A New Additive for Lithium Battery Electrolytes Based on an Alkyl Borate Compound

Tris[2H-hexafluoroisopropyl) borate (THFPB) was synthesized and studied as an additive for lithium battery electrolytes. The low polarity of its fluorinated isopropyl substitute imparts a low solubility of THFPB in the polar ethylene carbonate-dimethyl carbonate (EC-DMC) solvent system. THFPB shows dramatic effect on conductivity enhancement on lithium salts, such as CF 3 CO 2 Li and LiBr. The ambient-temperature ionic conductivity of these composite electrolytes reaches as high as ∼5 × 10 -3 S/cm. Although THFPB is a stronger anion complexing agent than its homologue tris(pentafluorophenyl) borate (TF-PBO), which has fluorinated phenyl substitutes, its effect on conductivity enhancement of LiF and LiCl is limited by its low solubility and is less than that of TFPBO. Composite electrolytes containing THFPB and LiF or CF 3 CO 2 Li salts display excellent electrochemical stability in the EC-DMC solvent, their electrochemical windows being 5.5 and 5.23 V for LiF or CF 3 CO 2 Li. respectively. THFPB-based electrolytes show high cycling efficiency and cycleability in Li/LiMn 2 O 4 and Li/LiNi 0.85 Co 0.15 O 2 cells.