1. Introduction The performance of lithium-ion batteries depends on the composition and properties of each component of the battery pole piece, including electroactive materials, conductive agents, binders, etc. The electrode preparation process determines the micromorphology of the electrode and is also very important. The progress of electrode preparation technology is not only... Keywords: lithium-ion battery, battery pole piece, electrode slurry layer.

  1 Introduction

The performance of lithium-ion batteries depends on the composition and properties of each component of the battery pole piece, including electroactive substances, conductive agents, binders, etc. The electrode preparation process determines the micromorphology of the electrode and is also very important. Advances in electrode preparation technology can not only reduce battery production costs, but also improve battery capacity and cycle stability. In academia, many methods have been tried for the preparation of lithium battery pole pieces, such as chemical vapor deposition, spray deposition, laser deposition, spin coating, etc. There are even researchers working on developing a dry powder mixture of electroactive particles, conductive agents, and binders that can be directly coated on a current collector substrate without using a liquid slurry. All of these methods have no commercial applications and are not discussed in this article.

At present, most lithium-ion battery pole pieces are produced by coating the metal current collector with an electrode slurry layer, then drying, and then rolling and compacting the dried pole pieces. These technologies are not only used in commercial production, but are also commonly used in academia, except that comma blade coating is generally used in scientific research. Electrode slurry is prepared by stirring and mixing electroactive materials, polymer binders, conductive agents and solvents. Table 1 shows common commercial battery electrode materials. Sometimes, the slurry process also requires the addition of some auxiliary mixing additives, such as water-based solvents, and dispersants are often added to adjust the rheological properties of the slurry to match the requirements of the coating equipment.

Slurry preparation technology, drying of wet pole pieces and rolling compaction process parameters of pole pieces control the microscopic morphology of the electrode, thus having a huge impact on the performance of the electrode. In addition, the properties of the slurry will also affect the performance of the electrode production process, thereby affecting the production efficiency of the equipment and the final electrode morphology, and even the usability of the entire battery.

2. The influence of slurry morphology and preparation process on electrode morphology characteristics

Lithium-ion battery pole pieces have a complex porous structure, including active material and conductive agent particles, which are connected together through a binder and adhere to the metal current collector. The performance of the electrode depends on the performance of each component and the morphology of the electrode. The conductive agent is usually a variety of carbon conductive material particles. It can also form interlocking with the active material particles and strengthen the bonding with the current collector. The ideal electrode particle coating morphology is shown in Figure 1 and should be like this:

(i) The active material particles are fine and uniformly dispersed without agglomeration. The conductive agent particles form a thin layer and disperse into a conductive network, and the maximum number of active material particles are interlocked and connected on the current collector. In fact, the conductive agent particles are generally various carbon material particles. In the optimal situation, a conductive agent system with multi-scale characteristics can be considered. The role of the binder is to ensure the mechanical stability of the electrode structure. The positive electrode is usually a PVDF-based polymer. In addition, the electrode needs to have enough holes to allow the electrolyte to penetrate all active material particles. The electrode structural characteristics also mean that the mass ratio of active material to conductive agent and binder should be as high as possible.

(ii) The active material particles should be small to ensure that the battery has a high current density. The traditional view is that lithium ion diffusion inside active material particles determines the battery rate performance. Small particles with short lithium ion diffusion paths can improve current characteristics and Coulomb efficiency. Recently, many research works have begun to revise the traditionally accepted concepts of lithium ion diffusion or lithium ion conduction, and believe that even if the active material particles are micron-scale, the diffusion of lithium ions is not the key process that determines the rate performance. At the same time, increasing the lithium ion diffusion rate has been considered not to be the only reason for reducing the size of active material particles. Reducing particle size can improve battery rate performance. Currently, there are two other explanations. Reducing the size of active material particles is:

a. Required for low electronic conductivity of active material particles. For example, the conductivity of lithium iron phosphate particles is about 10^-10 S/cm, and the specific conductivity of 2-micron particles is higher than that of 15-micron particles. Therefore, the low conductivity of active materials also requires smaller particles, so that electrons and ions conduct electricity. performance can be improved, thereby improving battery power performance.

b. The need for electrode coating morphology, especially conductive components. Active material particles are combined with conductive agent particles. The smaller the particles, the more likely it is theoretically possible to form a dispersed thin layer, allowing the conductive agent to be evenly distributed on the surface of the active material particles.

