context: Another critical property of an electrode material toward practical application is the ease of handling against a moist environment. Most of the O3-type transition metal oxide cathodes reported so far are moisture sensitive because of the low redox potential associated with 3d-metals. Once the cathode is exposed to air or moisture, water oxidizes the transition metal ions with the concomitant removal of Na+ ions, which further react with CO2 and H2O in air to form Na2CO3 or NaOH. Such undesirable reactions usually cause structural degradation and poor electrochemical performances and thus make them difficult to handle. As revealed in Fig. 7, after exposure to air with a relative humidity of ≈55%, the Na[Ni0.5Mn0.5]O2 exhibited an apparent structural change to a Na-deficient monoclinic O′3 phase together with formation of Na2CO3·nH2O on the surface. In contrast, Na0.98Ca0.01[Ni0.5Mn0.5]O2 remarkably retarded spontaneous phase transition and retained the original O3 structure. To confirm in detail the structural stability against a humid atmosphere, a comparative study of the XRD evolution of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 and Na[Ni0.5Mn0.5]O2 cathodes was conducted as a function of exposure time (1, 3, and 5 days and 1 week) (Fig. S16†). After 1 day, there were no significant structural changes in either cathode. However, after three days, the Na[Ni0.5Mn0.5]O2 cathode was readily oxidized in the presence of moisture, and subsequently, the intensity of (003)hex. and (104)hex. peaks decreased in the XRD patterns. After 5 days, splitting of the (003)hex. and (006)hex. diffraction lines was observed, indicating a slightly enlarged interslab distance for the new monoclinic O3 phase. Finally, after 1 week, the phase transformation from the hexagonal O3 to the monoclinic O′3 phase occurred in the Na[Ni0.5Mn0.5]O2 cathode. This monoclinic O′3 phase transformation can be observed in the sodium deficient structure when the sodium ions are extracted down to 20 mol% in the original O3 structure. In contrast, throughout the week, the intensified peaks, (003)hex., (006)hex., (101)hex., (102)hex., and (104)hex., in the O3 phase were well retained persistently for the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode, indicating less reactivity against moisture. These results suggest that the Ca2+ ions on the Na sites with strong Ca–O bonding prevent the removal of Na+ ions from the structure when exposed to air. As expected, in Fig. S17,† the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode demonstrated higher reversible capacity with lower voltage polarization and better cycling stability after prolonged exposure, while the Na[Ni0.5Mn0.5]O2 cathode showed poor electrochemical performance and even exhibited an irreversible reaction during charging, due to the decomposition of Na2CO3 which is a by-product of removed Na+ ions and CO2 in air. Such a comparison of the thermal properties and air-stability strengthens the merits of Ca-substituted cathodes in practical application.
answer: Na[Ni0.5Mn0.5]O2 