Practical Sulfolane-Based Electrolytes: Choice of Li Salt for Graphite Anode Operation
Authors: Zhang Tong , Kaymaksiz Serife , De Meatza Iratxe , Paillard Elie 
Published: February 2020

In the last years, continuous attempts have been made to increase further the energy density of Li-ion batteries and cathode materials operating above 4.5V versus Li+/Li have been proposed since the late 90’s, such as LiNi0.5Mn1.5O4 [1], or LiCoPO4F [2]. However, the so-called ‘5V’ Li-ion battery is still not on a reality, due to the lack of adapted electrolyte. Several reports deal with high voltage electrolytes such as ionic liquids, alkyl dinitrile compounds or fluorinated alkylcarbonates. Besides, alkyl sulfone-based electrolytes [3] are also promising and among them, Sulfolane (tetramethylene sulfone, SL), as a by-product of the oil industry and thus cheap and produced by tons, is an attracting candidate for practical electrolytes.
SL, regardless of its high viscosity, is known for leading to Li-ion electrolytes with high anodic stability. This is not only supported by experimental results [4] [5] but can also be explained by fundamental studies using Quantum Chemistry [6] and Molecular Dynamics [7]. Mixtures of sulfolane and low dielectric co-solvent seem able to allow room temperature operation of high voltage electrodes but to the best of our knowledge, no report can be found on a sulfolane-based electrolyte allowing efficient graphite cycling, which is a prerequisite for high energy density batteries. In fact, the SEI formed on graphite and its effect on battery performance is at least as influential as the electrolyte intrinsic properties such as its conductivity, given its role in capacity loss and rate performance.
Thus, we report not only on the intrinsic physico-chemical properties of SL/DMC mixtures including a variety of Li salts, both inorganic and organic (LiBF4, LiPF6, LiDFOB, LiBOB, LiTFSI, LiFSI, LiTDI), but also on the effect of the salts on the cycling behavior of graphite electrodes in these ‘high voltage’ electrolytes.
As shown in Figure 1, some of the electrolytes are as conductive as state-of-the-art alkyl carbonate-based electrolytes with values above 7 mS cm-1 at 20°C and most are still well conductive at -20°C.
Figure 2 shows an example of cycling results of 5.5 mg cm-2 graphite electrodes obtained with 1M LiFSI and LiDFOB-based electrolytes in half-cells without additive. As can be seen, both salts allow efficient cycling with first cycle efficiencies above 90% and LiFSI allows for the steadiest cycling.

Acknowledgements

The research presented is part of the ‘SPICY’ project funded by the European Union’s Horizon 2020 research and innovation program under grant agreement N° 653373.

References

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Figure 1