Stable cycling of high nickel Li-metal batteries with limited Li anode in fluorine rich flame retardant electrolytes

Abstract

Traditional carbonate-based electrolytes are not suitable for high voltage lithium metal batteries (LMBs), which seriously hinders the development of high-energy density LMBs. Here, a novel flame retardant fluorine-rich electrolyte (termed FFT) composed of 1.2 M lithium hexafluorophosphate (LiPF6) in a mixed solvents of fluoroethylene carbonate (FEC), 3,3,3-fluoroethylmethyl carbonate (FEMC) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) was rationally designed and studied. It exhibits noninflammability, low viscosity, good wettability and fluidity, and high oxidation stability. The designed electrolyte enables Li||NMC622 with limited Li anode (50 µm thickness) and high cathode active material loading of 13.8 mg cm−2 (2.5 mAh cm−2) to remain 80% capacity after 160 cycles and 135 cycles with a high cut-off of 4.4 V and 4.5 V under charge at C/5 and discharge at C/2 at 25 °C, respectively. Moreover, the Li||NMC622 using FFT also exhibits excellent low temperature discharge capacity, which could deliver 122 mAh g−1 (about 62% of the discharge capacity at 25 °C) even at −40 °C. Further analyses reveal the stable and conductive CEI and SEI with LiF-rich are formed on both cathode and anode, which contributes to the stable cycle of the battery.