Exploring Optimal Li-ion Substitution for High Na-content P2-Na0.67+a [Li x Ni0.33−y Mn0.67−z ]O2 Cathodes for Sodium-ion Batteries

P2-type Na0.67[Ni0.33Mn0.67]O2 layered oxide cathode is attractive for practical Na-ion battery application due to its high voltage Ni4+/Ni2+ 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 Na0.67[Ni0.33Mn0.67]O2 to tailor a series of high Na-content P2-type cathodes. Among them, Na0.85[Li0.14Ni0.29Mn0.57]O2 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 Ni4+/Ni2+ and Mn4+/Mn3+ redox couples during Na (de)intercalation of the Na0.85[Li0.14Ni0.29Mn0.57]O2. A full Na-ion cell (Na0.85[Li0.14Ni0.29Mn0.57]O2||hard carbon) is demonstrated with an energy density of 420 Wh kg−1.