Exploring Optimal Li-ion Substitution for High Na-content P2-Na 0.67+a [Li x Ni 0.33−y Mn 0.67−z ]O 2 Cathodes for Sodium-ion Batteries

P2-type Na 0.67 [Ni 0.33 Mn 0.67 ]O 2 layered oxide cathode is attractive for practical Na-ion battery application due to its high voltage Ni 4+ /Ni 2+ redox and good air-stability. However, it suffers from rapid capacity decay originating from high voltage P2-O2 transformation. To alleviate this issue, herein we explore optimum Li-substitution into the transition metal layer of Na 0.67 [Ni 0.33 Mn 0.67 ]O 2 to tailor a series of high Na-content P2-type cathodes. Among them, Na 0.85 [Li 0.14 Ni 0.29 Mn 0.57 ]O 2 cathode with optimal Li-substitution exhibits reversible capacities of ∼168 mAh g −1 at 0.1C rate and good cycling stability (82% of retention after 100 cycles at 1C rate). In-situ XRD measurement reveals the formation of complete solid-solution and X-ray absorption spectroscopy studies confirm the participation of Ni 4+ /Ni 2+ and Mn 4+ /Mn 3+ redox couples during Na (de)intercalation of the Na 0.85 [Li 0.14 Ni 0.29 Mn 0.57 ]O 2 . A full Na-ion cell (Na 0.85 [Li 0.14 Ni 0.29 Mn 0.57 ]O 2 ||hard carbon) is demonstrated with an energy density of 420 Wh kg −1 .