Phase separation of intercalation electrodes

Lithium ion ceils are widely used for portable equipment because of their high energy density. These cells employ lithium intercalation materials as their anode and cathode active materials. Lithium intercalation materials sometimes show phase separation as a function of the amount of inserted Li, and the cell voltage becomes a constant at the two-phase region. The thermodynamic criterion of the phase separation of binary mixtures is already known using Gibbs free energy. The criterion of the phase separation was applied to that of lithium intercalation materials. First, the phase separation of spinel LiMn ₂O4 and LiNiO₂ was studied, with the assumption that the cathode materials do not deform during Li insertion and extraction, are completely ionic, and only the Coulomb potential is effective in terms of changing the internal energy. Our calculated phase separation was in agreement with the experiments at 0 < x < 0.5 for Li_xMn₂O ₄ and 0 < x < 0.25 for Li_xNiO₂. Our calculated voltage of Li_xMn₂O₄ was very high compared with the experimental value, because of a very low value of the Coulomb potential at the lithium tetrahedral site in Li_xMn₂O ₄. Therefore, it seems that LiMn₂O₄ is not a perfect ionic crystal. Next, the phase separation of graphite was studied, considering a work (W_a) to expand the layer when Li is inserted. When W_a is small, it is expected that the phase separation is the co-existence of the Li-poor phase and the Li-rich phase. However, if W _a is large, it is expected that the two-phases are a phase without Li and a phase filled with Li completely. From these considerations, graphite seems to have a large w_a.