High-voltage electrochemical double layer capacitors enabled by polymeric ionic liquid

•A polymeric ionic liquid is synsthesized and served as an electrolyte additive.•PVEIm.TFSI can improve the electrochemical stability of EMIm.TFSI/PC based electrolytes.•The operational voltage of EDLC with PIL additive increases from 2.8 V to 3.2 V.•Equivalent circle model and bode plots demonstrate changes in interface behaviors.•PIL additive reduces each element of impedance and improves the ions diffusion.