Controlling Iron Versus Oxygen Redox in the Layered Cathode Na_(0.67)Fe_(0.5)Mn_(0.5)O_2: Mitigating Voltage and Capacity Fade by Mg Substitution

摘要

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 Fe3+ 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 Fe3+/Fe4+ 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.