1. A detection method for the content of a coating layer on the surface of a graphite negative electrode material of a lithium battery is characterized by comprising the following steps:

(1) drying the coated graphite powder samples with different coating contents, wherein the coating content is A_i；

(2) Carrying out multiple thermogravimetric tests on each powder sample to obtain a thermal weight loss rate range value B of the coated graphite_ij(Bimin, Bimax), rate of temperature rise per thermogravimetric test and N₂The flow is always kept consistent;

(3) with the content of coating layer A_iAs abscissa, rate of thermal weight loss B_ijLinear fitting analysis is carried out for the ordinate to obtain a relation curve Y ═ Y₁+Y₂*X；

(4) Drying the coated graphite with unknown coating content in the step (1), and performing thermogravimetric test, wherein the temperature rise rate and N of the test are₂The flow rate is kept consistent with that in the step (2) to obtain the thermal weight loss rate B_yObtaining the theoretical coating content A of the coated graphite according to the relation curve of the step (3)_y。

2. The method for detecting the content of the coating layer on the surface of the graphite negative electrode material of the lithium battery as claimed in claim 1, wherein in the step (1), the drying mode is air drying or vacuum drying, and the drying temperature is 60-110 ℃.

3. The method for detecting the content of the coating layer on the surface of the graphite cathode material of the lithium battery as claimed in claim 1, wherein in the step (1), i is the number of the coated graphite samples and is 1, 2 or 3 … ….

4. The method for detecting the content of the coating layer on the surface of the graphite cathode material of the lithium battery as claimed in claim 1, wherein in the step (2), i is the number of the coated graphite samples and is 1, 2 or 3 … …, and j is the number of repeated tests and is 1, 2 or 3 … ….

5. The method for detecting the content of the coating layer on the surface of the graphite negative electrode material of the lithium battery as claimed in claim 1, wherein in the step (3), Y is₁Is the shift coefficient associated with the volatiles of the coated graphite aggregate, Y₂Is the shift coefficient associated with the volatiles of both the coating material and the graphite material being coated.

Background

The large-scale application of high-end digital and power batteries puts higher requirements on the high-rate charge and discharge of the batteries. For the negative electrode graphite material, coating a layer of amorphous carbon layer with larger interlayer spacing, such as soft carbon, hard carbon and the like, on the surface is a main method for improving rate capability. However, the coating uniformity and the coating carbon content in different areas of the production process are not tested and evaluated effectively.

The prior art discloses a method for detecting whether a carbon coating layer exists on a graphite surface, which comprises the steps of calculating a deviation threshold value by utilizing a fitting relation curve of graphite crystal structure content and a disordered-layer structure characteristic value, comparing the deviation of a sample to be detected with the deviation threshold value, and judging that the carbon coating layer exists if the deviation is greater than the deviation threshold value. Although the method is simple and effective, the method has great limitation, whether the carbon coating layer exists on the surface of the graphite cathode material is difficult to judge if the deviation degree is between deviation degree threshold values, and the relationship between the content of the carbon coating layer and the deviation degree is not complicated.

In addition, the prior art discloses a method for detecting the coating integrity of an active material of a lithium ion battery, which comprises the steps of firstly cleaning and drying a sample to obtain a sample to be detected, then coating the sample by using elements contained in a non-coating material and a coated material, finally carrying out XPS analysis on the sample, and carrying out peak fitting calculation according to a returned binding energy curve to obtain the coating rate. The method can realize quantitative measurement of the coating rate of the carbon layer on the surface of materials such as lithium iron phosphate, lithium cobaltate, silicon oxide, nano silicon and the like, but if the coating material and the coated material are the same elements, for example, if the carbon layer is coated on the surface of graphite, the coating rate cannot be detected.

Therefore, how to judge the content of the carbon coating on the surface of the graphite material is a difficult problem, and no existing standard can be referred to at present.

Disclosure of Invention

In order to solve the technical problems, the invention provides a method for detecting the content of a coating layer on the surface of a graphite cathode material of a lithium battery, which is simple and effective and solves the problem that the content of a carbon coating layer on the surface of the graphite material cannot be detected by the conventional scheme.

The invention adopts the following technical scheme:

a detection method for the content of a coating layer on the surface of a graphite negative electrode material of a lithium battery comprises the following steps:

(1) samples of coated graphite powder containing different coating levels were dried,the content of the coating layer is A_i；

(2) Carrying out multiple thermogravimetric tests on each powder sample to obtain a thermal weight loss rate range value B of the coated graphite_ij(Bimin, Bimax), rate of temperature rise per thermogravimetric test and N₂The flow is always kept consistent;

(3) with the content of coating layer A_iAs abscissa, rate of thermal weight loss B_ijLinear fitting analysis is carried out for the ordinate to obtain a relation curve Y ═ Y₁+Y₂*X；

(4) Drying the coated graphite with unknown coating content in the step (1), and performing thermogravimetric test, wherein the temperature rise rate and N of the test are₂The flow rate is kept consistent with that in the step (2) to obtain the thermal weight loss rate B_yObtaining the theoretical coating content A of the coated graphite according to the relation curve of the step (3)_y。

The technical proposal is further improved in that in the step (1), the drying mode is air blast drying or vacuum drying, and the drying temperature is 60-110 ℃.

