A Design Approach to Lithium-Ion Battery Electrolyte Based on Diluted Solvate Ionic Liquids

Abstract

An equimolar mixture of lithium bis(trifluoromethylsulfonyl)amide (Li [TFSA]) and triglyme (G3) or tetraglyme (G4) yields the stable molten complexes, [Li(G3)] [TFSA] or [Li(G4)][TFSA], respectively, classified into solvate ionic liquids (SILs). The Li-conducting SIL electrolytes have favorable thermal and electrochemical properties, but their intrinsic high viscosities and low ionic conductivities impede widespread application. In this study, SILs were diluted with organic solvents, such as toluene, hydrofluoroether (HFE) and propylene carbonate (PC), to enhance their ionic conductivity. Subsequently, the performance of a battery consisting of diluted SILs, LiCoO 2 , and graphite electrodes was evaluated. The electrochemical stability and charge/discharge behavior of the LiCoO 2 cathode and graphite anode were greatly influenced by the stability of the complex cations, [Li(G3)] + or [Li(G4)] + , in the diluted SILs. Unfavorable ligand exchange between the glyme and PC occurred in PC-diluted SILs. Oxidative decomposition of the uncoordinated glyme and pitting corrosion of Al current collector deteriorated the battery performance of LiCoO 2 half-cell with PC-diluted SILs. We demonstrate that toluene-and HFE-diluted SILs, which do not contain chemicals such as carbonate solvent and LiPF 6 used in commercialized Li-ion batteries, allow both LiCoO 2 cathode and graphite anode to operate stably. Because of their high specific energy density, Li-based secondary batteries have been the most attractive candidates among a variety of energy storage systems. Research aimed at the development of Li-ion battery technologies has focused mainly on positive and negative electrode material, 1,2 while Li-conducting electrolytes have played a "supporting" role. Indeed, the major components of the electrolyte in commercialized Li-ion batteries, i.e., a mixture of polar ethylene carbonate (EC) and low-viscous linear carbonates such as diethyl carbonate (DEC) with ∼1 mol dm −3 of lithium hexafluorophosphate (LiPF 6 ), 3 have been in use since these batteries were first developed for practical application in 1991.The standard electrolyte (1 mol dm −3 LiPF 6 in EC/DEC) has been selected for the mature Li-ion battery technology, for several reasons, [3][4][5][6][7] and the battery reaction relies on each electrolyte component. The polar EC not only assists the supporting Li-salt to dissociate to a higher degree, but also forms a good solid electrolyte interphase (SEI) on the graphite anode. The SEI is formed via sacrificial initial decomposition and it allows reversible intercalation/deintercalation reactions of Li ions during charge/discharge. Less viscous linear carbonates such as DEC are mixed with the more viscous EC to improve the ionic conductivity of the electrolyte without compromising the high degree of salt dissociation and formation of the SEI. Despite its poor chemical stability, LiPF 6 is used as the supporting salt because of its high degree of dissociation. Moreover, LiPF 6 plays an important role in suppressing the corr...

Phosphonate‐functionalized Ionic Liquid: A Novel Electrolyte Additive for Eenhanced Cyclic Stability and Rate Capability of LiCoO2 Cathode at High Voltage

“…Linear sweep voltammetry (LSV) was used to test the electrochemical stability of electrolyte . As seen in Figure a, the onset voltage of electrochemical oxidation of PFIL‐based hybrid electrolyte is around 4.1 V (Li/Li + ), while the decomposition voltage of the standard electrolyte is around 4.2 V (Li/Li + ) .…”

High-Efficiency Lithium Metal Batteries with Fire-Retardant Electrolytes

“…To exploit the advantages of HCEs but minimize their disadvantages, several groups have attempted to dilute HCEs with a co-solvent. [36][37][38][39][40][41] For example, Watanabe's group diluted solvate ionic liquids (SILs, concentrated lithium bis(trifluoromethanesulfonyl)amide [LiTFSA] in glymes) with selected molecular solvents, and determined the stability of the complex cations in the diluted SILs. 40 They found that the addition of a hydrofluoroether (HFE), 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, did not break the solvation of the glyme-Li-salt molten complexes and greatly enhanced the power density of Li-S batteries.…”

Ionic Liquid-Based Electrolytes for Energy Storage Devices: A Brief Review on Their Limits and Applications

“…Recently, IL-based electrolytes (LiTFSI-PYR 14 TFSI) have shown an optimized upper cut-off voltage in the range of 4.8−5.1 V and 3.2−3.6 V for metallic lithium and LTO-based dual-ion cells, respectively [107][108][109]. A similar type of enhancement has been observed for trimethylhexylammonium bis(trifluoro-methane sulfonyl)imide (TMHA-TFSI), glyme, and oligomeric ILs [2,[109][110][111][112][113][114]. In contrast, Ferrari et al prepared electrochemically stable and benign electrolytes for LIBs and Li-O 2 batteries using 3-(2-(2-methoxy ethoxy)ethyl)-1-methylimidazolium TFSI (ImI 1,10201 TFSI) and 1-(2-methoxyethyl)-3-methylimidazolium TFSI (ImI 1,2O1 TFSI).…”