The electrode of lithium battery is a coating composed of particles. During the preparation of the electrode, a uniform wet slurry is applied to the metal collector, and then the solvent in the coating is removed by drying. Electrode paste often needs to add polymer binder or dispersant, as well as conductive agents such as carbon black. Although the solid content is generally greater than 30%, during the drying process, the coating always undergoes some shrinkage as the solvent evaporates, and the solid substances come close to each other in the wet coating, eventually forming a porous dry electrode structure.
The drying and coating processes of lithium ion battery electrodes are independent and interrelated. The properties of coating affect the design and operation of drying process. Coating speed and coating thickness determine the drying length; The coating has leveling process in drying process, which affects the uniformity of coating. Therefore, coating in the design process can accurately use the best coating, drying process, balance the relationship between the two, and ultimately affect the comprehensive technical performance of coating.
Pole sheet drying method
(1) far infrared radiation drying. The far infrared emitting element radiates the heat energy to the surface of the dry object to make the liquid evaporate and vaporize for drying. Features: its drying speed mainly depends on the radiation temperature, high temperature, drying speed. Its advantage is that the equipment is relatively simple, so it is used in the coating machine of lower gear. Its disadvantage is low drying efficiency, uneven drying, easy to produce drying malady.
(2) Double-sided air supply floating dry. Floating drying is to set a specially designed air nozzle on both sides of dry foil and send high-speed jet air. Under the effect of air flow and wall attachment effect, the dry sheet is dried in a floating state.
(3) conventional convection hot air drying. Convection drying is a traditional drying technology. The heated dry air is sent into the drying channel, and the heat energy in the dry air is transmitted to the object to be dried through the convection of the air, so that the liquid evaporates and vaporizes for drying. Its advantage is simple equipment, its disadvantage is low drying efficiency, in modern drying equipment gradually replaced by efficient hot air impact drying.
(4) circular hot air impact drying. High efficiency drying technology developed by using the principle of air jet hydrodynamics. The dry air is sprayed to the surface of the object to be dried at a high speed through a specially designed air nozzle. The static air layer of the object to be dried is damaged under the impact, thus speeding up the drying process and greatly improving the drying efficiency. The characteristics of circulating hot air impact drying are: drying speed and temperature, and related to the drying air volume. Part of the circulating dry air can be used to supply air to increase the air volume and improve the drying speed, and greatly improve the utilization of the heat of the dry air. Therefore, the circulating hot air impact drying has the characteristics of high efficiency and energy saving. In addition, by increasing the air supply to improve the drying speed, can avoid the high temperature drying may produce cracks and drying defects.
(5) superheated steam drying. Superheated vapors are vapors obtained by heating a liquid to saturated vapors that evaporate completely and then by further heating. Superheated steam drying is a new drying method in which the dry medium directly contacts with the wet coating, and the heat is mainly introduced into the material by convection, and the solvent precipitated by the dry medium is taken away by the dry medium. In the drying process, the superheated steam passes through the material surface as a drying medium, and the heat is transferred to the wet coating, and the free solvent on the coating surface is heated and vaporized, resulting in the difference of the moisture concentration between the material surface and the interior. Under this difference, the internal wet fraction diffuses from liquid or gaseous form to the surface, and the vaporized water vapor is carried away by the superheated steam stream. Its advantage is that the latent heat of steam can be used, high thermal efficiency, can achieve the effect of energy saving, superheated steam drying than hot air drying heat transfer coefficient.
(6) microwave drying. Microwave drying is a method that uses microwave energy with frequency of 915-2450mhz to make materials heat up and evaporate water to dry. Microwave drying is different from traditional drying in that the direction of heat conduction is the same as the direction of moisture diffusion. Compared with the traditional drying method, microwave drying has the advantages of fast drying rate, energy saving and environmental protection, high production efficiency, clean production, excellent drying effect, easy to realize automatic operation and control, and can improve the quality of products.
