Na3V2O2(PO4)2F nanoparticles@reduced graphene oxide: a high-voltage polyanionic cathode with enhanced reaction kinetics for aqueous zinc-ion batteries

Na3V2O2(PO4)2F is characterized by a high theoretical specific capacity, high-voltage plateaus, large ion diffusion channels, and stable three-dimensional structure, which is a promising cathode candidate for rechargeable aqueous zinc-ion batteries (AZIBs). Nevertheless, the low electronic conductivity hinders its cycling and rate performance. In this work, a facile microwave-assisted solvothermal approach with postheat treatment has successfully produced Na3V2O2(PO4)2F nanoparticles with the dimension of 20-50 nm wrapped by reduced graphene oxide (rGO). The high pseudocapacitive contribution, high diffusion coefficient, low electrochemical impedance, favorable electrode wettability, and stable structure can be obtained by optimizing the chemical components of electrolyte and the content of rGO, facilitating the Zn2+ storage performance. Hence, Na3V2O2(PO4)2F@rGO delivers excellent electrochemical properties such as a high capacity of 127.0 mAh g  1 at 0.5C, a stable capacity of 63.9 mAh g  1 at 30C after 5000 cycles, and a high average voltage plateau of 1.50 V in hybrid aqueous electrolyte (2 M zinc trifluoromethane sulfonate (Zn(OTf)2) and 4 M lithium trifluoromethane sulfonate (LiOTf)). Furthermore, exceptional cycling performance in the soft-pack batteries with the size of 3 x 3.5 cm2 includes a reversible capacity of 84.1 mAh g  1 at 10C after 1000 cycles. In situ X-ray diffraction, ex situ X-ray photoelectron spectroscopy, transmission electron microscopy, and scanning electron microscopy reveal reversible Zn2+ insertion/extraction process.