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The NASICON‐type Na₃V₂(PO₄)₃ (NVP) cathode material is investigated in an aqueous sodium‐ion battery, which is explored by using a three‐electrode system. The battery behaviors and capacitive properties of this electrode system are critically investigated by using 1 m Li₂SO₄, Na₂SO₄, and K₂SO₄ electrolytes, with an optimal performance found to arise in Na⁺‐based electrolyte, which exhibits a capacitance of 209 Fg⁻¹ at 8.5 C as well as enhanced ion diffusion. Larger, hydrated Li⁺ is less able to diffuse into the network of NVP, and the low conductivity and mobility leads to near noncapacitive behavior. In the case of K⁺‐based electrolyte, NVP presents asymmetric cyclic voltammograms, owing to weak solvation and the high conductivity of K⁺, making the ions more easily able to form electric double‐layer capacitance on the surface or pores of NVP, rather than insert into the network. The equivalent …