Application of Ultrasonic Equipment in Lithium-ion Battery Separator Defoaming

Application of Ultrasonic Equipment in Lithium-ion Battery Separator Defoaming

In lithium-ion battery separator production (such as wet biaxial stretching processes), polymer solutions (such as PP/PE slurry) are prone to generating microbubbles due to stirring, transportation, or formulation characteristics. If these microbubbles remain in the separator, they can lead to uneven porosity, decreased mechanical properties, and even affect battery safety. Ultrasonic defoaming, based on the acoustic cavitation effect, can efficiently remove dissolved gases and microbubbles from the slurry without damaging the separator material structure.

The specific application method is as follows:

I. Core Principle
When ultrasound acts on the polymer slurry, high-frequency pressure fluctuations (compression-expansion cycle) are generated inside the liquid:

Low-pressure stage: Micro-vacuum bubbles (cavitation nuclei) form in the slurry;

Gases dissolved in the slurry (such as air, solvent evaporation gases) continuously diffuse towards the cavitation nuclei, causing the bubbles to grow rapidly;

Under the influence of buoyancy and ultrasonic vibration, the bubbles aggregate and float to the surface, eventually escaping from the slurry surface, thus achieving defoaming.

Compared to traditional mechanical defoaming (which easily damages slurry homogeneity) and chemical defoamers (which may introduce impurities), ultrasonic defoaming has advantages such as no secondary pollution, thorough defoaming, and no impact on slurry performance.

II. Equipment Selection and Configuration

1. Core Equipment Selection

Equipment Type | Key Parameter Requirements | Selection Basis

Ultrasonic Generator | Frequency 20-80kHz (40kHz recommended, balancing defoaming efficiency and slurry stability); Power 500-3000W (adjustable according to slurry tank volume, 10-20W/L recommended); Supports continuously adjustable power and automatic frequency tracking. Too high a frequency (>80kHz) weakens the cavitation effect and reduces defoaming efficiency; too low a frequency (<20kHz) may cause localized overheating of the slurry or breakage of polymer molecular chains.

Ultrasonic Transducer | Piezoelectric ceramic material (high stability, high energy conversion efficiency); Installation method: immersion or wall-mounted. Immersion transducers directly contact the slurry for more direct defoaming; wall-mounted transducers are suitable for sealing the slurry tank to avoid contamination.

Slurry Tank / Reactor | Material: Stainless steel 316L (solvent-resistant); built-in baffle plate (to create a circulating flow of the slurry, avoiding dead zones in localized defoaming); equipped with a temperature control device (temperature ≤60℃, preventing excessive solvent evaporation or slurry gelation). Diaphragm slurries often contain organic solvents such as hexane and paraffin oil, requiring corrosion-resistant materials; temperature control prevents localized high temperatures generated by ultrasonic cavitation from affecting slurry performance.

2. Auxiliary Configuration

Vacuum System: Equipped with a vacuum pump with a vacuum level of -0.06~-0.08MPa, used in conjunction with ultrasonic waves (a vacuum environment reduces the solubility of gases in the slurry, accelerates bubble rise, and improves defoaming efficiency by more than 30%);

Stirring Device: Low-speed stirring (30-60r/min) avoids the generation of new bubbles from high-speed stirring, while promoting uniform contact of the slurry with ultrasonic energy;

Filtration Device: Before the slurry enters the extruder/casting machine, it passes through a 5-10μm precision filter to intercept a small number of large bubbles (diameter > 10μm) that have not escaped, further ensuring the flatness of the diaphragm.

III. Specific Operating Procedures

1. Pretreatment Stage

Slurry Preparation: Mix PP/PE powder, plasticizer, solvent, etc., according to the formula to form a homogeneous polymer solution (solid content 20%-40%);

Equipment Inspection: Confirm that there are no adhering substances (such as slurry residue, impurities) on the surface of the ultrasonic transducer, that the generator and transducer are properly connected, and that the slurry tank is well sealed (if using vacuum synergy).

2. Defoaming Operation Procedure

Slurry Injection: Inject the prepared polymer slurry into a slurry tank equipped with an ultrasonic device. Control the liquid level at 70%-80% of the tank volume (to avoid overflow during defoaming due to excessive liquid level).

Parameter Setting: Turn on the ultrasonic generator, set the frequency to 40kHz, power density to 15W/L, and initial running time to 30-60 minutes (adjust according to bubble content). If using a vacuum system, first evacuate to -0.07MPa before starting the ultrasonic process.

