The Effect of Support Salt on Penetration Depth of Reaction of Electrochemical Reduction of Sulfur into Volume of Positive Electrode of Lithium-Sulfur Batteries

Abstract

Lithium-sulfur batteries are batteries with a liquid cathode. Electrochemical transformations of sulfur and the products of its reduction (lithium polysulfides and lithium sulfide) occur on the surface of carbon particles, forming a porous carbon conductive matrix (porous carbon electrode).The depth and rate of electrochemical transformations of sulfur and lithium polysulfides are determined by both the properties of the carbon matrix (porosity, specific surface area, electrical conductivity, sorption capacity towards sulfur and lithium polysulfides) and the properties of electrolyte systems. One of the most important parameters of a porous electrode is the characteristic length - the penetration depth of the electrochemical process into the electrode volume, the value of which is determined by the ratio of the conductivity of electrolyte and charge transfer resistance (equation).We studied the effect of the conductivity of electrolyte solutions and the surface capacity of sulfur electrodes on the depth of the electrochemical reduction of sulfur (figure). The sulfur electrodes (d=2.85 cm, A=6.38 cm2) used in this study had the following composition: S (99.5%, Acros) - 70% wt., Ketjenblack® EC-600JD (Akzo Nobel) - 10% wt., Polyethylene oxide (ММ=4x106, Aldrich) - 20% wt. The negative electrodes (d=2.55 cm, A=5.10 cm2) were made of lithium foil (99.9%, Russia) with a thickness of 120 micrometers. 1M solution of LiSO3CF3 in sulfolane (specific conductivity is 0.87 mS/cm at 30 0C) and 1M solution of LiClO4 in sulfolane (specific conductivity is 2.23 mS/cm at 30 0C) were used as electrolytes. The content of electrolytes into lithium-sulfur cells was 4 microliters per 1 mAh, which is equivalent to 6.7 microliters per 1 mg of sulfur. The electrolyte was introduced into lithium-sulfur cells using a “MICROLITERTM Syringe” (Hamilton Company, USA) with a volume of 50 microliters. The accuracy of electrolyte dosing into the cells was ±5 %.It was established that the penetration depth of the reactions of electrochemical reduction of sulfur and lithium polysulfides into volume of the sulfur electrode is higher in 1M solution of LiClO4 in sulfolane than 1M solution of LiSO3CF3 in sulfolane. The observed patterns are explained from the standpoint of the theory of a porous electrode.This work was performed as part Government Order of the Ministry of Science and Higher Education of the Russian Federation (theme No. АААА-А17-117011910031-7) and was also financially supported by Russian Science Foundation (Project No 17-73-20115) and by Russian Foundation for Basic Research (Project No 16-29-06190). Figure 1