New Family of Lithium Salts for Highly Conductive Nonaqueous Electrolytes

New lithium salts of weakly coordinating anions were prepared by treating lithium imidazolates or LiN(CH3)2 with
2 equiv of BF3. They are LiIm(BF3)2, Li 2-MeIm(BF3)2, Li 4-MeIm(BF3)2, LiBenzIm(BF3)2, Li 2-iPrIm(BF3)2, and LiN(CH3)2(BF3)2 (Im = imidazolate, Me = methyl, iPr = isopropyl, BenzIm = benzoimidazolate). The salts were
characterized by NMR spectroscopy and mass spectrometry. The structure of LiBenzIm(BF3)2 consists of a dimeric
centrosymmetric unit with each lithium atom forming a bridge between the two anions through one fluorine contact
to each anion. The structure of a hydrate of LiN(CH3)2(BF3)2 consists of an infinite chain in which each anion
chelates two different lithium atoms through Li−F bonds. The conductivities of electrolyte solutions of these salts
were measured and are discussed in terms of different ion-pairing modes determined from the solid-state structures,
the anion's ability to distribute charge, and solution viscosity. Organic carbonate solutions of LiIm(BF3)2 partially
disproportionate at 85 °C forming LiBF4, LiBF2[Im(BF3)]2, and Li[(BF3)ImBF2ImBF2Im(BF3)], reaching equilibrium by
3 months at 85 °C but not disproportionating at room temperature after 9 months. A mechanism for the formation
of these disproportionation products is proposed. The lower conductivity of the 1 M LiIm(BF3)2 solution that has
undergone disproportionation is attributed to the formation LiBF4, which is less conductive, and LiBF2[Im(BF3)]2 and
Li[(BF3)ImBF2ImBF2Im(BF3)], which increase solution viscosity.