Replacing the liquid electrolyte in a lithium battery with a solid electrolyte can resolve many inherent safety issues as well as enable the use of next generation electrode materials. Recent research in solid electrolytes, however, has mainly focused on improving ionic conductivity while neglecting compatibility with energy dense anodes such as lithium metal. Herein, we report a new crystalline solid electrolyte devised to be inexpensive, highly conductive, and compatible with lithium metal. This study presents and characterizes an analog to Li10GeP2S12 using the isovalent ion of silicon to displace germanium as a cost effective constituent. The crystal Li10SiP2S12 displays a conductivity of 2.3 x 10(-3) S cm(-1), the highest reported conductivity for an unsintered silicon-based solid electrolyte. Impedance spectroscopy is used to probe interactions between the new superionic conductor and lithium metal revealing a more favorable decomposition than previous metal containing electrolytes. Due to the enhanced compatibility of Li10SiP2S12 with lithium metal, we present one of the longest cycling bulk solid-state lithium metal batteries to date. Discerning the nature of chemical decomposition at the electrolyte-lithium interface is essential to the design and synthesis of future solid electrolytes. (C) 2014 The Electrochemical Society. All rights reserved.