context: Although tremendous efforts have resulted in the great advances in recent years, O3-type layered oxide cathode materials have been plagued by the issue of poor thermal stability and structural stability against a humid atmosphere, which are favorable properties for practical application. To confirm the practical acceptability, we first checked the thermal stability and air-stability of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode by comparing it with the Na[Ni0.5Mn0.5]O2 cathode. Differential scanning calorimetry (DSC) was performed to observe the exothermic reactions of the deeply de-sodiated electrodes (highly unstable state) collected from cells charged to 4.3 V (vs. Na/Na+) after the first cycle. The results are plotted in Fig. S15,† showing both onset temperature of the exothermic reactions and the total specific heat generation. Although a similar amount of Na+ ions (according to charge capacities) was extracted from the crystal structure, the thermal reaction between NaxCa0.01[Ni0.5Mn0.5]O2 and Nax[Ni0.5Mn0.5]O2 (x is close to 0) cathodes was different. In contrast, the Nax[Ni0.5Mn0.5]O2 electrode displayed an exothermic peak at 254.1 °C with a heat generation of 634.5 J g−1, the NaxCa0.01[Ni0.5Mn0.5]O2 electrode showed a lower heat generation of 352 J g−1 and an exothermic peak at 258.9 °C. It should be noted that the thermal reaction is related to the evolution of oxygen from the crystal structure because the oxygen released from the cathode materials can react with the electrolyte solution and undergo a redox reaction. Therefore, it is strongly believed that the improved thermal stability of the Ca-substituted cathode likely resulted from the strong interaction of immobile Ca2+ with O2−, which suppresses the rate of release of thermal oxygen from the highly oxidized cathode material.
answer: O3-type layered oxide