The Li1.4Al0.4Ti1.6(PO4)3(LATP) nanocrystal powder is synthesized by citric acid assisted sol-gel method.The LATP powder is crystalized at 850℃ for 4 h,and the X-ray diffraction patterns show that the NASICON structure is obtained without any impurity phase.The LATP films are prepared by tape casting method through using as-synthesized LATP powder and subsequently recrystalized at various temperatures for 5 h.The impedance spectra of LATP film recrystalized at various temperatures indicate that the film sintered at 950℃ has the highest lithium ionic conductivity. Meanwhile,it is demonstrated that no impurity exists in LATP film recrystalizated at 950℃,and its lattice parameters are a=b=8.50236 Å and c=20.82379 Å.The high-purity LATP-epoxy films are prepared by modification with epoxy resin.The water permeation test proves that the LATP-epoxy film can prevent water from penetrating for 15 d,which indicates that epoxy resin fills the holes in LATP film.The fracture surface topography of LATP-epoxy film shows its dense structure with grain sizes from nano-scale to micro-scale.The energy dispersive X-ray spectrometer mapping of the fracture of LATP-epoxy film indicates that the carbon elements are uniformly distributed in grain boundary,which means that epoxy resin is soaked into LATP film.The relative density of 89.5% is obtained for LATP film,which is increased to 93.0% for LATP-epoxy (the nominal density is around 2.9624 g/cm3).The difference in relative density between LATP film and LATP-epoxy film indicates that the epoxy resin is immersed in LATP film already.The total,bulk,and grain boundary lithium ionic conductivities for the LATP film at 25℃ are 8.70×10-4 S·cm-1,2.63×10-3 S·cm-1 and 1.30×10-3 S·cm-1,respectively.The total,bulk,and grain boundary lithium ionic conductivities for the LATP-epoxy film at 25℃ are 3.35×10-4 S·cm-1,1.84×10-3 S·cm-1 and 4.09×10-4 S·cm-1,respectively.The decrease in the total conductivity of the LATP-epoxy film may be caused by the increase in its grain boundary resistance and its exposure to the atmosphere during modification with epoxy resin.The high lithium ionic conductivity for both LATP film and LATP-epoxy contributes to homogeneous mixture at sol-gel process and the decreasing of grain boundary impedance for this special structure.The activation energies for LATP film and LATP-epoxy film are 0.36 eV and 0.34 eV,respectively, based on Arrhenius equation.The water-impermeable high lithium ion conducting solid electrolyte of LATP modified with epoxy resin is likely to be used as protective film for lithium metal electrode of novel high energy density batteries.