In recent years, the lithium battery articles published in internationally renowned journals can either achieve breakthrough performance or thoroughly study the mechanism. The mechanism research is to test the academic ability of scientific researchers and the abundance of scientific research funds. In addition, mechanism research requires advanced equipment and even in-situ characterization equipment to study the reaction of materials. At present, materials research and characterization methods can be described as diverse. This editor only summarizes the mechanism research methods of some common lithium batteries and other energy storage materials. Limited to the level, there must be omissions, welcome everyone to add.
The editor is divided into four categories according to common material characterization analysis, material structural component characterization, material morphology characterization, material physical and chemical characterization and theoretical calculation analysis.
Material structural component characterization
At present, XRD, NMR, XAS and other advanced characterization techniques are involved in the characterization of common structural components of energy storage materials. In addition, current research is increasingly transitioning from ex-situ characterization to in-situ characterization. Use the advantages of real-time analysis of in-situ characterization to explore the changes in the material during the reaction. In addition, more and more research work began to involve the use of XAS and other characterizations that require the use of synchrotron radiation technology, and it is even more important to seize the limited synchrotron radiation source resources.
X-ray absorption near edge structure (XANES), also known as near edge X-ray absorption fine structure (NEXAFS), is a type of absorption spectrum. In the X-ray absorption spectrum, the spectrum in the low-energy region within 60eV above the threshold has strong absorption characteristics, which is called the near-edge absorption structure (XANES). It is caused by the excitation of photoelectrons undergoing multiple scattering from surrounding atoms. It not only reflects the geometrical configuration of atoms in the surrounding environment of absorbing atoms, but also reflects the structure of low-energy electronic states near the Fermi level of condensed matter. Therefore, it has become a useful tool for studying the chemical environment of materials and their defects. At present, the domestic synchrotron radiation light source devices mainly include the Beijing synchrotron radiation device (BSRF, the first-generation light source), the Hefei synchrotron radiation device of the University of Science and Technology of China (NSRL, the second-generation light source) and the Shanghai Light Source (SSRF, the third-generation light source) Light source), which has played a huge role in many domestic materials science research.
Recently, Wang Hailiang’s research team used XANES and other advanced characterization techniques to study defect-rich single crystal ultra-thin Co3O3 nanosheets and their electrochemical properties (Adv. Energy Mater. 2018, 8, 1701694), as shown in Figure 1. The research work used XANES and other technologies to analyze the chemical environment of the defect-rich cobalt tetroxide, thereby proving the existence and relative content of oxygen defects. In addition, it is proved by EAXFS that Co in the defect-rich Co3O4 has a lower coordination number. The existence of these conditions helps to reduce the surface energy, so that the material has good stability. The use of synchrotron radiation technology to characterize material defects, and the use of chemical environment for mechanism research has become a current research hotspot.
Figure 1. Analysis of O-vacancy defects on the reduced Co3O4 nanosheets. (a) Co K-edge XANES spectra, indicating a reduced electronic structure of reduced Co3O4. (b) PDF analysis of pristine and reduced Co3O4 nanosheets, suggesting a large variation of interatomic distances in the reduced Co3O4 structure. (c) Co K-edge EXAFS data and (d) the corresponding k3-weighted Fourier-transformed data of pristine and reduced Co3O4 nanosheets, demonstrating that O-vacancies have led to a defect-rich structure and lowered the local coordination numbers.
The full name of XRD is X-ray diffraction, which is to analyze the diffraction pattern of the material by X-ray diffraction to obtain the structure and composition of the material. It is a commonly used method for characterizing structural components of battery materials.
In-situ XRD technology is an important analysis method in the current energy storage field research. It can not only eliminate the influence of external factors on electrode materials, improve the authenticity and reliability of data, but also monitor the electrochemical process of electrode materials in real time. , In the real-time process of electrochemical reaction to characterize the changes in its structure and components, so that the overall reaction of the system can be analyzed and processed more clearly, and its intrinsic reaction mechanism can be revealed. Therefore, the introduction of in-situ XRD characterization technology can improve our understanding of the energy storage mechanism of electrode materials and will quickly promote the development of high-performance energy storage devices.
