Pyrrolidinium- and Imidazolium-stabilized 2,2,6,6-tetramethylpiperidin-1-oxyl nitroxyl radical electrolytes for high-voltage and redox-enhanced nonaqueous supercapacitors

Abstract

Nonaqueous organic redox electrolyte-enhanced electrochemical double-layer capacitors (NORECs) are promising candidates for high-energy and high-power grid applications. Unlike traditional aqueous ORECs, NORECs benefit from a significantly wider electrochemical stability window enabled by organic solvents. However, the electrochemical instability of organic redox electrolytes near marginal potentials restricts window optimization. Here, we develop two stable nitroxide radical-based redox electrolytes by introducing propoxy-linked, electron-conjugated methylimidazolium and methylpyrrolidinium cations into 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) cores and coupling with aluminum-stable BF₄⁻ anions, namely MimP-TBF₄ and MpyP-TBF₄, respectively. Theoretical calculations confirm that the electron-withdrawing nature of imidazolium and pyrrolidinium cationic groups delocalizes the electron density of the NO moiety in TEMPO, thereby enhancing the stability of its radical and oxoammonium states. Interestingly, the MpyP-TBF₄ first demonstrates higher ionic conductivity, diffusion coefficient and rate constant compared to MimP-TBF₄ because of the weaker interactions between aliphatic pyrrolidinium cations and acetonitrile, enabling it has faster mass/charge transport dynamics. Consequently, the MpyP-TBF₄ assembled NORECs acquire a 3.2 V high cell voltage, a notable energy density and power density (48.2 Wh kg⁻¹ and 23.9 kW kg⁻¹, respectively), as well as outstanding cycling stability with 88.5 % capacitance retention after 10,000 cycles. This study offers a promising design for organic redox electrolytes in high-performance NORECs.