Tailoring high concentration electrolytes for supercapacitors: the impact of anion structure on ion transport and charge storage

Electric double-layer capacitors (EDLCs) are promising energy storage devices due to their high power density and long cycle life, but their lower energy density compared to batteries remains a major limitation. Highly concentrated electrolytes present a safer and more sustainable alternative to conventional organic electrolytes, while retaining the ability to operate at high-voltage. Since many water-in-salt (WIS) electrolytes are based on fluorinated anions, the length of the fluorinated chain plays a critical role in determining electrolyte properties. In this study, we investigate the impact of anion structure by comparing three fluorinated imide-based salts, i.e. LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiPFSI (lithium bis(pentafluoroethanesulfonyl)imide), and LiNFSI (lithium bis(nonafluorobutanesulfonyl)imide), for supercapacitor applications. Electrochemical characterization, Raman spectroscopy, and solid-state nuclear magnetic resonance (NMR) reveal that LiPFSI, with its higher proportion of double-donor single-acceptor (DDA) hydrogen bonding profiles, forms a highly coordinated anionic network at an optimal concentration of 5 mol kg−1. Solid-state NMR spectroscopy shows that LiPFSI preferentially occupies the in-pore environment of nanoporous carbon, achieving a favourable ion to solvent balance that supports efficient charge storage and enhanced capacitance. In contrast, although LiNFSI can form dense ion agglomerates at high concentrations, the bulky anion and associated higher viscosity restrict ion transport and pore accessibility. These findings highlight the importance of anion selection and concentration, and pore-level electrolyte distribution in tailoring high-concentrated electrolytes for next-generation high-voltage, high-capacity supercapacitors.