Destabilized Passivation Layer on Magnesium-Based Intermetallics as Potential Anode Active Materials for Magnesium Ion Batteries

ORIGINAL RESEARCH published: 23 January 2019 doi: 10.3389/fchem.2019.00007 Destabilized Passivation Layer on Magnesium-Based Intermetallics as Potential Anode Active Materials for Magnesium Ion Batteries Masaki Matsui 1*, Hiroko Kuwata 2 , Daisuke Mori 2 , Nobuyuki Imanishi 2 and Minoru Mizuhata 1 1 Department of Chemical Science and Engineering, Kobe University, Kobe, Japan, 2 Department of Chemistry for Materials, Mie University, Tsu, Japan Edited by: Sai Gautam Gopalakrishnan, Princeton University, United States Reviewed by: Yushi He, Shanghai Jiao Tong University, China Bora Karasulu, University of Cambridge, United Kingdom *Correspondence: Masaki Matsui Specialty section: This article was submitted to Inorganic Chemistry, a section of the journal Frontiers in Chemistry Received: 17 October 2018 Accepted: 07 January 2019 Published: 23 January 2019 Citation: Matsui M, Kuwata H, Mori D, Imanishi N and Mizuhata M (2019) Destabilized Passivation Layer on Magnesium-Based Intermetallics as Potential Anode Active Materials for Magnesium Ion Batteries. Front. Chem. 7:7. doi: 10.3389/fchem.2019.00007 Frontiers in Chemistry | www.frontiersin.org Passivation of magnesium metal anode is one of the critical challenges for the development of magnesium batteries. Here we investigated the passivation process of an intermetallic anode: Mg3 Bi2 synthesized by solid-state and thin film process. The Mg3 Bi2 composite electrode shows excellent reversibility in magnesium bis(trifluoromethansulfonylamide) dissolved in acetonitrile, while Mg3 Sb2 , which has same crystal structure and similar chemical properties, is electrochemically inactive. We also fabricated the Mg3 Bi2 thin film electrodes, which show reversibility with low overpotential not only in the acetonitrile solution but also glyme-based solutions. Surface layer corresponding to the decomposed TFSA anion is slightly suppressed in the case of the Mg3 Bi2 thin film electrode, compared with Mg metal. Comparative study of hydrolysis process of the Mg3 Bi2 and the Mg3 Sb2 suggests that the both intermetallic anodes are not completely passivated. The bond valence sum mapping of the Mg3 Bi2 indicates that the fast Mg2+ diffusion pathway between 2d tetrahedral sites is formed. The electrochemical properties of the Mg3 Bi2 anode is mainly due to the less passivation surface with the fast Mg2+ diffusion pathways. Keywords: intermetallics, anode, passivation layer, hydrolysis, bond-valence sum mapping, Mg2+ diffusion pathway INTRODUCTION Beyond Li-ion batteries have been widely investigated last decade (Larcher and Tarascon, 2015). Alternative anode active material using electrochemical deposition-dissolution process of less-noble metal is one strategy for the development of high-energy battery system (Lin et al., 2017). Among various choices of the less-noble metal anodes, magnesium is one of the potential candidates as the high-energy anode active material, because the volumetric capacity: 3,800 mAh cm−3 is 1.9 times higher than that of lithium metal, and no dendritic growth during the deposition process (Matsui, 2011). On the other hand, the surface of the magnesium metal is easily passivated in conventional electrolyte solutions such as magnesium perchlorate dissolved in propylene carbonate (Lu et al., 1999). Therefore, the choice of the electrolyte solutions for rechargeable magnesium batteries is very limited. Typically the organohaloaluminate electrolyte solutions, which are widely studied by Aurbach et al. in early 2000s (e.g., Aurbach et al., 2000, 2003, 2007), 1 January 2019 | Volume 7 | Article 7 Matsui et al. Magnesium-Based Intermetallic Anode Active Materials films were characterized using X-ray photoelectron spectroscopy (XPS). In order to evaluate the stability of the surface layer on the intermetallic anodes, the hydrolysis process of the Mg3 Bi2 and the Mg3 Sb2 were investigated. The Mg2+ diffusion pathways in the Mg3 Bi2 and the Mg3 Sb2 were estimated by bond-valence sum (BVS) mapping (Adams, 2006) to discuss the reaction kinetics of these two intermetallic anodes. show highly reversible deposition/dissolution of magnesium metal, however since these electrolyte solutions contain halides: Cl− or Br− , the potential window of the organohaloaluminate electrolyte solutions is limited. In addition the corrosive properties of the halides initiate the dissolution of the cathode current collector at high cell voltage >2.5 V (Muldoon et al., 2012). Therefore, finding halide-free electrolyte solutions has been standing in the center of the electrolyte development for high-voltage magnesium batteries. Recently, Tutusaus et al. reported a new class of the magnesium salt having boron cluster anion monocarborane CB11 H12− dissolved in glymes showed reversible deposition/dissolution of magnesium without corrosive properties (Tutusaus et al., 2015). A fluorinated alkokyborate-based electrolyte: magnesium hexafluoroisopropylaluminate (Mg[B(hfip)4 ]2 ) also shows excellent reversibility without the corrosive properties (ZhaoKarger et al., 2017). Even with the excellent electrochemical properties of these new class of the electrolytes, the oxidation stabilities of these solutions are still limited up to 3.5 V vs. Mg due to the ether-based solvents. Our group has been working on intermetallic anodes such as Mg3 Bi2 and Mg2 Sn, because the intermetallic anodes are compatible with a conventional electrolyte solution: magnesium bis(trifluoromethansulfonylamide) (Mg(TFSA)2 ) dissolved in acetonitrile(AN) (Arthur et al., 2012; Singh et al., 2013). Despite the high equilibrium potential and the low specific capacity of the Mg3 Bi2 , the compatibility against wide variety of the electrolyte solutions could become an advantage for the development of a battery system. Moreover, since the bismuth is very heavy element, the volumetric capacity of the Mg3 Bi2 : 1906 mAh cm−3 is still comparable value to the theoretical capacity of lithium metal. Therefore, we think the “Mg-ion” battery using the intermetallic anode could be a more realistic direction to develop a practical battery system. Even with the several reports concerning the Mg3 Bi2 as the anode active material for Mg-ion batteries, there still remains room to understand why the Mg3 Bi2 shows the compatibility with various electrolytes solutions. Also the fast reaction kinetics of the Mg3 Bi2 among various Mg-based intermetallic anodes, are still under investigation. In the present study, we attempted to understand the origin of the wide compatibility and the fast kinetics of the Mg3 Bi2 as the alternate anode active material for magnesium batteries. We investigated the electrochemical properties of the Mg3 Bi2 composite electrodes, synthesized by a conventional solid-state process. In order to conduct comparative studies, another intermetallic anode: Mg3 Sb2 was also investigated, because of its structural and chemical similarities to the Mg3 Bi2 . The Mg3 Sb2 has same crystal (...truncated)