In this work, we have successfully synthesized a novel imidazolium polymeric ionic liquid containing polyhedral oligomeric silsesquioxane and use it as host polymer to develop novel solid polymer electrolytes with high room temperature ionic conductivity, good cycle performance and rate capability. The polymeric polyhedral oligomeric silsesquioxane ionic liquids based solid polymer electrolytes exhibit highest room temperature ion conductivity up to 4.82 × 10−4 S cm−1 and outstanding electrochemical stability up to 5 V relative to Li+/Li. Moreover, the Li/LiFePO4 cells containing the novel solid polymer electrolytes exhibit excellent cycle performance and rate capability, delivering an initial discharge capacity of about 151.8 mAh g−1 at a rate of 0.1 C at room temperature with excellent capacity retention after 100th cycles.