Bonding between vanadium and citric acid in solution affects the morphology of NVP/C and Na-ions storage performance

NASICON-type Na3V2(PO4)3/C (NVP/C) is a promising cathode material for sodium-ion batteries and has attracted the widespread interest due to its unique three-dimensional open crystal structure and good chemical stability. Researchers have focused on improving its electrochemical properties by modifying the components or morphology, and few reports have focused on the bonding of the V with the carbon source in the solution as well as its effects on morphology and electrochemical performance. In this work, the change in V valence state in the solution, with/without citric acid, as well as during heat treatment is investigated. Bonding between V and chelate affects the decomposition temperature of complexion, the morphology, the ratio of V4+/V3+, and the specific surface area, but not the crystal structure. The results illustrate that the NVP/C-pH 4 shows unique bowl-type carbon structure and bears excellent high rate dischargability. The initial capacity is 93.7 mAh g−1 at 5C and it remains at 80.0 mAh g−1 after 1000 cycles. Meanwhile, the CV curve, the reaction resistance, the coefficient of Na-ion, electrochemical impedance spectroscopy (EIS) and the content of V, Na and C vary with cycles. This work not only reveals the effects of bonding interactions in solution on the morphology of the materials as well as their electrochemical properties, but also provides a new idea for the optimization of carbon materials for SIBs.