Fluorine-doped Na0.65MnO2 with a layer-tunnel structure for high-rate and long-cycle-life sodium-ion cathodes

Abstract

Sodium-ion batteries (SIBs) exhibit comprehensive advantages in excellent cycling stability and high-rate performance. In this work, we present a novel cathode with a layered-tunnel coexisted structure with F ions, denoted as Na0.65MnO2-xFx. This material was confirmed the coexistence of layered and rod-like tunnel structures by advanced characterization methods. Due to the high specific capacity of the layered structure and the cycling stability of the rod-like tunnel structure, the Na0.65MnO2-xFx (x=0.2) cathode material exhibits outstanding comprehensive performance. At a rate of 0.1 C, the reversible discharge specific capacity is 124.6 mAh g−1, and notably, after 1000 cycles at a high rate of 10 C, the specific capacity is 55.9 mAh g−1 with a capacity retention rate of 98.2 %. This is significantly higher than the 34.1 mAh g−1 and 48.3 % capacity retention of pristine Na0.65MnO2. The introduction of F leads to an increase in the spacing between the oxide layers, the formation of strong metallic bonds and the formation of composite structures, which not only enhances the stability of the transition metal layer structure but also induces complex changes in the microstructure. This approach of ion doping and microstructure optimization provides a fresh perspective for the design and optimization of high-performance materials for SIBs.