Abstract
For various electrochemical devices in practical use, porous solid materials are widely fabricated for holding electrolyte solutions. It is reported that electrolyte solutions related to ion-solvent interaction indicate different properties by coexistence of solid phase[1], and an elucidation of liquid phase properties of solid-liquid interface in the non-aqueous electrolyte solution system is important for improving the performance of energy storage devices such as lithium ion batteries(LIBs). In addition, in late years the study of new electrolyte materials used highly-concentrated electrolytes attracts attention. Dahn et al.[2], report that highly-concentrated aqueous electrolyte solutions improve charge rate and wider overvoltage. It is also very important to investigate properties of the highly-concentrated electrolytes in aiming at the performance enhancement of LIBs. In this study, NMR spectroscopic behavior of non-aqueous LiClO4 solution mixed with fumed silica as model system of gel electrolyte for LIBs, and their dynamic properties and influences to solvent molecules from solid phase or Li+ ion by measurement of the spin-lattice(T1) and spin-spin (T2) relaxation phenomena.
Samples were prepared in Ar glovebox by mixing propylene carbonate(PC) or LiClO4/PC of predetermined concentration (1-3 M) and fumed silica(FS) (200 ± 50 m2 g-1), amorphous silica nanoparticles with self-cohesive properties in NMR tube. A content of liquid phase were ranged from 90-100 vol% by gravimetry. We measured 1H and 7Li NMR spectroscopy (Varian INOVA 400; 9.39T) , spin–lattice relaxation time; T1 measurements, and spin–spin relaxation time ;T2 measurements (XiGo Nanotools Acorn area ; 0.3T).
In 1H NMR spectra, little amount of solid phase (fumed silica) at around 1% affected to NMR signal which was deformed to the broadened band. It was shown that the dynamic properties of the PC molecules drastically decreased in comparison with the system of the liquid system only.
Owing to the shielding effect to the 1H nucleus of the PC molecules by the surface charge of fumed silica, a signal continues to shift to higher magnetic field with the increasing of the solid-phase content without the depending on the LiClO4 concentration. This phenomenon also observed in the water solution system equally. From the calculation of the integrated intensity of the 1H NMR signals, that is the 1H qNMR analysis, in 90-100 vol%, the relatively slight influence in comparison with the aqueous-solid system which the hydrogen nucleus of propylene carbonate affected by fumed silica was observed. It is suggested that the influence from the solid phase to the PC molecules is much smaller than the water molecules from the 1H spin-spin relaxation time (T2) as shown in Fig. 1. Dependences of T1 and T2 relaxation time on liquid content indicated that the interaction between the fumed silica surface and the PC molecules is much smaller than the case of the water molecules.
On the other hand, as for the electrolyte concentration dependence, more remarkable variation was observed in the system containing PC rather than that of the aqueous system. Because the molar ratio of water molecules to Li+ ions is ca. 53 in 1 mol L-1 LiClO4 aqueous solution, the water molecule participating in the hydration to Li+ ion is a small portion, and most water molecules form a hydrogen bonding network. Whereas most PC molecules solvate with Li+ ion because the molar ratio of water molecules to Li+ ions is ca. 11 in 1 mol L-1 PC LiClO4 solution. Therefore the network structure of the whole solvent is greatly affected by the addition of the Li+ ion, and it is thought that relaxation time largely decreased. This result is supported from the increase of viscosity with the increase of the electrolyte concentration of the LiClO4/PC solution. Furthermore, from result of a measurements of the 7Li NMR and the Li+ ion conductance, it was shown that the interaction between the fumed silica surface and the Li+ ion in the PC solvent was smaller than in the water solvent. The details of the liquid phase properties in other solvents and the comparison with the PC system will be also reported in this research.
This study was supported by the JST Core Research for Evolutional Science and Technology (CREST).
【Reference】
[1] M. Mizuhata et al., J. Mol. Liq., 83, 179, (1999).
[2] Wu Li, J. R. Dahn, D. S. Wainwright, Science, 264, 1115, (1994).
Figure 1