Triggering anionic redox activity in Fe/Mn-based layered oxide for high-performance sodium-ion batteries

Among various sodium-ion cathode materials, Fe/Mn-based layered oxides stand out due to cost-effectiveness and relatively high theoretical specific capacity. However, further enhancement in capacity and improvement in cyclability are still needed to meet the requirements for practical applications in sodium-ion batteries. Herein, we report that ruthenium-doped Na0.67Fe0.5Mn0.5O2 can not only achieve high reversible capacity but also deliver superior cycling stability. Owing to the substitution of 4d ruthenium heteroatoms, the cathode exhibits promising electrochemical performance with a higher reversible specific capacity of 170 mA h g(-1) at 0.2C be-tween 2 and 4 V as well as a stable cycling performance with 82.2% capacity retention at 2C after 100 cycles compared to its pristine counterpart. Advanced structural characterization techniques combined with theoretical calculations unveil that the presence of ruthenium ions can trigger anionic redox activity, thus leading to harvesting of extra capacity. Moreover, ruthenium ions can also play an important role in stabilizing and improving the structural framework, resulting in prominent cyclability and excellent rate performance. Overall, the present work demonstrates that anionic redox activity could be triggered by integration of trace 4d element in Fe/Mn-based layered oxides and represents an effective strategy to develop high-performance sodium cathodes.