Therefore, the control of coating uniformity is very strict in the production process of lithium iron phosphate battery. Those who know the formula and coating process know that the smaller the material particles, the more difficult it is to do uniform coating. As far as its mechanism is concerned, I have yet to see a relevant explanation. The coating line is believed to be caused by the non-Newtonian fluid properties of the electrode paste.
The electrode slurry should be a thixotropic fluid in a non-Newtonian fluid, which is characterized by viscous or even solid state at rest, but becomes thin and easy to flow after agitation. Binders are linear or network structures in the submicroscopic state. When agitated, these structures are destroyed and the fluidity is good. After resting, they are re-formed and the fluidity becomes poor. Lithium iron phosphate particles are small. Under the same mass, the number of particles increases. To connect them to form an effective conductive network, the amount of conductive agent required increases accordingly. As the particles are smaller and the amount of conductive agent increases, the amount of binder required also increases. When standing, it is easier to form a network structure, and the fluidity is worse than that of conventional materials.
In the process of removing the slurry from the agitator to the coating process, many manufacturers still use revolving barrels to transfer the slurry. During the process, the slurry is not stirred or the stirring intensity is low, and the fluidity of the slurry changes and gradually becomes viscous. Like jelly. The fluidity is not good, resulting in poor coating uniformity, which is manifested as an increase in the density tolerance of the pole piece and poor surface morphology.
The fundamental is to improve the material, such as increasing the electrical conductivity, increasing the particles, spheroidizing the particles, etc., and the effect may be limited in a short time. Based on the existing materials, from the perspective of battery processing, the ways to improve can be tried from the following:
1. Using "linear" conductive agent
The so-called "linear" and "particle-shaped" conductive agents are the author's image, and may not be described in this way academically.
"Linear" conductive agents are used, mainly VGCF (carbon fiber) and CNTs (carbon nanotube), metal nanowires, etc. at present. They have a diameter of several nanometers to tens of nanometers, and a length of more than tens of micrometers or even a few centimeters, while the size of the currently commonly used "particle-shaped" conductive agents (such as SuperP, KS-6) is generally tens of nanometers. The size is a few microns. In the pole piece composed of "particle-shaped" conductive agent and active material, the contact is similar to the point-to-point contact, and each point can only contact the surrounding points; in the pole piece composed of "linear" conductive agent and active material, It is the point-to-line, line-to-line contact, each point can be in contact with multiple lines at the same time, and each line can also be in contact with multiple lines at the same time. Even better. Using a combination of different types of conductive agents can play a better conductive effect. How to choose the conductive agent is a problem worth exploring for battery production.
Possible effects of using "linear" conductive agents such as CNTS or VGCF are:
(1) The linear conductive agent improves the bonding effect to a certain extent and improves the flexibility and strength of the pole piece;
(2) Reduce the amount of conductive agent (remember that it has been reported that the conductive efficiency of CNTS is 3 times that of conventional particle conductive agents of the same mass (weight)), in combination with (1), the amount of glue may also be reduced, and the content of active substances can be increased;
(3) Improve polarization, reduce contact resistance, and improve cycle performance;
(4) The conductive network has many contact nodes, the network is more perfect, and the rate performance is better than that of the conventional conductive agent; the heat dissipation performance is improved, which is very meaningful for high rate batteries;
(5) The absorption performance is improved;
(6) Material prices are higher and costs rise. For 1Kg conductive agent, the commonly used SUPERP is only tens of yuan, VGCF is about two or three thousand yuan, and CNTS is slightly higher than VGCF (when the addition amount is 1%, 1KgCNTs is calculated at 4000 yuan, about an increase of 0.3 yuan per Ah);
(7) The specific surface of CNTS, VGCF, etc. is high. How to disperse is a problem that must be solved in use. Otherwise, the performance of dispersing is not good. Ultrasonic dispersion and other means can be used. There are CNTs manufacturers that provide dispersed conductive liquids.
2. Improve dispersion effect
If the dispersion effect is good, the probability of particle contact agglomeration will be greatly reduced, and the stability of the slurry will be greatly improved. The dispersion effect can be improved to a certain extent through the improvement of the formula and batching steps, and the ultrasonic dispersion mentioned above is also an effective method.
3. Improve the slurry transfer process
When the slurry is stored, it can be considered to increase the stirring speed to avoid the viscosity of the slurry; for the use of turnover barrels to transfer the slurry, the time from discharging to coating should be shortened as much as possible, and the conditions should be changed to pipeline transportation to improve the phenomenon of slurry viscosity.
4. Using extrusion coating (spraying)
Extrusion coating can improve the surface texture and uneven thickness of blade coating, but the equipment is expensive and requires higher stability of the slurry.