High-voltage stability of O3-type sodium layered cathode enabled by preferred occupation of Na in the OP2 phase

Cobalt-free O3-type layered sodium nickel manganese oxide, Na[Ni0.5Mn0.5]O2, is one of the most attractive candidates as a cathode material for low-cost and practical sodium-ion batteries (SIBs). However, stable cycling with high reversible capacity in SIBs using Na[Ni0.5Mn0.5]O2 as a cathode is still hampered by an irreversible phase transition at voltages above 4.0 V (vs. Na/Na+). In this study, to suppress the detrimental phase transition, Mg substitution into Ni sites is proposed, and a series of Na[Ni0.5-xMgxMn0.5]O2 (x = 0, 0.05, 0.1 and 0.15) materials are synthesized and characterized. The substitution of Mg over 10 mol% into Ni site regulates sodium occupancy into P-type stacking, resulting in the OP2 phase upon high-voltage operation. This enables the material to tolerate strains in the crystal structure by alleviating the irreversible O3′−O3″ phase transition, which was evidenced by the results of operando X-ray diffraction and density functional theory calculation. Compared to its corresponding members, the optimized Na[Ni0.4Mg0.1Mn0.5]O2 cathode delivers a high reversible capacity of 177 mAh g−1 as well as improved cycling stability and rate capability in the wide voltage range of 2.0 − 4.3 V. Moreover, the Mg substitution strategy enhances the thermal and chemical stability against moisture, indicating its excellent practical applicability.