Impact of Calcium on Air Stability of Na[Ni_1/3Fe_1/3Mn_1/3]O₂ Positive Electrode Material for Sodium-ion Batteries

O3-type Na[Ni_1/3Fe_1/3Mn_1/3]O₂ is a promising positive electrode material for sodium-ion batteries. However, it suffers from structural degradation accompanied by surface-impurity growth during ambient storage and processing. In this study, we synthesized Na_1–2yCa_y[Ni_1/3Fe_1/3Mn_1/3]O₂ with y = 0 and 0.02, and studied their structural stability towards ambient exposure. Na_0.96Ca_0.02[Ni_1/3Fe_1/3Mn_1/3]O₂ demonstrated excellent air stability by retaining 0.93 of lattice Na in the original O3 structure after 6 days of ambient storage. Titration experiments confirmed that the presence of Ca in Na_0.96Ca_0.02[Ni_1/3Fe_1/3Mn_1/3]O₂ effectively suppressed the otherwise rigorous Na⁺/H⁺ ion exchange in the presence of water. Charge/discharge cycling in half cells suggested that Ca improved the active material’s specific capacity and capacity retention by retaining its structural integrity and eliminating surface impurity formation during ambient electrode processing. Finally, the cycling performance of Na_0.96Ca_0.02[Ni_1/3Fe_1/3Mn_1/3]O₂/hard carbon full cells was evaluated with upper cut-off voltages of 4.0 V and 4.1 V. Raising the upper cut-off voltage to 4.1 V resulted in a 20% gain in specific energy, but also accelerated capacity fade and voltage polarization, most likely due to an irreversible phase transition above 4.0 V.