Fast and stable Mg2+ intercalation in a high voltage NaV2O2(PO4)2F/rGO cathode material for magnesium-ion batteries

Sluggish kinetics of Mg2+ intercalation and low working potential seriously hinder the development of highenergy- density magnesium-ion batteries (MIBs). Hence developing cathode materials with fast Mg2+ diffusion and high working voltage is a key to overcome the obstacles in MIBs. Herein, a tetragonal NaV2O2(PO4)(2)F/reduced graphene oxide (rGO) is proposed as an effective Mg2+ host for the first time. It exhibits the highest average discharge voltage (3.3 V vs. Mg2+/Mg), fast diffusion kinetics of Mg2+ with the average diffusivity of 2.99x10(-10) cm(2) s(-1), and ultralong cycling stability (up to 9500 cycles). The Mg2+ storage mechanism of NaV2O2(PO4)(2)F/rGO is demonstrated as a single-phase (de) intercalation reaction by in situ X-ray diffraction (XRD) technology. Density functional theory (DFT) computations further reveal that Mg2+ ions tend to migrate along the a direction. X-ray absorption near edge structure (XANES) demonstrates a decrease in the average valence of vanadium, and the local coordination environment around vanadium site is highly conserved after magnesiation. Moreover, the assembled NaV2O2(PO4)(2)F//Mg0.79NaTi2(PO4)(3) Mg-ion full cell exhibits high power and energy densities, which indicates that NaV2O2(PO4)(2)F/rGO owns potential for practical applications. This work achieves a breakthrough in the working voltage of cathode materials for MIBs and provides a new opportunity for high-energy-density MIBs.