Lithium batteries are widely used in electronic products and automobiles as new energy sources. In recent years, the state has vigorously supported the new energy industry, and many domestic and foreign companies and research institutes have increased their input and continuously researched new materials to improve various aspects of lithium battery performance. Lithium-ion materials and related full-cell, half-cell, and battery packs undergo a series of tests before being put into production. Here’s a summary of several common test methods for lithium-ion materials.The most intuitive structural observations: scanning electron microscopy (SEM) and transmission electron microscopy (TEM)Scanning electron microscope (SEM)Since the observation scale of the battery material is in the sub-micron range of several hundreds of nanometers to several micrometers, the ordinary optical microscope cannot meet the observation requirements, and a higher magnification electron microscope is often used to observe the battery material.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, using a very narrow electron beam to scan the sample, through the electron beam and The interaction of the sample produces various effects, which are mainly the secondary electron emission of the sample. Scanning electron microscopy can observe the particle size and uniformity of lithium-ion materials, as well as the special morphology of nanomaterials themselves. Even by observing the deformation of materials during the cycle, we can judge whether the corresponding cycle-keeping ability is good or bad. As shown in Figure 1b, titanium dioxide fibers have a special network structure that provides good electrochemical performance.Fig. 1: (a) Scanning electron microscopy (SEM) structural schematic; (b) Photographs obtained by SEM testing (TiO2 nanowires)1.1 SEM scanning electron microscope principle:As shown in Figure 1a, SEM is the use of electron beam bombardment of the sample surface, causing secondary electrons such as signal emission, the main use of SE and amplification, transmission of information carried by SE, point-by-point imaging in time series, imaging on the tube.1.2 Scanning electron microscope features: (1) Strong stereoscopic image and observable thickness (2) Sample preparation is simple and larger samples can be observed (3) Higher resolution, 30 to 40? (4) The magnification can be continuously variable from 4 times to 150,000 (5) Can be equipped with accessories for quantitative and qualitative analysis of micro-area1.3 Observing objects:Powders, granules, and bulk materials can all be tested. No special treatment is required except that they are kept dry before testing. It is mainly used to observe the surface morphology of the sample, the structure of the split surface, and the structure of the inner surface of the lumen. It can intuitively reflect the specific size and distribution of the particle size of the material.2. TEM transmission electron microscopeFigure 2: (a) Structural schematic of a TEM transmission electron microscope; (b) TEM test photo (Co3O4 nanosheet)2.1 Principle: The incident electron beam is used to pass through the sample to produce an electronic signal that carries the cross-section of the sample. It is then imaged on a fluorescent plate after being amplified by a multi-level magnetic lens, and the entire image is established at the same time.2.2 Features: (1) Thin sample, h<1000 ? (2) 2D planar image, poor stereoscopic effect (3) High resolution, better than 2 ? (4) Complex sample preparation2.3 Observing objects:Nano-scale materials dispersed in the solution need to be dripped on the copper mesh before use, prepared in advance and kept dry. The main observation is the internal ultrastructure of the sample. The HRTEM high-resolution transmission electron microscope can observe the corresponding lattice and crystal plane of the material. As shown in Figure 2b, observing the 2D planar structure has a better effect, with a poor stereoscopic quality relative to the SEM, but with higher resolution, more subtle parts can be observed, and the special HRTEM can even observe the material Crystal surface and lattice information.3. Material Crystal Structure Test: (XRD) X-ray Diffraction TechnologyX-ray diffraction (XRD) technology. Through X-ray diffraction of the material, analysis of its diffraction pattern, to obtain the composition of the material, the internal atom or molecular structure or morphology of the material and other information research methods. X-ray diffraction analysis is the main method for studying the phase and crystal structure of a substance. When a substance (crystal or non-crystal) is subjected to diffraction analysis, the substance is irradiated with X-rays to produce different degrees of diffraction. The composition, crystal form, intramolecular bonding, molecular configuration, and conformation determine the production of the substance. Unique diffraction pattern. The X-ray diffraction method has the advantages of not damaging the sample, no pollution, rapidity, 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 for the analysis of material structure and composition has been widely used in research and production of various disciplines.Figure 3: (a) XRD spectrum of lithium-ion material; (b) Principle structure of X-ray diffractometer3.1 Principle of XRD: When X-ray diffraction is projected into a crystal as an electromagnetic