Mass Transfer Parameters and Ionic Association of Dual-Cation Electrolyte for Hybrid Capacitor System

[Introduction] Hybrid capacitors, which combine an activated carbon (AC) electrode with a large-capacity faradic (psuedocapacitive or battery) electrode, are the focus on an increase in energy density of electric double layer capacitors (EDLCs). One representative faradic electrode material for such hybrid systems is lithium titanate (Li4Ti5O12, LTO), which was used as a negative electrode in one of the first hybrid systems1) and has been extensively investigated since then 2). In this connection, we have reported a dual-cation electrolyte system composed of lithium tetrafluoroborate (LiBF4) and spiro-(1,1’)-bipyrrolidium tetrafluoroborate (SBPBF4) dissolved in propylene carbonate (PC), where the latter cation gives high ionic conductivity in the PC-based electrolytes3). The dual-cation electrolyte system raised rate capability of LTO/AC hybrid capacitors to a level comparable to EDLCs, thanks to the enhancement of LTO reaction kinetic. Still, the detailed mechanism of improved rate capability by dual-cation system has not been completely understood. The difficulty of the analysis lies in the electrolyte containing multiple ion spices, whose contribution to the rate capability cannot be simply divided into individuals. The purpose of this study was to evaluate the contribution of individual ionic species by using pulsed gradient spin-echo (PGSE) NMR technique. Specifically, by determining the self-diffusion coefficient of dual cation electrolyte system, we attempted to break down the ionic conductivity of the whole electrolyte system into individual ionic species and to calculate a carrier number of ions. [Experimental] Electrolytes used in this report were prepared by dissolving 1 M LiBF4 and x M SBPBF4 in PC (0<x<3). All the electrolyte was prepared by mixing components with magnetic stirrer overnight in the Ar-filled glove box. PGSE-NMR was conducted to determine self-diffusion coefficients of Li+(7Li), SBP+(1H) and BF4 -(19F) in electrolytes with maximum gradient strength of 13.5 T/m at different durations. The interval between pulse gradients was set as 50 ms. Ionic conductivity of prepared electrolytes was measured by electrochemical impedance spectroscopy (EIS). [Results and discussion] Self-diffusion coefficients of different ions obtained from PGSE-NMR were plotted in Figure 1, with different concentration of SBPBF4 (0, 1, 2, and 3 M) in the 1 M LiBF4/PC electrolyte. Diffusion coefficients for all three ions decreased with an increment of added SBPBF4 concentration, while the value of SBP+ is higher compared to other two ions, especially Li+, in all cases. Based on obtained self-diffusion coefficients, we calculated the transport number t x of ions (x = Li+, SBP+, or BF4 -) according to the following equation(1) t x = N x D x /(N[Li+]D[Li+] + N[BF4 -] D[BF4 -] + N[SBP+] D[SBP+]) (1) where N x and D x is a concentration and self-diffusion coefficient of corresponding ions, respectively. Using this t x, we further calculated ionic conductivity of each ion by simply multiplying electrolyte ionic conductivity by t x. The evaluated values are shown in Figure 2. Like diffusion coefficients shown in Figure 1, Li+ ionic conductivity decreased with an increment of SBPBF4 concentration. Interestingly, cations ionic conductivity, which is sum of Li+ and SBP+, kept increased with an addition of SBPBF4. In order to discuss cations dissociation in electrolytes with presentence of SBPBF4, we evaluated number of carrier for Li+ based on the peak shifts observed in 7Li NMR spectra and theory of Haven ratio where degree of dissociation is considered as λEIS/λNMR. Here again, with a SBPBF4 concentration increase, number of Li+ carrier decreased, while number of SBP+ carrier increased. In conclusion, a dual-cation electrolyte (Li+ and SBP+) decreases Li+ ionic conductivity and its number of carrier, however, increases the two parameters for the SBP+. The present result suggests that the presence of highly conductive and dissociated SBP+ accelerates charge compensation of LTO reaction, and thus enhanced its rate capability. [References] 1) G.G. Amatucci et al., J. Electrochem. Soc., 148 (8) A930 (2001). 2) K. Naoi et al., Acc. Chem. Res., 46 (5), 1075 (2013). 3) T. Ueda et al., 5th International Conference on Advanced Capacitors (ICAC 2016), 2O-03, Otsu, Japan (2016). Figure 1