Understanding Capacity Fading of MgH₂ Conversion-Type Anodes via Structural Morphology Changes and Electrochemical Impedance

Abstract

Previous studies have demonstrated that MgH2 is a promising conversion-type anode toward Li with high capacity (2037 mAh/g) and low discharge/charge overpotential-hysteresis. A major challenge is the capacity loss after few cycles. To improve the understanding of the complex conversion mechanism at the electrode/electrolyte interface and of its possible evolution, a systematic investigation of the morphology-property relation is undertaken. A multitude of MgH2 materials are obtained by mechanical milling using different devices, milling conditions and time intervals. Upon cycling, the performance of the assembled batteries is strongly dependent on the quality of the prepared MgH2 powders. Electrochemical discharge/charge profiles (MgH2 ← → 2LiH + Mg) are discussed according to the changes in microstructure and morphology revealed by powder X-ray diffraction and transmission electron microscopy. For all electrode composites, the loss of the capacity occurs typically during delithiation in agreement with a kinetically limited process namely of LiH "detachement". Electrochemical impedance spectroscopy is meaningfully carried out using the representative tape-casted electrodes in Li/MgH2 cells to monitor the evolution of resistance components; in particular the formation of SEI-like layer as function of particle size, state of charge and cycle number.

MgH2–CoO: a conversion-type composite electrode for LiBH4-based all-solid-state lithium ion batteries

“…TEM analysis indicates that the 24 h milled MgH 2 powders have an average particle size of about 150 nm. 8,9 The starting hand-mixed CoO powders are in the micron range. However, aer Spex-milling, Fig.…”

“…4,6 However, the reversible capacity is still limited owing to the poor conductivity of the involved entities (MgH 2 /LiH), along with transient aging issues of LiH in carbonate-based liquid electrolytes. [7][8][9] Using amorphous 80Li 2 S-20P 2 S 5 as a solid electrolyte (SE), Ikeda et al 10 reported $31% reversible capacity for MgH 2 for the 1 st cycle at room temperature, compared to $75% for liquid electrolytes. 6 Possible interaction at the electrode/SE (80Li 2 S-20P 2 S 5 ) interface is pointed out as a cause for the low reversible capacity.…”

Exploits, advances and challenges benefiting beyond Li-ion battery technologies

“…To improve the energy density, other reactions are foreseen like alloying with highly capacitive elements (Si, Sn) or conversion reactions [271]. For the latter, metallic hydrides have shown over the past decade that they offer an interesting alternative to other materials [272][273][274][275][276]. This was first demonstrated by Oumellal et al [277] that established the following conversion reaction:…”

Metal (boro-) hydrides for high energy density storage and relevant emerging technologies

“…Metal hydrides have been considered as potential anode materials for Li-ion batteries due to their large capacities and low average potential value versus Li + /Li 0 . Since the discovery of the electrochemical activity of metal hydrides towards lithium, MgH2 (theoretical capacity ~2000 mAh g -1 ) remains the most studied in this category of anodes for LIBs using a carbonate-based liquid electrolyte [17,18,[342][343][344][345][346][347][348][349].…”

“…NiMH [15,16]). Owing to their light-weight, low voltage and small hysteresis compared to oxide anodes, metal hydrides are now intensively studied for their applications as high energy density electrodes for LIBs with carbonate-based liquid electrolytes [17][18][19]. In addition, complex metal hydrides are studied for their application as solid-state electrolytes in next-generation all-solid-state batteries [20][21][22][23].…”