What are the Common Problems of Lithium Iron Phosphate Materials in Battery Processing

Lithium iron phosphate has a low diffusion coefficient and poor conductivity due to lithium ions. Therefore, the current approach is to make its particles smaller, or even make nanometer series, and increase its charging 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). However, this has caused a series of difficulties in battery processing.

The first problem is the dispersion of materials.

Pulping is one of the most critical processes in the battery production process. Its core task is to evenly mix the active substances, conductive agents, binders and other materials, so that the material performance can be better played. To mix, must be able to disperse. As the particles decrease, the corresponding specific surface increases, the surface energy increases, and the tendency of polymerization between particles increases. The greater the energy required to overcome the dispersion of surface energy. It is now commonly used for mechanical stirring. The distribution of mechanical stirring energy is uneven. Only in a certain area can the polymerized particles be separated by large enough shear strength and high enough energy. To improve the ability of dispersion, one is to optimize the structure of the mixing device and increase the spatial ratio of the effectively dispersed area without changing the maximum shear speed; One is to increase the stirring power(increase the stirring speed) and increase the shear speed, and the corresponding effective dispersion space will also increase. The former is a problem on the device, how much space there is to upgrade, and the coating is not commented on online. In the latter case, the lifting space is limited because the shear speed mentions a certain limit, which can cause damage to the material and cause damage to the particles.

The more effective method is to use ultrasonic dispersion technology. Ultrasonic equipment, however, is more expensive, and one that was contacted earlier has a price comparable to that of an imported Japanese machine mixer. Ultrasonic dispersion process time is short, overall energy consumption is reduced, the dispersion effect of slurry is good, the polymerization of material particles is effectively delayed, and the stability is greatly improved.

In addition, the dispersion effect can be improved by the use of dispersants.

Homogeneity of coating

Unequal coating, not only battery consistency is not good, but also related to the design, use of safety and other issues. Therefore, the uniform control of coating is very strict during the battery manufacturing process. As the recipe, coating process know, the smaller the material particles, the more difficult to do uniform coating. As far as its mechanism is concerned, I have not seen any relevant explanation. The coating is thought to be caused by the non-Newtonian fluid properties of the electrode slurry.

The electrode slurry should be a contact fluid in a non-Newtonian fluid. This type of fluid is characterized by viscous, even solid, but becomes dilute and easy to flow after stirring. The binders are linear or reticular structures in the sub-microscopic state. When stirred, these structures are destroyed and fluidity is good. After they are stationary, they are re-formed and fluidity becomes worse. 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 agents required increases accordingly. Small particles, the amount of conductive agent increased, the amount of binder required also increased. When static, it is easier to form a mesh structure, and the fluidity is worse than that of conventional materials.

In the process of removing the slurry from the mixer to the coating, many manufacturers still use the turnover bucket transfer. During the process, the slurry does not stir or the stirring strength is low. The fluidity of the slurry changes and gradually becomes viscous. Like jelly. Poor fluidity leads to poor uniformity of the coating, which is characterized by an increase in one-dimensional density tolerances and poor surface morphology.

The basic is to improve the material, such as increasing the conductivity of particles, particle spherical shape, etc., which may have a limited effect in a short period of time. Based on the existing materials, from the perspective of battery processing, the improved approach can be tried from the following points:

1. Use "linear" conductivity

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

The use of "linear" conductive agents, currently mainly VGCF(carbon fiber) and CNTs(carbon nanotubes), metal nanowires and so on. They are several nanometers in diameter and several tens of nanometers in length, and are even more than a few centimeters in length. The commonly used "granular" conductive agents(such as SuperP, KS-6) are generally in the size of dozens of nanometers. The size of the battery material is several microns. "Granular" electrode consisting of a conductive agent and an active substance, in contact with similar points and contacts between points, each point can only come into contact with the surrounding point; In the polar film composed of a "linear" conductive agent and an active substance, it is a contact between a point and a line, a line, and a line. 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. More nodes, The conductive channels are also more open and the conductivity is better. The use of a variety of different types of conductive agents combination, can play a better conductive effect, specifically how to select conductive agents, for the battery production is a very worth exploring problem.

