π lead

The lithium battery electrode slurry is the beginning and the most important part of the battery. Electrode slurry involves a lot of content, including materials science, particle science, fluid mechanics, physics and other multidisciplinary content. Although the quality of slurry is only expressed by parameters such as viscosity, solid content, particle size, etc., there are many influencing factors, which is the reason why I hesitate to summarize. In fact, looking at the essence through the phenomenon and understanding the core that affects the properties of the slurry will definitely be able to prescribe the right medicine and solve the problem of the bad slurry.

The production and manufacture of lithium-ion batteries is a process that is closely linked by one process step. On the whole, the production of lithium batteries includes the pole piece manufacturing process, the battery assembly process, and the final liquid injection, precharge, formation, and aging processes. In the three-stage process, each process can be divided into several key processes, and each step will have a great impact on the final performance of the battery.

In the pole piece manufacturing process stage, it can be subdivided into five processes: slurry preparation, slurry coating, pole piece rolling, pole piece slitting, and pole piece drying. In the battery assembly process, according to the different specifications of the battery, it is roughly divided into winding, shelling, welding and other processes. In the final liquid injection stage, it also includes various processes such as liquid injection, exhaust, sealing, pre-filling, formation, and aging. The pole piece manufacturing process is the core content of the entire lithium battery manufacturing, which is related to the electrochemical performance of the battery, and the quality of the slurry is particularly important.

1. Basic theory of slurry

Lithium-ion battery electrode slurry is a kind of fluid, usually fluid can be divided into Newtonian fluid and non-Newtonian fluid. Among them, non-Newtonian fluids can be divided into dilatation-plastic fluids, time-dependent non-Newtonian fluids, pseudoplastic fluids and Bingham plastic fluids. Newtonian fluid is a low-viscosity fluid that is easily deformed after being subjected to force, and the shear stress is proportional to the deformation rate. A fluid in which the shear stress at any point is a linear function of the shear deformation rate. Many fluids in nature are Newtonian fluids. Most pure liquids such as water and alcohol, light oils, solutions of low molecular compounds, and gases flowing at low speeds are Newtonian fluids.

Non-Newtonian fluids refer to fluids that do not satisfy Newton's experimental law of viscosity, that is, fluids whose shear stress and shear strain rate are not linearly related. Non-Newtonian fluids exist widely in life, production and nature. Concentrated solutions and suspensions of high molecular polymers are generally non-Newtonian fluids. The vast majority of biological fluids belong to what is now defined as non-Newtonian fluids. Various body fluids such as blood, lymph fluid, cyst fluid, and "semi-fluids" such as cytoplasm are all non-Newtonian fluids.

Electrode slurry is composed of a variety of raw materials with different specific gravity and different particle sizes, and is mixed and dispersed in solid-liquid phase. The formed slurry belongs to non-Newtonian fluid. Lithium battery slurries can be divided into positive electrode slurries and negative electrode slurries. Due to different slurry systems (oily, water-based), their properties must vary widely. However, judging the properties of the slurry is nothing more than the following parameters:

1. The viscosity of the slurry

Viscosity is a measure of the viscosity of a fluid, a representation of the fluid flow force on its internal friction phenomenon. When a liquid is flowing, the property of internal friction between its molecules is called the viscosity of the liquid. The viscosity is expressed by viscosity, which is a resistance factor used to characterize the properties of the liquid. Viscosity is further divided into dynamic viscosity and conditional viscosity.

Viscosity is defined as a pair of parallel plates of area A, separated by dr, and filled with a liquid between the plates. A thrust force F is now applied to the upper plate, causing it to produce a speed change du. Because the viscosity of the liquid transmits this force layer by layer, each layer of liquid also moves accordingly, forming a velocity gradient du/dr, called the shear rate, represented by r'. F/A is called shear stress and is represented by τ. The shear rate and shear stress have the following relationship:

(F/A)=η(du/dr)

Newtonian fluid conforms to Newton's formula, viscosity is only related to temperature, independent of shear rate, and τ is proportional to D.

Non-Newtonian fluids do not conform to Newton's formula τ/D=f(D), and ηa represents the viscosity under a certain (τ/D), which is called apparent viscosity. In addition to temperature, the viscosity of non-Newtonian liquids is also related to shear rate, time, and changes in shear thinning or shear thickening.

2. Slurry properties

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

①Good liquidity. Fluidity can be observed by agitating the slurry and allowing it to flow down naturally. Continuity is good, and continuous intermittent indicates good liquidity. The fluidity is related to the solid content and viscosity of the slurry,

② leveling. The leveling of the slurry affects the flatness and uniformity of the coating.

