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

• DOI: 10.1016/j.electacta.2013.05.130
• Publication Date: 2013-06-12T17:45:49Z
• Authors: T. Schedlbauer; U.C. Rodehorst; C. Schreiner; H.J. Gores; M. Winter
• Keywords: 01 natural sciences; 7. Clean energy; 0104 chemical sciences; 3. Good health

Abstract 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 (LiBF 4 ) 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/LiBF 4 blend electrolytes compared to the LiBOB-based and LiBF 4 -based electrolytes, increasing with increasing LiBF 4 content. The coulombic efficiencies of the LiBOB/LiBF 4 blend electrolyte with highest LiBF 4 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/LiBF 4 blend electrolytes. The composition of the solid electrolyte interphase (SEI) of the LiBOB/LiBF 4 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 LiBF 4 content. Nuclear magnetic resonance (NMR) measurements showed that the formation of LiDFOB from LiBF 4 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/LiBF 4 blend are responsible for the good performance of blended electrolytes.