Comparative study of NiSb2 and FeSb2 as negative electrodes for Li-ion batteries

Crystalline FeSb2 powder prepared by ceramic route is examined as negative electrodes for lithium-ion batteries. The complete reaction mechanism of FeSb2 is investigated by 121Sb and 57Fe Mössbauer spectroscopy as well as magnetic measurements and the results are correlated with a previous in situ XRD characterization. On the first discharge the reaction with Li proceeds through a biphasic process transforming FeSb2 into a new LixFeySb2 phase, and this ternary phase is then converted into fcc Li3Sb and metallic Fe nanoparticles. The combination of Mössbauer spectroscopy and magnetic analyses leads i) to a better understanding of the FeSb2 → ternary phase reaction and concomitantly allowed ii) to specify the stoichiometry of the new ternary phase. On charge, the extrusion of lithium includes the back conversion of the Li3Sb/Fe mixture into both Li4Fe0.5Sb2 and metallic Sb, which are the main active species for the following cycles, responsible for the poor cycling life of the FeSb2 electrode. The nature of these resulting products is quite different from that previously observed for the isotype NiSb2 electrode which is characterized by a highly reversible mechanism.