Due to the low diffusion coefficient of lithium ions, lithium iron phosphate has poor conductivity, so the current practice is to make its particles small, or even make them into nanoscale, and shorten the migration path of LI+ and electrons to increase its charge and discharge speed. (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 encountered is the problem of 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 materials can be better exerted. . To mix well, first to be able to disperse. As the particles decrease, the corresponding specific surface area increases, the surface energy increases, and the tendency of aggregation between particles increases. The energy required to overcome the surface energy dispersion is greater. Mechanical agitation is commonly used now, and the energy distribution of mechanical agitation 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 dispersing ability, 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 (increase the stirring speed), increase the shear speed, and correspondingly The effective dispersion space will also increase. The former belongs to the problem of equipment, how big is the room for improvement, and Duo Online will not comment. In the latter, the lifting space is limited, because the shear speed reaches a certain limit, which will cause damage to the material and cause particle breakage. A more effective method is to use ultrasonic dispersion technology. It's just that the price of ultrasonic equipment is relatively high. The price of the one I contacted a while ago is comparable to that of imported Japanese mechanical mixers. Ultrasonic dispersion process time is short, the overall energy consumption is reduced, the slurry dispersion effect is good, the aggregation of 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 Issues
　　Uneven coating not only affects battery consistency, but also affects design and use safety. Therefore, the control of coating uniformity in the battery manufacturing process is very strict. Those who do formula and coating process know that the smaller the material particles, the harder it is to coat evenly. As far as its mechanism, I haven't seen the relevant explanation. The coating line is believed to be caused by the non-Newtonian fluidic properties of the electrode paste. The electrode slurry should belong to the thixotropic fluid in the non-Newtonian fluid. The characteristic of this type of fluid is that it is viscous or even solid when it is still, but it becomes thinner and easy to flow after stirring. The binder is a linear or network structure in the submicroscopic state. When stirring, these structures are destroyed, and the fluidity is good. After resting, they are re-formed, and the fluidity becomes poor. Lithium iron phosphate particles are small, and the number of particles increases under the same mass. To connect them to form an effective conductive network, the amount of conductive agent required also increases accordingly. The smaller the particles, the higher the amount of conductive agent, and the higher the amount of binder required. When standing still, it is easier to form a network structure, and its fluidity is worse than that of conventional materials.
　　During the process from the slurry taken out of 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 intensity is low, and the fluidity of the slurry changes and gradually becomes viscous, so that it is like Like jelly. Poor fluidity leads to poor uniformity of coating, which is manifested by increased surface density tolerance of pole pieces and poor surface morphology.
　　The fundamental thing is to improve the material, such as increasing the conductivity, increasing the size of the particles, and 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 improvement methods can be tried from the following:
　　1. Use "linear" conductive agent
　　The so-called "linear" and "granular" conductive agents are the author's image, which may not be described academically. "Linear" conductive agents are used, mainly VGCF (carbon fiber) and CNTs (carbon nanotubes), metal nanowires, etc. at present. Their diameters range from several nanometers to tens of nanometers, and their lengths are more than tens of microns or even several centimeters, while the commonly used "granular" conductive agents (such as Super P, KS-6) are generally tens of nanometers in size. The size of the material is several micrometers. The pole piece composed of "granular" conductive agent and active material is similar to the contact between points, and each point can only contact with the surrounding points; in the pole piece composed of "linear" conductive agent and active material, It is the contact between point and line, line and 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. With more nodes in contact, the conductive channel will be smoother and the conductivity will be improved. Also better. The use of a variety of conductive agents in different forms can achieve a better conductive effect. Specifically, how to choose a conductive agent is a problem worth exploring for battery production. The possible effects of using "linear" conductive agents such as CNTS or VGCF are: (1) linear conductive agents 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 was It is reported that the conductive performance of CNTS is 3 times that of the same mass (weight) conventional particle conductive agent), comprehensively (1), the amount of glue may also be reduced, and the content of active substances can be increased; (3) improve polarization, reduce contact impedance, improve Cycle performance; (4) The conductive network has more 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 very meaningful for high-rate batteries; (5) the absorption performance is improved; (6) Material prices are higher and costs are rising. 1Kg conductive agent, commonly used SUPER P 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%, 1Kg CNTs is calculated at 4000 yuan, and the cost per Ah increases by about 0.3 yuan) ;(7) CNTS, VGCF, etc. have relatively high specific surfaces, how to disperse is a problem that must be solved in use, otherwise the dispersion is not good, and the performance will not be fully exerted. Ultrasonic dispersion and other means can be used. Some CNTs manufacturers provide dispersed conductive fluids.
　　2Improve the dispersion effect
　　If the dispersion effect is good, 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 formula and batching process, and the aforementioned ultrasonic dispersion is also an effective method.
　　3 Improve the slurry transfer process.
　　When storing slurry, consider increasing the stirring speed to avoid slurry viscosity; for those who use turnover barrels to transfer slurry, shorten the time from discharge to coating as much as possible, and use pipeline transportation if possible to improve Viscous slurry phenomenon.
　　4 Extrusion coating (spray coating)
　　Extrusion coating can improve the surface texture and uneven thickness of blade coating, but the equipment price is relatively high, and the stability of the slurry is required to be high.
　　Drying is difficult.
　　Due to the large specific surface area of ​​lithium iron phosphate and the large amount of binder used, the amount of solvent required for the preparation of the slurry is also large, and it is difficult to dry after coating. How to control the volatilization rate of the solvent is a problem worthy of attention. High temperature, large air volume, and fast drying speed will result in large voids. At the same time, it may also drive the migration of colloids, resulting in uneven distribution of materials in the coating. If colloids accumulate on the surface, it will hinder the conduction of charged particles. , increasing the 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 ratio is much larger than that of lithium cobaltate and lithium manganese oxide, and more binders are needed. However, if the binder is used too much, the energy density will decrease if the content of the active material is reduced. Therefore, if possible, the battery production process will try to reduce the amount of binder used. 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 (higher molecular weight, improved bonding ability, but the more difficult the dispersion and the higher the impedance), on the one hand, it is to increase the amount of binder. So far the results seem to be unsatisfactory.
　　Less flexible
　　At present, when the lithium iron phosphate pole piece is processed, it is generally felt that the pole piece is relatively hard and brittle, which may not have a slight impact on the lamination, but it is very unfavorable when it is wound. The flexibility of the pole piece is not good, and it is easy to drop powder and break when it is wound and bent, resulting in short circuit and other defects. The explanation of the mechanism in this respect is not yet clear. It is speculated that the particles are small and the elastic space of the coating is small. Reducing the compaction density can be improved, but the volumetric energy density is also reduced. Originally, the compaction density of lithium iron phosphate is relatively low, and reducing the compaction density is an unavoidable method.