At present, Chen Zhongwei’s research group has made breakthrough progress in the research of lithium-sulfur batteries. Researchers have used in-situ XRD technology to characterize the charging and discharging process of small molecule anthraquinone compounds as lithium-sulfur batteries and explain the reaction mechanism. (NATURE COMMUN., 2018, 9, 705), as shown in Figure 2. Various characterizations confirmed that the ketone functional group and polysulfide in the anthraquinone molecule formed Lewis acid through strong chemisorption, which is the key to improving the cycle stability of lithium-sulfur batteries. Through the “chemical adsorption” of small molecule anthraquinone and soluble lithium polysulfide during charge and discharge, insoluble products that cannot be dissolved in the electrolyte are formed, thereby effectively inhibiting the loss of active materials and significantly increasing the life of the battery.
Fig. 2 In-situ XRD analysis of the interactions during cycling. (a) XRD intensity heat map from 4 o to 8.5o of a 2.4 mg cm–2 cell’s first cycle discharge at 54 mA g–1 and charge at 187.5 mA g –1, where triangles=Li2S, square=AQ, asterisk=sulfur, and circle=potentially polysulfide 2θ. (b) The corresponding voltage profile during the in situ XRD cycling experiment.
Material morphology characterization
In the research field of materials science, the commonly used morphology characterization mainly includes SEM, TEM, AFM and other microscope imaging techniques. At present, the morphology characterization of materials is the necessary supporting data for most materials science research. A novel and fascinating electron micrograph of the morphology is also the only way to publish high-level papers. The current research papers are increasingly focused on the research of nanomaterials, and use ultra-high resolution electron microscopes such as spherical aberration TEM to characterize nanometer-sized materials, and use high-resolution electron microscopes supplemented by EDX, EELS And other element analysis plug-ins to analyze and test, in order to obtain clear images and data and do analysis and processing.
The full name of TEM is transmission electron microscope, which is to project an accelerated and concentrated electron beam onto a very thin sample. The electrons collide with the atoms in the sample and change direction, thereby generating solid angle scattering. The size of the scattering angle is related to the density and thickness of the sample, so images with different brightness and darkness can be formed. The images will be displayed on the imaging device after being enlarged and focused. Using in-situ TEM and other technologies, real-time changes in the morphology and structure of the material can be obtained, such as the transformation of the microstructure or the change of the chemical composition. In the research of lithium-sulfur batteries, it is of great practical significance to use in-situ TEM to observe the morphology and phase transition of materials. Kim’s group used in-situ TEM and other morphology and structure characterization in the research of lithium-sulfur battery cathodes, and in-depth study of the relationship between the electrochemical performance of the material and its morphology and structure (Adv. Energy Mater., 2017, 7, 1602078.), as shown in Figure 3.
This work uses a porous carbon nanofiber sulfur composite material as the positive electrode of a lithium-sulfur battery. When charging and discharging at a high rate, in-situ TEM is used to observe the morphological changes of the material and the volume expansion of sulfur, which provides a new method to study sulfur The electrochemical performance and the volume expansion effect are linked together.
Fig. 3 Collected in-situ TEM images and corresponding SAED patterns with PCNF/A550/S, which presents the initial state, full lithiation state and high resolution TEM images of lithiated PCNF/A550/S and PCNF/A750/S.
Material physicochemical characterization
UV-vis spectroscopy is called ultraviolet-visible light absorption spectroscopy. Absorption spectra can be used for qualitative analysis and simple material structure analysis using the characteristics of absorption peaks, and can also be used for quantitative analysis of material absorption. UV-vis is a simple and commonly used effective means of characterizing inorganic and organic substances. It is often used to characterize specific products and reaction processes in liquid phase reactions, such as the determination of polysulfides in lithium-sulfur battery systems.
