Lithium (Li)-metal batteries have garnered considerable attention as a promising high-energy-density power source beyond commercial Liion batteries. [1][2][3][4] In order to achieve the ultimate goal of high-energydensity, Li-metal anodes should be preferentially coupled with highvoltage/high-capacity cathodes. [5,6] However, the catalytic reactivity of high-voltage cathode materials leads to undesired interfacial side reactions with electrolytes, resulting in electrolyte decomposition and capacity decay upon cycling. [7,8] This issue of the high-voltage cathodes, together with strong reducing nature of Li-metal anodes, poses an obstacle to practical applications of Li-metal batteries. Besides these energy-density and longevity issues, another formidable problem of Li-metal batteries is their safety; Li dendrites grown from Li-metal anodes vigorously react with liquid electrolytes and can trigger internal short-circuit failures, leading to cell fire or explosion through uncontrolled thermal runaway. [9][10][11] The aforementioned challenges of Limetal batteries often stem from liquid electrolytes and their interfacial instability with electrodes. Conventional carbonate-based liquid electrolytes, widely used in commercial Li-ion batteries, are not stable with Limetal anodes [12] and show a limited electrochemical stability window (<4.4 V). [13,14] Ether-based (localized) high-concentration electrolytes were suggested to improve cycling performance of Li-metal batteries. However, their insufficient oxidation stability limited a cell voltage to 4.5 V. [15,16] Recently, nonflammable fluorinated electrolytes have been explored to widen electrochemical stability window and mitigate reactivity of Li-metal anodes. [17,18] However, these electrolytes contain volatile solvents with low boiling points, which thus hinders the application of resulting Li-metal batteries in hightemperature environments.In contrast to the previous studies mostly focusing on carbonate-or ether-based electrolytes that have shown the limitations in thermal tolerance and operating voltage window, herein, we propose a new class of nitrile electrolytes based on concentrated nitrile electrolytes (CNEs) containing co-additives (Li nitrate (LiNO 3 ) and indium fluoride (InF 3 )). This new nitrile electrolyte enables the stable operation of high-voltage (4.9 V-class) Li-metal cells over wide range of temperatures and even under exposure to flame. The CNE is composed of high-concentration lithium bis(fluorosulfonyl) imide (LiFSI) in a solvent mixture of succinonitrile (SN)/acetonitrile (AN). SN is known to show high oxidation stability (a prerequisite for high-voltage cells), along with ion-dissociating capability, thermal stability (boiling point of 266 °C), and nonflammability. [19][20][21] However, its poor reduction stability against Li metals leads to undesired electrochemical polymerization that forms a thick passivation layer, thereby hampering ion transport toward Li metals. [22,23] To resolve this issue, we exploit a concept of concentrated elect...
