Analysis of the properties and key influencing factors of lithium battery paste

The production of lithium-ion batteries is a closely linked process of process steps. Generally speaking, the production of lithium batteries includes pole piece manufacturing process, battery assembly process and the final liquid injection, pre-charge, formation, and aging process. 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, it can be subdivided into five processes: slurry preparation, slurry coating, pole piece rolling, pole piece cutting, and pole piece drying. In the battery assembly process, according to the different battery specifications, it can be roughly divided into winding, casing, welding and other processes. In the final liquid injection stage, various processes such as liquid injection, exhaust, sealing, pre-filling, formation, and aging are included. The pole piece manufacturing process is the core content of the entire lithium battery manufacturing, which is related to the quality of 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. Generally, fluids can be divided into Newtonian fluids and non-Newtonian fluids. Among them, non-Newtonian fluids can be divided into swelling plastic fluids, time-dependent non-Newtonian fluids, pseudoplastic fluids and Bingham plastic fluids. Newtonian fluid is a low-viscosity fluid that is extremely easy to deform after being stressed, and the shear stress is proportional to the rate of deformation. A fluid whose shear stress at any point has a linear function relationship with the shear deformation rate. Many fluids in nature are Newtonian fluids. Most pure liquids such as water and alcohol, light oils, low-molecular compound solutions, and low-speed flowing gases are all Newtonian fluids.

Non-Newtonian fluids refer to fluids that do not satisfy Newton’s experimental laws of viscosity, that is, fluids whose shear stress and shear strain rate are not linear. Non-Newtonian fluids are widely present in life, production and nature. Concentrated solutions and suspensions of high molecular polymers are generally non-Newtonian fluids. Most biological fluids belong to the non-Newtonian fluids currently defined. Human body fluids such as blood, lymph, cyst fluid, and "semi-fluid" like cytoplasm are all non-Newtonian fluids.

Electrode slurry is composed of a variety of raw materials with different specific gravities and different particle sizes, and is mixed and dispersed in a solid-liquid phase. The resulting slurry is a non-Newtonian fluid. Lithium battery slurry can be divided into two types: positive electrode slurry and negative electrode slurry. Due to the different slurry systems (oily and water-based), their properties will vary greatly. 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 fluid viscosity and an expression of fluid flow force on its internal friction phenomenon. When a liquid flows, the property of internal friction between its molecules is called the viscosity of the liquid. The viscosity is expressed by viscosity, which is used to characterize the resistance factor related to the properties of the liquid. Viscosity is divided into dynamic viscosity and conditional viscosity.

Viscosity is defined as a pair of parallel plates with an area of A and a distance of dr. The plates are filled with a certain liquid. Now a thrust force F is applied to the upper plate to produce a speed 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 the shear rate, which is represented by r'. F/A is called the shear stress and is represented by τ. The relationship between shear rate and shear stress is as follows:

(F/A)=η(du/dr)

Newtonian fluid conforms to Newton's formula. The viscosity is only related to temperature and has nothing to do with shear rate. τ is proportional to D.

Non-Newtonian fluids do not conform to the Newtonian 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

The slurry is a non-Newtonian fluid, 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:

① Good liquidity. The fluidity can be observed by stirring the slurry to let it flow down naturally. Good continuity, continuous intermittent, shows good liquidity. The fluidity is related to the solid content and viscosity of the slurry,

②Leveling. 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 the flow, and its quality affects the quality of the pole piece.

3. The basis of slurry dispersion

In the manufacture of electrodes for lithium-ion batteries, the positive electrode slurry is composed of binder, conductive agent, and positive electrode materials; the negative electrode slurry is composed of binder, graphite carbon powder, etc. The preparation of positive and negative slurry includes a series of technological processes such as the mutual mixing, dissolution and dispersion of liquid and liquid, liquid and solid materials, and this process is accompanied by changes in temperature, viscosity, and environment. The mixing and dispersion process of lithium-ion battery slurry can be divided into a macro-mixing process and a micro-dispersion process. These two processes will always be accompanied by the entire process of preparing the lithium-ion battery slurry. The preparation of slurry generally goes through the following stages:

① Mix dry powder. The particles are in contact with each other in the form of dots, dots, and dots,

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

③Dilution and dispersion stage. After the kneading is completed, the solvent is slowly added to adjust the slurry viscosity and solid content. At this stage, dispersion and reunion 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 the properties of slurry

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

1. The viscosity of the slurry

The electrode slurry needs to have a stable and appropriate viscosity, which has a vital influence on the electrode piece coating process. If the viscosity is too high or too low, it is not conducive to the coating of the pole piece. The high viscosity slurry is not easy to precipitate and the dispersibility will be better, but the too high viscosity is not conducive to the leveling effect and the coating; the viscosity is too low It is also not good. Although the slurry has good fluidity when the viscosity is low, it is difficult to dry, which reduces the drying efficiency of coating, and problems such as coating cracking, slurry particle agglomeration, and poor surface density consistency will occur.

The problem that often arises in our production process is that the viscosity changes, and the "change" here can be divided into instantaneous changes and static changes. Instantaneous change refers to a drastic change in the middle of the viscosity test process, and static change refers to a change in the viscosity of the slurry after a period of time. The change in viscosity is high or low, or sometimes high and sometimes low. Generally speaking, the factors that affect the viscosity of the slurry mainly include the speed of stirring the slurry, time control, batching sequence, environmental 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 affected by the binder. Suppose, if there is no adhesive PVDF/CMC/SBR (as shown in Figures 2 and 3), or the adhesive does not combine the active materials well, will the solid active materials and the conductive agent form a non-Newtonian fluid with uniform coating? ? will not! Therefore, to analyze and solve the reasons for the change of slurry viscosity, we must start from the nature of the binder and the degree of slurry dispersion.

