Factors Improving Lithium Sulfur Battery Performance with Mesoporous Carbon-Sulfur Cathode by Mixing Vinylene Carbonate Electrolyte with Fluoroethylene Carbonate

Abstract

1. IntroductionLithium-ion batteries (LIB) are used in a variety of applications, including portable devices such as smartphones. More recently, the development of rechargeable batteries is expected for applications such as next-generation electric mobility, requiring energy storage devices with higher energy density than conventional LIB, and lithium-sulfur (Li-S) batteries are attracting attention among these applications. To realize Li-S batteries with high energy density and safety, we have focused on mesoporous carbon-sulfur cathodes and carbonate-based electrolytes. In previous studies, we have reported that Li-S batteries using vinylene carbonate (VC) and fluoroethylene carbonate (FEC) as electrolyte solvents exhibit superior rate characteristics than using only VC solvent 1). This study investigates the factors that contribute to the improved performance when using a FEC-VC mixed electrolyte.2. Experimental method(1) We assembled the cells using 1.0 M lithium bis(trifluoromethanesulfonyl)imide / VC (Only VC electrolyte) or 1.0 M lithium bis(trifluoromethanesulfonyl)imide / FEC: VC (FEC-VC electrolyte). The cell was pre-cycled at 0.1C in the voltage range of 1 - 3 V, and then discharged at 10 C or 0.1 C and charged at 0.1 C for three cycles. After discharging and charging, each cell and electrode were evaluated using electrochemical impedance spectroscopy (EIS) or XPS. (2) Radical polymerization of VC with 2,2'-azobis(isobutyronitrile) (AIBN) was made by mixing VC and AIBN (VC radical polymer). Anionic polymerization of VC with lithium ethoxide (LiOEt) was also obtained by mixing VC and LiOEt (VC anionic polymer). These polymers were analyzed using XPS and MALDI-TOFMS. (3) The cathode materials after discharging and charging were stripped from the current collector. The powder was analyzed by APCI-TOFMS. (4) The cathode after 10 cycles was analyzed by STEM-EELS.3. Results and discussionWe conducted EIS on Li-S batteries charged and discharged with Only VC electrolyte or FEC-VC electrolyte at 10 C or 0.1 C, respectively. As a result, in the case of Only VC electrolyte, the interfacial resistance increased significantly after charge-discharge at 10 C when compared to that after charge-discharge at 0.1 C. On the other hand, when using FEC-VC electrolyte, the increase in interfacial resistance was suppressed. In addition, XPS investigation of the electrodes after discharging and charging showed that the SEI film thickness increased significantly after discharging and charging at 10 C when using Only VC electrolyte, whereas the increase in SEI film thickness was suppressed when using FEC-VC electrolyte. These suggest that when using FEC-VC electrolyte, a good SEI that suppresses electrolyte decomposition more is formed, and thin and low-resistance SEI is maintained even after rapid discharge, thereby improving rate performance. In addition, the XPS C1s spectra observed on the electrode film showed similar profiles to those of the VC radical polymer when using Only VC electrolyte, and to those of the VC anionic polymer when using FEC-VC electrolyte. Therefore, these polymers were used as models for the assumed SEI organic components when using respective electrolytes and were investigated by MALDI-TOFMS. As a result, the molecular weight of the monomer unit of the polymer was 250 for the VC radical polymer, while it was 60 for the VC anionic polymer. The results of the APCI-TOFMS investigation on the cathode materials after discharging and charging showed that the components with higher molecular weight decreased as using the electrolyte solvent with a higher mixing ratio of FEC to VC. Therefore, we considered that the mixing of FEC makes the SEI organic components less bulky and denser, which suppresses the continuous formation of SEI kinetically and reduces the interfacial resistance. We conducted further investigation about SEI that could suppress electrolyte degradation more when using FEC-VC electrolyte. STEM-EELS investigation of the cathode after 10 cycles of charge-discharge showed that the SEI layer is divided into two layers when using FEC-VC electrolyte, and the SEI inner layer is mainly composed of S, F and Li. The XPS results also revealed the presence of LiF and Li2Sx (x=2-8) inside the SEI. Therefore, it is suggested that when using FEC-VC electrolyte, the formation of a layer that mainly composed of LiF and Li2Sx (x=2-8) inside the SEI acts as a barrier between the electrode and the electrolyte, which can suppress excessive decomposition of the electrolyte.AcknowledgementsThis research was partly performed as a subcontract (JPNP15005) from the New Energy and Industrial Technology Development Organization (NEDO) (contractor: GS Yuasa Corporation; subcontractor: Kansai University).Reference1) K. Kishida et al., The 60th Battery Symposium in Japan, The Committee of Battery Technology, The Electrochemical Society of Japan, p. 2D04 (2019).