We present first-principles calculations of the solid-state portion of the Cu-Li phase diagram based on the cluster expansion formalism coupled with the use of bond length-dependent transferable force constant to model of vibrational disorder and lattice gaz Monte Carlo simulations to model configurational disorder. These calculations help settle the existence of additional phases in the Cu-Li phase diagram that have been postulated, but not yet clearly established. Our calculations predict the presence of at least one additional phase and the associated predicted phase transitions are consistent with our electrochemical measurements, which exhibit clear plateaus in the electromotive force-composition curve. While cluster expansions for both the bcc and fcc lattices in this system indicate an ordering tendency, we find that any ordered phase fully disorders below the temperature where the electrochemical measurements were made. Instead, our calculations indicate that phase transitions between the bcc and fcc solid solutions are at the origin of the plateaus in the measured electromotive force as a function of composition.