Despite the impressive progress in the development of sodium-air batteries for energy storage, the current sodium-air batteries are still far from practical application in electric vehicles. A proposed sodium-air battery was successfully constructed using a NASICON structured solid electrolyte and a liquid anode, which demonstrated low voltage gap, reversibility, and high safety. The liquid anode was fabricated by dissolving sodium metal into a mixed solution of biphenyl and ethers. Due to the fantastic conductivity of the liquid anode, the battery exhibits a high discharge voltage of 2.88 V and a low voltage gap of 0.14 V at a current density of 0.1 mA cm−2, which leads to a high round trip efficiency of 95.3%. In addition, due to the high ionic conductivity of the liquid anode and low interfacial resistance between the liquid anode and solid electrolyte, the battery demonstrated a power density of 39 mW cm−2, the highest value to date for alkaline metal-air batteries. Furthermore, the battery exhibited good cycling performance, and no significant degradation in energy efficiency was observed during the whole charge-discharge process. More importantly, no strong reaction was observed between the liquid anode and water, which significantly improved the safety of the battery. The highly conductive liquid anode can be extended to other sodium-based batteries. Therefore, this work provides an avenue for exploring new types of highly safe liquid anode batteries, which is a potential candidate for next-generation energy storage technology.