[Dry goods] Lithium-ion battery material testing technology

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

X-ray diffraction (XRD). By X-ray diffraction of the material, the diffraction pattern is analyzed to obtain information on the composition of the material, the structure or morphology of the atoms or molecules inside the material. X-ray diffraction analysis is the main method for studying the phase and crystal structure of a substance. When a substance (crystal or amorphous) is subjected to diffraction analysis, the substance is irradiated by X-rays to produce different degrees of diffraction. The composition of the substance, the crystal form, the intramolecular bonding mode, the configuration of the molecule, and the conformation determine the substance. A unique diffraction pattern. The X-ray diffraction method has the advantages of no damage to the sample, no pollution, fast, high measurement accuracy, and a large amount of information about crystal integrity. Therefore, X-ray diffraction analysis, as a modern scientific method for material structure and composition analysis, has been widely used in research and production in various disciplines.

Figure 3: (a) XRD spectrum of lithium battery; (b) Schematic diagram of X-ray diffractometer

3.1 Principle of XRD: When X-ray diffraction is projected into a crystal as an electromagnetic wave, it is scattered by atoms in the crystal. The scattered wave is emitted from the center of the atom. The scattered wave emitted from the center of each atom is similar to the source spherical wave. Since the atoms are periodically arranged in the crystal, there is a fixed phase relationship between the scattered spherical waves, which causes the spherical waves in some scattering directions to reinforce each other and cancel each other in some directions, thereby causing diffraction phenomenon. The arrangement of the atoms inside each crystal is unique, so the corresponding diffraction pattern is unique, similar to human fingerprints, so phase analysis can be performed. Among them, the distribution law of the diffraction line 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: 2dsin θ = nλ, we can obtain different characteristic materials by using X-rays excited by fixed targets to generate characteristic signals at special θ angular positions, that is, characteristic peaks marked on PDF cards.

3.2 XRD test features:

XRD diffractometer is widely used for measuring bulk materials such as powder, single crystal or polycrystalline crystal, and 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-electric materials, most of the crystal materials can be tested for their specific crystal form by XRD. Figure 3a shows the XRD spectrum of the lithium-electric material Co3O4. The crystal surface information of the material is indicated on the corresponding PDF card. The black corresponding block material has a narrow crystal peak and a high height, indicating that the crystallinity is good.

3.3 Test object and sample preparation requirements:

A powder sample or a flat sample with a smooth surface. Powder samples are required to be ground and the surface of the sample is flattened to reduce the stress effect of the measured sample.

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

Lithium battery materials are in 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 scans one or more times with a triangular waveform over time. The potential range is such that different reduction and oxidation reactions can alternately occur on the electrode, and the current-potential curve is recorded. According to the shape of the curve, the degree of reversibility of the electrode reaction, the possibility of intermediate, phase boundary adsorption or formation of a new phase, and the nature of the coupling chemical reaction can be judged. It is often used to measure electrode reaction parameters, determine its control steps and reaction mechanism, and observe which reactions can occur within the entire potential sweep range, and how they are. For a new electrochemical system, the preferred research method is often cyclic voltammetry, which can be called "electrochemical spectrum." In addition to mercury electrodes, this method can also use platinum, gold, glassy carbon, carbon fiber microelectrodes, and chemically modified electrodes.

Cyclic voltammetry is a useful electrochemical research method for the study of the nature, mechanism and electrode process kinetic parameters of electrode reactions. For a new electrochemical system, the preferred method of research is often cyclic voltammetry. Due to the large number of factors affected, 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 charge and discharge test of the battery

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 for the cycle performance test. The charging and discharging process is often carried out by means of constant current charging and discharging, discharging and charging at a fixed current density, limiting the voltage or specific capacity conditions, and performing a cyclic test. The testers commonly used in the laboratory are 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 set of lithium battery materials assembled with batteries. We can see that the black bulk material can be cycled for 60 cycles and the red NS material can be cycled for more than 150 cycles.

Summary: There are many testing techniques for lithium battery materials. The most common ones are SEM, TEM, XRD, CV and cycle testing. In addition, Raman, infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and electron spectroscopy (EDS), electron energy loss spectroscopy (EELS), to determine material size and porosity Rate BET specific surface area test method. Even characterization methods such as neutron diffraction and absorption spectroscopy (XAFS) can be used.

In the past 30 years, the lithium battery industry has developed rapidly and will gradually replace traditional fuels such as coal and petroleum in power equipment such as automobiles. The development of characterization and detection methods has also continuously improved and promoted the progress in the field of lithium batteries.