Abstract
The important electrochemical processes in a battery happen at the solid/liquid interfaces. Operando ambient pressure photoelectron spectroscopy (APPES) is one tool to study these processes with chemical specificity. However, accessing this crucial interface and identifying the interface signal are not trivial. Therefore, we present a measurement setup, together with a suggested model, exemplifying how APPES can be used to probe potential differences over the electrode/electrolyte interface, even without direct access to the interface. Both the change in electron electrochemical potential over the solid/liquid interface, and the change in Li chemical potential of the working electrode (WE) surface at Li-ion equilibrium can be probed. Using a Li4Ti5O12 composite as a WE, our results show that the shifts in kinetic energy of the electrolyte measured by APPES can be correlated to the electrochemical reactions occurring at the WE/electrolyte interface. Different shifts in kinetic energy are seen depending on if a phase transition reaction occurs or if a single phase is lithiated. The developed methodology can be used to evaluate charge transfer over the WE/electrolyte interface as well as the lithiation/delithiation mechanism of the WE.
Highlights
While traditional ultrahigh vacuum Photoelectron spectroscopy (PES) has been limited to the study of solids, the development of ambient pressure photoelectron spectroscopy (APPES) instruments has diminished the vacuum constraint, enabling the study of both solid/gas and solid/ liquid interfaces.[3−5] With pressures up to ∼100 mbar in the analysis chamber, APPES can be used to study most organic electrolytes used in Li-ion batteries (LIBs).[6−9] Together with sample holders designed for electrochemical measurements, operando APPES measurements can be performed under conditions resembling those during real battery operation.[10−13] By combining electrochemistry and photoelectron spectroscopy, both the chemical composition and the electrochemical potential differences can be probed
Studies performed during charge transfer are more scarce. In this case a 1 eV/V slope cannot generally be expected, as the equilibrium at the interface will be dominated by faradaic reactions rather than electrical double layer (EDL) charging.[27,30−32] In our previous work, the behavior of a Au working electrode (WE) and a Cu WE was studied during charge transfer, and the results showed a deviation from the 1 eV/V shift.[33]
We suggest a model to explain this based on the equilibration of Li-ions over the WE/electrolyte interface
Summary
Photoelectron spectroscopy (PES) is one of the most used techniques to study interfaces in Li-ion batteries (LIBs) due to its surface and chemical sensitivity.[1,2] While traditional ultrahigh vacuum PES has been limited to the study of solids, the development of ambient pressure photoelectron spectroscopy (APPES) instruments has diminished the vacuum constraint, enabling the study of both solid/gas and solid/ liquid interfaces.[3−5] With pressures up to ∼100 mbar in the analysis chamber, APPES can be used to study most organic electrolytes used in LIBs.[6−9] Together with sample holders designed for electrochemical measurements, operando APPES measurements can be performed under conditions resembling those during real battery operation.[10−13] By combining electrochemistry and photoelectron spectroscopy, both the chemical composition and the electrochemical potential differences can be probed. Previous studies of solid/liquid interfaces in electrochemical systems have shown that in order for the liquid layer to be electrochemically active, the thickness of the electrolyte needs to be at least 10−20 nm.[19−21] Due to this constraint, access to tender X-rays (∼4−6 keV) is essentially necessary to directly probe the solid/liquid interface of electrochemical systems operando.[18,19,21−23]
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