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

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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-V-class 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 Li2Sm is very low, and highly efficient charge/discharge of the Li–S battery is possible without severe side reactions.

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