An electrochemical study of Fe1.18Sb1.82 as negative electrode for sodium ion batteries

The mechanism of the electrochemical reaction between FeSb2 and sodium in sodium half cells has been very recently reported [1,L.Baggetto, H.-Y. Hah, C. E. Johnson, C. A. Bridges, J. A. Johnson, G. M. Veith, Phys.Chem.Chem.Phys. 16 (2014) 9538]. Its electrochemical activity, initially limited, seems based on an incomplete desodiation of Na3Sb formed during the first discharge and the occurrence of a “Fe4Sb” alloy inactive in the cell. However, no more than two charge/discharge cycles were shown. With this work we shed light on the sodium ion battery electrode properties of another solid in the Fe-Sb system (Fe1.18Sb1.82). Capacity retention properties in two different electrolyte configurations containing NaClO4 as sodium salt and by setting two cycling voltage limits are shown. A discussion about the impact of an additive such as fluoroethylene carbonate (FEC) in the electrode performance is assisted by applying electrochemical impedance spectroscopy on the electrode/electrolyte interfaces. When the additive is not present in the electrolyte, the occurrence of a surface film onto the electrode particles due to the electrolyte decomposition is noticed at low voltage values (0.3 V vs. Na/Na+). Further discharge leads to the growth of this layer and conversely to decrease in the charge transfer resistance as several well dispersed metallic products are present in the discharged electrode. Upon charging, the film is firstly decomposed and/or dissolved and the charge transfer resistance increases. Beyond 1.05 V vs. Na/Na+, a second film, of a different nature from the first one appears onto fresh antimony or FexSby particles formed from desodiation of poorly crystallized Na3Sb. When FEC is added to the electrolyte, the interface is influenced at each stage of the discharge or the charge. FEC suppresses the growth of the surface film at low voltages and decreases the charge transfer resistance at any stage. Upon charging beyond 1.05 V vs. Na/Na+, the surface films are less resistive than in the absence of FEC. Therefore, the additive has a constructive effect on the cell capacity retention upon cycling. Finally setting the lowest limit voltage at 0.2 V vs. Na/Na+ does not result in an improvement in the capacity retention upon cycling, but halves the capacity values compared to the ones obtained at a 0 V vs. Na/Na+ limit voltage.