The reaction of charged cathodes with nonaqueous solvents and electrolytes -: I.: Li0.5CoO2

The thermal decomposition of Li0.5CoO2 was studied by accelerating rate calorimetry and X-ray diffraction. Oxygen loss from the material according to the reaction Li0.5CoO2 --> 0.5 LiCo3O2 + 1/6Co(3)O(4) + O-2 occurs at temperatures above 200 degreesC. By contrast, the reaction of Li0.5CoO2 with ethylene carbonate: diethyl carbonate (EC: DEC) solvent initiates at temperatures as low as 130 degreesC, which is much lower than the decomposition temperature of Li0.5CoO2 itself, and reduction to CoO occurs. We believe that this is caused by the reducing power of the solvent. The heat generated by this reaction is consistent with that expected from the combustion of the solvent by the oxygen liberated during the decomposition of the solid. The reaction of Li0.5CoO2 with xM LiPF6/EC:DEC (0 < x < 1.5) electrolyte was also studied. As the salt concentration is increased, the solvent combustion reaction is suppressed, suggesting that increased LiPF6 concentration can slow the cathode/electrolyte reaction in practical Li-ion cells under conditions of electrical or mechanical abuse. (C) 2001 The Electrochemical Society.