Effect of Quaternary Phosphonium Salts in Organic Electrolyte for Lithium Secondary Batteries

Additive behaviors of several bis (trifluoromethylsulfonyl) amide-based phosphonium salts in an organic electrolyte used for lithium secondary batteries are reported. Electrolytic properties of a lithium hexafluorophosphatebased organic electrolyte mixed with the phosphonium salts were examined. It was found that the mixed electrolytes containing the phosphonium salts showed lower conductivities and higher viscosities than the organic electrolyte. However, in the case of the mixed phosphonium electrolytes based on asymmetrical cations, the charge-discharge cyclabilities of the lithium battery cells were superior to that of the cell containing the organic electrolyte. Furthermore, the thermal stability of LiCoO2 cathodes charged in the mixed phosphonium electrolytes was considerably improved. These results suggest that the phosphonium salts are regarded as effective electrolyte additives for lithium secondary batteries.