(Invited) Development of Concentration Profile of Li+ Ion Accompanied with Electrodeposition of Li Metal in Solvated Ionic Liquid

Abstract

Li metal is promising anode candidate for next-generation batteries because Li metal has very large specific capacity and Li/Li+ potential is the lowest. It is indispensable to control the surface morphological variation during cycles in order to utilize Li metal anode in next-generation batteries. In this study, the morphological variation of Li metal electrode in the electrodeposition is discussed from the viewpoint of Li+ ionic mass transfer rate in the vicinity of the electrode surface. In general, the metal electrodeposition and the morphology of electrodeposits are strongly affected by the electrode surface concentration. However, there are only a few researches focused on the relationship between the morphological variations of electrodeposited Li metal and the ionic mass transfer phenomenon. A digital holographic microscope (DHM) was utilized in order to visualize the Li+ concentration profile during the electrodeposition of Li metal. Our previous researches used a holographic interferometry to in-situ measure the surface concentration and concentration boundary layer thickness during the electrodeposition of Li metal. The principle of the technique is same, but the resolution of DHM was drastically improved. Figure 1 shows the DHM image in solvated ionic liquid electrolyte (LiTFSA:tetraglyme(G4) = 1:1). A working electrode was Cu plate, and a counter electrode was Li metal. Applied current density was 3.0 mA cm-2. The image was recorded at 100 seconds after the electrodeposition started. The color variation indicates the Li+ concentration profile near the working electrode, and the image shows that very uniform concentration profile was formed in this case (the precursor of Li dendrite was not detected.). Not only Li+ surface concentration but also concentration boundary layer thickness are focused to discuss the Li+ ionic mass transfer phenomenon. Especially, solvated ionic liquids are new electrolytes for batteries and then it is very important to understand the ionic mass transfer in the solvated ionic liquids. We would like to discuss the relationship between the morphological variations of electrodeposited Li metal and the developing the concentration profile in solvated ionic liquids systems. Figure 1