Analysis on the constant-current overcharge electrode process and self-protection mechanism of LiCoO2/graphite batteries

With the expanding applications, concerns about the charging safety of lithium-ion batteries (LIBs) have become more significant. In this paper, the constant-current (CC) overcharge electrode process of pouch LiCoO2/graphite batteries are analyzed at 0.25C, and then, a self-protection mechanism for decreasing the overcharge risk of batteries is evaluated. As for the batteries passing the safety test, their typical overcharge behaviors show the battery voltage and temperature begin to dramatically increase to about 5.2V and 65 degrees C from about 155% state of charge (SOC) and then decrease slowly after a short fluctuating period. The element analysis of two electrodes and separator reveals that besides the well-known metal lithium, cobalt precipitation pierces the separator and subsequently forms an internal micro-short circuit at about 155% SOC to consume the charge energy and subsequently avoid the overcharge explosion and ignition. As a conclusion, a self-protection mechanism based on an internal micro-short circuit model, which is closely related with the deposited electric lithium and cobalt and the separator porosity, is proposed and experimentally verified. These results offer an idea and method to decrease the CC overcharge risk of LIBs and can advance the safe application of LIBs.