The Rate Equation of Decomposition for Electrolytes with LiPF₆in Li-Ion Cells at Elevated Temperatures

Organic compounds that are unstable at high temperatures are used as electrolytes in Li-ion cells. Therefore, the heat generation by chemical reactions of these organic compounds within the cells is an important factor to be considered. The thermal stabilities of 1M LiPF 6 /PC and 1M LiPF 6 /EC+DMC (1:1 in vol.) electrolytes used in lithium cells were measured by differential scanning calorimetry (DSC) using airtight containers. Rate equations, which explain the heat generation, were studied. The salt LiPF 6 is in equilibrium with LiF and PF 5 in the electrolyte solutions. As a mechanism of electrolyte decomposition, Sloop et al. showed that the PF 5 reacts with solvents, generating heat. Based on Sloop's mechanism and Wang's rate equation for thermal LiPF 6 decomposition, rate equations were developed, and the rate of heat generation as a function of temperature was calculated using the rate equations at DSC scan rate of 3, 7, 10, 12, 15, 17, and 20°C min −1 . Unfortunately, these calculated curves did not fit the experimental data well. Therefore, an additional side reaction of PF 5 , which did not contribute to heat generation, was assumed. With the inclusion of this side reaction, the calculated curves showed good agreement with the experimental data.