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

3. Slurry dispersion basis

In the manufacture of electrodes for lithium-ion batteries, the positive electrode slurry is composed of binders, conductive agents, and positive electrode materials; the negative electrode slurry is composed of binders, graphite carbon powder, and the like. The preparation of positive and negative slurries includes a series of technological processes such as mutual mixing, dissolving and dispersing between liquid and liquid, liquid and solid materials, and these processes are accompanied by changes in temperature, viscosity, and environment. The mixing and dispersion process of lithium-ion battery slurry can be divided into macro-mixing process and micro-dispersion process, both of which are always accompanied by the whole process of lithium-ion battery slurry preparation. The preparation of the slurry generally goes through the following stages:

①Dry powder mix. The contact between particles is in the form of point, point surface, point line,

②Semi-dry mud kneading stage. At this stage, after the dry powder is mixed evenly, the binder liquid or solvent is added, and the raw materials are wetted and muddy. After the strong stirring of the mixer, the material is sheared and rubbed by mechanical force, and there will also be internal friction between the particles. Under each force, the raw material particles tend to be highly dispersed. This stage has a critical impact on the particle size and viscosity of the finished slurry.

③ dilution and dispersion stage. After the kneading is completed, the solvent is slowly added to adjust the viscosity and solid content of the slurry. 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 interaction force between the slurry and the container wall.

2. Analysis of parameters affecting slurry properties

The mixed slurry needs to have good stability, which is an important indicator to ensure the consistency of the battery in the battery production process. With the end of mixing and stirring, the slurry will settle, flocculate and coalesce, resulting in large particles, which will have a greater impact on subsequent processes such as coating. The main parameters that characterize the stability of slurry are fluidity, viscosity, solid content, density, etc.

1. Slurry viscosity

The electrode slurry needs to have a stable and proper viscosity, which has a crucial impact on the pole piece coating process. Too high or too low viscosity is not conducive to the coating of pole pieces. The slurry with high viscosity is not easy to precipitate and has better dispersibility, but too high viscosity is not conducive to the leveling effect and coating; too low viscosity It is also not good. When the viscosity is low, although the fluidity of the slurry is good, it is difficult to dry, which reduces the drying efficiency of the coating, and also causes problems such as cracking of the coating, agglomeration of slurry particles, and poor areal density consistency.

A problem that often occurs in our production process is the change in viscosity, and the "change" here can be divided into: instantaneous change and static change. Instantaneous change means that there is a drastic change in the middle of the viscosity test process, and static change means that the viscosity of the slurry changes after a period of time. The change in viscosity is high or low, or high and low. Generally speaking, the factors that affect the viscosity of the slurry mainly include the rotating speed of the stirring slurry, time control, batching sequence, ambient temperature and humidity, etc. There are many factors, how should we analyze and solve it when we encounter viscosity changes? The viscosity of the slurry is essentially determined by the binder. Hypothetically, there is no binder PVDF/CMC/SBR (as shown in Figure 2, 3), or the binder does not combine the active material well, will the solid active material and the conductive agent form a non-Newtonian fluid with a uniform coating? ? Won't! Therefore, to analyze and solve the reasons for the viscosity change of the slurry, we should start from the nature of the binder and the degree of dispersion of the slurry.

(1) Viscosity increases

Different slurry systems have different viscosity changes. At present, the mainstream slurry system is the positive electrode slurry PVDF/NMP oily system, and the negative electrode slurry is the graphite/CMC/SBR water-based system.

①The viscosity of the positive electrode slurry increases after being placed for a period of time. The first reason (placed for a short time) is that the stirring speed of the slurry is too fast, the binder is not fully dissolved, and the PVDF powder is fully dissolved after being placed for a period of time, and the viscosity increases. Generally speaking, PVDF needs at least 3 hours to be fully dissolved, no matter how fast the stirring speed is, this influencing factor cannot be changed. The second reason (standing for a long time) is that during the standing process of the slurry, the colloid changes from a sol state to a gel state. At this time, if it is homogenized at a slow speed, its viscosity can be recovered. The third reason is that a special structure is formed between the colloid, the active material and the conductive agent particles, which is irreversible and cannot be recovered after the viscosity of the slurry increases.

②The viscosity of the negative electrode slurry increases. The increase in the viscosity of the negative electrode slurry is mainly caused by the destruction of the molecular structure of the binder, and the viscosity of the slurry increases after the molecular chain is broken and oxidized. If the material is overdispersed, the particle size will be greatly reduced, which will also increase the viscosity of the slurry.

(2) Viscosity reduction

①The viscosity of the positive electrode slurry decreases. One of the reasons is that the properties of the binder colloid have changed. There are various reasons for the change, such as strong shearing force during slurry transmission, qualitative change caused by the absorption of moisture by the binder, structural change during the stirring process, and self-degradation. The second reason is that the uneven stirring and dispersion lead to large-scale settlement of solid substances in the slurry. The third reason is that the binder is subjected to strong shearing force and frictional force from the equipment and active substances during the stirring process, and its properties change under high temperature conditions, resulting in a decrease in viscosity.

