Unlocking high performance in Na-Fe-Mn-O Cathode through Phase-Transition-free Cycling

Abstract

Fe-Mn-based Na-layered oxide cathodes suffer severe phase transitions at high states of charge, accompanied by transition metal migration and large-volume collapse. We propose the incorporation of small amount of Li⁺ into the transition metal layer to enable phase-transition-free cycling across a wide state-of-charge (SOC) range. In contrast to the previously studied undoped or Li/Mg/Ni doped P2-type Na-Fe-Mn-O cathodes, our P2-Na0.7Li0.127Fe0.127Mn0.746O2 (NLFM) cathode which has an optimized Li, Fe and Mn content and a disordered transition-metal layer, exhibits the remarkable property of complete suppression of O-type layer formation up to the highest charge voltage of 4.5 V. The cathode demonstrates reversible high-voltage Fe and O redox reactions without any phase transition,. The P to O phase transition, accompanied by transition-metal-migration and □-O-□ (□ = vacancy) formation, is essential for activation of O-redox in undoped Fe-Mn cathodes. The highly ionic character of Li-O bonds in NLFM increases the electron density over the O orbitals, making O-redox feasible in P2-phase even in the absence of □-O-□. By retaining its P2 phase at the highest state of charge, the cathode shows a negligible volume change of less than 1.8% thereby yielding high energy density along with excellent retention. The improved electrochemical performance of Li-doped cathode arises from the dopant-induced static and dynamic disorder, ensuring structural stability across a wide SOC range.