Dual carbon skeleton supported NVPFO sodium-ion battery cathode with high rate capability, superior cycling performance, and wide temperature adaptability

The cathode material of sodium-ion batteries, Na3V2(PO4)2F3-yOz (NVPFO), with its NASICON structure, exhibits greater stability and a higher average discharge potential compared with sodium vanadium phosphate (NVP). Owing to its low intrinsic conductivity, the electrochemical performance and cycle stability of NVPFO is unsatisfactory. In this study, a sol–gel method was implemented, by using melamine-cyanic acid-lignin supermolecule precursor (MCL) and citric acid (CA) as carbon sources in the preparation of NVPFO process, in which a double-carbon coating structure was established. The distinctiveness of this dual carbon source can result in the tremendous pyrolysis gases, created by the MCL during the calcination process, exploding the outer CA layer, through which sodium vanadium oxyfluorophosphate (NVPFO/MCLC@C) was produced with a dual carbon coating containing an outer coral-like carbon layer and an interior nitrogen-doped carbon layer. Nitrogen doping enables MCLC to adsorb Na+ with a pseudocapacitive behavior. The specific capacity of the NVPFO is 117.3 mAh/g at a current density of 0.1 A/g, and 52.8 mAh/g at a current density of 10.0 A/g. The capacity retention rate was 80.0 % after 1100 cycles at a current density of 1.0 A/g. In this work, a dual carbon coating strategy has been proposed to enhance the rate performance and long cycle stability of NVPFO cathode materials, which could be a potential approach for the cathode coating engineering of sodium-ion batteries.