(78f) Solvate Ionic Liquid-Based Gel Electrolytes Containing Functionalized Polymer-Based Networks for Use in Lithium Metal Battery Applications

Lithium metal batteries (LMBs) offer a promising advancement in battery technology by replacement of the graphite anode in Li-ion batteries with a more energy dense Li metal electrode. The use of a Li metal anode allows for the production of a battery with an increased energy density estimated to be about 1200 Wh/L (400 Wh/kg). LMBs require the use of an electrolyte that exhibits high electrochemical stability (> 4.2 V vs. Li/Li⁺), high ionic conductivity (> 0.1 mS/cm), a Li-ion transference number of ~0.2-0.5, and a high elastic modulus (~10-1,000 MPa) to prevent dendrite formation at the Li metal anode-electrolyte interface. Ionic liquids (ILs) offer the potential to replace volatile aqueous- and organic-based electrolytes due to their moderate room temperature ionic conductivity (~1 mS/cm), negligible vapor pressure, and high electrochemical stability. Solvate ionic liquids (SILs) are an emergent subclass of IL electrolytes that offer the advantages of traditional ILs while incorporating an inorganic cation (or anion) species, which is coordinated by a neutral ligand to form a complex ion. For example, lithium cation-containing SILs have previously been demonstrated by complexing the Li⁺ cation of an appropriate lithium salt with oligoether ligands (such as tetraglyme) to form a nonvolatile liquid electrolyte that is suitable for lithium-based electrochemical energy storage devices.

While SIL electrolytes hold great promise for many future applications, incorporating these liquids into solid-state gel electrolytes is a compelling direction of investigation due to the leakproof nature and robust flexibility of gels that can also potentially prevent dendrite formation for LMB applications. Recently, we fabricated mechanically robust solvate ionogel electrolytes via in situ free-radical polymerization/cross-linking of poly(ethylene glycol) diacrylate (PEGDA) to immobilize the solvate ionic liquid [Li(G4)][TFSI], which consists of an equimolar mixture of lithium bis(trifluoromethanesulfonyl)imide (Li[TFSI]) and tetraglyme (G4).¹ Solvate ionogels with varying PEGDA content have been prepared, and their electrochemical, ion transport, and mechanical properties have been measured. The solvate ionogels displayed enhanced lithium ion diffusivity values, which increased from 6.0 x 10^-12 m²/s to 10.0 x 10^-12 m²/s as the PEGDA content was increased from the minimum gelation point (8 vol.%) up to 21 vol.% polymer; the corresponding Li⁺ transport numbers increased from 0.510 to 0.575, respectively. Meanwhile, room temperature ionic conductivity measurements reveal high conductivities, up to 0.89 mS/cm at 8 vol.% PEGDA and 0.39 mS/cm at 21 vol.% PEGDA. Compression testing verifies a widely tunable elastic modulus of these solvate ionogels, varying from 2 to 640 kPa. In this work, chemical modification of the polymer network has been investigated in order to increase the compressive elastic modulus up to ~10 MPa while maintaining a practical room temperature ionic conductivity of 0.43 mS/cm. Tensile testing, AC impedance spectroscopy, linear sweep voltammetry, DC polarization, and charge-discharge testing using a commercial cathode are performed in order to determine the most promising solvate ionogel electrolyte candidates for LMB applications.

¹D’Angelo, A. J.; Panzer, M. J. J. Phys. Chem. B 2017, 121, 890-895.