The physical and electrochemical properties of a 60% Li3PO4-40% Li4SiO4 (60LP) solid solution phase have been investigated with the goal of developing an ion conductive protective layer for metallic lithium in secondary battery configurations. This phase was chosen because it has the highest ionic conductivity of 4 x 10(-6) S/cm at room temperature in the Li3PO4-Li4SiO4 solid solution series. Addition of 0.5 wt.% lithium borate or a mixture of its precursors lowered the sintering temperature of 60LP by as much as 150 degrees C without compromising the ionic conductivity of the dense pellets. Both AC and DC experiments showed that a stable interface is formed between the lithium and the phosphosilicate layer, allowing reversible lithium stripping and plating. The results are consistent with the assumption that the primary mobile species in the ceramic layer are lithium ions. (c) 2012 Elsevier B.V. All rights reserved.