Lithium ion battery (LIBs) is indispensable to various electrochemical devices in practical use such as a cell phone, electric vehicle due to its high energy density, high voltage. Although the extended use of LIBs technology, batteries which have higher capacity and can stand up to high speed charge/discharge than those of LIBs are demanded. Various studies have reported that the liquid phase occurring adjacent to solid particles has an anomalous behavior when compared to bulk solution[1]. Solid-liquid interface is performed as a base in charge-discharge of LIBs, so 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 LIBs. In addition, properties of the highly-concentrated electrolytes for performance improvement of LIBs are attracted. In this study, NMR spectroscopic behavior of non-aqueous LiClO₄ solution mixed with fumed oxide nanoparticles 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-spin (T₂) relaxation time and electrochemical measurement.

Samples were prepared in Ar glovebox by mixing propylene carbonate(PC) or LiClO₄/PC of predetermined concentration (1-3 mol L^-1) and fumed silica(FS) (200 ± 20 m² g^-1), amorphous silica nanoparticles with self-cohesive properties, fumed alumina(FA) (130 ± 20 m² g^-1) and fumed titania(FT) (90 ± 20 m² g^-1) 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; 9.39 T) and ¹H spin–spin relaxation time; T₂measurements (XiGo Nanotools Acorn area ; 0.3 T).

In ¹H NMR spectra, a broad signal was confirmed under the existence solid-phase. It was shown that the dynamic property of PC molecules largely decreased in comparison with the case only for liquid phases. In PC/FS and PC/FA system, ¹H NMR spectra shifted to higher magnetic field with the decrease of liquid content. For the shielding effect to the ¹H nucleus of the PC molecules by the surface charge of fumed silica, a signal continues to shift to highly-magnetic field as a content of solid-phase increases. This result suggested that the apparent charge densities around the proton of PC molecules increased by fumed silica because fumed silica has a negative charge. In alumina dispersion, NMR signal shifted to higher magnetic field than that of fumed silica. From this result, alumina gives more influence on dynamics properties of liquid phase. A similar result was observed in 1-3 mol L^-1 LiClO₄/PC and solid coexisting system.

From the liquid content dependences of the ¹H NMR detection ratios of propylene carbonate, almost same amount of PC molecules were detected in PC/FS coexisting system. This result indicated that fumed silica formed network structure but H of PC molecules remained its dynamics. On the other hand, in fumed alumina or fumed titania dispersion, values of PC molecules detected by NMR decreased compared to the theoretical value of detected amount of PC by ¹H NMR. It suggests that the mobility of PC molecules in fumed alumina and titania system were restricted than that in silica system.

T₂ in solid-liquid coexisting system is also shown in Fig. 1. From Fig.1, in all systems, the T₂ value decreases because the dynamics of water molecules of the liquid phase decrease. The T₂ value decreased in order of PC/FA, PC/FT, PC/FS system. The influence of the solid surface to solvent mobility in fumed alumina dispersion is the least of the three. This result supports a signal of the NMR and the result of ¹H qNMR measurement.

We will also discuss the NMR relaxation behavior for LiClO₄-PC solution coexisting with te same fumed metal oxides.

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).