Electrical double-layer capacitors offer high power density and long cycle life but are limited by moderate energy density. We investigate a strategy to improve their performance using quaternary electrolytes containing two distinct cations and two distinct anions. Our theoretical analysis shows that such electrolytes outperform pure ionic liquids and conventional mixtures sharing a common ion. We validate this approach experimentally using [EMIM][BF4] mixed with lithium salts, characterizing their local structure and electrochemical behavior via NMR, Raman spectroscopy, conductivity measurements, and electrochemical testing. We further demonstrate that the enhancement depends sensitively on electrode microporosity, underscoring the interplay between electrolyte composition and pore structure.
2026, 9 (1), 79-85.