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 Li_xFe_ySb ₂ phase, and this ternary phase is then converted into fcc Li ₃Sb 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 Li₄Fe_0.5Sb₂ 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 NiSb ₂ electrode which is characterized by a highly reversible mechanism.