Structural response of P2-Type NaxMnO2 against Na+ intercalation

P2-type Na_xMnO₂ (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 Na_xMnO₂ (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 Na_xCoO₂. In addition, the site energy differences [Δε = ε ₁ -ε ₂; ε ₁ (ε ₂) is the site energy at Na1 (Na2)] in Na_xMnO₂ are much smaller than those in Na_xCoO₂. These observations are interpreted in terms of the screening effect of the Na⁺ charge in Na _xMnO₂.