Magnesium hydride as a high capacity negative electrode for lithium ion batteries

Abstract

Conversion reactions in lithium batteries have been proved for several classes of materials, such as oxides, fluorides, sulphides, nitrides, phosphides and recently for hydrides. Metal hydrides can be electrochemically reduced to a highly conductive composite material consisting of nanometric metallic particles dispersed in an amorphous LiH matrix. Magnesium hydride undergoes a reversible conversion reaction and it has very good theoretical performances, i.e. a theoretical specific capacity of 2038 mA h g -1 and a working potential of 0.5 V vs. Li +/Li. The purpose of our study is to investigate the MgH 2 redox activity by evaluating the effect of ball milling pre-treatments and by studying the conversion reaction mechanism. Three materials, prepared by submitting bulk MgH 2 to different ball milling procedures, are investigated. By coupling electrochemical tests, ex situ X-ray powder diffraction and transmission electron microscopy, we prove that the lithium incorporation does not follow a simple direct conversion path as it follows at least a sequence of four consecutive processes: (a) the hydride conversion reaction of MgH 2 to give Mg and LiH, (b) the alloying of Li in hcp Mg and (c and d) the formation and lithium enrichment of a bcc Li-Mg solid solution. Furthermore some experimental clues suggest that the mechanism is probably even more complex as it can imply the formation of other unknown intermediate Li-Mg-H phases. Moreover large morphological changes occur upon lithium incorporation in the electrodes: in particular an extended sintering of the metal nanoparticles occurs upon cycling. This effect leads to electrode pulverization and capacity fading. On the other hand MgH 2 shows a very limited potential hysteresis between discharge and charge and very promising kinetics at high current. © 2012 The Royal Society of Chemistry

Morphology effects in MgH2 anode for lithium ion batteries

“…MgH 2 as anode reacts with 2Li + in a conversion reaction, leading to the formation of Mg and precipitation of 2LiH with a high theoretical capacity of 2037 mAh.g -1 (compared to graphite-anode 372 mAh.g -1 ) and the lowest charge-discharge polarization of any tested conversion-type material [2,8]. These excellent properties indicate that hydride materials could be suitable for applications in the future batteries, such as electrical vehicles.…”

“…The cycling performances are similar to the one obtained in ref. [8], but as the samples are treated in different ways that makes the comparison difficult. However, only 38% of this capacity is reversible during the first cycle.…”

“…Thus, the electrode formulation, including shape, morphology and additives, has a large influence on the cycling performance [8,[11][12][13]. However no systematic study has been reported yet regarding the effect of the morphology and cycling conditions of the active material.…”

“…These excellent properties indicate that hydride materials could be suitable for applications in the future batteries, such as electrical vehicles. However, a major challenge is the capacity loss after few cycles [8]. It has been reported a beneficial effect of mechanochemical reaction of MgH 2 with Li metal leading to a composite of Mg embedded in a LiH matrix.…”

Hydrides as High Capacity Anodes in Lithium Cells: An Italian “Futuro in Ricerca di Base FIRB-2010” Project

“…In 2012, on the basis of hypothetical conversion reactions involving the most popular hydride materials, we proposed some very encouraging preliminary predictions [17].…”