Identifying fast Li ions at the interfaces in composites of ionic liquids (EMIMTFSI) and Li salts (LiTFSI) by long-time ⁷Li NMR

New electrolytes are needed in lithium-based battery research in order to increase both safety and electrochemical performance. The mixture of an ionic liquid with a lithium salt represents a conceptually new class of electrolytes for high-temperature lithium batteries, termed ?ionic liquid-in-salt?. We used ⁷Li NMR spectroscopy, see, e.g., to study both local electronic structures and Li self-diffu¬sion in LiTFSI and Li_xEMIM_(1-x)TFSI with x = 0.9. The NMR spectra, recorded under static conditions, perfectly agree with the results from differential scanning calorimetry. Upon heating to 513 K they clearly reveal several double phase regions; the known solid-state phase transformation of LiTFSI can be well recognized by the change of the quadruple powder pattern of the ⁷Li NMR spectra of LiTFSI. A rapid increase in long-range ion conductivity, within two orders of magnitudes, takes place when the 1/2 EMIMTFSI/LiTFSI phase starts to melt. This can also be monitored by tempera ture-variable ⁷Li spin-lattice relaxation (SLR) NMR. If recorded up to delay times of 1000 s, the pronounced bi-exponential ⁷Li SLR NMR transients found directly reveal a subset of highly mobile Li ions, partly identified as [Li(TFSI)₂]^-, which can be well discriminated from the response of pure LiTFSI. Most likely, this Li sub-ensemble, which is anticipated to be located at the LiTFSI:EMIMTFSI interfacial regions, is responsible for the enhanced ion conductivity observed.