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Ti3C2Tx MXenes have typically a mixed surface termination of oxygen, hydroxyl and fluorine groups (Tx). In this work, we investigate the influence of the surface termination on the vibrational properties of Ti3C2Tx by performing thermal desorption and in situ Raman spectroscopy in ultra-high-vacuum (UHV). Significant changes in the Raman spectra occur after annealing above 600 °C, correlated with the desorption of approximately 80% of the fluorine termination, as confirmed by mass spectrometry and X-ray photoemission spectra. In particular, the intense Raman mode at 203 cm−1, usually attributed to a Ti–C-layer stretching vibration, is strongly damped upon fluorine desorption, while the broad spectral features between 220 and 680 cm−1, usually attributed to surface group vibrations, are not changing significantly. We show that the Raman spectra and the change induced by fluorine desorption are well represented by the phonon density of states instead of zone-center phonon modes. Disorder-induced Raman scattering strongly contributes to the Raman spectra. Moreover, due to the metallic nature of MXenes, charge density fluctuation scattering contributes as well. We show that the two scattering mechanisms, deformation potential and charge density fluctuation, may lead to opposite interpretations concerning the symmetry of the fluorine-related mode at 203 cm−1. This study provides new insights into the interpretation of the Raman spectra of MXenes, especially regarding the relation between surface chemistry and vibrational spectroscopy. [ABSTRACT FROM AUTHOR]

2D layered materials, such as transition metal carbides or nitrides, known as MXenes, offer an ideal platform to investigate charge transfer processes in confined environment, relevant for energy conversion and storage applications. Their rich surface chemistry plays an essential role in the pseudocapacitive behavior of MXenes. However, the local distribution of surface functional groups over single flakes and within few‐ or multilayered flakes remains unclear. In this work, scanning X‐ray microscopy (SXM) is introduced with simultaneous transmission and electron yield detection, enabling multimodal nanoscale chemical imaging with bulk and surface sensitivity, respectively, of individual MXene flakes. The Ti chemical bonding environment is found to significantly vary between few‐layered hydrofluoric acid‐etched Ti3C2Tx MXenes and multilayered molten salt (MS)‐etched Ti3C2Tx MXenes. Postmortem analysis of MS‐etched Ti3C2Tx electrodes cycled in a Li‐ion battery further illustrates that simultaneous bulk and surface chemical imaging using SXM offers a method well adapted to the characterization of the electrode‐electrolyte interactions at the nanoscale. [ABSTRACT FROM AUTHOR]

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Faradaic reactions including charge transfer are often accompanied with diffusion limitation inside the bulk. Conductive two‐dimensional frameworks (2D MOFs) with a fast ion transport can combine both—charge transfer and fast diffusion inside their porous structure. To study remaining diffusion limitations caused by particle morphology, different synthesis routes of Cu‐2,3,6,7,10,11‐hexahydroxytriphenylene (Cu3(HHTP)2), a copper‐based 2D MOF, are used to obtain flake‐ and rod‐like MOF particles. Both morphologies are systematically characterized and evaluated for redox‐active Li+ ion storage. The redox mechanism is investigated by means of X‐ray absorption spectroscopy, FTIR spectroscopy and in situ XRD. Both types are compared regarding kinetic properties for Li+ ion storage via cyclic voltammetry and impedance spectroscopy. A significant influence of particle morphology for 2D MOFs on kinetic aspects of electrochemical Li+ ion storage can be observed. This study opens the path for optimization of redox active porous structures to overcome diffusion limitations of Faradaic processes. [ABSTRACT FROM AUTHOR]