(iii) The above analysis shows that the smaller the particles, the better. Therefore, micron, submicron, and nanoparticle active materials are now widely used. However, this also faces some challenges or areas that need attention:

a. The active materials and conductive agents of small particles, especially nanoparticles, have a large specific surface area. When the positive and negative electrode potentials are outside the thermodynamic stability window of the electrolyte, the electrolyte solvent is more likely to react and decompose, forming a thin layer on the surface of the particles. It blocks lithium ion transport and consumes electrolyte.

b. During the use of the battery, the SEI film continues to form on the surface of the electrode active material and conductive agent particles, continuously consuming lithium ions and electrolytes. Although the thickness of the SEI film has nothing to do with the particle size of the active material and conductive agent, it is related to the particle surface area. Nanoparticles are more prone to problems due to their high specific surface area.

c. Another problem that hinders the application of nanomaterials is that the tap density of nanoparticles is low, so the electrode coating composed of particles generally has a low density.

These problems prompt the need to comprehensively consider the electrolyte, particle material properties and electrode morphology when optimizing the particle size of active materials and conductive agents. In addition, from the perspective of the slurry preparation process, particle size optimization is also very important, because small particles are difficult to disperse and are more likely to agglomerate in the slurry.

(iv) The thickness of lithium-ion battery electrodes is generally 40 to 300 microns, with a deviation of 1 to 2 microns. Comma blade and die extrusion coating are the most commonly used electrode preparation processes. The thickness of the electrode plate coating is also an important factor affecting battery performance, and slurry wet coating becomes a prerequisite for obtaining uniformly dry electrodes. The slurry is a suspension containing solid particles. Not only the size of the solid particles must be smaller than the thickness of the coating, but the size of the powder particle agglomerates must also be smaller than the thickness of the wet coating, otherwise the electrode performance will be affected, as shown in Figure 2a. Large particle agglomerates can also cause extrusion coating defects, as shown in Figure 2b.

In addition to parameters such as viscosity and particle size, other parameters will affect the coating process. For example, the thickness of the coating is very important. The thicker the coating, the easier it is to have thickness unevenness and coating pore defects (Figure 2d).

(v) The conductive agent forms a conductive path in the lithium-ion battery pole piece, which requires the conductive agent to be evenly distributed in the slurry (macro-mixing) and coat the active material particles (micro-mixing). The distribution of conductive agent not only depends on the mixing process, but also on the characteristics of the conductive agent itself. A variety of carbon conductive materials can stabilize the slurry, prevent segregation (settlement and agglomeration) of the slurry, and maintain a uniform slurry. Therefore, the preparation of the optimal slurry is related to the size of the active material and conductive agent particles on the one hand, and is also affected by the partial properties of the conductive agent on the other hand.

When the slurry is not fully mixed, the morphology of the prepared electrode is as shown in Figure 1b. The active material and conductive agent particles are agglomerated, and the binder forms relatively large spheres. In this way, the active material cannot be completely firmly interlocked, and there is no good For the lithium ion channel, some conductive agents and binders did not play a role at all. Therefore, the performance of such electrodes is also poor. The slurry preparation should be thoroughly mixed microscopically, and the conductive agent should coat the active material to form an electrode structure as shown in Figure 1a. To prepare good electrodes, drying is also very important. Improper drying methods may lead to electrode morphology defects. But the most important thing is to prepare a good slurry. The preparation of fine particle slurry is laborious, difficult and takes a long time. Small particles such as micron particles and some nanoparticles are easy to form a slurry with a non-uniform structure, and delamination occurs during the preparation. clusters. Therefore, during the slurry preparation process, many parameters such as the size, morphology, and density of the active material and conductive agent particles need to be considered.

3. Basic morphology of micron and nanoparticle slurry

After dispersion, the powder forms a slurry in which clusters of particles are suspended in a solvent. These clusters have two scale structures: large aggregates (Agglomerate) and small aggregates (Aggregate, secondary particles), as shown in Figure 3. Large aggregates are composed of small aggregates, while small aggregates (secondary particles) are composed of primary particles. Small aggregates are combined into large aggregates by weak van der Waals forces, and secondary particles are formed by stronger forces (usually electrostatic forces) between primary particles. In the literature, large agglomerates are often referred to as soft agglomerates and small aggregates as hard agglomerates.

Table 1 shows that the particle size of commercially available active material powder is usually 2-10 microns, and the size of large agglomerates formed is 50-90 microns. During the preparation of the slurry, the powder is dispersed and the size is reduced. As mentioned earlier, the smaller the particle size, the better the electrochemical performance. Therefore, the purpose of the slurry preparation process is to:

a. Disperse active material and conductive agent particle agglomerates;

b. Finally, reduce the secondary particle size of the active material and conductive agent;

c. Form the most appropriate arrangement of active materials, conductive agents and binders;

d. Maintain the optimal structure and component stability of the slurry and prevent component segregation such as sedimentation and agglomeration.

