The lithium ion conductivity of lithium argyrodite can be improved by introducing Cl to tailor the S/Cl disorder in the structure. An ultrafast room temperature lithium ion conductivity of up to 6.4 mS/cm was achieved for Li5.7PS4.7Cl1.3. The synthesis parameters for Li7-xPS6-xClx (x = 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9) are systematically investigated to obtain pure lithium argyrodite phase with high ionic conductivity.