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, Li2O 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 FeF3 and LiFePO4 cathodes.
Highlights
Most state-of-the-art cathodes for Li-ion batteries are based on homogeneous Li ion intercalation/deintercalation reactions in lithium-containing transition metal compounds
Only metal fluorides could be studied as cathode materials due to their relatively high operating potential, which is induced by highly ionic metal-ligand bonds [20,21]
FeOF was ball-milled with acetylene black (AB, Denka Co., Ltd., Tokyo, Japan) and mixed with polyvinylidene fluoride (PVdF) binder (KF#7305, Kureha Corp, Osaka, Japan) at a weight ratio of 70:25:5
Summary
Most state-of-the-art cathodes for Li-ion batteries are based on homogeneous Li ion intercalation/deintercalation reactions in lithium-containing transition metal compounds. These cathodes provide good cyclabilities and long cycle life, but limited theoretical capacities. Various conversion-type compounds, such as metal oxides [2,3,4], fluorides [5,6,7,8,9,10,11], sulfides [12,13,14,15], and nitrides [16,17,18,19], have been investigated as active materials for Li-ion batteries.
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