Electronic Structure Engineering of Honeycomb Layered Cathode Material for Sodium‐Ion Batteries

Abstract In this work, the rational design of O′3‐type Na[Ni_2/3−xCo_xSb_1/3]O₂, a solid solution of Na[Ni_2/3Sb_1/3]O₂–Na[Co_2/3Sb_1/3]O₂, is introduced. Because of the difficulty of the Co^3+/2+ redox reaction, the electronic structures of Na[Ni_2/3−xCo_xSb_1/3]O₂ compounds are engineered to build electroconducting networks in the oxide matrix through electrochemical oxidation of Co²⁺ to Co³⁺, after which the formed Co³⁺ does not participate in the electrochemical reaction but improves the electrical conductivity in the structure. Density functional theory calculations reveal a reduced bandgap energy after the formation of Co³⁺ during desodiation of Na_1−y[Ni_2/3−xCo_xSb_1/3]O₂. Using the oxidized Co³⁺ species while improving the electrical conductivity, the Na[Ni_2/3−xCo_xSb_1/3]O₂ (x = 1/6) electrode exhibits excellent cyclability for 1000 cycles with ≈72.5% capacity retention at 2C (400 mA g⁻¹) and activity even at 50C (10 A g⁻¹) in Na cells. Operando X‐ray diffraction and ex situ X‐ray absorption near‐edge structure investigations reveal suppressed lattice variations upon charge and discharge compared with those of Na[Ni_2/3Sb_1/3]O₂ achieved by the presence of the electrochemical‐driven Co³⁺ in the structure. These findings offer a new strategy for the development of cathode materials for sodium‐ion batteries, providing important insight into their structural transformations and the electronic nature of advanced cathode materials.