Sodium (Na) super ionic conductor (NASICON) polyanionic compounds have recently attracted much attention from the battery community because of their electroactive properties and reasonably high ionic conductivities, leading to their use as a cathode in sodium-ion batteries. This article describes the compositional optimizations, crystallographic evaluations, and electrochemical behavior of a new mixed NASICON polyanionic compound, NaFe2−xVx(PO4)(SO4)2. By doping the characteristic Fe3+ sites of the FeO6 octahedrons with varying amounts of V3+, the electrochemical stability and charge transport in NaFe2(PO4)(SO4)2 were enhanced. The resulting best composition, with crystal structure NaFe1.4V0.6(PO4)(SO4)2 resolved through the Rietveld method, exhibited a stable capacity compared with the other synthesized compositions. In situ powder x-ray diffraction measurements, a single-phase intercalation/deintercalation mechanism of the NASICON structure in the measured sodium concentration window was observed with no impurity phase formation. Further electrochemical assessments revealed the interfacial charge transfer kinetics to be the rate-limiting step in the sodium concentration window. Also, the measured sodium-ion diffusivity values in the range of 6 × 10−11 to 7 × 10−11 cm2/s in the measured sodium concentration range. The results reported here highlight the potential of compositionally and morphologically optimized NaFe1.4V0.6(PO4)(SO4)2 with higher particle surface areas as a cathode material for high-performance sodium-ion batteries.