Reactivity of Sodium Alanates in Lithium Batteries

Abstract

Novel chemistries for secondary batteries are investigated worldwide in order to boost the development of next-generation rechargeable storage systems and especially of lithium-devices. High capacity anode materials for Li-ion cells are at the center stage of R&D in order to improve the performances. In this view, conversion materials are an exciting playground. Among the various proposed class of conversion anodes, metal hydrides are probably the most challenging and promising due to the high theoretical capacities, instability toward the standard carbonate-based electrolytes, large volume variations upon cycling, and moderately low working voltages. Among them lightweight hydrides, like alkaline alanates, are an almost unexplored family of materials. In this study, we present a fundamental study on the electrochemical conversion reaction of sodium alanates: NaAlH4, Na3AlH6, and Na2LiAlH6. Our goal is to improve the understanding of the basic solid-state electrochemistry that drives the conversion reactions of these materials in lithium cells. Samples have been prepared mechanochemically and characterized by X-ray diffraction (XRD), infrared spectroscopy, and transmission electron microscopy. All materials have been assembled in lithium cells with a commercial liquid electrolyte to test their electrochemical activity. The Li incorporation/deincorporation mechanism for all materials has been explored by in situ XRD and interpreted also in view of density functional theory thermodynamic calculations.

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

“…Ab initio calculations based on density functional theory (DFT) were performed to get structural and energy predictions at the atomistic level for all of the metal hydrides and metallic elements and the possible envisaged reactions in which they could be involved. The technical details of the used computational methods were discussed by us in [22][23][24][25][26]. The experimental capability or inability was then correlated to a robust physical-chemical characterization of the sample under investigation, i.e., to its purity, crystallinity and morphology, in order to evaluate a first possible optimization strategy.…”

“…While the first ones were commercially available, the latter were synthesized by mechanochemistry [22,26]. Furthermore, in this case, DFT was used to preliminarily verify the thermodynamic feasibility of alanates' conversion [22,26].…”

“…Milling produced the expected particle reduction and compositing with carbon; nevertheless, especially the milling step with carbon brought some hydrogen desorption. Optimizing milling time allowed reaching a recharge efficiency equal to 39% in the case of LiAlH 4 and more than 70% for NaAlH 4 , but cyclability remained unsatisfactory for both alanates [22,26].…”

“…Hexahydrides were successfully synthesized by ball milling proper amounts of tetrahydrides, LiH and/or NaH [22,26]. TEM observation revealed large micrometric round-shaped particles with smooth surfaces and low phase contrast on the particle edges, which from an XRD Rietveld analysis are composed by 30-40-nm average size of crystallites.…”

“…Electrodes were prepared and tested in lithium cells, obtaining interesting discharge profiles. In the case of LiNa 2 AlH 6 , the full theoretical capacity, i.e., 1.87 Ah·g −1 , was delivered upon discharge with nearly 40% recharge efficiency [22].…”