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

Using an accelerating rate calorimeter, the reaction between lithium-contai ning carbon samples and nonaqueous electrolyte has been studied. Six differ ent carbons, differing in morphology (fiber, spheres, flakes), heat-treatme nt 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, s howing an initial activated process, associated with decomposition of metas table components of the solid electrolyte interface, followed by reaction o f intercalated lithium with electrolyte. The activation energy for the firs t process is about 1.4 eV for the lithium-containing carbons in LiPF6 ethyl ene carbonate:diethyl carbonate electrolyte. The reaction rates, however, w ere strongly dependent on the surface area of the graphitized samples, incr easing by about two orders of magnitude from the lowest to the highest surf ace area sample. Surprisingly, a petroleum coke sample, heated to only near 1200 degrees C, showed reaction rates an order of magnitude lower than exp ected based on its surface area. These results point the way to better carb ons for safer Li-ion cells. (C) 1999 The Electrochemical Society. S0013-465 1(99)03-026-5. All rights reserved.