The composite polymer electrolytes, consisting of a flexible matrix and superb fast ion conductor are promising for large-scale practical application in safer lithium metal batteries. The quantification of polymer structure evolution induced by the inorganic filler was rarely reported. Here, a designer cubic argyrodite Li6PS5X sulfide electrolyte was fabricated by a solid reaction. A stoichiometry deviation by increasing S and Li, reducing Cl contents, leads to a novel electrolyte of Li6.25PS5.25Cl0.75 with high conductivity of 1.0 mS cm(-1) and low activation energy of 0.14 eV at room temperature. The off-stoichiometry Li6PS5X sulfide electrolyte Li6.25PS5.25Cl0.75 was used as active filler for polyethylene oxide. Small angle X-ray scattering experiment indicates that theLi(6.25)PS(5.25)Cl(0.75) triggers a pronounced structure evolution in the matrix. As a result, Li-ion diffusion pathway is expanded significantly by the Li6.25PS5.25Cl0.75 additives. An optimum ionic conductivity of 1.4 mS cm(-1) at 70 degrees C for CPEs-1 was obtained, and a LFP/Li demo battery delivers a high capacity retention of 91.1% for 100 cycles at 0.2 C.