基于软X射线光谱的锂电池材料的电子结构与演变的研究进展

开发新型高能量密度以及低成本的锂离子电池, 是有效应对能源危机和环境挑战的可行路径之一. 锂离子电池材料的电子结构与电子态的演化决定了材料诸多本征性质以及电池综合性能. 探测并操控电极材料电子态的演化对探求电极反应的物理机理、 促进电池材料发展具有重要意义. 基于同步辐射的软X射线光谱技术可以直接探测费米能级附近的电子态. 本文从阴阳离子氧化还原反应的不同角度对利用软X射线光谱对电子态演变的研究进行了总结, 获得了电极材料电化学循环过程中过渡金属与氧的电子态演化信息, 系统阐述并总结了不同锂离子电池材料中电子态的演化以及氧化还原反应机理的最新研究进展.     

A Probe of Valence and Conduction Band Electronic Structure of Lead Oxide Films for Photodetectors

We investigate how the electronic structure of amorphous lead oxide (a-PbO) films deposited on ITO substrate is changed after annealing at various temperatures. Both experimental soft X-ray spectroscopic and density functional theory (DFT) based computational techniques are used to explore the electronic structure of this material. X-ray emission, resonant X-ray inelastic scattering, and X-ray absorption spectroscopic techniques are employed to directly probe the valence and conduction bands. We discover that the films are very stable and remain amorphous when exposed to temperatures below 300 °C. An amorphous-to-polycrystalline (α-PbO phase) transformation occurs during annealing at 400 °C. At 500 °C, an alpha to beta phase change is observed. These structural modifications are accompanied by the band gap value changing from 1.4±0.2 eV to 2.0±0.2 eV upon annealing at 400 °C and to 2.6±0.2 eV upon annealing at 500 °C. A difference between surface and bulk structural properties is found for all samples annealed at 500 °C and above; these samples also exhibit an unexpected suppression of O : 2p density of states (DOS) near the bottom of the conduction band, whereas additional electronic states appear well within the valence band. This study provides a significant step forward to understanding the electronic properties of two polymorphic forms of PbO needed for optimization of this material for use in X-ray sensors.     

The application of synchrotron X-ray techniques to the study of rechargeable batteries

The increased use of rechargeable batteries in portable electronic devices and the continuous development of novel applications(e.g. transportation and large scale energy storage), have raised a strong demand for high performance batteries with increased energy density, cycle and calendar life, safety and lower costs. This triggers significant efforts to reveal the fundamental mechanism determining battery performance with the use of advanced analytical techniques. However, the inherently complex characteristics of battery systems make the mechanism analysis sophisticated and difficult. Synchrotron radiation is an advanced collimated light source with high intensity and tunable energies. It has particular advantages in electronic structure and geometric structure(both the short-range and long-range structure)analysis of materials on different length and time scales. In the past decades, synchrotron X-ray techniques have been widely used to understand the fundamental mechanism and guide the technological optimization of batteries. In particular, in situ and operando techniques with high spatial and temporal resolution, enable the nondestructive, real time dynamic investigation of the electrochemical reaction,and lead to significant deep insights into the battery operation mechanism.This review gives a brief introduction of the application of synchrotron X-ray techniques to the investigation of battery systems. The five widely implicated techniques, including X-ray diffraction(XRD), Pair Distribution Function(PDF), Hard and Soft X-ray absorption spectroscopy(XAS) and X-ray photoelectron spectroscopy(XPS) will be reviewed, with the emphasis on their in situ studies of battery systems during cycling.     

Star-shaped molecule of MnII4O6 core with an St=10 high-spin state. A theoretical and experimental study with XPS, XMCD, and other magnetic methods

We report a comprehensive study of the electronic and magnetic properties of a star-shaped molecule comprising a MnII4O6 core. One feature of this compound is weak magnetic coupling constants compared to other similar polyoxo compounds. This leads to complicated low-lying magnetic states in which the ground state is not well separated from the upper-lying states, yielding a high-spin molecule with a giant magnetic moment of up to 20 microB/formula unit. We apply X-ray diffraction and magnetometry as well as other X-ray spectroscopic techniques, namely, X-ray photoelectron spectroscopy, X-ray magnetic circular dichroism, and X-ray emission spectroscopy. We compare our experimental results with ab initio electronic band structure calculations as well as the localized electronic structure around the Mn2+ ions with charge-transfer multiplet calculations.     

