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− x Co x Sb 1/3 ]O 2 , a solid solution of Na[Ni 2/3 Sb 1/3 ]O 2 –Na[Co 2/3 Sb 1/3 ]O 2 , is introduced. Because of the difficulty of the Co 3+/2+ redox reaction, the electronic structures of Na[Ni 2/3− x Co x Sb 1/3 ]O 2 compounds are engineered to build electroconducting networks in the oxide matrix through electrochemical oxidation of Co 2+ to Co 3+ , after which the formed Co 3+ 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 Co 3+ during desodiation of Na 1− y [Ni 2/3− x Co x Sb 1/3 ]O 2 . Using the oxidized Co 3+ species while improving the electrical conductivity, the Na[Ni 2/3− x Co x Sb 1/3 ]O 2 ( 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 2/3 Sb 1/3 ]O 2 achieved by the presence of the electrochemical‐driven Co 3+ 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.