Synthesis of Novel Lithium Salts containing Pentafluorophenylamido-based Anions and Investigation of their Thermal and Electrochemical Properties

Abstract

Three novel lithium salts, lithium bis(pentafluorophenyl)amide LiN(Pfp)₂, lithium pentafluorphenyl(trifluormethylsulfonyl)imide LiN(Pfp)(Tf) and lithium pentafluorphenyl(nonafluorbutylsulfonyl)imide LiN(Pfp)(Nf) were synthesized and characterized with respect to their thermal and electrochemical properties. LiN(Pfp)₂ decomposes at 108 ºC, whereas Li-N(Pfp)(Tf) and Li-N(Pfp)(Nf) show a much higher thermal stability of 307 ºC and 316 ºC, respectively. The ionic conductivity at 100 ºC measured by means of impedance spectroscopy decreases in the order LiN(Pfp)(Tf) > LiN(Tf)₂> LiN(Pfp)(Nf). Both, the activation energy and entropy for ion conduction in the new salts are lower than in LiN(Tf)₂ (LiTFSI), most likely due to the lower symmetry of the new anions. The electrochemical stability and ionic conductivity of LiN(Pfp)(Tf) and LiN(Pfp)(Nf) solutions (0.1 mol/l) in ethylene carbonate/dimethyl carbonate (1:3 w/w) are slightly lower than those of the LiTFSI solution, but still sufficient for application in lithium ion batteries. The high thermal stability of the novel salts and their stability towards hydrolysis makes them attractive candidates for overcoming the drawbacks of LiPF₆-based electrolytes at elevated temperatures.