Picosecond X-ray absorption spectroscopy of a photoinduced iron(II) spin crossover reaction in solution

In this study, we perform steady-state and time-resolved X-ray absorption spectroscopy (XAS) on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption near-edge structure (XANES) and in the X-ray absorption fine structure (EXAFS). For the time-resolved studies, 100 fs, 400 nm pump pulses initiate a charge-transfer transition in the low-spin complex. The subsequent electronic and geometric changes associated with the formation of the high-spin excited state are probed with 70 ps, 7.1 keV, tunable X-ray pulses derived from the Advanced Light Source (ALS). Modeling of the transient XAS data reveals that the average iron-nitrogen (Fe-N) bond is lengthened by 0.21+/-0.03 A in the high-spin excited state relative to the ground state within 70 ps. This structural modification causes a change in the metal-ligand interactions as reflected by the altered density of states of the unoccupied metal orbitals. Our results constitute the first direct measurements of the dynamic atomic and electronic structural rearrangements occurring during a photoinduced FeII spin crossover reaction in solution via picosecond X-ray absorption spectroscopy.     

Ruthenium molecular wires with conjugated bridging ligands: onset of band formation in linear inorganic conjugated oligomers

We have prepared and characterized a series of multimetallic oligomers of Ru using the pi-conjugated bridging ligand tetra-2-pyridyl-1,4-pyrazine (tppz), as well as mixed-ligand complexes with terpyridine end caps, and analyzed their electrochemical and spectroscopic properties, comparing them with modern computational electronic structure methods. The results suggest that the high degree of metal-metal interunit communication in these linear oligomers yields low HOMO-LUMO gaps, high delocalization, and the onset of "quasi-band" features, all indicative that these compounds should be excellent molecular wire materials. Recent spectroscopic and excited-state analyses of these and related compounds focus on optically accessible states, which ignore optically silent frontier electronic states more relevant to nanoelectronic applications.     

The B 1sigma+ and X 1sigma+ electronic states of hydrogen fluoride: a direct potential fit analysis

All literature pure rotational and vibration-rotational spectroscopic data on the ground X (1)Sigma(+) electronic state of HF and DF, together with the entire set of spectroscopic line positions from analyses of the B (1)Sigma(+) --> X (1)Sigma(+) emission band systems of HF and DF, have been used in a global least-squares fit to the radial Hamiltonian operators, in compact analytic form, for both electronic states. With a data set consisting of 6157 spectroscopic line positions, the reduced standard deviation of the fit was sigma = 1.028. Sets of quantum mechanically significant rotational and centrifugal distortion constants were calculated for both electronic states using Rayleigh-Schr?dinger perturbation theory.     

The application of synchrotron techniques to the study of lithium-ion batteries

This paper gives a brief review of the application of synchrotron X-ray techniques to the study of lithium-ion battery materials. The two main techniques are X-ray absorption spectroscopy (XAS) and high-resolution X-ray diffraction (XRD). Examples are given for in situ XAS and XRD studies of lithium-ion battery cathodes during cycling. This includes time-resolved methods. The paper also discusses the application of soft X-ray XAS to do ex situ studies on battery cathodes. By applying two signal detection methods, it is possible to probe the surface and the bulk of cathode materials simultaneously. Another example is the use of time-resolved XRD studies of the decomposition of reactions of charged cathodes at elevated temperatures. Measurements were done both in the dry state and in the presence of electrolyte. Brief reports are also given on two new synchrotron techniques. One is inelastic X-ray scattering, and the other is synchrotron X-ray reflectometry studies of the surface electrode interface (SEI) on highly oriented single crystal lithium battery cathode surfaces. Dedicated to the 85th birthday of John O’M. Bockris     

光电流谱、光致发光光谱和紫外可见吸收光谱在纳米半导体光电器件研究中的联用

在纳米半导体中由于纳米效应(如量子尺寸效应),其电子结构与块体半导体有所不同。进一步地,当纳米半导体与基底和其他组分结合制成器件后,其性质又受到基底或其他组分的影响,这两点导致了基于纳米半导体的光电器件的性能以及相应表征方法也大不相同。将光电流谱、光致发光光谱和紫外可见吸收光谱三种技术有机地结合起来,可以更好地表征纳米半导体的电子性质和光电性能。本文根据纳米半导体材料与电极的电子性质特点及其测量,结合本课题组前期工作,举例介绍三种谱学方法相结合应用于探究光伏电池和电致发光器件的纳米半导体材料的性能,以及纳米半导体材料表面态的表征。     

First-principles-based calculations of vibrational normal modes in polyatomic materials with translational symmetry: application to PETN molecular crystal

Numerical studies of vibrational energy transport and associated (non)linear infrared and Raman response in polyatomic materials require knowledge of the multidimensional vibrational potential-energy surface and the ability to perform normal-mode analysis on that potential. The presence of translational symmetry, as in crystals, leads to the observed dispersion of the unit cell normal modes and has to be accounted for in calculations of energy transfer rates and other spectroscopic quantities. Here we report on the implementation of a computational approach that combines the generalized supercell method and density functional theory electronic structure calculations to investigate the vibrational structure in translationally symmetric materials containing relatively large numbers of atoms in the unit cell (58 atoms in the present study). The method is applied to calculate the phonon and vibron dispersion relations and the vibrational density of states in pentaerythritol tetranitrate (PETN) molecular crystal which is an important energetic material. The results set the stage for future investigations of vibrational energy transport and associated nonlinear spectroscopic signatures in this class of materials.     

