The first problem encountered was material dispersion.
Slurry preparation is one of the most critical processes in battery production. Its core task is to uniformly mix active materials, conductive agents, binders, and other materials to ensure optimal material performance. For uniform mixing, dispersion is essential. Smaller particles result in larger specific surface areas and surface energy, increasing the tendency for particle aggregation. The energy required to overcome this surface energy dispersion is also greater. Currently, mechanical stirring is commonly used. However, mechanical stirring results in uneven energy distribution. Only within a certain area, with sufficiently high shear strength and energy, can aggregated particles be separated. To improve dispersion, one approach is to optimize the structure of the stirring equipment, increasing the proportion of the effective dispersion area without changing the maximum shear rate; another is to increase the stirring power (increasing the stirring speed), thereby increasing the shear rate and consequently the effective dispersion space. The former is an equipment-related issue, and the extent of improvement is not discussed here. The latter has limited improvement potential because exceeding a certain limit in shear rate will damage the material, leading to particle breakage.
A more effective method is ultrasonic dispersion technology. However, ultrasonic equipment is expensive; one supplier I contacted recently had a price comparable to imported Japanese mechanical mixers. Ultrasonic dispersion processes are quick, reduce overall energy consumption, produce excellent slurry dispersion, effectively delay particle aggregation, and significantly improve stability.
In addition, the dispersion effect can be improved by using dispersants.
Coating uniformity problem
Uneven coating not only affects battery consistency but also impacts design and safety. Therefore, controlling coating uniformity is crucial during battery manufacturing. From my understanding of formulations and coating processes, the smaller the material particles, the more difficult it is to achieve uniform coating. Regarding the underlying mechanism, I haven't yet seen a relevant explanation. Some believe it's caused by the non-Newtonian fluid properties of the electrode slurry.
The electrode slurry should be classified as a thixotropic fluid, a type of non-Newtonian fluid. These fluids are characterized by being viscous, even solid, when at rest, but thinning and flowing easily upon agitation. The binder, in its submicroscopic state, has a linear or network structure. Agitation disrupts this structure, improving flowability; upon resting, it reforms, reducing flowability. Lithium iron phosphate particles are small, and for the same mass, the increased particle number necessitates a corresponding increase in the amount of conductive agent required to connect them into an effective conductive network. Smaller particles and increased conductive agent usage lead to a higher amount of binder needed. When stationary, it more easily forms a network structure, resulting in poorer flowability than conventional materials.
From the time the slurry is removed from the mixer to the coating process, many manufacturers still use transfer buckets for transfer. During this process, the slurry is not stirred or the stirring intensity is low, causing the slurry's fluidity to change, gradually becoming viscous, to the point of resembling jelly. Poor fluidity leads to poor coating uniformity, manifested as increased density tolerance of the electrode sheets and poor surface morphology.
The fundamental solution lies in improving the materials themselves, such as increasing conductivity, enlarging particle size, and sphericalizing the particles. However, these measures may have limited effects in the short term. Based on existing materials and from a battery manufacturing perspective, improvements can be explored through the following approaches:
1
Using "linear" conductive agents
The terms "linear" and "particulate" conductive agents are figurative and may not be used academically.
The most important "linear" conductive agents currently used are VGCF (carbon fiber), CNTs (carbon nanotubes), and metal nanowires. Their diameters range from a few nanometers to tens of nanometers, and their lengths are tens of micrometers or even several centimeters. In contrast, commonly used "particulate" conductive agents (such as SuperP and KS-6) are generally tens of nanometers in size, while battery materials are only a few micrometers in size. Electrodes composed of "particulate" conductive agents and active materials have contact similar to point-to-point contact, where each point only contacts its surrounding points. Electrodes composed of "linear" conductive agents and active materials have point-to-line and line-to-line contact; each point can contact multiple lines simultaneously, and each line can contact multiple lines simultaneously. With more contact points, the conductive channels are more open, resulting in better conductivity. Using combinations of different forms of conductive agents can achieve even better conductivity; how to select the appropriate conductive agent is a topic worthy of further exploration in battery manufacturing.
The potential 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 and enhance the flexibility and strength of the electrode sheet;
(2) Reduce the amount of conductive agent (I remember there was a report that CNTS has a conductivity that is 3 times that of conventional granular conductive agents of the same mass (weight)). In summary (1), the amount of adhesive may also be reduced, and the content of active materials may be increased;
(3) Improve polarization, reduce contact resistance, and improve cycle performance;
(4) The conductive network has more contact nodes and a more complete network, resulting in better rate performance than conventional conductive agents; the improved heat dissipation performance is significant for high-rate batteries.
(5) Absorption performance is improved;
(6) Higher material prices lead to increased costs. 1 kg of conductive agent, commonly used SUPERP, costs only tens of yuan, while VGCF costs about two to three thousand yuan. CNTS is slightly more expensive than VGCF (when the addition amount is 1%, 1 kg of CNTs costs 4,000 yuan, which increases the cost by about 0.3 yuan per Ah).
(7) CNTS and VGCF have high specific surface areas, and how to disperse them is a problem that must be solved during use; otherwise, poor dispersion will not allow them to perform well. Ultrasonic dispersion and other methods can be used. Some CNT manufacturers supply well-dispersed conductive liquids.
2
Improve dispersion effect
A well-dispersed slurry significantly reduces the probability of particle contact and agglomeration, thus greatly improving the slurry's stability. Improving the formulation and batching process can enhance the dispersion effect to some extent; ultrasonic dispersion, as mentioned earlier, is also an effective method.
3
Improve the slurry transfer process
When storing slurry, consider increasing the stirring speed to prevent the slurry from becoming viscous; for slurry transfer using turnover buckets, try to shorten the time from discharge to coating, and if possible, use pipeline transportation to improve the slurry viscosity.
4
Extrusion coating (spraying) is used.
Extrusion coating can improve the surface texture and thickness unevenness of doctor blade coating, but the equipment is expensive and requires high stability of the slurry.
Drying difficulties
Because lithium iron phosphate has a large specific surface area and requires a large amount of binder, a large amount of solvent is needed to prepare the slurry, making drying after coating more difficult. Controlling the solvent evaporation rate is a crucial issue. High temperature and high airflow result in rapid drying, but also larger voids and may promote the migration of adhesives, leading to uneven material distribution in the coating. If adhesives accumulate on the surface, they will hinder the conduction of charged particles, increasing impedance. Low temperature and low airflow result in slow solvent evaporation, longer drying time, and lower yield.
Poor adhesion
Lithium iron phosphate (LFP) materials have smaller particle sizes and a significantly larger specific surface area compared to lithium cobalt oxide and lithium manganese oxide, requiring more binder. However, excessive binder usage reduces the content of active materials, thus lowering energy density. Therefore, whenever possible, the amount of binder used in battery production is minimized. To improve bonding, current common practices in LFP processing include increasing the molecular weight of the binder (higher molecular weight improves bonding ability but also makes dispersion more difficult and increases impedance) and increasing the amount of binder used. However, the results so far do not seem to be satisfactory.
Poor flexibility
Currently, during the processing of lithium iron phosphate (LFP) electrodes, the electrodes are generally considered to be relatively hard and brittle. This may have a significant impact on stacking, but it is particularly detrimental during winding. The poor flexibility of the electrodes makes them prone to powder shedding and breakage during winding and bending, leading to short circuits and other defects. The underlying mechanism is not yet clear, but it is speculated that the small particle size limits the elastic space of the coating. Reducing the compaction density can improve this somewhat, but this also reduces the volumetric energy density. Since LFP already has a relatively low compaction density, reducing it is a last resort.
