Hybrid membranes composed of 90 wt% Li + -conducting inorganic electrolyte (lithium aluminum germanium phosphate, LAGP) and 10 wt% poly(vinylidene fluoride-co-hexafluoropropylene) (P(VdF-co-HFP)) polymer were prepared in the form of a flexible thin film and directly formed on the as-prepared negative electrode. The lithium-ion cells assembled with the hybrid membrane exhibited superior cycling performance in terms of discharge capacity, capacity retention, rate capability and high temperature cycling stability, as compared to the cell with polypropylene separator and liquid electrolyte. The use of hybrid membranes allowed improve thermal properties compared to conventional polyolefin separator and use less amount of flammable liquid electrolyte, resulting in enhancement of thermal safety of the cell. Lithium-ion batteries have been predominantly used as power sources for portable electronic devices and are now being extensively developed as potential power sources for electric vehicles and energy storage systems because of their high energy density and long cycle life.1-8 However, the full utilization of these batteries is still challenging for future large-capacity energy storage applications due to the safety issues caused by the flammable nature of the liquid electrolyte. In addition, the polyolefin separators used in current lithium-ion batteries may shrink and even melt at elevated temperatures, which causes a short circuit between two electrodes in cases where unusually high heat is generated, leading to fire and explosion. [10][11][12] Furthermore, the large difference in polarity between hydrophobic polyolefin separators without surface treatment and polar organic solvents leads to poor wettability, resulting in high ionic resistance during cycling. 13 In this respect, extensive studies have been carried out on inorganic solid electrolytes without a polymer separator as an alternative electrolyte for improving the safety of lithium-ion batteries.14-18 However, many inorganic materials require a thermal sintering at high temperatures to form a pellet-type solid electrolyte. In addition, a lack of flexibility results in poor interfacial contact between the inorganic solid electrolyte and solid electrodes in the cell during charge and discharge cycling. To solve these problems, the hybrid solid electrolytes composed of an inorganic electrolyte and flexible polymer have been investigated. [19][20][21][22][23] However, it is still challenging to secure competitive cycling performance when compared to liquid electrolyte-based lithium-ion batteries. The most critical issue to be solved in cells employing an inorganic electrolyte is high interfacial resistances between electrolyte and electrodes due to the solid-on-solid interface in the cells. Therefore, it is highly desirable to minimize the interfacial resistances at solid-solid interfaces while enhancing battery safety and maintaining a cycling performance competitive to that of lithium-ion batteries that employ conventional polyolefin separator and organic ...