Lithium-ion battery anode material series three - related testing knowledge of graphite materials during processing

The previous article mainly introduced the basic physical properties of graphite anode materials. As a lithium-ion battery company, the raw materials will eventually appear in front of consumers as products. When you get a negative electrode material, how can the material be It is the responsibility of the R&D department of a lithium-ion battery enterprise to use it well and give full play to the performance of the material itself. As an important part of the lithium-ion battery - graphite negative electrode material, what are the detection indicators in the process? The third article in this series will take you with you to find relevant answers.

1, the homogenization process

As the first step of the negative electrode material, it is also a relatively important step. Once there is a problem in the first step, it is useless to remedy it later. Therefore, there are many related detection parameters in the homogenization process, which will be briefly introduced one by one below.

(1) Solid content: the percentage of active substances, conductive agents and binders in the total mass of the slurry in the slurry. The measurement method is also relatively simple. Take a little slurry and weigh it on the copper foil. After drying Weigh and then calculate. However, there are already related automatic equipment, with its own drying and weighing, as long as the sample is put in, and the relevant parameters are input, the result will be automatically calculated.

(2) Fineness: the degree of dispersion of powdery particles in the solvent, the slurry with good dispersion degree, the solid particles can be well wetted, the prepared coating will have a smooth surface, and the particles will be smooth during storage. Precipitation is not easy to occur, and storage stability is good. If the slurry is not well dispersed, there will be problems such as large particles and slurry sedimentation. At present, the scraper fineness meter is basically used to measure the fineness. When measuring, drop the sample into the deepest part of the groove, then touch the plate vertically with a scraper, pull the slurry through the entire length of the groove at a suitable speed, and then observe the uniform exposure of the particles in the groove along the direction of the incident light at a 30° angle. The scale, and this thickness represents the fineness of the sample, generally a good fineness should be below 20μm.

(3) Viscosity: Viscosity is a physical and chemical property of a substance. It is defined as a pair of parallel plates with an area of A and a distance of dr. The plates are filled with a certain liquid. Now, a thrust F is applied to the upper plate to generate a speed. The force required to change the degree. It is generally measured with a rotational viscometer, and the unit is poise. According to the different coating equipment, choose different viscosity slurries for coating. It should be noted that if the slurry settles, the viscosity of the upper and lower layers of the homogenization tank will be different. At the same time, the viscosity of the slurry will also change after being placed for a period of time. In the actual production process, it is necessary to collect relevant information. Summarize and analyze the data, and work out a suitable viscosity range and slurry storage time.

1 poise (1P)=100 centipoise (100cP) 1 centipoise (1cP)=1 millipascal s (1mPa s) 1 millipascal s (1mPa s)=1000 micropascals s (1000 μPa.s )

(4) Rheology: Rheology appeared in the 1920s and has been widely used in the coatings industry. Its meaning is the deformation and flow of objects under the action of external force, and it has also been widely used in the slurry of lithium ion batteries. Lithium-ion battery slurries are shear-thin, non-Newtonian fluids, and the viscosity decreases with increasing shear rate. The rheological curve represents the functional relationship between the viscosity value of the material and the shear stress, and there are special testing equipment.

(5) Slurry stability: With the deepening of research, the stability of the slurry has been paid more and more attention by researchers in the lithium battery industry. What is the internal particle distribution of a finished slurry? How long it is stable to store will directly affect the production cycle and batch of a real battery. Therefore, the research on the stability of the slurry came into being.

The measurement principle uses the principle of multiple light scattering. When the particle concentration in the sample is relatively low, the light that can pass through the sample after being scattered by multiple particles is called projection light, and the light that cannot pass through is scattered light. The equipment uses two probes to scan from the upper layer to the lower layer of the slurry to be tested, and make a relevant curve. The curve can characterize the uniformity of the sample, and it can be scanned at intervals and obtained through the relevant software. A series of curves from which the Stability Kinetic Index (TSI) of the slurry can be derived.

(6) Zeta potential: the potential of the shear plane, which is an index to characterize the stability of the colloidal dispersion, and changes with the distance from the particle surface.

2, coating process

After the simple introduction of the homogenization process above, the following is a brief introduction to the coating process. The main detection points are the thickness of the coating, the thickness difference between the middle and the two sides, whether there are defects on the surface after drying, etc.; One by one a brief introduction.

(1) Thickness detection: At present, some larger lithium battery manufacturers are using online detection systems, and can analyze the data in real time. Once there is a difference in coating thickness, the equipment will alarm, and there is not much here. , Different batteries and different manufacturers have different thickness control standards and ranges. The difference in thickness will directly cause the difference in battery capacity. Therefore, the smaller the range, the better. With the increase of the current capacity, the negative electrode is also coated thicker and thicker. Therefore, due to the influence of the drying rate, the coating edge of the negative electrode will cause a thick edge. The general solution is to modify the shape of the coating gasket. There are many related literatures and materials, which are not introduced one by one here;

(2) Surface condition detection: This can only be visually inspected at present. Check whether there are scratches, large particles, pits, exposed foils, etc. on the surface after coating and drying. Improve slurry homogenization, including rotation speed, feeding sequence, temperature control, additive amount, etc.

3, rolling process

After the homogenization and coating process of qualified negative pole pieces are completed, the next process is rolling, why is rolling? Because the surface state of the coated pole piece is relatively loose, the thickness is relatively thick, and the bonding force is not necessarily ideal.

The parameters that need to be tested during the rolling process are similar to those of coating, mainly thickness control and consistency of thickness, etc. In order to achieve high energy density, the coating density of the negative electrode is now getting higher and higher, resulting in two rolling The techniques of pressing, heating and rolling are all to better press the negative electrode to the required thickness, and it needs to be applied according to the actual situation.

summary:
This paper mainly introduces the parameters and detection methods that need to be detected in the process of homogenization, coating and rolling of anode materials. It is believed that with the development of lithium-ion battery technology, the research on related parameters will become more and more in-depth. Parameters are also introduced into process testing of Li-ion batteries.