  The work presents the investigations of Li1.3Al0.3Ti1.7(PO4)3-xLiF Li-ion
conducting ceramics with 0 < x < 0.3 by means of X-ray diffractometry (XRD),
7Li, 19F, 27Al and 31P Magic Angle Spinning Nuclear Magnetic Resonance (MAS
NMR) spectroscopy, thermogravimetry (TG), scanning electron microscopy (SEM),
impedance spectroscopy (IS) and density method. It has been shown that the
total ionic conductivity of both as-prepared and ceramic Li1.3Al0.3Ti1.7(PO4)3
is low due to a grain boundary phase exhibiting high electrical resistance.
This phase consists mainly of berlinite crystalline phase as well as some
amorphous phase containing Al3+ ions. The electrically resistant phases of the
grain boundary decompose during sintering with LiF additive. The processes
leading to microstructure changes and their effect on the ionic properties of
the materials are discussed in the frame of the brick layer model (BLM). The
highest total ionic conductivity at room temperature was measured for
LATP-0.1LiF ceramic sintered at 800{\deg}C and was equal to {\sigma}tot = 1.1 x
10-4 Scm-1.
