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 Lilm(BF3)(2), Li 2-Melm(BF3)(2), Li 4-Melm(BF3)(2), LiBenzlm(BF3)(2), Li 2-(i)Prlm(BF3)(2), and LiN(CH3)(2)(BF3)(2) (CH3)(2)(BF3)(2) (lm = imidazolate, Me = methyl, Pr-i = isopropyl, Benzim = benzoimidazolate). The salts were characterized by NMR spectroscopy and mass spectrometry. The structure of LiBenzlm(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 Lilm(BF3)(2) partially disproportionate at 85 degreesC forming LiBF4, LiBF2[lm(BF3)](2), and Li[(BF3)lmBF(2)lmBF(2)lm(BF3)], reaching equilibrium by 3 months at 85 degreesC 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 Lilm(BF3)(2) solution that has undergone disproportionation is attributed to the formation LiBF4, which is less conductive, and LiBF2[lm(BF3)](2) and Li[(BF3)lmBF(2)lmBF(2)lm(BF3)], which increase solution viscosity.