Aiming at a molecular-scale understanding of the initial stages of the solid-electrolyte interphase (SEI) formation in Li-ion batteries, the chemical reaction of a monolayer of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) adsorbed on a graphite (0001) substrate during postdeposition of Li and subsequent annealing has been investigated using a combined experimental and theoretical approach. For comparison, also the reaction between a bulk-like multilayer BMP-TFSI film and postdeposited Li is investigated. Employing X-Ray photoelectron spectroscopy and density functional theory-based calculations, it is found that postdeposition of Li at room temperature leads to a significant modification of both monolayer film and bulk BMP-TFSI, including the formation of (adsorbed) molecular fragments, binary Li compounds, and desorption of volatile C- and F-containing species. The initial reaction with Li is highly exothermic and non- or little activated, and products are identified by comparison of experimental XP spectra and calculated core-level binding energies. Further reaction steps, leaving only binary Li compounds or comparable adsorbed species, are considerably activated and require annealing to >500-650 K, depending on the anion. Consequences of these results for the molecular-scale understanding of the initial stages of SEI formation in an electrochemical environment are discussed.

The anode/electrolyte interface behavior, and by extension, the overall cell performance of sodium-ion batteries is determined by a complex interaction of processes that occur at all components of the electrochemical cell across a wide range of size- and timescales. Single-scale studies may provide incomplete insights, as they cannot capture the full picture of this complex and intertwined behavior. Broad, multiscale studies are essential to elucidate these processes. Within this perspectives article, several analytical and theoretical techniques are introduced, and described how they can be combined to provide a more complete and comprehensive understanding of sodium-ion battery (SIB) performance throughout its lifetime, with a special focus on the interfaces of hard carbon anodes. These methods target various length- and time scales, ranging from micro to nano, from cell level to atomistic structures, and account for a broad spectrum of physical and (electro)chemical characteristics. Specifically, how mass spectrometric, microscopic, spectroscopic, electrochemical, thermodynamic, and physical methods can be employed to obtain the various types of information required to understand battery behavior will be explored. Ways are then discussed how these methods can be coupled together in order to elucidate the multiscale phenomena at the anode interface and develop a holistic understanding of their relationship to overall sodium-ion battery function. Here, several analytical methods across multiple time and length scales are discussed, covering a wide range of physical and (electro)chemical properties. To fully grasp the complexity of sodium-ion battery anodes, integrated studies on the same battery system, ranging from the cellular level to the atomic level, are required.image

In order to obtain atomistic insights into the initial stages of the formation of the solid electrolyte interphase (SEI) in Na ion or Na metal batteries, we employ surface chemistry experiments and DFT calculations to study the interactions and reactions between a Na surface and the ionic liquid (IL) 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP‐TFSI), a candidate to be used as electrolyte in batteries. Oxygen‐free Na thin films, which were grown on Ru(0001) and characterized by X‐ray and ultraviolet photoelectron spectroscopy (XPS, UPS), can be understood as model of a Na‐rich electrode. After deposition of submonolayer to multilayer BMP‐TFSI films on the Na thin films at room temperature, XPS measurements revealed partial decomposition and the formation of a ‘contact layer’ at the Na surface, consisting of mainly TFSI‐based decomposition products. By comparison to core level binding energies obtained from density functional theory calculations for energetically feasible reaction products, the constituents of the ‘contact layer’ were identified both as atomic fragments of TFSI (F, O, S) and as larger fragments of TFSI (NSO 2 CF 3 , NSO 2 CF 3 SO 2 ), presumably remaining at the surface due to kinetic barriers. Increasing the temperature results in cumulative decomposition towards the stable atomic species at or within the Na surface.

Structural diversity of different BTPs in exciplexes together with TCTA is investigated, to improve their performance in TADF OLEDs.

The Front Cover illustrates the chemical reactions of an ionic liquid with Li atoms. Combining experiment and theory, initial products in the solid-electrolyte interphase formation are clearly identified. More information can be found in the Research Article by K. Forster-Tonigold and co-workers.

