Hybrid Electrolytes Composed of Lithium-Ion Conducting Solid Particles and Liquid Electrolytes for Lithium-Ion Cells

Abstract

Lithium ion batteries for automotive applications such as electric vehicles (EVs) have required to enhance power density, low temperature performance, energy density and safety. Recently, all-solid-state batteries using lithium-ion conducting solid electrolytes have been extensively studied to respond to these demands. However, all-solid-state batteries using solid electrolytes have some issues. Sulfide based solid electrolytes have low chemical stability and oxide electrolytes have large resistance at interfaces between active materials and solid electrolytes.We have developed hybrid electrolytes consisting of lithium-ion conducting solid particles and a gel polymer or liquid electrolyte [1]. The cell with hybrid electrolytes exhibited a low internal resistance, particularly at low temperatures. Moreover, we proposed the mechanism for reducing of the resistance because cyclic carbonates such as PC and PF6 - anion on the surface of lithium-ion conducting solid particles enhance Li+ transference number [2].In this study, we have investigated the detail mechanism of reducing the resistance, and develop high-power lithium-ion batteries. We prepared 1Ah TiNb2O7/LiNi0.5Co0.2Mn0.3O2 pouch cells with hybrid electrolytes to reveal the effective position of lithium-ion conducting solid electrolytes. Figure 1 shows schematic illustrations of the cross-section of the hybrid electrolytes cells. In cell-A, the solid electrolytes were coated on anode. In cell-B, the solid electrolytes layer are not in contact with active materials by inserting a separator. We compared cell-A and B with cell-C without the solid electrolytes layer. Li1.2Zr1.9Ca0.1(PO4)3 (LZCP) was used as a lithium-ion conducting solid electrolytes because of its wide potential window, high chemical stability and relatively low cost of raw materials. Figure 2 and 3(a) show discharge curves of cell-A, B and C at 25℃ and -30℃. The discharge curves of cell-A, B and C showed almost the same at 25℃. On the other hand, the discharge performance of cell-A and B was significantly superior to that of cell-C at -30℃. Comparing cell-A with cell-B, the time to reach 1.5V of cell-A was longer than that of cell-B. Furthermore, typical ac-impedance spectra of the hybrid cells at -30℃ are shown in Figure 3(b). The impedance spectrum exhibits a depressed semicircle. The semicircles of cell-A and B are smaller than that of cell-C. The semicircle of cell-A is smaller than that of cell-B. The semicircle can be interpreted as resulting from the interface resistance such as the charge-transfer process on the electrodes. The interface resistance at low temperature decreased by using the hybrid electrolyte even if the solid electrolytes don’t contact the electrode, however the solid electrolytes in contact with the electrode had the effect of reducing the interface resistance. Considering the above mentioned mechanism of the increasing of Li+ transference number by hybrid electrolytes [2], it was indicated that the interface resistance decreased significantly by putting the solid electrolytes near the surface of the electrodes where the large concentration overpotential is caused during discharge at low temperature. The detail of this mechanism and the effect of reducing resistance by utilizing hybrid electrolytes will be discussed in this presentation.