Synthesis and Characterization of Lithium Bis( fluoromalonato) borate for Lithium-Ion Battery Applications

A new orthochelated salt, lithium bis(monofluoromalonato)borate (LiBFMB), is synthesized and purified 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 (sigma(exp)) and ion diffusivity coefficients (sigma(NMR)). Half-cell tests using 5.0 V lithium nickel manganese oxide (LiNi0.5Mn1.5O4) as a cathode and ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC)as a solvent reveals that the impedance of the LiBFMB cell is much larger than those of LiPF6- and lithium bis(oxalato)borate (LiBOB)-based cells, which results in lower capacity and poor cycling performance of the former. X-ray photoelectron spectroscopy (XPS) results for 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 a thicker SEI layer. Initial results also indicate that using a high dielectric constant solvent, propylene carbonate, alters the surface chemistry, reduces the interfacial impedance, and enhances the performance of LiBFMB-based 5.0 V cell.