Electrodeposition of Li Metal onto an Ulta-Micro-Electrode

Abstract

Li metal is very attractive candidate for a negative electrode of next-generation rechargeable batteries because Li metal has very large theoretical capacity and the lowest standard electrode potential. In order to utilize Li metal for negative electrode, it is indispensable to control the morphological variation during charging and discharging. However, Li metal forms dendrites very easily in the electrodeposition process. In general, the distribution of the current density will induce the dendrite formation in the electrochemical system. In addition to that, SEI (solid electrolyte interphase) should play an important role for the morphological variation of the 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 localization of the electrodeposition sites is an obstacle for the quantitative analysis in order to discuss the dendrite initiation and growth.In this study, ultra-micro-electrode was utilized to observe the morphological variation of the electrodeposited Li metal. Constant current electrodeposition was conducted to observe the electrodeposited Li metal. All electrochemical experimental was done in super dry room. The dewpoint of the room was kept lower than 213K. After the electrodeposition, the ultra-micro-electrode was rinsed with some solvents very carefully, and transferred to FE-SEM or TEM with transfer vessel without air exposure. Ex-situ SEM and TEM observation was done to discuss the electrolyte effects to the morphology of electrodeposited Li metal. For SEM observation, not only the observation of morphology but also the chemical composition of SEI layer was analyzed by EDS spectra. The SEM observation was conducted in room temperature. For TEM observation, the sample was cooled to be about 100 K in order to minimize the electron beam damage. STEM-EELS analysis was also conducted for analysis of SEI layer on the electrodeposited Li metal.The electrodeposited Li metal onto a ultra-micro-electrode in 1M LiPF6-EC:DEC=1:1 electrolyte was very smooth and uniform column shape. The chemical composition of the SEI layer on the electrodeposited Li metal was confirmed to be mainly LiF and Li2O by SEM-EDS and STEM-EELS analysis. In this presentation, we would like to discuss the relationship between the SEI layer and morphological variation based on the SEM and TEM observation results.