The synthesis of a new ionic liquid (IL), consisting of the symmetric tetra-butyl-phosphonium (P4444+) cation and the (nonafluorobutanesulfonyl)(trifluoromethanesulfonyl)imide (IM14–) anion, via a facile and environmentally-friendly aqueous route is reported. The novel P4444IM14 IL demonstrates excellent thermal and electrochemical stability (beyond 6 V vs. Li+/Li0 (against Ni-foil)) in combination with good near-room temperature conductivity and ionic liquid characteristics, such as non-measurable volatility and exceptional flame-retardant properties. Employed as electrolyte component in Li-metal cells together with the high-voltage, Li1.2Ni0.2Mn0.6O2 positive electrode material, enables an initial discharge capacity of 264 mA h g-1 at 12.5 mA g−1 and improved initial Coulombic efficiency. Additionally, the discharge capacity retention is promising (92.3% after 50 cycles and 84.4% after 100 cycles) when compared with using a conventional organic carbonate-based electrolyte (55.5% after 50 cycles). Further development of this ionic liquid electrolyte may facilitate the practical application of Li-metal anodes in high-voltage/high-energy lithium batteries. © 2021 Elsevier B.V.

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