Improved cycling performance of P2-type layered sodium cobalt oxide by calcium substitution

P2-type Na2/3−xCaxCoO2 is synthesized via a conventional solid-state reaction. The substituted calcium ions occupy the sodium ion layer and eliminate the lattice mismatches of the two phases in Na2/3−xCaxCoO2. Several voltage steps typically observed in the voltage profiles of NaxCoO2 are mostly disappeared associated with the expansion of single-phase regions, because the substituted calcium ions hinder the ordering of sodium ions and vacancies. Furthermore the Na2/3−xCaxCoO2 shows improved cycling performance especially at high charging-discharging rate. During the cycling test, the calcium-free Na0.74CoO2 shows phase separation to form an inactive sodium poor phase, while the Na5/8Ca1/24CoO2 maintained the single phase, suggesting that the calcium substitution suppress the structural change of the P2-type NaxCoO2 to prevent the phase separation, resulting in the improved cycling performance.