Decomposition kinetics of anhydrous and moisture exposed LiPF6 salts by thermogravimetry

Thermal decomposition of anhydrous and moisture exposed LiPF6 solids were studied using thermogravimetry (TG). The effects of experimental conditions such as sample mass and inert gas atmosphere on decomposition kinetics were investigated. It was found that an experimental sample mass range of 5–10 mg provided a reasonably constant activation energy value of approximately 116.0 kJ mol−1 for the decomposition of anhydrous LiPF6. The use of larger sample masses for the thermal decomposition study on TG resulted in lower activation energies and pre-exponential factors compared to smaller sample masses. This anomaly can be ascribed to inefficiencies in heat and mass transfer processes. Moisture exposed LiPF6 has an apparent activation energy value (123.7 kJ mol−1) that is higher than that of the anhydrous salt and this is thought to be associated with the partial formation of a LiPF6·HF adduct on the solid surface resulting from the absence of an efficient sweeping gas to remove decomposition by-products. The spontaneous decomposition rate constant of anhydrous LiPF6 at 298.15 K has been determined to be 4.2 × 10−5 s−1.

Moisture exposed LiPF6 (LiPF6·HF) Ea = 121 kJ mol−1, anhydrous LiPF6 (LiPF6) Ea = 116 kJ mol−1.Figure optionsDownload as PowerPoint slideHighlights
► We show that the Ea for the thermal decomposition of anhydrous LiPF6 is different from that of moisture exposed LiPF6.
► A decomposition rate constant of LiPF6 at 298.15 K is deduced.
► The effect of the TG experimental sample size on the activation energy and pre-exponential factor is discussed.
► It is shown that LiPF6 exposed to anhydrous HF shows an IR spectrum similar to LiPF6 exposed to atmospheric moisture.