Electrochemical Characteristics of Novel Fluorinated Ether Solvent for Lithium-Sulfur Batteries

Introduction In recent years, the development of electric airplanes, large drones, and other equipment has created a need for batteries with high energy density and high power density. Lithium-sulfur batteries have been investigated as such batteries. Electrolyte solutions applying carbonate solvents and ether solvents have been developed as electrolyte for lithium-sulfur batteries1. An electrolyte with ether-based solvents is lighter than that with carbonate-based solvents, so the former is expected to be an electrolyte for lithium sulfur batteries with high energy density. However, lithium-sulfur batteries based on ether solvents have difficulty in reversible charge-discharge reactions due to the dissolution of lithium polysulfide (Li2S x ) that occurs during the charge-discharge process2. In this study, we investigate the electrochemical characteristics of electrolytes to apply a new fluorinated ether (C14) which is resistant to elution of Li2S x . Experimental Sulfur-carbon composite (S-C) was prepared by compositing activated carbon (C-novel, Toyo Tanso) and sulfur in a ratio of 35:65 by weight using a thermal impregnation method. S-C was used as the active material and mixed with acetylene black (AB, DENKA), carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) to prepare a slurry. The cathode was obtained by coating the obtained slurry on an aluminum foil by a doctor blade method. An electrochemical cell was assembled with the obtained cathode as the working electrode, Li foil as the counter electrode, and LiTFSI dissolved in a mixture of dioxolane (DOL), dimethoxyethane (DME), and C14 at a concentration of 1 mol L-1 as the electrolyte. In order to clarify the electrochemical characteristics of the lithium sulfur battery with these electrolytes, constant current charge-discharge tests were conducted using the electrochemical cell described above. To clarify the coordination state of the solvent to lithium ions and lithium polysulfide in the electrolyte, IR spectra were measured for these electrolytes. Results and Discussion In the IR spectra for the solutions of Li2S x dissolved in DME, the intensity of the peak attributed to the asymmetric stretching of the C-O-C bond, which can be seen at around 1090 cm-1, increases with increasing concentration of Li2S x . The change in peak intensity appears to be due to the coordination of the structure of the diether in DME to the Li+ of Li2S x . On the other hand, in the IR spectra of Li2S x immersed in C14, the intensity of the corresponding peak was found to be similar to that of C14 only, regardless of the concentration of Li2S x . The results indicate that C14 is hard to coordinate to Li2Sx. This may be due to the fact that the functional group with fluorine in the structure of C14 is an electron-withdrawing group, which delocalizes the electrons located around O in the diether and weakens the coordinative force to Li2Sx. As a result of the constant-current charge-discharge test, it was confirmed that the voltage stagnated at about 2.3 V in the charge curves of the cell to which 1M LiTFSI / DOL : DME = 1 : 1 by vol.% was applied. The stagnation of the voltage is considered to be due to the shuttle reaction caused by the coordination of DME to lithium polysulfide and its dissolution into the electrolyte during the charge-discharge process. On the other hand, in the charge-discharge curves of the cell with 1M LiTFSI / C14, the stagnation of voltage that was observed in the cell with 1M LiTFSI / DME could not be confirmed, and it was turned out that the irreversible capacity was small. This should be due to the inability of C14 to coordinate with Li2S x . Acknowledgement A part of this research was conducted under a subcontract (JPNP15005) from the New Energy and Industrial Technology Development Organization (NEDO), a national research and development corporation. Reference 1) L. Gordin et al, ACS Appl. Mater. Interfaces, 6 (2014) 8006−8010 2) B. Zhang et al, ACS Energy Lett., 6 (2021) 537−546