Understanding the Li Local Environment in the (ACN)2-Litfsi Solvate Electrolyte
Authors: Miller Elizabeth C , Kasse Robert M , See Kimberly A , Lau Kah Chun , Gewirth Andrew A , Curtiss Larry A , Toney Michael F 
Published: February 2020

Lithium-sulfur (Li-S) batteries are a promising next generation energy storage technology owing to their high theoretical energy density (2,500 Wh/kg) and the abundance, non-toxicity, and low cost of sulfur. However, Li-S systems face a number of challenges including degradation of the Li anode, electrolyte consumption, and dissolution of soluble polysulfide intermediates from the cathode, all of which cause fast capacity fade and poor cyclability. One approach to solving these problems is the use of sparingly solvating electrolytes, which allows for control of polysulfide solubility and thus the prevention of these degradation modes while maintaining good ionic conductivity. One promising electrolyte is a combination of bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) in a mixture of acetonitrile (ACN) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE). It is theorized that dilution of the solvate electrolyte with TTE improves performance by decreasing viscosity and allowing TTE to take the place of some of the ACN in the solvation shell of Li, freeing up the ACN to solubilize polysulfides and improve cathode reaction kinetics. The solvation shell of Li influences the electrolyte viscosity, which is related to Li ion mobility, and the Li ion desolvation energy, which controls the charge transfer between Li in the solvent and the sulfur electrode. However, a better understanding of the local environment of the Li ion and the role of TTE in this system is needed to tailor the electrolyte properties for optimal battery performance. Additionally, investigating the molecular interactions in this electrolyte system will provide insight into how fluorinated electrolytes minimize the reaction between the Li anode and polysulfides and limit the polysulfide shuttle between electrodes; both of which are believed to lead to capacity loss. See et al. used in situ Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy to identify the average coordination number of Li ions, and ab initio molecular dynamics (AIMD) simulations have shed light on the solvation shell structure. However, these techniques are unable to directly measure the coordination lengths between Li and species in the first solvation shell.

In this work, we perform experimental verification of the solvation shell structure using pair distribution function (PDF) measurements employing both hard X-rays (Advanced Photon Source) and neutrons (Oak Ridge National Laboratory, Spallation Neutron Source) as a probe. While X-rays interact with the electron cloud surrounding an atom, neutrons interact with atomic nuclei, thus these techniques provide complimentary information, as neutrons are more sensitive to light elements. Neat electrolyte (without TTE) is measured as a function of LiTFSI concentration to determine the concentration at which solvate complexes form. The solvate electrolyte, (ACN)2-LiTFSI, is diluted to different levels with TTE to determine how the TTE molecule interacts with the first solvation shell of Li. PDF obtained from experimental data are compared to simulated patterns generated from the results of AIMD simulations both to aid in data analysis and to contribute to the model to help improve its predictive capability for use in studying yet to be developed electrolytes.