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-Na0.83K0.067Li0.10Fe0.15Mn0.75O2 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+/Na0 and faster Na-ion kinetics (10–9–10–11 cm2 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.