Investigation of Electrical Double Layers of Ionic Liquids in Graphene-Capped Liquid Cells by Multimodal Synchrotron Infrared Nanospectroscopy for Advanced Energy Storage Devices*

Electrical double layer capacitors (EDLCs) are prominent energy devices owing to their rapid charge/discharge processes and extremely long cycle life. However, the generally low energy density of EDLCs hinders their broad application. A major strategy to improve the energy density of EDLCs involves employing electrolytes capable of high operation voltages. Ionic liquids (ILs) are a novel class of electrolytes typically composed of asymmetric organic cations and weakly coordinated anions. They appear to be ideal candidates to replace the diluted aqueous and organic electrolytes in EDLCs for their wide electrochemical windows, which enable operation voltages much higher than conventional EDLCs and thus significantly improve the energy density. Moreover, the unique EDL structure of ILs may present great possibilities for superior performance to traditional electrolytes. The implementation of ILs as electrolytes in EDLCs for enhanced performance and functionalities is contingent upon understanding the behaviors of IL EDLs, a knowledge gap that exists thus far. Here, we incorporate ILs in custom-made graphene-capped liquid cells, which allow for synchrotron infrared nanospectroscopy (SINS) investigation of IL EDLs, in combination with multimodal atomic force microscopy characterizations. This approach effectively correlates the chemical and vibrational bond information of IL EDLs with their nanoscale ion ordering and charge distribution, leading to a comprehensive understanding of IL EDLs that was previously inaccessible.