Room temperature ionic liquids (RTILs) have recently been investigated for lithium and lithium ion battery (LIB) applications due to their nonvolatility and thus lack of (or much reduced) flammability and increased safety. RTIL can have ionic conductivities of 10^-2S/cm¹.  In LIBs, the only electroactive species is the Li⁺ ion, which is transported between the anode and the cathode, and therefore a lithium salt with the same anion as the RTIL is used, typically at 0.25- 1 M concentrations².

Solid polymer electrolytes have advantages such as processibility, ability to be laminated to electrodes and elimination of heavy containment materials that decrease gravimetric energy density, but have low ionic conductivities (< 10^-5 S/cm). RTILs have therefore been incorporated into polymers with a view towards increasing their ionic conductivity, but still maintaining sufficient mechanical strength to form flexible, thin membranes. Since a liquid is incorporated into the polymer, they are referred to as gel electrolytes, and when the liquid is a RTIL, they are often referred to as ion gels.

The addition of both LiX and a polymer into RTILs increase their viscosities and decrease their ionic conductivities. Further, low lithium ion transference numbers (t_Li⁺), the fraction of the charge carried by the electroactive Li⁺ cation, are typically measured in RTIL/LiX systems. High values of t_Li⁺ are desirable to increase the power density of LIBs, while low values cause polarization in the electrolyte. Diffusion coefficients, measured by  pulse field gradient NMR, of the ionic liquid anions and cations, as well as the Li⁺ cations decrease in the order D_cation > D_anion > D_Li+³.

Increases in t_Li⁺ can be achieved by increasing the number density and diffusion coefficients of lithium ions, or decreasing the diffusion coefficients of the anions or cations of the RTIL. For LiX/RTIL investigated to date, t_Li⁺ increases with the mole fraction of LiX (to ~0.3)³. Larger anions with greater charge delocalization would also enhance t_Li⁺ by decreasing ion pairs and aggregates, thus leading to higher lithium ion diffusivities. Last, interaction of the anions with a polymer can slow down their motions, and decrease their interaction with the Li ions, also enhancing t_Li⁺⁴.

Here, we have synthesized a new multi-ionic lithium salt (Figure 1), referred to as 4merLi-TFSI, and compared its electrochemical and mechanical properties with the lithium imide salt (LiTFSI). These salts are highly dissociative due to the electron-with-drawing nature of the trifluoromethyl substituents and delocalization of the negative charge that promotes low lattice energies in the solid state, so that these salts are more easily solvated by their hosts. Both have been incorporated into PYR₁₄TFSI, the RTIL shown in Figure 1, and formed as ion gels using methyl cellulose (MC). All components are soluble in dimethyl formamide (DMF). The solutions are liquids at high temperature, but form gels upon cooling. Removal of the DMF results in ion gels composed of PYR₁₄TFSI/LiTFSI/MC or PYR₁₄TFSI/4merLi-TFSI/MC. At the same Li⁺ molarity, ion gels prepared with either LiTFSI or 4merLi have very similar conductivities and mechanical properties (in the MPa range). However, the 4merLi, with its large (1270) anion, has lithium ion transference numbers, t_Li⁺ that are ~ 0.39, much larger than PYR₁₄TFSI/LiTFSI (t_Li⁺ ~ 0.05) or PYR₁₄TFSI/LiTFSI/MC (t_Li⁺ ~ 0.2). The improvement in t_Li⁺ is due to the bulkiness of the 4mer anion, and the interaction of the anions with the OH groups of MC. Interaction of the anions with the MC decreases their formation into aggregates with the Li⁺ cation. Unlike what is sometimes observed in the case of single ion conductors, the conductivity does not decrease as the t_Li⁺ increases.

These separators have high thermal stability, dictated by the degradation temperature of the MC and the selected Li salt, since the thermal decomposition of the RTIL is > 400 ^oC, and good electrochemical stability. The best ambient temperature ionic conductivity 3×10^-4 S/cm was achieved for the 80/20 PYR₁₄TFSI/MC 4merLi-TFSI.

References

1.            Galinski, M.; Lewandowski, A.; Stepniak, I., Ionic liquids as electrolytes. Electrochimica Acta 2006, 51, (26), 5567-5580.

2.            Lewandowski, A.; Swiderska-Mocek, A., Ionic liquids as electrolytes for Li-ion batteries-An overview of electrochemical studies. Journal of Power Sources 2009, 194, (2), 601-609.

3.            Jeremias, S.; Kunze, M.; Passerini, S.; Schoenhoff, M., Polymerizable Ionic Liquid with State of the Art Transport Properties. Journal of Physical Chemistry B 2013, 117, (36), 10596-10602.

4.            Gouverneur, M.; Jeremias, S.; Schoenhoff, M., Li-7 nuclear magnetic resonance studies of dynamics in a ternary gel polymer electrolyte based on polymeric ionic liquids. Electrochimica Acta 2015, 175, 35-41.