Employing density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS), we identify products of the reaction of the ionic liquid N,N-butylmethylpyrrolidinum bis(trifluoromethylsulfonyl)imide (BMP-TFSI) with lithium in order to model the initial chemical processes contributing to the formation of the solid electrolyte interphase in batteries. Besides lithium oxide, sulfide, carbide and fluoride, we find lithium cyanide or cyanamide as possible, thermodynamically stable product in the Li-poor regime, whilst Li3N is the stable product in the Li-rich regime. The thermodynamically controlled reaction products as well as larger fragments of TFSI persisting due to kinetic barriers could be identified by a comparison of experimentally and computationally determined core level binding energies.

We report results of a combined experimental and computational model study on the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) with a Mg thin film model electrode grown on a Ru(0001) substrate, which aims at a fundamental understanding of the solid electrolyte interphase formation at the electrode–electrolyte interface in postlithium batteries. Scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy were employed for the characterization of the Mg thin film model electrode, revealing oxygen-free and atomically flat Mg films. Room temperature XPS measurements after vapor deposition of a (sub)monolayer of BMP-TFSI on the Mg film revealed the formation of a “contact layer” on Mg(0001), created by the reactive decomposition of the IL. In agreement with computationally determined core level binding energies of stable reaction products (dispersion corrected density functional theory calculations), we identified mainly inorganic MgF2-, MgO-, and MgS-like surface compounds, but also other more complex (Mg2+-free) F-, O-, and/or S-containing “TFSI-like” and carbon-containing adsorbed species. The deposition of higher IL amounts (up to 6 monolayers) results in the overgrowth of the direct “contact layer” by molecularly adsorbed BMP-TFSI. Heating of the adsorbate covered surface to around 470 K leads to desorption of multilayer BMP-TFSI and the partial desorption and transformation of adsorbed (Mg2+-free) “TFSI-like” decomposition products on the Mg substrate into MgF2-, MgO-, and MgS species or the respective adsorbed Fad, Oad, and Sad species.

The performance of structurally and chemically well-defined single-crystalline cobalt- and iron-containing mixed oxide thin film model electrodes as bifunctional catalyst in the oxygen reduction and oxygen evolution reactions (ORR and OER) was investigated and compared with those of unary CoO(111) and Fe3O4(001) oxides in a combined surface science and electrochemistry approach. Pure and mixed cobalt- and iron- containing film electrodes were prepared by vapor deposition in an O-2 atmosphere and characterized under ultrahigh vacuum (UHV) conditions by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Electro-chemical/catalytical measurements were performed in an electrochemical cell directly coupled to the UHV system. XPS measurements of mixed binary oxides with different Co:Fe atomic ratios reveal solely Co2+ and Fe3+ states, pointing to CoFe2O4 in combination with excess Co or Fe either in a CoO or a Fe2O3 phase. For the CoFe2O4/CoO binary metal oxide electrodes the base CVs in 0.5 M KOH show clear differences compared to Fe3O4(001) and the CoFe2O4/Fe2O3 electrodes at potentials > 0.5 V, reflecting the formation of Co2+ to Co3+ transition. The mixed cobalt- and iron-containing thin film electrodes show a higher overpotential for the OER than pristine CoO(111); it is, however significantly lower compared to magnetite Fe3O4(001). Together with changes in the ORR performance Fe doping is found to lower the overall efficiency as bifunctional catalyst, as compared to the pure CoO(111) and Co3O4(111) electrodes, but it is much higher compared to that of a pristine Fe3O4(001) electrode.

AbstractIn this work we aim towards the molecular understanding of the solid electrolyte interphase (SEI) formation at the electrode electrolyte interface (EEI). Herein, we investigated the interaction between the battery‐relevant ionic liquid (IL) 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP‐TFSI), Li and a Co3O4(111) thin film model anode grown on Ir(100) as a model study of the SEI formation in Li‐ion batteries (LIBs). We employed mostly X‐ray photoelectron spectroscopy (XPS) in combination with dispersion‐corrected density functional theory calculations (DFT‐D3). If the surface is pre‐covered by BMP‐TFSI species (model electrolyte), post‐deposition of Li (Li+ ion shuttle) reveals thermodynamically favorable TFSI decomposition products such as LiCN, Li2NSO2CF3, LiF, Li2S, Li2O2, Li2O, but also kinetic products like Li2NCH3C4H9 or LiNCH3C4H9 of BMP. Simultaneously, Li adsorption and/or lithiation of Co3O4(111) to LinCo3O4 takes place due to insertion via step edges or defects; a partial transformation to CoO cannot be excluded. Formation of Co0 could not be observed in the experiment indicating that surface reaction products and inserted/adsorbed Li at the step edges may inhibit or slow down further Li diffusion into the bulk. This study provides detailed insights of the SEI formation at the EEI, which might be crucial for the improvement of future batteries.

