Probing Cold Sintering-Regulated Interfaces and Integration of Polymer-in-Ceramic Solid-State Electrolytes

Solid-state batteries (SSBs) have recently been revived to increase energy density and eliminate safety concerns of current Li-ion batteries that use flammable liquid electrolytes. However, fabricating solid-state electrolytes (SSEs) faces many challenges, particularly the lack of effective methods to integrate reactive and sensitive solid ceramic conductors and address interfacial issues. To overcome these challenges, we introduce a cold sintering process (CSP) to integrate dissimilar conducting materials into polymer-in-ceramic (PIC) composite SSEs under external stress and mild heating conditions. Specifically, a NASICON phase Li1.3Al0.3Ti1.7(PO4)3 (LATP)-based composite SSEs were co-sintered, resulting in well-distributed polymer-in-ionic liquid gel (PILG) along the ceramic boundaries. Using electrochemical impedance spectroscopy (EIS), we monitored real-time impedance during the CSP, revealing the effect of the transient liquid on boundary formation in the PIC composite. The cold sintering-regulated composite structure demonstrated excellent charge transport capabilities. For example, it achieved a room temperature ionic conductivity of 4.2×10-4 S cm-1 and was able to reversibly transport Li+ for hundreds of hours. Additionally, the co-sintering of multi-layered Li metal SSBs (e.g., LiFePO4-PILG||LATP-PILG|PILG||Li) reduced interface resistance, thereby exhibiting stable cycling.