Stabilized Phase Transition Process of Layered NaxCoO2 via Ca-Substitution

We investigated the suppressed capacity fading of a Ca-substituted P3-type Na x CoO 2 during the charge/discharge process. The Ca-substituted Na x CoO 2 shows similar phase transition behavior to the Ca free one, with an expanded P′3 phase region. The capacity fading of the Na x CoO 2 is highly correlated with the phase transition at 4.0 V of P′3–O3′ phase transition and at 2.7 V of O′3–P′3 phase transition. The amount of Co 3 O 4 observed in the cycled electrodes corresponds to the capacity loss during the cycling test. Thus, the migration of the Co 3+/4+ ions during the phase transition process causes the capacity fading. The decomposition of the electrolyte solution also accelerates the migration of the Co 3+/4+ ions. Though the Ca-substitution does not prevent the phase transition, the Ca-substituted Na x CoO 2 shows improved capacity retention.