X-Ray CT Measurement of Lithium Metal Morphological Changes of All-Solid-State Lithium Battery

Abstract

Global warming, air pollution, and depletion of fossil fuels stimulate the transition of energy sources from conventional fuels to renewable energies. Electric vehicle (EV) with longer cruising range is desired to enhance the utilization of renewable energies in transportation. In order to develop the EV with longer cruising range, it is important to improve the energy density of rechargeable batteries due to the limited space in EV. All solid state battery (ASSB) is expected to realize higher energy density compared to conventional lithium-ion battery because it is able to use a higher voltage cathode and/or lithium metal anode. Hence, ASSB is promising as a power source of the next-generation EV. In order to maximize the energy density of ASSB, active material with a higher capacity is necessary. This instigates the research toward the application of lithium metal anode, which possesses the highest theoretical capacity among anode materials. The key point for realizing lithium metal anode is to secure a good contact between lithium metal anode and solid electrolyte, since the decrease of reaction area would result in the increase of interfacial resistance and the decrease of power output. In other words, lithium metal deposition and dissolution should play important roles on the cell performance through the morphological changes of lithium metal at the interface. Previous research suggested that lithium metal dissolution causes deterioration in the contact [1][2]. On the other hand, the behavior of deposition has not been revealed. In this study, we try to understand the effect of lithium metal morphological behavior during deposition on the cell performance. Here, we develop a nondestructive visualizing method of lithium metal with X-ray and visualization of lithium metal’s morphological changes under dissolution/deposition conditions is conducted. In order to observe the morphological behavior of lithium metal, it is required to distinguish solid electrolyte (SE), lithium metal, and void. However, it is difficult to distinguish void from lithium metal due to the similarity of absorption coefficients between lithium metal and argon used as inert gas. Therefore, in this study, high pressure xenon method is developed. In this method, the X-ray imaging jig (and the void in the cell) is filled with high pressure xenon (8 atm) that has significant difference in absorption coefficient compared to lithium metal, making enough contrast to distinguish lithium metal from void. By using this high pressure xenon method, the distribution of SE, lithium metal, and void (xenon) is successfully visualized. In this study, a cylindrical symmetry cell (lithium metal/SE (LPS-glass)/lithium metal, diameter of 2 mm) is used. The symmetry cell is installed into X-ray imaging jig, sandwiched by stainless-steel current collectors and pressurized to 0.5 MPa with a spring. In order to induce lithium dissolution/deposition, constant current of 0.2 mA/cm2 for 15 hours is supplied to the cell. Before and after the current supply, three dimensional images of the cell are captured by using X-ray CT. Then, these raw images are segmented into SE, lithium metal, and void regions respectively with the intensity of X-ray transmission. Figure 1 shows X-ray CT images of lithium deposition side electrode (a) before the current supply and (b) after the current supply. SE, lithium metal, and void are shown as yellow, white, and transparent respectively. Especially, the contact area of SE and lithium metal is colored by green. Before the current supply, good contact condition is achieved between the SE and lithium metal. However, after current is applied, the contact remains only around the center of the cell. Figure 2 shows the trend of the cell voltage when current is applied. Meanwhile, the voltage gradually increases. It is assumed that the decrease in the contact area between lithium metal and SE during lithium deposition would lead to this increase of voltage.