Blends of lithium bis(oxalato)borate and lithium tetrafluoroborate: Useful substitutes for lithium difluoro(oxalato)borate in electrolytes for lithium metal based secondary batteries?

This work was inspired by the observation that some borates exchange their ligands that are attached to boron. Therefore, we investigated the effect of blends of two salts, lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4) on lithium cycling on copper with solutions based on ethylene carbonate (EC) and diethyl carbonate (DEC) (3:7, by wt.). Coulombic efficiencies of dissolution rate (D-rate) tests demonstrated an enhanced performance of LiBOB/LiBF4 blend electrolytes compared to the LiBOB-based and LiBF4-based electrolytes, increasing with increasing LiBF4 content. The coulombic efficiencies of the LiBOB/LiBF4 blend electrolyte with highest LiBF4 content reached almost the coulombic efficiencies of the lithium difluoro(oxalato)borate (LiDFOB) based electrolyte at high current densities in D-rate tests. Voltage drop values, conductivity measurements, and AC impedance measurements show the good performance of the LiBOB/LiBF4 blend electrolytes. The composition of the solid electrolyte interphase (SEI) of the LiBOB/LiBF4 blend electrolytes studied by X-ray photoelectron spectroscopy (XPS) becomes more similar to the composition of the SEI of the LiDFOB-based electrolyte with increasing LiBF4 content. Nuclear magnetic resonance (NMR) measurements showed that the formation of LiDFOB from LiBF4 and LiBOB occurs already at room temperature by ligand exchange reactions at the boron atom. Therefore, we conclude that traces of LiDFOB in the LiBOB/LiBF4 blend are responsible for the good performance of blended electrolytes.