Ionic Liquid Electrolyte for Lithium Metal Batteries: Physical, Electrochemical, and Interfacial Studies of N-Methyl-N-butylmorpholinium Bis(fluorosulfonyl)imide

The ionic liquid (IL) N-methyl-N-butylmorpholinium bis(fluorosulfonyl)imide (C4mmor FSI) is examined from physical and electrochemical perspectives. Pulsed field gradient NMR spectroscopy shows that ion diffusivities are low compared with similar, non-ethereal ILs. Ionicity values indicate that above room temperature, less than 50% of ions contribute to conductivity. Lithium cycling in symmetrical cells using a C4mmor FSI-based electrolyte is best demonstrated at elevated temperatures. Specific capacities of 130 mAh g−1 are achieved in a Li−LiFePO4 battery at 85 °C. FT-IR spectroscopic investigations of lithium electrodes suggest the presence of alkoxide species in the solid electrolyte interphase (SEI), implying a ring-opening reaction of C4mmor with lithium metal. In contrast, the SEI derived from N-methyl-N-propylpiperidinium FSI lacks the alkoxide signature but shows signs of alkyl unsaturation, and the activation energy for Li+ transport through this SEI is slightly lower than that for the C4mmor-derived SEI. Our detailed findings give insight into the capabilities and limitations of rechargeable lithium metal batteries utilizing a C4mmor FSI electrolyte.