Modulating solvation and electric double-layer configuration for high-voltage supercapacitors

Supercapacitors (SCs) are considered promising next-generation energy storage devices due to their high power density, fast charge / discharge capabilities and long cycle life. However, in traditional acetonitrile (ACN) -based electrolytes, the energy density of SCs is severely limited by the decomposition of ACN and its side reactions with activated carbon electrodes at high voltages. In this work, we report a localized high-concentration electrolyte (LHCE, 2 M spiro(1,1′)-bipyrrolidinium bis(fluorosulfonyl)imide (SBP-FSI) / (ACN and fluorobenzene (FB)), with a molality ratio of 1: 3.38), which exhibits an exceptionally wide electrochemical stability window of 5.73 V. Molecular dynamics (MD) simulations of the planar graphene and slit-pore electrode system using constant potential method (CPM) reveal strong "SBP⁺ - ACN" and "FSI⁻ - ACN" solvation, along with the extensive adsorption of "inert" FB molecules onto the electrode surface. This forms a protective electric double layer (EDL) structure, effectively isolating ACN and enhancing voltage tolerance. Cylindrical SCs retained 88.7 % of its capacitance after 15,000 cycles at 3.2 V with the LHCE-2 M electrolyte, closely matching the cycling stability of commercial cylindrical SCs at 2.7 V. These results highlight the improved electrochemical performance of the novel electrolyte formulation, offering a promising solution for next-generation high-voltage SCs.