Super Charge Separation and High Voltage Phase in Na _x MnO₂

Na _x MnO₂ shows Mn³⁺ and Mn⁴⁺ charge separation with the charge stripe ordering upon Na deintercalation at x = 5/8. In this paper it is shown that, surprisingly, at lower Na compositions of 5/8 > x ≥ 1/18 the phase evolution pathway of Na _x MnO₂ upon Na deintercalation shows a unique phenomenon of super charge separation, where the Mn³⁺ and Mn⁴⁺ ions fully charge-separate into charge superplanes formed by succession of charge stripes in the third dimension. The Mn³⁺ superplanes attract Na ions electronically, and dominate the antiferromagnetic interactions in NaMnO₂. Na ions in Mn³⁺ superplanes also naturally pillar the MnO₂ layers to form the unusual O1 phases with large interlayer distances at x < 1/3, which dominates the unique electrochemical behavior of NaMnO₂.