Current Density Induced Microstructure Evolution on Li Dendrite and Solid Electrolyte Interphase Revealed By Cryogenic Transmission Electron Microscopy
Authors: Xu Yaobin , Wu Haiping , Jia Hao , Zhang Ji-Guang , Xu Wu , Wang Chongmin 
Published: July 2021

The ultralow negative electrochemical potential of -3.040 V versus standard hydrogen electrode, extremely high theoretical specific capacity of 3860 mAh g−1, and low gravimetric density of 0.534 g cm−3 make lithium (Li) metal has re-emerged as an ideal choice as anode material. However, the aggressive dendritic growth of Li metal during cycling makes it too dangerous for practical use. To address this critical issue, we need not only understanding on the macroscopic but also on the microscopic, nanoscopic and atomistic features to probe more fundamental aspects of dendritic growth process. [1-4]

Herein, by applying high resolution transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) techniques under cryogenic (cryo) conditions, [5-8] we characterize the detailed structure of electrochemically deposited Li metal (EDLi) and its solid electrolyte interphase (SEI) layers with systematic control of current density, to establish the correlation between electrochemical performance (interfacial impedance) and current density induced structure and chemical evolution.
We discover that increasing current density leads to increased overpotential for Li nucleation and growth, leading to the transition from growth-limited to nucleation-limited mode for Li dendrite. Independence of current density, the EDLi exhibits crystalline whisker-like morphology. The SEI formed at low current density (0.1 mA cm-2) is monolithic amorphous; while, a current density of above 2 mA cm-2 leads to a mosaic structured SEI, featuring an amorphous matrix with Li2O and LiF dispersoids, and the thickness of the SEI increases with the increase of current density. Specifically, the Li2O particles is spatially located at the top surface of the SEI, while LiF is spatially adjacent to the Li-SEI interface. [9] These results offer possible ways of regulating crucial microstructural and chemical features of EDLi and SEI through altering deposit conditions and consequently direct correlation with battery performance.

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