XPS on Li-Battery-Related Compounds: Analysis of Inorganic SEI Phases and a Methodology for Charge Correction

We systematically investigated the arsenic (As) 3d core-level x-ray photoelectron spectroscopy (XPS) binding energy and formation energy for As defects in silicon by first-principles calculation with a high accuracy of 0.1 eV by careful evaluation of the supercell size. For As, we adopt a pseudopotential with 3d states as the valence and the spherical hole approximation to ensure the convergence of self-consistent calculation for the XPS binding energy with large size systems. Some of the examined model defects have threefold coordinated As atoms. The XPS binding energies of these As atoms are distributed in the narrow region from −0.66 eV to −0.73 eV in neutral charge states. Such defects in negative charge states have a lower XPS binding energy by about 0.1 eV. From the XPS binding energy and electrical activity, negatively charged defects of a vacancy and two adjacent substitutional As atoms (As2V) are the most probable candidates for the experimentally observed peak at −0.8 eV called BEM from the reference substitutional As peak. Under the experimental condition, we find that As2V−,2− do not deeply trap electrons and are electrically inactive. We also demonstrate the surface effect that surface states near the bandgap decrease the XPS binding energy, which may generate defects with low binding energies similarly to the experimental peak at −1.2 eV called BEL.

Core-level spectra and binding energies of transition metal nitrides by non-destructive x-ray photoelectron spectroscopy through capping layers

We propose an efficient theoretical method to take into account the core-hole spin density in the projector augmented wave method, combined with spin un-polarized pseudopotentials and the planewave basis set. We apply this method to the calculation of absolute core level X-ray photoelectron spectroscopy binding energies of WS2 and its related materials, and find the following points. First, inclusion of core-hole spin in the core-exited state is essential for accurate description of the binding energies, especially for light elements. Second, the calculated absolute binding energies show excellent agreement with experimental results. Finally, when oxygen atoms are incorporated in the basal plane of WS2 in the metallic phase, the O 1s binding energy is expected to appear at lower energy than the corresponding value in the semiconducting phase.

X-ray photoelectron spectroscopy (XPS) shows that dramatic changes in the core level binding energies can provide strong indications of transitions between more dielectric and more metallic CoFe2O4 and NiCo2O4 thin films. These significant variations in the XPS core level binding energies are possible with a combination of annealing and oxygen exposure; however, the behaviors of the CoFe2O4 and NiCo2O4 thin films are very different. The XPS Co and Fe 2p 3/2 core levels for the CoFe2O4 thin film at room temperature show large photovoltaic surface charging, leading to binding energy shifts, characteristic of a highly dielectric (or insulating) surface at room temperature. The photovoltaic charging, observed in the XPS binding energies of the Co and Fe 2p 3/2 core levels, decreases with increasing temperature. The XPS core level binding energies of CoFe2O4 thin film saturated at lower apparent binding energies above 455 K. This result shows that the prepared CoFe2O4 thin film can be dielectric at room temperature but become more metallic at elevated temperatures. The dielectric nature of the CoFe2O4 thin film was restored only when the film was annealed in sufficient oxygen, indicating that oxygen vacancies play an important role in the transition of the film from dielectric (or insulating) to metallic. In contrast, the XPS studies of initially metallic NiCo2O4 thin film demonstrated that annealing NiCo2O4 thin film led to a more dielectric or insulating film. The original more metallic character of the NiCo2O4 film was restored when the NiCo2O4 was annealed in sufficient oxygen. Effective activation energies are estimated for the carriers from a modified Arrhenius-type model applied to the core level binding energy changes of the CoFe2O4 and NiCo2O4 thin films, as a function of temperature. The origin of the carriers, however, is not uniquely identified. This work illustrates routes to regulate the surface metal-to-insulator transition of dielectric oxides, especially in the case of insulating NiCo2O4 thin film that can undergo reversible metal-to-insulator transition with temperature.

