Thermal and electrochemical behaviour of C/LixCoO2 cell during safety test

Thermal and electrochemical processes in a 1000 mAh lithium-ion pouch cell with a graphite anode and a LixCoO2 cathode during a safety test are examined. In overcharge tests, the forced current shifts the cell voltage to above 4.2 V. This causes a cell charged at the 1 C rate to lose cycleability and a cell charged at the 3 C rate to undergo explosion. In nail penetration and impact tests, a high discharge current passing through the cells gives rise to thermal runaway. These overcharge and high discharge currents promote joule heat within the cells and leads to decomposition and release of oxygen from the de-lithiated LixCoO2 and combustion of carbonaceous materials. X-ray diffraction analysis reveals the presence of Co3O4 in the cathode material of a 4.5 V cell heated to 400 °C. The major cathode product formed after the combustion process cells abused by forced current is Co3O4 and by discharge current the products are LiCoO2 and Co3O4. The formation of a trace quantity of CoO through the reduction of Co3O4 by virtue of the reducing power of the organic solvent is also discussed.