Ionic liquid-based solutions and membranes as safe electrolytes for lithium batteries

Safety is one of the major issues still preventing the wide spread commercialization of lithium ion batteries as storage systems for renewable energy plants as well as power sources for low emission, hybrid electric vehicles or, even, for no emission, full electric vehicles. Recently, the research aimed at reducing the safety hazard in lithium batteries has successfully considered the replacement of the conventional, volatile and flammable organic electrolytes with highly stable ionic liquid (IL)-based solutions [1,2]. Further improvements, in terms of safety and reliability, can be achieved by moving from the liquid solutions to the polymer electrolytes [3]. Goal of the present work is to define novel, lithium conducting electrolyte mixtures formed by an IL, which contributes to safety and electrochemical stability, together with polar-aprotic solvents, providing low viscosity and good wetting of the electrodes’ active mass, to finally develop advanced gel polymer electrolyte (GPE) based on selected IL-containing mixtures. To this purpose, the behaviour of various ILs, both commercial and in-house synthesized, used as additives to lithium-salt solutions formed by conventional ethylene- and dimethyl- carbonate (EC/DMC) solvents, has been compared. Different IL cations, of the family of small saturated quaternary ammonium, have been considered. Also, two anion formulations, for both IL and Li salt, have been investigated, i.e. hexafluorophosphate (PF6-) and bis(trifluoromethanesulfonyl)imide (TFSI-). The electrolyte mixture solutions, optimized in their composition, as well as the related GPEs, have been thermally and electrochemically characterized. The safety concern has been also addressed by means of ex-situ flammability tests. Interesting improvements have been revealed by the IL-based mixtures, with respect to more conventional Li-salt/EC/DMC solutions, below 0 °C, opening possible usage in low temperature Li battery devices. Applicability of selected electrolyte compositions has been proven together with LiFePO4 cathode and Li4Ti5O12 anode materials.