Process Monitoring:

Observe the slurry surface: If uniform bubbles continue to escape, defoaming is normal. If the bubble quantity decreases sharply, reduce the power or shorten the time.

Detect Slurry State: Detect the bubble particle size in the slurry using a laser particle size analyzer (residual bubble diameter < 5μm, and number ≤ 10/mL); detect slurry viscosity changes using a rotational rheometer (fluctuation ≤ 5%, to avoid polymer molecular chain breakage).

Temperature Control: If the slurry temperature exceeds 50℃, reduce the ultrasonic power or turn on the cooling system to maintain the temperature. 40-50℃;

Subsequent Processing: After defoaming, turn off the ultrasonic and vacuum systems, maintain low-speed stirring, and convey the slurry through a filter to the next process (casting, stretching, etc.) to prevent the slurry from generating bubbles again after settling.

3. Key Operating Points

Avoid prolonged continuous operation of the ultrasonic system (it is recommended to stop for 10 minutes every 60 minutes of operation) to prevent localized overheating of the slurry or transducer fatigue damage;

Transducer Installation Position: Immersion transducers should be 10-20cm from the bottom of the tank and 5-10cm from the tank wall, and evenly distributed (one transducer per 1-2m² tank bottom area) to avoid concentrated energy causing slurry splashing;

Timing of Vacuum and Ultrasonic Coordination: First, evacuate the tank for 10 minutes to remove air, then start the ultrasonic system. This reduces the generation of new bubbles and improves the defoaming effect.

IV. Common Problems and Solutions

Problem Phenomenon | Possible Cause | Solution

Numerous microbubbles remain in the slurry after defoaming | 1. Insufficient ultrasonic power or improper frequency; 2. Slurry viscosity is too high (gas has difficulty diffusing); 3. No vacuum coordination | 1. Increase power density to 18-20W/L, adjust frequency to 40kHz; 2. Appropriately increase solvent ratio, reduce slurry viscosity (control at 1000-5000mPa・s); 3. Turn on the vacuum system and maintain a vacuum level of -0.07MPa

Abnormally high/low slurry viscosity | 1. Excessive ultrasonic power (leading to polymer molecular chain breakage or gelation); 2. Excessively high slurry temperature | 1. Reduce power density to 10-12W/L, shorten single run time; 2. Strengthen temperature control, keep temperature below 40℃

Scale/adhesion of slurry on transducer surface | Polymer residue adheres after solvent evaporation After each use, clean the transducer surface with the corresponding solvent (e.g., hexane) to avoid residue affecting energy transfer.

The finished diaphragm still has pinholes/uneven pore size. 1. Incomplete defoaming, leaving residual bubbles; 2. Insufficient filtration device precision.
1. Extend the ultrasonic running time to 60 minutes and optimize vacuum parameters; 2. Improve filter precision to 5μm and replace the filter membrane regularly.

V. Process Optimization Directions

Parameter Synergistic Optimization: Determine the optimal combination through orthogonal experiments (e.g., frequency 40kHz + power density 15W/L + vacuum degree -0.07MPa + temperature 45℃), which can reduce the residual bubble amount to below 5 bubbles/mL;

Multi-stage Defoaming Design: Install ultrasonic devices in the slurry preparation tank, transfer tank, and pre-extrusion buffer tank to achieve "segmented defoaming + step-by-step purification," further reducing the risk of residual bubbles;

Intelligent Control: Introduce an online bubble detection sensor (based on laser scattering principle) to monitor the bubble content in the slurry

in real time, automatically adjust the ultrasonic power, vacuum degree, and stirring speed to achieve closed-loop control.

VI. Precautions

Safety Protection: Wear protective gloves and goggles during operation to avoid inhaling volatile organic solvent gases; ensure the ultrasonic generator is properly grounded to prevent electrical leakage.

Equipment Maintenance: Regularly (every 1-2 months) calibrate the ultrasonic frequency and power, and check the transducer's sealing performance (to prevent solvent seepage leading to short circuits).

Slurry Compatibility: New slurry formulations require small-scale testing (500mL) to verify the impact of ultrasonic parameters on polymer molecular weight and slurry viscosity, avoiding quality issues during mass production.

Through the above measures, ultrasonic equipment can efficiently remove air bubbles from lithium battery separator slurries, significantly improving the separator's pore uniformity, tensile strength, and breakdown voltage, thus ensuring the safety and cycle life of lithium batteries.