Preparation, crystal structure and Li ion conductivity of Li10.8Ba1.85Al3.5O12F

Abstract

Polycrystalline ceramics comprising the newly discovered compound Li10.8Ba1.85Al3.5O12F were synthesized by heating a compacted mixture of Li2O, BaO and Al2O3 together with LiF as a fluorine source at 1023 K for 6 h in an Ar atmosphere. Single crystals of this oxide were subsequently obtained from a melt-solidified sample prepared by heating and slow cooling of the compact from 1133 K. Single-crystal X-ray diffraction established that this compound had a trigonal crystal structure (a = 6.03346(10) Å, c =10.13780(17) Å, space group P-3) containing Li split sites around the (001) plane at z = 0 with approximately 1/3 occupancy and large anisotropic atomic displacement parameters. The Li ion conductivities for the samples prepared by reaction sintering at 1023 K (relative density 55%) and by sintering of a powder compact at 1123 K (relative density 87%) were determined to be 1.6(3) × 10−8 and 2.8(2) × 10−8 S·cm−1 at 305 K and 3.9(2) × 10−6 and 1.0(2) × 10−5 S·cm−1 at 423 K, respectively, while the activation energies for conduction of the samples was found to be 0.52(2) and 0.55(3) eV, respectively. Cyclic voltammetry data acquired using Li and Ta electrodes indicated that this compound was stable in contact with Li metal and demonstrated a Li ion transport number >99%.