Improved electrochemical properties of linear carbonate-containing electrolytes using fluoroethylene carbonate in Na4Fe3(PO4)2(P2O7)/Na cells

Abstract

Sodium-ion batteries (NIBs) have attracted considerable attention as promising next-generation rechargeable batteries, especially for large-scale energy storage systems (ESS), because of the natural abundance of Na and the similarities of these batteries to lithium-ion batteries (LIBs). Much effort has been made to improve the electrochemical performances of NIBs through the development of high-performance cathodes, anodes, and electrolytes. One efficient and desirable strategy for practical applications of NIBs is to utilize materials that are adopted in commercialized LIBs. Electrolytes in most studies are composed of polar solvents such as ethylene carbonate (EC) and propylene carbonate (PC). Accordingly, instead of conventional polyethylene (PE) membranes, glass fiber filters (GFF), which easily uptake polar solvents, have been used as separators. However, the too thick, mechanically weak, and porous GFF is not suitable as a separator because it can reduce the volumetric energy density and cannot guarantee the safety of batteries. In this study, for the introduction ofa PE separator into NIBs, the inclusion of linear carbonate as a cosolvent was attempted, motivated by the fact that this material has been widely used owing to its low viscosity and good compatibility with conventional PE separators. However, due to their high reactivity toward Na metal electrodes in half cells, linear carbonate-containing electrolytes are not electrochemically stable at Na4Fe3(PO4)2(P2O7) cathodes during cycling. Undesirable reactions between linear carbonates and Na metal electrodes are examined using 13C nuclear magnetic resonance (NMR) and possible mechanisms for the detrimental effect of byproducts formed by linear carbonate decomposition at the Na metal electrode on the cathode are proposed. To alleviate severe decomposition of linear carbonates at the Na metal electrode, fluoroethylene carbonate (FEC) has been exploited as a functional additive. Our investigation reveals that remarkable enhancement in electrochemical properties of electrolytes with linear carbonates in Na4Fe3(PO4)2(P2O7)/Na half cells is achieved in the presence of FEC additive.