(Na,□)5[MnO2]13nanorods: a new tunnel structure for electrode materials determinedab initioand refined through a combination of electron and synchrotron diffraction data

Abstract

(Nax□1 − x)5[MnO2]13has been synthesized withx= 0.80 (4), corresponding to Na0.31[MnO2]. This well known material is usually cited as Na0.4[MnO2] and is believed to have a romanèchite-like framework. Here, its true structure is determined,ab initio, by single-crystal electron diffraction tomography (EDT) and refined both by EDT data applying dynamical scattering theory and by the Rietveld method based on synchrotron powder diffraction data (χ2= 0.690,Rwp= 0.051,Rp= 0.037,RF2= 0.035). The unit cell is monoclinicC2/m,a= 22.5199 (6),b= 2.83987 (6),c= 14.8815 (4) Å, β = 105.0925 (16)°,V= 918.90 (4) Å3,Z= 2. A hitherto unknown [MnO2] framework is found, which is mainly based on edge- and corner-sharing octahedra and comprises three types of tunnels: per unit cell, two are defined by S-shaped 10-rings, four by egg-shaped 8-rings, and two by slightly oval 6-rings of Mn polyhedra. Na occupies all tunnels. The so-determined structure excellently explains previous reports on the electrochemistry of (Na,□)5[MnO2]13. The trivalent Mn3+ions concentrate at two of the seven Mn sites where larger Mn—O distances and Jahn–Teller distortion are observed. One of the Mn3+sites is five-coordinated in a square pyramid which, on oxidation to Mn4+, may easily undergo topotactic transformation to an octahedron suggesting a possible pathway for the transition among different tunnel structures.