Application of Spray Drying Technology in Preparation of Lithium Ion Battery Materials
Spray drying is a process that has both granulation and drying processes, and the properties of the powder can be controlled to be stable through continuous operation. Spray drying technology has a history of more than 100 years, and its application in industry has a history of nearly one hundred years. It began to be limited to the production of a few products such as egg powder, milk powder and detergents. With the continuous deepening of research, it is now in various industries. It is widely used in the production of ultra-fine powders and nano-powders. The use of spray drying technology can produce spherical powders with good quality and reproducibility, which can shorten the preparation process of powders, and it is also conducive to automatic and continuous production. It is an effective method for large-scale preparation of fine powders and has become a variety of products. One of the best ways to miniaturize and dry. 1. The basic principle and characteristics of spray drying There are three types of commonly used sprayers: pressure sprayers, air sprayers, and centrifugal sprayers. Pressure sprayers are used in many applications. Therefore, the pressure spray drying technology is explained in detail in this paper. Spray drying technology is a better method for obtaining ultrafine dry powders from feed liquids. Feed liquids can be in the form of solutions, suspensions, emulsions, and other pumps. The basic principle is to use a nebulizer to spray a certain concentration of material liquid into a mist of droplets, falling into a flow of hot gas at a certain flow rate, so that it can be rapidly dried to obtain a powdery product. The basic principle diagram is shown in FIG. 1 . The specific process of the spray drying method (shown in Figure 2) is as follows: The hot gas generated by the heater (heat drying medium) enters the tower through the hot air pipe from the top of the drying tower. At the same time, the feed liquid from the injection pump passes through the fog. The nozzle sprays a very fine spherical droplet and falls in a hot gas flow at a certain flow rate for heat exchange. Since the droplet is fine and the specific surface area is large, the solvent quickly evaporates, and the drying and granulation processes are instantaneously completed. After drying, the finished product enters the cyclone separator from the cone-shaped discharge outlet under the tower body, and is separated from the drying medium and collected at the product receiver. Hot exhaust gas is discharged from the exhaust port through the induced draft fan. Compared with other powder preparation methods, spray drying has the following advantages: 2. Particle size distribution control Massive production practice shows that the process parameters that affect the particle size distribution of powder particles are: feed concentration, feed rate, atomization pressure, temperature, nozzle structure, flow rate of drying medium, gas-liquid contact mode, and nature of solvents and solutes. These factors affect the atomization effect and the dry granulation mechanism of the liquid from different angles. The following will discuss and analyze some of the major influencing factors. 2.1 Effect of Feed Concentration Studies have shown that the particle size of the particles obtained in the high concentration liquid is larger than that of the low concentration material liquid. This is because the feed liquid concentration is an important property parameter that affects the formation and size of droplets. The concentration is high, the corresponding solid content is high, the viscosity is large, and the energy required to form droplets is also high. Therefore, the droplets formed by the high-concentration liquid are larger, the gas-liquid contact area is reduced, the mass transfer effect is weakened, the time for the droplets to reach the oversaturated state is prolonged, the instantaneous nucleation amount is reduced, and the deposition of the particles is the main mechanism for growth, so the formation The larger particle size. In addition, the particles in the high concentration solution are prone to agglomeration. However, there is an upper limit to the increase of feed concentration, and if it exceeds this limit, no particles can be obtained. 2.2 Effect of feed rate The experimental results show that with the increase of feed rate, the grain size has a tendency of increasing gradually. This may be due to the fact that under other conditions being equal, as the feed rate increases, the diameter of the atomized droplets increases, and the larger droplets contain more solute, so the resulting powder particle size is correspondingly smaller. Big. In addition, as the feed rate increases, the number of undried droplets increases, and the droplets collide with each other to cause agglomeration, which aggravates product agglomeration. And the injection speed is too fast to be detrimental to the liquid atomization, which will affect the drying effect. 2.3 Influence of atomization pressure Atomization pressure is a key parameter of spray drying technology. Only when a certain pressure is reached can the droplets form. As the spray pressure increases, the particle size decreases. This is because the evaporation drying speed of the solvent in the droplet increases as the pressure increases while other conditions remain unchanged. On the other hand, the pressure drop at the nozzle increases, the atomized droplet size decreases as the impact energy of the airflow increases, the gas-liquid contact area is increased, the mass transfer effect between gas and liquid is enhanced, and the drying speed of the droplet is increased. . The combination of the two results in the shortening of the time required for the droplets to reach supersaturation, the rapid nucleation rate, and the increase in the number of nucleation. The deposition of particles is dominated by uniform precipitation. As a result, the particle size of the resulting final product decreases and the particle size distribution narrows. However, if the pressure is too high, it will have a negative impact on the micro-shape, such as broken, voids, dents, and other irregular shapes, affecting product performance. 