Electrodeposition of Li Metal Onto a Micro-Electrode in Various Electrolytes

Wednesday, 4 October 2017: 15:30
National Harbor 1 (Gaylord National Resort and Convention Center)
K. Nishikawa (National Institute for Materials Science) and K. Kanamura (Tokyo Metropolitan University)
Li metal is very attractive candidate for next-generation rechargeable batteries because Li metal has very large theoretical capacity and the lowest standard electrode potential. However, Li metal forms dendrite very easily by repeating the electrodeposition and electrochemical dissolution. In general, the distribution of the current density will cause the metal dendrite formation in the electrochemical system. In case of Li metal electrode, SEI (solid electrolyte interphase) covered on the Li metal surface. The SEI layer is very important parameter for the morphological variation of electrodeposited Li metal. Many researches were conducted based on the surface chemistry in order to reveal the relationship between the nature of SEI layer and the morphology of electrodeposited Li metal. However, the distribution of the electrodeposited Li metal is an obstacle for the quantitative analysis in order to discuss the dendrite initiation and growth. In other words, the dendrite growth phenomenon is strongly localized, especially in cm-scale electrode.

In this study, micro-electrode was utilized to observe the morphological variation of electrodeposited Li metal. Ex-situ SEM observation was done to compare the morphology of Li metal in various kinds of electrolyte. The Pt wire with 10 or 20 mm diameter was covered by glass capillary, and the tip was polished very carefully. And then, Cu was electrodeposited onto the Pt micro-electrode because Pt forms alloy with Li. The Cu coated Pt micro-electrode was utilized as the working electrode, and counter electrode was Li metal in an electrochemical cell. Constant current electrodeposition was conducted to observe the electrodeposited Li metal. Applied current density was 2 mA/cm2. After the electrodeposition, the micro-electrode was rinsed with some solvents very carefully, and transferred to FE-SEM with transfer vessel without air exposure.

Figure 1 demonstrates the time variation of electrodeposited Li metal in 1M LiPF6-EC:DEC(1:1 volume) electrolyte. Although applied current density (2 mA/cm2) is quite large, Li metal electrodeposited onto the micro-electrode with very smooth morphology. Micro-electrode system can offer enough Li+ mass transfer to the electrode surface with semi-sphere diffusion mode. And LiPF6 salt supplies small amount of HF with hydrolysis reaction. HF can react with Li metal to LiF as uniform SEI layer on the electrodeposited Li metal surface. These are reason why the very smooth electrodeposited Li metal formed onto the micro-electrode. We will show not only FE-SEM images but also 3D images obtained by FIB-SEM system for electrodeposited Li metal from various electrolyte in this presentation.