In the step (1), i is the number of the coated graphite samples, and is 1, 2, 3 … ….

In the step (2), i is the number of the coated graphite samples and takes values of 1, 2 and 3 … …, and j is the number of repeated tests and takes values of 1, 2 and 3 … ….

The technical proposal is further improved in that in the step (3), Y₁Is the shift coefficient associated with the volatiles of the coated graphite aggregate, Y₂Is the shift coefficient associated with the volatiles of both the coating material and the graphite material being coated.

The invention has the beneficial effects that:

the invention establishes the linear fitting relationship Y-Y between the thermogravimetric weight loss rate of the coated graphite and the adding proportion of the coating agent₁+Y₂X, then according to the test sample thermal weight loss rate, can carry out quantitative evaluation to coating content to carry out multizone sample test to the material, can also evaluate the homogeneous of materialThe method is simple and effective, and solves the problem that the content of the carbon layer coated on the surface of the graphite material cannot be detected by the conventional scheme.

Drawings

Fig. 1 is a graph showing a relationship between a thermal weight loss rate and a coating layer content in a method for detecting the coating carbon layer content on the surface of a graphite anode material of a lithium ion battery.

Detailed Description

The technical solution of the present invention is described in detail by examples, which are only a part of the technical solution of the present invention, and other examples performed according to the technical solution of the present invention are within the scope of the present invention.

A method for detecting the content of a carbon layer coated on the surface of a graphite cathode material of a lithium ion battery comprises the following steps:

(1) drying the coated graphite powder samples with different coating contents in a blowing drying mode or a vacuum drying mode, wherein the drying temperature is 60-110 ℃, and the coating content is A_iI is the number of the coated graphite samples, and the values are 1, 2 and 3 … …;

(2) carrying out multiple thermogravimetric tests on each powder sample to obtain a thermal weight loss rate range value B of the coated graphite_ij(Bimin, Bimax), i is the number of the coated graphite samples and takes values of 1, 2 and 3 … …, j is the number of repeated tests and takes values of 1, 2 and 3 … …, and the temperature rise rate and N of each thermogravimetric test₂The flow is always kept consistent;

(3) with the content of coating layer A_iAs abscissa, rate of thermal weight loss B_ijLinear fitting analysis is carried out for the ordinate to obtain a relation curve Y ═ Y₁+Y₂*X，Y₁Is the shift coefficient associated with the volatiles of the coated graphite aggregate, Y₂Is the shift coefficient associated with the volatiles of both the coating material and the graphite material being coated;

(4) drying the coated graphite with unknown coating content in the step (1), and performing thermogravimetric test, wherein the temperature rise rate and N of the test are₂The flow rate is kept consistent with that in the step (2), and the thermal weight loss is obtainedRate B_yObtaining the theoretical coating content A of the coated graphite according to the relation curve of the step (3)_y。

Thermogravimetric analysis is a technique for measuring the relationship between the mass of a substance and temperature under programmed temperature control, has strong quantification performance, and can accurately measure the change of the mass of the substance along with the temperature, regardless of whether the change is chemical or physical. The volatile components are important indexes for characterizing the properties of materials, and each material has a specific index. The graphite aggregate and the coating agent such as asphalt, tar and the like have larger difference in volatile components, the graphite aggregate has very low volatile components, the coating agent has higher volatile components, and the volatile component content and the high thermal weight loss rate are also high. The graphite surface is coated with a certain content of asphalt and other binders, the higher the content of the coating layer is, the higher the thermal weight loss rate is, so that a relation curve between the thermal weight loss rate and the content of the coating layer can be established, as shown in fig. 1.

Rate of thermal weight loss Y ═ X ═ F_Asphalt*C₁+(1-X)F_Graphite*C₂+C₃

→Y＝C₂+C₃+(F_Asphalt*C₁-F_Graphite*C₂)*X

I.e. Y is₁+Y₂*X

F_AsphaltIs the volatile component of the coating agent asphalt; f_GraphiteIs the volatile component of graphite aggregate; c₁、C₂、C₃Setting the deviation coefficient as a constant value; x is the coating content; y is the thermal weight loss rate; y is₁Is the deviation coefficient related to the volatile component of the coated graphite aggregate; y is₂Is the shift coefficient associated with the volatiles of both the coating material and the graphite material being coated.

The invention establishes the linear fitting relationship Y-Y between the thermogravimetric weight loss rate of the coated graphite and the adding proportion of the coating agent₁+Y₂And X, quantitatively evaluating the content of the coating layer according to the thermal weight loss rate of a sample to be tested, performing multi-region sampling test on the material, and evaluating the uniformity and consistency of the material.

Appropriate changes and modifications to the embodiments described above will become apparent to those skilled in the art from the disclosure and teachings of the foregoing description. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and variations of the present invention should fall within the scope of the claims of the present invention. Furthermore, although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.