Systematic Investigation on Ion Transport Mechanism in Glyme-Based Electrolytes for Li-Air Batteries

Abstract

In order to improve the cell performance of Li-air batteries, it is quite important to understand the Li+ ion transport mechanism in glyme-based electrolytes. In this study, we selected mainly two type of Li salts, i.e. LiCF3SO3(LiTfO) and LiN(SO2CF3)2(LiTFSI) and prepared various concentration electrolyte solutions (0.5 M, 1 M, 2 M, etc.) by dissolving in tetraglyme(G4) as solvent. Based on the view point of the mobility of carrier ions, the viscosity η of electrolytes, self-diffusion coefficients D of cation (3Li), anion (9F) and G4 (1H) were measured by viscometer (Lovis2000ME, AntonPaar) and a pulsed-field gradient spin echo (PGSE-)NMR [1-3] in the temperature range of 30°C to 60°C. The ionic conductivity σ of electrolytes was also examined and the apparent degree of dissociation α app of Li salts was estimated by using Nernst-Einstein equation to consider the concentration of carrier ions in the electrolytes. In addition, the D values were reproduced by a molecular dynamics (MD) simulation (Forcite Plus, BIOVIA) to investigate the dissociation of Li salts and solvation structure of Li+ion in the glyme-based electrolytes. From the result of viscosity, the electrolyte with higher Li salt concentration exhibited higher viscosity for both Li salts and the LiTFSI-based electrolytes gave higher η values at the same concentrations. This indicates that at least a part of Li salts was dissociated in the G4 to form Li+-G4 complexes and the LiTFSI salt is easier to dissociate in the G4. As the results, the mobility of carrier ions would be assumed to decreases with increasing the concentration especially for the LiTFSI-based electrolytes. Fig. 1 shows the D values of ions and G4 solvent in the electrolytes at 30oC. In fact, the D value decreased with an increase in the concentration of Li salts, and the LiTFSI-based electrolytes exhibited lower D values than the LiTfO-based ones. On the other hand, the highest σ values were obtained at the concentration of 1 M and the opposite trend was confirmed in comparison between the Li salts. This means that the amounts of carries ions increased with an increase in the concentration especially for the LiTFSI-based electrolytes. The α app values estimated from Nernst-Einstein equation actually increased by the concentration of both Li salts up to the molar ratio 1:1 and the magnitude of α app values became 3 to 4 times larger for the LiTFSI-based electrolytes. Consequently, the highest σ values were attained for 1 M LiTFSI/G4. Moreover, the D values for ions and solvents were clearly reproduced by the MD simulation at 30oC. Fig. 2 shows the radial distribution function from Li+ ion. The distance between Li+ ion and anion was much expanded for the LiTFSI compared with that for the LiTfO, implying the better dissociation of LiTFSI salt in the G4. Based on those results, we further investigated on the effects of other glyme (G3 and G5) and new Li salt, LiN(SO2F)2(LIFSI), and influence of impurity such as H2O from air on the ion transport. The reports will be presented in the meeting. This study was supported by JST “Next Generation Batteries Area in Advanced Low Carbon Technology Research and Development Program (ALCA)” and “A Tenure-track Program” from MEXT, Japan. [1] M. Saito et al., in the 56thBattery Symposium in Japan, 3G25 (2015). [2] K. Hayamizu et al., J. Phys. Chem. B, 103, 519 (1999). [3] Y. Aihara et al., J. Chem. Phys., 113, 1981 (2000). Figure 1