2.4 Effect of temperature The temperature can be divided into the inlet temperature and the outlet temperature, wherein the inlet temperature is more important, the particle size of the obtained fine particles at a low temperature is larger than that at a high temperature, and the particle size distribution is also wide, that is, when the other conditions are constant, the temperature is low, the solution The time for the droplet to reach supersaturation is prolonged, the instantaneous nucleation rate is reduced, and the number of nucleation is reduced. Therefore, the particle size of the obtained fine particles is increased. At this time, since the precipitation of the particles is mainly based on growth, the time for forming the final product accordingly is prolonged, and the agglomeration and collision of the particles also cause the uniformity of the product to be deteriorated, so that the particle size distribution becomes wider. As the temperature rises, the evaporation speed of the solvent is accelerated, and the time for the droplets to reach the supersaturation state is shortened, so that the particle size of the formed particles is accordingly reduced and the particle size distribution is narrowed. It has been found that when the temperature is too high, the formed particles tend to agglomerate, which increases the particle size. 2.5 Effect of nozzle outlet diameter The nozzle, also known as the atomizer, is a key component of the spray drying equipment. The different structure of the nozzle directly affects the atomization and dispersion effect of the liquid, thereby affecting the particle size and performance of the particles. With the same injection rate, the larger the diameter of the nozzle outlet is, the smaller the diameter is. This may be due to the fact that when the outlet diameter is large, the formed liquid film is thin, and the droplets dispersed after being impacted by air flow and rubbed are smaller. At the same time, the mass transfer between the gas and liquid is enhanced by the increase of the total surface area of ​​the droplets. The evaporation rate of the solvent is accelerated and the droplets reach the supersaturated state faster. Therefore, in the case of a certain liquid flow rate, the deposition of particles is mainly nucleation and precipitation, the number of instantaneous nucleation increases, and the particle size decreases. It can be seen that changing the structure of the nozzle can control the particle size and particle size distribution within a certain range and make it meet different granulation requirements. 3 Powder particle morphology control The products obtained by the spray drying technique are generally spherical particles, but due to improper control of the process parameters, the particles are often deformed, such as hollow spheres, hollow rings or apple shapes. The formation of deformed particles is affected by various factors such as the type of raw materials, the concentration of raw materials, the size of droplets, the residence time in the high temperature zone, and the air flow velocity. The deformation process is shown in the figure below. The low raw material concentration is the main reason for the formation of hollow particles. During the drying process, moisture migrates to the droplet surface and carries solid particles to form a partially hollow interior. If a low-permeability elastic film is formed at the periphery of the droplet, the temperature of the droplet will increase due to the low evaporation rate and the moisture will evaporate from the inside. So that the droplets rise up. Both of these conditions can destroy the spherical shape of the particles and produce deformed particles. In addition, when the droplets stay in the high temperature zone, the solute rapidly precipitates on the surface of the droplets and forms a shell layer. The existence of a solid shell impedes the mass transfer of vaporized molecules in the solution, while the heat transfer changes little, and thus the shell layer The temperature of the solution continues to rise and it may reach boiling state. The shell expands under internal pressure and the concentration of the center solute decreases. When the internal pressure is greater than the mechanical strength of the shell, the internal gasification molecules overcome the resistance at the weakest part of the shell and rush out of the shell, causing the shell to produce holes or hollow particles. The droplets evaporate first at high temperature to form a hard shell and the liquid in the shell continues to evaporate. If the hard shell is impermeable, the shell is blown to form a hollow sphere. In some cases, hollow spheres can be blown to form shell fragments. 4 Application of Spray Drying Technology in Preparation of Lithium Ion Battery Materials Li Yangxing et al. used a mixed solution of lithium acetate and cobalt acetate as the precursor, and the LiCoO2 ultrafine powder was obtained through heat treatment of the mixed powder prepared by the spray drying method. The LiCoO 2 powder has a uniform distribution with a particle size of 200-700 nm. It has been found that the LiCoO 2 prepared by this method has excellent electrochemical activity by testing and testing its charge and discharge capacity. Using Li2C03 and rutile TiO2 as precursors, Zhaoyin Wen et al. prepared spherical porous Li4Ti5O12 powders with a particle size of several micrometers by spray-drying and heat-treating process, which is 100 to 300 times lower than the commonly used mechanical mixture solid-state reaction method. An excellent electrode material was obtained at a heat treatment temperature of °C. The spray-drying method can also obtain ultrafine powders coated with a multi-component precursor, such as TiO2-Ce02, Ti02-Sn02, and Ti02-ZnO coated ultrafine powders having a particle size of 10 μm or less, as prepared by Huang Chen et al. . The spray drying method has the advantages of rapid drying process, high production efficiency and large output, and has certain advantages in large-scale preparation of electrode materials. However, the spray drying method also has some technical difficulties that need to be solved: to ensure that the obtained material has a high sphericity, a suitable particle size, improve the product tap density and particle fluidity; so that the preparation of the product has a uniform chemical composition, The proportion of the elements meets the requirements to ensure the electrochemical performance of the material is stable; ensure that this method is universal and can be promoted in the preparation of different electrode materials. MW02 Smart Watch MW02 Smart Watch everyone enjoys luck , https://www.eeluck.com
General spray drying includes 4 stages: 1 liquid atomization of the material; 2 contact of the mist group with the thermal drying medium; 3 evaporative drying of the droplets; 4 separation of the dried product from the drying medium. The dried product can be powdered, granular, or particulate agglomerates. 
(1) Spray drying allows granulation and drying to be completed in one step. The production process is simple and the operation and control are convenient. It is suitable for continuous industrial production and is easy to realize automation.
(2) The drying rate is high, the time is short, and it has little effect on the heat-sensitive components, so it is particularly suitable for the drying of heat-sensitive materials.
(3) When spray drying, the feed liquid is sprayed into an atomized dispersion under constant stirring, and drying is completed instantaneously, so the uniformity is good.
(4) In spray drying, because the solvent is quickly vaporized, the finished product is loose and fine particles, and when in contact with the solvent, the solvent easily enters the inside of the particle, and good dispersion and good solubility can be obtained without further treatment.
(5) Since spray drying is a continuous, closed production process, the purity of the product is high, the opportunity of exposure in the production environment and contact with the operator is eliminated, and environmental pollution is reduced. 