Effects of Lithium-Ion Conducting Solid Particles in Hybrid Electrolytes on Lithium-Ion Cell Performance

Abstract

Development of lithium-ion batteries for automotive applications such as electric vehicles (EVs) has been focused on enhancement of energy density, power, safety, and life. In particular, all-solid-state batteries have been extensively studied in order to respond to these demands. However, the solid electrolytes such as sulfide or oxide materials have important subjects for practical uses. Sulfide solid electrolytes have low chemical stability in air and low mechanical strength for thin separator. Oxide solid electrolytes have low ionic conductivity and large resistance at interfaces between active material and solid electrolyte. We proposed a thin hybrid electrolyte system instead of solid electrolytes [1]. The thin hybrid electrolyte consisting of lithium-ion conducting solid particles of a cubic garnet-type Li7La3Zr2O12 (LLZ) and a gel polymer electrolyte exhibits smaller activation energy for ionic conductivity than a gel polymer electrolyte without LLZ. The fabricated cells using the hybrid electrolyte show high-rate charge-discharge performance [1, 2]. We have also investigated electrochemical properties of other hybrid electrolytes consisting of NASICON-type Li-Al-Ti-P-O (LATP) particles wetted with liquid electrolytes of LiPF6-PC/DEC. Figure 1 shows fast-charge performance of 1 Ah TiNb2O7/LiNi0.5Co0.2Mn0.3O2 cells using the hybrid and the liquid electrolyte at 10 C rate. The charge performance of the cell using the hybrid electrolyte was significantly superior to that of the cell using the liquid electrolyte, which indicates the existence of LATP particles contributing to enhancement of migration of lithium ions in the hybrid electrolyte. Figure 2 shows high-rate discharge properties of the cells using the hybrid and the liquid electrolytes containing different LiPF6 concentration. The voltage decline of the cells using the hybrid electrolytes were more gradual than that of the cell using the liquid electrolytes with LiPF6 concentration of 0.5 M and 1 M. The difference between hybrid electrolytes and liquid electrolytes was clearly observed in the cell with LiPF6 concentration of 0.5 M. In the case of a high LiPF6 concentration of 2 M, the voltage decline of the cells using the hybrid and the liquid electrolytes showed almost no difference. Electrochemical kinetics on the discharge were more effectively enhanced by the existence of LATP particles wetted with the liquid electrolyte containing smaller amount of LiPF6. From these results, we considered that lithium-ionic conducting solid particles in hybrid electrolyte have effects to enhance lithium-ion conductivity by promoting lithium-ion migration on the surface of the solid particles and the amount of lithium-ions for charge transfer on the electrodes. However the detail of this mechanism for effects has not been cleared yet. In order to understand the phenomenon, it is determined that properties of lithium-ion conducting and electrochemical performances of hybrid electrolytes with different ratio of solid particles and liquid electrolyte and so on. In this presentation, we will discuss the role of lithium-ion conducting solid particles in hybrid electrolytes.