Fluorine-Free Ionic Liquid-Based Electrolytes
Faiz Ullah SHAH1, Andrei FILIPPOV1, Patrik JOHANSSON2
1Chemistry of Interfaces, Luleň University of Technology, Luleň, Sweden
2Department of Physics, Chalmers University of Technology, Gothenburg, Sweden
The electrolytes of our modern energy storage devices contain flammable organic solvents and fluorinated lithium/sodium salts that are intrinsically unsafe when exposed to heat and fire – this can be very problematic in confined spaces. The decomposition of these fluorinated salts produce hydrofluoric acid (HF), among other toxic gases. Another challenge is the recycling and handling of spent devices with high fluorine content posing a chemical hazard and a serious threat to occupational safety, human health and environment. To develop next-generation energy storage devices, the development of new fluorine-free and non-flammable electrolytes is indispensable; these electrolytes should conduct ions efficiently and also directly influence the safety, service life and overall performance of the energy storage devices.
Ionic liquids (ILs) with desired properties can potentially replace the conventional electrolytes. We develop novel fluorine-free IL-based electrolytes that can be used as solvent-free electrolytes with unique properties; they are non-flammable, liquid at low temperature, thermally and electrochemically stable. The synthesis and characterizations including multinuclear (7Li, 23Na, 31P, 13C, 11B, etc.) solution-state NMR, solid-sate MAS-NMR, MS, FTIR and Raman techniques, thermal properties (TGA and DSC), ionic conductivity and NMR diffusivity are performed. A range of new classes of IL-based electrolytes have been developed that can potentially be used as electrolytes in various energy storage devices.
One such example is the recently synthesized IL-based electrolytes; the IL contains tetrabutyl-phosphonium, (P4,4,4,4)+, coupled to a new anion: 2-[2-(2-methoxyethoxy)ethoxy]acetate anion (MEEA)-, and doped with 10-40 mol% of Li(MEEA). The PFG NMR data suggested faster diffusion of the (MEEA)- anion than (P4,4,4,4)+ in the neat IL, but the addition of Li-salt resulted in a slightly lower mobility of the anion than the cation. The ionic conductivity is also decreased with the addition of Li-salt. This agreed well with the combined 7Li NMR and ATR-FTIR spectroscopic data, which unambiguously revealed preferential interactions between the lithium cations and the carboxylate groups of the IL anions. Altogether, these systems provide a stepping-stone for further design of fluorine-free and low glass transition temperature IL-based electrolytes.