Dual-strategy of Cu-doping and O3 biphasic structure enables Fe/Mn-based layered oxide for high-performance sodium-ion batteries cathode

O3-type Fe/Mn-based layered oxides are considered as the competitive cathode materials for sodium-ion batteries because of environmental friendly metal elements and low cost. However, a lower capacity, poor cycling stability and irreversible phase transitions hinder their commercial application. Herein, the Cu-doped O3 biphasic Na[Mn0.6Fe0.4](1-x)CuxO2 (NFMCu-x) layered oxides are successfully synthesized by high temperature calcination using Fe/Mn-based Prussian blue analogues as precursor. The results indicate that the different stoichiometric of Cu2+ substitution in P2 NaMn0.6Fe0.4O2 can cause the transformation of P2 phase to O3 monophase or O3 biphase. The NFMCu-x samples exhibit O3 monophase at x = 0.1, 0.15, while O3 biphase coexists in the compound at x = 0.2, 0.25, and 0.3. Benefiting from the synergetic effect of Cu-doping and O3 biphasic structure, the optimized O3 biphasic NaMn0.48Fe0.32Cu0.2O2 exhibits a high Na + diffusion coefficient and an excellent sodium storage performance with a high discharge specific capacity of 103.76 mAh g−1 at 24 mA g−1 and good capacity retention of 82.9% after 100 cycles. Moreover, Cu-doped O3 biphasic NaMn0.48Fe0.32Cu0.2O2 features an excellent air stability. Thus, this work proposes a strategy in development of O3 Fe/Mn-based layered oxide as high-performance cathode materials for sodium-ion batteries.