Lithium-ion battery anode bonding system
main content
In the manufacturing of lithium-ion batteries, the negative electrode bonding system is a core element to ensure the structural stability and electrochemical performance of the electrode. The combination of sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) has become the mainstream bonding system for graphite anodes due to its unique complementarity. The two achieve a balance of dispersibility, bonding strength and flexibility with a ratio of 2.25%. The following analysis is conducted from aspects such as material properties, synergy mechanisms, and technical challenges.
I. Functional Characteristics of CMC and SBR
1. CMC: The cornerstone of slurry stability
CMC is a water-soluble cellulose derivative. The carboxymethyl (COO⁻) group on its molecular chain uniformly disperses graphite particles through electrostatic repulsion, preventing the slurry from settling. Experiments show that the degree of substitution (DS) and viscosity of CMC directly affect the dispersion effect: the higher the DS value, the stronger the hydrophilicity, but excessive unadsorbed CMC may cause agglomeration. For example, when the addition amount of CMC exceeds the critical value (usually 1.5% or 2.5%), the excess CMC molecules cross-link with each other through hydrogen bonds, resulting in a sudden increase in the viscosity of the slurry. In addition, CMC can form hydrogen bonds with the hydroxyl groups on the surface of graphite, enhancing the adhesion between the active substance and the current collector.
2. SBR: The Key to Flexibility
SBR is an elastic emulsion polymer formed by the copolymerization of styrene and butadiene. Its glass transition temperature (Tg) is regulated by the proportion of monomers. The SBR with a low Tg (50 ° C to 10 ° C) endows the electrode sheet with high elasticity and alleviates the volume expansion of graphite during charging and discharging. The particle size of SBR (150-200 nm) makes it distributed in a "point-like" manner between graphite particles. It combines with copper foil through van der Waals forces and chemical bonds, enhancing the peel strength of the electrode sheet (up to 3.5 N/mm). However, when SBR is used alone, it is prone to uneven migration due to solvent evaporation and needs to be used in combination with CMC.
Ii. Synergistic Mechanism and Process Optimization
The balance between dispersion and adhesion
The pre-adsorption effect of CMC: In the early stage of pulping, CMC preferently adsorbs on the surface of graphite, stabilizes the slurry through electrostatic repulsion, and avoids the agglomeration of conductive agents (such as Super P). Experiments show that when the mass ratio of CMC to graphite is 1:100, the Zeta potential of the slurry can reach 30 mV, and the dispersion effect is the best.
The reinforcing effect of SBR: On the basis of CMC pre-dispersion, SBR fills the particle gaps through an elastic network, enhancing the mechanical strength of the electrode sheet. For example, adding 2.25% SBR can increase the peel strength of the electrode sheet by 40%.
2. Process adaptability
Step-by-step addition technology: In the traditional process, the CMC gel solution needs to be prepared in advance (taking 48 hours), while the dry mixing process shortens the pulping time to 3.5 hours by adding CMC powder in three steps, and at the same time avoids the residue of the gel solution in the pipeline.
Drying condition control: SBR is sensitive to temperature. If the oven temperature exceeds 120℃, it may cause demulsification and form "black spot" defects. Optimizing the drying curve (with a gradient temperature rise of 80℃) can suppress the migration of SBR and ensure the uniformity of the electrode sheet.
Iii. Technical Challenges and Innovation Directions
1. Existing bottleneck
Dispersion stability: Graphite with a high specific surface area is prone to causing CMC adsorption saturation, which needs to be improved through surface oxidation (such as hydrogen peroxide treatment) or the introduction of wetting agents.
Low-temperature performance: The elastic modulus of SBR increases at low temperatures, resulting in an increase in lithium-ion transport resistance (RCT). Studies show that the SBR modified with acrylic acid can increase the discharge capacity at 20℃ by 4%.
2. Technological innovation
Development of composite binders: By compounding CMC with lithiated PAA (polyacrylic acid), a three-dimensional network can be formed in the silicon-carbon anode to suppress volume expansion (capacity retention rate > 80% after 500 cycles).
Green manufacturing: Use bio-based solvents (such as ethyl lactate) to replace NMP, reduce VOC emissions, and be compatible with existing coating equipment.
Iv. Market Application and Development Trends
At present, the CMC/SBR system occupies more than 90% of the market share of anode binders, and the domestic production rate has increased from less than 5% in 2017 to 40% in 2025. With the penetration of silicon-based anodes (expected to account for more than 15% by 2030), PAA/CMC composite systems with high bonding strength will gradually replace traditional combinations. In addition, the EU's restrictions on NMP solvents have accelerated the promotion of water-based binders, and the cost advantage of CMC/SBR (280,000 yuan per ton) has further consolidated its mainstream position.
Conclusion
The synergistic effect of CMC and SBR embodies the design philosophy of "combining rigidity and flexibility" : the rigid framework of CMC ensures the stability of the slurry, while the elastic network of SBR responds to volume deformation. In the future, through molecular modification and process optimization, this classic system will continue to play a core role in high-energy-density batteries, while providing a paradigm reference for the development of new binders.
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