Use of organic esters as cosolvents in electrolytes for lithium-ion batteries with improved low temperature performance

The electrolyte composition plays a strong role in determining the low temperature performance of lithium-ion cells, both in terms of ionic mobility in the electrolyte solution, as well as forming suitable surface films on the electrode surfaces. A series of ester solvents was chosen for incorporation into multicomponent electrolyte formulations due to their favorable physiochemical properties (i.e., low viscosity, low melting point, and high permittivity!, as well as good compatibility with carbonaceous anodes and mixed metal cathodes (i.e., LiCoO2 and LiNiCoO2). In addition to determining the relative facility of lithium intercalation and deintercalation in Li-carbon cells as a function of temperature, a number of conventional electrochemical methods were employed to further enhance the understanding of the nature of the electrode surface films in these ester-based electrolytes, including dc polarization and ac impedance measurements. A distinct trend was observed with respect to the stability of the surface films formed. In solutions containing low molecular weight cosolvents (i.e., methyl acetate and ethyl acetate! the surface films appear resistive and inadequately protective, whereas electrolytes containing higher molecular weight esters resulted in surface films with more desirable attributes. Promising electrolyte formulations were further evaluated in prototype lithium-ion cells (AA-size) and fully characterized in terms of their low temperature discharge performance. (C) 2002 The Electrochemical Society.