Unlocking the Potential of Lithium Iron Phosphate: Overcoming Processing Challenges for Faster Charging
Lithium iron phosphate’s conductivity is hindered by the low diffusion coefficient of lithium ions. But fear not! The solution lies in reducing particle sizes – even to the nanoscale. By shortening the migration path of LI+ ions and electrons, it’s possible to enhance the battery’s charging and discharging speed. However, this cutting-edge approach presents a few challenges when it comes to battery processing. Find out how we’re navigating these obstacles to advance the potential of lithium iron phosphate.
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Pulping is one of the most critical processes in the production of batteries. Its core task is to mix the active substances, conductive agents, binders, and other materials evenly so that the material performance can be better. To mix well, it is necessary to be able to disperse first. The particle size decreases, the corresponding specific surface increases, the surface energy increases, and the tendency of inter-particle polymerization increases. The more energy required to overcome the surface energy dispersion, the greater the energy required. Nowadays, mechanical stirring is commonly used, and the energy distribution of mechanical stirring is not uniform. Only in a certain area, the shear strength is large enough and the energy is high enough to separate the aggregated particles. To enhance the dispersion capacity, one is to optimize the structure of the mixing equipment, without changing the maximum shear speed to improve the proportion of space in the effective dispersion area; one is to increase the mixing power (increase the mixing speed), enhance the shear speed, the corresponding effective dispersion space will also increase. The former is a problem with the equipment, how big the lifting space is, and the coating online does not comment. The latter, the lifting space is limited, because the shear speed mentioned a certain limit, it will cause damage to the material, resulting in particle breakage.
A more effective method is to use ultrasonic dispersion technology. Only the ultrasonic equipment is more expensive, contacted a while ago, and its price and imported Japanese mechanical mixer equivalent. The ultrasonic dispersion process is short, the overall energy consumption is reduced, the slurry is well dispersed, the polymerization 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
Achieving a uniform coating is pivotal to ensuring the consistency and safety of battery production. However, the process is complicated, as the smaller the material particles, the more challenging it is to achieve uniformity. Manufacturers must control the flow and viscosity of the electrode slurry, which is a thixotropic fluid that becomes viscous when not stirred. To form a conductive network, a conductive agent and binder are required, and the amount increases with small particles, making it harder to achieve uniformity. Furthermore, poor flowability caused by the lack of stirring during transfer to the coating process results in a non-uniform coating, leading to increased surface density tolerance of the pole piece and poor surface morphology. By focusing on improving conductivity, particle size, and spherification, we can enhance the battery production process effectively.
Table of Contents
1. Use of “linear” conductive agents
The so-called “linear” “particle-shaped” conductive agent is the author’s image, academic may not be so described.
The use of “linear” conductive agents, currently the main VGCF (carbon fibers) and CNTs (carbon nanotubes), metal nanowires, etc. They are a few nanometers to tens of nanometers in diameter, length in tens of microns or even a few centimeters, while the current commonly used “particle-shaped” conductive agent (such as SuperP, KS-6) size generally in the tens of nanometers, the size of the battery material for a few microns. “Granular” conductive agent and the active material composition of the pole, contact similar to the contact between the point and the point, each point can only be in contact with the surrounding points; “linear” conductive agent and the active material composition of the pole, is the point and line, line and line contact, each point can be in contact with multiple wires at the same time, and each wire can be in contact with multiple wires at the same time, more contact nodes, the conductive channel will be more smooth, the conductivity will be better. The use of a variety of different forms of conductive agent combination, you can play a better conductive effect, how to make the specific choice of conductive agent, for battery production is a very worthwhile exploration of the issue.
The possible effects of using “linear” conductive agents such as CNTS or VGCF are:
- linear conductive agent to a certain degree to enhance the bonding effect, and improve the flexibility and strength of the pole piece.
- reduce the amount of conductive agent (remember that there are reports that CNTS conductive efficiency for the same mass (weight) of conventional granular conductive agent 3 times), comprehensive (1), the amount of adhesive may also be reduced, the active substance content can be increased.
- improve polarization, lower contact impedance, and improve circulation performance.
- conductive network contact nodes, the network is more complete, multiplier performance than the conventional conductive agent is more excellent; heat dissipation performance is improved, which is very meaningful for high multiplier batteries.
- absorption performance is improved.
- higher material prices, and rising costs. 1Kg conductive agent, commonly used SUPERP only tens of dollars, VGCF about two to three thousand dollars, CNTS than VGCF slightly higher (when the addition of 1%, 1Kg CNTs calculated at 4,000 yuan, about 0.3 yuan per Ah cost increase).
- CNTS, VGCF, and another high specific surface, how to disperse is a problem that needs to be solved in use, otherwise, poor dispersion performance will not play. With the help of ultrasonic dispersion and other means. There are CNTs manufacturers to provide a good dispersion of conductive liquid.
2. Improve the dispersion effect
If the slurry is well dispersed, the probability of particle contact 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 by the improvement of formulation and batching process, and the ultrasonic dispersion mentioned earlier is also an effective method.
3. Improve the slurry transfer process
Paste storage can be considered to improve the mixing speed to avoid sticky paste; for the use of turnover buckets to transfer the paste, as far as possible to shorten the time from the material to the coating, conditional on the use of pipeline transport to improve the paste sticky phenomenon
4. Using extrusion coating (spraying)
Extrusion coating can improve squeegee coating surface grain, thickness unevenness, etc., but the equipment is more expensive and requires higher stability of the slurry.
Drying difficulties
Due to the large specific surface of lithium iron phosphate, the binder dosage is large, the amount of solvent required to prepare the slurry is also large, and drying after the coating is also more difficult. How to control the evaporation rate of the solvent is a matter of concern. High temperature and high air volume, fast drying speed, resulting in large voids, but also may drive the migration of gum, resulting in uneven distribution of materials in the coating, if the gum in the surface layer produces aggregation, will impede the conduction of charged particles, increasing the impedance. Low temperature, low air volume, slow solvent escape, long drying time, and low capacity.
Poor bonding performance
Lithium iron phosphate material particles are small, and the specific surface ratio of lithium cobaltate, and lithium manganate increased by a lot, the need for more binder. But the binder with more, reducing the content of the active material, energy density is reduced, so where possible, the battery production process will try to reduce the amount of binder. In order to improve the bonding effect, the currently common practice of lithium iron phosphate processing on the one hand to increase the molecular weight of the binder (high molecular weight, bonding capacity increases, but the more difficult to disperse, the higher the impedance), on the other hand, to increase the amount of binder. The current results do not seem to be satisfactory.
Poor flexibility
In the current lithium iron phosphate wafer processing, the general feeling is that the wafer is harder and more brittle, and the impact on the stack may not be slightly smaller but in the winding, it is very unfavorable. Poor flexibility of the pole piece, winding, and bending is easy to drop powder, and fracture, resulting in short circuits and other bad. The explanation of this mechanism is not clear, the guess is that the particles are small, and the coating of the elastic space is small. Lower compaction density can be improved, but so the volume energy density is also reduced. The original lithium iron phosphate compaction density is relatively low, reducing the compaction density is the last resort that will take the means.
