Simultaneous Reduction of Co<sup>3+</sup> and Mn<sup>4+</sup> in P2-Na<sub>2/3</sub>Co<sub>2/3</sub>Mn<sub>1/3</sub>O<sub>2</sub> As Evidenced by X‑ray Absorption Spectroscopy during
Electrochemical Sodium Intercalation
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Abstract
Sodium intercalation in P2-Na<sub>2/3</sub>Co<sub>2/3</sub>Mn<sub>1/3</sub>O<sub>2</sub> (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<sup>3+</sup>/Co<sup>2+</sup> and Mn<sup>4+</sup>/Mn<sup>3+</sup>, are both involved in the reaction