Controlling Iron Versus Oxygen Redox in the Layered Cathode Na0.67Fe0.5Mn0.5O2: Mitigating Voltage and Capacity Fade by Mg Substitution

Abstract Layered oxides for Na‐ion batteries containing Fe have attracted strong interest mainly due to their low cost. However, full oxidation of Fe 3+ to Fe 4+ is rarely seen before O‐redox sets in and is typically accompanied by voltage and capacity fade on cycling. On charging P2‐Na 0.67 [Fe 0.5 Mn 0.5 ]O 2 , Fe 3+ is oxidized to only ≈ Fe 3.3+ before the onset of O‐redox. O‐redox occurs when the Na content is sufficiently low (Na ≈ 0.3) to permit the transition from P‐type to O‐type stacking, thus enabling Fe 3+ migration to the Na layer. Fe 3+ migration generates cation vacancies in the transition metal layer, forming □‐O‐□ configurations, which trigger the onset of O‐redox. In contrast, doping this material with Mg 2+ to form P2‐Na 0.67 [Fe 0.25 Mn 0.6 Mg 0.15 ]O 2 allows full oxidation of Fe 3+ to Fe 4+ before the Na content is low enough to favor O‐type stacking. During O‐redox, Mg 2+ is displaced into the Na layers instead of Fe. Mg substitution enables greater reversibility of the Fe 3+ /Fe 4+ redox couple and significantly suppresses Fe migration, which is responsible for the voltage and capacity fade observed for P2‐Na 0.67 Fe 0.5 Mn 0.5 O 2 .