Solid-state electrolytes with high ionic conductivity, large electrochemical window, good mechanical properties, and easy processability are needed for high-energy solid-state lithium batteries. In this work, composite membranes consisting of lithium garnet (i.e. Li6.4La3Zr1.4Ta0.6O12, LLZTO) particles and Li-salt-free polyethylene oxides (PEOs) are produced as solid-state electrolytes. Li-ion-conducting particles in nano-scale are crucial for the enhancement of conductivity and the membranes containing ~40nm LLZTO particles exhibit conductivities nearly two orders of magnitude larger than those with the micro-scale ones, which is attributed to the difference in specific surface area related to the percolation effect. Compared to the conventional PEO doped with lithium salt, the insulating PEO in PEO:LLZTO membrane electrolyte is conducive to the suppression of lithium dendrite growth owing to prohibition of current flow. With PEO:LLZTO membrane electrolytes in conductivity of 2.1×10−4 Scm−1 at 30°C and 5.6×10−4 Scm−1 at 60°C, the solid-state LiFePO4/PEO:LLZTO/Li and LiFe0.15Mn0.85PO4/PEO:LLZTO/Li cells deliver energy densities of 345Whkg−1 (662WhL−1) and 405Whkg−1 (700WhL−1) (without the package weight or volume) with good rate capability and cycling performance. This study suggests that the conjunction of nano-scale Li-ion-conducting particles and an insulating polymer provides a promising solution to produce powerful solid-state electrolytes for high-performance solid-state lithium batteries.