context: To better understand the good cycling stability of the full cell in the Na–H2O–urea–DMF electrolyte, inductively coupled plasma emission spectrometry-atomic emission spectroscopy (ICP-AES) analysis of the vanadium concentration in the Na–H2O–urea–DMF electrolyte and analysis of electrochemical impedance parameters were carried out before and after cycling. As shown in Fig. 4b and Table S3,† it is found that the concentration of V-ions in the Na–H2O–urea–DMF electrolyte before cycling is 0.05 μg mL−1. After 50 cycles and after 100 cycles, the concentrations of V-ions in the Na–H2O–urea–DMF electrolyte are 16.65 μg mL−1 and 20.12 μg mL−1, respectively. In 1 M Na2SO4 solution, the concentrations of V-ions are 0.08 μg mL−1, 60.68 μg mL−1 and 72.34 μg mL−1, respectively. Compared to 1 M Na2SO4 solution, the Na–H2O–urea–DMF electrolyte can obviously inhibit the dissolution of the electrode materials. Upon comparison of the V-ion concentrations in NaClO4 solution, Na–H2O–DMF, and Na–H2O–urea electrolyte of the NVP//NTP full cell after 50 cycles (Fig. S14†), it is found that the Na–H2O–urea–DMF electrolyte can suppress the dissolution of V-ions in the charge–discharge process. This result is consistent with the cycling performances of the NVP//NTP full cell in different electrolytes (Fig. S13†). Thus, it can be concluded that the NVP/C cathode is much more stable in the Na–H2O–urea–DMF electrolyte, which is beneficial for the long lifespan of the full cell. Similarly, as illustrated in Fig. S15,† the NTP anode in the Na–H2O–urea–DMF electrolyte displays high coulombic efficiency and good cycling stability relative to that in 17 M NaClO4 and 1 M Na2SO4 solution.
answer:  Na–H2O–urea–DMF