Demystifying lithium-ion battery material testing technology

　　As a new energy source, lithium batteries are widely used in electronic products and automobiles. In recent years, the state has vigorously supported the new energy industry, and many domestic and foreign related companies and research institutes have increased their investment to continuously research new materials to improve the performance of lithium batteries in all aspects.

　　Lithium batteries as a new energy source are widely used in electronic products and automobiles. In recent years, the state has vigorously supported the new energy industry, and many domestic and foreign related companies and research institutes have increased their investment to continuously research new materials to improve the performance of lithium batteries in all aspects. The lithium battery materials and related full-cell, half-cell, and battery packs need to undergo a series of tests before they are put into production and application. Let me summarize several commonly used testing methods for lithium battery materials below.

　　1. Scanning Electron Microscope (SEM)

　　Since the observation scale of battery materials is in the range of sub-micron, that is, a few hundred nanometers to several microns, ordinary optical microscopes cannot meet the needs of observation, and electron microscopes with higher magnification are often used to observe battery materials.

　　Scanning electron microscope (SEM) is a relatively modern cell biology research tool invented in 1965. It mainly uses secondary electron signal imaging to observe the surface morphology of the sample. That is, a very narrow electron beam is used to scan the sample. The interaction of the sample produces various effects, among which is the secondary electron emission of the sample. Scanning electron microscopy can observe the particle size and uniformity of lithium battery materials, as well as the special morphology of the nanomaterials themselves, and even by observing the deformation of the material during the cycle, we can judge its corresponding cycle retention ability. As shown in Figure 1b, the special network structure of the titanium dioxide fiber can provide good electrochemical performance.

　　1.1 Principle of SEM scanning electron microscope:

　　As shown in Figure 1a, SEM uses electron beams to bombard the sample surface to cause the emission of secondary electrons and other signals. It mainly uses SE to amplify and transmit the information carried by SE, image by point in time sequence, and image on the kinescope.

　　1.2 Features of Scanning Electron Microscope:

　　⑴The image has a strong sense of three-dimensionality and can observe samples of a certain thickness

　　⑵The sample preparation is simple, and larger samples can be observed

　　⑶ Higher resolution, 30~40?

　　⑷The magnification is continuously variable, from 4 times to 150,000

　　⑸ can be equipped with accessories for quantitative and qualitative analysis of micro-area

　　1.3 Observed objects:

　　Powder, granule, and block materials can be tested. Except for keeping dry before testing, no special treatment is required. It is mainly used to observe the surface morphology of the sample, the structure of the split surface, the structure of the inner surface of the lumen, etc. It can directly reflect the special structure and distribution of the particle size of the material.

　　2.TEM Transmission Electron Microscope

　　2.1 Principle:

　　mainly uses the incident electron beam to pass through the sample to generate an electronic signal carrying the inside of the cross section of the sample, which is magnified by a multi-stage magnetic lens and then imaged on the fluorescent plate, and the entire image is established at the same time.

　　2.2 Features:

　　⑴The sample is ultra-thin, h<1000?

　　⑵Two-dimensional plane image, poor stereo perception

　　⑶High resolution, better than 2?

　　⑷Complicated sample preparation

　　2.3 Observed objects:

　　The nano-scale materials dispersed in the solution need to be dropped on the copper net before use, prepared in advance and kept dry. Mainly observe the internal ultrastructure of the sample. HRTEM high-resolution transmission electron microscope can observe the corresponding crystal lattice and crystal plane of the material. As shown in Figure 2b, the observation of the two-dimensional planar structure has a better effect. Compared with the SEM, the three-dimensional perception is poor, but it can have a higher resolution and observe more subtle parts. The special HRTEM can even observe the material. The crystal plane and lattice information.

　　3. Material crystal structure test: (XRD) X-ray diffractometer technology

　　X-ray diffractometer technology (X-ray diffraction, XRD). A research method to obtain information such as the composition of the material, the structure or morphology of the internal atoms or molecules of the material by performing X-ray diffraction on the material and analyzing its diffraction pattern. X-ray diffraction analysis is the main method to study the phase and crystal structure of substances. When a substance (crystalline or amorphous) is subjected to diffraction analysis, the substance is irradiated by X-rays to produce different degrees of diffraction. The composition, crystal form, intramolecular bonding method, molecular configuration, conformation, etc. determine the production of the substance Unique diffraction pattern. The X-ray diffraction method has the advantages of no damage to the sample, no pollution, quickness, high measurement accuracy, and a large amount of information about the integrity of the crystal. Therefore, X-ray diffraction analysis, as a modern scientific method of material structure and composition analysis, has gradually been widely used in the research and production of various disciplines.

