Abuse testing of lithium-ion batteries - Characterization of the overcharge reaction of LiCoO2/graphite cells

The short-circuit and overcharge behavior of prismatic lithium-ion batterie s containing LiCoO2 cathodes and graphite anodes were studied in detail. In ternal thermocouples were used to characterize the thermal profiles of the cells under abusive conditions. Differences between the internal and surfac e temperatures of the cells during the safety tests highlighted the importa nce of the internal measurement for obtaining more meaningful data. Under s hort-circuit conditions the cells remained hermetically scaled, reached an internal temperature of 132 degreesC (the shutdown temperature of the separ ator), and then slowly cooled to ambient temperature. However, on extreme o vercharge testing different results were obtained depending on the current used to charge the battery. At low currents (less than or equal toC/5) the cells remained hermetic, but swelled significantly. When higher currents we re used, the cells ruptured during overcharge. Experimental cells were cons tructed with a systematic variation in cell balance and the point of cell r upture tracked to the amount of cathode in the cell, independent of the amo unt of anode material. The internal dc resistance of the cell was also meas ured during the overcharge reaction and remained low throughout most of the test, although a large increase was observed at the end of the test due to the melting of the shutdown separator. The cells overcharged with high cur rents all reached high temperatures (greater than or equal to 195 degreesC) immediately prior to rupturing, which suggests that the melting of lithium is a key underlying factor leading to the rupture of the cells. To test th is proposal, cells were assembled with lithium removed from the LiCoO2 cath ode, so that lithium metal would not plate on the anode during the overchar ge test. These cells reached a significantly higher temperature (similar to 280 degreesC) prior to rupture. (C) 2001 The Electrochemical Society.