Mechanism research method for common energy storage materials such as lithium battery

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/ 2019-05-07 / News

President's Comments:

In recent years, lithium-electric articles published in internationally renowned journals are capable of doing Breakthrough performance, or the ability to thoroughly study the mechanism. The mechanism research is to test the academic ability base of scientific researchers and the abundance of research funding. In addition, mechanism research requires advanced instrumentation and even in-situ characterization equipment to study the reaction of materials. At present, the research and characterization methods of materials can be described as various. In this small series, only some common research methods of energy storage materials such as lithium batteries are summarized. Limited to the level, there must be some omissions, welcome to add.

Xiaobian is divided into four major categories based on common material characterization analysis, material structure component characterization, material morphological characterization, material physicochemical characterization and theoretical calculations. analysis.

Material structure component characterization

The current characterization of common structural components of energy storage materials involves advanced characterization techniques such as XRD, NMR, XAS, etc. The current research is also increasingly transitioning from ex situ characterization to in situ characterization. The advantages of real-time analysis of in situ characterization are used to explore the changes in the material during the reaction. The 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 capture limited synchrotron source resources.

XANES

X-ray absorption near-edge structure (XANES), also known as near-edge X-ray absorption fine structure (NEXAFS), is a type of absorption spectrum. X-ray absorption spectrum Among them, the spectrum of the low energy region within 60 eV above the threshold exhibits strong absorption characteristics, which is called the near-edge absorption structure (XANES), which is caused by the multiple scattering of the excitation photoelectrons by surrounding atoms.It not only reflects the geometric configuration of the atoms in the environment surrounding the absorption atom, but also reflects the structure of the low-energy electronic state near the Fermi level of the condensed matter, and thus becomes a useful tool for studying the chemical environment of the material and its defects. At present, domestic synchrotron radiation source devices mainly include Beijing Synchrotron Radiation Facility (BSRF, first generation light source), Hefei Synchrotron Radiation Facility (NSRL, second generation light source) and Shanghai Light Source (SSRF, third generation) of University of Science and Technology of China. Light source) has played a huge role in the research of many materials sciences in China.

Recently, Wang Hailiang's research group used XANES and other advanced characterization techniques to study the single-crystal ultra-thin four-dimensional oxidation of tri-cobalt nanoparticles The sheet and its electrochemical properties (Adv. Energy Mater. 2018, 8, 1701694) are shown in Figure 1. The research used XANES and other technologies to analyze the chemical environment of the defect-rich tricobalt tetroxide, which proved the existence and relative content of oxygen deficiency.Furthermore, it was demonstrated by EAXFS that Co in the defect-rich tricobalt tetroxide has a lower coordination number. The presence of these conditions helps to reduce the surface energy and give the material good stability. The use of synchrotron radiation technology to characterize the defects of materials, the study of the chemical environment for the mechanism has become a hot research topic.

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.

XRD

The full name of XRD is X-ray diffraction, that is, by analyzing the diffraction pattern of the material by X-ray diffraction to obtain the structure and composition of the material, it is the current battery. u> The structural component characterization methods commonly used in materials.

In-situ XRD technology is an important analytical tool in the field of energy storage research. It can not only eliminate the influence of external factors on the electrode material, but also improve the reality of the data. Sexuality and reliability, the electrochemical process of the electrode material can be monitored in real time, and the changes in its structure and composition are characterized in the real-time process of the electrochemical reaction.Therefore, it is possible to analyze and process the overall reaction of the system more clearly and reveal its intrinsic reaction mechanism. Therefore, the introduction of in-situ XRD characterization technology can enhance our understanding of the energy storage mechanism of electrode materials and will rapidly promote the development of high-performance energy storage devices.

At present, Chen Zhongwei's research team has made breakthroughs in the research on lithium-sulfur batteries. Researchers use in-situ XRD technology for small molecule ruthenium compounds. The charge and discharge process of the positive electrode of lithium-sulfur battery was characterized and its reaction mechanism (NATURE COMMUN., 2018, 9, 705) was explained, as shown in Figure 2. It is confirmed by various characterizations that the formation of Lewis acid by strong chemisorption of ketone functional groups and polysulfides in ruthenium molecules is the key to improve the cycle stability of lithium-sulfur batteries. By "chemically adsorbing" small molecule hydrazine and soluble lithium polysulfide during charge and discharge, an insoluble product that cannot be dissolved in the electrolyte is formed, thereby effectively suppressing the loss of the active material, 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 topography

In the field of materials science research, the commonly used topographical characterization mainly includes SEM, TEM, AFM and other microscope imaging techniques. At present, the morphological characterization of materials is the necessary supporting data for most material science research. A novel and fascinating SEM image is also the only way to publish high-level papers. The current research papers are increasingly focused on the research of nanomaterials, and the use of ultra-high resolution electron microscopy such as spherical aberration TEM to characterize nanoscale materials, coupled with EDX by high resolution electron microscopy, EELS A plugin for elemental analysis to analyze the test,In this way, clear images and data are obtained and analyzed.

