The production and manufacture of lithium ion battery is a process closely linked by one technological step. As a whole, the production of lithium battery includes the electrode manufacturing process, battery assembly process and the final liquid injection, precharge, formation and aging process. In these three stages of the process, each process can be divided into several key processes, each step will have a great impact on the final performance of the battery.

In the process stage, it can be subdivided into five processes: paste preparation, paste coating, roller pressing, cutting and drying. In the battery assembly process, and according to the different battery specifications and models, roughly divided into winding, shell, welding and other processes. In the final stage of liquid injection, including liquid injection, exhaust, sealing, prefilling, formation, aging and other processes. The electrode manufacturing process is the core content of the whole lithium battery manufacturing, which is related to the electrochemical performance of the battery, and the quality of slurry is particularly important.

One, the basic theory of slurry

Lithium ion battery electrode slurry is a kind of fluid, usually can be divided into Newtonian fluid and non-Newtonian fluid. Among them, non-Newtonian fluid can be divided into dilatancy plastic fluid, time dependent non-Newtonian fluid, pseudoplastic fluid and bingham plastic fluid. Newtonian fluid is a low viscosity fluid which is easy to deform under stress and the shear stress is proportional to the deformation rate. Fluid in which the shear stress at any point is a linear function of the rate of shear deformation. Many fluids in nature are Newtonian fluids. Most pure liquids such as water and alcohol, light oil, low molecular compound solutions and low-velocity flowing gases are Newtonian fluids.

Non-newtonian fluid refers to the fluid that does not satisfy Newton’s experimental law of viscosity, that is, the relationship between shear stress and shear strain rate is not linear. Non-newtonian fluids are widely found in life, production and nature. Polymers concentrated solutions and suspensions of polymers are generally non-Newtonian fluids. Most biological fluids are now defined as non-Newtonian fluids. Non-newtonian fluids include blood, lymph, and cystic fluids, as well as “semi-fluids” such as cytoplasm.

Electrode slurry is composed of a variety of raw materials with different specific gravity and particle size, and is mixed and dispersed in solid-liquid phase. The slurry formed is a non-Newtonian fluid. Lithium battery slurry can be divided into positive slurry and negative slurry two kinds, due to the slurry system (oily, water) different, its nature will vary. However, the following parameters can be used to determine the properties of slurry:

1. Viscosity of slurry

Viscosity is a measure of fluid viscosity and an expression of fluid force on its internal friction phenomenon. When liquid flows, it produces internal friction between its molecules, which is called viscosity of liquid. Viscosity is expressed by viscosity, which is used to characterize the resistance factor related to liquid properties. Viscosity is divided into dynamic viscosity and conditional viscosity.

Viscosity is defined as A pair of parallel plates, area A, Dr Apart, filled with A liquid. Now apply a thrust F to the upper plate to produce a velocity change DU. Because the viscosity of the liquid transfers this force layer by layer, each layer of liquid also moves accordingly, forming a velocity gradient du/ Dr, called shear rate, represented by R ‘. F/A is called shear stress, expressed as τ. The relationship between shear rate and shear stress is as follows:

(F/A) = eta (du/Dr)

Newtonian fluid conforms to Newton’s formula, viscosity is only related to temperature, not shear rate, τ is proportional to D.

Non-newtonian fluids do not conform to Newton’s formula τ/D=f(D). The viscosity at a given τ/D is ηa, which is called apparent viscosity. The viscosity of non-Newtonian liquids depends not only on temperature, but also on shear rate, time, and shear thinning or shear thickening.

2. Slurry properties

Slurry is a non-Newtonian fluid, which is a solid-liquid mixture. In order to meet the requirements of subsequent coating process, slurry needs to have the following three characteristics:

① Good liquidity. Fluidity can be observed by agitating the slurry and allowing it to flow naturally. Good continuity, continuous off and off means good liquidity. Fluidity is related to the solid content and viscosity of slurry,

(2) leveling. The smoothness of the slurry affects the flatness and evenness of the coating.

③ Rheology. Rheology refers to the deformation characteristics of slurry in flow, and its properties affect the quality of pole sheet.

3. Slurry dispersion foundation

Lithium ion battery electrode manufacturing, cathode paste by adhesive, conductive agent, cathode material composition; The negative paste is composed of adhesive, graphite powder and so on. The preparation of positive and negative slurry includes a series of technological processes, such as mixing, dissolving and dispersing between liquid and liquid, liquid and solid materials, and is accompanied by changes in temperature, viscosity and environment in this process. The mixing and dispersion process of lithium ion battery slurry can be divided into macro mixing process and micro dispersion process, which are always accompanied by the whole process of lithium ion battery slurry preparation. The preparation of slurry generally goes through the following stages:

① Dry powder mixing. Particles contact each other in the form of dots, dots, planes, and lines,

② Semi-dry mud kneading stage. At this stage, after the dry powder is mixed evenly, the binder liquid or solvent is added, and the raw material is wet and muddy. After the strong stirring of the mixer, the material is subjected to the shear and friction of mechanical force, and there will be internal friction between the particles. Under each force, the raw material particles tend to be highly dispersed. This stage has a very important effect on the size and viscosity of the finished slurry.

