On the use of LiPF3(CF2CF3)3 (LiFAP) solutions for Li-ion batteries. Electrochemical and thermal studies |
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| Institution: | 1. Department of Chemical System Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan;2. Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30, Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan;3. Center for Green Research on Energy and Environmental Materials (GREEN) and International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan;4. Mitsubishi Materials Electronic Chemicals Co., Ltd., 3-1-6, Barajima, Akita City, Akita 010-8585, Japan;5. Advanced Products Company, Mitsubishi Materials Corporation, 1-6-1, Yokoami, Sumida-ku, Tokyo 130-0015, Japan;6. Central Research Institute, Mitsubishi Materials Corporation, 1002-14, Mukohyama, Naka-shi, Ibaraki 311-0102, Japan;1. State Key Laboratory of Advanced Electromagnetic Engineering and Technology, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China;2. School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei, 430074, China |
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| Abstract: | Electrolyte solutions comprising a mixture of LiPF6 and LiPF3(CF2CF3)3 (LiFAP) in alkyl carbonates (ethylene, dimethyl and diethyl carbonate) were found to be superior to single salt LiFAP or LiPF6 solutions for lithium–graphite anodes at elevated temperatures. Graphite electrodes could be cycled (Li insertion–deinsertion) more than hundred times at 80 °C with high and stable capacity in the two-salt solutions, while in the single-salt solutions this was impossible. Preliminary studies by voltammetry and impedance spectroscopy indicate that the combination of the two salts in solution has a unique influence on the electrodes surface (not yet defined). Thermal studies by accelerating rate and differential scanning calorimetry show that thermal decomposition of LiFAP solutions has a higher onset, but very high heat and pressure developing rates, compared to LiPF6 solutions. The presence of LiPF6 in LiFAP solutions decreased their self-heating and pressure-developing rates pronouncedly. From product analysis of the thermal reactions by NMR, FTIR and MS, we can suggest possible unique bulk reactions that occur in LiPF6–LiFAP solutions. One of these is a nucleophilic reaction between F− and PF3(CF2CF3)3−, which may neutralize the effect of trace HF in solutions (thus forming new P–F bonds and HCF2CF3). Such a reaction should have a positive effect on both the performance of the Li–graphite electrodes and the thermal behavior of the solutions. |
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