“Ionic liquids-in-salt” – a promising electrolyte concept for high-temperature lithium batteries?

Abstract

A novel electrolyte concept for lithium-ion batteries, termed “ionic liquid-in-salt”, is introduced. Our feasibility study on (1 − x)EMIMTFSI:(x)LiTFSI, 0.66 ≤ x ≤ 0.97, showed that at elevated temperatures the various dual liquid and solid phase regions are characterized by a wide thermal stability window, high ionic conductivities and appreciable mechanical integrity. The highest conductivity values are obtained for the compositions x = 0.70 and x = 0.75 (σ ≈ 6 × 10⁻³ S cm⁻¹) and are related to the final melting of the materials. Overall, high conductivities are observed for 0.70 < x < 0.90 while low ones are found for x > 0.90. Raman and NMR spectroscopies reveal the presence of highly mobile Li-containing species, partly identified as [Li(TFSI)₂]⁻, albeit rather unexpected for these high x values, which might explain the high ionic conductivities observed. To prove the general value of our concept in more detail, some first results on BMIMTFSI and PY₁₃TFSI based systems are also presented.