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

摘要

In this work, the rational design of O ' 3-type Na[Ni2/3-xCoxSb1/3]O-2, a solid solution of Na[Ni2/3Sb1/3]O-2-Na[Co2/3Sb1/3]O-2, is introduced. Because of the difficulty of the Co3+/2+ redox reaction, the electronic structures of Na[Ni2/3-xCoxSb1/3]O-2 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)[Ni2/3-xCoxSb1/3]O-2. Using the oxidized Co3+ species while improving the electrical conductivity, the Na[Ni2/3-xCoxSb1/3]O-2 (x = 1/6) electrode exhibits excellent cyclability for 1000 cycles with approximate to 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[Ni2/3Sb1/3]O-2 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.