Synthesis and ionic conductivity of Li boracites, Li4B7O12Cl and Li4B4Al3O12Cl1-Br

Li 4 B 7 O 12 Cl, Li 4 B 4 Al 3 O 12 Cl, and new boracite, Li 4 B 4 Al 3 O 12 Cl 1- x Br x ( x = 0.1, 0.2 and 1.0) were synthesized using solid-state reaction method. The crystal structures of Li 4 B 7 O 12 Cl and Li 4 B 4 Al 3 O 12 Cl were investigated through high-intensity synchrotron X-ray diffraction (SXRD) analysis. The ionic conductivity was elucidated for the ceramics and green compacts to discuss their potential as a solid electrolyte of all-solid-state Li-ion batteries. The ionic conductivity of the bulk part was 2.0 × 10 −5 S/cm for Li 4 B 4 Al 3 O 12 Cl 0.2 Br 0.1 and Li 4 B 4 Al 3 O 12 Cl 0.8 Br 0.2 at 294 K and the total ionic conductivity of these compounds was approximately 10 −8 S/cm. Furthermore, we succeeded to prepare a high-density green compact for Li 4 B 7 O 12 Cl. The total ionic conductivity of this sample was higher than that of Li 4 B 7 O 12 Cl glass-ceramics at high temperatures. The conduction pathway was determined by bond valence sum mapping for Li 4 B 7 O 12 Cl and Li 4 B 4 Al 3 O 12 Cl, which was conducted based on the refined structural data of the high-intensity SXRD data. The results showed that an additional conduction pathway is formed for Li 4 B 4 Al 3 O 12 Cl and its larger unit cell promotes Li + migration around the halogen ion. Although the substitution of Cl − with Br − suppresses the migration of Li + around halogen ion due to the large ionic radius of Br − in Li 4 B 4 Al 3 O 12 Cl 1- x Br x , appropriate substitution enhances ion migration via the additional pathway without suppressing the ion transfer around the halogen ion. Thus, Li 4 B 4 Al 3 O 12 Cl 0.2 Br 0.1 and Li 4 B 4 Al 3 O 12 Cl 0.8 Br 0.2 showed high ionic conductivity. • Green compacts of Li-boracites shows high ionic conductivity. • Li-boracites are proven worthy candidates of a solid electrolyte of all-solid-state Li-ion batteries. • The conduction pathways of these Li-boracites are visualized by bond valence map.