Unlocking high performance in a Na–Fe–Mn–O cathode through phase-transition-free cycling

Abstract

Fe–Mn-based Na-layered oxide cathodes suffer from severe phase transitions at high states of charge, accompanied by transition metal migration and large-volume collapse. We propose the incorporation of a 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-Na_0.7Li_0.127Fe_0.127Mn_0.746O₂ (NLFM) cathode, which has optimized Li, Fe and Mn contents 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 the 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 the 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 the Li-doped cathode arises from dopant-induced static and dynamic disorder, ensuring structural stability across a wide SOC range.