The capacity fading of lithiated and delithiated graphite electrodes has been studied using lithium manganese oxide as a counter electrode. Higher storage temperatures and longer storage periods give larger capacity losses and larger amounts of manganese (Mn) deposition on the graphite. The capacity losses appear to be related to the amount of deposited Mn in storage experiments in dry electrolyte solutions containing various concentrations of Mn2+ ions, but definite evidence has not been found that the Mn deposit is a simple and direct cause of the capacity fading. The capacity losses estimated from quantities of Mn deposition on the graphite, assuming a Mn2+­Li+ exchange process, appear negligible when compared with the experimental findings. Scanning electron microscope and transmission electron microscope observations show the presence of some amounts of fluoride compounds accompanying some Mn contents on the graphite. The capacities of the graphites are recovered to some extent during the cycles. © 2002 The Electrochemical Society. All rights reserved.

Concentrated LiBF4 solutions in propylenecarbonate (PC) or PC−trimethyl phosphate (TMP) were studied regarding the ion-association and solvation of Li+ ions in each solution from the measurements of conductivities, viscosities, Raman spectra, and diffusion coefficients by field gradient NMR spectroscopies. From the Walden product evaluated, the dissociation degree of LiBF4 in the LiBF4−PC solution was roughly deduced to be about 30% in concentrations over 1 mol dm-3. About 1.8 PC molecules were found bound strongly to a Li+ ion in the LiBF4−PC solution, and this number was smaller than 3 for LiPF6 in PC solutions. The small solvation number could be evidence of the formation of contact ion-pairs of Li+−BF4 -. The diffusion coefficients of BF4 - came closer to those of Li+ ions as concentrations increased, and this is also in accordance with the assumption that the contact ion-pairs are formed. In LiBF4−PC−TMP solutions, about 50% of LiBF4 dissociates at 1 mol dm-3. TMP as a Lewis base is expected to suppress the contact ion-pairs of Li+−BF4 - and to improve the conductivity.

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