Na⁺-Conductive Na₂Ti₃O₇-Modified P2-type Na_2/3Ni_1/3Mn_2/3O₂ via a Smart in Situ Coating Approach: Suppressing Na⁺/Vacancy Ordering and P2–O2 Phase Transition

Abstract

Sodium-ion batteries (SIBs) have shown great superiority for grid-scale storage applications because of their low cost and the abundance of sodium. P2-type Na_2/3Ni_1/3Mn_2/3O₂ 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 and a concomitant doping of Ti⁴⁺ into the bulk structure. Based on the combined analysis of ex situ X-ray diffraction, scanning electron microscopy, electrochemical performance tests, and electrochemical kinetic analyses, Na₂Ti₃O₇ coating and Ti⁴⁺ doping effectively refrain 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 Na_2/3Ni_1/3Mn_2/3O₂ exhibits excellent cycling stability (almost no capacity decay after 200 cycles at 5 C) and outstanding rate capability (31.1% of the initial capacity at a high rate of 5 C 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 applications.