Common problems in the processing of lithium iron phosphate batteries

Lithium iron phosphate has a low diffusion coefficient of lithium ions and poor conductivity, so the current practice is to make its particles small, or even made into nanometers. By shortening the migration path of LI + and electrons, its charge and discharge speed is improved. (In theory, 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 problem encountered is material dispersion

Pulping is one of the most critical processes in the battery production process. Its core task is to mix active materials, conductive agents, binders and other materials uniformly, so that the performance of the materials can be better played. To mix well, first disperse. When the particles are reduced, the corresponding specific surface is increased, the surface energy is increased, and the tendency of polymerization between the particles is increased. The greater the energy required to overcome the surface energy dispersion. Mechanical stirring is now commonly used. The energy distribution of mechanical stirring is uneven. Only in a certain area, the shear strength is large enough and the energy is high enough to separate the aggregated particles. To improve the dispersion capacity, one is to optimize the structure of the stirring equipment and increase the space ratio of the effective dispersion area without changing the maximum shear speed; the other is to increase the stirring power (increasing the stirring speed) and increase the shear speed, corresponding The effective dispersion space will also increase. The former is a problem on the device, how much room for improvement, coating online does not comment. In the latter, the lifting space is limited, because the shear speed mentioned a certain limit, will cause damage to the material, resulting in particle damage.

A more effective method is to use ultrasonic dispersion technology. It is just that the price of ultrasonic equipment is relatively high. The price of the one we contacted earlier was comparable to that of imported Japanese mechanical mixers. The ultrasonic dispersion process 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 is not only bad for battery consistency, but also related to design and safety issues. Therefore, the uniformity of the coating is strictly controlled during the battery manufacturing process. Knowing the recipe and coating process, the smaller the material particles, the harder it is to make the coating uniform. As for its mechanism, I have not seen relevant explanations. The coating line is considered to be caused by the non-Newtonian fluid characteristics of the electrode paste.

The electrode slurry should be a thixotropic fluid in a non-Newtonian fluid. This type of fluid is characterized by viscousness at rest, or even solid state, but it becomes thinner and easy to flow after agitation. The binder is a linear or network structure in the submicroscopic state. When agitated, these structures are destroyed and the fluidity is good. After rest, they are re-formed and the fluidity becomes poor. Lithium iron phosphate particles are small. Under the same quality, the number of particles increases. To connect them to form an effective conductive network, the amount of conductive agent required also increases accordingly. The particles are small, the amount of conductive agent is increased, and the amount of binder required is also increased. When standing, it is easier to form a network structure, and the fluidity is worse than conventional materials.

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

The fundamental is to improve from the material, such as increasing the conductivity to increase the particles, particle spheroidization, etc., may have a limited effect 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 items:

1. Use "linear" conductive agent

The so-called "linear" and "particulate" conductive agent is the author's image, which may not be described academically.

Using "linear" conductive agent, currently mainly VGCF (carbon fiber) and CNTs (carbon nanotubes), metal nanowires, etc. They have a diameter of a few nanometers to tens of nanometers, a length of more than tens of micrometers or even a few centimeters, and the size of currently used "particulate" conductive agents (such as Super P, KS-6) is generally tens of nanometers, batteries The size of the material is a few microns. A pole piece composed of a "granular" conductive agent and an active substance is in contact with similar points and points, and each point can only make contact with surrounding points; in a pole piece composed of a "linear" conductive agent and an active substance, It is the point and line, line and line contact, each point can be in contact with multiple lines at the same time, each line can also be in contact with multiple lines at the same time, there are more nodes in contact, the conductive channels are more smooth, and the conductive ability It's better. The use of a variety of different forms of conductive agent combinations can play a better conductive effect. How to choose the conductive agent specifically is a question worth exploring for battery production.

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

(1) The linear conductive agent enhances 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 the conventional conductive agent of the same mass (weight)). Comprehensive (1), the amount of glue may also be reduced, and the content of active substances may be increased;

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

(4) The conductive network has many contact nodes, the network is more complete, and the rate performance is better than conventional conductive agents; the heat dissipation performance is improved, which is meaningful for high rate batteries;

(5) The absorption performance is improved;

(6) The price of materials is high and the cost is rising. 1Kg conductive agent, the commonly used SUPER P is only tens of yuan, VGCF is about two or three thousand yuan, CNTS is slightly higher than VGCF (when the addition amount is 1%, 1Kg CNTs is calculated at 4000 yuan, and the cost per Ah increases by 0.3 yuan) ;

(7) The specific surface area of ​​CNTS, VGCF, etc. is relatively high. How to disperse is a problem that must be solved during use. Otherwise, the dispersion is not good and the performance will not be exerted. Means such as ultrasonic dispersion can be used. Some CNTs manufacturers provide dispersed conductive fluids.

2. Improve dispersion effect

With a slurry with good dispersion effect, the probability of particles contacting with agglomeration will be greatly reduced, and the stability of the slurry will be greatly improved. The improvement of the formulation and batching steps can improve the dispersion effect to a certain extent. The aforementioned ultrasonic dispersion is also an effective method.

3. Improve slurry transfer process

When the slurry is stored, consider increasing the stirring speed to avoid the viscosity of the slurry; for those who use the revolving barrel to transfer the slurry, the time from discharging to coating should be shortened as much as possible, and the pipeline should be used to transfer the condition to improve the viscosity of the slurry.

4. Using extrusion coating (spraying)

Extrusion coating can improve the surface texture and uneven thickness of the blade coating, but the price of the equipment is higher, and the stability of the slurry is higher.

Difficult to dry

Due to the large specific surface of lithium iron phosphate and the large amount of binder, the amount of solvent required when preparing the slurry is also large, and it is difficult to dry after coating. How to control the evaporation rate of the solvent is a problem worthy of attention. High temperature, large air volume, fast drying speed, large gaps, and may also promote the migration of colloids, resulting in uneven distribution of materials in the coating, if the colloids accumulate on the surface, it will hinder the conduction of charged particles To increase the impedance. Low temperature, low air volume, slow solvent escape, long drying time, and low productivity.

Poor adhesion

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 more binders are needed. However, the binder is used more, reducing the content of the active material, the energy density is reduced, so when possible, the battery production process will try to reduce the amount of binder. In order to improve the bonding effect, the current general practice 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 resistance), on the one hand is to increase the amount of binder. It seems that the results are not satisfactory.

Poor flexibility

At present, when processing lithium iron phosphate pole pieces, it is generally felt that the pole pieces are harder and more brittle, which may not have a slight effect on the lamination, but it is very disadvantageous when it is wound. The pole piece has poor flexibility, 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. It is guessed that the particles are small and the elastic space of the coating is small. Reducing the compacted density can improve, but the volumetric energy density is reduced. Originally, the compaction density of lithium iron phosphate was relatively low, and reducing the compaction density was not a means to be adopted.

The above are the common problems in the processing of Lithium Iron Phosphate Batteries and some views on the coating line. Here I am bravely proposed to do the tricks. If there is a generous home, please point me right. If you have any other findings or opinions, you are welcome to discuss them.