Impact of the sintering additive Li₃PO₄ on the sintering behaviour, microstructure and electrical properties of a ceramic LATP electrolyte

Competitive all solid-state batteries (ASSBs) require particulate, ternary composite cathodes, consisting of a ceramic active material, ceramic solid-state electrolyte (SSE) and an electrical conductor, to achieve high energy densities. Firmly bonded contacts between the active material and SSE are necessary to obtain fast Li-ion transfer, so that sintering processes are unavoidable. Since the sintering temperatures of oxidic ceramic SSEs, such as Li1+xAlxTi2−x(PO4)3 (LATP), are above 950 °C, decomposition and mixed phase formation take place during sintering with active materials, so a reduction of the SSE's sintering temperature is necessary. This study investigates the impact of Li3PO4 (3, 5 and 10 vol%) as a sintering additive on the sintering behaviour, microstructure and electrical properties of LATP. The obtained liquid phase sintering results in the acceleration of the densification process, so that the start of sinter neck formation and shrinkage could be reduced by 50 °C and 150 °C, respectively. Effects related to the sintering process, such as LiTiOPO4 and AlPO4 formation, densification, grain size distribution and crack formation, are correlated with the electrical properties. Microscopic effects like changes in the lattice stoichiometry and nature of the grain boundary as well as macroscopic effects like pores, grain sizes and cracks have an influence on the percolation network for Li-ion migration. Finally, the addition of Li3PO4 to LATP results in a high ionic conductivity of ca. 2 × 10−4 S cm−1 along with a reduced sintering temperature of 800 °C.