Abstract: Through the self-designed ac impedance monitoring platform for thermal runaway of lithium ion battery, the impedance change rule in the process of thermal abuse of lithium ion battery was studied. Based on ac impedance technology and thermal runaway mechanism of lithium ion battery, the changes of impedance, phase Angle, ohmic resistance, polarization resistance and voltage with the surface temperature of lithium ion battery during thermal abuse were analyzed. The results show that in the process of heat abuse, the impedance of the lithium ion battery first decreases and then increases rapidly several minutes before the battery is disconnected, and the battery voltage remains stable until the battery is disconnected. The phase angle is going to decrease in the heat and then it's going to be stable. The impedance of 18650 lithium ion battery is mainly affected by ohmic resistance. The polarization resistance of lithium ion battery is stable before the battery breaks, which proves that there is no chemical reaction in the positive electrode material at this stage.

Key words: lithium ion batteries, ac impedance, thermal runaway, early warning