Analysis of the electronic structure of Hf(Si 0.5As 0.5)As by X-ray photoelectron and photoemission spectroscopy

The interaction of ultrathin films of Ni and Pd with W(110): an XPS study

X-ray photoelectron spectroscopy (XPS) has been used to investigate the reaction of YBa2Cu3O7−x films with solutions of HF, HCl, Br2, HBr, I2, and HI in absolute ethanol (EtOH). The XPS core level and x-ray excited Auger spectra from untreated and halogen-treated surfaces are used to identify surface species by comparison with XPS data from the literature and with XPS spectra from more than 20 Y, Ba, and Cu halides, oxides, hydroxides, and carbonates measured in this work. XPS measurements on a number of these materials are being reported for the first time. Treatment of films with HF/EtOH results in the formation of an oxyfluoride with Y:Ba:Cu relative concentrations of 1:4:3. Additional features in the XPS spectra from HF-treated films are also consistent with the formation of CuF, a compound which does not exist in bulk form. Treatment of films with HCl/EtOH results primarily in the formation of BaCl2 (∼75%), with smaller amounts of YCl3, CuCl, and CuCl2. Treatment of films with Br2/EtOH or HBr/EtOH results in the formation of YBr3, BaBr2, and CuBr with relative concentrations 1:4:3. YBa2Cu3O7−x films were found to have no discernible reaction with I2/EtOH solutions, but treatment of films with HI/EtOH results in the formation of CuI on the surface.

Although self-assembled monolayers (SAMs) of alkylthiols on planar gold (2D SAMs) and on gold nanoparticles (3D SAMs) have been intensely studied, the actual nature of the Au−S bonding remains poorly characterized. Comparison of the X-ray photoelectron spectroscopy (XPS) spectra of 2D and 3D SAMs and “reference” Au(I) complexes, sometimes referred to as Au(I) thiolate polymers, provides detailed insight into this problem. We report high-resolution XPS spectra and Au 4f7/2 and S 2p3/2 binding energies (BE) in 2D SAMs, 3D SAMs and the Au(I) thiolate complexes for two short-chain thiols (n-C4SH and n-C5SH). Sulfur 2p3/2 BE shifts are used to compare the different states of bonding in the SAM systems and the Au(I) complexes and establish that the S atom in the SAM systems bears a charge of about 0.2e. The 2D and 3D SAMs exhibit similar XPS characteristics and are both distinguishable from the Au(I) complexes. The origins of the observed BE values are discussed in the context of the nature of the gold substrate and the oxidation state of the chemisorbed sulfur atom. Comparison of 13C NMR chemical shift data and XPS BE data further clarifies the nature of the surface interactions as well as the use of the Au(I) complexes as reference materials.

The interaction of ultrathin films of Cu with Rh(100) and Ru(0001) has been examined by using X-ray photoelectron spectroscopy (XPS). The effects of surface annealing temperature, adsorbate coverage (film thickness), and CO chemisorption were investigated. The XPS data show that the atoms in a monolayer of Cu supported on Rh(100) or Ru(0001) are electronically perturbed with respect to the surface atoms of Cu(100). The magnitude of the electronic pertubations is larger for Cu/Rh(100). Measurements of the Cu(2p{sub 3/2}) XPS peak position of Cu/Rh(100) and Cu/Ru(0001) as a function of film thickness show that the Cu-Rh and Cu-Ru interactions affect the electronic properties of two or three layers of Cu atoms. The present results show a correlation between the shifts in the XPS surface core-level binding energies and the variations in the desorption temperatures of CO from Cu adlayers. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of (1) variations that occur in the Cu-Cu interaction when Cu adopts the lattice parameters of Rh(100) or Ru(0001) in a pseudomorphic adlayer and (2) modifications in the electronic properties of the Cu adatoms, caused by the Cu-Rh and Cu-Ru interactions. Chemisorption of CO induces a large more » decrease in the electron density of the Cu adlayers. « less

The quantitative evaluation of the carbon hybridization state by x-ray photoelectron spectroscopy (XPS) has been a surface-analysis problem for the last three decades due to the challenges associated with the unambiguous identification of the characteristic binding energy values for sp2- and sp3-bonded carbon. Here, we computed the binding energy values of C(1s) core electrons on the absolute energy scale for model structures of amorphous carbon (a-C) using density functional theory (DFT). The DFT calculations show that in the case of hydrogen-free a-C, the C(1s) binding energy for sp3 carbon atoms is a distribution found approximately 1 eV higher than the binding energy distribution of sp2-hybridized carbons. However, the introduction of hydrogen in the a-C network reduces the distance between the characteristic signals of sp3- and sp2-bonded carbon due to the increased ability to screen the core hole by neighboring hydrogen atoms as compared to carbon atoms. This effect hinders the unambiguous quantification of the carbon hybridization state on the basis of C(1s) XPS data alone. This work can assist surface scientists in the use of XPS for the accurate characterization of carbon-based materials.

