P2 Structure Sodium Manganese Oxides for Na-Ion Batteries

Lithium ion batteries have dominated the world of portable electronic devices over the past two decades and are making their way into the electric vehicle market due to their high energy density. However, recently sodium ion batteries have regained the interest of the scientific community due to the high and uniform abundance of sodium across the world and consequent low cost compared to lithium, making them attractive for grid storage. Various electrode materials have been studied for sodium ion batteries. Layered oxides, NaMO₂, where M is one or more transition metals, represent an attractive class of cathodes for Na batteries. Here we show that Na_0.67MnO₂ with the P2 structure, Fig 1, exhibits a high capacity (175 mAhg^-1), close to the best reported for a Na cathode, and with good capacity retention, in contrast to previous studies of Na_0.67MnO₂ where the capacity faded rapidly.¹ Due to the presence of Jahn–Teller active Mn³⁺ in Na_0.67MnO₂, the structure undergoes various structural transitions during sodiation/desodiation leading to many voltage steps in the charge/discharge profile.^1-3 We have investigated reducing the phase transitions by substituting Mn³⁺ ions with electrochemically inactive dopant ions, which have a strong preference for octahedral sites in the layered oxide framework. This leads to the formation of a highly stable framework that shows a smooth charge/discharge profile with very low polarization even for a substitution level as low as 5%, Fig. 2, and a capacity of ~ 175 mAhg^-1. Such an observation is significant for the future development of sodium ion battery cathode materials.

Lattice parameters of all the substituted compositions could be refined using hexagonal (P6₃/mcm) and orthorhombic (Cmcm) space group symmetries, whilst the situation pertaining to Na_0.67MnO₂ samples proved more complex.

References

[1]  A. Caballero, L. Hernan, J. Morales, L. Sanchez, J. Santos Pena and M. A. G. Aranda, J. Mater. Chem., 2002, 12, 1142.

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

[3]  X.F. Wang, M. Tamaru, M. Okubo and A. Yamada, J. Phys. Chem. C, 2013, 117, 15545.