P3-Type Layered Sodium-Deficient Nickel-Manganese Oxides: A Flexible Structural Matrix for Reversible Sodium and Lithium Intercalation

Sodium-deficient nickel-manganese oxides exhibit a layered structure, which is flexible enough to acquire different layer stacking. The effect of layer stacking on the intercalation properties of P3-NaxNi0.5Mn0.5O2 (x=0.50, 0.67) and P2-Na2/3Ni1/3Mn2/3O2, for use as cathodes in sodium- and lithium-ion batteries, is examined. For P3-Na0.67Ni0.5Mn0.5O2, a large trigonal superstructure with 23ax23ax2c is observed, whereas for P2-Na2/3Ni1/3Mn2/3O2 there is a superstructure with reduced lattice parameters. In sodium cells, P3 and P2 phases intercalate sodium reversibly at a well-expressed voltage plateau. Preservation of the P3-type structure during sodium intercalation determines improving cycling stability of the P3 phase within an extended potential range, in comparison with that for the P2 phase, for which a P2-O2 phase transformation has been found. Between 2.0 and 4.0V, P3 and P2 phases display an excellent rate capability. In lithium cells, the P3 phase intercalates lithium, accompanied by a P3-O3 structural transformation. The in situ generated O3 phase, containing lithium and sodium simultaneously, determines the specific voltage profile of P3-NaxNi0.5Mn0.5O2. The P2 phase does not display any reversible lithium intercalation. The P3 phase demonstrates a higher capacity at lower rates in lithium cells, whereas in sodium cells P3-NaxNi0.5Mn0.5O2 operates better at higher rates. These findings reveal the unique ability of sodium-deficient nickel-manganese oxides with a P3-type structure for application as low-cost electrode materials in both sodium- and lithium-ion batteries.