Electronic State of P2-Type NaxMO2 (M = Mn and Co) as Investigated by In situ X-ray Absorption Spectroscopy

P2-type NaxMO2 (M = Mn and Co) with a layered structure is a promising cathode material for low-cost sodium ion secondary batteries (SIBs). Here, we performed an in situ X-ray absorption spectroscopy (XAS) of Na _xMO₂ and Na_xMO₂ near the Mn and Co K-edges against Na concentration (x). In both compounds, the red shift of the main peak with x indicates the electron doping on the Mn 3d (Co 3d) orbital. We further analyzed the pre-edge XAS profiles with several configuration models. In NaxMnO2, the spectral weights of the doublet components suggest the high-spin (HS) state of Mn. In Na_xMO₂, the single peak feature suggests that the ground-state configurations of Co⁴⁺ and Co ³⁺3+ have significant weights of d⁶R and d⁷R, where R denotes an oxygen 2p hole.