Structural and electrical properties of Garnet-type structure Li₇La₃Ce₂O₁₂ as solid electrolytes for solid state Li-ion batteries

Technology development nowadays leads to a growing need for rechargeable batteries with superior performance and are safe to be used. The safety issues of conventional Li-ion batteries become a driving force for developing solid-state Li-ion batteries where liquid electrolytes were replaced with solid electrolytes materials. Many types of solid electrolytes have been investigated in detail in the past years and garnet-type structure is the most promising solid electrolyte. Hence, this work is embarked to study the garnet-type structure Li₇La₃Ce₂O₁₂ as potential candidates for solid electrolytes in solid-state Li-ion batteries. The structural, electrical properties and microstructure properties of Li₇La₃Ce₂O₁₂ were systematically studied. The samples were synthesized using aconventional solid-state reaction method at temperature 600 oC – 900 oC in the air for 12 hours. These compositions were characterized by using X-ray Diffraction (XRD), Impedance Spectroscopy (IS) and Scanning Electron Microscopy (SEM) techniques. This research involves three main investigations. Firstly, the study was initiated by evaluating the purity phase of Li₇La₃Ce₂O₁₂ and structural stability up to 900C. All the XRD patterns could be indexed with the tetragonal structure and the space group of I4/mmm. The electrical properties revealed that Li₇La₃Ce₂O₁₂ exhibits frequency plateau with a dispersion at high frequency region in the conductivity plot indicating solid electrolytes’ properties. Nevertheless, the conductivity is considered low which is about 4.07 × 10-6 S cm-1 at 300 oC measured at 1 kHz. In addition, it exhibits a small grain size with a large number of pores that might be contributed to low conductivity. Hence, further study was conducted with different amounts of lithium excess: 10%, 20%, 30% and 40% in order to enhance the conductivity of Li₇La₃Ce₂O₁₂. The results showed that Li₇La₃Ce₂O₁₂ exhibited a relatively higher conductivity of 7.14 × 10-6 S cm-1 with 30% amount of lithium excess. Furthermore, the microstructure was observed with a larger grain size without any pores. Since the processing method affects the properties of ceramic materials, therefore the comparison between Li7La3Ce2O12 was prepared using pestle mortar and planetary ball-mill method. The properties of both samples were compared. Surprisingly, Li₇La₃Ce₂O₁₂ prepared using pestle mortar has better properties than the sample prepared using the planetary ball-mill method. Samples that used pestle mortar exhibited slightly higher conductivities with well-connected grain. But the sample that was prepared using a planetary ball-mill exhibited poorly connected grain with a large number of pores observed that might contribute to the low conductivity. It can be summarized that Li₇La₃Ce₂O₁₂ prepared using pestle and mortar with 30% as the optimum amount of lithium excess exhibit the best properties compared to others. Since the high conductivity achieved at high temperature, the solid electrolytes Li₇La₃Ce₂O₁₂ potentially can be used for high temperature battery application.