③ Dilution and dispersion stage. After kneading, solvent was added slowly to adjust slurry viscosity and solid content. At this stage, dispersion and agglomeration coexist, and finally reach stability. At this stage, the dispersion of materials is mainly affected by mechanical force, frictional resistance between powder and liquid, high-speed dispersion shear force, and the impact interaction between slurry and container wall.

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Analysis of parameters affecting slurry properties

It is an important index to ensure the consistency of the battery in the process of battery production that the slurry should have good stability. With the end of the combined slurry, mixing stops, slurry will appear settlement, flocculation and other phenomena, resulting in large particles, which will have a greater impact on the subsequent coating and other processes. The main parameters of slurry stability are fluidity, viscosity, solid content and density.

1. Viscosity of slurry

The stable and appropriate viscosity of electrode paste is very important to the coating process of electrode sheet. The viscosity is too high or too low is not conducive to polar piece coating, the slurry with high viscosity is not easy to precipitate and the dispersion will be better, but the high viscosity is not conducive to leveling effect, is not conducive to coating; Viscosity too low is not good, viscosity is low, although the slurry flow is good, but it is difficult to dry, reduce the drying efficiency of coating, coating cracking, slurry particle agglomeration, surface density consistency is not good.

The problem that often occurs in our production process is the change of viscosity, and the “change” here can be divided into instantaneous change and static change. Transient change refers to the drastic change in the viscosity testing process, and static change refers to the viscosity change after a period of time. The viscosity varies from high to low, from high to low. Generally speaking, the main factors affecting slurry viscosity are the speed of mixing slurry, time control, ingredients order, environmental temperature and humidity, etc. There are many factors, when we meet viscosity change should be how to analyze and solve it? The viscosity of slurry is essentially determined by the binder. Imagine that without the binder PVDF/CMC/SBR (FIG. 2, 3), or if the binder does not combine the live matter well, will the solid live matter and the conductive agent form a non-Newtonian fluid with uniform coating? Don’t! Therefore, to analyze and solve the reason of slurry viscosity change, we should start from the nature of binder and slurry dispersion degree.

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FIG. 2. Molecular structure of PVDF

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Figure 3. Molecular formula of CMC

(1) the viscosity increases

Different slurry systems have different viscosity change rules. At present, the mainstream slurry system is positive slurry PVDF/NMP oily system, and negative slurry is graphite /CMC/SBR aqueous system.

① The viscosity of positive slurry increases after a period of time. One reason (short time placement) is that the slurry mixing speed is too fast, the binder is not fully dissolved, and the PVDF powder is fully dissolved after a period of time, and the viscosity increases. Generally speaking, PVDF needs at least 3 hours to fully dissolve, no matter how fast the stirring speed can not change this influencing factor, the so-called “haste makes waste”. The second reason (long time) is that in the process of slurry standing, the colloid changes from the sol state to the gel state. At this time, if it is homogenized at a slow speed, its viscosity can be restored. The third reason is that a special structure is formed between colloid and living material and conductive agent particles. This state is irreversible, and the slurry viscosity cannot be restored after increasing.

The viscosity of the negative slurry increases. The viscosity of the negative slurry is mainly caused by the destruction of the molecular structure of the binder, and the viscosity of the slurry is increased after the oxidation of the molecular chain fracture. If the material is excessively dispersed, the particle size will be greatly reduced, and the viscosity of the slurry will also be increased.

(2) the viscosity is reduced

① The viscosity of positive slurry decreases. One of the reasons, adhesive colloid changes in character. There are many reasons for the change, such as strong shear force during slurry transfer, qualitative change of water absorption by binder, structural change and degradation of itself in the process of mixing. The second reason is that the uneven stirring and dispersion leads to the large area settlement of solid materials in the slurry. The third reason is that in the process of stirring, the adhesive is subjected to strong shear force and friction of equipment and living material, and changes in properties at high temperature, resulting in a decrease in viscosity.

The viscosity of the negative slurry decreases. One of the reasons is that there are impurities mixed in CMC. Most of the impurities in CMC are insoluble polymer resin. When CMC is miscible with calcium and magnesium, its viscosity will be reduced. The second reason is sodium hydroxymethyl cellulose, which is mainly the combination of C/O. The bond strength is very weak and easily destroyed by shear force. When the stirring speed is too fast or the stirring time is too long, the structure of CMC may be destroyed. CMC plays a thickening and stabilizing role in the negative slurry, and plays an important role in the dispersion of raw materials. Once its structure is destroyed, it will inevitably cause slurry settlement and viscosity reduction. The third reason is the destruction of SBR binder. In the actual production, CMC and SBR are usually selected to work together, and their roles are different. SBR mainly plays the role of binder, but it is prone to demulsification under long-term stirring, resulting in bond failure and viscosity reduction of slurry.

