Dual-cation electrolytes for low H2 gas generation in Li4Ti5O12//AC hybrid capacitor system

•Dual-cation system (LiBF4 with SBPBF4) suppressed H2 gas in LTO//AC hybrid capacitor.•50% reduction of generated H2 volume was achieved by use of dual-cation electrolyte.•Generated H2 reduction owing to the suppressed PC/H2O decomposition on LTO surface.•SBP+-derived Lewis-based passivates LTO surface to slow PC/H2O decomposition.•Dual-cation simultaneously achieves high power and low H2 gas generation for LTO//AC. H2 gas generation with associated swelling using Li4Ti5O12 (LTO)-based hybrid capacitors upon high-voltage operation over 3.0 V has been a major challenge owing to catalytic reductive decomposition of solvent such as propylene carbonate (PC) at the Lewis-acid sites on the LTO surface. Herein, we report a method to suppress gassing behavior by adding spiro(1,1′)-bipyrrolidinium tetrafluoroborate (SBPBF4) to the lithium-based electrolyte [1 M lithium tetrafluoroborate (LiBF4) in PC]. The dual-cation (Li+/SBP+) electrolyte, which has previously been reported as the electrolyte composition used for rate-capability enhancement, reduced 35–50% volume of generated H2 compared to that for the single-cation (Li+) electrolyte under high-voltage floating conditions (3.5 V, 60 °C). This behavior was attributed to the LTO surface passivated by the Lewis-based layer via Hofmann elimination of SBP+. The concept of passivation of the LTO Lewis-acid site in the presence of SBP+ was further confirmed by applying alternative quaternary ammonium cations, such as tetraethylammonium (TEA+) and triethylmethylammonium (TEMA+), resulting in reduced H2 generation to the same level as that obtained using the SBP+-based dual-cation. [Display omitted]