Impact of Total Ionic- and Individual Li+-Conduction in Dual-Cation Electrolytes on Power Performances for Thick Electrode Li4Ti5O12//AC Hybrid Capacitors

The utilization of dual-cation electrolytes constructed with a Li-based electrolyte (LiBF4) and an additional supporting electrolyte (quaternary ammonium salts or ionic liquids) is a promising strategy toward simultaneously achieving thick-electrode (∼100 μm) Li-based energy storage devices with high power and high energy densities. We observed that 1-ethyl-3-methylimidazolium tetrafluoroborate was suitable for the dual-cation electrolyte in a Li4Ti5O12 (LTO)//activated carbon (AC) (LTO//AC) hybrid capacitor, showing 64% capacity retention at a high current density of 200 mA cm–2. The EMI-based dual-cation system exhibited the highest total ionic conductivity and output characteristics compared to other dual-cation systems even with lowered Li+ transport in the bulk electrolyte. The simulation of the charge-transfer resistance (Rct) based on the transmission line model shows that the dual-cation electrolyte with high ionic conduction mitigates the Rct distribution within the macropores of a 200 μm thick LTO electrode, resulting in a decrease of the mean value of Rct. A combination of spectroscopic analysis and electrochemical characterization at different electrolyte compositions revealed that a good balance between the total ionic and individual Li+-conductivities should be found by controlling the concentration ratio between Li-based and supporting electrolytic salts in order to extract the maximum performance of the thick-electrode LTO//AC systems.