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 1/3 Fe Mn ]O 2 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 1–2y Ca y [Ni with = 0 0.02, studied their stability towards exposure. 0.96 0.02 demonstrated excellent air retaining 0.93 of lattice in the original O3 structure after 6 days storage. Titration experiments confirmed that presence effectively suppressed otherwise rigorous + /H ion exchange water. Charge/discharge cycling half cells suggested improved active material’s specific capacity retention its integrity eliminating surface impurity formation Finally, performance /hard carbon full was evaluated upper cut-off voltages 4.0 V 4.1 V. Raising voltage to resulted 20% gain energy, but also accelerated fade polarization, most likely due an irreversible phase transition above