Role of the cathode and anode in heat generation of Li-ion cells as a function of state of charge

Abstract

Thermal stability of Li-ion cells (from two different manufacturers, A and B) and their cathodes and anodes were evaluated as a function of state of charge (SOC) using accelerating rate calorimetry (ARC). Cell A was rated at 750 mAh and consisted of Sn-doped LiCoO2 cathode and meso-carbon micro-fiber (MCMF) anode. Cell B was rated at 790 mAh and was comprised of a cathode and an anode made of LiCoO2 and graphite. The electrolytes in cells A and B were mixtures of EC:EMC + 1MLiPF6 and EC:EMC:DMC + 1MLiPF6, respectively. ARC results indicated that between 50 and 125% SOC, the total heat generated by cell A is mostly dominated by self-heating of the cathode, while between 125 and 200% SOC the total heat generated by the cell was dominated by the anode. The total heat generated in cell B, however, was dominated by self-heating of the graphite anode, over the entire 50–200% SOC range. Also, the self-heating in cell B was higher than that for cell A. Heat generation of the LiCoO2 increases with increasing SOC, and the Sn-LiCoO2 heat generation reaches a maximum near 125% SOC. At low SOC, the temperature of the onset of chemical reaction (TOSCR) for cell A was higher than that for the cell B. For both cells, at higher SOC, TOSCR decreased with a sharp drop between 100 and 150% SOC. Overall results show that cell A offers a higher thermal stability than cell B. Also, the ARC test is more suitable than differential scanning calorimetry for correlation of cathode and anode heat generations to the thermal performance of full Li-ion cells.