C, respectively), and excellent cycling stability (85.2% at 10C after 500 cycles). The substitution of Co by Mn 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.