Stabilized phase transition process of layered na_xcoo₂ via ca-substitution

We investigated the suppressed capacity fading of a Ca-substituted P3-type Na_xCoO₂ during the charge/discharge process. The Ca-substituted Na_xCoO₂ shows similar phase transition behavior to the Ca free one, with an expanded P′3 phase region. The capacity fading of the Na_xCoO₂ 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 Co₃O₄ observed in the cycled electrodes corresponds to the capacity loss during the cycling test. Thus, the migration of the Co³+/4⁺ ions during the phase transition process causes the capacity fading. The decomposition of the electrolyte solution also accelerates the migration of the Co³+/4⁺ ions. Though the Ca-substitution does not prevent the phase transition, the Ca-substituted Na_xCoO₂ shows improved capacity retention.