A Study of Three Ester Co-Solvents in Lithium-Ion Cells

Abstract

The effects of three esters incorporated as co-solvents in 1.2MLiPF6 EC:EMC:DMC (25:5:70 by volume %) electrolyte were studied in Li[Ni1-x-yCoxAly]O2/Graphite-SiO pouch cells. The esters: methyl propionate (MP), ethyl acetate (EA) and methyl butyrate (MB) were compared in a variety of tests on the cells. Storage tests at 60°C at both 4.2 V and 2.5 V demonstrated that MB and MP outperformed EA and cells containing 20% MP showed the least voltage drop during the 500 h storage period. In long-term cycling tests at 40°C, cells containing up to 20% ester exhibited similar capacity retention compared to cells with only EC:EMC:DMC solvent. Unwanted lithium plating could be suppressed in cells with 20% MP during charging above 2C compared to ester-free cells, which showed the onset of unwanted lithium plating at 1.8C. This improvement is due to the increased Li+ conductivity of electrolytes containing MP. In addition, the use of up to 40% MP did not enhance reactivity between the charged electrode materials and electrolyte at elevated temperatures according to accelerating rate calorimetry measurements.