Failure mechanism of Li-ion battery at overcharge conditions

The overcharge kinetics of a commercial prismatic Li-ion battery at different current rates (1 C, 2 C, and 3 C) has been studied. Battery surface temperature, heat output, and voltage were monitored and analyzed during overcharge testing. It has been shown that the heat rate of the battery surface does not increase in proportion to the applied current rate. Separator shutdown properties may be realized for heat rates less than 3 °C/min. Li-ion batteries have been submitted to different stages of overcharge by a “soft” overcharge technique (1 C to 4.4, 4.6, and 5.0 V). Differential scanning calorimetry (DSC) tests of the charged anode, cathode, and separator recovered from overcharged cells have been performed. It was found that the anode at different overcharged states has two main exothermic peaks at 120 and 300–320 °C. At a higher state of overcharge (SOOC), the second peak shifts to a lower temperature. DSC for overcharged cathodes has more complicated profiles depending on SOOC. Increasing the cutoff voltage from 4.4 to 5.0 V shifts the maximum of the first temperature peak from 235 to 200 °C and the second from 345 to 320 °C. Electrical impedance spectroscopy and scanning electron microscopy have been used to characterize electrode materials at different SOOC and overcharge conditions. The heat rate (related to current), cell construction, and design are considered as the main factors of Li-ion battery failure at overcharge.