Solvent Activity in Electrolyte Solutions Controls Electrochemical Reactions in Li-Ion and Li-Sulfur Batteries

Abstract

Solvent−ion and ion−ion interactions have significant effects on the physicochemical properties of electrolyte solutions for lithium batteries. The solvation structure of Li + and formation of ion pairs in electrolyte solutions composed of triglyme (G3) and a hydrofluoroether (HFE) containing 1 mol dm −3 Li[TFSA] (TFSA: bis(trifluoromethanesulfonyl)amide) were analyzed using pulsed-field gradient spin−echo (PGSE) NMR and Raman spectroscopy. It was found that Li + is preferentially solvated by G3 and forms a [Li(G3)] + complex cation in the electrolytes. The HFE scarcely participates in the solvation because of low donor ability and relatively low permittivity. The dissociativity of Li[TFSA] decreased as the molar ratio of G3/Li [TFSA] in the solution decreased. The activity of G3 in the electrolyte diminishes negligibly as the molar ratio approaches unity because G3 is involved in 1:1 complexation with Li + ions. The negligible activity of G3 in the electrolyte solutions has significant effects on the electrochemical reactions in lithium batteries. As the activity of G3 diminished, the oxidative stability of the electrolyte was enhanced. The corrosion rate of the Al current collector of the positive electrode was suppressed as the activity of G3 diminished. The high oxidative stability and low corrosion rate of Al in the G3/Li[TFSA] = 1 electrolyte enabled the stable operation of 4-Vclass lithium batteries. The activity of G3 also has a significant impact on the Li + ion intercalation reaction of the graphite electrode. The desolvation of Li + occurs at the interface of graphite and the electrolyte when the activity of G3 in the electrolyte is significantly low, while the cointercalation of Li + and G3 takes place in an electrolyte containing excess G3. The activity of G3 influenced the electrochemical reaction process of elemental sulfur in a Li−S battery. The solubility of lithium polysulfides, which are reaction intermediates of the sulfur electrode, decreased as the activity of G3 in the electrolyte decreased. In the G3/ Li[TFSA] = 1 electrolyte, the solubility of Li 2 S m is very low, and highly efficient charge/discharge of the Li−S battery is possible without severe side reactions.

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

“…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.…”

“…[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. 121,122 Figure 8 shows how the anionic structures of lithium salts affect the formation of solvate ILs. 99 The amount of free glyme, determined by Raman spectroscopy, strongly depends on the anionic structures [ Fig.…”

“…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. 122 …”

Electrochemical Deposition and Dissolution of Lithium on a Carbon Fiber Composite Electrode in a Solvate Ionic Liquid

“…It has been reported that the stable charge-discharge operation of these anodes is possible in the LiTFSA-G4 solvate ionic liquid. [9][10][11][12][13] However, these anode materials have to be lithated before combining with the S cathode. The Li metal anode is preferred to these anode materials without Li.…”

Si/Li₂S Battery with Solvate Ionic Liquid Electrolyte

“…5,6 We have previously reported that ionic liquid and solvate ionic liquid electrolytes suppress the dissolution of lithium polysulfides. [7][8][9][10][11] The Li metal anode also has serious disadvantages. The dendritic growth of Li metal occurs in the cell during the charging process, especially at high current densities.…”