Unveiling the Reactivity of Li1+xAlxTi2–x(PO4)3 with Lithium Salts to Reduce Its Sintering Temperature

Abstract

NaSICON-type materials, such as Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), are considered promising solid electrolytes due to their good total ionic conductivity of 1 × 10^–4 S cm^–1 at room temperature and their stability at high potentials (4.1 V vs Li/Li⁺). However, decreasing their densification temperature is crucial for their integration into all-solid-state batteries (ASSBs). The minimum required heat treatment temperature for densification of LATP is 900 °C, which is incompatible with its integration in the composite electrode of ASSBs due to reactivity with the positive electrode material (cathode). To lower this temperature, lithium salts are often proposed as sintering aids to promote liquid-phase sintering. However, the systematic formation of impurities, such as LiTiOPO₄ and Li₄P₂O₇, suggests that chemical reactivity plays a significant role in LATP densification. In this work, the chemical reactivity mechanism of lithium salts with LATP during densification and sintering was investigated. Various characterization techniques, including in situ and ex situ X-ray diffraction, TGA–DTA–MS, DSC, ex situ Raman and solid-state NMR spectroscopy (⁷Li, ²⁷Al, and ³¹P), were employed to elucidate the mechanism. The formation of intermediate decomposition products Li₃PO₄ and TiO₂ is identified for the first time via the reactivity of the lithium salt with LATP prior to the melting temperature of the salt. These intermediates subsequently react with LATP at a higher temperature, resulting in the formation of final impurities LiTiOPO₄ and Li₄P₂O₇. This unified mechanism provides important insights on the enhanced densification of LATP at lower temperatures with the use of Li salt sintering aids.