Working Toward Faster Charging Lithium-Ion Cells through Electrolyte Chemistry

Abstract

As the adoption of electric vehicles continues to increase, there is a growing demand for faster charging lithium-ion cells to compete with the convenience of fuel-powered vehicles. Yet even for modern designs, rapidly charging lithium-ion cells can cause a decrease in cycle and calendar lifetimes. This is largely attributable to the large overpotentials required to apply large currents, which can lead to unwanted lithium metal plating at the graphite electrode surface. Moreover, as electrode thickness increases for higher energy density cells, lithium-ion transport in the electrolyte becomes limited, leading to even larger overpotentials. This work will present novel electrolytes with improved lithium-ion transport properties. Several co-solvent systems were identified based on the viscosity, permittivity (dielectric constant), and DFT-calculated electrochemical stability windows of the pure compounds. Several formate, nitrile, ketone, and amide co-solvents will be shown to increase the ionic conductivity of LiPF6 in conventional organic-carbonate-based solutions (Figure 1). Based on gas production during the first formation cycle in Li[Ni1-x-yCoxAly])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), ultrahigh-precision coulometry (UHPC), and long-term cycling results will be presented showing that MF offers the greatest balance between conductivity increase and cell lifetime. PN may prove useful for low temperature (< 40°C) applications. Figure 1 – a) Ionic conductivity of 1.2 mol L−1 LiPF6 solutions prepared with 20% of co-solvent and 80% of a 25:5:70 mixture by weight of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate, respectively. b) Solution conductivities relative to control solution, i.e., no co-solvent. Co-solvents include isobutyl acetate (iBA), methyl propionate (MP), methyl butyrate (MB), methyl formate (MF), ethyl formate (EF), n-propyl formate (nPF), isobutyl formate (iBF), propionitrile (PN), isobutyl nitrile (iBN), methyl ethyl ketone (MEK), and dimethyl formamide (DMF). Figure 1