Comparison of the reactions between Li7/3Ti5/3O4 or LiC6 and nonaqueous solvents or electrolytes using accelerating rate calorimetry

The reactions of discharged Li4/3Ti5/3O4 (Li7/3Ti5/3O4) or discharged mesocarbon microbead (Li0.81C6) with ethylene carbonate/ diethyl carbonate (EC/DEC) (1: 2 by volume) solvent, 1 M LiPF6 EC/DEC or 0.8 M LiBOB EC/DEC are compared using accelerating rate calorimetry (ARC). We find that Li7/3Ti5/3O4 (1.5 V vs. Li) shows lower reactivity than Li0.81C6 (0.05 V vs. Li) in EC/DEC solvent or in LiPF6-based electrolyte. The reactions between both electrodes and 1 M LiPF6 EC/DEC proceed in a clear stepwise fashion, where first, intercalated lithium reacts with PF5 (from the decomposition of LiPF6) to produce LiF, and then, when the PF5 is consumed, the remaining lithium reacts with the solvents. The addition of 0.8 M LiBOB to EC/DEC solvent greatly increases the thermal stability of both Li7/3Ti5/3O4 and Li0.81C6. X-ray diffraction (XRD) studies show that both Li7/3Ti5/3O4 and Li0.81C6 react with EC/DEC or LiBOB EC/DEC to produce Li2CO3 at the end of the ARC experiments. XRD studies show that Li7/3Ti5/3O4 reacts with 1 M LiPF6 EC/DEC to form LiF, Li4/3Ti5/3O4, Li2CO3, and some TiO2. In fact, even a fresh sample of Li4/3Ti5/3O4 reacts with 1 M LiPF6 EC/DEC to produce substantial quantities of TiO2 and LiF, but little heat is evolved during this reaction since the Li in Li4/3Ti5/3O4 is so tightly bound. The results in this paper suggest that safer lithium-ion cells could be built using Li4/3Ti5/3O4 negative electrodes than with graphite negative electrodes. (C) 2004 The Electrochemical Society.