Citation: Jin, O.; Shang, Y.; Huang, X.; Mu, X.; Szabó, D.V.; Le,T.T.; Wagner, S.; Kübel, C.; Pistidda, C.; Pundt, A. Microstructural Study of MgB 2 in the LiBH 4 -MgH 2 Composite by Using TEM. Nanomaterials 2022, 12, 1893. https://doi.org/10.3390/ nano12111893 Academic Editor: Cheol-Min Park Received: 28 April 2022 Accepted: 30 May 2022 Published: 31 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Article Microstructural Study of MgB 2 in the LiBH 4 -MgH 2 Composite by Using TEM Ou Jin 1,2 , Yuanyuan Shang 3 , Xiaohui Huang 2 , Xiaoke Mu 2 , Dorothée Vinga Szabó 1,2,4 , Thi Thu Le 3 , Stefan Wagner 1 , Christian Kübel 2,4,5 , Claudio Pistidda 3 and Astrid Pundt 1, * 1 Institute of Applied Materials, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany; ou.jin@kit.edu (O.J.); dorothee.szabo@kit.edu (D.V.S.); stefan.wagner3@kit.edu (S.W.) 2 Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany; xiaohui.huang@partner.kit.edu (X.H.); xiaoke.mu@kit.edu (X.M.); christian.kuebel@kit.edu (C.K.) 3 Institute of Hydrogen Technology, Helmholtz-Zentrum Hereon GmbH, 21502 Geesthacht, Germany; yuanyuan.shang@hzg.de (Y.S.); thi.le@hzg.de (T.T.L.);claudio.pistidda@hzg.de (C.P.) 4 Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany 5 Joint Research Laboratory Nanomaterials, Technical University of Darmstadt, 64206 Darmstadt, Germany * Correspondence: astrid.pundt@kit.edu; Tel.: +49-721-608-42345 Abstract: The hampered kinetics of reactive hydride composites (RHCs) in hydrogen storage and release, which limits their use for extensive applications in hydrogen storage S1and energy conversion, can be improved using additives. However, the mechanism of the kinetic restriction and the additive effect on promoting the kinetics have remained unclear. These uncertainties are addressed by utilizing versatile transmission electron microscopy (TEM) on the LiBH 4 -MgH 2 composite under the influence of the 3TiCl 3 ·AlCl 3 additives. The formation of the MgB 2 phase, as the rate-limiting step, is emphatically studied. According to the observations, the heterogeneous nucleation of MgB 2 relies on different nucleation centers (Mg or TiB 2 and AlB 2 ). The varied nucleation and growth of MgB 2 are related to the in-plane strain energy density at the interface, resulting from the atomic misfit between MgB 2 and its nucleation centers. This leads to distinct MgB 2 morphologies (bars and platelets) and different performances in the dehydrogenation kinetics of LiBH 4 -MgH 2 . It was found that the formation of numerous MgB 2 platelets is regarded as the origin of the kinetic improvement. Therefore, to promote dehydrogenation kinetics in comparable RHC systems for hydrogen storage, it is suggested to select additives delivering a small atomic misfit. Keywords: hydrogen storage; transmission electron microscopy; crystallography; reactive hydride composite; additive 1. Introduction Hydrogen is a clean and reproducible energy carrier with the highest gravimetric energy density of ~120 kJ g −1 . For extensive applications of hydrogen, advanced hydrogen storage materials are demanded to store hydrogen safely and efficiently. Reactive hydride composites (RHCs) have been studied intensively due to their exceptionally reversible hydrogen storage capacity [1]. These materials were initially derived from light metal complex hydrides (e.g., LiBH 4 , LiNH 2 , NaAlH 4 , etc.) in combination with a second hydride (e.g., LiH, MgH 2 , etc.) [2–4]. Among various RHCs, the LiBH 4 -MgH 2 composite is one of the most competitive candidates for both on- and off-board applications, based on the International Energy Agency Task 22 [5]. According to prior studies, the related decomposition reaction occurs in two steps [6]: 2LiBH 4 + MgH 2 → 2LiBH 4 + Mg + H 2 → 2LiH + MgB 2 +4H 2 (1) Compared with the hydrogen capacity of ~18.5 wt% in pristine LiBH 4 , about 11.4 wt% of hydrogen can still be yielded with the LiBH 4 -MgH 2 composite, while the thermodynamic Nanomaterials 2022, 12, 1893. https://doi.org/10.3390/nano12111893 https://www.mdpi.com/journal/nanomaterials