Non-Fluorinated Diluent Making Localized High-Concentration Electrolyte for Lithium Metal Anode Battery

Abstract

Localized high-concentration electrolyte (LHCE) has been adopted to derive resilient inorganic-rich solid-electrolyte interphase (SEI) layer for lithium-metal anodes (LMA) and increase the ionic conductivity of high-concentration electrolyte (HCE). While the ideal diluent molecules used in LHCE should be miscible in electrolyte system, it should not solvate lithium-ion effectively. To date, systematic investigation to reveal the design rule of diluent molecules remain poorly investigated. In addition, the choice of diluents is largely limited to fluorinated organic molecules which are expensive and environmentally hazardous and unstable at the reductive potential. Because of poor cathodic stability, fluorinated diluents (FDs) have failed to achieve cycle lives longer than 500 cycles in lithium metal half-cell.Herein, we establish the design rule of diluents for the ideal LHCE, which allows to derive three non-fluorinated diluents (NFDs) systematically. We present that two solvatochromic parameters, HBA basicity (β) and normalized electronic transition energy (ET N), unravel the generalization rule for the lithium-ion solvation capability and miscibility of diluents. Our design rule discovers three NFDs: anisole, ethoxybenzene, and furan. These NFDs have the unique resonance structures which effectively delocalize the oxygen lone pair electrons and thus enhance non-solvating characteristics, confirmed by the β value and the density functional theory (DFT) calculation. In addition, nuclear magnetic resonance (NMR) spectroscopy studies confirm that our NFDs remains stable and unreacted after long-term cycling. Finally, NFD-containing electrolytes achieve superior cyclability up to 1400 cycles, the record for LHCE to date. The results obtained in this study may pave the way for designing advanced electrolyte for next generation LMA.