Study on aging and external short circuit mechanisms of Li-ion cells with different electrode thicknesses

Thermal runaway is one of the challenges associated with the widespread use of Li-ion batteries. The thermal runaway characteristics of Li-ion batteries are closely associated with the electrode materials, electrode structure design and degree of cycling deterioration. The energy density of Li-ion cells can be efficiently increased by expanding the electrode thickness, but cycle and thermal performance will be deteriorated with the increase of electrode thickness. The cycle performance and aging mechanisms of cells with different electrode thicknesses during cycling has been investigated in present research. Afterwards, the external short-circuit was conducted on aged cells to explore the effect of electrode thickness on cell thermal runaway behavior. The results showed that the ohmic internal resistance and polarization internal resistance of the thick-electrode cell increased continuously with cycling, and the capacity faded significantly. The increase in Li+ migration distance in a thick electrode will make the state of charge (SOC) distribution uneven along the thick direction. The thick electrode cell had the largest absolute temperature rise and temperature rise rate when it was short-circuited, which were 104.07 °C and 1.89 °C/s, respectively. The temperature of the area near the negative electrode of the cell was higher than that of the center and the positive electrode. The micro structure of electrode by SEM revealed that cycling can cause thick electrode material to crack, resulting in capacity loss. When thermal runaway developed, thick electrodes had the highest absolute temperature rise and temperature rise rate. As the current density increased, the thermal runaway would aggravate the damage to the positive electrode material, and the separator would close and shrink, thereby cutting off the reaction between the positive and negative electrodes.