How to solve the coating uniformity problem of LiFePO4 battery?

LiFePO4 is poor in electrical conductivity because of the low diffusion coefficient of Lithium ion. The current approach is to make the particles smaller or even nanoscale. Through shortening the migration path of Li+ and electrons, the speed of charging and discharging is increased (theoretically, the migration time is inversely proportional to the square of the migration path). But this brings a series of difficult problems to battery processing. Let’s take a look at how to solve the problem of coating uniformity of lithium iron phosphate battery .

The uneven coating of LiFePO4 battery not only results in poor consistency of the battery, but also affects the design and safety of use. So during its producing process, the uniformity of coating is strictly controlled in the manufacture of lithium battery. The people engaged in formulation and coating process know that the smaller the material particles, the more difficult it is to do uniform coating.

The electrode slurry belongs to Thixotropic fluids in non-Newtonian fluids, the characteristic of this kind fluid is thick even solid when it remains static, but after stirring, it turns to be thinned and easy to flow. The binding agent is linear or network structure in the submicroscopic state, these structures will be destroyed when stirring, the liquidity turns to be fine, but after keeping static, the liquidity will be bad because of the reform of structures. The recomposition of LiFePO4 is fine, under the same mass, the number of particles increase to put them together to form an effective conductive network, the amount of conductive agent required is also increased. When the particle size is small and the amount of conductive agent increases, the amount of required binding agent also increases. When standing, it is easier to form a network structure and the fluidity is worse than that of conventional materials.

During the process of taking out the slurry from the blender to coating, many manufactures still use the turnover bucket transfer, the slurry is not stirred or not stirred strongly in the process at low degrees, the fluidity of the slurry changes and gradually becomes thick, so that it resembles jelly. The fluidity is not good, resulting in the coating uniformity is not good, which is represented by the increasing of pole sheet surface density tolerance, surface topography is not good.

The fundamental improvement is from the material, such as increasing the conductivity of particles, spherification of particles, etc. There may be a limited effect in a short time. Based on the existing materials, from the perspective of battery processing, the improvement can be attempted from the following aspects.

1. Use a “linear” conductive agent.

The so-called “linear” or “granular” conductive agent is the author’s image, which may not be described in a academic terms.

“Linear” conductive agent are used, which are mainly VGCF (carbon fiber), CNTS (carbon nanotubes), metal nanowires, etc. Their diameter in a few nanometers to tens of nanometers, the length of tens of microns or even a few centimeters, and the commonly used “granular” conductive agent (such as SuperP, KS-6) size is generally in dozens of nanometers, the size of battery materials is a few microns. The contact of pole sheet which consists of a “granular” conductive agent and an active substance is like point to point contact, each point can only contact the surrounding points; The contact of pole sheet which consists of “linear” conductive agent and active material is point-to-line, line-to-line contact, each point can be contacted at the same time and by many lines, each line can also contact with many lines at the same time, the more the contact section are, the better the conductive channel will be. The use of variety of different forms of conductive agent combinations will have better electrical conductivity.

The possible effects of using a “linear” conductive agent such as CNTS or VGCF are:

(1) The linear conductive agent can improve the bonding effect and the flexibility and strength of the pole sheet to a certain extent;  

(2) Reduce the amount of conductive agent (remember that it was reported that the conductive efficiency of CNTS was 3 times that of conventional particle conductive agent with the same mass (weight)), and (1),  

The amount of glue may also be reduced, the content of active substances can be increased;  

(3) Improve polarization, reduce contact impedance and improve cycle performance;  

(4) The conductive network has more contact nodes, the network is more perfect, and the rate performance is better than the conventional conductive agent; the heat dissipation performance is improved, which is very meaningful for high times rate batteries.  

(5) The absorption performance is improved;  

(6) Higher material prices and rising costs. 1Kg conductive agent, commonly used SUPERP is only tens of Yuan, VGCF is about two or three thousand Yuan, CNTS is slightly higher than VGCF (1KgCNTs at 4000 Yuan at 1% addition, approximately 0.3 Yuan per Ah cost will increase);  

(7 )CNTS, VGCF and other specific surfaces are high, so how to disperse them is a problem that must be solved in use, otherwise the performance will not be great if the dispersion is not good  

Flick. Ultrasonic dispersion and other means can be used. Some CNTs manufacturers provide dispersed conductive solution.

2. Improve dispersion effect

For the slurry with good dispersion effect, 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 by the improvement of formula and batching steps, and the ultrasonic dispersion mentioned above is also an effective method.

3. Improve slurry transfer process  

When the slurry is stored, it can be considered to increase the stirring speed to avoid the viscosity of the slurry; if the slurry is transfered by turnover bucket, shorten the time of coating as far as possible, use the pipeline transportation if conditions allow, improve the viscosity of slurry phenomenon.

4. Extrusion coating (spraying)  

Extrusion coating can improve the blade coating surface grain, thickness nonuniformity phenomenon, but the equipment price is higher, the stability requirement of the slurry is also higher.