Decomposition of LiBH4-MgH2-Al Bjarne R. S. Hansena, Dorthe B. Ravnsbæka, Carsten Gundlachb & Torben R. Jensena

aCenter for Materials Crystallography and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark

b MAX-II laboratory, Lund University, Ole Römers väg 1, 223 63, S-22100 Lund, Sweden

Abstract LiBH4 is an interesting hydrogen storage material, as it has a high gravimetric hydrogen content of 18.5 wt% [1]. However the utilization is hampered by lack of reversibility and high decomposition temperatures. One way of improving metalborohydrides, is the utilization of reactive hydride composites (RHC), which can be achieved by adding for instance Al [2] or MgH2 [3]. The ternary LiBH4-MgH2-Al composite further improves the hydrogen release and uptake properties of LiBH4 [4]. In this study the decomposition reactions of LiBH4-MgH2-Al in molar ratios (4:1:1) and (4:1:5) are investigated using in situ Synchrotron Radiation Powder X-ray Diffraction (SR-PXD) and thermal analysis (TGA/DSC) coupled with mass spectroscopy (MS).

Acknowledgements and references Sincere acknowledgements should be directed to iNANO, Danscatt , MAX-Lab

and Center for Materials Crystallography (CMC)

During desorption the transformation from o-LiBH4 to h-LiBH4 is observed at T = 100 °C. At T = 265 °C diffraction from h-LiBH4 disappear. MgH2 decompose at T = 290 °C after which diffraction from the intermediate compounds Mg17Al12 and Mg0.9Al0.1 are observed. These compounds were not expected from the proposed decomposition reactions [4]. MgAlB4 is formed at T ≈ 390 °C and LiAl is observed at T = 460 °C.

The transformation from o-LiBH4 to h-LiBH4 and the disappearance of h-LiBH4 is observed at T = 100 °C and T = 250 °C respectively. MgH2 decompose at T = 290 °C, but unlike the LiBH4-MgH2-Al (4:1:1) sample, Mg0.9Al0.1 and Mg17Al12 are not observed. An unknown intermediate (denoted 1) is however observed, that could be similar to intermediates observed in LiBH4-Al systems [2]. The formation of MgAlB4 and of LiAl are observed at T = 330 °C and 410 °C respectively. Another unknown compound, denoted 2 is observed at T = 490 °C. Both samples were heated from RT 500 °C (10 °C/min) and kept at 500 °C for 10 min prior to cooling.

Mechanochemical Synthesis: Fritsch Pulverisette no. 4 with main disk speed: 400 rpm., Relative ratio: -2.25, Mill time: 5 min, Pause time: 2 min, Ball-to-Powder mass ratio: 35:1, Repetitions: 24 (Total mill time: 120 min).

[1] L. Schlapbach, Nature 2009, 460, 809-811; [2] D. B. Ravnsbæk, T. R. Jensen, J. Appl. Phys. 2012, 111, 112621 ; [3] U. Bösenberg, S. Doppiu, L. Mosegaard, G. Barkhordarian, N. Eigen, A. Borgschulte, T. R. Jensen, Y. Cerenius, O. Gutfleisch, T. Klassen, M. Dornheim, Acta Materialia 2007, 55, 3951–3958; [4] Y. Zhang, Q. Tian,

H. Chu, J. Zhang, L. Sun, J. Sun, Z. Wen, J. Phys. Chem. C 2009, 113, 21964–21969

λ = 1.103671 Å λ = 1.102050 Å

TGA/DSC-MS analysis complement the pathway observed from in situ data. The phase transformation and melting of LiBH4 is observed at 107 °C and 110 °C and 265 °C and 267 °C (peak temp) for the samples. At T = 300 °C the decomposition of MgH2 is observed in sample LiBH4-MgH2-Al (4:1:1), but not in the (4:1:5)-sample, although a release of hydrogen is observed in the MS data for both samples. The formation of MgAlB4 is observed as a broad endothermic event and the formation of LiAl is revealed at T = 410 °C. No diborane is released during the decomposition of either sample (not shown).

Sieverts measurements for LiBH4-MgH2-Al (4:1:5) reveal capacity loss during hydrogen release and uptake cycles. PXD show the anticipated decomposition product of LiAl and MgAlB4, and LiBH4, Al and MgH2 are formed during hydrogen absorption, which indicate a degree of reversibility in the system. However, stable amorphous compounds such as Li2B12H12 might form during hydrogen cycling, which is the case for LiBH4-Al systems. 11B MAS-NMR measurements will hopefully help determine this in the near future.

LiBH4-MgH2-Al (4:1:5)

λ = 1.54056 Å