(Invited) Solvate Ionic Liquids for Advanced Lithium Batteries

Abstract

Certain concentrated mixtures of salts and solvents are not simply "solutions" anymore, but they may be described as "solvate ionic liquids", in which the solvents strongly coordinate the cation and/or the anion of the salts to form stable “solvate ions”. The new family of ionic liquids can be obtained by simply mixing glyme (triglyme (G3) or tetraglyme (G4)) with lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) in a molar ratio of 1:1 [1-3]. The equimolar complex [Li(G3 or G4)1][TFSA] maintains a stable liquid state over a wide temperature range and can be regarded as a solvate ionic liquid consisting of a [Li(glyme)1]+ complex cation and a [TFSA]- anion, exhibiting high self-dissociativity (ionicity) at room temperature [1, 2]. The electrochemical oxidation of [Li(glyme)1][TFSA] takes place at the electrode potential of ~4.6 V vs. Li/Li+, while the oxidation of solutions containing excess glyme molecules ([Li(glyme)x][TFSA], x > 1) occurs at lower than 4 V [1]. This enhancement of oxidative stability is due to the donation of lone pairs of ether oxygen atoms to the Li+ cation, resulting in the highest occupied molecular orbital (HOMO) energy level lowering of a glyme molecule, which is confirmed by ab initio molecular orbital calculations. The solvation state of a Li+ cation and ion conduction mechanism in the [Li(glyme)x][TFSA] solutions strongly suggests that Li+ cation conduction in the equimolar complex takes place by the migration of [Li(glyme)1]+ cations, whereas the ligand exchange mechanism is overlapped when interfacial electrochemical reactions of [Li(glyme)1]+ cations occur. The ligand exchange conduction mode is typically seen in a lithium battery with a configuration of [Li anode | [Li(glyme)1][TFSA] | LiCoO2 cathode] when the discharge reaction of a LiCoO2 cathode, i.e., de- solvation of [Li(glyme)1]+ and insertion of the resultant Li+ into the cathode, occurs at the electrode-electrolyte interface. The battery can be operated for more than 200 charge-discharge cycles in the cell voltage range of 3.0–4.2 V, regardless of the use of ether-based electrolyte, because the ligand exchange rate is much faster than the electrode reaction rate [1].Another intriguing aspect of the solvate ionic liquids is unusual solubility. 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. In the equimolar complexes [Li(G3 or G4)][TFSA] consisting of [Li(glyme)1]+ complex cation and [TFSA]- anion, both cations and anions are low coordinating ions with low Lewis acidity and basicity, respectively. The [Li(G3 or G4)][TFSA] molten complexes do not readily dissolve other ionic solutes due to the low coordinating nature of the cation and anion, which leads to the stable operation of the Li–S battery over more than 400 cycles with discharge capacities higher than 700 mAh g-S-1 and with coulombic efficiencies higher than 98% throughout the cycles [4]. 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 [4].An important electrode reaction, which is enabled by the use of the solvate ionic liquids, is electrochemical Li+ intercalation into graphite electrodes [5]. Li+-intercalated graphite was successfully formed in an equimolar molten complex, [Li(G3)1][TFSA]. The desolvation of Li+ ions took place at the graphite/[Li(G3)1][TFSA] interface in the electrode potential range 0.3‒0 V vs Li. In contrast, the cointercalation of G3 and Li+ (intercalation of solvate [Li(G3)1]+ cation) into graphite occurred in [Li(G3) x ][TFSA] electrolytes containing excess G3 (x > 1). This cointercalation causes exfoliation of the graphite. The electrode potential for the formation of Li+-intercalated graphite (desolvation of solvate [Li(G3)1]+ cation) changes greatly, depending on the activities of free G3 in the electrolyte. In the solvate ionic liquid, the activity of the free solvent is very low, and consequently, the electrode potentials for the formation of Li+-intercalated graphite become higher than that for cointercalation, and the cointercalation of G3 is inhibited in [Li(G3)1][TFSA].