How Much Do You Know About Slurry Preparation?

Lithium-ion power batteries primarily consist of four components: the cathode, anode, electrolyte, and separator. Currently, the main types of lithium-ion power batteries on the market are lithium iron phosphate (LFP) batteries and ternary lithium batteries. Although these two battery types differ significantly in material composition and process parameters, their manufacturing processes are similar.

The coating machine applies electrode slurry evenly on metal foils. It then bake­s them to create positive­ and negative battery parts. Xiaowei is committed to being an enabler of slurry preparation by providing advanced coating machines for over 10 years. If you are interested and want more detailed quotations, please contact the sales team by email at **sales@xiaoweitop.com**.

Slurry preparation is the starting point of battery manufacturing and is also the most easily underestimated step; it can be said that slurry quality has a direct impact on coating yield. Production experience shows that fluctuations in slurry quality are one of the most common causes of decreased coating yield. In this article, I will focus on explaining the slurry preparation.

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What Are the Three Major Parts of Production Process for Lithium-Ion Batteries?

1. Electrode sheet manufacturing, which mainly includes four processes: slurry preparation, coating, calendering and slitting, and laser tab cutting — collectively known as the front-end process;

2. Cell manufacturing, which primarily includes four processes: winding, assembly, laser welding, and baking — collectively known as the middle section;

3. Capacity sorting and testing, which primarily includes processes such as electrolyte filling and sealing, cleaning, formation, and capacity sorting — collectively known as the back-end section.

The equipment required for the front-end section primarily includes slurry mixers, coating machines, roll-press slitting machines, and laser tab forming machines; the equipment required for the middle-end section primarily includes winding machines, assembly line equipment, laser welding machines, and drying ovens; and the equipment required for the back-end section primarily includes electrolyte filling machines, cleaning machines, formation and capacity testing equipment, and automated process warehousing and logistics equipment.

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1. Slurry Preparation Process

The formulation and coating of lithium-ion battery electrodes are the initial processes in battery manufacturing and also the most critical stages. Understanding the characteristics of electrode raw materials and the corresponding processes is of great significance for optimizing the design of battery formulation and coating parameters.

Slurry preparation, also known as homogenization, involves thoroughly mixing cathode or anode materials — such as lithium iron phosphate (LFP) cathode material or graphite anode material — with a dispersing solvent and necessary binders and conductive agents using mixing equipment to create a uniform slurry ready for the next process step.

The slurry formulation is straightforward, consisting of the following four components:

For the cathode, NCM or LFP serves as the active material, carbon black as the conductive agent, PVDF as the binder, and NMP as the solvent.

For the anode, graphite is used, with SBR + CMC as the binder and water as the solvent.

Once the slurry is prepared, it is coated onto aluminum foil (cathode) or copper foil (anode), dried, and calendered to form a cathode or anode sheet.

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2. Why is Slurry Quality so Difficult to Control?

There are three reasons:

First, slurry is neither a liquid nor a solid — it is something “in between.” It must be thin enough to be applied (like paint) yet thick enough to prevent sagging after application (like toothpaste). Striking this balance is extremely difficult.

Second, the particles in the slurry are extremely small. Active material particles are approximately 5–15 μm (one-tenth the width of a human hair), while conductive agent particles are only 30–50 nm (smaller than a virus). These nanoscale particles spontaneously clump together when dry. The slurry process essentially involves breaking up these clumps and uniformly dispersing them in the binder. How thoroughly must they be dispersed to be considered adequate? This cannot be determined by the naked eye and must be measured using specialized equipment.

Third, the slurry is extremely sensitive to moisture. When LiPF₆ in the electrolyte comes into contact with water, it produces HF — a highly corrosive acid that can damage the cathode material and destroy the SEI film. Therefore, every drop of water in the slurry poses a potential safety hazard. The NMP solvent used for the cathode readily absorbs moisture, so humidity must be strictly controlled on the production line.

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3. Basic Composition and Control Points of the Slurry Formulation

(1) Active Materials

• Control Points: PSD, BET, Moisture Content, Tap Density
• Actual Production Line Issues:

• Countermeasure: Test the PSD of each incoming batch and fine-tune the formulation’s solids content or binder dosage based on the measured values.

(2) Conductive Additives

• Control Points: Dispersion State, Addition Amount, Moisture Content
• Poor carbon black dispersion: The direct manifestation is failure to meet fineness specifications (>25 μm). Possible causes include insufficient linear speed of the dispersion disc (<10 m/s) or inadequate dispersion time.

Consequences: Incomplete conductive network → high electrode internal resistance → poor battery rate performance. This is particularly evident in LFP slurries — when the conductive agent is poorly dispersed, the DC internal resistance of LFP batteries can increase by 30–50%.

• Batch Variations in CNT Stock Solution: The degree of dispersion may vary between different batches of CNT stock solution (due to differences in CNT bundle size).

Countermeasure: Upon receipt of each batch of CNT stock solution, first measure its viscosity (at the same solid content, viscosity reflects the degree of dispersion). Excessively high viscosity indicates poor dispersion, and the product should be returned to the supplier.

• Moisture in conductive agents: Carbon black and CNTs have large specific surface areas (BET surface area of carbon black is approximately 60–80 m²/g, and that of CNTs is approximately 200–300 m²/g), making them highly prone to adsorbing moisture.

Consequence: Moisture is introduced into the slurry. Countermeasure: Measure the moisture content of conductive agents before use; carbon black must be <500 ppm, and CNTs must be <1,000 ppm.

(3) Binder (PVDF)

• Control Points: Molecular weight, dissolution state, solution viscosity, water content
• Incomplete dissolution of PVDF: Manifested as transparent gel particles in the coating solution.

Cause: Dissolution temperature is too low (dissolution is very slow at 30°C), insufficient time, or inadequate stirring.

Consequences: Gel particles form bumps during coating; these bumps are crushed during calendering, resulting in pinholes.

Countermeasures: Control the dissolution temperature at 45–55°C, the stirring speed at 500–800 rpm, and the dissolution time at ≥2 hours.

Acceptance Criteria: Place a drop of the adhesive solution on a glass plate for observation; it is considered qualified if it is completely transparent and free of particles.

(4) Solvent (NMP)

• Control Points: Moisture content, purity, quality of recycled NMP
• Deterioration in the quality of recycled NMP: In the NMP recycling system, after multiple cycles, oligomers and decomposition products accumulate in the NMP, causing its color to change from colorless to pale yellow.

Consequences: The viscosity of the slurry prepared with recycled NMP differs from that prepared with fresh NMP, and impurities in the recycled NMP may remain on the electrode sheets after drying.

Countermeasures: Establish quality standards for recycled NMP (water content < 500 ppm, purity ≥ 99.5%, color ≤ 50 APHA); if standards are not met, downgrade or discard the material.

• NMP Temperature Control: NMP heats up due to friction during stirring.

Consequences: When the NMP temperature exceeds 50°C, the viscosity of the PVDF solution undergoes irreversible changes (gelation may occur), NMP evaporation intensifies, and the solids content changes.

Countermeasure: Circulate cooling water through the mixing tank jacket to maintain the slurry temperature below 40°C.

• NMP Leaks and Environmental Safety: NMP is toxic (reproductive toxicity; its use is restricted in Europe), and the concentration of NMP in workshop air must be below 50 ppm.

Countermeasure: Install NMP concentration detectors and alarms in the mixing tank area; operators must wear gas masks and protective gloves.