A nanoscaled Na3SbS3.75Se0.25 solid electrolyte with less grain-boundary resistance was synthesized using a liquid/solid fusion technology. The Na3SbS3.75Se0.25 solid electrolyte shows a high ionic conductivity of 4.03 × 10−3 S cm−1 at room temperature due to the significantly decreased amorphous phase in the electrolyte. Moreover, the small particle size of the solid electrolytes enhances the contact between solid electrolyte and electrode, reducing the interfacial contact resistance. As a result, FeS2/Na3SbS3.75Se0.25/Na all-solid-state sodium batteries achieve a high specific capacity of 140.6 mAh g−1 for 300 cycles at a high current of 500 mA g−1. In addition, FeS2/Na3SbS3.75Se0.25/Na cells also demonstrate a high rate-capacity of 365.3, 301.8, 210.1 and 96.0 mAh g−1 at current densities of 50, 300, 500 and 1000 mA g−1, respectively. The liquid/solid fusion technology is a unique synthesis strategy to develop superionic electrolytes for room temperature all-solid-state sodium secondary battery.