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 LiPF6/PC and 1M LiPF6/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 LiPF6 is in equilibrium with LiF and PF5 in the electrolyte solutions. As a mechanism of electrolyte decomposition, Sloop et al. showed that the PF5 reacts with solvents, generating heat. Based on Sloop's mechanism and Wang's rate equation for thermal LiPF6 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 PF5, 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.