Polymer electrolytes have attracted great attention due to their promising abilities, however, only on further improving their safety and electrochemical performance can they be used as next generation electrolytes. Here, a novel composite polymer electrolyte (CPE) with nano-SiO₂ acting as a cross-linking agent to form a 3D cross-linked network was synthesized to improve the electrochemical stability and mechanical ability. The novel CPE exhibited some excellent properties such as high ionic conductivity (4.65 × 10⁻³ S cm⁻¹ at room temperature) with a broad electrochemical stability window (∼5.4 V vs. Li/Li⁺), high ionic transfer number (0.45), high mechanical robustness and flexibility (Young’s module ∼8.9 MPa, breaking strain 181%), and excellent interface stability. The lithium metal battery LiFePO₄/CPE/Li was assembled and delivered a specific capacity of 160 mA h g⁻¹ with a capacity retention of above 90% after 500 cycles at 1C. Furthermore, to cooperate with high voltage cathode materials LiNi_0.8Co_0.1Mn_0.1O₂ and LiNi_0.5Mn_1.5O₄, the novel CPE still delivered superior cycling performance compared to traditional electrolytes.