Na3V2(PO4)3/C·Na3V2(PO4)2F3/C@rGO blended cathode material with elevated energy density for sodium ion batteries

Abstract

Na3V2(PO4)3 (NVP), a promising cathode material of sodium ion batteries, has gained much attention due to its competitive capability of sodium storage. However, the relatively low discharge potential of ~3.4 V (vs. Na+/Na) needs to be improved and the current energy density cannot satisfy the practical applications. Herein, the introduction of Na3V2(PO4)2F3 with high voltage plateaus is adopted to elevate the energy density. Therefore, the mixed composite seems to be more attractive because of the prospective high energy character. Nevertheless, this mixture still suffers from the intrinsic poor electronic conductivity. Accordingly, a novel Na3V2(PO4)3/C·Na3V2(PO4)2F3/C @reduced graphene oxide (rGO) (denoted as B@rGO) blended cathode material is successfully synthesized through a facile sol-gel method. The coated carbon layers and rGO are employed to construct a conductive network for promoting the ionic and electronic diffusion efficiently. Detailed kinetic analyses demonstrate that the NVP phase dominates the kinetic reaction in the blended composite. With respect to the electrochemical properties, this modified B@rGO composite can deliver a discharge capacity of 115.1 mAh g-1 at 0.1 C, corresponding to a high energy density of 418.3 Wh kg-1, which is far more than the theoretical one of pure NVP (399.8 Wh kg-1). Moreover, the cyclic characteristics are significantly improved, resulting from the boosted structural stability. Even at a high rate of 30 C, it can release a reversible capacity of 85.9 mAh g-1, and a high capacity retention of 81.6% is obtained after 200 cycles. Furthermore, the superior electrochemical properties are determined and demonstrated by the improved kinetics, including the large capacitive contribution (79.6% at 0.5 mV s-1) and rapid ionic migration. Therefore, this novel co-modification strategy with high-voltage material and conductive rGO can be an efficient way for Na3V2(PO4)3 to optimize the electrochemical performance and push its application in sodium ion batteries.