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S C I E N T I F I C H I G H L I G H T S

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Unravelling the complexity of magnesium borohydride- ethylenediamine solid-state electrolytes

By combining synchrotron X-ray and neutron diffraction experiments, the intricate architecture of a solid-state electrolyte made from magnesium borohydride and ethylenediamine has been unravelled. The structural complexity offers valuable insights into enhancing magnesium-ion battery technology.

Fig. 68: Crystal structure of Mg(en)1.2(BH4)2: the isolated binuclear anion [{Mg(κ3-BH4)(κ2-BH4)2}2(μ2-en)]2− is represented on the right and the [Mg3(en)5(BH4)4]2+ cation constituting the coordination polymer chains, [Mg2+(κ2-en)2(μ2,κ1,2:κ2-BH4−)Mg2+(κ2-en)(κ2-BH4−)(μ2-en)Mg2+(κ2-en)(κ2-BH4−)(μ2,κ2:κ1,2-BH4−)], on the left. Colour code: Mg: aquamarine; N: blue; B: olive; C: grey; H: white.

Magnesium-ion batteries (MIBs) are emerging as a promising alternative to traditional lithium-ion batteries due to their higher energy density and the abundance of magnesium in the Earth s crust [1]. The key to realising the full potential of MIBs lies in developing efficient solid-state electrolytes that facilitate the transport of magnesium ions within the battery. Single-crystal X-ray diffraction at beamline BM01 and neutron powder diffraction at J-PARC proton accelerator (Japan) were used to unveil the structural intricacies of the magnesium borohydride- ethylenediamine system, a promising candidate solid- state electrolyte for MIBs.

Annealing a ball-milled sample resulted in single crystals of about 20 μm, suitable for structure determination using synchrotron X-ray diffraction. The structure of the conductive compound (Figure 68), initially believed to be Mg(en)1(BH4)2 [2], was revealed to be far more complex than originally thought. Contrary to the initial assumption, it was determined to be Mg5(en)6(BH4)10, indicating a 5:6 stoichiometric ratio of magnesium to ethylenediamine. Magnesium atoms exhibited three different coordination numbers: 4, 5, and 6. This variation in coordination numbers confirms the diverse roles of the ligands in the structure: ethylenediamine plays a dual role as both a chelating and bridging ligand in the cation and anion; borohydride ions (BH4-) serve as terminal and bridging ligands, contributing to the compound s structural complexity.

Remarkably, the structure of Mg5(en)6(BH4)10 is not densely packed, revealing the presence of small voids that occupy nearly 3% of the structure volume. These voids could potentially play a crucial role in facilitating magnesium ion mobility within the solid-state electrolyte. These newly evidenced structural features open new avenues for understanding the mechanism of magnesium ion diffusion in solid-state electrolytes, a crucial factor for MIB performance.