Properties of BMIBF4-LiBF4 Electrolytes for Lithium Ion Batteries

International audienceIonic liquids (IL) present unique properties that make them interesting for one part as battery electrolyte. As a result, ionic conductivity, dynamic viscosity and electrochemical and thermal stability of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) with or without lithium tetrafluoroborate (LiBF4) are studied. Thermogravimetric and differential scanning calorimetry investigations allows the observation of the degradation and glass transition temperatures. Temperature and concentration in LiBF4 dependences of ionic conductivity and dynamic viscosity are analyzed thanks to impedance spectroscopy and rheometric methods, respectively. Furthermore, fitting laws are proposed to describe the thermal behaviors. Cyclic voltammetry experiments indicate that the reduction of BMI+ on an iron working electrode occurs at 0.9 V vs. a Li+/Li reference electrode, whereas BF4- is oxidized on a platinum working electrode at 5.7 V vs. a Li+/Li. Then LiFePO4/Li4Ti5O12 batteries using BMIBF4-LiBF4 with various concentrations as the electrolyte are assembled for cycling tests