Preparing solid electrolytes through solid reaction processes in certain compositions and temperatures leaves the possibility of forming impurity phases in the reaction products, which affect their conductivity. However, the type and quantity of the impurity phase cannot be predicted from the beginning of the reaction. In this study, the Montmorillonite was mixed as a measured parameter with the precursors of the 1.5Li2O-P2O5 compound, to determine its effect on the quality of the reaction product and its conductivity. The material synthesis was carried out by the solid-state reaction method through the melt quenching technique. The characterization was carried out by using Electrochemical Impedance Spectrometry (EIS) and X-Ray Diffractometer (XRD). The value of conductivity is dominated by the conductivity of the dominant phase Li4P2O7. The addition of MMT affected the lattice parameter of the dominant phase, increased the conductivity, and reached a maximum at the addition of 25 wt%. The minor phases have a filler effect in the compound, affected the conductivity, in the form of a matrix to filler ratio of a composite.