Lithium iron phosphate has a low diffusion coefficient of lithium ions and poor conductivity, so the current approach is to make its particles smaller, or even nanometers, to improve the charge and discharge speed by shortening the migration path of LI+ and electrons. (Theoretically, the migration time is inversely proportional to the square of the migration path). But this brings a series of problems to battery processing.

The first thing we encounter is the problem of material dispersion

Pulping is one of the most critical processes in the battery production process. Its core task is to uniformly mix active materials, conductive agents, binders and other materials so that the material performance can be better played. To mix well, it must first be able to disperse. As the particles decrease, the corresponding specific surface area increases, so the surface energy increases, and the tendency of particles to polymerize increases. The greater the energy required to overcome the surface energy dispersion. Nowadays, mechanical stirring is commonly used, and the energy distribution of mechanical stirring is uneven. Only in a certain area, the shear strength is high enough and the energy is high enough to separate the aggregated particles. To improve the dispersing capacity, one is to optimize the structure of the mixing equipment to increase the space ratio of the effective dispersion area without changing the maximum shear speed; the other is to increase the mixing power (increase the mixing speed) and increase the shearing speed. The effective dispersion space will also increase. The former is a problem on the device, so I don’t comment on how much room for improvement. In the latter, the lifting space is limited, because when the shear speed reaches a certain limit, it will cause damage to the material and cause the particles to break.

A more effective method is to use ultrasonic dispersion technology. It's just that the price of ultrasonic equipment is relatively high, and its price is comparable to imported Japanese mechanical mixers. Ultrasonic dispersion process time is short, the overall energy consumption is reduced, the slurry dispersion effect is good, the polymerization of the material particles is effectively delayed, and the stability is greatly improved.

In addition, the dispersion effect can be improved by using a dispersant.

Coating uniformity problem

Uneven coating not only causes poor battery consistency, but also concerns issues such as design and use safety. Therefore, the uniformity of coating is strictly controlled during the battery manufacturing process. To understand the formulation and coating process, the smaller the material particles, the more difficult it is to achieve uniform coating. Regarding its mechanism, I have not yet seen the relevant explanation. The online coating is believed to be caused by the non-Newtonian fluid properties of the electrode slurry.

The electrode slurry should be a thixotropic fluid in a non-Newtonian fluid. The characteristic of this type of fluid is that it is viscous at rest, or even solid, but becomes thin and easy to flow after being stirred. The binder has a linear or network structure in a sub-microscopic state. When agitated, these structures are destroyed and the fluidity is good. After being static, they are re-formed and the fluidity becomes poor. Lithium iron phosphate particles are small, and under the same quality, the number of particles increases. To connect them to form an effective conductive network, the amount of conductive agent required is also increased accordingly. The particle size is small, the amount of conductive agent is increased, and the amount of binder required also increases. When standing still, it is easier to form a network structure, and the fluidity is worse than that of conventional materials.

After the slurry is taken out from the agitator to the coating process, many manufacturers still use turnover barrels to transfer the slurry. During the process, the slurry is not stirred or the stirring strength is low. The fluidity of the slurry changes and gradually becomes viscous. The jelly is the same. Poor fluidity results in poor coating uniformity, which is manifested by increased tolerance of pole piece surface density and poor surface morphology.

The fundamental thing is to improve the material, such as increasing the conductivity of the particles, making the particles spherical, etc., which may have limited effects in a short period of time. Based on the existing materials, from the perspective of battery processing, the ways to improve can be tried from the following items:

1Using "linear" conductive agent

The so-called "line-shaped" and "particle-shaped" conductive agents are vivid expressions, which may not be described in academic terms.

