Reactivity of complex hydrides Mg2FeH6, Mg2CoH5 and Mg2NiH4 with lithium ion: Far from equilibrium electrochemically driven conversion reactions

The electrochemical reaction of lithium ion with Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄ complex hydrides prepared by reactive grinding is studied here. Plateaus at an average potential of 0.25 V, 0.24 V and 0.27 V corresponding to discharge capacities of 6.6, 5.5 and 3.6 Li can be achieved respectively for Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄. From in situ X-ray diffraction (XRD) characterizations of complex hydride based electrodes, dehydrogenation leads to a decrease of the intensities of the diffraction peaks suggesting a strong loss of crystallinity since formation of Mg and M (M = Fe, Co, Ni) peaks is not observed. ⁵⁷Fe Mössbauer spectroscopy confirms the formation of nanoscale Fe or an amorphous Mg–Fe alloy during the decomposition of Mg₂FeH_6. Interestingly, lattice parameter variations suggest phase transitions in the Mg₂NiH₄ system involving the formation of low hydrogen content hydride Mg₂NiH, while an increase of lattice parameters of Mg₂CoH₅ hydride could be attributed to the formation of a Mg₂CoH₅Li_x solid solution compound up to x = 1.