| 1. |
- Kotronia, Antonia, et al.
(författare)
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Nature of the Cathode–Electrolyte Interface in Highly Concentrated Electrolytes Used in Graphite Dual-Ion Batteries
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:3, s. 3867-3880
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Tidskriftsartikel (refereegranskat)abstract
- Dual-ion batteries (DIBs) generally operate beyond 4.7 V vs Li+/Li0 and rely on the intercalation of both cations and anions in graphite electrodes. Major challenges facing the development of DIBs are linked to electrolyte decomposition at the cathode–electrolyte interface (CEI), graphite exfoliation, and corrosion of Al current collectors. In this work, X-ray photoelectron spectroscopy (XPS) is employed to gain a broad understanding of the nature and dynamics of the CEI built on anion-intercalated graphite cycled both in highly concentrated electrolytes (HCEs) of common lithium salts (LiPF6, LiFSI, and LiTFSI) in carbonate solvents and in a typical ionic liquid. Though Al metal current collectors were adequately stable in all HCEs, the Coulombic efficiency was substantially higher for HCEs based on LiFSI and LiTFSI salts. Specific capacities ranging from 80 to 100 mAh g–1 were achieved with a Coulombic efficiency above 90% over extended cycling, but cells with LiPF6-based electrolytes were characterized by <70% Coulombic efficiency and specific capacities of merely ca. 60 mAh g–1. The poor performance in LiPF6-containing electrolytes is indicative of the continual buildup of decomposition products at the interface due to oxidation, forming a thick interfacial layer rich in LixPFy, POxFy, LixPOyFz, and organic carbonates as evidenced by XPS. In contrast, insights from XPS analyses suggested that anion intercalation and deintercalation processes in the range from 3 to 5.1 V give rise to scant or extremely thin surface layers on graphite electrodes cycled in LiFSI- and LiTFSI-containing HCEs, even allowing for probing anions intercalated in the near-surface bulk. In addition, ex situ Raman, SEM and TEM characterizations revealed the presence of a thick coating on graphite particles cycled in LiPF6-based electrolytes regardless of salt concentration, while hardly any surface film was observed in the case of concentrated LiFSI and LiTFSI electrolytes.
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| 2. |
- Källquist, Ida
(författare)
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Combining Electrochemistry and Photoelectron Spectroscopy for the Study of Li-ion Batteries
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis photoelectron spectroscopy (PES) is combined with electrochemistry to investigate the electrochemical processes that occur at the electrode/electrolyte interfaces in lithium-ion batteries (LIBs). LIB systems are studied by the use of both ex situ PES, where electrodes are electrochemically pre-cycled and subsequently measured by PES, and operando PES, where electrodes are cycled during PES measurements. Ex situ PES is used to determine the main degradation mechanisms of a novel high capacity material, Li2VO2F. The capacity fade seen for Li2VO2F. is found to be related to an irreversible oxidation of the active material at high voltages, and a continuous surface layer formation at low voltages. To decrease the capacity fading three strategies for optimizing the interface are investigated. The results show that a surface coating of AlF3 most efficiently can mitigate electrolyte reduction, while boron containing electrolyte additives and transition metal substitution more successfully limit the oxidation of the active material. A large part of the work performed in this thesis has been devoted towards developing a methodology suitable for conducting operando ambient pressure photoelectron spectroscopy (APPES) measurements on LIB systems. A general connection between the theory of PES and electrochemistry is made, where in particular a model suitable for interpreting operando APPES results on solid/liquid interfaces is suggested. The model is further developed for the specific case of LIB interfaces. The results from the operando studies show that the kinetic energy shifts of the liquid electrolyte measured by APPES can be correlated to the electrochemical reactions occurring at the interface. If no charge transfer occurs, the kinetic energy shift is proportional to the applied voltage. During charge transfer the behavior is more complex, and the kinetic energy shifts are related to the change in chemical potential of the working electrode. In summary, this thesis exemplifies how both ex situ and operando PES are highly useful techniques for the study of LIB battery interfaces. The possibilities of both techniques are highlighted, and important considerations for an accurate interpretation of the PES results are also discussed.
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| 3. |
- Källquist, Ida, et al.
(författare)
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Probing Electrochemical Potential Differences over the Solid/Liquid Interface in Li-Ion Battery Model Systems.
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:28, s. 32989-32996
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Tidskriftsartikel (refereegranskat)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.
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