Photoelectron Spectroscopy for Chemical Analysis

Photoelectron spectroscopy started its modern development in the fifties based on techniques for studies of nuclear decay. Since then, photoelectron spectroscopy has undergone a dramatic expansion of application and is now a prime research tool in basic and applied science. This progress has been largely due to the concomitant development of photon sources, sample handling and electron energy analyzers. The present article describes some of the salient features of modern photoelectron spectroscopy and its applications with particular emphasis on energy relevant issues

Probing Electrochemical Potential Differences over the Solid/Liquid Interface in Li-Ion Battery Model Systems

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

Potassium- and Rubidium-Passivated Alloyed Perovskite Films: Optoelectronic Properties and Moisture Stability.

Halide perovskites passivated with potassium or rubidium show superior photovoltaic device performance compared to unpassivated samples. However, it is unclear which passivation route is more effective for film stability. Here, we directly compare the optoelectronic properties and stability of thin films when passivating triple-cation perovskite films with potassium or rubidium species. The optoelectronic and chemical studies reveal that the alloyed perovskites are tolerant toward higher loadings of potassium than rubidium. Whereas potassium complexes with bromide from the perovskite precursor solution to form thin surface passivation layers, rubidium additives favor the formation of phase-segregated micron-sized rubidium halide crystals. This tolerance to higher loadings of potassium allows us to achieve superior luminescent properties with potassium passivation. We also find that exposure to a humid atmosphere drives phase segregation and grain coalescence for all compositions, with the rubidium-passivated sample showing the highest sensitivity to nonperovskite phase formation. Our work highlights the benefits but also the limitations of these passivation approaches in maximizing both optoelectronic properties and the stability of perovskite films.Engineering and Physical Sciences Research Council (grant number: EP/M005143/1

Vibrationally-resolved RIXS reveals OH-group formation in oxygen redox active Li-ion battery cathodes †

Vibrationally-resolved resonant inelastic X-ray scattering (VR-RIXS) at the O K-edge is emerging as a powerful tool for identifying embedded molecules in lithium-ion battery cathodes. Here, we investigate two known oxygen redox-active cathode materials: the commercial LixNi0.90Co0.05Al0.05O2 (NCA) used in electric vehicles and the high-capacity cathode material Li1.2Ni0.13Co0.13Mn0.54O2 (LRNMC) for next-generation Li-ion batteries. We report the detection of a novel vibrational RIXS signature for Li-ion battery cathodes appearing in the O K pre-peak above 533 eV that we attribute to OH-groups. We discuss likely locations and pathways for OH-group formation and accumulation throughout the active cathode material. Initial-cycle behaviour for LRNMC shows that OH-signal strength correlates with the cathodes state of charge, though reversibility is incomplete. The OH-group RIXS signal strength in long-term cycled NCA is retained. Thus, VR-RIXS offers a path for gaining new insights to oxygen reactions in battery materials

Growth of Transition-Metal Dichalcogenides by Solvent Evaporation Technique

Due to their physical properties and potential applications in energy conversion and storage, transition-metal dichalcogenides (TMDs) have garnered substantial interest in recent years. Among this class of materials, TMDs based on molybdenum, tungsten, sulfur, and selenium are particularly attractive due to their semiconducting properties and the availability of bottom-up synthesis techniques. Here we report a method which yields high-quality crystals of transition-metal diselenide and ditelluride compounds (PtTe2, PdTe2, NiTe2, TaTe2, TiTe2, RuTe2, PtSe2, PdSe2, NbSe2, TiSe2, VSe2, ReSe2) from their solid solutions, via vapor deposition from a metal-saturated chalcogen melt. Additionally, we show the synthesis of rare-earth-metal polychalcogenides and NbS2 crystals using the aforementioned process. Most of the crystals obtained have a layered CdI2 structure. We have investigated the physical properties of selected crystals and compared them to state of the art findings reported in the literature. Remarkably, the charge density wave transition in 1T-TiSe2 and 2H-NbSe2 crystals is well-defined at TCDW ≈ 200 and 33 K, respectively. Angle-resolved photoelectron spectroscopy and electron diffraction are used to directly access the electronic and crystal structures of PtTe2 single crystals and yield state of the art measurements. © 2020 American Chemical Society.M.A.-H. acknowledges support from the VR starting grant 2018-05339 and KL1824/6. The crystal growth experiments were supported by the Russian Science Foundation, Project 19-12-00414. The work has been supported by the program 211 of the Russian Federation Government agreements 02.A03.21.0006 and 02.A03.21.0011, by the Russian Government Program of Competitive Growth of Kazan Federal University. We acknowledge MAX IV Laboratory for time on Beamline Bloch under Proposal 20190335. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152 the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. We acknowledge ARPES experiment support from Craig Polley (MAX IV), Maciej Dendzik (KTH) Antonija Grubisic-Cabo (KTH) and Oscar Tjernberg (KTH). H.R., D.P. and G.J.M. acknowledge the Swedish Research Council (2018-06465, 2018-04330) and the Swedish Energy Agency (P43549-1) for financial support