Oxidative-Stability Enhancement and Charge Transport Mechanism in Glyme–Lithium Salt Equimolar Complexes

Abstract

The oxidative stability of glyme molecules is enhanced by the complex formation with alkali metal cations. Clear liquid can be obtained by simply mixing glyme (triglyme or tetraglyme) with lithium bis(trifluoromethylsulfonyl)amide (Li[TFSA]) in a molar ratio of 1:1. The equimolar complex [Li(triglyme or tetraglyme)₁][TFSA] maintains a stable liquid state over a wide temperature range and can be regarded as a room-temperature ionic liquid consisting of a [Li(glyme)₁]⁺ complex cation and a [TFSA]⁻ anion, exhibiting high self-dissociativity (ionicity) at room temperature. The electrochemical oxidation of [Li(glyme)₁][TFSA] takes place at the electrode potential of ∼5 V vs Li/Li⁺, while the oxidation of solutions containing excess glyme molecules ([Li(glyme)_x][TFSA], x > 1) occurs at around 4 V vs Li/Li⁺. 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 is elucidated by means of nuclear magnetic resonance (NMR) and electrochemical methods. The experimental results strongly suggest 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, that is, desolvation 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.