Design localized high concentration electrolytes via donor number and solubility

The salt-concentrated electrolytes offer superior properties beyond conventional dilute electrolytes yet suffer from high cost and viscosity that hinder their practical applications. A key strategy to address this challenge is to introduce a secondary solvent as a diluent that reduces the salt content while maintaining the local structure of salt-concentrated electrolytes, giving rise to localized high concentration electrolytes (LHCEs). Through a thorough investigation involving ~700 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. Correlating DN with solvent solubility leads to a simpler screening rule: Li-salt-insoluble solvents are diluents while Li-salt-soluble solvents become co-solvents. Both DN- and solubility-based design principles can be understood in an atomistic model of LHCE and are applicable to other electrolyte systems.