Nax(Fe,Mn)O2 Layered Oxides Used in Sodium Batteries : Structural Transformations Induced By the Electrochemical Process

Layered sodium oxides with the formula Na_xMO₂ (where x is comprised between 0 and 1, M is a transition metal) have been intensively studied these last thirty years either for their unique physical properties (high thermoelectric power, superconductivity …) or with a view for their use in large sodium-ion batteries. More recently, advanced electrochemical properties of some Na_xMO₂ systems containing only one transition metal have been re-investigated and new complex materials with several transition metals in the MO₂ layers have been synthesized and studied. In particular, systems containing non-toxic and abundant elements such as manganese and iron appear very attractive and first results in the P2-NaxMn_1/2Fe_1/2O₂ system have showed a capacity as high as 190 mAh/g [1]. Other promising results were obtained in other compositions in manganese- and iron-based systems [2,3].

In this context, we recently studied some Na_xMO₂ phases used as positive electrode in Na cells with a special focus on the structural transformations and redox processes occurring during cycling in P2- and O3-Na_x(Fe,Mn)O₂ systems. The structures of all starting materials were determined by Rietveld refinement from powder X-Ray diffraction data. The electrochemical study was carried out in sodium batteries with a solution of NaPF6 in PC as electrolyte. For all materials, a very good reversibility of the electrochemical process was observed. In situ and ex situ X-ray powder diffraction experiments were carried out to understand better the structural transformations induced by the electrochemical (de)intercalation process. In parallel, redox processes were studied by ex situ and in situ Mössbauer spectroscopy.

[1] N. Yabuuchi, M. Kajiyama, J. Iwatate, H. Nishikawa, S. Hitomi, R. Okuyama, R. Usui, Y. Yamada, S. Komaba Nature Materials, 2012, 11, 512.

[2] J. Zhao, J. Xu, D. H. Lee, N. Dimov, Y. S. Meng, S. Okada J. Power Sources, 2014, 264, 235.

[3] D. Yuan, X. Hu, J. Qian, F. Pei, F. Wu, R. Mao, X. Ai, H. Yang, Y. Cao Electrochimica Acta, 2014, 116, 300.