Electrical properties of lithium–lanthanum silicate oxyapatites

Abstract

Lithium–lanthanum silicate oxyapatite exhibits high lithium-ion conductivity at temperatures above 400 °C. To further enhance its ionic conductivity, the relationships among the microstructure, bulk conductivity, and grain boundary conductivity were investigated for Li-rich compositions. LixLa10−xSi6O27−x (x = 1–8), Li5La5Si5O20, Li5La5Si4.5O19, Li6La4Si4.5O18, and Li7La3Si4O16 were synthesized, and their ionic conductivities were determined. Using X-ray diffraction, Li2La8Si6O25, Li5La5Si5O20, Li5La5Si4.5O19, Li6La4Si4.5O18, and Li7La3Si4O16 were analyzed to be close to the apatite phase with few impurities. Scanning electron microscopy/energy-dispersive X-ray spectroscopy and nuclear magnetic resonance analyses revealed that the lithium–silicate glass phase increases with respect to the apatite phase in the following order: Li2La8Si6O25 < Li5La5Si5O20 < Li5La5Si4.5O19 < Li6La4Si4.5O18 < Li7La3Si4O16. Further, as the La/Si ratio of the apatite phase increases, the chemical composition changes from Li1La9Si6O26 to Li3La7Si6O24. Both the bulk (apatite phase) and the grain boundary (glass phase) resistivities at 400 °C decreased in the order Li5La5Si5O20 > Li5La5Si4.5O19 > Li6La4Si4.5O18 > Li7La3Si4O16. The apparent activation energies for the bulk ion conductivity of Li5La5Si5O20, Li5La5Si4.5O19, Li6La4Si4.5O18, and Li7La3Si4O16 were 57.3, 47.6, 43.0, and 40.4 kJ mol−1, respectively. The total conductivities (bulk + grain boundary conductivities) of Li7La3Si4O16 (8.1 × 10−3 S cm−1 at 400 °C) was approximately four times higher than that of Li1.08LaSiO4.04 (2.2 × 10−3 S cm−1 at 400 °C), which has been previously reported to have the highest ionic conductivity.