The commercial use of lithium-sulfur (Li-S) batteries is hampered by the shuttle phenomenon in cathodes and the uncontrolled growth of Li dendrites in anodes. Designing functional material-coated separators is gaining importance in the effort to tackle these issues in both of the electrodes simultaneously. Here, an initiated chemical vapor deposition (iCVD) technique is introduced in Li-S batteries for the first time to homogeneously deposit a polyvinylimidazole (pVIDZ) nanolayer on the separator. An ultrathin and ultralight-weight polymer coating nanolayer with a thickness of 70-100 nm and a weight of 0.055 mg cm(-2) was achieved. Furthermore, the pVIDZ layer on the separator was observed to perform a bifunctional role in stabilizing the anode by alleviating Li dendrite growth and also to improve the cycle stability of the cathode by inhibiting the shuttle phenomenon. Consequently, even with high sulfur loading electrodes of 4 mg cm(-2), the use of iCVD-derived bifunctional separators exhibits an initial discharge capacity of 881 mA h g(-1) at a rate of 1.0 C while maintaining 84.5% of its initial capacity after 300 cycles corresponding to a capacity decay of only 0.051% per cycle.