SEM

Scanning electron microscopy (SEM) is between TEM and optical microscopy A microscopic topography observation method can directly perform microscopic imaging using the material properties of the sample surface material. The advantage of scanning electron microscope is that 1 has a higher magnification and is continuously adjustable between 20 and 200,000 times; 2 has a large depth of field, a large field of view, and a three-dimensional image, which can directly observe the uneven surface of various samples. Fine structure; 3 sample preparation is simple. The current scanning electron microscopes are equipped with an X-ray energy spectrometer device, which can simultaneously observe the microstructure morphology and micro-component composition analysis, so it is a very useful scientific research instrument today.

Detailed description of what shape the sample has and which micro-structures are formed. Briefly describe the shape, size, etc. of the sample.SEM can also be used as evidence to illustrate the previously confirmed conclusions.

Fig. 3: High-magnification SEM images show that the rods that compose these tubular structures are highly ordered, with the gold ends on the exterior of the structure and the polymerends all pointing toward the interior of The tubes.Rods with a 3:2 gold/polymer ratio also yield tubular structures but with ~ 29 μm diameters.

(Ref. Park, S. et. al. Science , 2004, 303, 348-351)

TEM

TEM is called TEM, The accelerated and concentrated electron beam is projected onto a very thin sample, and the electrons collide with atoms in the sample to change direction, thereby generating solid angle scattering. The size of the scattering angle is related to the density and thickness of the sample, so that images with different brightness and darkness can be formed, and the image will be displayed on the imaging device after being enlarged and focused. Real-time changes in material morphology and structure can be obtained by techniques such as in-situ TEM.Such as the transformation of microstructure or changes in chemical composition. In the research of lithium-sulfur batteries, it is of great practical significance to observe the morphology and phase transformation of materials by in-situ TEM. Kim's research group used in-situ TEM and other characterizations of the morphology and structure of the lithium-sulfur battery in the positive electrode research, and deeply studied the relationship between the electrochemical properties 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 nano-sulfur composite as the positive electrode of a lithium-sulfur battery, which is used when charging and discharging at a large rate. In situ TEM observation of the morphology change of the material and the volume expansion of sulfur provides a new method to study the electrochemical properties of sulfur and relate it to the volume expansion effect.

Fig. 4 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

UV-vis spectroscopy is called UV-Vis absorption spectroscopy.Absorption spectroscopy can be used for qualitative analysis and simple material structure analysis using the characteristics of the absorption peaks, and can also be used for quantitative analysis of material absorption. UV-vis is a simple and commonly used means for the effective characterization of inorganic and organic materials. It is commonly 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 lithium-sulfur battery charging by in-situ UV-Visible spectroscopy. During the formation of polysulfides during discharge, the changes in the types and contents of polysulfides during charging and discharging were obtained in real time according to the peak intensities at different positions in the map, as shown in Figure 4. The researchers found that when selenium doping is introduced into the material, the formation of long-chain polysulfide in the lithium-sulfur battery is significantly reduced during the discharge process, thereby effectively suppressing the shuttle effect of the polysulfide and improving the coulombic efficiency and capacity retention. The rate has opened up a new way for the research and practical application of lithium-sulfur batteries.

Figure 5 (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.

As the development of resources matirals,Computational materials science, such as density functional theory calculations, molecular dynamics simulation and other fields of computational applications have also been greatly improved, and now has become an important foundation and core technology of material calculation simulation on the atomic scale, providing a solid foundation for the development of new materials. The basis of theoretical analysis.

Density functional theory calculation (DFT)

Using DFT calculations to obtain system energy The change is used to calculate the difference in energy of the material from the initial state to the final state. Energy values such as adsorption energy, activation energy, and the like can be obtained by different systems or calculations. In addition, molecular dynamics simulations and Monte Carlo simulations of the dynamic behavior and structural characteristics of materials can be used. Recently, Ceder's research group has made important achievements in the research of new lithium-rich material anodes (Nature 2018, 556, 185-190), as shown in Figure 5.This study used Monte Carlo simulation to explain the changes in Li2Mn2/3Nb1/3O2F materials during charge and discharge and their effects on material structure and chemical environment. The study also broadened its new applications in the field of batteries for high-performance manganese-rich cathodes.

Fig. 6 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 is 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 Illustrated band structure of Li2Mn2/3Nb1/3O2F.

summary

current lithium Research in ion batteries and other battery fields is still in full swing. However, most research papers still focus on the analysis of materials using conventional characterization.Some mechanisms are difficult to prove by the data obtained by conventional characterization equipment, and further research on the depth of the mechanism remains to be explored. Therefore, it is possible to deeply study the reaction mechanism in materials, combine the use of difficult experimental work and use powerful techniques 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 comprehensively explain the reaction mechanism is a high level of publication. The main way of the article. In addition, combined with various research methods, combined with multidisciplinary fields and mutual evidence to give perfect experimental evidence to prove their views is even more important.

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