Electronic states and the influence of oxygen addition on the optical absorption behaviour of manganese phthalocyanine

The effects of electronic states and air exposure on the spectroscopic properties of manganese phthalocyanine (MnPc) have been examined. The observed features of the Q-band in the absorption spectra can be explained by intrinsic electronic properties of MnPc, i.e., the formation of singly charged molecules by charge transfer excitations. However, the reaction of MnPc with atmospheric molecular oxygen leads to deviations in peak intensities but does not change the fundamental characteristics of the spectra. Nevertheless, the reaction with oxygen changes the spin state from S = 3/2 to S = 1/2. X-ray diffraction measurements also indicate a slow diffusion process of the oxygen into the MnPc crystal. We discuss both influences to explain the behaviour of MnPc in various spectroscopic methods (EELS, ellipsometry, PES). Furthermore, we support the experimental investigations by detailed ab-initio calculations of spectroscopic properties using methods of the density functional theory framework.     

On the electronic structures and spectroscopic properties of polyyne and its derivatives

We present a theoretical study on the electronic structure and spectroscopic properties of polyyne and its derivatives using periodic density functional theory study and semiempirical ZINDO/s methods. The electronic density of states were calculated, which indicates that polyyne behaves like a semiconductor and the insertion of aromatic groups into the polymer chain gives rise to significant metallic characters in the materials. The vibrational and UV–vis spectra of polyyne and its derivatives were simulated using a monomer or oligomer model. The effects of aromatic insertion into polyyne chain on their electronic and spectroscopic properties were discussed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Electronic structure of A- and B-site doped lanthanum manganites: a combined X-ray spectroscopic study

The electronic properties of a series of colossal magnetoresistance (CMR) compounds, namely LaMnO3, La(1-x)Ba(x)(MnO3 (0.2 < or = x < or = 0.55), La(0.76)Ba(0.24)Mn(0.84)Co(0.16)O3, and La(0.76)Ba(0.24)Mn(0.78)Ni(0.22)O3, have been investigated in a detailed spectroscopic study. A combination of X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) was used to reveal a detailed picture of the electronic structure in the presence of Ba, Co, and Ni doping in different concentrations. The results are compared with available theory. The valence band of La(1-x)()Ba(x)MnO3 (0 < or = x < or = 0.55) is dominated by La 5p, Mn 3d, and O 2p states, and strong hybridization between Mn 3d and O 2p states is present over the whole range of Ba concentrations. Co-doping at the Mn site leads to an increased occupancy of the e(g) states near the Fermi energy and an increase in the XPS valence band intensity between 0.5 and 5 eV, whereas the Ni-doped sample shows a lower density of occupied states near the Fermi energy. The Ni d states are located in a band spanning the energy range of 1.5-5 eV. XAS spectra indicate that the hole doping leads to mixed Mn 3d-O 2p states. Furthermore, RIXS at the Mn L edge has been used to probe d-d transitions and charge-transfer excitations in La(1-x)Ba(x)MnO3.     

Extended triphenylamine conjugated systems derivatized by perfluorophenyl groups

Two triphenylamine derivatives bearing terminal perfluorophenyl groups have been synthesized. Their HOMO, LUMO levels and electronic band gap have been evaluated by spectroscopic and electrochemical measurements and rationalized with theoretical calculations. X-ray structure analysis of crystals allowed the observation of multiple intermolecular interactions due to the presence of the perfluorophenyl pendant groups. The multiplication of these interactions explains the differences between calculated (in gas phase) and observed (in solid states) structures.     

