Understanding The Reactivity between Li_1+xAl_xTi_2-x(PO₄)₃ and Sintering Aids to Minimize Densification Temperatures in All-Solid-State Batteries

NaSICON-type materials like Li_1+xAl_xTi_2-x(PO₄)₃(LATP) are considered promising solid electrolytes for all-solid-state batteries due to their good total ionic conductivity of 10^-4 S.cm^-1 at room temperature. However, a critical issue concerns the processability of the solid electrolyte in the composite positive electrode¹. Indeed, LATP must be densified by heat treatment (&gt;900°C) to approach its maximum ionic conductivity and form an intimate contact with the active material. Nevertheless, chemical reactivity in the composite cathode has been observed as low as 700°C for some systems^2,3.<br/>To prevent this reactivity, one strategy is to lower the densification temperature of this solid electrolyte. Multiple reports in the literature propose to use lithium salts as a sintering aid to achieve this goal. For example, LiBF₄ and LiF have been shown to densify the solid electrolyte LATP to relative densities above 90% of the theoretical density at temperatures ranging from 800°C to 900°C, respectively^4,5. However, these sintering temperatures remain too high and close observation of powder X-ray diffraction data reported in the literature, confirmed by our own experimental results, shows that resistive impurity phases such as LiTiOPO₄ and Li₄P₂O₇ already form.<br/>In this work, we investigated the chemical reactivity mechanism during the densification and sintering processes. We first studied the reactivity of several lithium salts with LATP (x = 0.3), using <i>in situ</i> and <i>ex situ</i> X-ray diffraction, Raman spectroscopy and <i>ex situ</i> solid-state NMR spectroscopy (⁷Li, ²⁷Al, ³¹P). We identified new intermediate phases for the first time, bringing a new light on the reactivity and sintering mechanisms. We show that lithium salts react with LATP at low temperature, long before they melt. This result interrogates the liquid phase sintering mechanism proposed in the literature and explain that the sintering temperature is not correlated to the melting point of the salt.<br/>Decreasing the sintering temperature of LATP is a key challenge to achieve a viable composite positive electrode for all-solid-state batteries. Our work shows that the chemical reactivity between lithium salts and LATP is a limiting factor in decreasing the sintering temperature of the material below 700°C. This new knowledge enables us to reexamine sintering of oxide ceramics and explore new paradigms to address this challenge.<br/>(1) Banerjee, A.; et al. Interfaces and Interphases in All-Solid-State Batteries with Inorganic Solid Electrolytes. <i>Chem. Rev.</i> <b>2020</b>, <i>120</i> (14), 6878–6933.<br/>(2) Miara, L.; et al. About the Compatibility between High Voltage Spinel Cathode Materials and Solid Oxide Electrolytes as a Function of Temperature. <i>ACS Appl. Mater. Interfaces</i> <b>2016</b>, <i>8</i> (40), 26842–26850.<br/>(3) Yu, C.-Y.; et al. High-Temperature Chemical Stability of Li_1.4Al_0.4Ti_1.6(PO₄)₃ Solid Electrolyte with Various Cathode Materials for Solid-State Batteries. <i>J. Phys. Chem. C</i> <b>2020</b>, <i>124</i> (28), 14963–14971.<br/>(4) Dai, L.; et al. Influence of LiBF₄ Sintering Aid on the Microstructure and Conductivity of LATP Solid Electrolyte. <i>Ceramics International</i> <b>2021</b>, <i>47</i> (8), 11662–11667.<br/>(5) Kwatek, K.; et al. Structural and Electrical Properties of Ceramic Li-Ion Conductors Based on Li_1.3Al_0.3Ti_1.7(PO₄)₃-LiF. <i>Journal of the European Ceramic Society</i> <b>2020</b>, <i>40</i> (1), 85–93.