Constructing layered/tunnel interlocking oxide cathodes for sodium-ion batteries based on breaking Mn3+/Mn4+ equilibrium in Na0.44MnO2 via trace Mo doping

Sodium-ion batteries (SIBs) with Mn-based oxide cathodes have attracted much interest because of their cost effectiveness, minimal toxicity, and remarkable electrochemical activity. The typical tunnel Na0.44MnO2 cathode for SIBs exhibits stable cycling performance, but its low sodium content hinders the practical application. And the other NaxMnO2 cathodes with layered structures provide higher capacity but less satisfactory cycling. Therefore, it is of great significance to construct Mn-based oxide cathodes to integrate high specific capacity layered structure and high stability tunnel structure. Herein, the strategy of breaking the Mn3+/Mn4+ equilibrium in Na0.44MnO2 via doping trace amounts of Mo was proposed to promote the transformation from the original tunnel to the layered structure and to in-situ construct a layered/tunnel biphasic interlocking structure. The optimized interlocking layered/tunnel Na0.44Mn0.97Mo0.03O2 (NMO-3M) cathode integrates the advantages of layered and tunnel structures, and achieves highly reversible phase transition and excellent electrochemical performance, which delivers a high specific capacity of 178.9 mAh g−1 at 0.1 C and capacity retention of 77.8 % after 100 cycles at 1 C. The well-maintained P2 phase was revealed by the in-situ X-ray diffraction (XRD), demonstrating the highly reversible charge compensation and structural evolution. In addition, NMO-3M cathode shows a good storage performance in air over 270 days. This research designed a layered/tunnel interlocking structure induced by trace Mo doping to construct a stable and high capacity oxide cathode, providing a guideline for the preparation of excellent oxide cathodes for SIBs in the future.