A Novel Mechanochemical Approach to Form an Effective SEI on Lithium Metal Anodes

Abstract

Lithium metal has the most negative deposition potential of all metals (-3.04 V vs. SHE) and a very high theoretical specific capacity of 3861 mA h g-1, which makes it a promising anode material for next generation batteries. However, the commercialization of lithium metal anodes is still impaired by several drawbacks.1 Lithium metal reacts with the electrolyte, forming a solid electrolyte interphase (SEI). Due to the inhomogeneity of this SEI, electrodissolution/-deposition of lithium is favored where the SEI is less resistive or cracked, leading to protrusions and dendrite growth. This does not only lower the coulombic efficiency (CE) and cell specific capacity but also raises the risk of short circuits and thermal runaway.2-3 Therefore, an effective SEI is required to enable safe high energy batteries with lithium metal anodes by limiting lithium protrusions. An ideal SEI is electronically insulating and highly conductive for Li+ but blocking for other ionic species in the electrolyte. Furthermore, it does not react with the electrolyte, is homogeneous in terms of Li+ transport and mechanically stable.4 To enable those characteristics, there are different approaches to grow an effective SEI, such as the use of electrolyte additives, mechanical methods (roll-pressing, micro-patterning) and chemical modification (immersion).5-7 Herein, we present a novel approach to form an effective SEI on the lithium metal surface by combining mechanical (roll-pressing) and chemical modification utilizing various ionic liquids (ILs) and salts prior to cell assembly for application in high voltage, low temperature lithium metal batteries with liquid electrolytes. Applying this mechanochemical method leads to significantly decreased impedance and low overvoltage during electrodissolution/-deposition, even at high current densities of 10 mA cm-2. In addition to electrochemical tests, X-Ray photoelectron spectroscopy (XPS) was utilized to shed light on the correlation between improved electrochemical performance and the composition of the artificial SEI layer. Acknowledgements: The research presented is part of the ‘VIDICAT’ project funded by the European Union's Horizon 2020 research and innovation program under grant agreement n° 829145.The authors would like to thank Dr. Uta Rodehorst for conducting the XPS measurements.