Optimization of sintering process on Li1+Al Ti2-(PO4)3 solid electrolytes for all-solid-state lithium-ion batteries

In this study, a NASICON-structured Li1.3Al0.3Ti1.7(PO4)3 (LATP) powder is prepared by hydrothermal methods followed by calcination, cold pressing and post-sintering processes. The white, solid product is characterized thoroughly using powder X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS). The conductivity of the material is measured by a impedance spectroscopy as a function of temperature. Initially, hydrothermal synthesis yields a material isostructural with the orthorhombic oxyphosphate, LiTiOPO4. EDS analysis shows that the distribution of aluminum throughout this material is uniform. A systematic study is then performed to investigate how altering the sintering parameters (such as powder pre-sintering temperature and pellet sintering temperature) affect the formation of LATP. The structure is determined by Rietveld refinement against XRD data and the effects of sintering temperature on porosity, microstructure and electrical conductivity were resolved. The experimental results show that the optimum pre-sintering and sintering temperatures of LATP powders and pellets respectively are 900 °C and 1100 °C. These conditions produce materials with the highest density (99.07% of theoretical), superior conductivity (grain-, grain boundary- and total lithium-ion conductivities of 6.57 × 10−4, 4.59 × 10−4 and 2.70 × 10−4 S cm−1, respectively) and with an activation energy for Li motion of 0.17 eV.