Addressing Self-Discharge in Lithium-Sulfur Batteries By Electrolyte Modification

Despite recent advances in lithium-ion batteries, the limits of modern energy storage still place bounds on other up-and-coming technologies such as renewable energy generation and electric vehicles.  In an effort to overcome these limitations, many researchers have turned to lithium-sulfur (Li-S) batteries as the next big step in energy storage technology.  Given the high theoretical capacity (1675 mAh/g) and high energy density (2600 Wh/kg) of sulfur, along with its low cost and eco-friendliness compared with traditional Li-ion cathode materials, the advantages of Li-S batteries are clear.^1,2

     Despite these advantages, there are several major barriers that must still be addressed: low practical capacity, poor cycling stability, and low efficiency are endemic to Li-S batteries.  These problems are largely due to the dissolution and side-reaction of soluble lithium polysulfides in the electrolyte.^1–3  Similarly critical, this behavior can lead to high self-discharge.  Compared to the many attempts to improve efficiency and cycling stability of Li-S batteries,^1,4–10 many fewer efforts have focused on characterizing and mitigating the self-discharge.^11–15

     In this work, the self-discharge of Li-S cells was studied and found to be quite severe.  Using coin cells with high-sulfur-loading cathodes (>5 mg S/cm²) and a conventional electrolyte of 0.5M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-dimethoxyethane/1,3-dioxolane (DOL/DME, 1:1 v/v), self-discharge after resting for two weeks at 45°C averaged around 33%.  Addition of 0.2M LiNO₃ did not significantly decrease self-discharge.  Taking a cue from work on fluorinated additives for improving solid electrolyte interphase (SEI) stability in silicon anodes for Li-ion batteries,¹⁶ we tested the self-discharge of cells with a fluorinated ether (FE) containing electrolyte.  Although cells with FE co-solvent alone did not show lower self-discharge, those with both FE and LiNO₃had a lower average self-discharge of 25%, as shown in Fig. 1a.

     To understand the mechanism by which these additives function, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were carried out on the lithium anodes after cycling.  Anodes from cells with FE and LiNO₃had a significant amount of surface film, as shown in Fig. 1b.  FTIR indicates that new components formed at the surface of electrodes with these combined additives.  These traits are believed to indicate formation of a more robust SEI layer, which can mitigate the shuttle effect and thus decrease self-discharge.  This work identifies a direction for future efforts to mitigate the self-discharge of Li-S batteries by tuning the choice of FE and the electrolyte composition.

     This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-EE0005475.

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