Lithium iron phosphate has a low diffusion coefficient of lithium ions and poor conductivity. Therefore, the current approach is to make its particles smaller, even in the nanoscale, by shortening the migration path of LI+and electrons to improve its charging and discharging speed (theoretically, the migration time is inversely proportional to the square of the migration path). But this brings a series of difficulties to battery processing.

The first issue encountered is material dispersion

Pulping is one of the most critical processes in battery production, and its core task is to evenly mix active substances, conductive agents, binders, and other materials, so that the material performance can be better utilized. To mix well, one must first be able to disperse. As the particles decrease, the corresponding specific surface area also increases, the surface energy also increases, and the trend of aggregation between particles strengthens. The greater the energy required to overcome surface energy dispersion. Mechanical stirring is commonly used nowadays, and the energy distribution of mechanical stirring is uneven. Only in a certain area, with sufficient shear strength and high energy, can the aggregated particles be separated. To improve the dispersion ability, one is to optimize the structure of the mixing equipment and increase the spatial proportion of the effective dispersion area without changing the maximum shear speed; One is to increase the stirring power (increase stirring speed), increase the shear speed, and the corresponding effective dispersion space will also increase. The former is a problem with the device, how much room for improvement is there, and no comments will be made online for coating. The latter has limited space for improvement, as reaching a certain limit in shear speed can cause damage to the material and lead to particle damage. A more effective method is to use ultrasonic dispersion technology. It's just that the price of ultrasonic equipment is relatively high. A few days ago, I came into contact with a company whose price is comparable to imported Japanese mechanical mixers. The ultrasonic dispersion process has a short time, overall energy consumption is reduced, the slurry dispersion effect is good, the polymerization of material particles is effectively delayed, and the stability is greatly improved. Additionally, the dispersion effect can be improved by using dispersants.

Coating uniformity issue

Uneven coating not only affects the consistency of the battery, but also relates to issues such as design and safety in use. So, the control of coating uniformity is very strict during the battery production process. Knowing the formulation and coating process, the smaller the material particles, the more difficult it is to achieve uniform coating. I haven't seen any relevant explanation for its mechanism yet. The online coating is believed to be caused by the non Newtonian fluid properties of the electrode slurry. The electrode slurry should belong to the thixotropic fluid in non Newtonian fluids. The characteristic of this type of fluid is that it is viscous at rest, even in a solid state, but becomes thinner and easier to flow after stirring. In the submicroscopic state, the binder has a linear or network structure. When stirred, these structures are destroyed, resulting in good fluidity. However, when stationary, they re form, and the fluidity deteriorates. Lithium iron phosphate particles are small, and with the same mass, the number of particles increases. To connect them together to form an effective conductive network, the amount of conductive agent required also increases accordingly. As the particle size increases and the amount of conductive agent used increases, the required amount of adhesive also increases. When standing still, it is easier to form a mesh structure and has poorer fluidity than conventional materials.

During the process of removing the slurry from the mixer and spreading it, many manufacturers still use turnover buckets for transfer. During the process, the slurry is not stirred or the stirring intensity is low, and the fluidity of the slurry changes, gradually becoming viscous, like jelly. Poor fluidity leads to poor uniformity of coating, manifested by an increase in density tolerance on the extreme surface and poor surface morphology.

The fundamental improvement is to improve the material, such as increasing conductivity, increasing particle size, and spheroidizing particles, which may have limited effects in the short term. Based on existing materials, from the perspective of battery processing, improvement methods can be attempted from the following aspects:

