The last article in the series of negative electrode materials for lithium ion batteries - the introduction of basic knowledge of graphite materials mainly introduces the basic knowledge of negative electrode materials.

When we get a negative electrode material, how can we test its performance? What is the basis? Some basic physical properties are used to select negative electrode materials. The second part of this series will take you to understand the basic knowledge and basic judgment indicators of graphite negative electrode performance testing.

It is difficult for us to judge what type of negative electrode material it is from the appearance. It does not matter. The negative electrode manufacturer has already classified us. The appearance color is black or just gray, and it has a metallic luster. Graphite (Artificial Graphite), generally abbreviated as NG and AG, will understand what this abbreviation means in the future.

The basic properties of graphite include particle size distribution, specific surface area, tap density, magnetic substance content, degree of graphitization, appearance, initial charge and discharge capacity, and initial efficiency. The following will describe the test in detail according to each basic parameter. methods and basic judgment indicators.

Particle size distribution test:

The laser particle size analyzer measures the particle size distribution according to the physical phenomenon that particles can scatter laser light. When a beam of parallel light encounters the blockage of particles, a part of the beam will be scattered, and the propagation direction of the scattered beam and the propagation direction of the main beam form scattering. Its size is related to the size of the particle. The larger the particle, the smaller the scattering angle, and vice versa. At the same time, the intensity of the scattered light represents the number of particles of this size, so the scattering is measured at different angles. The particle size distribution of the sample can be obtained by the intensity of the light. Generally, the parameters of the particle size of D10, D50 and D90 will be obtained, which means that the particle size of 10%, 50% and 90% is in the measured size value. Hereinafter, the parameters of the particle size volume distribution of D10, D50, and D90 are also obtained, and the meanings are also the same.

Specific surface area test:

The determination of solid specific surface and pore size distribution based on nitrogen adsorption method is based on the adsorption law of gas on solid surface. Pressure can change the amount of adsorption. The curve in which the equilibrium adsorption capacity changes with pressure is called the adsorption isotherm. The study and measurement of the adsorption isotherm can not only obtain information about the properties of the adsorbent and adsorbate, but also calculate the specific surface area and pore size distribution of the solid. Generally speaking, the specific surface area of negative electrode materials with small particles is larger than that of negative electrode materials with large particles, and these physical indicators need to be considered when used in combination with positive electrodes and electrolytes.

Tap Density Test:

Its test principle The graduated cylinder with powder or particles is fixed on the mechanical vibration device, the vibration motor drives the mechanical vibration device to vibrate vertically up and down, and the graduated cylinder with powder or particles vibrates with the mechanical vibration device in a rhythmic manner. As the number of times increases, the powder or particles in the graduated cylinder are gradually vibrated. After the number of vibrations reaches the set number of times, the mechanical vibration device stops vibrating, and the volume of the graduated cylinder is read out. According to the definition of density: the mass is divided by the volume, so as to obtain Density of powder or granules after tap. This parameter is closely related to the size and shape of the particles, and also directly affects another important indicator of the negative electrode material during processing-the compaction density, which in turn affects the thickness of the battery.

Magnetic Substance Test:

Since it is difficult to avoid magnetic impurities in the production process of negative electrode materials, these impurities will affect the self-discharge of the battery, and in severe cases will cause a short circuit of the battery. Therefore, the test of magnetic substances is also essential, generally by passing The content of iron, chromium, nickel and zinc in the samples was tested by inductively coupled plasma emission spectrometer.

Graphitization degree test:

XRD is used for testing. The principle of XRD has been explained before, so I will not repeat it here. In addition, it should be noted that d002 (layer spacing) can also be calculated by XRD, which is also a relatively important indicator. , directly on the formula:

Degree of graphitization: degree of graphitization (%)=(3.44-d002)/(3.44-3.354)

Interlayer spacing d002: Bragg equation 2dsinƟ=nλ, where λ is the wavelength of X-ray, λ=1.54056 Å, and the diffraction order n is any positive integer;

Through this test, the difference between the graphite material and the ideal graphite can be understood, and different kinds of anode graphite materials can be obtained by controlling different conditions.

Appearance test:
Generally, the appearance and distribution of the material can be seen through SEM. If you want to see it more clearly, you need a TEM with more magnification to analyze the internal structural state and the coating state of the surface.

The coating state of the surface of the graphite material and even the number of layers of graphene can be seen by TEM. With the improvement of detection equipment, AFM (atomic force microscopy), optical microscopy, scanning tunneling microscopy (STM) and some in-situ detection techniques have been widely used in the characterization of material morphology.

The first charge and discharge efficiency test:
This picture is believed to be familiar to the majority of lithium battery colleagues. Assemble button batteries, perform the first charge and discharge and the first efficiency test and calculation. Generally, at the beginning of assembly, due to the method and proficiency, the assembled button battery is often very consistent. Poor, the test data is not good, don't worry about this, install a few more times, each time you install a few more, you can find out the key points, so as to achieve the desired results.

Summary: As the second part of this series, this paper mainly introduces the testing principles and testing methods of the basic properties of graphite anode materials. With the deepening of research, some more in-depth research methods have begun to be applied to anode materials, such as Raman Spectroscopy (Raman), X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), etc. It is believed that with the development of science and technology, more, newer, better and simpler test methods will also be used in the lithium battery industry. widely used.