Five-Membered Cyclic Sulfur Compounds as (Co-)Solvents for Lithium-Ion Battery Electrolytes
Authors: Oldiges Kristina , Grünebaum Mariano , von Aspern Natascha , Cekic-Laskovic Isidora , Brunklaus Gunther , Winter Martin 
Published: February 2020

Lithium-ion batteries (LIBs) are known for their domination in the market for rechargeable energy storage systems. However, the state-of-the-art non-aqueous aprotic electrolytes, as inevitable components thereof, generally comprising organic carbonates such as ethylene carbonate (EC) and dimethyl carbonate (DMC) with lithium hexafluorophosphate (LiPF6) as conducting salt, still cope with well-known drawbacks and limitations1. In order to realize a substantial improvement, it is of significant importance to tailor new electrolyte formulations and to understand the structural impacts of the electrolyte components, especially solvents, on the electrolyte’s properties. The established organic carbonates, e.g., EC or propylene carbonate (PC), have a structural similarity to five-membered cyclic sulfur compounds. A variation of the oxygen atom number and position leads to five commercially available modifications with constant geometry, i.e., tetrahydrothiophene, tetrahydrothiophene 1-oxide, ethylene sulfite, sulfolane, 1,3-propane sultone and 1, 3, 2- dioxathiolane 2, 2- dioxide. Most of them have already been approved as solid electrolyte interphase (SEI) additives, whereas sulfolane has been used as single or co-solvent, as well2, 3. To reveal the impact of the aforementioned compounds on physicochemical and electrochemical properties of the formulated electrolytes, they were investigated as co-solvents with PC to achieve dissolution of every compound, as 1,3-propane sultone and 1, 3, 2- dioxathiolane 2, 2- dioxide are solid at room temperature. Lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) was used as conducting salt due to its good solubility, thermal and electrochemical stability, and inertness to water4. Viscosity, conductivity, permittivity, Raman and pulsed-field gradient nuclear magnetic resonance (PFG NMR) measurements as well as linear sweep voltammetry (LSV) were conducted to characterize the performance of the formulated electrolytes. It was demonstrated that not the amount of oxygen atoms but the type of functional group (sulfinyl or sulfonyl) plays a decisive role in ionic mobility. Tetrahydrothiophene 1-oxide was selected as promising solvent candidate due to its excellent physicochemical properties. Therefore, the focus for further electrochemical, analytical and spectral characterization was set on this sulfur compound containing electrolytes. The convincing physicochemical properties have a direct impact on the electrochemical behavior, enabling stable galvanostatic cycling of cells containing the PC-based electrolyte. With this in line, interfacial electrochemistry regarding formation, dynamics and also stability of the protective layer was considered, as well. The obtained results pointed out that tetrahydrothiophene 1-oxide as co-solvent in organic carbonate-based electrolytes has a highly positive effect on its ionic mobility as well as on the overall cell performance.

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