New covalent salts of the 4+V class for Li batteries

摘要

There is urgent action required for replacing LiPF6 as a solute for Li-ion batteries electrolytes. This salt, prone to highly Lewis acidic PF5 release and hydrolysis to HF is responsible for deleterious reaction on carbonate solvents, corrosion of electrode materials leading to safety problems then release to toxic chemicals. A major advantage of LiPF6 is that it passivates aluminium. Most attempts to replace LiPF6 with hydrolytically-stable salts have been unsuccessful because of Al corrosion. We present here two "Huckel" type salts, namely lithium (2-fluoroalkyl-4,5-dicyano-imidazolate); flu-oroalkyle = CF3 (TDI), C2F5 (PD1) with high charge delocalization. These thermally stable salts give both appreciably conductive solutions in EC/DMC (>6mScm~1 at 20 C) with a lower decrease with temperature than LiPF6. Non fluorinated lithium (4,5-dicyano-l,2,3-triazolate) is comparatively less than half as conductive. The lithium transference number T+ measured by PFG-NMR is also higher. Voltammetry scans with either platinum or aluminium electrodes show an oxidation wall at 4.6 V versus Li+:Li . These two salts are thus the first examples of strictly covalent, non-corroding salts allowing 4+V electrode material operation. This is demonstrated with experimental Li/LiMn2O4 cells as beyond the third cycles, the fade of the three electrolytes were quasi-identical, though LiPF6 had a sharper initial decrease.