In recent years there has been a resurgence in the use of Li metal as an anode, because such use can to lead to rechargeable batteries with higher energy than currently exist. Using a Li anode with a liquid electrolyte can lead to dendrite shorting and flammability issues. To overcome these problems Li-ion conducting solid electrolytes are attracting considerable attention. The major requirements for the solid Li-ion conducting electrolyte are: high Li-ion conductivity, low electronic conductivity, high relative density, chemical/electrochemical stability with the Li anode and cathodes, adequate mechanical properties. One possible electrolyte under consideration based on the NaSCION structure is Li_1.2Zr_1.9Sr_0.1(PO₄)₃. It is the purpose of this presentation to discuss the following: 1] ionic/electronic conductivity 2] elastic modulus/hardness/fracture toughness and 3] Li /water stability of dense (~97%) fine-grain (~10 mm) Li_1.2Zr_1.9Sr_0.1(PO₄)₃ consolidated by reactive hot-pressing. The results will compared to other Li-ion solid-state electrolyte conductors based on the NaSICON, perovskite and garnet structures.