Optimal battery electrolyte materials provide for high transport of a single ionic species. Here we report solid single-ion conductors (SICs) based on composite polymer electrolytes consisting of poly(ethylene glycol) (PEG) and an organic−inorganic component (OIC) formed in situ. The resulting solid films showed transport properties with conductivity as high as 10-4 S/cm at room temperature and a cation transference number of 0.9. The materials were characterized using solid-state NMR, differential scanning calorimetry, X-ray diffraction, transmission electron microscopy (TEM), and impedance spectroscopy. The OIC was formed by sol−gel reaction of the three precursors: the sodium salt of 3-trihydroxysilylpropylmethylphosphonate (SPMP), (3-glycidylopropyl)trimethoxysilane (GLYMO), and tetramethoxysilane, resulting in a high degree of three-dimensional network character of OIC within SIC solid polymer electrolytes as indicated by solid-state NMR. TEM examination of calcined OIC shows that the silicate parti...