Lithium-Ion Electrolytes Containing Ester Cosolvents for Improved Low Temperature Performance

As part of our continuing efforts to develop advanced electrolytes to improve the performance of lithium-ion cells, especially at low temperatures, we have identified a number of electrolyte cosolvents that can be incorporated into multicomponent electrolyte formulations for enhanced performance, especially at very low temperatures (down to -70 degrees C). In the current work, we investigated a number of ester cosolvents, namely methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and butyl butyrate, in multicomponent electrolytes of the following composition: 1.0 M LiPF6 in ethylene carbonate (EC) + ethyl methyl carbonate + X (20:60:20 vol. %) [where X = ester cosolvent]. Emphasis was placed upon determining the effect of electrolyte type upon the low temperature performance in experimental meso-carbon microbeads-LiNixCo1-xO2 cells (equipped with reference electrodes) and characterizing their influence upon the lithium intercalation/deintercalation kinetics. These cells were subjected to electrical characterization (charge and discharge at different temperatures and rates) as well as electrochemical characterization (electrochemical impedance spectroscopy, linear polarization, and Tafel polarization measurements). These electrolytes have been optimized to provide good low temperature performance (down to -60 degrees C), while still offering reasonable high temperature resilience to produce the desired wide operating temperature systems (-60 to +60 degrees C). This has primarily been achieved by fixing the EC-content at 20% and the ester cosolvent at 20% in contrast to the previously developed systems. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3501236] All rights reserved.