Analysis of Ion Transport in Glyme-Based Electrolyte Solutions for Li-Air Batteries

Abstract

In order to elucidate the Li+ ion transport in glyme-based electrolyte solutions for Li-air batteries, LiCF3SO3(LiTfO) or LiN(SO2CF3)2(LiTFSI) dissolved glyme (triglyme(G3), tetra-glyme(G4) and pentaglyme(G5)) electrolytes were prepared, and the self-diffusion coefficients D of Li+ ion (7Li), anion (9F) and glyme (1H) solvent were measured individually by a pulsed-field gradient spin echo (PGSE-)NMR [1-3] in the temperature range of 30oC to 60oC. The viscosity η, density d and ionic conductivity σ were also measured to analyze the mechanism of ion transport in the glyme-based electrolytes. Moreover, molecular dynamics (MD) simulation was applied to reproduce the D values and investigate the dissociation of Li salts and solvation structure of Li+ion in the glyme solvents. As the results, it was found that the glyme with shorter polyethylene glycol (PEG) chains gave lower η (Fig. 1) and higher D values (Fig. 2) and LiTFSI-based electrolytes exhibited higher σ values (Fig. 3) because of high apparent degree of dissociation α app (Fig. 4), which were estimated by using Nernst-Einstein equation, for an increase in the amount of Li+ carriers. The trend was kept in the examined temperature range from 30oC to 60oC. However, the a value for the Li salts was decreased by an increase in the temperature, implying a reduction of the interaction energy between the Li+ ion and glyme solvents to form a metal complex. On the other hand, the a value was increased by the concentration of both Li salts in the electrolytes up to the molar ratio of 1:1 although the viscosity was also increased. Consequently, the highest σ values were attained both for 1 M LiTfO and LiTFSI-based electrolytes. From the results of MD simulation, the above trend of D values for ions and solvents were clearly reproduced and the larger amount of ion pairs of LiTfO was also confirmed than those of LiTFSI in the glyme solvents. The distance between Li+ ion and anion was much expanded for the LiTFSI compared with that for the LiTfO. Based on those results, we further investigated on the other new Li salt, LiN(SO2F)2(LiFSI), and influence of impurity such as H2O from air to the ion transports. 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.