Inspired by the concept of“polymer-in-ceramic”,a composite poly(ε-caprolactone)(PCL)/ceramic contain-ing LiTFSI is prepared and investigated as a solid electrolyte for all-solid-state batteries.The composite with the optimum concentration of 45 wt％LiTFSI and 75 wt％Li1.5Al0.5Ge1.5(P04)3(LAGP,NASICON-type structure)exhibits a high ionic conductivity(σi=0.17 mS cm-1)at 30 ℃,a transference number of 030,and is stable up to 5.0 V.The composite electrolyte is a flexible and self-standing membrane.Solid-state LiFeP04//Li batteries with this composite electrolyte demonstrate excellent cycling stability with high dis-charge capacity of 157 mA h g-1,high capacity retention of 96％and coulombic efficiency of 98.5％after 130 cycles at 30 ℃ and 0.1 C rate.These electrochemical properties are better than other PCL-based all-solid-lithium batteries,and validate the concept of“polymer-in-ceramic”by avoiding the drawback of lower conductivity in prior“polymer-in-ceramic”electrolyte at high concentration of the ceramic.