<i>In-situ</i> surface coating and subsurface gradient doping contrives P2-Na _0.67 Ni _0.33 Mn _0.67 O ₂ single crystal with highly stable interface and structure

P2-Na_0.67Ni_0.33Mn_0.67O₂ cathode holds the merits of high working voltage/capacity, facile manufacture, and similar large-scale production to Li layered oxides. However, it suffers from issues of irreversible P2–O2 phase transition at a high voltage (&gt;4.0 V), interfacial instability, and particle cracks after repeated cycle. Herein, <em>in-situ</em> formed MgO surface coating layer and Mg²⁺ subsurface gradient doping is obtained by Mg₃(PO₄)₂ decomposition under 900 ℃. The as-formed <em>in-situ</em> surface coating and subsurface doping effects simultaneously guarantee the high-stability of material interface and structure. HRTEM and HAADF-STEM images clearly show the surface coating layer is 2‒5 nm and subsurface gradient doping depth is 3‒5 nm, rather than bulk doping. <em>In-situ</em> XRD patterns and <em>in-situ</em> DRT analysis profoundly clarify the enhanced electrochemical reaction stability and structural reversibility. Theoretical calculations elucidate superior electronic and spatial structures after <em>in-situ</em> surface coating and subsurface doping engineering. As a result, the optimized cathode shows ascendant discharge capacity of 100.3 mAh·g^–1 at 1C with 80.8% retention during 500 cycles. It displays much improved rate capability of 81.5 mAh·g^–1 at 5 C. Revealing excellent cycling stability and potential applications for high-performance sodium-ion batteries.&nbsp;