Aiming at a detailed understanding of the formation of the solid electrolyte interphase (SEI) at the electrode-electrolyte interface, which plays a critical role in the performance of Li-ion batteries (LIBs), we studied the interaction between Li, ultrathin films of ethylene carbonate (EC, main electrolyte component in LIBs), and CoO(111) thin films grown on Ru(0001), where the latter serves as a model for a conversion electrode, under ultrahigh vacuum conditions. Employing Xray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Fourier transform infrared spectroscopy, we found that vapor deposition of EC on CoO(111) at 80 K results in partial decomposition of EC for films in the monolayer range, most likely because of interaction with defect sites, while it adsorbs molecularly in the multilayer regime. In both cases, desorption sets in at 170 K. Between 220 and 240 K, competing desorption and decomposition take place. To mimic the electrolyte, 0.5-2 ML of Li was stepwise postdeposited on a preadsorbed EC adlayer at 80 K and at 300 K, which leads to EC decomposition, most likely into Li-containing -C.O, -C-O-C-, -C-H, -C-C- species, and Li2O2 or LiOH. This can be considered as the initial stage of the chemical SEI formation (open-circuit conditions). CoO conversion, which is essential for Li storage in the electrode, is observed after postdeposition of Li onto a surface precovered with EC decomposition products at 300 K. In these measurements, we could resolve molecular details on the SEI formation on a CoO model anode and on the conversion of CoO, both of which are important processes in conversion-based LIBs.

The performance of structurally and chemically well-defined Ni-free and Ni-modified single-crystalline Co3O4(1 1 1) thin-film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni-modified Co3O4(1 1 1) film electrodes were prepared and characterized under ultrahigh-vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Both Ni decoration (by post-deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O-2) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co3O4(1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 V-RHE (RHE: reversible hydrogen electrode) at 0.1 mA cm(-2) was almost unchanged.

The process of solid-electrolyte interphase (SEI) formation is systematically investigated along with its chemical composition on carbon electrodes in an ionic liquid-based, Li-containing electrolyte in a combined surface science and electrochemical model study using highly oriented pyrolytic graphite (HOPG) and binder-free graphite powder electrodes (Mage) as model systems. The chemical decomposition process is explored by deposition of Li on a pre-deposited multilayer film of 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) under ultrahigh vacuum conditions. Electrochemical SEI formation is induced by and monitored during potential cycling in [BMP][TFSI]+0.1 m LiTFSI. The chemical composition of the resulting layers is characterized by X-ray photoelectron spectroscopy (XPS), both at the surface and in deeper layers, closer to the electrode|SEI interface, after partial removal of the film by Ar+ ion sputtering. Clear differences between chemical and electrochemical SEI formation, and also between SEI formation on HOPG and Mage electrodes, are observed and discussed.

The Cover Feature maps the ongoing endeavor to bridge the gap between well-defined but strongly simplified model studies and realistic but convoluted applied battery research, by stepwise increasing the complexity of the electrode materials and conditions. Here, this starts with structurally well-defined electrode surfaces such as highly oriented pyrolytic graphite (HOPG)-based substrates, which are characterized first by surface science methods, and next by electrochemical characterization methods. They are then compared to more realistic, binder-free graphite powder electrodes. More information can be found in the Full Paper by I. Weber et al. on page 2589 in Issue 10, 2020 (DOI: 10.1002/cssc.202000495).

The performance of structurally and chemically well‐defined Ni‐free and Ni‐modified single‐crystalline Co$_{3}$O$_{4}$(1 1 1) thin‐film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni‐modified Co$_{3}$O$_{4}$(1 1 1) film electrodes were prepared and characterized under ultrahigh‐vacuum conditions by scanning tunneling microscopy and X‐ray photoelectron spectroscopy. Both Ni decoration (by post‐deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O$_{2}$) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co$_{3}$O$_{4}$ (1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 V$_{RHE}$ (RHE: reversible hydrogen electrode) at 0.1 mA cm$^{-2}$ was almost unchanged.