Recently, Yan Chenglin’s research group (Nano Lett., 2017, 17, 538-543) used the reflection mode of in-situ ultraviolet-visible light spectroscopy to detect the formation of polysulfides during the charging and discharging of lithium-sulfur batteries, according to the peak intensity at different positions in the spectrum. Real-time acquisition of polysulfide types and content changes during charging and discharging, as shown in Figure 4. Researchers found that when selenium doping is introduced into the material, the amount of long-chain polysulfides produced during the discharge of lithium-sulfur batteries is significantly reduced, which effectively inhibits the shuttle effect of polysulfides and improves the coulombic efficiency and capacity retention. The efficiency of lithium-sulfur batteries has opened up a new way for the study of the mechanism of lithium-sulfur batteries and their practical application.
Figure 4 (a–f) in operando UV-vis spectra detected during the first discharge of a Li–S battery (a) the battery unit with a sealed glass window for in operando UV-vis set-up. (b) Photographs of six different catholyte solutions; (c) the collected discharge voltages were used for the in situ UV-vis mode; (d) the corresponding UV-vis spectra first-order derivative curves of different stoichiometric compounds; the corresponding UV-vis spectra first- order derivative curves of (e) rGO/S and (f) GSH/S electrodes at C/3, respectively.
Theoretical calculation analysis
With the vigorous development of energy materials, computational materials science such as density functional theory calculations, molecular dynamics simulation and other fields have also been greatly improved, and now it has become an important foundation and core technology for material calculation and simulation on the atomic scale. , Provide a solid theoretical analysis basis for the research and development of new materials.
Density functional theory calculation (DFT)
The energy change of the system can be obtained by DFT calculation, which can be used to calculate the energy difference of the material from the initial state to the final state. Through different systems or calculations, energy values such as adsorption energy, activation energy and so on can be obtained. In addition, molecular dynamics simulation and Monte Carlo simulation of the dynamic behavior and structural characteristics of materials can also be used. Recently, Ceder’s research group has achieved important results in the research of new lithium-rich material cathodes (Nature 2018, 556, 185-190), as shown in Figure 5. This study uses Monte Carlo simulation calculations to explain the changes of Li2Mn2/3Nb1/3O2F materials during charging and discharging and their effects on the material structure and chemical environment. This research has also broadened its new applications in the battery field for high-performance manganese-rich cathodes.
Fig. 5 Ab initio calculations of the redox mechanism of Li2Mn2/3Nb1/3O2F. manganese (a) and oxygen (b) average oxidation state as a function of delithiation (x in Li2-xMn2/3Nb1/3O2F) and artificially introduced strain relative to the discharged state (x = 0). c, Change in the average oxidation state of Mn atoms that are coordinated by three or more fluorine atoms and those coordinated by two or fewer fluorine at
Fig. 5 Ab initio calculations of the redox mechanism of Li2Mn2/3Nb1/3O2F. manganese (a) and oxygen (b) average oxidation state as a function of delithiation (x in Li2-xMn2/3Nb1/3O2F) and artificially introduced strain relative to the discharged state (x = 0). c, Change in the average oxidation state of Mn atoms that are coordinated by three or more fluorine atoms and those coordinated by two or fewer fluorine atoms. d, Change in the average oxidation state of O atoms with three, four and five Li nearest neighbours in the fully lithiated state (x = 0). The data in c and d were collected from model structures without strain and are representative of trends seen at all levels of strain. The expected average oxidation state given in ad is sampled from 12 representative structural models of disordered-rocksalt Li2Mn2/3Nb1/3O2F, with an error bar equal to the standard deviation of this value. e, A schematic band structure of Li2Mn2/3Nb1/3O2F.
At present, research in the field of lithium-ion batteries and other batteries is still in full swing. However, most research papers still focus on the use of conventional characterization to analyze materials. Some mechanisms are difficult to prove by the data obtained by conventional characterization equipment. In addition, in-depth mechanism research still needs to be explored in depth. Therefore, we can thoroughly study the reaction mechanism in the material, combine the use of difficult experimental work and use powerful technical means such as in-situ characterization to monitor the reaction process in real time, and at the same time increase the intensity of basic research and fully explain the reaction mechanism is a high-level publication The main way of the article. In addition, it is even more important to combine various research methods and multi-disciplinary fields to provide perfect experimental evidence to prove one’s own views.
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