Design of Localized High-Concentration Electrolytes via Donor Number

Salt-concentrated electrolytes offer properties beyond conventional dilute electrolytes yet suffer from high cost and viscosity which hinder their practical applications. Introducing a secondary solvent as a diluent could reduce the salt content while maintaining the local solution structure of salt-concentrated electrolytes, giving rise to localized high-concentration electrolytes (LHCEs). Through a comprehensive investigation involving over 500 samples, we find that the dielectric constant of solvent, a widely used parameter for electrolyte design, does not serve as a useful screening criterion for diluents; instead, donor number (DN) is an effective design parameter to achieve LHCE structure-i.e., the primary solvent must have DN > 10 and the diluent must have DN = 10. Based on this simple rule, a new LHCE using low-cost m-fluorotoluene diluent is formulated, enabling high-voltage (>4.6 V) and wide-temperature (-40-100 degrees C) operation of lithium batteries.