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 parallel to Na2S5 full cell is finely tuned resulting a 45 mAh g(-1) cycling capacity over 200 cycles.