Variation of Ionic Conductivity of LiClO₄ Solution Coexisting with SiO₂ Nanoparticles in Binary Solvents Induced By Disproportionation

Abstract

A mixed solvent containing two or more solvents is used for the electrolyte of lithium-ion batteries for increasing the conductivity. It is well known that an increase of conductivity by addition of low viscose solvent such as DME solvent1). On the other hand, we focus the influence of the solid phase for the liquid properties; for example, the ionic conductivity of the 1 mol L-1 LiClO4 propylene carbonate (PC)- 1,2-dimethoxyethane (DME) solution was hindered by the existence of LiCoO2 powder.2) In previous studies of Boyer et al., the solution composition on the carbon surface varies from that of the original one of the binary EC-DMC solution due to the affinity of DMC to carbon surface depending on the surface potential by MD simulation.3) In these studies, the solid surface properties influence the structure of solvation and ionic interaction and so on within a few – 100 nanometers. However the influenced distance is still not clear and depends on the measured properties. In this study, the variation of ionic conductivity and its related properties of LiClO4 solution coexisting with SiO2 nanoparticles in binary solvents induced by disproportionation of solvent composition, since SiO2 is conventionally utilized for separator, solvophilic surface modification and so on, and obtained result might be useful. We used PC-DME solution in which LiClO4 was dissolved as liquid phase, and used SiO2 with a particle size of 12 nm as the solid phase. Both of them were mixed at a volume ratio of 15 to 70% to obtain a paste-like samples. By using the particle size of this size, the thickness of the liquid phase existing on the SiO2 surface was several nm to several tens nm, and the solution information at the SiO2 interface could be obtained. The thickness of the liquid phase was obtained by dividing the volume of the liquid phase by the surface area of the solid phase. We performed AC impedance measurement and quantitative 1HNMR measurement on this sample. Furthermore, resonance shear viscosity measurement (RSM) was performed to obtain the local viscosity of the electrolyte near the SiO2 surface. Dependence of the electric conductivity; σ on the molar fraction of DME in binary solvent for the SiO2/1 molL-1 LiClO4-PC1-x DME x coexisting system in several liquid content was shown in Fig 1. In the SiO2/1 mol L-1 LiClO4 PC-DME coexisting system, the composition ratio at which the electric conductivity has the maximum value shifted toward the lower DME content as the proportion of SiO2 increased. At the liquid content of 20 vol%, the electrical conductivity simply decreased with DME content. From the behavior of electric conductivity, it is suggested that the effect of lowering the viscosity of DME cannot be obtained in the solid-liquid coexisting system. From the results of 1H-qNMR measurement adding SiO2 to each solution, it became clear that the mobility of the solvent molecules, which exists near the SiO2 surface, decreased due to the interaction with the SiO2 surface. Liquid volume fraction dependences of the 1H-qNMR detection amount of SiO2/PC-DME sample showed that the detection amount of DME significantly decreased with the addition of SiO2, whereas the decrease of PC detection amount is suppressed. It is suggested that DME molecules preferentially interacted with silica surfaces and was concentrated to solid surface. From the results of the RSM, the local viscosity of the PC-DME solution increased from a distance farther from SiO2 surface than those of the PC and DME pure solutions. Since the exchange of solvent molecules existing on the SiO2 surface and the solvent molecules existing in the bulk was suppressed by the phase separation of the solution, a long-range DME layer having a high viscosity was formed on the SiO2 surface. However, in high concentration solutions, DME molecules formed a stable solvation structure with Li+ and are not concentrated on the SiO2 surface and do not cause disproportionation of the solution composition. Therefore, in the RSM measurement, the distance from the SiO2 surface where the local viscosity increases in the PC-DME solution decreased with increasing concentration. [Reference] Y. Matsuda et al., J. Electrochem. Soc., 128, 2552 (1981). Y. Suzuki et al., Electrochemistry, 87, 294 (2019). M. J. Boyer et al., Phys. Chem. Chem. Phys., 18 (2016) Figure 1