The Effects of Interfacial Compression on Lithium Cycling Behavior Revealed through in-Situ and Ex-Situ Electron Microscopy
Authors: Harrison Katharine Lee , Jungjohann Katherine L , Zavadil Kevin R , Hahn Nathan T , Goriparti Subrahmanyam , Boyce Brad , Perdue Brian Robert , Fischer Robert , Meng Xiangbo , Elam Jeffrey W. , Leenheer Andrew Jay , Zhang Ji-Guang 
Published: May 2020

Lithium-ion batteries are important for applications including portable electronics, electric vehicles, and grid storage. Lithium metal anodes can significantly increase energy density beyond lithium-ion batteries because their theoretical storage capacity is 10x larger than that for state-of-the-art graphite anodes. Development of functional lithium metal anodes is critically important for implementation of next-generation battery concepts like Li-S, Li-O2, and Li-metal batteries paired with traditional insertion cathodes. Unfortunately, lithium metal anodes suffer from parasitic reactions with electrolytes, high aspect ratio morphological evolution, and poor cycling behavior. Many strategies have been proposed to improve lithium metal anode cycling including the development of electrolytes that enable more favorable lithium morphology and minimize side reactions. One of the very promising electrolytes that have been developed is 4 M lithium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane (4 M LiFSI/DME).1

To better understand lithium metal plating and stripping processes in 4 M LiFSI/DME, we plated and stripped lithium metal using in-situ electrochemical scanning transmission electron microscopy (EC-STEM).2 We performed EC-STEM experiments using a customizable, sealed liquid cell STEM chip designed and built at Sandia National Laboratories. The Coulombic efficiency and morphology observed in the in-situ EC-STEM experiments2 differed from that in published coin cell results.1 Through macroscale (not TEM) experiments, we determined that the discrepancy could be explained by the absence of a separator and interfacial pressure at the lithium-electrolyte interface for the in-situ EC-STEM experiments. Thus, we concluded that lithium metal plating and stripping performance depends significantly on interfacial compression. Our data also suggests that compression changes how the solid electrolyte interphase forms, which could in turn affect how lithium metal deposits.
Motivated by our observations that lithium plating and stripping behavior depends on interfacial compression, we systematically studied the role of interfacial compression on lithium plating and stripping. We performed electrochemical testing in pouch cells under varied loading and disassembled the cells for subsequent characterization. The electrode stacks were studied by cross sectioning the electrodes with cryo focused ion beam milling. These samples were then imaged using cryo scanning electron microscopy experiments to understand the impact of interfacial compression on morphological evolution. We confirm through these experiments that interfacial compression plays an important role in lithium plating and stripping behavior.
This work was supported by the Joint Center for Energy Storage Research and the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.
1. Qian, J., Henderson, W.A., Xu, W., Bhattacharya, P., Engelhard, M., Borodin, O. and Zhang, J.G., 2015. High rate and stable cycling of lithium metal anode. Nature communications, 6, p. 6362.
2. Harrison, K.L., Zavadil, K.R., Hahn, N.T., Meng, X., Elam, J.W., Leenheer, A., Zhang, J.G. and Jungjohann, K.L., 2017. Lithium self-discharge and its prevention: direct visualization through in situ electrochemical scanning transmission electron microscopy. ACS nano, 11(11), pp.11194-11205.