At present, some manufacturers production of coater with hot air drying, air supply nozzle, from the form of similar and impact drying, but the structural design of the nozzle and air volume and wind speed are not impact effect, drying process is still convection drying, drying efficiency is not high.
Classification of moisture in materials
Figure 1 shows the relationship among total water content, balanced water content, free water content, combined water content and uncombined water content of materials.
Balanced moisture: Moisture that can be removed by drying. Free moisture: Moisture that cannot be removed by drying.
The combined water includes the water in the cell wall of the material, the water in the capillary of the material, and the water in the form of crystal water in the solid material, etc.
Unbound water includes water mechanically attached to solid surfaces, such as adsorbed water on material surfaces, water in larger pores, etc.
The basic principles of drying
Drying: Removal of moisture from a solid material by heating water or other solvent to vaporize and removing the resulting vapors.
As shown in Figure 2, water vaporizes on the surface of the material, and there is a gas film near the surface. The partial pressure of water vapor in the gas film is equal to the vapor pressure of water in the material. The mass transfer driving force of water in the gas phase is the difference between the vapor pressure of the film and the partial pressure of water vapor in the main body of the gas phase. At the same time, the hot air heats up the material and transfers the heat to the wet material. The driving force is the temperature gradient between the hot air and the material. For convective drying, the constant flow of the medium takes away the vaporized water, thus forming a partial pressure difference.
The necessary condition for the drying process to proceed is that the partial pressure of water vapor generated by the water in the dried material is greater than that in the hot air. If the two are equal, it means that evaporation reaches equilibrium and drying stops; If the partial pressure of water vapor in hot air is high, the material will absorb water instead.
The drying process of materials is a combination of heat and mass transfer:
(1) Hot air heats up the material;
(2) Evaporation and vaporization process of liquid on material surface;
(3) The process of internal liquid diffusion to the surface through pores.
The kinetic process of drying
Drying curve: the curve of the relationship between water content x of material, drying time T and material surface temperature T in the drying process, as shown in Figure 3.
Drying rate curve: the relationship curve between material drying rate U and material moisture content X, as shown in Figure 4.
The internal diffusion and surface vaporization of water occur simultaneously, but the rate is different at different stages of drying process, and the mechanism controlling drying rate is also different. The drying process is divided into preheating and heating stage AB, constant speed drying stage BC and decreasing speed drying stage CDE.
(1) Preheating heating section AB: The material is heated and heated
(2) the constant speed drying stage BC: be dry material surface is always kept wet water evaporation, the steam in the quantity of heat is absorbed by the material, all this heat is used to evaporate moisture on the surface of the material, material surface moisture evaporating speed and material internal moisture diffusion velocity is almost equal, drying rate remained stable at this point, constant speed dry state.
(3) The first deceleration stage (CD section) : the diffusion rate of water inside the material is less than the vaporization rate of water on the surface at the wet-bulb temperature. At this time, the surface of the material cannot maintain overall wetting and forms a "dry zone", leading to the drying rate decline.
(4) The second deceleration stage (DE section) : the vaporization surface of water gradually moves to the inside of the material, thus lengthening the heat and mass transfer path, increasing the resistance and causing the drying rate to decrease.
Lithium battery electrode paste composition is uniform distribution, then, the solvent evaporation induced wet coating thickness decreases, the graphite particles gradually close to each other, until the formation of the accumulation of the most densely populated state, coating contract termination (FIG. 5 c), then pushed further solvent evaporation forced the gas-liquid interface to the pore structure inside, eventually forming porous coating on dry electrodes (FIG. 5 e). Large pores tend to preferentially empty liquid phase. In the process of coating shrinkage, the tiny pores on the surface are filled with liquid phase until the coating shrinkage stops (FIG. 5C) and the pores are filled with solvent. The solvent is then further removed, resulting in the first larger hole in the coating (FIG. 5d), which is more difficult to empty due to capillary force.
