Thermal Stability of Surface Film Formed on a Graphite Negative Electrode in Lithium Secondary Batteries

Abstract

The stability at elevated temperatures of a surface film formed on a graphite electrode in lithium secondary batteries was investigated by transmission electron microscopy (TEM) and electrochemical AC impedance spectroscopy (EIS). TEM analysis revealed that the surface film partly dissolved in the electrolyte solution during storage at 60 degrees C, resulting in a decrease in the thickness of the surface film and a change in its morphology to a porous structure. On the other hand, an increase in the impedance of the surface film which is attributable to a change in composition of the surface film was confirmed by EIS analysis during the storage at 60 degrees C. It was also shown that the addition of vinylene carbonate or 1,3-propane sultone or etylene sulfite, even if limited, improves the stability of the surface film at elevated temperatures.