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

A new orthochelated salt, lithium bis(monofl uoromalonato)borate (LiBFMB), is synthesized and purifi ed for application in lithium-ion batteries. The presence of fl uorine in the borate anion of LiBFMB increases its oxidation potential and also facilitates ion dissociation, as refl ected by the ratio of ionic conductivity ( σ exp ) and ion diffusivity coeffi cients ( σ NMR ). Half-cell tests using 5.0 V lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O 4 ) 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 LiPF 6 - 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 signifi cantly different from those of LiPF 6 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.