Dual-cation electrolyte systems, which contain two cations [Li + and spiro-1,1′-bipyrrolidinium (SBP + ), are proposed to enhance the power capability of hybrid capacitors composed of thick Li 4 Ti 5 O 12 (LTO) negative (200 μm) and activated carbon (AC) positive electrodes (400 μm), which thus reduces the resistive overvoltage in the system. Detailed studies of the mass transport properties based on the combination of spectroscopy and electrochemical analysis have shown that the presence of SBP + , despite slower Li + transport in the electrolyte bulk, further reduces overvoltage associated with migration limitation in the thick LTO electrode macropores. This study on the dual-cation electrolyte quantifies the influence of the addition of a supporting electrolyte and shows interest in SBPBF 4 addition for increasing the output power density of hybrid capacitors with a thick electrode configuration.

To achieve lithium ion-based energy storage devices having high power densities and stabilities, the use of a low-viscosity and low-dielectric-constant solvent [such as dimethyl carbonate (DMC)] as well as a chemically stable BF4 –-based salt (such as LiBF4 or quaternary ammonium salts [spiro-(1,1)-bipyrrolidinium tetrafluoroborate (SBPBF4))] is promising for application in next-generation electrolytes. However, these combinations are impractical for several reasons, including the low ionic conductivity of LiBF4/DMC and the phase separation of SBPBF4/DMC systems. Thus, we developed a DMC-based dual-cation system (1 M LiBF4 + 1 M SBPBF4/DMC) possessing a higher ionic conductivity (5.7 mS cm–1) than that of single-cation systems (1 M LiBF4/DMC, 0.5 mS cm–1) and realizing a stable single-phase solution. Raman measurements suggest that the dual-cation system constitutes one DMC and two or three BF4 – complexes (not neutral-charged states), which should result in high ionic conductivity. Furthermore, the DMC-based dual-cation system exhibited a higher power performance in a Li4Ti5O12//activated carbon hybrid capacitor than the single-cation system (88 and 13% capacity retention at 50 mA cm–2, respectively) and demonstrated high Li+ conductivity (dual-cation: 1.6 mS cm–1, single-cation: 0.2 mS cm–1). Therefore, the dual-cation strategy could aid the development of diverse electrolyte combinations involving salts and solvents that have been considered impracticable.

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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 (R ct) based on the transmission line model shows that the dual-cation electrolyte with high ionic conduction mitigates the R ct distribution within the macropores of a 200 μm thick LTO electrode, resulting in a decrease of the mean value of R ct. 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.

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Full cells employing Li 3 VO 4 (LVO) and Li 3 V 2 (PO 4 ) 3 (LVP) as anode and cathode, respectively, are energy storage devices offering high power and cyclability. Such full cells, termed as LVO//LVP, were constructed in this study, and they exhibited low capacity retention (72 %) over 1000 cycles at a high temperature of 50°C. We clarified the capacity degradation mechanism using chargedischarge cycling simulations based on a difference in coulombic efficiency (CE) between two electrodes with/without a capacity decay at electrode materials. Simulation results indicate that the low CE of LVO accompanied with a cyclic capacity decay of LVO was responsible for the full cell capacity degradation. The LVO capacity decay was further elucidated by experimental evidences, showing that the cycled LVO was covered by resistive polymeric films derived from the electrolyte reductive decomposition. Indeed, the capacity retention of full cell cycling was improved to 86-96 % by mitigating the effect of such side reaction, demonstrating the credibility and effectivity of our simple cycling simulation. Our finding may help to elucidate the degradation mode of the full cell cycling with less experimental efforts and work out own strategy to mitigate the degradation.

Mg2+ secondary batteries are remarkably safe, resourceful, and exhibit high energy density. However, the excessively slow reaction kinetics at Mg2+-battery cathode materials results in charge–discharge over 10–60 h at room temperature, hindering the performance evaluation and mechanistic analysis of the electrode materials. In this study, we developed a dual-salt electrolyte comprising a conventional magnesium salt, magnesium bis(trifluoromethanesulfonyl)imide [Mg(TFSA)2], and a quaternary ammonium salt, spiro-(1,1′)-bipyrolidinium tetrafluoroborate (SBPBF4), for achieving high-rate performance in the cathode reaction. In a charge–discharge test conducted using a highly defective FePO4 cathode, the dual-salt system [0.5 M Mg(TFSA)2 + 0.5–2.0 M SBPBF4] showed a high capacity of over 150 mAh g–1 at 0.5C-rate, even at room temperature. In situ X-ray absorption fine structure measurements demonstrated the Fe2+/Fe3+ redox reaction of the FePO4 cathode during the charge–discharge, whereas Raman analysis and molecular dynamics simulation indicated that the multiple-anion-coordinated [Mg2+–BF4 –] structure was more effective in facilitating Mg2+ insertion/extraction than the [Mg2+–TFSA–] structure, which has a lower number of coordinated anions. These findings indicate that the Mg2+ insertion/extraction at the cathode/electrolyte interface is drastically improved by using a combination of typically used electrolytic salts as the electrolyte. This strategy enables rapid evaluation of the electrochemical performance of various Mg2+-battery cathodes without high-temperature and prolonged operation.