Abstract
A new orthochelated salt, lithium bis(monofluoromalonato)borate (LiBFMB), has been synthesized and purified for the first time for application in lithium ion batteries. The presence of fluorine in the borate anion of LiBFMB increases its oxidation potential and also facilitates ion dissociation, as reflected by the ratio of ionic conductivity measured by electrochemical impedance spectroscopy (σexp) and that by ion diffusivity coefficients obtained using pulsed field gradient nuclear magnetic resonance (PFG-NMR) technique (σNMR). Half-cell tests using 5.0 V lithium nickel manganese oxide (LiNi0.5Mn1.5O4) as a cathode and EC/DMC/DEC as a solvent reveals that the impedance of the LiBFMB cell is much larger than those of LiPF6 and LiBOB based cells, which results in lower capacity and poor cycling performance of the former. XPS spectra of the cycled cathode electrode suggest that because of the stability of the LiBFMB salt, the solid electrolyte interphase (SEI) formed on the cathode surface is significantly different from those of LiPF6 and LiBOB based electrolytes, resulting in more solvent decomposition and thicker SEI layer. Initial results also indicate that using high dielectric constant solvent PC alters the surface chemistry, reduces the interfacial impedance, and enhances the performance of LiBFMB based 5.0V cell.
