The low ionic conductivity and poor interfacial contact are the main obstacles restricting the practical application of all-solid-state polymer electrolyte in lithium metal batteries. Herein, four-armed and imidazolium cation-tethered polymeric ionic liquid (IMFPIL) is prepared through atom transfer radical polymerization of hydroxyethyl acrylate and subsequent functionalization. The branching architecture of the organic scaffold endows IMFPIL with high thermal stability, low glass transition temperature and loosely packed polymer backbones. The derived more free paths and volumes for Li-ion migration confer much enhanced ion conductivity on the as-prepared all-solid-state polymer electrolyte (IMFSPE), which is 22 times that of the linear counterpart with the same composition. High-temperature thermal integration of IMFSPE into the lithium metal battery effectively eliminates the gaps between the electrolyte and the two electrodes, rendering excellent interfacial contact and stability. The volume variation of the electrodes during the charge-discharge can be effectively mediated. As a result, in sharp contrast to the battery failure of the linear counterpart, such an integrated LiFePO4/IMFSPE/Li all-solid-state battery presents a high discharge capacity of 153 mAh g−1 at 0.2 C with 99% of coulombic efficiency, and the capacity retention ratio reaches 73% after 150 cycles. Compared with IMLSPE, IMFSPE-2 efficiently weakens the formation of dendritic lithium. The branching design combining with the thermal integration strategy and superior electrochemical features of imidazole make IMFSPE behave as an all-solid-state polymer electrolyte, which paves a new way for developing PILs as ideal SPEs in energy storage device.