The stirring of lithium battery pack slurry is a key step in preparing high-performance batteries, directly affecting the coating quality of the electrode sheets, the internal resistance of the battery and the cycle life. The following is a technical analysis from three dimensions: stirring mechanism, optimization of process parameters, common problems and solutions:

First, the stirring mechanism and core objective

Dispersion and homogenization

The agglomeration between the particles of active materials (such as NCM/LFP at the positive electrode and graphite at the negative electrode) is disrupted by mechanical shear force, allowing them to be uniformly dispersed in the three-dimensional network formed by conductive agents (such as SP, CNT) and binders (such as PVDF, CMC).

If the stirring is not thorough, the graphite particles may agglomerate locally, resulting in uneven resistance of the negative electrode sheet and causing local lithium plating.

Viscosity control

The viscosity of the slurry needs to be controlled within a reasonable range (for example, the viscosity of the positive electrode slurry is usually 3000-8000 mpa ·s). If it is too high, it will cause uneven thickness of the electrode sheet during coating; if it is too low, it is easy to cause particle sedimentation.

Consistency of solid content

The fluctuation of solid content (the mass proportion of active substances, conductive agents and binders) should be less than ±0.5%; otherwise, it will affect the density of the electrode surface and the consistency of the battery.

Second, optimization strategies for process parameters

Stirring speed and time

Low-speed premixing stage: Stir at 50-100 RPM for 5-10 minutes to initially wet the powder and prevent dry powder splashing.

High-speed dispersion stage: Raise the speed to 1000-2000 RPM and stir for 1-2 hours to break the particle agglomerations through high shear force.

Deaeration stage: Reduce the speed to 200-500 RPM and stir for 30 minutes. Use a vacuum environment (vacuum degree ≤-0.09MPa) to remove bubbles.

Key point: The rotational speed needs to match the type of stirring paddle (for example, the double planetary paddle is suitable for high-viscosity systems, while the dispersion disc paddle is suitable for low-viscosity systems).

Solvent addition strategy

Stepwise addition method: First, add a portion of the solvent (such as NMP or deionized water) to moisten the powder, and then gradually add it to the target viscosity, which can reduce particle agglomeration.

Temperature control: The solvent temperature should be consistent with the stirring environment temperature (usually 25±3℃) to avoid viscosity fluctuations caused by temperature differences.

Binder dissolution and activation

For binders such as PVDF, they need to be pre-dissolved with a solvent before stirring (for example, it takes 1-2 hours for NMP to dissolve PVDF), and then the powder can be added after forming a uniform adhesive solution.

For water-based binders such as CMC, they need to be fully activated through high-speed stirring (≥1500rpm) to avoid insufficient peel strength of the electrode sheet.

Third, Common Problems and Solutions

Slurry sedimentation and stratification

Reasons: Low solid content, insufficient viscosity or too short stirring time.

Solution: Increase the solid content to the design value (for example, the solid content of the cathode slurry is usually 60-70%), and extend the high-speed dispersion time until the particle size D50 is ≤5μm.

Particle agglomeration and large particle residue

Reason: Insufficient shear force of the stirring paddle or inadequate pre-dispersion of the powder.

Solution: Replace the high-shear force stirring paddle (such as a serrated dispersion disc), or add a powder pre-mixing step (such as ball milling pre-dispersion).

Abnormal viscosity of the slurry

Excessively high viscosity: Insufficient solvent addition or excessive binder. Solvent needs to be replenished and the mixture stirred again.

Low viscosity: Incomplete dissolution of the binder or excessive addition of powder. The dissolution time needs to be extended or additional binder should be added.

Residual bubbles

Reason: The stirring speed is too high or the vacuum degassing is insufficient.

Solution: Reduce the stirring speed to below 800rpm and extend the vacuum degassing time to more than 1 hour.

Fourth, quality inspection of the slurry after stirring

Viscosity test: Use a rotational viscometer to measure at 25℃ to ensure that the viscosity meets the process requirements.

Particle size distribution: The D50 and D90 of particles in the slurry are detected by a laser particle size analyzer to ensure uniform particle size distribution.

Solid content detection: The solid content is measured by the drying method (drying at 105℃ until constant weight), and the fluctuation should be ≤±0.5%.

Fifth, cases of process optimization

Optimization of the cathode slurry stirring process in a certain enterprise

Original process: Single planetary paddle stirring, rotational speed 800rpm, stirring time 3 hours. The viscosity of the slurry fluctuates greatly, and the thickness of the coated electrode sheet is uneven.

After optimization: A combination of dual planetary paddles and dispersion discs is adopted, and the stirring is carried out in three stages (low-speed pre-mixing for 5 minutes → high-speed dispersion for 1.5 hours → degassing for 30 minutes). The viscosity stability of the slurry is increased to ±5%, and the consistency of the coated electrode sheet thickness is improved by 90%.