Operando Electrochemical Atomic Force Microscopy of Solid–Electrolyte Interphase Formation on Graphite Anodes: The Evolution of SEI Morphology and Mechanical Properties

Abstract

Understanding and ultimately controlling the properties of the solid–electrolyte interphase (SEI) layer at the graphite anode/liquid electrolyte boundary are of great significance for maximizing the performance and lifetime of lithium-ion batteries (LIBs). However, comprehensive in situ monitoring of SEI formation and evolution, alongside measurement of the corresponding mechanical properties, is challenging due to the limitations of the characterization techniques commonly used. This work provides a new insight into SEI formation during the first lithiation and delithiation of graphite battery anodes using operando electrochemical atomic force microscopy (EC-AFM). Highly oriented pyrolytic graphite (HOPG) is investigated first as a model system, exhibiting unique morphological and nanomechanical behavior dependent on the various electrolytes and commercially relevant additives used. Then, to validate these findings with respect to real-world battery electrodes, operando EC-AFM of individual graphite particles like those in commercial systems are studied. Vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are shown to be effective additives to enhance SEI layer stability in 1 M LiPF 6 /ethylene carbonate/ethyl methyl carbonate (EC/EMC) electrolytes, attributed to their role in improving its structure, density, and mechanical strength. This work therefore presents an unambiguous picture of SEI formation in a real battery environment, contributes a comprehensive insight into SEI formation of electrode materials, and provides a visible understanding of the influence of electrolyte additives on SEI formation.

Model Studies on Solid Electrolyte Interphase Formation on Graphite Electrodes in Ethylene Carbonate and Dimethyl Carbonate II: Graphite Powder Electrodes

“…Individual, HOPG-supported graphite particles, which were studied by electrochemical atomic force microscopy, displayed an enhanced SEI formation activity and a better cell performance, which was attributed to the higher concentration of defect sites. [45] This is in line with our observations of the significant influence of the HOPG surface roughness on the SEI formation process. [33] Finally, the influence of the individual electrolyte components was investigated in further model studies using single-solvent electrolytes [15,33,39,46] instead of the typical carbonate solvent blends.…”

“…Several studies reported exfoliation of the HOPG electrode, most likely induced by solvent co‐intercalation (especially in DMC [15,33] ). Individual, HOPG‐supported graphite particles, which were studied by electrochemical atomic force microscopy, displayed an enhanced SEI formation activity and a better cell performance, which was attributed to the higher concentration of defect sites [45] . This is in line with our observations of the significant influence of the HOPG surface roughness on the SEI formation process [33] .…”

Scanning Probe Microscopy Facility for Operando Study of Redox Processes on Lithium ion Battery Electrodes

“…The most advanced and complex way of measuring is achieved by operating the battery during the measurement, the so-called operando measurements (Breitung et al, 2016;Wang et al, 2017;Zhang et al, 2020). By performing a measurement this way, the dynamic processes that occur can be followed real-time and related to all kinds of battery parameters and settings, bringing this type of measurements closest to the practical behavior of the battery.…”

“…Several research groups have developed scanning probe microscopy techniques to investigate electrochemical processes (Arreaga-Salas et al, 2012;Kim et al, 2018;Benning et al, 2019;Zhang et al, 2020), and a commercial AFM-SECM instrument has recently become available which is suitable for battery research (Bruker, 2020). However, all these scanning probe techniques are used to analyze battery processes on micro to nano level, while charge and discharge conditions are controlled at bulk level.…”

The Importance of Interphases in Energy Storage Devices: Methods and Strategies to Investigate and Control Interfacial Processes

“…AFM generates a tomographic image of the surface of the investigated sample. For these reasons it is particularly suitable for the study of SEI formation and evolution, particularly for the evaluation of the thickness and stability with time [98][99][100][101].…”