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.
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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