We carried out a comprehensive study on the B1s core-level X-ray photoelectron spectroscopy (XPS) binding energies for B clusters in crystalline Si using a first-principles calculation with careful evaluation of the local potential boundary condition for the model system, where convergence within 0.1 eV was confirmed for the supercell size. For ion-implanted samples, we identified experimental peaks due to B clusters and threefold B as icosahedral B12 and 〈001〉B-Si defects, respectively. For as-doped samples prepared by plasma doping, it was found that the calculated XPS binding energies for complexes of vacancies and B atoms were consistent with the experimental spectra.

Angle-resolved core-level and valence-band x-ray photoelectron spectroscopy (XPS) data for GaAs(110), Ge(110), and Ge(111) surfaces are analyzed to determine core-level to valence-band maximum binding-energy differences to a precision of the order of the room-temperature thermal energy. A method for markedly improving the precision with which the position of the valence-band maximum in XPS data can be located is presented. This method is based on modeling the XPS valence-band spectrum in the vicinity of the valence-band maximum by an instrumentally broadened theoretical valence-band density of states and fitting this model to the experimental data by using the least-squares method. The factors which influence the attainable precision for determining core-level to valence-band maximum binding-energy differences are quantitatively discussed. These factors include the presence of occupied surface states, band bending, surface chemical shifts, background effects associated with inelastic processes, instrumental line shape, and spectrometer calibration accuracy. The spin-orbit-split components of the Ga, As, and Ge $3d$ core lines are resolved and binding energies of these components, measured relative to the valence-band maxima in GaAs and Ge, are reported.

Novel nanostructured titania nanotubes and hydrogen titanate nanoribbons were synthesised using hydrothermal treatment of Degussa P25. The nanostructure types formed were monitored as a function of the hydrothermal conditions. Changes in structure was evaluated using X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) of Ti(2p) and O(1s) binding energies in titania nanotubes were measured and a systematic trend in the XPS binding energies was observed. This indicated a strengthening of the Ti-O bond occurred as the material phase changed from titania nanotube to the titanate ribbon form. The changes in binding energies for both the Ti and O XPS peaks were consistent with changes observed in the Raman spectra of nanostructured titania.

The instability of the solid electrolyte interphase (SEI) on the lithium (Li) metal anode is a major challenge towards improving the Coulombic efficiency (CE) and cycle life of Li batteries. In classical SEI models with mosaic and layered structures, ionic phases (e.g., Li2O and LiF) are enriched closest to the Li|SEI interface, while the outer SEI are highly dependent on electrolytes and typically assigned to less-reduced species, such as semi-carbonates and organic Li salts.1, 2 The formation of SEI on Li is often interpreted as a consequence of reactivity between Li metal and electrolytes; however, there remains a lack of understanding about the interplay between electrolytes and specific SEI phases once they are formed. Some recent computational studies have shed light on the chemical reactivity between thin (~1 nm) single-phase inorganic SEI (Li2O, LiOH, and Li2CO3) on Li metal and DME electrolytes containing LiTFSI or LiFSI salts, using ab initio molecular dynamics (AIMD) calculations;3, 4 experimental insights are still much-needed.Herein, we study two ionic SEI phases, Li2O (Fig. 1a-b) and LiF (Fig. 1c-d), that are nearly ubiquitously found across many native SEIs, and investigate their stability at the SEI|electrolyte interface. We find, by using a combination of electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and non-destructive X-ray absorption near-edge spectroscopy (XANES), that the ionic SEI|electrolyte interfaces can undergo significant chemical evolution as a function of electrolyte. As shown in Fig. 1a, distinctive lower-frequency semicircles emerged in the Nyquist plots of EIS when 1 M LiTFSI EC/DEC and 1 M LiPF6 EC/DEC were used, indicating significant changes at the interface between the ionic Li2O SEI and carbonate-based electrolytes, especially in the presence of LiPF6 salt. The surface atomic concentration of F also increased significantly from 4 at.% in 1 M LiTFSI DOL/DME to 44 at.% in 1 M LiPF6 EC/DEC, suggesting that a F-rich Li2O|electrolyte interface leads to an additional charge transfer barrier. Non-destructive Li K-edge XANES of the Li2O SEI soaked in 1 M LiPF6 EC/DEC electrolyte displayed a dominant LiF peak at 62.3 eV with an evident blue shift compared with the standard LiF peak at 62.0 eV (Fig. 1b),5 which could be attributed to the solvation of F-rich species at the interface by organic phases from carbonates decomposition, forming an “organic/F-rich” outer SEI layer. Similar organic/F-rich outer SEI was also observed on the LiF SEI soaked in 1 M LiPF6EC/DEC electrolyte, as evidenced by EIS (Fig. 1c), XPS (Fig. 1d) and XANES results. The organic/F-rich outer SEI induced by reactivity between ionic SEI phases and carbonate-based electrolytes can significantly exacerbate the subsequent plating overpotentials. Our experimental results suggest that electrolyte solvent and salt selection is of great importance to optimize transport in ionic-rich Li interfaces, which can have important implications ultimately for SEI stability and thus CE. Figure 1. (a) Nyquist plot of EIS spectra of Li|Li2O SEI in the three different electrolytes, where the thickness L and conductivity σ of the Li2O SEI were acquired by fitting the higher-frequency semi-circle (or the sole semi-circle in the case of DOL/DME).6 (b) Li K-edge XANES fluorescence yield (FLY) of Li|Li2O interface after soaking in EC/DEC solvent or 1 M LiPF6 EC/DEC electrolyte for 20 hours. (c) Nyquist plot of EIS spectra of Li|LiF SEI in the three different electrolytes. (d) C1s and F1s XPS of the surface layer on Li|LiF SEI after soaking in each electrolyte.