Aiming at a molecular-level understanding of the processes at the electrodel electrolyte interface (EEI), we investigated the interaction between the battery-releyant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP](+)[TESI](-)), Li, and CoO(111) thin films on Ru(0001) as a model study of the solidlelectrolyte interphase (SEI) in Li-ion batteries. Employing mainly angle-dependent X-ray photoelectron spectroscopy and scanning tunneling microscopy, in combination with dispersion-corrected density functional calculations for characterization of the CoO (111) surface, we found that vapor deposition of metallic Li on CoO (111) at 300 K results in the conversion of Co2+ to Co-0, together with the formation of Li2O and adsorbed surface Li2O2. The conversion starts in the near-surface region (1-2 nm) and proceeds in the extended-surface region (6-8 nm). If the surface is precovered by molecularly adsorbed [BMP] TFSI] species (solvent/electrolyte), stepwise postdeposition of small amounts of Li results in gradual decomposition of [TEST] and [BMP] (=SEI formation), forming products such as Li3N, Li2S, LiF, LiwCxHyNz, and other Li-bound fragments of the anion (e.g., LiNSO2CF3). For higher amounts of Li deposition, relative to the IL precoverage, IL decomposition is followed by conversion of Co0(111). Hence, the SEI resulting from IL decomposition is permeable for Li, which is essential for the storage of Li in the CoO(111) anode. This study demonstrates the potential of model studies for a molecular-scale understanding of the initial stages of SEI formation at the EEI and its role in Li storage in a CoO(111) model anode.

Aiming at a detailed molecular understanding of the initial stage of the solid|electrolyte interphase (SEI) formation in Li-ion batteries, we have investigated the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]) (solvent/electrolyte) and Li (Li+ ion shuttle) on well-defined Li-poor Li4Ti5O12(111) and Li-rich Li4.3Ti5O12(111) surfaces/electrodes in a combined surface science and electrochemical model study. X-ray photoelectron spectroscopy (XPS) measurements reveal that postdeposition of Li-0 under ultrahigh vacuum (UHV) conditions on a Li-poor Li4Ti5O12(111) surface precovered with a molecularly adsorbed [BMP][TFSI] adlayer leads to little IL decomposition at 80 and 300 K. We assume that most of the Li diffuses through the IL adlayer and rapidly inserts into the Li4Ti5O12(111) bulk. More pronounced IL decomposition was obtained upon IL deposition on a Li-rich Li4.3Ti5O12 phase at 80 K and subsequent heating to 300 K. Cyclic voltammograms (CVs) recorded on the Li4Ti5O12(111) electrodes in Li-TFSI/[BMP][TFSI] indicate an almost reversible Li (de-)insertion, with a slight decay of the amount of (de-)inserted Li with increasing cycle number. XPS measurements performed on the electrode after potential cycling show low intensity signals of IL decomposition products, in addition to dominant signals from residual IL electrolyte, which are related to reaction of the adsorbed IL with Li inserted into/extracted from Li4Ti5O12 during the CV. The results indicate a close similarity between IL decomposition products formed under UHV and under electrochemical conditions, underlining the validity of this experimental approach and the potential of such kind of model studies for obtaining detailed understanding of the SEI formation.

Aiming at a detailed, molecular-scale understanding of the initial stages of the solid|electrolyte interphase (SEI) formation in Li-ion batteries, the interaction of the common electrolyte solvent component ethylene carbonate (EC) with fully lithiated LiCoO2 and reduced LiCoO2-delta films as model electrodes for the cathode is investigated. The results are compared with previous findings for pristine and lithiated highly oriented pyrolytic graphite, serving as model anode. Employing X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy measurements, it is found that vapor deposition of EC on LiCoO2 and LiCoO2-delta at 80 K results in molecularly adsorbed EC, both in the monolayer and in the multilayer regime. XPS measurements detect significant changes of the adlayer between 170 and 255 K, indicating competing desorption and decomposition. Synchrotron-based XPS measurements reveal a very similar decomposition pattern upon EC deposition on LiCoO2 at close to ambient temperatures. In both cases, the remaining adlayer is mostly composed of Li-containing-C(sic)O,-C-O-C-,-C-H, and-C-C-moieties such as Li2CO3, ROCO2Li, (CH2OCO2Li)(2), and Li2O2. The activated decomposition of EC is caused by interaction with the oxide surface or, more specifically, with surface Li. This process can be considered as the initial stage of the chemical SEI formation.

