Performance Improvement of the All-Solid-State Battery Using Porous Li0.33La0.56TiO3 Membrane By Sol-Gel Method
Authors: Oikawa Hijiri , Arachi Yoshinori 
Published: December 2020

Introduction
The all-solid-state battery using a solid electrolyte has no risk of liquid leakage or ignition and it is highly reliable in terms of safety. Unlike conventional organic electrolytes, they have no fluidity, so they can be stacked. Furthermore, solid electrolytes show single conduction in which only Li+ ions move in the electrolyte. Therefore, it is expected that a high energy density can be obtained, and it is expected to be load in an electric vehicle as a next-generation storage battery. However, in all-solid-state battery, since each material is solid, it is not possible to form an interface as easily as a solid-liquid interface. In other words, the biggest challenge of all-solid-state batteries is the reducing of electrode / electrolyte interface resistance. In our laboratory, it was found that LiCoO2 (LCO) formed on a LLTO substrate using a molten salt of Li and Co has a low interface resistance (about 40 Ω･cm2) and high electrochemical activity. However, the film thickness obtained by this method is as thin as 1 μm. It is the active material that stores energy, and even if it is a thin film, it cannot move an electric vehicle. Thus, the purpose of this paper is to increase the amount of active material by using a porous LLTO membrane to three-dimensionally fill the LLTO with molten salt LCO.
Experimental
Li0.33La0.56TiO3 (LLTO) was used as the solid electrolyte. Lithium nitrate, lanthanum nitrate hexahydrate and titanium bisammonium lactate were dissolved in a mixed solvent of water and acetic acid with Polyvinylpyrrolidone(PVP) and stirred at 298K for 12 hours. Then, obtain a coating solution. The coating liquid was formed on the LLTO substrate by a spin coating method, dried and sintered to obtain a porous film. Then, molten salt prepared by mixing and deliquescing lithium nitrate, lithium chloride, and cobalt nitrate hexahydrate (88: 12: 100, mol ratio) was added dropwise to the LLTO membrane, followed by sintered at 973K for 1 h, filled with LCO. The powder X-ray diffraction measurement by CuKα radiation was performed to identify the crystalline phase and examine their crystallinity. Electrochemical testing was carried out with LCO / porous LLTO + LLTO / sulfide solid electrolyte / In-Li cell (Φ10 mm) at 323K. The rate of constant current was 1/100 C in the range of 2.0-3.6 V.
Results and discussion
Fig. 1 is a Surface structure image of the LLTO film prepared by the sol-gel method. Its particle size is about 1 μm and pores size is about 1 μm distributed throughout the structure. By changing its preparation condition, it was possible to prepare different porosities and film thicknesses. The target LLTO single phase was confirmed from the powder XRD pattern of the LLTO membrane, and the calculated tetragonal lattice constants almost matched the literature values. The result of constant current charge-discharge measurement of this sample is shown in Fig. 2. For comparison, the result of LCO formed on a conventional LLTO sintered body is shown. It is considered that this is because both the charge and discharge specific capacity increased, and the electrochemically active LCO amount increased with the increase in the LCO-LLTO contact area. On the other hand, it has been reported that the LLTO film obtained by the sol-gel method has low ion conductivity, and here is also considered to have the possibility that the utilization rate of the active material is low. In order to solve this problem, it is necessary to improve the conductivity of the LLTO membrane. At that time, it is necessary to estimate the balance between the porosity and the conductivity of the membrane. From the above, it was clarified that the use of a porous membrane can improve the battery characteristics while increasing the LCO filling amount.

Figure 1