The ever-increasing demand for safe and high-energy-density batteries urges the exploration of ultrathin, lightweight solid electrolytes with high ionic conductivity. Solid polymer electrolytes (SPEs) with high flexibility, reduced interfacial resistance and excellent processability have been attracting significant attentions. However, reducing the thickness of SPEs to be comparable with that of commercial separators increases the risk of short-circuiting. Herein, an ultrathin (≈7 μm), flexible and mechanical robust SPE was constructed from a rationally designed multi-functional polymer network,