Robust cathode-ether electrolyte interphase on interfacial redox assembled fluorophosphate enabling high-rate and ultrastable sodium ion full cells

Abstract

The promising ether electrolytes, which greatly improve the performance of anodes of sodium-ion batteries (SIBs), are usually incompatible with high-voltage cathodes due to the oxidation instability. In addition, little is known about the cathode-ether electrolyte interphase (CEIether) and its key effects on battery performance, hindering practical development of ether-based SIBs. Herein, we provide the direct microscopic and spectroscopic evidences for forming beneficial CEIether on 4.3 V cut-off-voltage fluorophosphate cathode in ether electrolyte and elucidate its key role in SIB chemistry. The synergy of ether solvent and salt is unveiled to be very crucial to electrochemical stability window and CEIether generation, which is directly supported by density functional theory calculations. Using NaPF6-DIGLYME as the optimized ether electrolyte, robust fluorine-rich inorganic-organic interphase is identified, which effectively ameliorates the interface, facilitates ultrafast charge transfer and stabilizes high-voltage cathode over 10,000 times with the help of cathode engineering. The cathode is attained by using a novel interfacial redox self-assembly approach via interfacial redox reaction between trivalent vanadium precursor (V3+) and graphene oxide (GO), giving rise to chemically bonded sodium vanadium fluorophosphates/rGO 3D sub-microsphere that facilitates fast ion/electron transport and stability. Consequently, using fluorophosphate 3D sub-microsphere cathode and Na2Ti2O5 nanosheets anode, a full cell is further designed with high initial coulombic efficiency (90%), outstanding rate performance (40 C) and ultrastable cycling performance (>4000 cycles with no decay) that cannot be achieved in ester electrolytes. This work provides inspiration for the rational design of CEIether on cathode to enable practical high-performance high-voltage SIBs using ether electrolytes.