NOVELTY - A substantially impervious composite solid electrolyte, comprises: a base component comprising a continuous inorganic solid electrolyte matrix having through pores; and a filler component contained in the base component through pores and providing a fluid barrier. The composite layer has metal ion conductivity of at least 10-6 (preferably 10-6-10-2) S/cm.    USE - In a protective membrane architecture that is useful for protecting active metal anode of a battery cell e.g. a primary cell, a rechargeable cell (claimed) such as active metal/aqueous cell, active metal water cells, active metal/air cells, active metal/metal hydride cells (e.g. lithium/nickel battery, lithium-silver battery, Li/water battery and hydrogen generator for fuel cell).    ADVANTAGE - The presence of through-porosity renders the base component pervious to fluids, such as air and liquid electrolytes (e.g. non-aqueous and aqueous electrolytes). By impregnating the base component with filler material to form a filler component that is able to plug the pathways and effectively close off through-porosity, the composite solid electrolyte is rendered substantially impervious while retaining active metal ion conductivity. The composite solid electrolyte has room temperature (RT) metal ion conductivity of 10-6-10-2 S/cm. Since the composite solid electrolyte acts as a material layer in protected anodes such that an active metal anode is protected from adverse reaction with aggressive environments e.g. air, moisture, and battery cell components that are incompatible in contact with the anode (e.g., aqueous and non-aqueous catholyte).    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) a method of fabricating the electrolyte, involving impregnating the base component with the filler material to form a fluid barrier filler component in the through pores of the base component such that the composite solid electrolyte is rendered substantially impervious;    (2) a protected anode, comprising an active metal anode having the two surfaces; a protective membrane architecture on at least the first surface of the anode. The architecture has an ionic conductivity of the active metal of at least 10-6 S/cm and comprises the composite solid electrolyte;    (3) a method of fabricating the protected anode, involving forming a laminate of the active metal anode, a first component layer adjacent to the active metal anode that is ionically conductive and chemically compatible with an active metal, and the composite solid electrolyte layer adjacent to the first layer that is substantially impervious, active metal ion conductive and chemically compatible with the first layer material;    (4) a battery cell, comprising the protected anode; and a cathode structure.