In-situ formation of pure olivine NaFePO4 from chemically synthesized LiFePO4 nanoparticles via an electrochemical ion-exchange route in sodium salt containing aqueous electrolyte is reported. Both in-situ electrochemical quartz crystal microbalance (EQCM) and in-situ X-ray diffraction (XRD) measurements are performed to monitor the formation of NaFePO4. Subsequently, a rechargeable Na-ion aqueous polysulfide battery is demonstrated where NaFePO4 and dissolved Na2S5 solution are used as cathode and anolyte, respectively; and are separated from each other by an ion-exchange polymeric membrane. In order to prevent diffusion of Na2S5 polysulfide from the anode to the cathode side, salt concentration at both sides of the NaFePO4||Na2S5 full cell is finely tuned resulting a 45 mAh g−1 cycling capacity over 200 cycles.