Figure 2. PVDF molecular arrangement structure

Figure 3. CMC molecular structure

(1) Increased viscosity

Different slurry systems have different viscosity changes. The current mainstream slurry system is the positive electrode slurry PVDF/NMP oily system, and the negative electrode slurry is the graphite/CMC/SBR aqueous system.

① The viscosity of the positive electrode slurry increases after being left for a period of time. One of the reasons (short-term storage) is that the slurry stirring speed is too fast and the binder is not fully dissolved. After a period of time, the PVDF powder is fully dissolved 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 "hasty is not fast". The second reason (long-term storage) is that the colloid changes from a sol state to a gel state when the slurry is standing still. If it is homogenized at a slow speed, its viscosity can be restored. The third reason is that a special structure is formed between the colloid, the living substance, and the conductive agent particles. This state is irreversible and cannot be recovered after the slurry viscosity 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. The viscosity of the slurry increases after the molecular chain is broken and oxidized. If the material is over-dispersed, the particle size will be greatly reduced, which will also increase the viscosity of the slurry.

(2) Reduced viscosity

① The viscosity of the positive electrode slurry decreases. One of the reasons is that the properties of the adhesive colloid have changed. There are many reasons for the changes, such as strong shearing force during the slurry transfer process, qualitative change due to moisture absorption by the binder, structural changes during the stirring process, and degradation of itself. The second reason is that the uneven mixing and dispersion causes the solid matter in the slurry to settle in a large area. The third reason is that the adhesive is subjected to strong shear and friction forces from equipment and living materials 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 CMC contains impurities. Most of the impurities in CMC are poorly soluble 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 is very weak and easily damaged by shearing force. When the stirring speed is too fast or the time is too long, the structure of CMC may be destroyed. CMC plays a role in thickening and stabilizing the negative electrode slurry, and at the same time it plays an important role in the dispersion of raw materials. Once its structure is damaged, it will inevitably cause the slurry to settle and reduce its viscosity. The third reason is the destruction of SBR adhesive. 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 failure of cohesion and reduction of slurry viscosity.

(3) Special circumstances (jelly shape is high and sometimes low)

       During the preparation of the positive electrode slurry, sometimes the slurry becomes "jelly". There are two main reasons for this situation: First, moisture. Consider that the moisture absorption of the living material, poor moisture control during the mixing process, and the high humidity of the raw material after the raw material absorbs moisture or the mixing environment will cause the PVDF to absorb moisture and become jelly. Second, the pH value 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 high and low. One of the reasons may be that the slurry is not completely stabilized during the testing process, and the slurry viscosity is greatly affected by temperature. Especially after being dispersed at a high speed, the internal temperature of the slurry has a certain temperature gradient, and the different viscosities of the samples are not the same. The second reason is that the dispersion of the slurry is poor, the active material, binder, and conductive agent are not well dispersed, the slurry does not have good fluidity, and the viscosity of the natural slurry fluctuates high and low.

2. Size of slurry

After mixing, the particle size needs to be measured. The method of particle size measurement usually adopts the scraper method. The 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. Theoretically, the smaller the slurry particle size, the better. When the particle size is too large, the stability of the slurry will be affected, resulting in sedimentation and poor consistency of the slurry. In the extrusion coating process, there will be material blocking, pitting after drying of the pole piece, which causes the quality of the pole piece. In the subsequent rolling process, the uneven force on the poor coating can easily cause the pole piece to break and local microcracks, 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, adhesives, conductive agents, etc. have different particle sizes and different densities, and various contact methods such as mixing, squeezing, friction, and agglomeration will occur during the stirring process. In the stages of raw materials being gradually mixed, wetted by solvents, large pieces of materials broken and gradually stabilizing, there will be uneven mixing of materials, poor dissolution of the adhesive, severe agglomeration of fine particles, and occurrence of adhesive properties. Changes, etc., will lead to the production of large particles.

When we understand the cause of the particles, we must prescribe the right medicine to solve these problems. Regarding the dry powder mixing of materials, I personally feel that the speed of the mixer has little effect on the degree of dry powder mixing, but the two need enough time to ensure the dry powder mixing. Now some manufacturers choose powdery adhesives and some choose liquid-dissolved adhesives. The two different adhesives determine the difference in the process. The use of powdered adhesives takes longer to dissolve, otherwise Swelling, rebound, viscosity change, etc. will occur in the later stage. Agglomeration between fine particles is inevitable, but we must ensure that there is enough friction between the materials to 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 at different stages. Too low solid content will affect the friction and dispersion between particles.

3. The solid content of the slurry

The solid content of the slurry is closely related to the stability of the slurry. With the same process and formula, the higher the solid content of the slurry, the greater the viscosity, and vice versa. Within a certain range, the higher the viscosity, the higher the slurry stability. When we design batteries, we generally reverse the thickness of the 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 adjusted by the coating machine and the roller press, and the solid content of the slurry has no direct influence on it. So, is the solid content of the slurry irrelevant?

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

(2) The solid content has certain requirements for the equipment. High solid content slurry has higher loss of equipment, because the higher the solid content, the more serious the equipment wear.

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

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

4. Paste density

The density of the slurry is an important parameter for the consistency of the reaction 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, and through the above summary, I believe everyone can prepare a good electrode slurry.