Surface Structure Reconstruction to Suppress Heterogeneous Phase Transformation for Air-stable Single Crystalline O3-type Sodium Oxide

O3-type layered oxide cathode material for sodium-ion batteries (SIBs) has attracted much attention as one of the most viable candidates due to its high specific capacity and mature synthesis process, while the moisture sensitivity and harmful phase transformation lead to poor processing properties and unsatisfactory life-span, hindering its large-scale and commercial application. Herein, single crystallization strategy is adopted to enhance air stability and processing performance, and surface structure reconstruction for single crystalline cathode material O3-NaNi_1/3Fe_1/3Mn_1/3O₂ by ammonium tetraborate pretreatment is employed to further remove residual alkali and improve sodium ions diffusion dynamics and suppress heterogeneous phase transformation, achieving superior structure stability. Surface residual alkali is in-situ converted into a protective coating layer of Na₂B₄O₇ and meanwhile partial B atoms enter into the interstitial site of sub-surface or near surface, which accelerates sodium ions transport as well as enhances TM-O bonding and hybridization of surface O (2p)-Fe (3d-t_2g) orbital, inhibits TMO6 slabs gliding and strengthens structure on the surface and near surface. Additionally, the formed boron-rich surface exhibits high stability, effectively alleviating structural degradation from surface to bulk and enhancing air stability. Benefiting from the reconstructed surface structure, the modified single crystalline oxides (NFM@B) exhibit distinguished processing performance and electrochemical properties.