New Family of Lithium Salts for Highly Conductive Nonaqueous Electrolytes
dataset
posted on 2004-11-29, 00:00 authored by Thomas J. Barbarich, Peter F. Driscoll, Suzette Izquierdo, Lev N. Zakharov, Christopher D. Incarvito, Arnold L. RheingoldNew 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.
