Exploring Classes of Co-Solvents for Fast-Charging Lithium-Ion Cells

Fast-charging lithium-ion cells require electrolyte solutions that balance high ionic conductivity and chemical stability. The introduction of an organic ester co-solvent is one route that can improve the rate capability of a cell. Several new co-solvent candidates were identified based on viscosity, permittivity (dielectric constant), and DFT-calculated electrochemical stability windows. Several formate, nitrile, ketone, and amide co-solvents are shown to increase the ionic conductivity of lithium hexafluorophosphate in conventional organic-carbonate-based solutions. Based on gas production during the first formation cycle in Li[Ni 1-x-y Co x Al y ]O 2 /graphite-SiO pouch cells, five candidates were identified: methyl formate (MF), ethyl formate (EF), propionitrile (PN), isobutyronitrile (iBN), and dimethyl formamide (DMF). High temperature storage (60°C), long-term cycling, and ultrahigh-precision coulometry results indicate that MF offers the greatest balance between conductivity increase and cell lifetime. Future work is encouraged to develop more stable solution chemistries that incorporate MF. PN may prove useful for low temperature (< 40°C) applications.