The above goals are achieved through appropriate mixing processes, including appropriate feeding sequence, adding appropriate surfactants, appropriate slurry solvents, appropriate mixing equipment, etc. Lithium-ion battery slurry is in a non-equilibrium state. Active materials and conductive agents tend to agglomerate over time, and stability requires high molecular weight long-chain binders to maintain.

4. Stirring methods and equipment

The slurry preparation process for dispersed particle clusters is similar to the solid particle comminution process in that both require the application of the same type of stress. Agglomerates are combined by van der Waals forces, and secondary particles are combined by electrostatic forces. These forces are smaller than the forces between solid particle crystals, so the stress intensity required for stirring and dispersion is smaller, so the equipment and processes for solid particle crushing are complete. Sufficient for slurry preparation. Shear fluid flow and ultrasonic stirring are often used for slurry preparation. Dispersing equipment can be divided into two categories: the first type of mixing equipment, through solid grinding, shear force acts on the clusters to achieve material dispersion, such as stirring ball mill; the second type of equipment, through the liquid medium to implement shear force, such as fluid-based Mechanical shear mixer, disc ball mill, 3-roller ball mill, kneader, ultrasonic homogenizer. The schematic diagram of various types of mixer equipment is shown in Figure 4.

Kneaders and three-roller ball mills are often used for dispersing slurries with high solid content and high viscosity, and are basically not used in lithium-ion battery slurry preparation. Disc ball milling is also rarely used for battery slurry preparation. The shear stress intensity of these two types of mixers is related to the cluster size of the dispersion process. The shear force generated by fluid flow shear stirring is not related to the cluster size. The shear force generated by ball milling is inversely proportional to the cluster size. The relationship between various stirring shear forces and cluster size is shown in Figure 5.

Therefore, in addition to ultrasonic mixing, the dispersion efficiency of ball mill mixers is higher than other dispersing equipment. The relationship between the minimum cluster size of the alumina powder slurry dispersed by different stirring methods and the stirring and dispersion specific energy is shown in Figure 6. Publicly published patents and information provided by lithium-ion slurry mixing equipment manufacturers indicate that the industrial production process of lithium-ion battery slurry is mainly based on the hydrodynamic shear mixing method, and ball mill mixing is also used in battery slurry production.

4.1. Fluid shear mixing

This type of mixer mainly includes low-energy magnetic stirrer/dissolver, drum mixer, high-energy homogenizer, turbine mixer, static mixer, etc. Slurry preparation often uses the shear force generated by fluid mechanics, which is determined by the flow shear rate. , cluster cross-sectional area, hydrodynamic viscosity control. Slurry preparation generally involves two processes: the breakup of clusters and the reorganization of suspended aggregates.

Cluster fragmentation is a complex process, including three ways: abrasion, fracture, and fragmentation, as shown in Figure 7. Cluster fragmentation specifically relies on particle-particle interaction, slurry solvent-particle interaction, and most importantly, shear force, which in turn depends on the viscosity and motion speed of the solvent. Abrasion usually occurs at lower energies, where small fragments are gradually sheared away from large agglomerates. When the stirring energy is high, the clusters break into several parts. Fragmentation is a special variation of fracture in which a cluster breaks into a large number of small fragments simultaneously. Parameters relevant to the reorganization of agglomerates are particle-particle interaction, slurry solvent-particle interaction, and slurry solids content.

The balance between cluster reorganization and dispersion speed dominates the equilibrium size of clusters in the slurry. There is a critical size under which the cluster dispersion speed is very small. Existing literature reports that in the slurry prepared by hydrodynamic shear stirring under appropriate processing time and stirring energy, the size of the agglomerates cannot be less than 100 nanometers. Therefore, this kind of stirring can only occur when the primary particle size is not less than 100 nanometers. It is possible to completely disperse the powder to the primary particle size. Complete dispersion of nanoparticles is not possible. When using a Ramond high-speed mixer, dispersion of medium-sized clusters down to 40-60 nm is also possible. In addition, surfactants can also change the balance of agglomerate combination and dispersion, making the slurry cluster size smaller.

Finally, when preparing the slurry, the most important thing for the morphology of the lithium-ion battery pole piece is that the reassembled agglomerates of the slurry are often denser and have lower porosity than the original active material and conductive agent clusters. The electrode performance is closely related to the porosity of the coating. On the one hand, high-intensity stirring can more fully disperse the active material and conductive agent clusters, but on the other hand, high-intensity stirring will reduce the molecular weight of the binder and change its initial bonding. characteristics, making it unable to maintain the stability of the slurry structure. Therefore, the optimal selection of stirring intensity also requires balancing the interaction between the particle dispersion characteristics and the stability of the slurry structure.

