Effect of Swelling in Electrolyte Polymer Binder Positive Electrode on the Characteristics of Lithium-Sulfur Batteries

Abstract

Lithium-sulfur (Li–S) batteries attract great attention of researchers due to their high theoretical energy density (2600 W∙h∙kg-1) and economic efficiency compared to state-of-art lithium-ion batteries [1]. The depth of the electrochemical reduction of sulfur and the cycle life of lithium-sulfur batteries are determined by the amount of electrolyte retained in the positive electrode [2]. In turn, the amount of electrolyte in the positive electrode is determined by its porosity and the ability of binders to swell in the electrolyte. Therefore, polymer binders used in the positive electrode can significantly affect the characteristics of lithium-sulfur batteries. The aim of the work was to study the effect of the swelling of polymer binders of the positive electrode in the electrolyte solution on the characteristics of lithium-sulfur batteries. The sulfur electrodes, with the composition of 70 wt.% Sulfur (99.5%, Russia), 10 wt.% multi-walled carbon nanotubes (Sigma Aldrich) and 20 wt% binder was the object of study. Polymeric binders were PEO (ММ 4∙106), PVDF-HFP (ММ 3∙105), LA-132. Lithium-sulfur cells were assembled as described in [3]. Electrolyte was1M LiCF3SO3 in sulfolane. Sulfolane is promising solvent for lithium–sulfur batteries. Electrolyte solutions based on sulfolane have high chemical and electrochemical stability [4], and moderate conductivity [5]. Sulfolane has a high flash point (166°C) [6]. Lithium trifluoromethanesulfonate was chosen as a support salt since electrolytes based on it have good scattering ability and provide lithium–sulfur cells with the smallest capacity depletion compared to electrolyte systems based on other salts [4]. Studies have shown that all investigated sulfur electrodes had good elasticity, mechanical strength and adhesion. Swelling of the electrode layer of sulfur electrodes was estimated by weight method at room temperature (23-25 °C) as follows. Pre-weighted sulfur electrodes were placed into the electrolyte solution for 48 hours. Then the sulfur electrodes were removed from the electrolyte, the excess of electrolyte was removed from the surface of the electrodes by a filter paper, and weighed. The mass of the electrolyte sorbed by the sulfur electrode was calculated as the difference in the masses of the sulfur electrodes after and before their storage into the electrolyte solution. It is established that the composition of the polymer binder has a significant effect on the swelling of the positive electrode in the electrolyte (Table). Sulfur electrodes containing PEO as a polymer binder can soak up to 2.8 µL/mAh(S). Sulfur electrodes containing PVDF-HFP and LA-132 can soak 1.5 and 1.1 µL/mAh(S), respectively. The discharge voltage profiles of lithium-sulfur cells with sulfur electrodes containing various polymeric binders differ significantly (Figure). The shape of the discharge voltage profile of lithium-sulfur cells, with the polymer binder PEO, is traditional: there are two voltage plateau: high voltage and low voltage. And in the case of polymer binders LA132 and PVDF-HFP, the discharge capacity corresponding to the end of the high-voltage stage is significantly lower than for cells with PEO binder, and the low-voltage plateau is almost indistinguishable. The difference in the swelling of sulfur electrodes containing various polymeric binders in the electrolyte solution explains the differences in the characteristics of the lithium-sulfur cells. This work was performed as part of a Government Order to Ufa Institute of Chemistry of the Russian Academy of Sciences by the Ministry of Science and Higher Education of the Russian Federation (Theme No. AAAA-A17-117011910031-7) Russia and was also financially supported by the Russian Science Foundation (project No 17-73-20115).