All compounds present in the lithium-silicon binary phase diagram were synthesized and analyzed by electron energy-loss spectroscopy. In order to limit beam damage, and to develop a fast and local method of characterizing silicon negative electrodes, the valence energy-loss spectrum region was investigated, in particular the very intense plasmon peak in these alloys. Experimental spectra are in strong agreement with theoretical ones obtained from density functional theory. These results constitute a database for Li(x)Si alloys' plasmon energies. The method is applied to the study of the first discharge of a silicon electrode, thus identifying a Li(2.9+/-0.3)Si phase in equilibrium with Si on the voltage plateau. A nucleation process of this phase in the pristine Si is revealed, as well as a possible over-lithiation beyond the end of discharge Li(15)Si(4) crystalline phase.