Rational solvent molecule tuning for high-performance lithium metal battery electrolytes

Electrolyte engineering improved cycling of Li metal batteries and anode-free cells at low current densities; however, high-rate capability and tuning of ionic conduction in electrolytes are desirable yet less-studied. Here, we design and synthesize a family of fluorinated-1,2-diethoxyethanes as electrolyte solvents. The position and amount of F atoms functionalized on 1,2-diethoxyethane were found to greatly affect electrolyte performance. Partially fluorinated, locally polar -CHF2 is identified as the optimal group rather than fully fluorinated -CF3 in common designs. Paired with 1.2 M lithium bis(fluorosulfonyl)imide, these developed single-salt-single-solvent electrolytes simultaneously enable high conductivity, low and stable overpotential, >99.5% Li||Cu half-cell efficiency (up to 99.9%, +/- 0.1% fluctuation) and fast activation (Li efficiency >99.3% within two cycles). Combined with high-voltage stability, these electrolytes achieve roughly 270 cycles in 50-mu m-thin Li||high-loading-NMC811 full batteries and >140 cycles in fast-cycling Cu||microparticle-LiFePO4 industrial pouch cells under realistic testing conditions. The correlation of Li+-solvent coordination, solvation environments and battery performance is investigated to understand structure-property relationships. Cycling capability, especially at high rates, is limited for lithium metal batteries. Here the authors report electrolyte solvent design through fine-tuning of molecular structures to address the cyclability issue and unravel the electrolyte structure-property relationship for battery applications.

Automated summary

The authors designed and synthesized a family of fluorinated-1,2-diethoxyethanes as electrolyte solvents, addressing the issue of cycling capability in lithium metal batteries and uncovering the relationship between electrolyte structure and performance.