Ab initio configuration-interaction study of the ground and low-lying electronic states of NiI

For the first time, we have studied the potential-energy curves, spectroscopic terms, vibrational levels, and the spectroscopic constants of the ground and low-lying excited states of NiI by employing the complete active space self-consistent-field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified six low-lying electronic states of NiI with doublet spin multiplicities, including three states of Delta symmetry and three states of Pi symmetry of the molecule within 15 000 cm(-1). The lowest (2)Delta state is identified as the ground state of NiI, and the lowest (2)Pi state is found at 2174.56 cm(-1) above it. These results fully support the previous conclusion of the observed spectra although our computational energy separation of the two states is obviously larger than that of the experimental values. The present calculations show that the low-lying excited states [13.9] (2)Pi and [14.6] (2)Delta are 3 (2)Pi and 3 (2)Delta electronic states of NiI, respectively. Our computed spectroscopic terms, vibrational levels, and spectroscopic constants for them are in good agreement with the experimental data available at present. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.     

Unveiling the High-valence Oxygen Degradation Across the Delithiated Cathode Surface

Charge compensation on anionic redox reaction (ARR) has been promising to realize extra capacity beyond transition metal redox in battery cathodes. The practical development of ARR capacity has been hindered by high-valence oxygen instability, particularly at cathode surfaces. However, the direct probe of surface oxygen behavior has been challenging. Here, the electronic states of surface oxygen are investigated by combining mapping of resonant Auger electronic spectroscopy (mRAS) and ambient pressure X-ray photoelectron spectroscopy (APXPS) on a model LiCoO₂ cathode. The mRAS verified that no high-valence oxygen can sustain at cathode surfaces, while APXPS proves that cathode electrolyte interphase (CEI) layer evolves and oxidizes upon oxygen gas contact. This work provides valuable insights into the high-valence oxygen degradation mode across the interface. Oxygen stabilization from surface architecture is proven a prerequisite to the practical development of ARR active cathodes.     

Resonant inelastic X-ray scattering and X-ray absorption spectroscopy on the negative electrode material Li0.5Ni0.25TiOPO4 in a Li-ion battery

We have studied the first lithiation/delithiation cycle of the Li-ion battery electrode material Li(x)Ni(0.25)TiOPO(4) applying X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS). A set of ten identical Li(x)Ni(0.25)TiOPO(4) battery electrodes have been cycled and left in different states of charge in the range of x = 0.5 … 2.5, before disassembly in an Ar filled glove box. We find that Ni-, Ti-, and O-ions are affected simultaneously, rather than sequentially, upon lithiation of the material. In particular, Ni is reduced from Ni(2+) to Ni(0) but only partially re-oxidized to Ni(1+), again, by delithiation. Overall, there is considerable "crosstalk" between the different atomic species and non-linearity in the response of the electronic structure during the lithiation/delithiation process. Fortuitously, the background variation in Ni L-XAS shows to contain valuable information about solid-electrolyte interface (SEI) creation, showing that the SEI is a function of the degree of lithiation.     

High-resolution spectroscopy of methyl 4-hydroxycinnamate and its hydrogen-bonded water complex

The electronic structure and spectroscopic properties of the lower excited singlet states of methyl 4-hydroxycinnamate, a model for the chromophore of the photoactive yellow protein in neutral form, have been investigated using various high-resolution gas-phase spectroscopic techniques and quantum-chemical calculations. The experiments show that under our experimental conditions the molecule can adopt four conformations with similar spectroscopic properties. From the detailed assignment of the vibrationally active modes in excitation and emission spectra, it is concluded that the S(1) and S(2) states should be assigned to the V' and V pipi* states that are characterized by, respectively, small and large contributions of the HOMO --> LUMO excitation. We find that complexation with a single water molecule affects the spectroscopic properties of methyl 4-hydroxycinnamate considerably in terms of stabilization of the lowest excited singlet state but in particular with respect to the transition intensities. The latter observation is tentatively interpreted as being caused by an increase in the oscillator strength of the respective electronic transition as well as by a rise/removal of conical intersections with the pisigma* state.     

Electron-vibrational dynamics of photoexcited polyfluorenes

The highly polarizable pi-electron system of conjugated molecules forms the basis for their unique electronic and photophysical properties, which play an important role in numerous biological phenomena and make them important materials for technological applications. We present a theoretical investigation of the dynamics and relaxation of photoexcited states in conjugated polyfluorenes, which are promising materials for display applications. Our analysis shows that both fast (approximately 20 fs) and slow (approximately 1 ps) nuclear motions couple to the electronic degrees of freedom during the excited-state dynamics. Delocalized excitations dominate the absorption, whereas emission comes from localized (self-trapped) excitons. This localization is attributed to an inherent nonlinear coupling among vibronic degrees of freedom which leads to lattice and torsional distortions and results in specific signatures in spectroscopic observables. Computed vertical absorption and fluorescence frequencies as well as photoluminescence band shapes show good agreement with experiment. Finally, we demonstrate that dimerization such as spiro-linking does not affect the emission properties of molecules because the excitation becomes confined on a single chain of the composite molecule.