Introduction: Sulfolane (SL), having an edge of low melting point over other sulfones, has been adopted as an electrolyte co-solvent for lithium-ion battery (LIB), as it exhibits high stability against oxidation and combustion while not causing much side effects to the battery electrochemistry. It is therefore expected that SL may serve as a safety-enhancing agent in sodium-ion battery (SIB). To evaluate the effect of SL content on the behavior of common carbonate-based sodium electrolytes as well as the compatibility of SL-based electrolytes with NaNi1/3Mn1/3Co1/3O2 (NaNMC) cathode, mixtures of 0, 10, 20, 30 or 50% vol. SL and each of the following, EC:PC 1:1 vol. (EP11), EC:DMC 1:1 vol. (ED11), EC:PC:DMC 1:1:3 vol. (EPD113) and EC:PC:DMC 3:1:1 vol. (EPD311), with or without 1M NaClO4, were studied with regard to both inherent properties and performance in NaNMC half-cells. Methods: Solvent flammability was evaluated via the self-extinguishing time (SET) and ignition time indexes. Conductivity and viscosity were respectively measured by Electrochemical Impedance Spectroscopy (EIS) and Ostwald method. Electrochemical techniques, i.e. Cyclic Voltammetry (CV) and Galvanostatic Cycling with Potential Limitation (GCPL), were used to test the sodium-ion battery performance. Results: A moderate amount of SL (typically below 30% vol.) proved to enhance both electrolyte non-flammability and self-extinguishing behavior, while maintaining an acceptable compromising rate in viscosity and conductivity. Amongst 30%-SL electrolytes, EPD311-based ones allow the best Na+ diffusion when combined with NaNMC cathode in sodium half-cell configuration. The corresponding system gives satisfactory performance: initial specific capacity of 97 mAh.g−1 , 92% capacity retention, and above 90% reversibility after 30 cycles at C/10 rate. Conclusion: SL can be used as a stabilizing co-solvent for SIB, but its content should be limited to below 30% vol. to ensure its effectiveness.