XAS investigations on nanocrystalline Mg2FeH6 used as a negative electrode of Li-ion batteries

In the frame of research on new metallic hydrides as conversion reaction materials for negative electrodes of Li-ion batteries, the MgFe2H6 complex hydride has been investigated in and ex situ using XRD, XAS and magnetic measurements at different states: initially ball milled complex hydride (CH), electrochemically desorbed (ED) and thermally desorbed (TD). Fe-H bonding is clearly evidenced by EXAFS in the CH sample. It is also observed that the electrochemical reaction leads to a nanocrystalline state that needs local probe analyses to be fully investigated and interpreted. From the XAS and magnetic data, the different routes (ED and TD) for dehydrogenation of the complex hydride are compared. For both electrochemically and thermally driven reactions, the hydrogen depletion from Mg2FeH6 hydride leads to decomposition into its constituting elements Mg and Fe. However, different Fe structures are observed: bulk alpha-Fe and amorphous Fe nanoparticles for TD and ED samples, respectively.