Wetting of Nano-Confined Electrodes By Lithium-Ion Battery Electrolytes Using Multiple Beam Interferometry

Abstract

The interface with the electrolyte is a decisive feature in the design of composite battery electrodes, which typically contain active material particles, conductive material, and polymer binder. A detailed understanding of the electrolyte layering and wetting of materials such as graphene, graphites, metals such as aluminum or even gold, silicon, and metal oxides are ultimately important for improving crucial lithium-ion battery mechanisms. In particular the solid electrolyte interphase (SEI) and lithium-ion mobility at electrode interfaces are still not well understood.

Here we use the (Electrochemical) Surface Forces Apparatus (SFA) (1) to address the wetting behavior of nano-scale-confined lithium-ion battery electrodes by battery electrolytes. After its introduction in the 1970s by Israelachvili and Adams (2), the SFA has been used to investigate interfacial phenomena in fields varying from geoscience to microbiology (3). Figure 1 shows a detail of the SFA-setup that was used for the experiments. White light is thereby guided through a flat contact area of two semi-transparent SFA disks, causing interference patterns which are analyzed using a spectrometer. Monitoring the pattern throughout the experiments allows to follow electrolyte wetting behavior in the confined contact area.

Some of the investigated surfaces were mica, gold and graphene. For instance for graphene, a unique combination of rigid and mobile lithium-ion containing electrolyte layers was observed. Figure 2 shows a typical population mapping of a SFA-experiment which focused on the initial wetting behavior of a battery electrolyte (EC/DEC - LiPF6) on dry mica, gold and graphene surfaces. After an initial drop of about 1.8 Å in the inter-mirror distance, which is due to the excretion of a monolayer of adsorbed water from the initially dry interface, the wetting process of the electrolyte was only in the case of graphene able to overcome the contact force.

Similar observations were made when nano-confined gaps were wetted with ionic liquids. Molecular beam interferometry is a versatile and powerful tool for studying such nanoscaled wetting processes using battery electrodes including ionic liquids or organic solvents.