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.