Sodium-ion batteries (SIBs) have aroused intense research and academic interest due to the natural abundance and cost-effectiveness of sodium resources. Presently, cathode materials based on the Na 3 (VO 1−x PO 4 ) 2 F 1+2x (0 ≤ x ≤ 1, NVPF 1+2x ) polyanionic framework show intriguing electrochemical performances toward practical and advanced SIBs due to its high operating voltage (>3.9 V) and high energy density (>500 Wh kg −1 ). Different from conventional approaches focusing on delicate morphology design, metal ion substitution, and the conductive matrix's incorporation to overcome the low intrinsic electrical conductivity, here we adopt a one-step microwave-assisted hydrothermal approach to optimize the electrochemical performances of NVPF 1+2x via manipulating its phase compositions with different vanadium sources and distinguishing the tetragonal (I4/mmm) symmetry of the Na 3 (VOPO 4 ) 2 F phase from the orthorhombic symmetry (Amam) of the Na 3 V 2 (PO 4 ) 2 F 3 phase. The introduction of the conductive reduced graphene oxide (rGO) framework and its impacts on the phase compositions were systematically investigated. The rGO framework with different calcination temperature can alter the phase composition and the electrical conductivity of NVPF 1+2x cathodes significantly, thus having a great impact on their electrochemical performances. Galvanostatic charge/discharge, cyclic voltammetry, electrochemical impedance spectroscopy, and the galvanostatic intermittent titration technique are adopted to compare their electrode polarization and kinetics difference and show that NVPF@ rGO-600 °C possesses a high rate, small polarization, and fast kinetics electrochemical properties. This work provides new insights into manipulating phase compositions of the NVPF 1+2x cathode by modulating the synthesis conditions and revealing their synergistic effect with a rGO conductive framework toward a superior rate capability and more realistic practical applications for SIBs.

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Sodium-ion batteries (SIBs) have been regarded as promising alternative to lithium-ion batteries (LIBs) due to the abundance of sodium resource and cost-effectiveness of electrode manufacture. Na 3 (VO 1-x PO 4 ) 2 F 1 + 2x (0 � x � 1, NVPF 1 + 2x ) polyanionic material, a potential high-energy-density cathode, has shown superior electrochemical performances for advanced SIBs due to its high working voltage (> 3.9 V). Electrolyte composition, which plays an indispensable and critical role in determining the cycle stability and the electrode/electrolyte interfacial properties, is of great significance to possess good compatibility with electrode materials, especially the NVPF 1 + 2x cathode. Here, different electrolyte systems, including commonly used 1.0 m NaPF 6 /diglyme (NP-005), 1.0 m NaPF 6 /propylene carbonate (PC)/ 5.0 % fluoroethylene carbonate (FEC) (NP-009), 1.0 m NaClO 4 / ethylene carbonate-dimethyl carbonate (EC-DMC; 1 : 1 v/v)/ 5.0 % FEC (NC-019), and 1.0 m NaClO 4 /PC (NC-013), were systematically investigated and compared for NVPF 1 + 2x cathode. NVPF 1 + 2x electrode with NP-009 electrolyte showed a superior cycle stability and rate capability at 1-10 C (1 C = 130 mA g À 1 ) than that of NC-019 and NC-013, while NVPF 1 + 2x electrode with NP-005 electrolyte showed the best high-rate capability at 20-50 C. The cathode/electrolyte interphase (CEI), post-mortem electrode morphology, and electrochemical kinetic characteristics of NVPF 1 + 2x electrode with different electrolytes were profoundly investigated and compared. It demonstrated that NVPF 1 + 2x electrode with NP-005 exhibited a thin, efficient, and NaF-rich CEI layer with less polarization, smaller interfacial resistance, and faster Na + diffusion than that of NC-019 and NC-013 since they suffered from a thick, overgrown CEI layer due to the consecutive decomposition of FEC, NaClO 4 , and/or linear DMC, resulting in inferior electrochemical performance. This work provides new insights for the battery community to gain more comprehensive understanding about the compatibility and interfacial chemistry between different electrolyte systems and various electrode surfaces.

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