context: To investigate the electrochemical Na ion storage mechanism on the Na[Ni0.5Mn0.5]O2 and Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathodes upon Na ion extraction/insertion, cyclic voltammetry was performed (Fig. S7†). Both cathodes underwent a series of phase transitions (O3hex. → O′3mon. → P3hex. → P′3mon. → → ). The intensity of the redox peaks of the Na[Ni0.5Mn0.5]O2 cathode, especially during the hexagonal O3′–hexagonal O3′′ phase transition, became gradually polarized and reduced in height with cycling, indicating high capacity fading continuously with structural degradation. In comparison, the redox peaks of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode hardly changed, which is consistent with the excellent Na intercalation stability of the cathode. In addition, ex situ X-ray absorption near edge structure (XANES) analysis was conducted (Fig. 3a and S8†) to examine the oxidation state of transition metals during the charge–discharge process. For both cathodes in the as-prepared state, Ni and Mn were divalent (2+) and tetravalent (4+), respectively. The Ni K-edge absorption spectrum clearly shifted toward the higher energy region after charging at 4.3 V, indicating that a change in the oxidation state of nickel from the divalent state to the tetravalent state occurred due to electrochemical oxidation in the Na cell. In comparison, although a shape change of the white line was observed, which is associated with a local geometry change due to the redox reaction of the surrounding electrochemically active metals, no significant edge shift was observed at the Mn K-edge, suggesting that manganese ions are electrochemically inactive in the tetravalent state, which is consistent with other previous reports. On discharge, the average oxidation state of Ni returned to its original value. Furthermore, this reaction mechanism was also confirmed through prediction of the net magnetic moments on Mn and Ni ions of Nax[Ni0.5Mn0.5]O2 based on first-principles calculations. As presented in Fig. S9,† in the case of O3-Na1[Ni0.5Mn0.5]O2, the integrated spin moments of Ni and Mn atoms were approximately 0 and +3, respectively, which indicates the existence of Ni2+ and Mn4+ ions in O3-Na1[Ni0.5Mn0.5]O2. During 1 mol Na+ extraction from the structure, the integrated spin moment of the Mn atoms remained unaltered and that of the Ni atom gradually increased from 0 to +2, which indicates the Ni2+/Ni4+ redox reaction of O3-Na1[Ni0.5Mn0.5]O2 during Na ion extraction/insertion. This prediction is consistent with the experimental results based on the XANES analysis. These results suggest that 0.01 mol of Ca2+ ions was successfully incorporated into the Na layer rather than a transition metal layer without interrupting the charge-transfer reaction.
answer: Na0.98Ca0.01[Ni0.5Mn0.5]O2