Freestanding Na₃V₂O₂(PO₄)₂F/Graphene Aerogels as High-Performance Cathodes of Sodium-Ion Full Batteries

Abstract

Although sodium vanadium fluorophosphate, Na₃(VO_1-<i>x</i>PO₄)₂F_1+2<i>x</i> (0 ≤ <i>x</i> ≤ 1), is a highly promising cathode candidate for sodium-ion batteries because of its stable structure and high working voltage, the low charge diffusion dynamics and the inactive materials used in traditional coating electrodes reduce the energy density of a sodium-ion full battery. Hence, Na₃V₂O₂(PO₄)₂F/graphene aerogels (NVPF/GAs) with a three-dimensional continuous porous network are first prepared by coassembly and freeze-drying. The three-dimensional porous network helps to obtain a high NVPF content of 81 wt %, relieve the volume change for improving the cyclability, and enhance the wettability of the electrode with the electrolyte for accelerating the diffusion dynamics of sodium ions and electrons. As a directly used freestanding cathode without the use of any binder/collector, an optimized freestanding NVPF/GA electrode exhibits excellent cycling and rate performances compared to traditional coating electrodes. The average capacities at current densities of 0.2, 0.5, 1.0, 2.0, and 5.0 C are 135.4, 128.0, 125.1, 121.9, and 115.1 mA h g^-1, respectively. Especially, it maintains a capacity retention of 100% after 1000 cycles at an ultrahigh current of 40 C. A sodium-ion full battery with the NVPF/GA cathode and the Sb/graphene/carbon anode attains a of 82.1 mA h g^-1 without an obvious decline after 100 cycles.