Synergistic Effect of (K⁺+Li⁺) Cosubstitution on P2-Type Fe–Mn Oxide Cathodes for Sodium-ion Batteries

Layered Na–Fe–Mn-O cathodes are promising for sodium-ion batteries due to their low-cost and earth-abundant precursors. However, their practical realization is limited by multiple phase transitions, Jahn-Teller (J.T.) distortion associated with Mn-redox, and lower cycle life. To address these issues, herein, we develop a series of Li + - and K + -ion substituted Na–Fe–Mn-O cathodes. Among them, the (K + + Li + ) cosubstituted-Na 0.83 K 0.067 Li 0.10 Fe 0.15 Mn 0.75 O 2 cathode displays superior performance with a capacity retention of 83% after 50 cycles at 0.1 C in 4.5–1.5 V vs Na + /Na 0 and faster Na-ion kinetics (10 –9 –10 –11 cm 2 s –1 ). Electrochemical, in situ X-ray diffraction and X-ray absorption spectroscopy analyses suggest that the combined effect of (K + + Li + ) cosubstitution alleviates phase transitions, reduces the J.T. distortion, and stabilizes the cathode during cycling. The cosubstitution approach can serve as a universal strategy for developing higher cycle life cathodes for sodium-ion batteries.