Na+-conductive Na2Ti3O7 modified P2-type Na2/3Ni1/3Mn2/3O2 via a smart in situ coating approach: Suppressing Na+/Vacancy Ordering and P2-O2 Phase Transition.

Sodium-ion batteries (SIBs) have shown great superiority for grid-scale storage applications due to the low cost, and the abundance of sodium. P2-type Na2/3Ni1/3Mn2/3O2 cathode materials have attracted much attention for their high capacities and operating voltages as well as their simple synthesis processes. However, Na+/vacancy ordering and the P2-O2 phase transition are unavoidable during Na+ insertion/extraction, leading to undesired voltage plateaus and deficient battery performances. We show that this defect can be effectually eliminated by coating a moderate Na+ conductor-Na2Ti3O7-with a smart in situ coating approach. Based on the combined analysis of ex situ x-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical performance tests, and electrochemical kinetic analyses, coating with Na2Ti3O7 effectively refrains Na+/vacancy ordering and P2-O2 phase transition during cycling. Additionally, the Na2Ti3O7 coating layer suppresses particle exfoliation and accelerates Na+ diffusion at the cathode and electrolyte interface. Hence, Na2Ti3O7 coated Na2/3Ni1/3Mn2/3O2 exhibits excellent cycling stability (almost no capacity decay after 200 cycles at 5C) and outstanding rate capability (31.1% of the initial capacity at a high rate of 5C compared to only 10.4% for the pristine electrode). This coating strategy can provide a new guide for the design of prominent cathode materials for SIBs that are suitable for practical application.