②The viscosity of the negative electrode slurry decreases. One of the reasons is that there are impurities mixed in CMC. Most of the impurities in CMC are insoluble polymer resins. When CMC is miscible with calcium, magnesium, etc., its viscosity will be reduced. The second reason is that CMC is sodium hydroxymethyl cellulose, which is mainly a combination of C/O. The bond strength is very weak and it is easily damaged by shear force. When the stirring speed is too fast or the time is too long, the structure of CMC may be damaged. CMC plays a role in thickening and stabilizing the negative electrode slurry, and also plays an important role in the dispersion of raw materials. Once its structure is destroyed, it will inevitably cause the slurry to settle and the viscosity will decrease. The third reason is the destruction of the SBR binder. In actual production, CMC and SBR are usually selected to work together, and the roles of the two are different. SBR mainly acts as a binder, but it is prone to demulsification under long-term stirring, resulting in the failure of cohesiveness and the reduction of slurry viscosity.

(3) Special circumstances (jelly-like high and low in time)

 During the preparation of the positive electrode slurry, sometimes the slurry becomes "jelly". There are two main reasons for this: First, moisture. Considering the moisture absorption of the living material and the poor control of moisture during the stirring process, after the raw materials absorb moisture or the humidity of the stirring environment is high, PVDF absorbs moisture and becomes jelly-like. Second, the pH of the slurry or material. The higher the pH value, the stricter the control of moisture, especially the stirring of high nickel materials such as NCA and NCM811.

The viscosity of the slurry fluctuates up and down. One of the reasons may be that the slurry is not completely stabilized during the testing process, and the viscosity of the slurry is greatly affected by temperature. Especially after being dispersed at a high speed, there is a certain temperature gradient in the internal temperature of the slurry, and the viscosity of the slurry is different for different samples. The second reason is that the dispersibility of the slurry is poor. If the active material, binder and conductive agent are not well dispersed, the slurry has no good fluidity, and the natural slurry viscosity fluctuates.

2. Slurry particle size

After mixing the pulp, its particle size needs to be measured, and the method of particle size measurement usually adopts the scraper method. Particle size is an important parameter to characterize the quality of the slurry. The particle size has an important influence on the coating process, the rolling process and the battery performance. In theory, the smaller the slurry particle size, the better. When the particle size is too large, the stability of the slurry will be affected, and sedimentation and poor consistency of the slurry will occur. In the extrusion coating process, there will be material blockage and pitting after the pole piece is dried, which will cause the quality of the pole piece. In the subsequent rolling process, due to the uneven force on the poorly coated part, it is easy to cause the pole piece to break and local micro-cracks, which cause great harm to the cycle performance, rate performance and safety performance of the battery.

The main materials such as positive and negative active materials, binders, and conductive agents have different particle sizes and different densities. During the stirring process, there will be various contact methods such as mixing, extrusion, friction, and agglomeration. In the stages that the raw materials are gradually mixed, wetted by the solvent, the bulk material is broken and gradually stabilized, there will be uneven mixing of the material, poor dissolution of the adhesive, serious agglomeration of fine particles, and occurrence of adhesive properties. Changes, etc., will lead to the generation of large particles.

When we figure out the reason for the appearance of particles, we must prescribe the right medicine to solve these problems. Regarding the mixing of dry powder, I personally think that the mixer speed has little effect on the degree of dry powder mixing, but both require sufficient time to ensure the mixing of dry powder. Now some manufacturers choose powdered adhesives and some choose liquid-dissolved adhesives. Two different adhesives determine the difference in the process. Using powdered adhesives takes longer to dissolve, otherwise Swelling, springback, viscosity changes, etc. will occur in the later stage. The agglomeration between fine particles is unavoidable, but we must ensure that there is enough friction between the materials, which can promote the extrusion and crushing of the agglomerated particles, which is conducive to mixing. This requires us to control the solid content of the slurry in different stages. Too low solid content will affect the friction and dispersion between particles.

3. Solid content of slurry

The solid content of the slurry is closely related to the stability of the slurry. The same process and formula, the higher the solid content of the slurry, the higher the viscosity, and vice versa. Within a certain range, the higher the viscosity, the higher the slurry stability. When we design a battery, we generally push the thickness of the winding core from the battery capacity to the design of the pole piece. Then the pole piece design is only related to parameters such as areal density, active material density, and thickness. The parameters of the pole piece are the result of adjusting it by the coater and roller press, and the solid content of the slurry has no direct effect on it. So, is it irrelevant whether the solid content of the slurry is high or low?

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

(2) The solid content has certain requirements for the equipment. High-solids slurries have higher losses on equipment, because the higher the solids content, the more severe the equipment wear.

(3) The slurry with high solid content has higher stability. Some slurry stability test results (as shown in the figure below) show that the TSI (instability index) of conventional stirring is 1.05 higher than the TSI value of high-viscosity stirring process of 0.75, so The slurry stability obtained by the high viscosity stirring process is better than that obtained by the conventional stirring process. However, the high solid content of the slurry can also affect its flowability, which is very challenging for the equipment and technical personnel of the coating process.

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

4. Slurry density

The density of the slurry is an important parameter to reflect the consistency of the slurry, and the dispersion effect of the slurry can be verified by testing the density of the slurry at different positions. I won't go into details here. Through the above summary, I believe that everyone has prepared a good electrode paste.