All Solid State Battery with LLZ Solid Electrolyte and Li Metal Anode
Authors: Kanamura Kiyoshi , Kimura Takeshi , Shoji Mao , Munakata Hirokazu 
Published: February 2020

Several new generation batteries have been extensively studied by many research groups. Among new generation batteries, all solid state battery is one of promising candidates. There are two types of all solid batteries with solid sulfide and oxide electrolytes. The sulfide material has higher ionic conductivity than oxide material. On the other hand, a chemical stability of oxide material is higher than sulfide one. In this study, the oxide solid electrolyte was employed to prepare all solid state batteries. There are several solid oxide electrolytes. Among them, LLZ (Li7La6Zr8O12) is one of promising candidates for all solid state battery. Lithium ion battery, which has been applied to portable, stationary and electric vehicle applications. Anode is generally graphite. Graphite has 372 mA h g-1. For the next generation battery, anode should have higher capacity density. Several kinds of anodes have been investigated. Among them, Li metal anode is the most suitable anode material for next generation battery. Li metal can be used in all solid state battery in which solid electrolyte is used as separator. Problems for Li metal anode can be solved.
The fabrication of all solid state battery has been investigated to confirm the electrochemical performance of all solid state battery with LLZ and Li metal anode. In this study, LLZ powder was prepared from LiOH, ZrO2, and La(OH)3
1). These starting materials were mixed and heated at 900 °C to obtain precursor for pellet preparation. The obtained powder was pressed into a pellet type shape and then heated at 1150 °C. The obtained pellet had 95 % or higher density and the crystal structure was a cubic. The ionic conductivity of the pellet was 3×10-4 S cm-1. The thickness of the LLZ pellet was about 500 mm. In order to prepare all solid state battery, Aerosol Deposition (AD) process was utilized to form cathode layer on the LLZ pellet. LiCoO2 cathode powder or a mixture of LiCoO2 (LCO) and Li3BO3 (LBO) was deposited on the LLZ by using AD method. The thickness of cathode layer was controlled by repeating time of AD process. Figure 1 shows a scanning electron microscopic images for the cathode layers deposited on the LLZ. The thickness of the cathode layer prepared from the mixture of LiCoO2 and Li3BO3 was two times larger than that obtained from LiCoO2 only. The dense layers were formed on the LLZ pellet for both cases. From the SEM observation for the interface between cathode layer and LLZ solid electrolyte, it can be seen that the connection between cathode layer and LLZ solid electrolyte depends on the presence of Li3BO3 additive. The connection between cathode layer and solid electrolyte interface was improved by adding Li3BO3. In fact, the interfacial resistance of the cell prepared from LiCoO2 and Li3BO3 was smaller than that prepared from LiCoO2 only. In both cases, all sold state battery with Li metal anode has been successfully prepared. The prepared all solid state cells were evaluated with a constant current discharge and charge conditions, as follows, temperature: 30 °C, discharge and charge current: 10 mA corresponding to 0.3 C rate, cut off voltage for charge and discharge: 4.2 V and 2.5 V. Figure 2 shows the discharge and charge curves of all solid state cells with or without Li3BO3 additive. When using cathode layer without Li3BO3, the cell delivered 30 mA h g-1 based on LiCoO2 cathode. When using LiCoO2 and Li3BO3 mixture cathode layer, the cell capacity was 95 mA h g-1. The difference is related to the connection between the cathode layer and LLZ solid electrolyte. Li3BO3 is soft material, so that the shape can be easily changed. This property may provide a good contact between the cathode layer and LLZ solid electrolyte layer. By the way, an irreversible capacity of cells was observed at the first cycle. This is a problem for practical battery. At this moment, there are some possibilities for this irreversible capacity. Further researches have to be done to solve this problem.
1) M. Kotobuki, K. Kanamura, Y. Sato, T. Yoshida, J. Power Sources, 196 (2011) 7750-7754

Figure 1