The use of "linear" conductive agents, currently important are VGCF (carbon fiber) and CNTs (carbon nanotube), metal nanowires, etc. They have a diameter of a few nanometers to tens of nanometers, and a length of more than tens of microns or even a few centimeters. At present, the commonly used "particle-shaped" conductive agents (such as SuperP, KS-6) are generally tens of nanometers in size. The size is a few microns. A pole piece composed of a "granular" conductive agent and an active material is in contact with similar points and points, and each point can only contact the surrounding points; in a pole piece composed of a "line-shaped" conductive agent and an active material, It is the contact between points and lines, lines and lines. 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. There are more nodes in contact, and the conductive channel is more smooth. It's better. Using a combination of various conductive agents in different forms can exert a better conductive effect. How to choose a conductive agent in detail is a question worth exploring about the production of batteries.

The possible effects of using "linear" conductive agents such as CNTS or VGCF include:

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

(2) Reduce the amount of conductive agent (remember that there have been reports that the conductivity of CNTS is 3 times that of conventional particle conductive agents with the same mass (weight)), comprehensively (1), the amount of glue may also be reduced, and the content of active materials may be increased;
(3) Improve polarization, reduce contact resistance, and improve cycle performance;
(4) The conductive network has more contact nodes, the network is more complete, and the rate performance is better than that of conventional conductive agents; the heat dissipation performance is improved, which is of great significance for high rate batteries;
(5) The absorption performance is improved;
(6) Higher material prices and rising costs. 1Kg conductive agent, the commonly used SUPERP is only tens of yuan, VGCF is about two to three thousand yuan, CNTS is slightly higher than VGCF (when the addition amount is 1%, 1KgCNTs is calculated at 4000 yuan, and the cost is about 0.3 yuan per Ah);
(7) The specific surface area of ​​CNTS, VGCF, etc. is relatively high. How to disperse is a problem that must be solved in use, otherwise the performance of poor dispersion will not be exerted. Can use means such as ultrasonic dispersion. There are CNTs manufacturers supplying dispersed conductive liquids.

2Improve the dispersion effect

With a good dispersion effect, the probability of particle contact and 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 the batching process. The ultrasonic dispersion mentioned above is also an effective method.

3Improve the slurry transfer process

When storing the slurry, consider increasing the stirring speed to prevent the slurry from sticking; regarding the use of turnover barrels to transfer the slurry, shorten the time from discharging to coating as much as possible, and use pipeline transportation if possible to improve the viscosity of the slurry.

4Using extrusion coating (spraying)
Extrusion coating can improve the surface texture and uneven thickness of the knife coating, but the equipment is more expensive and requires higher slurry stability.

Difficult to dry

Because the lithium iron phosphate has a large surface area and a large amount of binder, the amount of solvent required when preparing the slurry is also large, and it is more difficult to dry after coating. How to control the volatilization rate of solvents is a problem worthy of attention. High temperature, large air volume, and fast drying speed will cause large voids. At the same time, it may drive the migration of colloids, resulting in uneven distribution of materials in the coating. If colloids aggregate on the surface, it will hinder the conduction of charged particles. , Increase impedance. The temperature is low, the air volume is low, the solvent escapes slowly, the drying time is long, and the output is low.

Poor bonding performance
The particles of lithium iron phosphate materials are small, and the specific surface ratio is much larger than that of lithium cobaltate and lithium manganate, and it requires more binders. However, if the binder is used too much, the energy density will be reduced if the content of the active material is reduced. Therefore, when possible, the amount of binder will be reduced as much as possible during the battery production process. In order to improve the bonding effect, the current common method of lithium iron phosphate processing is to increase the molecular weight of the binder on the one hand (high molecular weight, improved bonding ability, but the more difficult the dispersion, the higher the impedance), on the other hand, it is to increase the amount of binder. It seems that the results are not satisfactory.

Poor flexibility
At present, when the lithium iron phosphate pole piece is processed, it is generally felt that the pole piece is hard and brittle, which may not have a small impact on the laminate, but it is very unfavorable when it is wound. The flexibility of the pole piece is not good, and it is easy to fall off and break when winding and bending, resulting in short-circuit and other defects. The explanation of the mechanism in this respect is not clear, but the guess is that the particles are small and the elastic space of the coating is small. Reducing the compaction density can improve, but in this way the volumetric energy density will also be reduced. Originally, the compaction density of lithium iron phosphate is relatively low, and reducing the compaction density is a must.