In a classic example of stability from instability, we show that Li2OHCl solid electrolyte forms a stable solid electrolyte interphase (SEI) layer with a metallic lithium anode. The Li2OHCl solid electrolyte can be readily achieved through simple mixing of LiOH and LiCl precursors at a mild processing temperature <400 °C. Additionally, we show that continuous, dense Li2OHCl membranes can be fabricated at temperatures <400 °C, standing in great contrast to current processing temperatures of >1600 °C for most oxide-based solid electrolytes. The ionic conductivity and Arrhenius activation energy were explored for the LiOH-LiCl system of crystalline solid electrolytes, where Li2OHCl with increased crystal defects was found to have the highest ionic conductivity and reasonable Arrhenius activation energy. The Li2OHCl solid electrolyte displays stability against metallic lithium, even in extreme conditions past the melting point of lithium metal. To understand this excellent stability, we show that SEI formation is critical in stabilizing the interface between metallic lithium and the Li2OHCl solid electrolyte.