Probing cold sintering-regulated interfaces and integration of polymer-in-ceramic solid-state electrolytes

Solid-state batteries (SSBs) are considered the next-generation energy storage technology, offering safer and more stable alternative to conventional Li-ion batteries with flammable liquid electrolytes. Among various solid-state electrolytes (SSEs), the NASICON-phase Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) stands out as a promising candidate due to its high ionic conductivity at room temperature and excellent stability in air. However, densifying oxide-based LATP SSEs typically requires high-temperature sintering, while ion depletion across grain boundaries significantly limits the practical performance of SSBs. To overcome these challenges, we introduce a transient liquid-assisted cold sintering process (CSP) to seamlessly integrate dissimilar ionic conducting materials into polymer-in-ceramic (PIC) composite SSEs under pressure and mild heating. This process enables the uniform distribution of a highly conductive poly(ionic liquid) gel (PILG) phase at the boundaries of LATP particles, effectively reducing interfacial resistance. In-situ electrochemical impedance spectroscopy (EIS) was employed to monitor real-time impedance changes during densification process, providing insights into dynamic interface behaviors. The LATP-PILG composite SSE achieved high ionic conductivities of 4.2 × 10⁻⁴ S cm⁻¹ and 5.15 × 10⁻⁴ S cm⁻¹ in a coin cell and a split cell under 20 MPa at room temperature, respectively. Furthermore, it demonstrated reversible plating/stripping for hundreds of hours. The integrated LiFePO₄-PILG||LATP-PILG|PILG||Li cell exhibited excellent cycling stability.