Optimizing the Electrolyte Systems for Na₃ (VO_1-x PO₄ )₂ F_1+2x (0≤x≤1) Cathode and Understanding their Interfacial...

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. Na₃ (VO_1-x PO₄ )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 NaPF₆ /diglyme (NP-005), 1.0 m NaPF₆ /propylene carbonate (PC)/5.0 % fluoroethylene carbonate (FEC) (NP-009), 1.0 m NaClO₄ /ethylene carbonate-dimethyl carbonate (EC-DMC; 1 : 1 v/v)/5.0 % FEC (NC-019), and 1.0 m NaClO₄ /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^-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, NaClO₄ , 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.