NCM523 || graphite lithium ion cells operated at 4.5 V are prone to an early "rollover" failure, due to electrode cross‐talk, that is, transition metal (TM = Mn, Ni, and Co) dissolution from NCM523 and deposition at graphite, subsequent formation of Li metal dendrites, and, in the worst case, generation of (micro‐)short‐circuits by dendrites growing to the cathode. Here, the impact of different separators on the high‐voltage performance of NCM523 || graphite cells is elucidated focusing on the separators' structural properties (e.g., membrane vs fiber) and their reactivity toward LiPF6 (e.g., ceramic‐coated separators). First, the separator architecture has a major impact on cycle life. Fiber‐structured separators can prevent the "rollover" failure by a more homogeneous deposition of TMs and formation of Li metal dendrites, thus, hindering penetration of dendrites to the cathode. In contrast, porous membrane‐structured separators cannot prevent the cell failure due to inhomogeneous TM deposits/Li metal dendrites. Second, it is demonstrated that different types of ceramic‐coated separators (Boehmite (γ‐AlO(OH)) vs α‐Al2O3) exhibit different reactivities toward LiPF6. While α‐Al2O3 shows a minor reactivity toward LiPF6, the γ‐AlO(OH) coating leads to in situ formation of the beneficial difluorophosphate anion in high amounts due the high reactivity toward LiPF6 decomposition, which significantly improves cycle life. [ABSTRACT FROM AUTHOR]

Lithium ion battery cells operating at high‐voltage typically suffer from severe capacity fading, known as 'rollover' failure. Here, the beneficial impact of Li2CO3 as an electrolyte additive for state‐of‐the‐art carbonate‐based electrolytes, which significantly improves the cycling performance of NCM523 ∥ graphite full‐cells operated at 4.5 V is elucidated. LIB cells using the electrolyte stored at 20 °C (with or without Li2CO3 additive) suffer from severe capacity decay due to parasitic transition metal (TM) dissolution/deposition and subsequent Li metal dendrite growth on graphite. In contrast, NCM523 ∥ graphite cells using the Li2CO3‐containing electrolyte stored at 40 °C display significantly improved capacity retention. The underlying mechanism is successfully elucidated: The rollover failure is inhibited, as Li2CO3 reacts with LiPF6 at 40 °C to in situ form lithium difluorophosphate, and its decomposition products in turn act as 'scavenging' agents for TMs (Ni and Co), thus preventing TM deposition and Li metal formation on graphite. [ABSTRACT FROM AUTHOR]

Layered oxides, particularly including Li[NixCoyMnz]O2 (NCMxyz) materials, such as NCM523, are the most promising cathode materials for high‐energy lithium‐ion batteries (LIBs). One major strategy to increase the energy density of LIBs is to expand the cell voltage (>4.3 V). However, high‐voltage NCM∥ graphite full cells typically suffer from drastic capacity fading, often referred to as "rollover" failure. In this study, the underlying degradation mechanisms responsible for failure of NCM523∥ graphite full cells operated at 4.5 V are unraveled by a comprehensive study including the variation of different electrode and cell parameters. It is found that the "rollover" failure after around 50 cycles can be attributed to severe solid electrolyte interphase growth, owing to formation of thick deposits at the graphite anode surface through deposition of transition metals migrating from the cathode to the anode. These deposits induce the formation of Li metal dendrites, which, in the worst cases, result in a "rollover" failure owing to the generation of (micro‐) short circuits. Finally, approaches to overcome this dramatic failure mechanism are presented, for example, by use of single‐crystal NCM523 materials, showing no "rollover" failure even after 200 cycles. The suppression of cross‐talk phenomena in high‐voltage LIB cells is of utmost importance for achieving high cycling stability. [ABSTRACT FROM AUTHOR]

Front Cover: Exploiting the Degradation Mechanism of NCM523 Graphite Lithium-Ion Full Cells Operated at High Voltage (ChemSusChem 2/2021) Keywords: lithium-ion batteries; degradation mechanisms; electrode materials; metal deposition; single-crystals EN lithium-ion batteries degradation mechanisms electrode materials metal deposition single-crystals 487 487 1 01/28/21 20210121 NES 210121 B The Front Cover b shows the cross-talk phenomena in high-voltage NCM523 graphite lithium ion battery cells, that is, the dissolution of transition metals (Mn, Co, Ni) from the NCM523 cathode and subsequent deposition at the graphite anode. Lithium-ion batteries, degradation mechanisms, electrode materials, metal deposition, single-crystals. [Extracted from the article]

Invited for this month′s cover is the group of Tobias Placke and Martin Winter at the MEET Battery Research Center (University of Münster). The image shows the failure mechanism of high‐voltage operated NCM523 || graphite lithium‐ion cells, that is, the dissolution of transition metals (Mn, Co, Ni) from the NCM523 cathode and subsequent deposition at the graphite anode, resulting in formation of Li metal dendrites. The Full Paper itself is available at 10.1002/cssc.202002113. [ABSTRACT FROM AUTHOR]

