Thermal Characteristics of Conversion-Type FeOF Cathode in Li-ion Batteries

Abstract

Rutile FeOF was used as a conversion-type cathode material for Li-ion batteries. In the present study, 0.6Li, 1.4Li, and 2.7Li per mole lithiation reactions were carried out by changing the electrochemical discharge reaction depth. The thermal characteristics of the FeOF cathode were investigated by thermogravimetric mass spectrometric (TG-MS) and differential scanning calorimeter (DSC) systems. No remarkable HF release was detected, even up to 700 • C, which indicated a low toxic risk for the FeOF cathode. Changes in the thermal properties of the FeOF cathode via different conversion reaction depths in the associated electrolyte were studied by changing the cathode/electrolyte ratio in the mixture. LiFeOF was found to exothermically react with the electrolyte at about 210 • C. Similar exothermic reactions were found with charged FeOF cathodes because of the irreversible Li ions. Among the products of the conversion reaction of FeOF, Li 2 O was found to exothermically react with the electrolyte at about 120 • C, which induced the main thermal risk of the FeOF cathode. It suggests that the oxygen-containing conversion-type cathodes have a higher thermal risk than the oxygen-free ones, but controlling the cathode/electrolyte ratio in cells successfully reduced the thermal risk. Finally, the thermal stability of the FeOF cathode was evaluated in comparison with FeF 3 and LiFePO 4 cathodes.

Electrochemical Performance and Thermal Stability of Iron Oxyfluoride (FeOF) for Sodium-Ion Batteries

“…During cycling, the discharge capacity was controlled at 300 mAh g´1, while the charge cut-off potential was set at 4.0 V. Figure 5 shows the DSC curves of the mixtures of the cycled FeOF cathodes and the associated LiClO 4 /PC electrolyte. Unlike that in the 1 mol dm´3 LiPF 6 /EC-DMC solution [13], both sodiated and desodiated FeOF cathodes started their exothermic heat distribution from a low temperature of 100˝C, which was very similar to the pattern observed for the Na-ion battery. Obviously, reactions between FeOF and perchlorate salts and/or PC solvent were responsible for the heat generation.…”

“…However, the exothermic onset temperature was very close to that of the sodiated cathodes. In addition, fresh FeOF cathodes gave out heat at low temperature in the associated 1 mol dm´3 NaClO 4 /PC electrolyte, while no similar reaction was found when FeOF was heated together with 1 mol dm´3 LiPF 6 /EC-DMC electrolyte [13]. This phenomenon indicated that FeOF might be unstable in ClO4 /PC solutions.…”

“…According to the MS spectra, all sodiated and desodiated FeOF cathodes showed slight HF evolution at temperatures higher than 500~600 °C , while the signals were even lower than that from FeOF/Li samples. Clearly, FeOF did not present a risk of toxicity in Na-ion batteries, and in fact, was even FeOF has been shown to have no higher risk of hypertoxic HF gas release than other fluorine-free electrodes in Li-ion batteries [13]. Nonetheless, because it is a fluorine-containing active material, its toxicity in Na-ion batteries must still be investigated.…”

“…Based on the results of the XANES and XRD analyses, a reversible insertion/extraction of Na + into the FeOF cathode could be confirmed, and could be described by the following reactions: Na + + e − + FeOF (Rutile) → NaFeOF (Cubic) ⇄ Na + + e − + FeOF (Amorphous or nanocrystal). FeOF has been shown to have no higher risk of hypertoxic HF gas release than other fluorinefree electrodes in Li-ion batteries [13]. Nonetheless, because it is a fluorine-containing active material, its toxicity in Na-ion batteries must still be investigated.…”

“…On the other hand, the sodiated FeOF cathodes showed less HF evolution, with a 50 °C higher onset temperature than the desodiated ones. This indicated that NaFeOF might be more thermally stable than FeOF, as in the case of LiFeOF [13]. The thermal behavior in the associated electrolyte is one of the most important thermal characteristics of an electrode material.…”