In recent years, Na-ion batteries are expected as a potential post Li-ion battery system because of the abundant resources.  A lot of layered materials: Na_xMO₂ (M=Cr, Mn, Fe, Co, Ni) has been investigated as candidate positive electrode materials for Na-ion batteries.  Substitution of the transition metal in Na_xMO₂ is common strategy for the improvement of their cycling performances.  On the other hand, an elemental substitution of Na sites has never been investigated because the substituted atoms easily hinder the smooth diffusion of Na+ in the active materials. In the present study, we intentionally substituted the Na⁺ sites in the P2-type Na_xCoO₂ with Ca²⁺ and investigated how it affected to the electrochemical properties. 

The XRD study showed that the calcium substitution suppressed the lattice mismatch of the co-existing two phases with some shrinkage of the lattice constant along c-axis.  The several voltage steps observed at the peculiar compositions of the Na_xCoO₂ were eliminated by the calcium substitution.  We suspect the calcium should have hindered the sodium ions and the vacancies to take the ordered form.  As a consequence the most of the biphasic regions in the voltage profile were disappeared.  The suppression of the phase transformation during the charging-discharging process improved the cycling performance of the Na_xCoO₂ especially at high charging-discharging rate as shown in Fig.1.  The XRD patterns after the cycling test showed that the Na_xCoO₂ formed a sodium poor phase, while the substituted calcium ion mostly suppressed the phase separation.  The results showed that the cycleability of the layered Na_xMO₂ could be improved not only by the materials design of MO₂ slab, but the sodium ion layer ordering.