Design of Electrochemcial Interface Between Cathode/Solid Electrolyte By Using Liquid Type Li+ Conducting Materials

Lithium ion secondary batteries have been used widely as power sources of various electric devices, and recently used for larger applications such as electric vehicles. According to this movement, the safety of battery becomes more important. However, it is difficult to realize high safety in current batteries using liquid electrolytes containing flammable organic solvents. Thus, all-solid-state batteries using ceramics electrolytes have been focused in recent years. We have studied about the all-solid-state batteries using Li₇La₃Zr₂O₁₂ (LLZ), which has Li⁺ conductivity higher than 10^-4 S cm^-1 at room temperature and chemical stability against lithium metal. Thus, LLZ is a promising solid electrolyte to realize the all-solid-state batteries with high safety and energy density. However, new insights and technologies are needed to form an electrochemical electrolyte/electrode interface in all-solid-state batteries since solid electrolytes have no fluidity. In this study, formation of an appropriate electrochemical interface for all-solid-state batteries is investigated by applying various Li⁺ conducting materials to the composite cathode formed on a LLZ pellet.

LLZ powder was synthesized by solid state reaction and pelletized. LiFePO₄ was used as cathode and its slurry containing acetylene black and polyvinilidene difluoride in a weight ratio of 80: 10: 10 in N-methylpyrrolidone was coated on LLZ pellet. After vacuum drying at 85 ^oC, a small amount of electrolyte materials was impregnated in to the cathode. Then, lithium-metal was attached onto the back side of LLZ pellet as anode. As the electrolyte materials, 1 mol dm^-3 LiPF₆ in propylene carbonate and lithium bisfluorosulfonylimide-tetraglyme complex (LiFSI-G4) were used.

Fig. 1 shows a cross-sectional SEM image of the LLZ pellet with LiFePO₄ composite cathode. The composite cathode with a thickness of about 10 μm was well attached onto the LLZ pellet. Fig. 2 shows the cyclic voltammogram of the cell without liquid electrolyte impregnation to the cathode. A couple of oxidation and reduction peaks corresponding to the charge and discharge of LiFePO₄ were observed with a relatively high symmetry, centered at around 3.5 V, which agrees with the operating potential of LiFePO₄. However, those peak currents were as low as 1 μA, suggesting that only a small amount of LiFePO₄ near the LLZ/cathode interface worked. 

Fig. 3 shows cyclic voltammograms of the cells with LiFePO₄ composite cathodes impregnated with (a) 1 mol dm^-3 LiPF₆ in propylene carbonate and (b) LiFSI-G4. In the case of 1 mol dm^-3 LiPF₆ in propylene carbonate, the oxidation and reduction peaks corresponding to the charge and discharge of LiFePO₄ were observed, and those currents were increased compared with the cell without the electrolyte impregnation (Fig. 2). This improvement can be understood by the formation of Li⁺ conducting pathways between LiFePO₄ in the cathode and LLZ pellet. Similarly, the currents were increased by applying LiFSI-G4. However, the charge and discharge peaks became broader and shifted to positive and negative, respectively. This difference may be due to the lower ionic conductivity and higher viscosity of LiFSI-G4 than those of propylene carbonate electrolyte solution. Further optimization on LLZ/LiFePO₄ cathode interface is now underway and the details will be discussed in the meeting.