Understanding the Connection between Li-Ion Transport Number and Electrochemical Performance in Fluorinated Ether-Based Electrolytes

Abstract

As high-performance batteries increase in demand, lithium metal batteries (LMBs) have attracted increasing attention in recent years. Nonetheless, LMBs are limited in their performance due to a low cation transport number (tLi+) of non-aqueous liquid electrolytes, which is the ratio of charge transport of Li-ions to its counterion. This study investigated fluorinated ether-based electrolytes (1,2-(1,1,2,2-tetra fluoroethoxy) ethane, TFEE) with 1,2-dioxolane (DOL) as a co-solvent that were found to have tLi+ of 0.70, which was determined through electrochemical impedance spectroscopy, EIS. This electrolyte mixture gives a low electrolyte coulombic efficiency and poor stability tested in both Li-Cu half and Li – Li4Ti5O12 full cells. The same fluorinated electrolyte with dimethoxy ethane (DME) as a co-solvent showed better coulombic efficiency and stability despite its low tLi+ of 0.18. A significant migration resistance through the porous solid electrolyte interface (SEI) in the case of TFEE-DOL electrolytes is suspected to explain the observed discrepancy, as demonstrated by EIS and X-ray photoelectron spectroscopy. We propose that the properties/process of the Li anode are more crucial for LMBs’ performance than the ionic transport through the bulk electrolyte. Furthermore, the transport number should be determined carefully to avoid any spurious effects associated with SEI properties. Acknowledgments This work has received funding from the European Union Horizon 2020 research and innovation DESTINY program under grant agreement No. 945357. Co-financing from Slovenian Research Agency (ARRS) is fully acknowledged.