　　3.1XRD principle:

　　X-ray diffraction, as an electromagnetic wave, is projected into a crystal, it will be scattered by the atoms in the crystal, and the scattered wave appears to be emitted from the center of the atom, and the scattered wave emitted from each atom center is similar to the source spherical wave. Because the atoms are arranged periodically in the crystal, there is a fixed phase relationship between these scattered spherical waves, which will cause the spherical waves in some scattering directions to strengthen each other, and to cancel each other in some directions, resulting in diffraction. The arrangement of atoms inside each crystal is unique, so the corresponding diffraction pattern is unique, similar to a human fingerprint, so phase analysis can be carried out. Among them, the distribution law of diffraction lines in the diffraction pattern is determined by the size, shape and orientation of the unit cell. The intensity of the diffraction line is determined by the type of atoms and their position in the unit cell. Through the Bragg equation: 2dsinθ=nλ, we can get different materials to generate characteristic signals at special θ angle positions by X-rays excited by a fixed target, that is, the characteristic peaks marked on the PDF card.

　　3.2XRD test features:

　　XRD diffractometer has a wide range of applicability. It is usually used to measure bulk materials such as powder, single crystal or polycrystal. It has the advantages of fast detection, simple operation, and convenient data processing. It is a standard "conscience product". Not only can it be used to detect lithium battery materials, most of the crystal materials can be tested for their specific crystal form by XRD. Figure 3a is the XRD spectrum corresponding to the lithium battery material Co3O4. The crystal plane information of the material is marked on the figure according to the corresponding PDF card. The black color of the figure corresponds to the narrow crystal peak of the bulk material and its height is obvious, indicating that its crystallinity is very good.

　　3.3 Test object and sample preparation requirements:

　　Powder samples or block samples with a flat surface. Powder samples need to be ground evenly, and the surface of the sample should be leveled to reduce the stress effect of the measured sample.

　　4. Electrochemical performance (CV) cyclic voltammetry and cyclic charge and discharge

　　Lithium battery materials belong to the electrochemical range, so a series of corresponding electrochemical tests are indispensable.

　　CV test: a commonly used electrochemical research method. This method controls the electrode potential to scan one or more times in a triangular waveform at different rates over time. The potential range is to allow different reduction and oxidation reactions to occur alternately on the electrode, and to record the current-potential curve. According to the shape of the curve, the degree of reversibility of the electrode reaction, the possibility of intermediates, phase boundary adsorption or new phase formation, and the nature of the coupling chemical reaction can be judged. It is commonly used to measure electrode reaction parameters, determine its control steps and reaction mechanism, and observe which reactions can occur within the entire potential scanning range and their properties. For a new electrochemical system, the preferred research method is often cyclic voltammetry, which can be called the "electrochemical spectrum". In addition to mercury electrodes, platinum, gold, glassy carbon, carbon fiber microelectrodes and chemically modified electrodes can also be used in this method.

　　Cyclic voltammetry is a very useful electrochemical research method, which can be used to study the nature and mechanism of electrode reaction and the kinetic parameters of electrode process. For a new electrochemical system, the preferred research method is often cyclic voltammetry. Because there are many affected factors, this method is generally used for qualitative analysis and is rarely used for quantitative analysis.

　　Constant current cycle charge and discharge test: After the lithium battery material is assembled into the corresponding battery, it needs to be charged and discharged to test the cycle performance. The charging and discharging process often adopts the method of constant current charging and discharging, discharging and charging with a fixed current density, limiting the voltage or specific capacity, and carrying out the cycle test. There are two testers commonly used in the laboratory: Wuhan Landian and Shenzhen Xinwei. After setting up a simple program, the cycle performance of the battery can be tested. Figure 4b is a cycle diagram of a set of lithium battery materials after assembling the battery. We can see that the black bulk material can circulate 60 times, and the red NS material can circulate more than 150 times.

　　Such traditional fuels are used in power equipment such as automobiles, and the subsequent development of characterization and detection methods have also been continuously improved and promoted the progress of the lithium battery field.