wave, it will be scattered by atoms in the crystal. Scattered waves are emitted from the center of the atom. The scattered waves emitted from the center of each atom resemble the source spherical wave. Since 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 reinforce each other and cancel each other in some directions, resulting in diffraction phenomena. The arrangement of atoms inside each crystal is unique, so the corresponding diffraction pattern is unique, similar to human fingerprints, so that phase analysis can be performed. Among them, the distribution of diffraction lines in the diffraction pattern is determined by the size, shape, and orientation of the unit cell. The intensity of the diffraction lines is determined by the type of atoms and their position in the unit cell. By using the Bragg equation: 2dsinθ=nλ, we can obtain X-rays excited by different materials using fixed targets to generate characteristic signals at special θ-angles, ie characteristic peaks marked on the PDF card.3.2 XRD test features:The XRD diffractometer has a wide applicability and is usually used to measure powder, monocrystalline or polycrystalline bulk materials, and has the advantages of rapid detection, simple operation, and convenient data processing. It is a standard conscience product. Not only can be used to detect lithium materials, most crystal materials can use XRD to test its specific crystal form. Figure 3a shows the XRD spectrum corresponding to the lithium-ion material Co3O4. The crystal plane information of the material is marked on the figure according to the corresponding PDF card. The crystallization peak of the corresponding black block material in this figure is narrow and highly apparent, indicating that its crystallinity is very good.3.3 Test object and sample preparation requirements:Powder samples or flat samples with a smooth surface. Powder samples require grinding, the sample surface to be flattened, reducing the stress effect of the measured sample.4. Electrochemical Performance (CV) Cyclic Voltammetry and Cyclic Charge and DischargeLithium battery materials belong to the electrochemical range, so a corresponding series of electrochemical tests is essential.CV test: A commonly used electrochemical research method. The method controls the electrode potential at different rates and repeatedly scans with the triangular waveform one or more times over time. The potential range is to alternately generate different reduction and oxidation reactions on the electrode and record the current-potential curve. According to the shape of the curve, the degree of reversibility of the electrode reaction, the possibility of adsorption of the intermediate or phase boundary or the formation of a new phase, and the nature of the coupling chemical reaction can be judged. Commonly used to measure the electrode reaction parameters, determine the control steps and reaction mechanism, and observe what reaction can occur within the entire potential scan range, and how their nature. For a new electrochemical system, the preferred method of study is often cyclic voltammetry, which can be referred to as “electrochemical spectroscopy.” In addition to using mercury electrodes, this method can also use platinum, gold, glassy carbon, carbon fiber microelectrodes, and chemically modified electrodes.Cyclic voltammetry is a useful electrochemical method for the study of the nature, mechanism, and kinetic parameters of electrode processes. For a new electrochemical system, the preferred method of study is often cyclic voltammetry. Due to the large number of affected factors, this method is generally used for qualitative analysis and is rarely used for quantitative analysis.Figure 4: (a) CV cycle diagram of the reversible electrode; (b) Constant current cycle charging and discharging test of the batteryConstant Current Cycling Charging and Discharging Test: After the lithium battery is assembled into the corresponding battery, charge and discharge are required to test the cycle performance. The charge-discharge process often uses a galvanostatic charge-discharge method, discharges and charges at a fixed current density, limits voltage or specific capacity conditions, and performs cycle testing. There are two kinds of testers commonly used in laboratories: Wuhan Blue Power 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 group of lithium battery assembled batteries. We can see that the black bulk material can be circulated for 60 circles, and the red NS material can be circulated over 150 circles.Summary: There are many test techniques for lithium battery materials. The most common ones are the above-mentioned SEM, TEM, XRD, CV and cycle test. There are also Raman spectroscopy (Raman), infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and energy spectrum analysis (EDS) of electron microscope attachments, electron energy loss spectroscopy (EELS) to determine the material particle size and porosity. Rate of BET surface area test. Even neutron diffraction and absorption spectroscopy (XAFS) can be used in some cases.In the past 30 years, the lithium battery industry has developed rapidly and gradually replaced traditional fuels such as coal and petroleum for use in automotive and other power equipment. The characterization and detection methods developed along with it have also continued to improve and promote progress in the field of lithium batteries.
Fuente: Meeyou Carbide