A safe and low-cost aqueous electrolyte, in which maximum amount of sodium trifluoroacetate (NaTFA) is dissolved (26 mol kg-1), showed high ionic conductivity of 23 mS cm-1 and provided wide electrochemical stability window of 3.1 V. These attractive features may originate from so-called “water-in-salt” effect of the highly concentrated electrolyte in addition to a robust fluoride layer formed on anode thereof. This novel electrolyte enabled reversible operation of NASICON-type Na2VTi(PO4)3 as symmetrical operation based on V3+/4+ at cathode and Ti3+/4+ and V2+/3+ at anode, respectively.
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