Rationally-Designed Solvate Ionogel Electrolytes for Improved Lithium-Sulfur Battery Performance
Authors: Hubble Dion , Qin Jiaxu , Lin Francis , Murphy Ian Alexander , Li Yun , Yang Jihui , Jen Alex 
Published: February 2020

High-capacity battery technology has become a dire need in an increasingly energy-hungry world, especially if mass transportation without fossil fuels is to be realized. Lithium-sulfur batteries are an intriguing solution to this need, stemming from their theoretical energy density 5x that of current lithium-ion systems, as well as the relative abundance of their active materials. However, commercialization of lithium-sulfur batteries has yet to be effectively realized due to a multitude of intrinsic challenges with this chemistry, ranging from dendrite growth on the lithium metal anode to dissolution of polysulfide intermediates in the electrolyte, resulting in capacity loss, redox shuttling, and eventual cell failure.

Ionic liquid electrolytes have recently emerged as a popular method for addressing these challenges, since they generally have a far lower solubility for lithium polysulfides than organic electrolytes, and also tend to suppress lithium dendrite growth, while still maintaining relatively high ionic conductivity. So-called solvate ionic liquids (SILs), based on stoichiometric complexes of tetraglyme with lithium salts, have proven particularly effective at addressing lithium-sulfur performance challenges, since the transference number of Li+ in these solvents tends to be 0.5 or higher, unlike ternary ionic liquids containing organic cations in addition to Li+. SILs can also be diluted with low-basicity solvents in order to modulate various properties without changing their ionic-liquid-like character.

Combining organic electrolytes, or even ionic liquids, with solid particles or polymers to form free-standing gel electrolytes has also proven popular for improving lithium-sulfur performance. Immobilized electrolytes are not only useful to prevent leakage in larger cells, but can also produce effects such as increased lithium dendrite resistance, trapping or blocking of dissolved lithium polysulfides, and elastic buffering of mechanical stress from electrode volume change during operation. However, introduction of a solid matrix almost always lowers ionic conductivity significantly, which negatively affects performance at moderate or high current density.

In this work, we demonstrate ionogel electrolytes based on SILs and polymerizable methacrylate groups with varying functionality. This materials platform allows us to achieve very high conductivity for an ionogel (>1 mS/cm), while also addressing the unique challenges of lithium-sulfur chemistry through rational molecular design of polymer structure. Our strategy allows us to adjust electrolyte properties such as lithium transference number and polysulfide affinity for optimum battery performance, while requiring no elaborate fabrication steps or costly chemical reagents. We discuss both the properties of, and rationale behind, initial gel designs, as well as the implications for lithium-sulfur battery performance and creative designs we plan to pursue in the near future.

Figure 1