Enhanced performance of lithium metal batteries via cyclic fluorinated ether based electrolytes

To address the challenges associated with applying high-voltage cathodes in lithium metal batteries (LMBs) there is a need for new electrolytes enabling stable interphases at both electrodes. Here we attack this by using a dioxolane-derived cyclic fluorinated ether, 2,2-bis(trifluoromethyl)-1,3-dioxolane (BTFD), as a fluorinated diluent to a 1,2-dimethoxyethane (DME) based electrolyte. The cells using the resulting BTFD-based electrolytes exhibit higher Coulombic efficiencies for lithium stripping and plating as compared to those using the non-fluorinated ether-based electrolyte. This originates from the reduced formation of 'dead Li' at the anode, as shown by using electrochemical impedance spectroscopy (EIS). In practice, the BTFD-based electrolytes are shown to improve the performance of Li||NMC cells, which is due to the formation of a predominantly inorganic cathode electrolyte interphase (CEI) that suppresses the cathode degradation during cycling. We used X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) to characterize the CEIs' overall composition and structure. To obtain more details on the CEI speciation, Raman and nuclear magnetic resonance (NMR) spectroscopies were employed, assisted by molecular level computations. Overall, we demonstrate how the very design of the electrolyte composition influences the performance of LMBs.