A study of highly conductive ester co-solvents in Li[Ni0.5Mn0.3Co0.2]O2/Graphite pouch cells

The effect of low viscosity esters on rate capability and lifetime of Li[Ni0.5Mn0.3Co0.2]O2/graphite pouch cells was studied using a variety of methods including ultra high precision coulometry, isothermal calorimetry and long term cycle testing. Methyl acetate (MA) and methyl propionate (MP) were selected as the ester co-solvents in ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) (25:5:70 vol%) blended solvent along with 2% vinylene carbonate (VC) or 2% fluoroethylene carbonate (FEC) additives. Cells containing electrolytes with 20% or 40% MA or MP could support higher charging rates without unwanted lithium plating than those without esters. All electrolytes with 2% FEC could support higher charging rates without unwanted lithium plating compared to corresponding electrolytes with 2% VC. However, UHPC and microcalorimetry measurements indicate that both the use of esters and the use of FEC over VC lead to lifetime penalties which were confirmed by long term cycling tests. Useful electrolytes, detailed in this report, that yield a good compromise between high charging rates and long lifetime are those that contain 20% MA by weight.