As the demand for safer and more sustainable energy storage systems continues to rise, all solid-state batteries (ASSBs) have emerged as a promising alternative to traditional lithium-ion batteries. By replacing liquid electrolytes with solid-state ones, the resulting solid batteries offer the potential for enhanced safety operation and higher energy density, as well as the elimination of harmful and flammable organic compounds. In this line, the development of a green solid-state electrolyte is a fundamental part of the transition to a new generation of solid-state batteries, since solid electrolytes can prevent and minimize the failure mechanisms derived from dendrite growth in lithium-metal batteries, while reducing the amount of critical organics and liquids. Among all the types of solid electrolytes within the inorganic families (NaSICON, LiSICON, garnet, perovskites…), the Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) a NaSICON-like phosphate, stands out due to its theoretical high ionic conductivity (3 mS cm^-1), air and humidity stability and lack of scarce transition metals. ^[1]

However, ceramic bodies like the LATP have traditionally been processed following solid-state reactions involving  high-temperature routes, which result in a high energy consumption and large CO₂ emissions. To assess these problems, the Cold Sintering Process (CSP) has emerged as a new sintering route that enables the obtention of dense ceramics like LATP employing a low processing temperature (less than 300 ºC), mid-to-large pressures (hundreds of MPa) and a small amount of a transient liquid phase (TLP).^[2] Furthermore, with the CSP it is possible to incorporate polymeric additives to form composites with enhanced electrochemical properties taking advantage of the low operating temperatures. In this way, this new processing technique opens the door for the obtention of new hybrid materials acting as solid electrolytes for the new ASSBs.

In this work, authors present a comprehensive study of the application of the Cold Sintering Process in the manufacture of composite solid electrolytes (CSEs) based on LATP and a 10% PEO₂-LiTFSI polymer matrix additive, by focusing on some of the most important processing variables such as the TLP content, the ceramics particle size, the nature of the dopants and the sintering pressure, which will determine the final behavior of the electrolyte in a lithium-metal battery. It is worth mentioning that by optimizing the above-mentioned variables, CSEs with a maximum ionic conductivity of 0.5 mS cm^-1, a low activation energy (0.296 eV) and good cycling stability (160.1 mAh g^-1_LFP at C/10 and 75.8 mAh g^-1_LFP at 1C) are obtained at a maximum pressure and temperature of 300 MPa and 150 ºC, respectively. Furthermore, in operando electrochemical impedance spectroscopy (EIS) is employed as a powerful characterization technique, since it is not only used during the sintering process as a control tool to monitor the densification, but also during the cycling of CSP-obtained half-cells (with an LFP|CSE|Li structure) to identify potential failure mechanisms derived from the stacking of the layers.