Correlative methods to characterize single entities by electrochemistry and microscopy/spectroscopy are increasingly needed to elucidate structure‐function relationships of nanomaterials. However, the technical constraints often differ depending on the characterization techniques to be applied in combination. One of the cornerstones of correlative single‐entity electrochemistry (SEE) is the substrate, which needs to achieve a high conductivity, low roughness, and electrochemical inertness. This work shows that graphitized sputtered carbon thin films constitute excellent electrodes for SEE while enabling characterization with scanning probe, optical, electron, and X‐ray microscopies. Three different correlative SEE experiments using nanoparticles, nanocubes, and 2D Ti3C2Tx MXene materials are reported to illustrate the potential of using carbon thin film substrates for SEE characterization. The advantages and unique capabilities of SEE correlative strategies are further demonstrated by showing that electrochemically oxidized Ti3C2Tx MXene display changes in chemical bonding and electrolyte ion distribution. [ABSTRACT FROM AUTHOR]

Ti 3 C 2 T x MXenes have typically a mixed surface termination of oxygen, hydroxyl and fluorine groups (T x ). In this work, we investigate the influence of the surface termination on the vibrational properties of Ti 3 C 2 T x by performing thermal desorption and in situ Raman spectroscopy in ultra-high-vacuum (UHV). Significant changes in the Raman spectra occur after annealing above 600 °C, correlated with the desorption of approximately 80% of the fluorine termination, as confirmed by mass spectrometry and X-ray photoemission spectra. In particular, the intense Raman mode at 203 cm -1 , usually attributed to a Ti-C-layer stretching vibration, is strongly damped upon fluorine desorption, while the broad spectral features between 220 and 680 cm -1 , usually attributed to surface group vibrations, are not changing significantly. We show that the Raman spectra and the change induced by fluorine desorption are well represented by the phonon density of states instead of zone-center phonon modes. Disorder-induced Raman scattering strongly contributes to the Raman spectra. Moreover, due to the metallic nature of MXenes, charge density fluctuation scattering contributes as well. We show that the two scattering mechanisms, deformation potential and charge density fluctuation, may lead to opposite interpretations concerning the symmetry of the fluorine-related mode at 203 cm -1 . This study provides new insights into the interpretation of the Raman spectra of MXenes, especially regarding the relation between surface chemistry and vibrational spectroscopy.

Faradaic reactions including charge transfer are often accompanied with diffusion limitation inside the bulk. Conductive two-dimensional frameworks (2D MOFs) with a fast ion transport can combine both-charge transfer and fast diffusion inside their porous structure. To study remaining diffusion limitations caused by particle morphology, different synthesis routes of Cu-2,3,6,7,10,11-hexahydroxytriphenylene (Cu 3 (HHTP) 2 ), a copper-based 2D MOF, are used to obtain flake- and rod-like MOF particles. Both morphologies are systematically characterized and evaluated for redox-active Li + ion storage. The redox mechanism is investigated by means of X-ray absorption spectroscopy, FTIR spectroscopy and in situ XRD. Both types are compared regarding kinetic properties for Li + ion storage via cyclic voltammetry and impedance spectroscopy. A significant influence of particle morphology for 2D MOFs on kinetic aspects of electrochemical Li + ion storage can be observed. This study opens the path for optimization of redox active porous structures to overcome diffusion limitations of Faradaic processes., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

MXenes, as an emerging class of 2D materials, display distinctive physical and chemical properties, which are highly suitable for high-power battery applications, such as lithium ion batteries (LIBs). Ti 3 C 2 T x (T x = O, OH, F, Cl) is one of the most investigated MXenes to this day; however, most scientific research studies only focus on the design of multilayered or monolayer MXenes. Here, we present a comprehensive study on the synthesis of few-layered Ti 3 C 2 T x materials and their use in LIB cells, in particular for high-rate applications. The synthesized Ti 3 C 2 T x MXenes are characterized via complementary XRD, Raman spectroscopy, XPS, EDX, SEM, TGA, and nitrogen adsorption techniques to clarify the structural and chemical changes, especially regarding the surface groups and intercalated cations/water molecules. The structural changes are correlated with respect to the acidic and basic post-treatment of Ti 3 C 2 T x . Furthermore, the detected alterations are put into an electrochemical perspective via galvanostatic and potentiostatic investigations to study the pseudocapacitive behavior of few-layered Ti 3 C 2 T x , exhibiting a stable capacity of 155 mAh g -1 for 1000 cycles at 5 A g -1 . The acidic treatment of Ti 3 C 2 T x synthesized via the in situ formation of HF through LiF/HCl is able to increase the initial capacity in comparison to the pristine or basic treatment. To gain further insights into the structural changes occurring during (de)lithiation, in situ XRD is applied for LIB cells in a voltage range from 0.01 to 3 V to give fundamental mechanistic insights into the structural changes occurring during the first cycles. Thereby, the increased initial capacity observed for acidic-treated MXenes can be explained by the reduced co-intercalation of solvent molecules.

