Understanding theNew Crucial Role of a Calcium Ionas a Pillar Dopant in Stabilizing O3-Type Na[Ni_1/3Fe_1/3Mn_1/3]O₂ Cathodes

The pillar dopants occupying the sodium sites show great potential for stabilizing O3-type oxide cathodes, but the specific mechanism of action needs to be further uncovered. Herein, a series of Ca²⁺-doped O3-type Na_{1–2<i>x</i>}Ca<i>_x</i>[Ni_1/3Fe_1/3Mn_1/3]O₂ compounds were synthesized using coprecipitation and solid-state reaction methods. Density functional theory calculation confirms a lower formation energy for the Ca dopant occupying the sodium sites compared to that of transition metal sites. In-situ XRD results reveal that pristine O3-type Na[Ni_1/3Fe_1/3Mn_1/3]O₂ cathodes undergo a similar O3 - O3/P3 - P3 - P3/O3 - O3 phase transition within the first and second cycles. Different from the pristine O3-type cathode, the Ca²⁺-doped counterpart shows a distinct O3 - O3/P3 - P3 - P3/O3 phase transition in the first cycle, indicating that the transformation from the P3 phase to the O3 phase is partly suppressed during discharging. In the second cycle, a reversible P3/O3 - P3 - P3/O3 phase transition with weak volume changes is observed for the Ca²⁺-doped electrode, suggesting improved structural stability. Consequently, the Ca²⁺-doped Na_0.94Ca_0.03[Ni_1/3Fe_1/3Mn_1/3]O₂ shows an enhanced cycling stability, retaining 92.4% of its initial capacity after 100 cycles at 0.1 C and 85% after 300 cycles at 1 C, which is much better than that of the pristine electrode. These results reveal a new crucial role of calcium ions as pillar dopants in regulating the phase transition and stabilizing the structure of O3-type cathodes for advanced sodium-ion batteries.