1. Using "linear" conductive agents

The so-called "linear" and "granular" conductive agents are the author's vivid description, which may not be the case in academia. Linear conductive agents are mainly used, such as VGCF (carbon fiber) and CNTs (carbon nanotubes), metal nanowires, etc. They have a diameter ranging from a few nanometers to tens of nanometers, and a length ranging from tens of micrometers to even a few centimeters. Currently, commonly used "particle shaped" conductive agents (such as Super P, KS-6) generally have a size of tens of nanometers, and the size of battery materials is several micrometers. A polar plate composed of granular conductive agents and active substances, with contact similar to that between points, where each point can only make contact with surrounding points; In the electrode composed of linear conductive agents and active substances, there is contact between points and lines, and between 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. With more nodes in contact, the conductive channels are smoother and the conductivity is better. The use of various combinations of different forms of conductive agents can achieve better conductivity. The specific selection of conductive agents is a question worth exploring for battery production. The possible effects of using linear conductive agents such as CNTS or VGCF include: (1) Linear conductive agents can improve the bonding effect to a certain extent, enhance the flexibility and strength of the electrode; (2) Reduce the amount of conductive agent used (remember there were reports that the conductive efficiency of CNTS is three times that of conventional particle conductive agents of the same mass (weight)). Overall, (1) the amount of adhesive used may also be reduced, and the content of active substances can be increased; (3) Improve polarization, reduce contact impedance, and improve cycling performance; (4) The conductive network has multiple contact nodes, a more complete network, and better rate performance than conventional conductive agents; Improving heat dissipation performance is meaningful for high rate batteries; (5) Improved absorption performance; (6) The material prices are high, resulting in an increase in costs. 1Kg conductive agent, commonly used SUPER P is only a few tens of yuan, VGCF is about two to 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 increases by about 0.3 yuan per Ah); (7) CNTS, VGCF and other materials have higher specific surface area, so how to disperse them is a necessary problem to be solved in use. Otherwise, poor dispersion may not fully utilize their performance. Ultrasound dispersion and other methods can be used. CNTs manufacturers provide well dispersed conductive liquids.

2. Improve dispersion effect

A slurry with good dispersion effect will greatly reduce the probability of particle contact agglomeration, and the stability of the slurry will be greatly improved. The improvement of formula and ingredient steps can to some extent improve the dispersion effect, and the use of ultrasonic dispersion mentioned earlier is also an effective method.

3. Improve the slurry transfer process

When storing slurry, it is possible to consider increasing the stirring speed to avoid the viscosity of the slurry; For the use of turnover buckets to transfer slurry, try to shorten the time from discharge to coating as much as possible, and if conditions permit, switch to pipeline transportation to improve the viscosity of the slurry.

4. Use extrusion coating (spray coating)

Squeezing coating can improve the surface texture and uneven thickness of scraper coating, but the equipment price is high and requires high stability of the slurry.

Difficulty in drying

Due to the large surface area of lithium iron phosphate and the large amount of binder used, the amount of solvent required for preparing the slurry is also large, making it difficult to dry after coating. How to control the evaporation rate of solvents is a noteworthy issue. High temperature, high air volume, and fast drying speed can create large gaps, and may also drive the migration of resin, resulting in uneven distribution of materials in the coating. If the resin aggregates on the surface, it will hinder the conduction of charged particles and increase impedance. Low temperature, low air volume, slow solvent escape, long drying time, and low production capacity.

Poor bonding performance

The particles of lithium iron phosphate material are small, and the specific surface area is much larger than that of lithium cobalt oxide and lithium manganese oxide, requiring more binders. However, if too much adhesive is used, reducing the content of active substances will result in a decrease in energy density. Therefore, in possible cases, efforts will be made to reduce the amount of adhesive used during battery production. To improve the bonding effect, the current common practice for processing lithium iron phosphate is to increase the molecular weight of the binder (with higher molecular weight, improved bonding ability, but more difficult to disperse and higher impedance), and to increase the amount of binder used. At present, it seems that the result is not satisfactory.

Poor flexibility

At present, when processing lithium iron phosphate electrodes, it is generally felt that the electrodes are harder and more brittle, which may not have a slight impact on the stacking, but it is very unfavorable for winding. The flexibility of the polarizer is not good, and when it is wound and bent, it is easy to lose powder and break, resulting in short circuits and other defects. The mechanism explanation in this regard is not yet clear, and it is speculated 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 decrease. The original compaction density of lithium iron phosphate was relatively low, and reducing the compaction density is a necessary measure.