Simultaneous Reduction of Co³⁺ and Mn⁴⁺ in P2-Na_2/3Co_2/3Mn_1/3O₂ As Evidenced by X-ray Absorption Spectroscopy during Electrochemical Sodium Intercalation

Sodium intercalation in P2-Na 2/3 Co 2/3 Mn 1/3 O 2 (obtained by a coprecipitation method) was investigated by ex situ and in situ X-ray absorption spectroscopy. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and the Mn edges, suggesting that the redox pairs, that is, Co 3+ /Co 2+ and Mn 4+ /Mn 3+, are both involved in the reaction.