Lithium Batteries Research Papers - Academia.edu (original) (raw)

Lithium-ion solvate ionic liquids (SILs), consisting of complexed Li + cations and a weakly basic anion, represent an emergent class of nonvolatile liquid electrolytes suitable for lithium-based electrochemical energy storage. In this report, solid-state, flexible solvate ionogel electrolytes are synthesized via UV-initiated free radical polymerization/cross-linking of poly-(ethylene glycol) diacrylate (PEGDA) in situ within the [Li(G4)][TFSI] electrolyte, which is formed by an equimolar mixture of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and tetraglyme (G4). Ion diffusivity measurements reveal enhanced Li + diffusion in PEGDA-supported solvate ionogels, as compared to poly(methyl methacrylate)-supported gels that lack ethylene oxide chains. At 21 vol% PEGDA, a maximum Li + transport number of 0.58 and a room temperature ionic conductivity of 0.43 mS/cm have been achieved in a solvate ionogel electrolyte that exhibits an elastic modulus of 0.47 MPa. These results demonstrate the importance of polymer scaffold selection on solvate ionogel electrolyte performance for advanced lithium-based batteries. ■ INTRODUCTION Ionic liquids (ILs), or room temperature molten salts, are composed entirely of ions and demonstrate unique properties with great potential as electrolytes for electrochemical energy applications, including supercapacitors, fuel cells, dye-sensitized solar cells, and lithium-ion batteries. 1,2 ILs possess moderate room temperature ionic conductivity (∼1−10 mS/cm), high electrochemical stability (3−5 V), and negligible vapor pressure. 3,4 The first manifestations of Li +-containing IL electrolytes for lithium-ion battery prototypes were prepared via dissolution of a solid lithium salt (with a typical concentration of 1 M) into an IL, resulting in a substantial increase in solution viscosity at a relatively low concentration of lithium cations. 5 This increase in viscosity yields maximum room temperature ionic conductivities in the range of only ∼0.1 mS/cm, while also limiting the lithium-ion transport number (t Li +) to a value of 0.3 or lower due to the presence of at least three different ionic species in the electrolyte. 6 Recently, a new class of lithium-containing electrolytes has emerged within the IL research community, namely, solvate ionic liquids (SILs). 7−11 SILs offer some of the same advantages of traditional ILs, such as ultralow vapor pressure and non-flammability, and consist of a salt combined with a ligand that is strongly coordinated with either the cation or anion to form a complex ion. 8 An early example of a lithium-ion SIL was reported by Pappenfus et al. via an equimolar combination of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) with the organic ligand tetra(ethylene glycol) dimethyl ether (tetra-glyme, G4) to form [Li(G4)][TFSI]. 9 SILs can exhibit room temperature ionic conductivities of at least 1 mS/cm, high electrochemical stability up to 4.5 V vs Li/Li + , 7,10,11 and, importantly, larger t Li + values (∼0.5), 11 making them strong candidates for lithium-based energy storage devices. While lithium-ion SILs show promise for developing safer batteries, these liquid electrolytes must still be contained to prevent leakage. Few investigations have been conducted to date on developing free-standing SIL gel composite (i.e., solvate ionogel) electrolytes. Kitazawa et al. fabricated a self-assembled physical gel network using an ABA-triblock copolymer consisting of polystyrene and poly(methyl methacrylate) blocks to form physical cross-links and ion conduction pathways, respectively, in [Li(G4)][TFSI]. 7 At 10 wt% copolymer loading, the solvate ionogel demonstrated a room temperature ionic conductivity of ∼0.4 mS/cm and a t Li + value of 0.52, although the mechanical stiffness was low (shear modulus ∼2 kPa). 7 Quasi-solid-state solvate ionogels have also been fabricated by blending fumed silica nanoparticles into [Li-(G4)][TFSI], as reported by Unemoto and co-workers. 12 A solvate ionogel containing 25 vol% SiO 2 nanoparticles exhibited a room temperature ionic conductivity of 0.15 mS/cm and a t Li + of 0.53. 12 Obtaining a high t Li + value is critical for the realization of a high power lithium-ion battery, where lithium ions are capable of carrying a greater fraction of current through the device. A recent study by Porcarelli et al. attempted to leverage