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

The thermal decomposition of Li0.5CoO2 was studied by accelerating rate cal orimetry 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 tempe ratures above 200 degreesC. By contrast, the reaction of Li0.5CoO2 with eth ylene carbonate: diethyl carbonate (EC: DEC) solvent initiates at temperatu res as low as 130 degreesC, which is much lower than the decomposition temp erature of Li0.5CoO2 itself, and reduction to CoO occurs. We believe that t his is caused by the reducing power of the solvent. The heat generated by t his reaction is consistent with that expected from the combustion of the so lvent by the oxygen liberated during the decomposition of the solid. The re action 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 re action is suppressed, suggesting that increased LiPF6 concentration can slo w the cathode/electrolyte reaction in practical Li-ion cells under conditio ns of electrical or mechanical abuse. (C) 2001 The Electrochemical Society.