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Solid-state Li metal batteries (SSLMBs) are promising solutions for the next-generation energy storage devices with high energy densities and safety. Accordingly, the advanced solid-state electrolytes are further needed to address the challenges-low ionic conductivity, poor interfacial compatibility and uncontrollably Li dendrites, boosting the electrochemical and safety performances of SSLMBs. Herein, a "flexible and rigid" strategy is proposed to enhance the electrochemical and mechanical properties of polyethylene oxide (PEO)-based electrolytes. Specifically, the flexible poly-1,3-dioxolane (poly-DOL) and rigid graphitic carbon nitride (g-C 3 N 4 ) are coordinated by a coupling reaction to prepare g-C 3 N 4 -poly-DOL, which is further employed as the filler for the PEO matrix to fabricate a composite polymer electrolyte g-C 3 N 4 -pDOL-PEO. The flexible poly-DOL and rigid g-C 3 N 4 together endow the PEO-based electrolyte with good interfacial stability, high ion-conductivity and strong mechanical strength. Consequently, the Li/g-C 3 N 4 -pDOL-PEO/LiFePO 4 cell delivers high cyclability with a capacity retention ratio of 89.7 % after 150 cycles and an average Coulombic efficiency over 99.9 %, and, the Li/g-C 3 N 4 -pDOL-PEO/Li cell can stably cycle beyond 300 h at 0.2 mAh cm -2 with small polarization (13 mV). The "flexible and rigid" strategy coupling the polymer with the filler provides an effective electrolyte design for high-performance SSLMBs., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)