NOVELTY - A substantially impervious composite solid electrolyte, comprises: a base component containing 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, where the composite layer has metal ion conductivity of at least 10-6 S/cm.    USE - As substantially impervious composite solid electrolyte layer or as substantially impervious composite solid structure in protected anode. The protected anode used in battery cell (where the cell is a primary cell or rechargeable cell) (all claimed) for portable electronic devices (such as cellular phones and laptop computers); and in electrochemical cell.    ADVANTAGE - The substantially impervious composite solid electrolyte has metal ion conductivity of at least 10-6 to 10-2 S/cm; has a thickness of 1-25 (preferably 10) microns; allows increased ease of fabrication of cells; maintains properties needed for cell construction such as flexibility, low interfacial resistance and stability to the electrolyte; provides efficient exclusion of undesirable species from the lithium surface, which can lead to dendrite formation, self discharge, and loss of cycle life; and provides many benefits such as increased tolerance of the volumetric changes which accompany the migration of lithium back and forth from the anode during the cycles of discharge and charge of the cell, and improved robustness to withstand stresses during manufacturing processes; and also provides cells with long cycle life, high lithium cycling efficiency, and high energy density.    DETAILED DESCRIPTION - A substantially impervious composite solid electrolyte, comprises: either a base component containing 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; or separator containing an inorganic solid electrolyte matrix having pores connected in a continuous fashion from one outermost surface of the matrix through to the other outermost surface of the matrix, and a filler component contained in the pores of the separator and providing a barrier to fluids, where the composite layer has metal ion conductivity of at least 10-6 S/cm. INDEPENDENT CLAIMS are included for following:    (1) a protected anode (A1), comprising either an active metal anode having a first and second surface, a protective membrane architecture on at least the first surface of the anode, where the architecture having ionic conductivity of the active metal of at least 10-6 S/cm, where the protective membrane architecture comprises the impervious composite solid electrolyte; or an active metal anode having a first and second surface, a protective structure on at least the first surface of the anode, where the protective structure having ionic conductivity of the active metal of greater than 10-7 ohm-1cm-1, where the protective structure comprises a impervious composite solid structure; and    (2) a battery cell (C1), comprising the protected anode (A1) and a cathode structure.