Formation of an Intermediate Layer Between Lithium Metal and Li7La3Zr2O12 to Reduce Its Interfacial Resistance

Abstract

Introduction Li-ion secondary batteries have been used as energy sources of small electronic devices such as cellular phones and laptop personal computers because of their high energy density. The recent progress of their application requires high safety in addition to high energy density. Hence, nonflammable Li-ion conducting ceramics have attracted much attention as safe electrolytes. We have focused on Li7La3Zr2O12 (LLZ) with cubic garnet structure as a promising ceramic electrolyte to realize safe all-solid-state batteries due to its Li-ion conductivity higher than 10-4 S cm-1at room temperature and stability against lithium metal. The formation of an electrochemical interface between electrode and electrolyte with low resistance is very important to obtain good electrochemical performance of batteries. In this study, we focused on the interface between lithium-metal anode and LLZ, and investigated thoroughly the effect of Au intermediate layer on the interfacial resistance forming interface between lithium metal and LLZ. Especially, in order to reduce the interface resistance, the introduction of Au layer was investigated. Experimental LLZ pellets were prepared by sintering of LLZ powder, which was made from LiOH･H2O, La(OH)3 and ZrO2by solid-state reaction. 2032 coin-type cells with symmetric Li/LLZ/Li and Li/Au/LLZ/Au/Li configurations were prepared by attaching lithium-metal sheets on both sides of LLZ pellets with or without a sputtered Au thin layer. A cell resistance was characterized by AC impedance measurement at several temperatures. Results and discussion Figure 1 shows Nyquist plots of the Li/Au/ LLZ/Au/Li cell and Li/ LLZ/ Li cell at 60 oC. Each Nyquist plot shows one semicircle. The edge of semi-circle at high frequency region corresponds to the resistance of LLZ pellet, and the diameter of semicircle corresponds to the interfacial resistance between LLZ and lithium metal. The latter resistance was different for each cell from 100 ohm to 3800 ohm. LLZ is not well attached to lithium leading to high resistance of Li/LLZ/Li cell. On the other hand, when a sputtered Au layer was used for current collector for impedance measurement, a good interface between Au and LLZ was easily obtained. Au alloy with lithium metal. Therefore, Li/Au/LLZ/Au/Li cell shows a smaller interfacial resistance compared with Li/LLZ/Li cell. Figure 2 (a) shows Arrhenius plot of Li ion conductivity of LLZ pellet in Li-Au/LLZ/Au-Li cell. Figure 2 (b) shows temperature dependence of interface resistance between LLZ and lithium metal. The interface resistance between LLZ and lithium metal became smaller immediately at 100 oC, indicating that alloying reaction between Au and lithium metal took place with increasing temperature. And the interface resistance reached less than 1 ohm cm-2 at 100 oC. In cooling process, the interface resistance became smaller compared with before the heating process. This indicates that the good contact between Li metal and LLZ is supported by alloy reaction between Li and Au. Figure 1