Ammonia is an indispensable component of modern fertilizers and a key nitrogen source in chemical synthesis, making it one of the highest-volume commodity chemicals.1–3 However, its synthesis via the Haber-Bosch process contributes significantly to global carbon emissions, motivating the development of alternative pathways for ammonia synthesis.4–6 Of many alternative methods, an electrochemical lithium-mediated process is one of the most promising. Since its initial implementations,7–9 novel cell designs10–12 and electrolytes13–15 have greatly improved the selectivity and productivity of lithium-mediated electrochemical ammonia synthesis (LiMEAS), but questions persist regarding the mechanism of nitrogen fixation and the role of lithium in this process (Figure 1a). In particular, many in the literature have alluded to the possible importance of lithium passivation species, commonly known as the solid electrolyte interphase (SEI), but their small length-scale and reactivity make direct observation challenging.11,13,15 In this work, we leveraged a multiscale approach to reveal connections between device-scale performance and nanoscale passivation behavior in LiMEAS. We used four model systems, varying the presence of proton donor (no proton donor or 0.1 M ethanol), and the feed gas (Ar or N2), which allowed us to examine the influence of the two reactive components in LiMEAS which are not usually present for lithium metal electrodeposition in the literature: proton donor and nitrogen. For each model system, we quantified major reaction products (Figure 1b, f). Ammonia and lithium nitride were measured by the salicylate colorimetric assay,16 hydrogen by online gas chromatography (GC), and residual lithium by galvanostatic stripping. We used scanning electron microscopy to observe microscale morphology, and to gain deeper insights into nanoscale surface morphology, we implemented cryo-electron microscopy (cryo-EM) (Figure 1c-d, g-h). Cryo-EM is a powerful technique that enables high-resolution observation of beam-sensitive metallic lithium and SEI materials by holding them at cryogenic temperatures, which preserve their native state.17 In addition to nanoscale morphological information, cryo-EM provided chemical information in the form of selected-area electron diffraction (SAED), energy-dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS), which were paired with X-ray photoelectron spectroscopy (XPS) to develop an understanding of the chemistry of the interphase.Our results indicate that the proton donor governs reactivity toward nitrogen reduction in LiMEAS via its influence on the SEI. In the absence of proton donor, the lithium deposit forms a passivating, mosaic-structured SEI that preserves the deposited lithium, regardless of whether the feed gas is argon or nitrogen (Figure 1e). With the addition of ethanol, the surface changes significantly. The resultant SEI does not passivate the lithium, allowing it to react with nitrogen and other electrolyte components (Figure 1i). This permeable SEI appears to be mosaic in structure, with organic phases dominated by ethanol breakdown products, and its formation is key to the functionality of lithium-mediated ammonia synthesis. These findings can help guide the development of optimal SEIs for selectivity and stability of LiMEAS, and beyond that, offer insights into the characteristics of the lithium SEI at a reactive rather than passivated interface. Figure 1. (a) Schematic of possible lithium reactions. 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