LAUSR

Among the cathodes of the sodium ion batteries, the manganese based oxide materials present many advantages due to their high energy density, low-cost and low-toxicity. In particular, numerous layered materials have been reported in the system Na-Mn-O. These materials are interesting because they show weak interlayer interactions with free space allowing sodium diffusion. Here, we report for the first time the electrochemical activity of another lamellar phase of this system: Na2Mn3O7 [1]. This material, synthesized by a conventional solid-state method, consists of [Mn3O7 2] layers built up with edge sharing MnO6 octahedra, separated by NaO6 and NaO5 polyhedra. The galavanostatic cycling of this material have been performed at rate of C/20 (i.e. 1 Na + in 10h) in the potential window 1.5–3.0 V versus Na + /Na. Starting from Na2Mn3O7, a reversible capacity of 2 Na/f.u. (160 mAh/g) is obtained through a plateau at 2.1 V with a low polarization of 100 mV. The PITT curve reveals a bell shape type response on the reversible phenomena, which illustrates, together with the sharpness of the peaks in the derivative curve dQ/dE, that the reversible process is biphasic. Thus, the electrochemical process allows a reduced phase Na4Mn3O7 to be obtained, which cans intercalate/de-intercalate two sodium per f.u., reversibly. The ex situ XRD patterns obtained for this reduced phase suggest a large atomic rearrangement with the formation of a new structure with the composition Na4Mn3O7, without amorphization and/or increase of disordered and stacking faults. The mechanism of insertion as well as the structures of the asprepared and reduced phase will be discussed in this presentation.