New Insights into Inactive-Element Substitution in Fe/Mn Anionic Redox Cathodes

Fe/Mn-based cathodes are cost-effective for sodium-ion batteries (SIBs) but suffer from slow kinetics and poor air stability. This work detailed Ca/Mg codoped P2/O3–Na0.67Ca0.02Mn0.5Fe0.38Mg0.12O2 (NFM-CM2), elucidating how Ca/Mg codoping balanced the negative capacity impact of inert elements with enhanced anionic redox reversibility (ARR). Thus, Ca/Mg codoping allowed for a delicate trade-off between the negative impact of inert elements on capacity and the positive effect of ARR on extra capacity contribution. This issue has rarely been tackled until recently. Building upon this, NFM-CM2 demonstrated better capacity (202.9 mAh g–1 at 0.1 C), stability (97.5%/85.7%, 100/300 cycles), rate performance (91.9%), and energy density (401.6 Wh kg–1), along with admirable air stability (151.4 mAh g–1, 81.9%). Additionally, there is a more complete picture of textural evolution and charge compensation mechanisms. This research might reshape new perspectives on inactive element doping, inspiring ideas for reversible anionic redox chemistry in designing Fe/Mn-based materials.