P2-type NaxMnO2 (P63=mmc, Z = 2) with a layered rock-salt structure is a promising cathode material for low-cost sodium ion secondary batteries (SIBs). Here, we performed an ex situ synchrotron radiation x-ray structural analysis of NaxMnO2 (0:43 ≤ x ≤ 0:71) against Na+ concentration (x). We found that the thicknesses (dNaO2 ) of the NaO2 layer is nearly independent of x, in a sharp contrast to the rapid decrease observed in isostructural NaxCoO2. In addition, the site energy differences [Δε = ε 1 -ε 2; ε 1 (ε 2) is the site energy at Na1 (Na2)] in NaxMnO2 are much smaller than those in NaxCoO2. These observations are interpreted in terms of the screening effect of the Na+ charge in Na xMnO2.
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