Layered oxides, particularly including Li[Ni x Co y Mn z ]O 2 (NCMxyz) materials, such as NCM523, are the most promising cathode materials for high-energy lithium-ion batteries (LIBs). One major strategy to increase the energy density of LIBs is to expand the cell voltage (>4.3 V). However, high-voltage NCM ∥ graphite full cells typically suffer from drastic capacity fading, often referred to as "rollover" failure. In this study, the underlying degradation mechanisms responsible for failure of NCM523 ∥ graphite full cells operated at 4.5 V are unraveled by a comprehensive study including the variation of different electrode and cell parameters. It is found that the "rollover" failure after around 50 cycles can be attributed to severe solid electrolyte interphase growth, owing to formation of thick deposits at the graphite anode surface through deposition of transition metals migrating from the cathode to the anode. These deposits induce the formation of Li metal dendrites, which, in the worst cases, result in a "rollover" failure owing to the generation of (micro-) short circuits. Finally, approaches to overcome this dramatic failure mechanism are presented, for example, by use of single-crystal NCM523 materials, showing no "rollover" failure even after 200 cycles. The suppression of cross-talk phenomena in high-voltage LIB cells is of utmost importance for achieving high cycling stability., (© 2020 The Authors. ChemSusChem published by Wiley-VCH GmbH.)

Because of their high specific capacity and rather low operating potential, silicon-based negative electrode materials for lithium-ion batteries have been the subject of extensive research over the past 2 decades. Although the understanding of the (de)lithiation behavior of silicon has significantly increased, several major challenges have not been solved yet, hindering its broad commercial application. One major issue is the low initial Coulombic efficiency and the ever-present self-discharge of silicon electrodes. Self-discharge itself affects the long-term stability of electrochemical storage systems and, additionally, must be taken into consideration for inevitable prelithiation approaches. The impact of the crystalline Li 15 Si 4 phase is of great interest as the phase transformation between crystalline ( c ) and amorphous ( a ) phases not only increases the specific surface area but also causes huge polarization. Moreover, there is the possibility for electrochemical over-lithiation toward the Li 15+ a Si 4 phase because of the electron-deficient Li 15 Si 4 phase, which can be highly reactive toward the electrolyte. This poses the question about the impact of the c -Li 15 Si 4 phase on the self-discharge behavior in comparison to its amorphous counterpart. Here, silicon thin films used as model electrodes are lithiated to cut-off potentials of 10 mV and 50 mV versus Li|Li + ( U 10mV and U 50mV ) in order to systematically investigate their self-discharge mechanism via open-circuit potential ( U OCP ) measurements and to visualize the solid electrolyte interphase (SEI) growth by means of scanning electrochemical microscopy. We show that the c -Li 15 Si 4 phase is formed for the U 10mV electrode, while it is not found for the U 50mV electrode. In turn, the U 50mV electrode displays an almost linear self-discharge behavior, whereas the U 10mV electrode reaches a U OCP plateau at ca. 380 mV versus Li|Li + , which is due to the phase transition from c -Li 15 Si 4 to the a -Li x Si phase. At this plateau potential, the phase transformation at the Si|electrolyte interface results in an electronically more insulating and more uniform SEI ( U 10mV electrode), while the U 50mV electrode displays a less uniform SEI layer. In summary, the self-discharge mechanism of silicon electrodes and, hence, the irreversible decomposition of the electrolyte and the corresponding SEI formation process heavily depend on the structural nature of the underlying lithium-silicon phase.