Wet homogenization process for Lithium-ion Battery Anodes
main content
Wet homogenization, as one of the core processes in the manufacturing of lithium-ion battery anodes, has long occupied a mainstream position due to its advantages of simple operation and good fluidity of the slurry. This process takes "preparing the gel first and then dispersing" as its core concept. It achieves the stability of the slurry through the pre-dispersion effect of sodium carboxymethyl cellulose (CMC) gel solution. However, its high energy consumption and long cycle have also attracted much attention. The following is an analysis from the aspects of process flow, technical bottlenecks and optimization directions.
I. Traditional wet homogenization process flow
1. Preparation of CMC gel
The starting point of the wet process is to dissolve CMC powder in deionized water to form a gel. At this stage, the solid content (usually 5% to 10%) and stirring conditions must be strictly controlled. For instance, patent CN112467087B indicates that the preparation of the gel solution requires maintaining a stirring rate of 2035r/min for 36 hours to form a uniform system with a viscosity of 2000 to 8000 mpa ·s. The carboxymethyl groups (COO⁻) on the CMC molecular chain form a three-dimensional network structure through electrostatic repulsion, providing framework support for subsequent dispersion.
2. Dispersion of conductive agents
Conductive agents (such as Super P, carbon nanotubes) are added to the gel solution for primary dispersion. Due to the large specific surface area of the conductive agent (for example, Super P is above 60m²/g), it is prone to absorb the solvent to form the "dry island effect", and the agglomeration needs to be broken through high-speed shearing (1500-3000r/min). Experiments show that stepwise addition of conductive agents can shorten the dispersion time by 30%. However, at this stage, the solvent evaporation loss can reach 5% to 8%, and additional NMP or water needs to be supplemented to maintain the stability of the system.
3. Graphite mixing and viscosity adjustment
The active substance graphite was added to the conductive adhesive in batches, and the solid content was adjusted to 45% to 55% through gradient dilution. The traditional process adopts a single-stage dilution, which leads to a sudden change in viscosity (such as a sharp drop from 12,000 mpa ·s to 4,000 mpa ·s), causing particle sedimentation and the breakage of the conductive network. Webpage 13 points out that stepwise dilution (such as first reducing to 58% to 61%, and then to 53% to 55%) can reduce viscosity fluctuations and lower the sedimentation rate by 40%.
Ii. Process Bottlenecks and Energy Consumption Analysis
1. Time cost and energy consumption
The total time consumption of the traditional wet process can reach 46 hours, among which the dissolution of CMC takes 48 hours, the dispersion of the conductive agent takes 35 hours, and the mixing of graphite takes 10 to 15 hours. Take the double planetary mixer as an example. The energy consumption of a single batch can be as high as 80,120 KWH, mainly concentrated in the high-speed shearing stage (accounting for more than 60% of the total energy consumption). In addition, the operating energy consumption of the NMP solvent recovery system accounts for 25% to 30% of the total energy consumption of the production line.
2. Uneven dispersion and stability defects
The density difference between the conductive agent and graphite (Super P density 0.2g/cm³ vs.) Graphite (2.2g/cm³) leads to slurry stratification, and particle agglomeration intensifies when the absolute value of Zeta potential is lower than 20mV. Data from Webpage 13 shows that the TSI (Instability Index) of the slurry prepared by the traditional wet method reaches 1.05, and the solid content fluctuates by more than 3% after standing for 24 hours. Electron microscope analysis shows that the conductive agent forms an "island" distribution on the graphite surface, resulting in a 20% to 30% increase in the resistance of the electrode sheet.
3. Solvent evaporation and environmental protection pressure
As a mainstream solvent, NMP has a boiling point as high as 202℃. The drying process requires a gradient temperature increase (80℃→120℃→150℃), and the evaporation loss rate is approximately 8% to 12%. The EU REACH regulation restricts the use of NMP (VOC emission threshold < 10ppm), and enterprises need to install additional condensation recovery devices, increasing the processing cost per ton of slurry by 500,800 yuan.
Iii. Directions for Technological Optimization
1. Modification of pre-dispersed gel solution
Patent CN112467087B proposes the adoption of a step-by-step gluing technology: dividing the glue solution into three parts and mixing them with different proportions of active substances respectively can reduce the fineness of the slurry from 20μm to 12μm and improve the uniformity of dispersion by 25%. In addition, the introduction of nanocellulose (CNF) compounded with CMC and the utilization of the CNF nanofiber network to enhance the suspension stability can extend the sedimentation time to more than 72 hours.
2. Intelligent process parameters
Adjust the stirring strategy based on rheological properties:
Flow curve optimization: When the slurry shear thinning index (n value) is less than 0.4, intermittent high-speed shearing (such as a cycle of 2000r/min×2min→500r/min×5min) is adopted to prevent structural collapse.
Thixotropic recovery control: The dilution rate is dynamically adjusted through an online viscosity sensor to ensure that the viscosity recovery time after high shear is less than 30 minutes.
3. Green solvent substitution
Development of waterborne alcohol complex systems: Adding 2% to 5% isopropyl alcohol can reduce the surface tension of the solvent (from 72mN/m to 35mN/m), lower the wetting Angle of graphite from 85° to 40°, and shorten the dispersion time to 28 hours. In addition, the introduction of bio-based solvents (such as ethyl lactate) can reduce VOC emissions by 70% and is compatible with existing coating equipment.
Iv. Future Development Trends
Although dry homogenization has risen rapidly due to its environmental advantages, the wet process remains irreplaceable in the field of high-end graphite anodes (such as single crystal graphite). It is expected that by 2030, the wet process will be upgraded through the following paths:
1. Equipment integration: By integrating ultrasonic dispersion (2040kHz) with microreactor technology, the total process time is compressed to within 30 hours.
2. Binder innovation: Develop lithiated CMC (such as LiCMC), which increases the ionic conductivity by three times and reduces the amount of conductive agent used by 10% to 15%.
3. Digital twin application: By using AI models to predict the rheological behavior of slurry, parameter self-optimization is achieved, and energy consumption is reduced by 40%.
Conclusion
The traditional wet homogenization process has laid the foundation for anode manufacturing through the pre-dispersion mechanism of CMC gel, but its problems of high energy consumption and long cycle urgently need to be broken through. In the future, through material modification, process innovation and intelligent upgrading, wet technology will continue to play a core role in scenarios with high consistency requirements, and at the same time provide a technical paradigm for the development of new homogenization systems.
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