https://doi.org/10.15255/KUI.2004.035
Published: Kem. Ind. 54 (4) (2005) 205–214
Paper reference number: KUI-35/2004
Paper type: Original scientific paper
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Ionic Liquids as Electrolyte in Lithium-ion Batteries: Imidazolium and Pyrazolium Tetrafluoroborate in LiMn2O4/Metal Oxide Cells

J. Čaja, T. D. J. Dunstan, H. A. Dyar, H. Krall and V. Katovic

Abstract

This paper describes the application of ionic liquids, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and 1-ethyl-2-methylpyrazolium tetrafluoroborate (EMPBF4) as electrolytes in lithium-ion cells. Lithium-ion cells employed LiMn2O4 as the cathode and metal oxide (MoO2, Li4Ti5O12 ) as the anode. The physicochemical properties of ionic liquids, such as: melting point, thermal stability, conductivity, and electrochemical stability, were determined. Melting point for EMPBF4 was 47.5 °C to 49 °C and for 0.8 molal LiAsF6 in EMPBF4 21 °C to 26 °C. EMPBF4 is thermally stable to 300 °C. Conductivity of neat ionic liquids as well as ionic liquids containing dissolved lithium salts (LiBF4, LiAsF6 ) was determined. The conductivity is much lower for the latter than for the neat ionic liquids; in addition, it decreases with an increase in the lithium salt concentration. Electrochemical window was > 4.3V for all examined ionic liquid compositions except for neat EMIBF4 which was slightly lower (4.2V). Reduction and oxidation potential vs. lithium was lower for EMIBF4 containing ionic liquids than for the EMPBF4 ones. The cells were cycled at several temperatures. It was observed that the cells using EMPBF4 were less affected by increase in temperature. Both MoO2/LiMn2O4 and Li4Ti5O12/LiMn2O4 cells demonstrated stable capacity with cycling at room temperature and remained stable to 60 °C for the cell employing EMIBF4 ionic liquid. The cells employing EMPBF4 showed stable capacity with cycling to 80 °C. Coulombic efficiency showed similar behavior. It was 96% for both type of cells at room temperature. It decreased to 90 % at 80 °C for the cells employing EMPBF4 while the cells using EMIBF4 achieved coulombic efficiency of ~90 % at 60 °C or below.

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Keywords

ionic liquids, low temperature molten salts, lithium-ion batteries, nonflammable solvents, MoO2/LiMn2O4 cells, Li4Ti5O12/LiMn2O4 cells