As the demand for high energy-density and safe Li-based batteries, the development of composite solid polymer electrolytes (CSPEs) with high ionic conductivity and high electrochemical stability are significant. Herein, the amorphous infinite coordination polymer (ICP) nano-network of Ce-coordinate polyphenol ellagic acid (EACe2) is introduced into PEO matrix to achieve highly endurable Li metal batteries (LMBs). Different from conventional metal-organic frameworks (MOFs) with discrete crystalline grains, ICP network with high interconnectivity and self-assembly capability enables the better penetration effect and multi-scale interaction of filler in PEO. EACe2 nano-filler promotes the improvement of ionic conductivity and Li-ion transference number of CSPEs, as well as the construction of robust solid-electrolyte interphase (SEI) with the concentration of LiF, Li2O and Li2S, leading to ultrastable and kinetically favorable Li/Li cells with dendrite-mitigated cycling over 8800 h. The low interfacial resistances (16 Ω cm2 and 20.5 Ω cm2 for CSPE-EACe2/Li and CSPE-EACe2/LiFePO4 respectively) can achieve highly reversible Li/LiFePO4 cells (with capacities of 102.6 mAh g−1 after 2000 cycles at 0.5 C and 103.2 mAh g−1 after 1200 cycles at 1 C). This CSPE-EACe2 membrane also enables the tolerance of high-voltage cathode (LiNi0.8Co0.1Mn0.1O2, up to 4.3 V), room temperature operation, and ultrathin thickness (25-μm-thick). Download : Download high-res image (232KB) Download : Download full-size image