The effect of electrolyte type upon the resilience of lithium-ion cells to high-temperature storage has been investigated in experimental mesocarbon microbead carbon-LixNiyCo1-yO2 three-electrode cells. Specifically, electrolytes have been studied where the solvent mixtures have been varied, with the intention of determining the impact of ethylene carbonate (EC) content upon performance. In addition to determining the reversible and irreversible capacity losses sustained as a result of high-temperature storage (55 to 70° C), a number of electrochemical measurements (ac impedance, Tafel polarization, and linear polarization) have been performed to determine the impact of the high-temperature exposure upon the electrode kinetics and the nature of the electrode surface films. It was observed that cells containing electrolytes with high EC content (i.e., 70% EC by volume) displayed superior resilience to high-temperature storage, in contrast to cells containing low EC content electrolytes (i.e., 30% EC by volume), which displayed much larger irreversible capacity losses and poorer lithium intercalation/deintercalation kinetics after exposure to high temperatures. Solid-state Li-7 nuclear magnetic resonance measurements were used to determine quantitatively the fraction of Li in the irreversible solid electrolyte interphase (SEI) as compared to Li in the active electrode (both anode and cathode) material. In addition, the electrodes were characterized using a scanning electron microscope equipped with an X-ray energy-dispersive spectrometer to examine the film morphology and composition. The results indicate that the nature of the SEI formed on the anode in low EC content cells correlates with the poor electrochemical performance observed after being subjected to high temperatures. © 2005 The Electrochemical Society. All rights reserved.