Lithium-metal-based batteries, owing to the extremely high specific energy, have been attracting intense interests as post-Li-ion batteries. However, their main drawback is that consumption/de-activation of lithium metal can be accelerated when O2 or S used in the cathode crosses over to the metal, reducing the lifetime of the batteries. In use of ceramic solid state electrolyte (SSE) separator, despite the capability of gas blocking, thick and heavy plates (~0.3 mm) are necessitated to compensate its mechanical fragility, which ruin the high specific energy of the batteries. Here, we demonstrate fabrication of a new membrane made of micron-sized SSE particles as Li-ion channels embedded in polymer matrix, which enable both high Li-ion conduction and gas-impermeability. Bimodal surface-modification was used to control the energy of the particle/polymer interface, which consequently allowed channel formation via a simple one-step solution process. The practical cell with the new membrane provides a cell-specific energy of over 500 Wh kg-1, which is the highest values ever reported.