Na-ion batteries have gained a remarkable interest during the last decade due to their attractive features of reduced costs and possible use of more environmentally friendly materials. These characteristics could promote the development of this type of electrochemical storage technology as a possible alternative to Li-ion cells for a cost-effective large-scale stationary storage of electricity. Yet, the advancement of suitable Na-ion batteries for such a purpose has to face more severe challenges than those occurred during the initial development of Li-ion battery technology. In fact, the noticeable differences in mass and size between Li⁺ and Na⁺ ions have considerable effects on the associated electrochemical performances for the corresponding battery systems. These factors heavily affect also the choice of convenient materials for Na-ion cells, especially for negative electrodes. In fact, graphite cannot be employed as insertion host [1], although possible Na⁺ incorporation via solvent co-intercalation phenomena [2] has been reported.

The most suitable materials for negative electrodes in Na-ion batteries so far are disordered carbons [3], since they can offer a convenient trade-off in terms of specific capacities, rate capabilities and cycle performances. Nevertheless, such specifications could be advanced and definitely there is room for further improvements. In this regard, more abundant and cheaper materials, which can be synthesized in simpler ways at lower temperatures, have also been proposed. Among these, iron oxide is certainly one of the most attractive because of its earth-abundance and environmental friendliness. For these reasons, this compound has been investigated in a number of preliminary studies [4-6] in Na-half cells.

The reaction pathway of iron oxide upon its initial phases of Na⁺ uptake resembles to a good extent that occurring upon similar Li⁺ incorporation. However, noticeable differences characterize the respective electrochemical mechanisms, being the subsequent conversion reaction of iron oxide into Fe and Na₂O clearly hampered [4-6] compared to its analogue mechanism upon lithiation, in which the extensive formation of Fe nanoparticles surrounded by a Li₂O matrix is involved [7].

The objectives of this study are to identify the limiting factors occurring during the conversion reaction of iron oxide upon sodiation and to assess its feasibility for possible enhancement of Na storage. These aims are pursued by comparing its features to those displayed by its analogous mechanism driven by Li⁺. A systematic analysis via cyclic voltammetry (CV) has been conducted on different types of iron oxide nanostructures (e.g. nanopowders, nanowires and thin films) cycled in parallel in Li- and Na-half cells containing analogous electrolytes (i.e. LiClO₄ and NaClO₄ respectively). In this approach, the characteristic behaviours of these particular electrochemical systems have been compared. The roles played by their intrinsic structures, textures, as well as by the different electrical wiring throughout the associated electrodes embedding these active materials, will be presented and highlighted. The limiting factors influencing the conversion reactions of these iron oxide nanostructures upon their respective lithiation and sodiation will be discussed and the issues arising from such limitations will be identified and addressed.

It will be shown that the porosity of the active materials and their mutual electrical contact, as well as that with the current collector, play a crucial role in the electrochemical processes. Finally, Na storage in iron oxide via conversion reactions is demonstrated to be intrinsically limited, irrespectively of the type of nanostructure employed for this specific purpose, thus posing restrictions on the ultimate usage of this type of material in combination with Na⁺ ions.

References

[1] P. Ge, M. Fouletier, Sol. St. Ionics 28-30 (1988) 1172.

[2] B. Jache, P. Adelhelm, Angew. Chem. Int. Ed. 53 (2014) 10169.

[3] M. Dahbi, N. Yabuuchi, K. Kubota, K. Tokiwa, S. Komaba, Phys. Chem. Chem. Phys. 16 (2014) 15007.

[4] M. Valvo, F. Lindgren, U. Lafont, F. Björefors, K. Edström, J. Power Sources 245 (2014) 967.

[5] B. Philippe, M. Valvo, F. Lindgren, H. Rensmo, K. Edström, Chem. Mater. 26 (2014) 5028.

[6] B. Huang, K. Tai, M. Zhang, Y. Xiao, S.J. Dillon, Electroch. Acta 118 (2014) 143.

[7] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496.