High Ionic Conductivity, Mechanically Strong Ion Gels Made from Methyl Cellulose/PYR14tfsi
Authors: Chinnam Parameswara Rao , Mantravadi Ramya , Wunder Stephanie 
Published: February 2020

Room temperature ionic liquids (RTILs), which are salts with low temperature melting points, have been investigated as electrolytes in electrochemical devices1, since they are nonvolatile (and as a result often non-flammable), and therefore safer than conventional aprotic liquid electrolytes, and have useful electrochemical stability windows (> 4V)2. However, they are typically more viscous (30-200 cP vs ~ 1 cP) than conventional aprotic or aqueous solvents, and therefore less conductive (0.1 to 18 mS/cm). Ion gels are ion conducting liquids such RTILs immobilized in a polymer matrix, forming a solid polymer electrolyte (SPE). They have become increasingly important as components in polymer electrolytes3 for potential use as flexible solid-state electrochemical devices such as separators in lithium ion batteries and gate insulators in organic thin-film transistors (OTFTs). Immobilization of the RTIL can be achieved using gelators that participate in chemical crosslinking reactions (e.g. methylmethacrylate)7 or form physical crosslinks. Physical gelation can be achieved with block copolymers, where one component (polyethylene oxide) solvates and the other component (polystyrene) is immiscible in the IL, or poly(vinylidene fluoride)-hexafluoropropylene copolymers (PVDF-HPF), where the crystalline component forms the crosslink sites.

Here, methyl cellulose, a natural, renewable, environmentally friendly, abundant and inexpensive natural polymer, is used as the gel former,  and combined with the ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (referred to here PYR14TFSI) to form tough, temperature stable ion gels. The detailed procedure for making the ion gels is presented in figure 2.The morphology of the ion gels, shown for the 90/10 composition (Figure 2), is a volume spanning, random 3-D network of nanometer diameter MC fibrils. Linear fibers can form gels through their topological interactions alone, i.e., without cross-links, provided the fibers are sufficiently long. Although the network looks dense (Figure 2), it is the result of its collapse upon removal of the 90% liquid component.

The phase separated morphology of the PYR14TFSI/MC ion gels results in both excellent mechanical properties as well as high ionic conductivities (Figure 1). As expected, the storage moduli increase and ionic conductivities decrease with MC content. Room temperature moduli > 1 GPa are achieved for all compositions with < 60% PYR14TFSI and the PYR14TFSI/MC = 90/10 and 80/20 have respectable RT moduli of 0.15 GPa and 0.75GPa, respectively. The frequency dependence of both the storage and loss moduli for the PYR14TFSI/MC = 90/10 composition, show solid behavior over the whole frequency range.

                In summary, we have developed a facile route for the preparation solid polymer electrolyte ion gels from PYR14TFSI/MC. These ion gels have the highest combined moduli and ambient ionic conductivities (> 1 x 10-3S/cm) to date. These favorable properties are attributed to the immiscibility of PYR14TFSI in MC, which permits the ionic conductivity to be independent of the MC, and the high Tm and Tg of the MC fibrils.

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.            Susan, M. A.; Kaneko, T.; Noda, A.; Watanabe, M., Ion gels prepared by in situ radical polymerization of vinyl monomers in an ionic liquid and their characterization as polymer electrolytes. Journal of  the American Chemical Society
2005, 127, (13), 4976-4983.

Figure 1