Unveiling Low-Temperature Sintering of Solid-State Ceramic Composite Electrolytes via In-Situ Electrochemical Impedance Spectroscopy

The increasing safety concerns surrounding high-energy density rechargeable batteries have spurred interest in the development of solid-state electrolytes, aiming for nonflammable and tolerance to extreme conditions. However, conventional fabrication methods of ceramic-based composites face two primary challenges: limited ability to simultaneously tune the intergranular phase and achieve high material density, as well as a lack of efficient methodology to address interfacial issues under mild processing conditions. Consequently, producing stable solid-state electrolytes with low interface resistance, particularly solid-state oxide electrolytes, without resorting to high-temperature sintering for extended periods remains a challenge.<br/>To overcome these challenges, we introduce a cold sintering process (CSP), a multistage nonequilibrium approach combining dissolution−precipitation under external stress, viscous flow of saturated solutions, and species diffusion for the fabrication of highly ionic conductive solid-state electrolytes at low processing temperatures. Additionally, CSP bridges the temperature gap between high melting point constituents (<i>e.g.</i>, ceramics) and low melting point constituents (<i>e.g.</i>, nano-sized additives, or polymers), enabling the co-sintering of ceramic-based composites for a wide range of solid-state electrolytes. In this study, CSP was utilized to prepare a solid-state NASICON-Halide (nanosized Li₃InCl₆in LATP matrix) composite electrolyte. To gain further insight into the sintering dynamics and the process-structure-property correlation, we employed electrochemical impedance spectroscopy (EIS) as an in-line monitoring tool to study the real-time impedance during the CSP. We considered the mixed ceramic powders with a small volume of transient solvent as an effective dielectric medium. Through simulations using an equivalent circuit comprising a constant phase element and a resistive element, we captured the dielectric properties and resistance sensitive to CSP, which vary as evaporation and densification occur. Decoding the resulting resistance data provided a profound understanding of the temperature and pressure effects.<br/>Furthermore, we investigated the associated microstructures and ionic conductivities to understand the process-structure-property correlation. The highly dense composite electrolyte, with a small fraction of Li₃InCl₆, exhibited an impressive ionic conductivity of 10^-4 S cm^-1 at room temperature, along with excellent reversibility in transporting Li ions for thousands of hours at a cycling capacity of 1 mAh cm^-2.