Abstract
The electrochemical potential difference (Δμ̅) 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), Δμ̅ 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 Δμ̅ 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 Δμ̅e 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 ∼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.
Highlights
The driving force for redox reactions in a Li-ion battery (LIB) is the differences in electrochemical potentials between different phases, where a transferable species will strive to move from a phase with higher electrochemical potential to a phase with lower electrochemical potential
We investigate the change in electron electrochemical potential difference Δμe over the working electrode (WE)/electrolyte interface as a function of applied voltage ΔV to the WE
This work demonstrates that fundamental properties such as Δμe over the solid/liquid interface can be probed by operando ambient pressure photoelectron spectroscopy (APPES), even without direct access to the interface itself
Summary
The driving force for redox reactions in a Li-ion battery (LIB) is the differences in electrochemical potentials between different phases, where a transferable species will strive to move from a phase with higher electrochemical potential to a phase with lower electrochemical potential. When applying an external voltage between the LIB electrodes, the first process to occur is usually charging of electrical double layers (EDLs) at the electrode/ electrolyte interfaces.[4,5] This process will alter the electrostatic potential difference (Δφ) between the electrode and electrolyte. When the external voltage is further increased, decomposition of components in the EDL region can occur, and a solid electrolyte interphase (SEI) forms on the surface of the negative electrode as a result of redox reactions.[6−9] The onset of SEI formation depends on the electrolyte solvents and salts used; for typical organic electrolytes used in LIBs, the electrolyte reduction occurs primarily below ∼1 V vs Li+/Li.[9,10] Depending on the reduction potential of the electrode, lithiation can occur before or after SEI formation. If lithiation occurs at a voltage below electrolyte reduction, a well-functioning SEI is essential to obtain a stable and safe battery performance.[10,11]
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