Uneven coating of lithium iron phosphate batteries not only affects battery consistency but also impacts design and safety. Therefore, strict control over coating uniformity is crucial during lithium iron phosphate battery manufacturing. Those familiar with formulation and coating processes know that the smaller the material particles, the more difficult it is to achieve uniform coating.
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; 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 also lead to a higher binder requirement. 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 areal density tolerance 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. Use a "linear" conductive agent
The terms "linear" and "particulate" conductive agents are figurative and may not be used academically.
Linear conductive agents are currently mainly 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 involve 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 various conductive agent forms can achieve even better conductivity.
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 approximately two to three thousand yuan. CNTS is slightly more expensive than VGCF (when the addition amount is 1%, 1 kg of CNTs costs 4000 yuan, which increases the cost by approximately 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 provide 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 avoid slurry thickening; for slurry transferred using turnover buckets, shorten the time from discharge to coating as much as possible, and if possible, use pipeline transportation to improve slurry viscosity.
4. Use extrusion coating (spraying)
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.
