Analysis of Ion Transport in Solution Electrolytes for Electrochemical Capacitors

In recent years, electric double layer capacitors (EDLCs) have attracted much attention not only for small electric devices as smart phone and tablet PC, but also for large power sources in electric vehicles. The advantage of EDLCs is a high-rate capability and good cycleability, which are due to the fast charge/discharge mechanism by using electric double layer regardless of Faradaic reaction with electrodes. Also, lithium ion capacitors (LICs) with Faradaic reaction electrodes such as graphite anode on the one side have been intensively developed to solve the small energy density of EDLCs. The performances of capacitors (EDLCs and LICs) strongly depend on the electrolyte properties, i.e. the ionic conductivity and electrochemical stability. In this study, we separately measured self-diffusion coefficients D of ions and solvent in the solution electrolytes for EDLCs and LICs by a pulse gradient spin-echo (PGSE) NMR. The viscosity and ionic conductivity were also examined to discuss the relationship with D values. Moreover, the interactions between ions and ion-solvent were evaluated from the DFT calculations.

1 M solutions of (C₂H₅)₄NBF₄(TEABF₄) and LiBF₄ in propylene carbonate (PC) were obtained from Kishida Chemical Co, Ltd. TEAPF₆ (99%, Aldrich), TEATfO (98%, Wako Pure Chemicals) and (C₂H₅)₃CH₃NBF₄ (TEMABF₄) (98%, Tokyo Chemical Industry Co., Ltd.) were used as supporting salts, and dissolved in PC (Wako Pure Chemicals) as a solvent in an Ar-filled dry box to prepare their 1 M solutions. The solution electrolytes were subjected to the PGSE-NMR for measuring the D values of cations (Li⁺(⁷Li), TEA⁺(¹H), TEMA⁺(¹H)), anions (BF₄^-(⁹F), PF₆^-(⁹F)) and solvent (PC(¹H)) [1]. The viscosity and ionic conductivity of electrolytes were evaluated by a viscometer (anton paar) and an AC impedance analyzer (Bio-Logic), respectively. The interaction energies between cation-anion and ion-solvent were estimated by DFT calculations with Gaussian09.

Fig. 1 plots the D values of ions at 30^oC against the ion radius, where the D value of Li⁺ was also plotted at the point of solvated radius by PC [2]. For the EDLC electrolytes, the smaller size of anions (BF₄^-, PF₆^-, TfO^-) showed relatively higher D values than the larger cations (TEA⁺, TEMA⁺). In addition, as for the kind of cation and anion, smaller size of TEMA⁺ and BF₄^- were also exhibited a higher D values than the other larger ions. These facts indicate that there is a clear dependence of ion radius for the EDLC electrolytes. As a result, it was found that the TEMABF₄ was the best supporting salt among them. On the other hand, for the LIC electrolytes smaller size of Li⁺ exhibited lower D value than the larger BF₄^-. Moreover, considering the solvated radius of Li⁺ by PC suggested the reasonable results on the transport behaviors as well as the EDLC electrolytes. This implies that Li⁺ in the LIC electrolytes was strongly solvated by PC, and suppressed the smooth transport compared with BF₄^-. In contrast, the PC solvation of TEA⁺ in the EDLC electrolytes is suggested to be weaker than that of Li⁺ in the LIC ones. Fig. 2 shows the concentration dependence of D values of ions in the representative electrolytes for EDLC (TEABF₄/PC) and LIC (TEABF₄/PC) at 30^oC. For all the concentration, the EDLC electrolyte exhibited higher D values than the LIC one, and the trend observed in Fig. 1 was not changed by the concentration. For both electrolytes, the D values became higher with a decrease in the concentration. This is due to a decrease in the viscosity of electrolytes. In addition, for the LIC electrolyte, promotion of the dissociation of LiBF₄ salt was also suggested to enhance the D values. To elucidate the detail, ion radius against PC (r_ion/r_PC) in the solution electrolytes at 30^oC were estimated by using the Stokes-Einstein equation (Fig. 3) [1]. The r_ion/r_PC values of Li⁺ and BF₄^- for the LIC electrolyte were higher than those of ions in the EDLC electrolytes. This implies that Li⁺ in the LIC electrolyte strongly interacts with PC and BF₄^- to form large solvated ions and ion pairs, respectively. Especially in comparison of the r_ion/r_PC value of BF₄^-, it was found that the LiBF₄ was more difficult to dissociate in PC than the TEABF₄ and the dissociation of LiBF₄ was promoted by the decrease in the concentration. Further analysis of Dvalues and comparison with the other parameters of viscosity, ionic conductivity, etc. will be discussed in the meeting.

This study was supported by JST “A Tenure-track Program” from MEXT, Japan.

[1] M. Saito et al., Abstracts in ACEPS-7, 2P-32 (2013).  [2] M. Ue, J. Electrochem. Soc., 141, 3336 (1994).