The sizes of currently commercially available positive and negative electrode materials are generally at the micron level, or although the primary particles are at the nano level, the raw material powder itself is also composed of nano primary particles and micron level secondary particles. Therefore, shear dispersion and mixing based on fluid mechanics Technology is the most widely used. However, this dispersion technology cannot completely disperse the nanoparticles, and too high a strength will break the binder molecular chain. Therefore, whether to choose this mixing method in actual production depends on the fineness of the active material and conductive agent, as well as the binder. Determined by nature.

4.2. Ball mill stirring

Ball mill stirring is also often used in the preparation of lithium-ion battery slurry. Like the mixing method based on fluid mechanics, the dispersion ability of the ball mill process is determined by the speed balance between cluster breakage and agglomerate reorganization. This balance is related to the properties of the powder particles. It will also be changed by the addition of surfactant.

During the ball milling process, the powder particles undergo a large number of surface and volume changes, which may lead to mechanochemical transformations of the material (for example, carbon nanotubes may split and their aspect ratio and structure change). Moreover, reactions may occur between particles, between powder and dispersion medium (solvent and binder), and even between powder and grinding balls. The collision of grinding balls and local fluid high shear turbulence will also cause the molecular weight of the binder to change. of division.

All mechanochemical changes have been observed in the slurry ball milling process and have been studied accordingly. Research shows that changes in the active material and conductive agent caused by ball milling may be beneficial to the performance of lithium-ion battery pole pieces, but this can also damage the initial properties of the active material and conductive agent. Moreover, the ball milling process is usually complex and has contradictory characteristics. For example, ball milling can impair low-rate performance and enhance high-rate performance.

In short, compared with the hydrodynamic stirring process, the ball milling process provides smaller active material and conductive agent cluster sizes, while also destroying the active material and conductive agent particle morphology. When the morphology of the active material and conductive agent particles themselves is beneficial to the electrode performance, the ball milling process is not good.

4.3. Ultrasonic stirring

Currently, ultrasonic waves are used for microscopic stirring due to the instantaneous acoustic cavitation effect. This effect occurs under very high ultrasonic intensity. A large number of microscopic bubbles form and grow. When the bubble size reaches a certain critical value, the bubble growth rate increases rapidly, and then the bubble bursts instantly, forming a shock wave. This bubble burst is almost adiabatic. , so this will form local high temperature and high pressure.

Another process that occurs during ultrasonic stirring is the macroscopic flow of liquid. The concentration of cavitation bubbles gradually decreases along the axis with the generator as the center. The bubbles diffuse to the low concentration area and drive the liquid to flow with a flow speed as high as 2m/s. This fluid flow is sufficient to provide adequate mixing without the need for additional equipment.

Relatively low ultrasonic frequency is beneficial to slurry preparation. Usually under lower input energy conditions, ultrasonic stirring can achieve the same effect as stirring based on fluid mechanics technology (as shown in Figure 6). The combination of ultrasonic technology and ball milling, as well as ultrasonic stirring with the addition of surfactants, are particularly beneficial for slurry preparation.

The characteristics of ultrasonic mixing technology show that it is possible to achieve uniform dispersion of slurry particles under low solvent content conditions. This high solid content technology is also more energy-saving. In the case of lithium-ion battery slurries, high solid content is also advantageous because slurries with low solid content are more likely to settle, resulting in uneven distribution of active materials, conductive agents, and binders, which can also occur during the drying process of electrode plates. This results in uneven distribution of holes along the thickness direction of the pole piece. The slurry solids settle to the bottom and are concentrated near the current collector, which also limits the transport of lithium ions in this area. When the solid content is high, the drying time of the pole piece is short, undesired active material distribution changes, and the uneven distribution changes of the conductive agent and binder are also small, which can also increase the bonding strength of the pole piece coating.

One issue that needs attention is that chemical reactions may occur under the action of high-intensity ultrasonic waves. Especially in water-based slurries, whether ultrasonic waves will produce free radicals such as H, OH, O and HO2, when lithium-ion battery slurries are dispersed by ultrasonic waves, whether the molecular chains of polymer binders are broken, and whether the binders are in contact with active substances. React with conductive agent particles. Commonly used lithium-ion battery slurry binders such as sodium methylcellulose, polyacrylic acid and polyvinyl alcohol are prone to polymerization under the action of ultrasound, and the molecular chain length of the polymer binder is an important parameter for controlling the morphological characteristics of the electrode. They can maintain the bonding strength of the pole pieces and eliminate the influence of volume changes of active materials during the electrochemical process. Ultrasonic mixing technology has few applications and is still in the research and development stage.

Related suggestion：

13300-400mAh 3C

13300-400mAh 10C

18350-900mAh 3C

[Technology] Reasons why electric buses should not be equipped with ternary lithium batteries

New water treatment technology doubles lithium battery production