Comparison of the reactivity of various carbon electrode materials with electrolyte at elevated temperature

Using an accelerating rate calorimeter, the reaction between lithium-containing carbon samples and nonaqueous electrolyte has been studied. Six different carbons, differing in morphology (fiber, spheres, flakes), heat-treatment temperature (1200 to 3000 degrees C), and surface area (0.4 to 9.2 m(2)/g) were studied. The reaction processes for all six samples were similar, showing an initial activated process, associated with decomposition of metastable components of the solid electrolyte interface, followed by reaction of intercalated lithium with electrolyte. The activation energy for the first process is about 1.4 eV for the lithium-containing carbons in LiPF6 ethylene carbonate:diethyl carbonate electrolyte. The reaction rates, however, were strongly dependent on the surface area of the graphitized samples, increasing by about two orders of magnitude from the lowest to the highest surface area sample. Surprisingly, a petroleum coke sample, heated to only near 1200 degrees C, showed reaction rates an order of magnitude lower than expected based on its surface area. These results point the way to better carbons for safer Li-ion cells. (C) 1999 The Electrochemical Society. S0013-4651(99)03-026-5. All rights reserved.