Rechargeable battery cells having a liquid electrolyte require a separator permeable to the electrolyte between the two electrodes. The volume changes of the electrodes during charge and discharge have previously limited porous separators to polymers with an electronic energy gap E_g large enough for the Fermi levels of the two electrodes to be within it. We demonstrate that the plasticity of a costly polymer membrane is not needed for the separator in a liquid electrolyte. SiO₂ is an insulator with the required large energy gap, and SiO₂ particles are easy to prepare, cheap, and environmentally friendly. We demonstrate that removal of adsorbed species from the surface of a film of SiO₂ nanoparticles leads to weak bonding between the particles to give a self-assembled, porous layer that has the needed plasticity for a liquid-electrolyte separator that is wet by an alkali-metal anode.