Solid composite electrolytes are prepared by polymerizing a liquid precursor containing Li 6.25 Ga 0.25 La 3 Zr 2 O 12 (Ga-LLZO) nanoparticles. The composite electrolyte shows an ionic conductivity of 1.8 × 10 −4 S cm −1 and a Li-ion transference number of 0.58 at 30 °C. Moreover, polymerized composite electrolytes (PCEs) are electrochemically stable up to 6.5 V versus Li/Li + at room temperature. Li/PCEs/Li symmetric cells maintain stable cycling for more than 1000 h at 0.5 mA cm −2 , and the dendrite formation is effectively inhibited, showing compatibility with lithium anodes. Using in situ polymerization, PCEs retain conformal interfacial contacts with all electrodes, and the interfacial resistance of solid-state batteries is decreased by half. Coupled with LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) and LiNi 0.85 Co 0.05 Al 0.1 O 2 (NCA) cathodes, solid-state Li metal batteries with PCEs exhibit good cycling performances, including superior specific capacity, high coulombic efficiency and a long cycle lifetime at room temperature. This work offers an approach to meet both the bulk and interfacial requirements of solid-state Li metal batteries and extends the applications of solid composite electrolytes to high-voltage cathodes.