Because of their potentially superior safety characteristics, room temperature ionic liquids (RTILs or ILs) have been vigorously researched as a potential replacement for current commercial lithium battery electrolytes, which are based on volatile and flammable organic carbonates. However, relatively poor battery performance, which is a consequence of the higher viscosity and lower conductivity of these materials, has prevented them becoming mainstream electrolytes for commercial lithium batteries. Amongst various RTILs, those containing the bis(fluorosulfonyl)imide (FSI) anion exhibit high conductivities and diffusivities, making them interesting potential electrolytes for lithium metal batteries. Here, we evaluate the electrochemical stability, lithium electrochemistry, and Li+ transference numbers of FSI-based ionic liquid electrolytes intended for use in rechargeable Li metal batteries. We show that ILs containing high concentrations of lithium, up to 3.2 mol.kg−1 in C3mpyr FSI, have excellent rate capability (higher than that of standard battery electrolytes) with both the lithium metal electrode and LiCoO2 cathode, in spite of their significantly higher viscosities and lower conductivities. This unusual behavior is ascribed to the concomitant increase in transference number with increasing Li-salt concentration.
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