A cyanate-functionalized polymer composite electrolyte with a self-healing gradient SEI affords ultra-thermally...

Solid-state electrolytes can significantly enhance the thermal stability of batteries and reduce the risk of thermal runaway. Polymer/ceramic composite electrolytes hold promise for addressing the solid/solid interface contact issues in solid-state batteries. However, high-temperature environments can exacerbate the interfacial phase separation of polymer/ceramics, hindering barrier lithium-ion transport and intensifying side reactions on the anode. To extend the working temperature up to 160 °C, a high-thermal-stability polymer electrolyte containing cyano and ester-urea groups was in situ polymerized with porous lithium iron phosphate (Li_1.3Al_0.3Ti_1.7(PO₄)₃). The cyano groups in PPEM suppress interfacial phase separation through strong coordination with Ti⁴⁺ sites on LATP, whereas the ester-urea segments promote Li⁺ transport. This dual-functional design achieved a lithium-ion transference number of 0.78 at room temperature. Moreover, a self-healing gradient solid electrolyte interphase (SEI) layer formed spontaneously during cycling, featuring a highly ion-conductive inner Li₃N layer and an outer crosslinked polymer layer for mechanical reinforcement, which effectively suppresses lithium dendrite growth and side reactions at elevated temperatures. The synergistic effect of phase separation inhibition and SEI healing enables the Li/PPEM-LATP/Li symmetric cell to cycle stably for 2400 h at 0.2 mA cm^-2 and the Li/PPEM-LATP/LFP pouch cell (0.5 Ah) to retain 91% capacity after 100 cycles at 160 °C/0.5C. Furthermore, the PPEM-LATP electrolyte exhibited a wide electrochemical window (>5.4 V) and exceptional thermal resilience, with no thermal runaway observed below 312.3 °C in abuse tests. This study establishes a paradigm for designing high-temperature-resistant solid-state electrolytes via interface-engineering strategies.