In Situ Observation of Lithium Dendrite Growth and SEI By Optical Microscopy and Raman Spectroscopy

Li metal negative electrode is expected for the next-generation Li battery electrode material due to its extremely high theoretical specific capacity (3,860 mAhg-1), low density (0.534 g cm -3 ) and the lowest negative electrochemical potential (-3.040 vs standard hydrogen electrode). However, the critical challenges of Li metal anodes (poor cycle life and safety) remain unsolved. This is attributed to electrolyte decomposition and the Li dendrite growth during electrochemical cycling. In this study, we developed a transparent two-electrode cell to carry out an in situ observation of the Li dendrite growth on Cu surface. Galvanostatic plating and stripping cycle(s) are applied at same current density on each cell. Meanwhile, the Raman spectroscopy is employed to take a mapping image along with the grown lithium surface. The morphology evolution of the Li dendrite growth and nucleation mechanism is observed in different electrolytes and additives. The behavior of lithium dendrite and SEI were analyzed comparatively with 1M LiPF6 EC/ DEC (v: v=1:1), 1M LiTFSI, DME/DOL (v : v=1:1), and 1M LiClO 4 , PC electrolyte at the same current density. We have clearly observed that after a few cycles of platting and stripping lithium dendrite grew dramatically on Cu surface with carbonate electrolyte and the Cu surface still remained some lithium dendrite due to the dead lithium formation, which causes low Coulombic efficiency. In addition, Raman spectroscopy is used to probe SEI composition. The correlation of lithium growth and its surface composition are established in this work, which can help to further understand the mechanism of lithium growth.