Lithium intercalation materials are widely used in lithium ion cells as cathode and anode active materials. Lithium intercalation materials show sometimes phase separation as a function of the amount of inserted Li+. 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. We demonstrated the phase separation condition of the Li cathode for the cases of Gas Model and Coulomb Potential Model. By using Gas Model, the cathode potential is written as E = E0+Ky+ (RT/F) In [(1 -y)/y], where E is the cathode potential (V vs. Li), y is Li occupancy. In this case, two phase area is exp (-KF/2RT)<y<1-exp (-KF/2RT) and the potential is E0+K/2. We have been considering the contribution of the internal energy change to the cell voltage (E vs. Li/Li+) assuming that the cathode material was completely ionic and only the Coulomb potential was effective in terms of changing the internal energy. From this model, we calculated E0 =3.879 V and K = 1.020 V for Lix NiO2 where y = 4 x (0<x<0.25). In this case, two phase area is 2.396 × 10-9<y<1-2.396 × 10-9 and the potential is 4.384 V.
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