Toward Safe and Sustainable Batteries: Na4Fe3(PO4)2P2O7 as a Low Cost Cathode for Rechargeable Aqueous Na-Ion Batteries

The electrochemical properties of Na4Fe3(PO4)(2)P2O7 in aqueous and organic electrolyte are compared under similar conditions.. Na4Fe3(PO4)(2)P2O7 is able to deliver almost the same capacity in both types of electrolytes despite the smaller electrochemical window entailed -by the aqueous one. As shown by electrochemical impedance spectroscopy (EIS), this is possible thanks to the lower overpotential that this material exhibits in aqueous electrolyte. It is shown here that the-main Contribution to-overpotential in organic electrolyte mainly originates from a SPI (Solid Permeable Interphase) layer formed below 3.5 V vs Na+/Na that acts, as a blocking layer and hinders Na+ diffusion:and that is absent in aqueous electrolyte. Overall; the obtained, result highlight the positive attributes of using low-cost and environmentally friendly aqueous electrolytes and the challenges to be overcome in terms of air and moisture stability of the studied material.