(3) Special circumstances (jelly-shaped timely high and low)

In the process of preparing positive paste, the paste sometimes turns into jelly. There are two main reasons for this: first, water. Considering that the moisture absorption of living substances and the moisture control in the mixing process are not good, the moisture absorption of raw materials or the humidity of the mixing environment is high, resulting in the absorption of water by PVDF into jelly. Second, the pH value of slurry or material. The higher the pH value is, the control of moisture is more strict, especially the mixing of high nickel materials such as NCA and NCM811.

The viscosity of slurry fluctuates, one of the reasons may be that the slurry is not completely stabilized in the testing process, and the viscosity of slurry is greatly affected by temperature. Especially after being dispersed at high speed, there is a certain temperature gradient in the internal temperature of the slurry, and the viscosity of different samples is not the same. The second reason is poor dispersion of slurry, live material, binder, conductive agent is not good dispersion, slurry is not good fluidity, natural slurry viscosity is high or low.

2. Size of slurry

After the slurry is combined, it is necessary to measure its particle size, and the method of particle size measurement is usually scraper method. Particle size is an important parameter to characterize the slurry quality. Particle size has an important influence on the coating process, rolling process and battery performance. Theoretically, the smaller the slurry size is, the better. When the particle size is too large, the stability of slurry will be affected, sedimentation, slurry consistency is poor. In the process of extrusion coating, there will be blocking material, pole dry after the pitting, resulting in pole quality problems. In the following rolling process, due to the uneven stress in the bad coating area, it is easy to cause pole breakage and local micro-cracks, which will cause great harm to the cycling performance, ratio performance and safety performance of the battery.

Positive and negative active substances, adhesives, conductive agents and other main materials have different particle sizes and densities. In the process of stirring, there will be mixing, extrusion, friction, agglomeration and other different contact modes. In the stages of raw materials being gradually mixed, wetted by solvent, large material breaking and gradually tending to stability, there will be uneven material mixing, poor adhesive dissolution, serious agglomeration of fine particles, changes in adhesive properties and other conditions, which will lead to the generation of large particles.

Once we understand what causes the particles to appear, we need to address these problems with appropriate drugs. As for the dry powder mixing of materials, I personally think that the mixer speed has little influence on the degree of dry powder mixing, but they need enough time to ensure the uniformity of dry powder mixing. Now some manufacturers choose powdery adhesive and some choose liquid solution good adhesive, two different adhesives determine the different process, the use of powdery adhesive needs a longer time to dissolve, otherwise in the late will appear swelling, rebound, viscosity change, etc. The agglomeration between fine particles is inevitable, but we should ensure that there is enough friction between materials to enable the agglomeration particles to appear extrusion, crushing, conducive to mixing. This requires us to control the solid content in different stages of slurry, too low solid content will affect the friction dispersion between particles.

3. Solid content of slurry

The solid content of slurry is closely related to the stability of slurry, the same process and formula, the higher the solid content of slurry, the greater the viscosity, and vice versa. In a certain range, the higher the viscosity, the higher the stability of slurry. When we design the battery, we generally deduce the thickness of the core-core from the capacity of the battery to the design of the electrode sheet, so the design of the electrode sheet is only related to the surface density, live matter density, thickness and other parameters. The parameters of electrode sheet are adjusted by coater and roller press, and the solid content of slurry has no direct influence on it. So, does the level of solid content of slurry matter little?

(1) Solid content has a certain influence on improving the stirring efficiency and coating efficiency. The higher the solid content, the shorter the stirring time, the less solvent consumption, the higher the coating drying efficiency, saving time.

(2) The solid content has certain requirements for equipment. Slurry with high solid content has a higher loss to equipment, because the higher the solid content, the more serious the equipment wear.

(3) The slurry with high solid content is more stable. The stability test results of some slurry (as shown in the figure below) show that the TSI(instability index) of 1.05 in conventional stirring is higher than that of 0.75 in high-viscosity stirring process, so the slurry stability obtained by high-viscosity stirring process is better than that obtained by conventional stirring process. But the slurry with high solid content will also affect its fluidity, which is very challenging for the equipment and technicians of the coating process.

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(4) The slurry with high solid content can reduce the thickness between the coatings and reduce the internal resistance of the battery.

4. Pulp density

The density of size is an important parameter to reflect the consistency of size. The dispersion effect of size can be verified by testing the density of size at different positions. In this will not be repeated, through the above summary, I believe that we prepare a good electrode paste.