The transition from conventional lithium-ion batteries to all-solid-state batteries (ASSBs) is currently at the forefront of energy storage research. A critical component in this shift is the development of efficient solid-state electrolytes (SSEs), particularly ceramic-based ones, which combine high ionic conductivity with the mechanical strength needed to mitigate dendrite growth. Among these, lithium aluminium titanium phosphate (LATP) stands out due to its high bulk ionic conductivity (~3 mS cm^-1) and chemical stability under ambient conditions. However, the fabrication of LATP typically requires high sintering temperatures (>1000 ºC), limiting its scalability and integration with other battery components.

To overcome this challenge, dense LATP SSEs are produced at significantly reduced temperatures (~150 ºC) using the Cold Sintering Process (CSP). A key feature of this research is the use of in operando Electrochemical Impedance Spectroscopy (EIS) as a real-time monitoring tool during the sintering process. To enable this, a novel in operando EIS setup was specifically designed and implemented for this purpose.

Through this setup, several critical parameters influencing the CSP process and the final electrochemical properties of the LATP SSEs have been systematically investigated:

1. Optimization of Transient Liquid Phase (TLP) content, to achieve the best balance between densification and ionic conductivity.
2. Tailoring Bi₂O₃ additive concentration, which acts as a sintering aid, enhancing both microstructure and electrochemical performance.
3. Control of starting LATP particle size, to balance densification and ionic transport properties.
4. Chemical engineering of intergranular phases by introducing a LiOH:LiNO₃ eutectic mixture, enabling further tuning of the grain boundary properties.
5. Analysis of key process variables, such as pressure, dwell time, and heating profile, which directly influence the sintering mechanism and the resulting microstructure.

The combination of low-temperature CSP and in operando EIS monitoring has proven to be a powerful approach for understanding and controlling the complex interplay between processing conditions, microstructure evolution, and electrochemical performance. The methodology developed here not only provides fundamental insights into the densification mechanisms at play but also opens new routes for the scalable fabrication of high-performance SSEs for next-generation ASSBs.