In-situ tracking of NaFePO₄ formation in aqueous electrolytes and its electrochemical performances in Na-ion/polysulfide batteries

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.