Lithium metal batteries (LMBs) have attracted extensive attention for their ultrahigh energy density. However, the uncontrollable growth of Li-dendrites results in poor cyclability and potential safety risks, thus preventing their practical application. Herein, a flexible and cost-effective aqueous polyethylene oxide (PEO)-based solid-state electrolyte is prepared, which enables uniform and dendrite-free Li deposition by introducing Cs+ with an electrostatic shielding mechanism at high current densities. The self-assembly of PEO and bacterial cellulose by hydrogen bonding reduces the crystallinity of PEO and increases uniformly the distribution of lithium ions. With excellent flexibility and thermal stability, such a 3D polymer solid-state electrolyte exhibits an enhanced electrochemical stability window of 5.8 V versus Li/Li+ potential and a high ionic conductivity of 1.28 × 10-4 S cm-1 at 60 °C. The Li|BC-PEO-Cs+|Li symmetric cells operate stably for more than 1000 h. Furthermore, Li|BC-PEO-Cs+|LiFePO4 (LFP) cells show remarkable enhancement in capacity (163.4 mA h g-1 at 0.1 C), cycling stability (with a capacity retention of 96% after 500 cycles at 1 C) and high functionality and safety (withstanding folding and cutting) in practical applications.