Lithium batteries are widely used as new energy sources in electronics and automobiles. In recent years, the state has vigorously supported the new energy industry. Many related enterprises and research institutes at home and abroad have increased investment and constantly researched new materials to improve the performance of various aspects of lithium batteries. Lithium-ion materials and related full-cell, half-cell, and battery packs require a series of tests before they are put into production. The following are some of the commonly used test methods for lithium battery materials.
The most intuitive structural observation: scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
1. Scanning electron microscopy (SEM)
Since the viewing dimensions of battery materials are in the submicron range of a few hundred nanometers to a few micrometers, ordinary optical microscopes cannot meet the observation requirements, and higher magnification electron microscopes are often used to observe battery materials.
Scanning Electron Microscopy (SEM) is a 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, to scan the sample with a very narrow electron beam, through electron beam and The interaction of the samples produces various effects, primarily secondary electron emission from the sample. The scanning electron microscope can observe the particle size and uniformity of the lithium-electric material, as well as the special morphology of the nano-material itself. Even by observing the deformation of the material during the cycle, we can judge the corresponding cycle retention ability. As shown in Figure 1b, the special network structure of the titanium dioxide fibers provides good electrochemical performance.
Figure 1: (a) Schematic diagram of scanning electron microscopy (SEM); (b) Picture obtained by SEM test (nanowire of TiO2)
1.1 SEM scanning electron microscope principle:
As shown in Fig. 1a, the SEM uses an electron beam to bombard the surface of the sample, causing the emission of signals such as secondary electrons. The SE is mainly used to amplify and transmit the information carried by the SE, and images are imaged by time series and imaged on the picture tube.
1.2 Features of SEM:
(1) The image has a strong stereoscopic effect and can observe a certain thickness.
(2) Sample preparation is simple, and large samples can be observed.
(3) High resolution, 30~40Å
(4) The magnification is continuously variable from 4 times to 150,000
(5) Can be equipped with accessories for quantitative and qualitative analysis of micro-areas
1.3 Observed objects:
Powders, granules, and bulk materials can be tested and require no special treatment until dry before testing. It is mainly used to observe the surface topography of the sample, the structure of the split surface, the structure of the inner surface of the lumen, and the like. It can visually reflect the specific structure and distribution of the particle size of the material.
2, TEM transmission electron microscope
Figure 2: (a) Structural schematic of a TEM transmission electron microscope; (b) TEM test photograph (Co3O4 nanosheet)
2.1 Principle: The incident electron beam is mainly used to pass through the sample to generate an electronic signal carrying the inside of the cross section of the sample, and is amplified by a multi-stage magnetic lens and imaged on the fluorescent plate, and the whole image is simultaneously established.
(1) The sample is ultra-thin, h<1000 Å
(2) Two-dimensional plane image, poor stereoscopic
(3) High resolution, better than 2 Å
(4) Complex sample preparation
2.3 Observed objects:
The nano-scale material dispersed in the solution needs to be dropped on the copper mesh before use, prepared in advance and kept dry. The internal ultrastructure of the sample was mainly observed, and the corresponding lattice and crystal plane of the material were observed by HRTEM high-resolution transmission electron microscope. As shown in Fig. 2b, the observation of the two-dimensional planar structure has a better effect, and the stereoscopic effect is poor relative to the SEM, but can have a higher resolution, and a finer portion is observed, and a special HRTEM can even observe the material. Information such as crystal faces and lattices.
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.