context: Fig. 4c shows the electrochemical impendence spectra of the full cell in the Na–H2O–urea–DMF electrolyte before cycling, after the first cycle and after 100 cycles. The Rs value of the battery in the Na–H2O–urea–DMF electrolyte, connected to the intercept on the real impedance axis at a high frequency in the EIS spectra, is relatively small and equal for the pristine state and after cycling, which suggests good ion diffusion in the electrolyte and good stability of the electrode materials before and after cycling. The Rct value represented the charge transfer resistance increase after the first cycle and after 100 cycles, indicating that a passivation layer may be formed on the NTP anode, which could be a significant factor in the improvement of the cycling performance of the NVP/NTP full cell. Moreover, the XRD patterns (Fig. S16†) and SEM images (Fig. S17†) of NVP/C and NTP/C before and after 500 cycles were recorded, respectively. The results indicate that the Na–H2O–urea–DMF electrolyte is conducive to achieving excellent stability of the NVP cathode. In addition, a NiHCF//NTP full cell displays 80% capacity retention after 2000 cycles at 2C rate (Fig. 4d).
answer: Na–H2O–urea–DMF