Iron-mediated reversible lattice-oxygen redox enables stable 200 Wh kg−1 sodium-ion batteries

Lattice-oxygen redox in layered oxides can enhance energy density, but its limited reversibility causes structural instability during deep cycling. Here we develop an iron-mediated strategy to regulate lattice-oxygen redox in layered oxide cathodes. In the Na 2/3 Mn 7/12 Mg 1/4 Fe 1/6 O 2 cathode, Fe ions act as redox mediators, with Fe 4+ capturing electrons from lattice oxygen during charging and Fe 2+ donating electrons back to oxidized oxygen during discharging through chemical pathways. With the assistance of iron mediation, the reversibility of lattice-oxygen redox is dramatically improved from 75% to 99%. As a result, the lattice-oxygen-activated cathode enables a sodium-ion pouch cell to achieve an energy density of 206 Wh kg −1 and operate stably for 100 cycles at 50 mA g −1 , with a capacity retention of 87.8%.