Disclosed are electrochemical device separator structures which include a substantially impervious active metal ion conducting barrier layer material, such as an ion conducting glass, is formed on an active metal ion conducting membrane in which elongation due to swelling on contact with liquid electrolyte is constrained in at least two of three orthogonal dimensions of the membrane. The non-swelling character of the membrane prevents elongation in the x-y (or lateral, relative to the layers of the composite) orthogonal dimensions of the membrane when it is contacted with liquid electrolyte that would otherwise cause the barrier layer to rupture. Substantial swelling of the membrane, if any, is limited to the z (or vertical, relative to the layers of the composite) dimension.