The influence of water on the electrical conductivity of aluminum-substituted lithium titanium phosphates

Li1+xAlxTi2−x(PO4)3 (0.1 ≤ x ≤ 0.6) powders were prepared by a novel sol-gel method with high phase purity, densification activity and conductivity. Diffraction analyses showed that the solubility limit of Al3+ was reached at x = 0.5. The highest ionic conductivity was obtained for Li1.5Al0.5Ti1.5(PO4)3, which reached 1.0 × 10−3 S cm−1 at 25 °C when tested in ambient air. However, measurements in dry argon resulted in a conductivity of only 5 × 10−4 S cm−1 at 25 °C. Hence, the influence of moisture or water on microstructure and grain-boundary conductivity was investigated by impedance spectroscopy, scanning electron microscopy, Raman spectroscopy, X-ray and neutron diffraction. An ion exchange of Li+ by protons could not be unambiguously achieved by exposure of LATP powder in water. Neutron diffraction of humidified samples did not clearly indicate the presence of water or protons in the crystal structure of LATP, whereas μ-Raman measurements confirmed the presence of water/protons on the sample surface and in the bulk material. A higher signal of the vibrational modes of H2O was measured on grain boundaries than in the grain interior on the sample surface as well as on the fractured surface of sintered specimens. Therefore the higher apparent conductivity of LATP samples may predominantly result from adsorbed water from ambient air at grain boundaries. Hence, the conductivity tested in dry argon represents the correct conductivity of LATP samples. The highest density after sintering is not necessarily leading to high conductivity but rather the microstructure plays the dominant role.