Mitigating the voltage fading and lattice cell variations of O3-NaNi0.2Fe0.35Mn0.45O2 for high performance Na-ion battery cathode by Zn doping

O3-type layered oxides have attracted considerable interest as cathode materials for sodium ion batteries. However, the oxides' poor rate capability, inferior cycling stability and voltage decay impede the use of these oxides in practical applications. In this study, a series of O3-type NaNi0.2Fe0.35Mn0.45-xZnxO2 (x = 0, 0.02, 0.05, 0.07 and 0.1) cathode materials were synthesized via a solid state reaction. With an optimized Zn2+ content of 0.05, NaNi0.2Fe0.35Mn0.4Zn0.05O2 cathode exhibits better electrochemical performance compared to the undoped-NaNi0.2Fe0.35Mn0.45O2 in terms of rate capability, cycling stability and suppressed voltage decay. The role of Zn has been elucidated. First, the substitution of Zn for Mn reduces the content of unfavourable Mn3+ and hence improves the cycling stability. Second, the introduction of Zn2+ into the TM-O layer stabilizes the crystal structure and mitigates the irreversible migration of Fe3+ into Na+ layer upon cycling, thereby alleviating the voltage fading during Na+ extraction/insertion. Third, Zn2+ doping promotes the O3-P3 reversible phase transformation. Moreover, Zn2+ doping reduces the lattice cell variations during Na+ extraction/insertion and improves the structure stability. The proposed insights into the role of Zn are also instructive for designing other high-performance cathode materials for sodium-ion batteries through lattice doping.