The possible effects of the use of "linear" conductors such as CNTS or VGCF are:

(1) Linear conductive agents enhance the bond effect to a certain extent, improving the flexibility and strength of the electrode;

(2) Reduction in the amount of conductive agents(it is recalled that CNTS has been reported that the conductivity of the same mass(weight) conventional particle conductive agent is three times), synthesis(1), the amount of adhesive may also be reduced, and the content of active substances can be increased;

(3) Improved polarization, reduced contact impedance and improved cyclic performance;

(4) There are many contact nodes in the conductive network, the network is more perfect, and the ratio performance is better than that of conventional conductive agents; The improvement of heat dissipation performance is very meaningful for high-magnification batteries;

(5) Improved absorption performance;

(6) Higher prices and higher costs for materials. 1Kg conductive agent, commonly used SUPERP is only a few tens of yuan, VGCF is about two or three thousand yuan, CNTS is slightly higher than VGCF(when the addition is 1 %, 1KgCNTs are calculated at 4,000 yuan, approximately an increase of 0.3 yuan per Ah cost);

(7) CNTS, VGCF, etc. are higher than the surface. How to disperse is a problem that must be solved in use. Otherwise, poor performance of dispersion is not played. Can use ultrasound dispersion and other means. There are CNTs manufacturers to provide scattered conductive fluids.

2. Improve dispersion

If the slurry is dispersed well, the probability of particle contact will be greatly reduced and the stability of the slurry will be greatly improved. By improving the formula and ingredients step, the dispersion effect can be improved to a certain extent. The ultrasonic dispersion mentioned above is also an effective method.

3. Improve slurry transfer process

When the slurry is stored, the mixing speed can be increased to avoid the viscosity of the slurry. For the use of a swing bucket to transfer the slurry, as much as possible to shorten the expected coating time, conditional use of pipe transport to improve the slurry viscous phenomenon.

4. Adopt extrusion coating(spray)

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

Hard to dry

Since lithium iron phosphate has a larger surface area and a larger amount of binder, the amount of solvent required for preparing the slurry is large, and drying after coating is more difficult. How to control the evaporation rate of the solvent is a problem worthy of attention. The temperature is high, the air volume is large, the drying speed is fast, and the generated voids are large. At the same time, the migration of the colloid may be driven, resulting in uneven distribution of materials in the coating. If the colloid is aggregated on the surface layer, the conduction of charged particles is hindered. Increase the impedance. The temperature is low, the air volume is low, the solvent escapes slowly, the drying time is long, and the production capacity is low.

Poor bonding performance

The particles of lithium iron phosphate materials are small, which is much larger than the surface than lithium cobalt and lithium manganese, and more binders are needed. However, more binders are used, reducing the content of active substances and reducing the energy density, so if possible, the battery production process will try to reduce the amount of binders. In order to improve the adhesion effect, the current common practice for the processing of lithium ferric phosphate is to increase the molecular weight of the binder(high molecular weight, higher adhesion capacity, but more difficult to disperse and higher impedance), and on the one hand, to increase the amount of binder. The results do not seem to be satisfactory.

Poor flexibility

At present, when the lithium iron phosphate electrode is processed, it is generally felt that the electrode is harder and more brittle. It may not have a slightly smaller effect on the laminate, but it is very unfavorable when it is wound. Extreme film flexibility is not good, when winding and bending, it is easy to fall off powder and break, resulting in short circuit and other poor. The explanation of the mechanism is not yet clear. The guess is that the particles are small and the elastic space of the coating is small. Reducing compaction density can be improved, but the volume energy density can be reduced. The compaction density of the original lithium iron phosphate is relatively low, and reducing the compaction density is a means that can not be taken.

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