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

Improving the energy density of electrochemical double layer capacitors (EDLC) is an important but challenging research topic. Electrolytes with wide operating potential windows are key materials for current EDLC tech-nology. In this work, an ether branch polymeric imidazole ionic liquid (PVEIm.TFSI) is synsthesized and served as an electrolyte additive in common electrolyte to optimize the working potential ranges of carbon-based EDLC. The addition of PVEIm.TFSI can improve the electrochemical stability of electrolytes and effectively adjust the upper limit potential (PU), lower limit potential (PL). Under the optimal addition ratio of 10%, the operational voltage of EDLC increases from 2.8 V to 3.2 V, which lead to an approximately 41.5% energy density increase compared with EDLC without additive. Cycling stabibity of EDLC containing 10% PVEIm.TFSI is significantly enhanced under both 3.0 V and 3.2 V. Furthermore, an equivalent circuit model and Bode plots are proposed from the electrochemical impedance spectroscopy (EIS) measurements. Both simulated and measured date dedicate that PVEIm.TFSI reduces each element of resistance, which result in more ions movement and smaller energy barrier for the diffusion processes, thereby improving the electrochemical performance of cells.