Elsevier

Solid State Ionics

Volume 361, March 2021, 115566
Solid State Ionics

Synthesis, crystal structure and ionic conductivity of MgAl2X8 (X = Cl, Br)

https://doi.org/10.1016/j.ssi.2021.115566Get rights and content

Highlights

  • MgAl2Cl8-yBry was also synthesized at the ratio y = 0–8.

  • MgAl2Cl8-yBry had the same crystal structure as MgAl2Cl8 and the arrangement of Cl and Br in the crystal had no order.

  • MgAl2Cl8-yBry are the Mg2+ ionic conductors and MgAl2Cl2Br6 shows the highest conductivity of 1.3 × 10−6 S/cm at 400 K.

  • Temperature dependence of 27Al NMR spectra are obtained and the fast ionic motion was not detected.

Abstract

MgAl2X8 (X = Cl, Br) was synthesized from MgX2 and AlX3, and the physical properties were evaluated by XRD, AC conductivity, and NMR. MgAl2Br8 has the same crystal structure as MgAl2Cl8 and belongs to the space group C2/c. MgAl2Cl8-yBry was also synthesized in the same manner as MgAl2Br8. MgAl2Cl8-yBry was found to have the same crystal structure as MgAl2Cl8 and forms a solid solution in all the synthesized compositions. From the Rietveld analysis of the XRD pattern, it was found that the arrangement of Cl and Br in the crystal had no order and occupied the halogen sites almost randomly. The AC conductivity of the obtained compound was lowest for MgAl2Cl8 and highest for MgAl2Cl2Br6. The conductivity of MgAl2Cl2Br6 at 400 K is 1.3 × 10−6 S/cm, and the Nyquist plot suggests that MgAl2Cl8-yBry is a magnesium ionic conductor. In the temperature range from room temperature to 412 K, there was no significant change in the 27Al NMR spectra of a MgAl2Br8 single crystal, and the effects of Mg2+ ionic conduction and the reorientation motion of AlX4ˉ anion were not observed.

Graphical abstract

MgAl2Cl2Br6 shows the highest Mg ion conductivity among MgAl2Cl8-yBry(y = 0–8), and its value is 1.3 × 10−6 S/cm at 400 K.

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Introduction

Among commercially available secondary batteries lithium-ion secondary batteries have special features such as high energy density and high cycle characteristics, and are used as batteries for portable devices and electric vehicles [1,2]. Because the size and demand of batteries are expected to increase in the future, and the safety and supply of lithium-based batteries are uncertain, alternative secondary batteries have been actively researched. Magnesium ions are the charge carriers in Mg secondary batteries. Magnesium is more stable than alkali metals elements, is abundant in nature, and is expected to have a high capacity because of the two-electron reaction. Therefore, the Mg secondary battery is expected as one of the post-lithium ion secondary batteries [3,4].

The Grignard reagent and acetonitrile have been reported as organic electrolytes for Mg secondary batteries, and Mg deposition and dissolution reaction on the working electrode have been observed using magnesium metal as counter electrode [3,[5], [6], [7], [8]]. Some oxides and sulfide-based glasses have been reported for magnesium ion conductive solid electrolytes. It has been reported that solid oxide electrolytes have low ionic conductivity, and that MgHf(WO4)3 and (Mg0.1Hf0.9)4/3.8Nb(PO4)3 have conductivity of 2.5 × 10−4 S/cm at 873 K and 2.1 × 10−6 S/cm at 573 K, respectively [9,10]. Sulfide-based glass containing Mg2+ ion synthesized in a similar manner as lithium-ion conductive glass exhibits ionic conductivity as the magnesium ion conductor [11]. Some synthesized Mg(BH4)-based compounds also exhibit relatively high ionic conductivity (1.3 × 10−5 S/cm at 303 K) [12,13]. Since it is essential to improve safety when considering the increase in size of secondary batteries, it is expected that the development of all-solid-state batteries for Mg secondary batteries will be required as same as current lithium ion secondary batteries. There are reports of MoS2 and MgMn2O4 as positive electrode active materials [14,15], and many new related compounds are expected to be synthesized in the future. The suitable combination of active material and electrolyte is also necessary to sufficiently exhibit the battery characteristics, and the development of various kinds of solid electrolytes is very important for practical battery use.

Halo complexes of group 13 elements such as MAlX4 and MGaX4(M = Li, Na, Cu, Ag; X = Cl, Br) have been reported as M+ ion conductors, and the compound with high conductivity show values of 10−5 S/cm at room temperature [16,17]. The crystal system of LiAlX4 is a monoclinic, and its space group is P21/c [18]. In the crystal, Al3+ ion is tetracoordinated to Xˉ ions to form AlX4ˉ anions, and the Li+ ion occupies the octahedral site and is hexacoordinated to Xˉ ions. An identical crystal structure is obtained when M is Na or Ag. When M is Cu, the Cu+ ion is tetracoordinated to Xˉ ions and the crystal system is tetragonal, although the anion structure is the same. In these compounds, the self-diffusion rate of M+ ions increases with the reorientation motion of AlX4ˉ anions, and the improvement of M+ ion conductivity is observed [16,19]. The electrolyte salt of the above-mentioned acetonitrile solution, MgAl2Br8, is the double salt of MgBr2 and the halide of group 13 elements, AlBr3, similar to MAlX4. So far, only MgAl2Cl8 has been reported as the crystal structure of the compound of MgX2 and AlX3, and its crystal structure is mainly similar to LiAlX4 and NaAlX4 [20]. Compared to Li+ and Na+ ions, Mg2+ ion has a larger charge and become difficult to diffuse, but the cation conduction is expected to be observable, similar to MAlX4. In this study, MgAl2Cl8-yBry was synthesized, the crystal structure was analyzed, and the Mg2+ ion conductivity was evaluated in order to investigate the potential of the Mg secondary battery as an electrolyte. We also investigated the correlation between changes in Mg2+ ion conductivity and changes in crystal lattice due to halogen substitution.

Section snippets

Experimental

MgAl2Cl8-yBry was synthesized using MgCl2, AlCl3, MgBr2, AlBr3 as starting materials and melting their mixture in a furnace. MgCl2 and MgBr2 were used after drying in vacuum at 473 and 573 K for 24–48 h. AlCl3 was purified by sublimation and AlBr3 was purified by the Bridgman method. Those purified raw materials were mixed in a stoichiometric ratio using a mortar and then sealed in a Pyrex glass test tube. The sealed mixed samples were melted in an electric furnace at 573 K for 24 h and

XRD patterns and Rietveld analysis

Fig. 1 shows the XRD patterns of the synthesized MgAl2Cl8-yBry. The XRD pattern of the obtained MgAl2Cl8 is in good agreement with that of the previous report [20]. Although a slight peak of the raw material MgCl2 was observed, no other starting material peaks were observed, indicating that the target compound was obtained. The crystal system was monoclinic and the space group was C2/c (No. 15). Compared with MgAl2Cl8, the peaks in the XRD pattern of MgAl2Br8 shifted to a lower angle, and there

Conclusions

MgAl2Cl8-yBry was synthesized using MgX2 and AlX3. In XRD measurement, as the amount of Br increases, the diffraction peak shifts to the lower angle side, indicating that the crystal lattice becomes larger. From Rietveld analysis of the XRD patterns of the synthesized compounds, it was clarified that MgAl2Cl8-yBry has the same crystal structure and its space group is C2/c. It was clarified that four kinds of halogen sites in the MgAl2Cl8-yBry crystal were occupied by both Brˉ ion and Clˉ ion,

Declaration of Competing Interest

The authors declare no conflict of interest, financial or otherwise.

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