The Effect of Electrolyte Composition on Lithium Plating During Low Temperature Charging of Li-Ion Cells

A number of additives have been investigated for their effect on the charge characteristics of cells containing low temperature, high ester content electrolytes (1.0 M LiPF6 in ethylene carbonate (EC), ethyl methyl carbonate (EMC), methyl propionate (MP) 20:20:60 vol% + additive). These experimental three-electrode cells are composed of graphite anodes, LiNiCoAlO2 cathodes, and a lithium metal reference electrode. Cells were first characterized at various temperatures by performing Electrochemical Impedance Spectroscopy (EIS), linear micropolarization and Tafel polarization measurement to determine the kinetic parameters for the anodes and cathodes. Following characterization, the cells were subjected to charging at decreasing temperatures using C/5 rates to 4.10V, with a constant potential C/50 taper. Analysis of the C/20 discharge following the charge at low temperature revealed either the presence or absence of a high voltage plateau during the initial stages of the discharge. This plateau has been linked to lithium metal on the anode and was used as a measure for the extent of lithium plating. Differential analysis (dV/dQ) was applied clearly visualize the position of the high voltage plateau and use the peak position (x-axis) in the dV/dQ plot as a gauge for the amount of plating which took place in the cell. Most additives investigated increased the anode resistance and decreased the cathode resistance, thereby leading to increased plating at low temperature. One additive, LiFSI, was found to be beneficial during low temperature charging, however studies of electrode kinetics from the cell containing LiFSI did not reveal significant improvements compared to the baseline.