Probing Electrochemical Potential Differences Over The Solid/Liquid Interface In Li-Ion Battery Model Systems

ACS APPLIED MATERIALS & INTERFACES(2021)

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Abstract
The electrochemical potential difference (Delta(mu) over bar) is the driving force for the transfer of a charged species from one phase to another in a redox reaction. In Li-ion batteries (LIBs), Delta(mu) over bar values for both electrons and Li-ions play an important role in the charge-transfer kinetics at the electrode/electrolyte interfaces. Because of the lack of suitable measurement techniques, little is known about how Delta(mu) over bar affects the redox reactions occurring at the solid/liquid interfaces during LIB operation. Herein, we outline the relations between different potentials and show how ambient pressure photoelectron spectroscopy (APPES) can be used to follow changes in Delta(mu) over bar (e) over the solid/liquid interfaces operando by measuring the kinetic energy (KE) shifts of the electrolyte core levels. The KE shift versus applied voltage shows a linear dependence of similar to 1 eV/V during charging of the electrical double layer and during solid electrolyte interphase formation. This agrees with the expected results for an ideally polarizable interface. During lithiation, the slope changes drastically. We propose a model to explain this based on charge transfer over the solid/liquid interface.
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Key words
electrical double layer, ambient pressure photoelectron spectroscopy, operando spectroscopy, electrochemical potentials, lithium-ion batteries, electrochemical reactions, electrode/electrolyte interface
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