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

Abstract

Li4B7O12Cl, Li4B4Al3O12Cl, and new boracite, Li4B4Al3O12Cl1-xBrx (x = 0.1, 0.2 and 1.0) were synthesized using solid-state reaction method. The crystal structures of Li4B7O12Cl and Li4B4Al3O12Cl 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 Li4B4Al3O12Cl0.2Br0.1 and Li4B4Al3O12Cl0.8Br0.2 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 Li4B7O12Cl. The total ionic conductivity of this sample was higher than that of Li4B7O12Cl glass-ceramics at high temperatures. The conduction pathway was determined by bond valence sum mapping for Li4B7O12Cl and Li4B4Al3O12Cl, 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 Li4B4Al3O12Cl 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 Li4B4Al3O12Cl1-xBrx, appropriate substitution enhances ion migration via the additional pathway without suppressing the ion transfer around the halogen ion. Thus, Li4B4Al3O12Cl0.2Br0.1 and Li4B4Al3O12Cl0.8Br0.2 showed high ionic conductivity.