The homogenization and coating process of lithium-ion battery production have an important influence on the performance of lithium-ion battery, so people pay more attention to the homogenization and coating process.Li-ion battery slurry mainly consists of active material and conductive agent particles, Solvent, binder and other components. During the drying process, the solvent will volatilize and the binder will precipitate on the surface of the active material and the conductive agent. Therefore, the drying process will affect the pore structure and adhesion of the electrode Agent distribution.We have previously introduced the results of the study at Karlsruhe Industrial University in Germany and found that drying rate will have a significant impact on the distribution of PVDF binder in the electrode, the electrode surface will be enriched during fast drying More PVDF binder.
In fact, coating is a widely used technology, such as film printing, diaphragm coating and other fields will involve a variety of slurry coating process, coating is a very large subject, and lithium-ion Battery is only involved in a very small area .Today, for everyone to introduce the coating after the drying process for the coating (this is not the introduction of lithium-ion battery paste coating, but we can learn from Experience).
Single component solution drying
Although the single-component solution is much simpler than that of the complex system, the drying process is still very complicated. Firstly, it involves the migration of solvent molecules in the liquid phase and the diffusion of the gas phase, In the process of vapor phase evaporation from liquid to vapor phase, solvent molecules will accumulate towards the liquid surface. The distribution of solvent molecules in solution is affected by two competing forces, which are diffusion and evaporation respectively. The two forces Which is the characteristic length, D is the diffusion coefficient, the evaporation time function is Tev = l / Vev, Vev is the evaporation rate, and therefore can be based on the diffusion time function and The evaporation time function defines a Peclet parameter for film formation as shown in the following equation.
If the film formation parameters Pe <1, 那么扩散速度较快, 那么由于蒸发形成的浓度梯度会很快在扩散的作用下消失, 从而保证形成均匀的膜结构. 但是如果Pe>1, then the evaporation rate is faster than the diffusion rate, thus resulting in the concentration gradient increasing with time during the drying process, leading to a decrease in the uniformity of the membrane structure.
As can be seen from the above description, the lower drying rate is conducive to the formation of a more uniform film, but in actual production, we prefer to quickly dry in order to improve production efficiency in the high-speed drying process will result in pulp The concentration of the material surface rapidly increases, resulting in an increase of the surface viscosity and the formation of a gel-like material, which we generally refer to as the 'epidermis', which needs to be avoided in the coating. The 'epidermis' not only causes the final film Rough surface, affecting the drying, but also cause pits, micropores and other defects on the surface of the membrane.
In the fast drying process, the appearance of 'epidermis' on the surface of the membrane will cause instability of the solution. An unstable phenomenon is that the appearance of the 'epidermis' will block the solution underneath and affect the drying of the solution film. One instability is the presence of microporosity defects in the 'epidermis' (studies have shown that the 'epidermal' energy released during the drying process releases in the form of micropores.) Therefore, in actual production, the drying speed can not be pursued blindly According to the characteristics of the solution, the appropriate choice of the appropriate drying speed.
Two-component solution drying
The one-component solutions discussed above are ideal, and are often used in the actual production of multi-component mixing, such as a variety of features, size of the particles, surfactants and polymers, each component Will now give different properties to the slurry.We now assume a glue solution containing two sizes of particles, which are initially distributed evenly in solution (as shown in Panel A below) .During the drying process, since the solution There are two types of particles, so we also have two Pe numbers: Pe1, Pe2, Pe1> 1 in one case, and Pe2<1, 此时大颗粒由于扩散比较慢, 因而会在溶液的表层会富集较多的大颗粒, 而小颗粒因为扩散速度比较快, 因而分散比较均匀, 最终会形成大颗粒在上层的分层结构. 此外, 在硬颗粒体系中, 如果两个Pe参数都大于1 (Pe1>1, Pe2> 1), and the size difference between the size of particles is relatively large (more than 7 times difference), may also cause small particles in the upper stratification structure.
We also observed drying delamination in some nanosystems, for example, at lower drying rates, weaker particle-polymer forces may cause particle agglomeration, but stronger particle-polymer The force can promote the formation of a more uniform film structure.In the fast drying process, the weaker particle-polymer force will cause the polymer in the upper-particle under the layered structure in the stronger particle-polymerization Under the force of the material, there is a structure where the particles are on the upper polymer, and during high speed drying, a layer of 'epidermis' is formed on the surface of the solution film, which may be the reason for the heaviest film delamination.
Real Understanding It is a very complicated issue to understand the structure change of slurry during drying. Especially in the complex slurry system of Li-ion battery, there are many kinds of hard particles with different surface characteristics. The difference of particle size Still very poor (active material particles in 10-30um, conductive agent from a few hundred nanometers to a few microns), but also increase the complexity of the electrode in the drying process, improper drying system is prone to electrode layered structure, Thus affecting the electrochemical properties of the electrode, so we need to constantly explore the experience in practice, to improve electrode uniformity.
