Polymeric ionic liquid electrolytes have received great attention for use in next-generation solid lithium-ion batteries. However, most polymeric ionic liquid electrolytes require a high proportion of organic liquid plasticizer to improve the room temperature ionic conductivity. This comes at the expense of sacrificing the mechanical strength of the electrolytes. In addition, it is also difficult to establish a clear relationship between polymeric ionic liquid segments and properties of the electrolyte. Here, a copolymeric ionic liquid poly(N-(1-vinylimidazolium-3-butyl)-ammonium bis(trifluoromethanesulfonyl)imide)-co-poly(poly(ethylene glycol) methyl ether methacrylate) is synthesized and the corresponding self-standing flexible all-solid electrolyte (PVIMTFSI-co-PPEGMA/LiTFSI) is prepared via a solution-casting method. The effect of the polymeric ionic liquid segments on comprehensive properties of the electrolyte is investigated in detail by comparing the performance of PVIMTFSI-co-PPEGMA/LiTFSI with PVIM-co-PPEGMA/LiTFSI (poly(N-vinlyimidazole)-co-poly(poly(ethylene glycol) methyl ether methacrylate) based electrolyte) which contains un-ionized imidazole groups. As a result, the electrochemical window slightly decreases from 5.5 V to 5.3 V, but the ionic conductivity and interfacial behavior are both greatly improved. In Particular, the polymeric ionic liquid segments can restrain the formation of lithium dendrites. Moreover, a discharge capacity of 136 mAh g−1 at 0.1 C can be realized for the PVIMTFSI-co-PPEGMA/LiTFSI-based LiFePO4/Li battery that is maintained at approximately 70 mAh g−1 at 1 C at 60 °C, which is superior to that obtained for the PVIM-co-PPEGMA/LiTFSI electrolyte.