Aiming at a detailed understanding of the Li adsorption and insertion behavior on/into lithium titanate (Li4Ti5O12, LTO), which is a promising anode material in Li-ion batteries, we have investigated the interaction of vapor deposited Li with LTO in the temperature range between 80 K and room temperature by angle-resolved X-ray photoelectron spectroscopy (ARXPS). The experiments were performed under ultrahigh vacuum (UHV) conditions, and the presence of additional Li species was detected by the formation of Ti3+ in the Ti 2p core level signal due to charge transfer from adsorbed/inserted Li to Ti. Even at 80 K most of the deposited Li diffuses into the bulk of LTO, reflecting the facile insertion and diffusion of Li into and in LTO. Deposition of up to 3 monolayers equivalent (MLE) of Li at 80 K results in an increase in Li concentration in the surface region (topmost 6 nm), up to a stoichiometry of Li4+xTi5O12 with x > 0.3, and slightly lower values in the near-surface region (topmost 1 nm). For higher Li doses, the amount of Li in the near-surface region, including adsorbed Li, increases more than the concentration in the underlying surface region. This is attributed to the blocking of diffusion channels by inserted Li at a stoichiometry of Li4.3Ti5O12 and above. Upon increasing the temperature, Li+ starts to diffuse into the LTO bulk at temperatures between 120 K and 175 K, depending on the concentration in the surface region. It has completely disappeared at 260 K. The consequences of these results for the understanding of physical Li insertion will be discussed.

We report results of a combined experimental and computational model study on the interaction of the battery-relevant ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis-(trifluoromethylsulfonyl)imide ([BMP](+)[TFSI](-)) with Li on pristine highly oriented pyrolytic graphite (HOPG), which aims at a molecular-/atomic-level understanding of the processes at the electrodelelectrolyte interface of Li-ion batteries. Employing mainly X-ray and ultraviolet photoelectron spectroscopy as well as dispersion-corrected density functional calculations (DFT-D), we find intact anion-cation pairs for adsorbed [BMP](+)[TESI](-) (sub)monolayers on HOPG at 300 K and also on lithiated HOPG at 80 K, that is, under conditions where the mobility of Li+ in the bulk is low. Vapor deposition of [BMP](+)[TESI](-) on lithiated HOPG at 300 K results in rapid accumulation of Li delta+ at the surface or in the surface region, indicating that deintercalation is activated under these conditions. This is explained by a dynamic equilibrium between bulk Li+ and surface Li delta+, which is established independent of whether Li is deposited as metallic Li-0 from the vacuum side or segregates as Li+ from the bulk of lithiated HOPG to the surface and which is shifted to the side of surface Li delta+ by stabilization of these species. Stabilization occurs either by formation of stable Li-containing surface compounds by reactive decomposition mainly of the [TEST](-) anions (Li3N, Li2S, LiF, etc.) or by interaction of partially charged Li delta+ species with [TESI](-) anions in the adlayer. DFT-D calculations reveal that a possible initial step in the reactive decomposition is the transfer of electrons from the HOPG surface covered with Li delta+ into the lowest unoccupied molecular orbital of [TESI](-), resulting in elongation and cleavage of the S-N bond and finally insertion of Li into it. Alternatively, stabilization of Li delta+ is possible by formation of a polar bond with the oxygen atoms of [TFSI](-) within the IL adlayer. The resulting calculated work function decrease Delta Phi with respect to that of the bare graphite (0001) surface is in excellent agreement with experimental observations. The interaction of [BMP](+)[TESI](-) and Li at the HOPG interface is considered as the initial stage of the solidlelectrolyte interphase formation at the electrodelelectrolyte interface in Li-ion batteries.