Solid polymer electrolytes (SPEs) are considered to be an important move to revive high-energy-density lithium-metal batteries due to their good flexibility and high safety. However, the low room-temperature ionic conductivity of SPEs has always been a stumbling block for their practical applications. Herein, a composite SPE has been fabricated by the cooperation of polyethylene oxide (PEO) and a type of natural nanoclay-halloysite nanotube (HNT) in a poly(vinylidene fluoride) (PVDF) matrix. It is expected that the sticky PEO can improve the interfacial stability of the SPE and lithium foil, while the special structure and surface charge properties of the HNTs changed the coordination environment of lithium ions and facilitated Li + running on highways along the HNT outer surface. The optimized SPE composite showed an outstanding roomtemperature ionic conductivity of 2.45 × 10 −4 S cm −1 and high ion transference number of 0.67 at 25 °C. A LiFePO 4 /SPE-H5/Li full battery retained a discharge capacity of 142 mAh g −1 after 200 cycles at a rate of 0.2 C (25 °C). In addition, SPE-H5 can also be used in a 4.3 V high voltage NCM/SPE-H5/Li battery due to its excellent electrochemical stability window (more than 5 V). This work also reveals that lower-cost natural clay minerals are excellent nanoceramic fillers in realizing sustainable high-energy-density energy storage.