Room-temperature Na-ion batteries (NIBs) have recently attracted attention as potential alternatives to current Li-ion batteries (LIBs). The natural abundance of sodium and the similarity between the electrochemical properties of NIBs and LIBs make NIBs well suited for applications requiring low cost and long-term reliability. Here, the first successful synthesis of a series of Na3(VO1−x PO4)2F1+2x (0 ≤ x ≤ 1) compounds as a new family of high-performance cathode materials for NIBs is reported. The Na3(VO1−x PO4)2F1+2x series can function as high-performance cathodes for NIBs with high energy density and good cycle life, although the redox mechanism varies depending on the composition. The combined first-principles calculations and experimental analysis reveal the detailed structural and electrochemical mechanisms of the various compositions in solid solutions of Na3(VOPO4)2F and Na3V2(PO4)2F3. The comparative data for the Na y (VO1−x PO4)2F1+2x electrodes show a clear relationship among V3+/V4+/V5+ redox reactions, Na+−Na+ interactions, and Na+ intercalation mechanisms in NIBs. The new family of high-energy cathode materials reported here is expected to spur the development of low-cost, high-performance NIBs.