Understanding the Behavior of Lithium Bis(fluorosulfone)Imide and Lithium Bis(trifluoromethanesulforyl)Imide in Localized Superconcentrated Electrolytes
Authors: Mai Sebastian , Ivanov Svetlozar , Bund Andreas , Thompson Travis , Cabelguen Pierre Etienne , Vasseur Karolien , Levasseur Stephane 
Published: February 2020

High concentration Electrolytes (HCEs) exhibit a small molar ratio of solvent to salt, usually in the range of 2…4:1. Localized Superconcentrated Electrolytes (LSEs) are derived from HCEs but contain a co-solvent that is used to dissipate the solvent-salt complexes, leading to significantly reduced viscosity and therefore improved conductivity. One of these co-solvents is 1,1,2,2-Tetrafluoroethyl 2,2,3,3-Tetrafluoropropyl Ether (TTE). They can be prepared using solvents that have a high anodic stability such as Sulfolane (SL) making them ideal candidates for cycling lithium metal in conjunction with advanced cathode materials above 4,5 V. Since the introduction of the LSE concept, proposed by Jiangfeng et al.
[1] most published variations used Lithium bis(fluorosulfone)imid (LiFSI) as the conducting lithium salt [2]. The structurally similar salt Lithium bis-(trifluoromethanesulforyl)imid (LiTFSI) is commonly used in classical electrolytes, especially for Lithium-Sulfur batteries because of its good ion separation and resulting high conductivity. It has however rarely been used in LSEs to date [3]. Although LiFSI and LiTFSI are very similar in structure, in contact with lithium metal their decomposition products display significant differences and can therefore lead to drastic divergencies in lithium passivation and deposition behavior [4, 5].

In this study we present a comparison of LSEs containing LiFSI and/or LiTFSI. The kinetics of Solid Electrolyte Interphase (SEI) formation is investigated by Electrochemical Impedance Spectroscopy (EIS) and differences in SEI composition are discerned using X-Ray Photoelectron Spectroscopy (XPS). Resulting divergencies in SEI stability and morphology of cycled Lithium metal are being investigated by Galvanostatic Cycling and Scanning Electron Microscopy (SEM) while principal conclusions are being drawn from Nernst-Potentials and solvent complex geometries obtained by Density Functional Theory (DFT).

Our results show that the main LiTFSI decomposition fragment form a SEI that is unsuitable for efficient cycling of lithium metal in a HCE. This however can be remedied by applying the LSE principle to it where the stoichiometric addition of TTE improves the electrolytes electrochemical performance, so that a stable SEI can be formed on lithium metal. Because of its comparatively high conductivity it would be advantageous to use the LiTFSI based LSE over a FSI-based one. To further enhance the LSE multiple commonly employed electrolyte additives are being considered. In this regard results from EIS, XPS and galvanostatic cycling are correlated with theoretical predictions obtained from DFT calculations.

In conclusion, this study highlights the influence of TTE on the formation of the SEI in the presence of LiTFSI. The co-solvent TTE serves a double purpose: dissipating solvent-salt complexes while simultaneously acting as additive during the initial passivation phase. Further, our results show that additives, that are applied in common electrolytes do not necessarily work the same way when used in an LSE, sometimes deteriorating the SEI rather than improving it. The reason for this is that the chemical nature of a SEI significantly differs when formed from SL based electrolytes compared to ones containing 1,3-Dioxolan or ethylene carbonate.

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