Solvate Ionic Liquid Electrolyte for Li–S Batteries

Abstract

Innovation in the design of electrolyte materials is crucial for realizing next-generation electrochemical energy storage devices such as Li–S batteries. The theoretical capacity of the S cathode is 10 times higher than that of conventional cathode materials used in current Li–ion batteries. However, Li–S batteries suffer from the dissolution of lithium polysulfides, which are formed by the redox reaction at the S cathode. Herein, we present simple solvate ionic liquids, glyme–Li salt molten complexes, as excellent electrolyte candidates because they greatly suppress the dissolution of lithium polysulfides. The molten complexes do not readily dissolve other ionic solutes, which leads to the stable operation of the Li–S battery over more than 400 cycles with discharge capacities higher than 700 mAh g-sulfur−1 and with coulombic efficiencies higher than 98% throughout the cycles. Such high performance has not been realized to the best of our knowledge. Furthermore, the addition of a nonflammable fluorinated solvent, which does not break the solvate structure of the glyme–Li salt molten complexes, greatly enhances the power density of the Li–S battery. The strategic design of electrolyte properties provides opportunities for the development of new electrochemical devices with many different electrode materials.

Ionic Conductivity and Viscosity of Solvate Ionic Liquids Composed of Glymes and Excess Lithium Bis(Trifluoromethylsulfonyl)Amide

“…1 Recently, the electrolytes containing lithium salts at high concentrations have attracted attention due to the distinct characteristics emerged from unique coordination environment of lithium ion and/or unusually low activity of free solvent molecules. [2][3][4][5][6][7][8][9][10][11][12] Some lithium salts are able to be dissolved in glymes (Gn, CH 3 O(CH 2 CH 2 O) n CH 3 ), such as triglyme (G3, n = 3) and tetraglyme (G4, n = 4), at extremely high concentrations (> 3 mol dm ¹3 ). 13,14 It has been known that stoichiometric LiTFSAglyme mixtures (TFSA ¹ = bis(trifluoromethylsulfonyl)amide), of which the mole fractions of LiTFSA, X LiTFSA , are 50 mol%, are able to be regarded as solvate ionic liquids since the mixtures consist of a complex cation of lithium ion and glyme, [Li(glyme)] + , and TFSA ¹ anion according to the following equilibrium.…”

Design and Materialization of Ionic Liquids Based on an Understanding of Their Fundamental Properties

“…[115][116][117][118][119][120] The theoretical capacity of the S cathode is ten times greater than that of conventional cathode materials used in current Li-ion batteries. However, Li-S batteries suffer from the dissolution of lithium polysulfides, which are formed by the redox reactions at the S cathode.…”

“…98 Furthermore, the transport properties of solvate ILs are much better than conventional Li-doped ILs 113 and can be further improved by dilution with solvents that do not disrupt the solvate structures. 117,122 Consequently, solvate ILs are suitable for use as electrolytes in lithium ion batteries [111][112][113][114] and lithium-sulfur batteries. [115][116][117][118][119][120] The most striking and significant findings of our research on solvate ILs are the formation criteria of solvate ILs, 101 the enhanced oxidation stability of coordinating solvents, 95 the low coordinating properties of solvate cations and anions, and their electrode reactions with graphite.…”

“…Such low solubilities lead to the stable operation of Li-S batteries for more than 400 cycles with discharge capacities higher than 700 mAh g ¹1 S ¹1 and coulombic efficiencies higher than 98% throughout the cycles. 117 Furthermore, the addition of low polar solvents, which do not disrupt the structure of the [Li(glyme)] solvates, greatly enhances the power densities of Li-S batteries. 117 Electrochemical Li + intercalation into graphite electrodes is solvent-selective, and the formation of a favorable solid electrolyte interphase (SEI) that enables desolvation of Li + at the interface and conduction Li + in the layer is considered to be essential.…”

“…117 Furthermore, the addition of low polar solvents, which do not disrupt the structure of the [Li(glyme)] solvates, greatly enhances the power densities of Li-S batteries. 117 Electrochemical Li + intercalation into graphite electrodes is solvent-selective, and the formation of a favorable solid electrolyte interphase (SEI) that enables desolvation of Li + at the interface and conduction Li + in the layer is considered to be essential. 125 121 The success of the electrochemical intercalation/deintercalation can be correlated with the activity the Li + -solvating free solvent, and very low activity of such solvent makes electrochemical intercalation/deintercalation possible.…”