In lithium battery manufacturing, electrode rolling is a crucial step connecting the preceding and following processes. Many rolling defects originate not from the electrode calendering machine itself, but from incoming materials and basic maintenance. This article systematically analyzes the core causes and key prevention measures for twelve common defects after rolling, helping you build an efficient and stable rolling process.
I. The Root of the Problem: 80% of Defects Originate Before Rolling
There's an industry saying: "Uneven coating renders rolling adjustments useless." This profoundly reveals the characteristic of the rolling process-it's more like a "quality inspector" than a "magician." The vast majority of appearance defects, thickness issues, and tape breakage risks are sown during the coating and baking stages and germinate under inappropriate rolling processes.
Successful rolling control must be built upon strict access control for incoming materials, meticulous maintenance of equipment status, and a clear understanding of process boundaries.
II. Tracing the Root Causes of Defects from Phenomena
1. Electrode Breakage (Strip Breakage)
This is the most serious production interruption problem, usually triggered by five major hidden dangers:
Hard Object Injection: Hard particles, metal shavings, or dry slurry introduced during coating act like "nails" piercing the foil under high pressure.
Sudden Changes in Incoming Material: Sudden changes in areal density or thickness create localized stress concentration during rolling, essentially "tearing" at weak points.
Equipment Malfunction: Instantaneous overload of rolling pressure (e.g., exceeding the set value by more than 15%) or violent fluctuations in the tension system directly break the electrode.
Roller Surface Damage: Dry material or mechanical damage adhering to the roller surface creates periodic "blades" that continuously scratch the electrode.
Key Control: The cleanliness and stability of upstream processes are crucial. A regular inspection and cleaning system for the roller surface must be established, and the stability of closed-loop pressure and tension control must be ensured.
2. Uneven Thickness and Rebound:
Thickness issues directly affect the consistency and safety of cell capacity.
Lateral Unevenness: The main root cause is poor lateral coating uniformity and uneven gap between the left and right rollers.
Longitudinal Fluctuation/Drift: This may originate from coating surface density drift, roller eccentricity, or bearing clearance.
Thickness Rebound: Electrode rebound after rolling is mostly due to internal stress release or insufficient baking. A rebound rate exceeding 5% usually warrants attention.
Key Control Measures: Using an online thickness gauge in conjunction with an automatic thickness control (AGC) system for real-time adjustment is a temporary solution, while ensuring coating uniformity and roller mechanical precision is the fundamental solution. Simultaneously, the formulation and baking process need to be optimized to reduce internal stress in the material from the source.
3. Surface Appearance Defects
These defects affect electrode consistency and may become the starting point for safety hazards.
Roller Sticking (Powder Shedding): The root cause is that the electrode is not completely dry, with excessive residual solvent in the coating (>0.5%), causing the binder to be picked up during rolling. Using a hot rolling process at 85-110℃ can effectively soften the binder and reduce adhesion.
Roller Imprints/Pockmarks: Caused by foreign matter adhering to the roller surface or pitting and scratches on the roller surface itself, forming periodic marks on the electrode.
Dark Spots/Cracks/Pinholes: These are mostly dark spots, impurities, or pinholes existing during the coating stage, which are exposed and magnified under the high pressure of the rollers.
Key Control Measures: Strictly control residual solvents after coating and drying, and establish strict roller surface cleaning and maintenance procedures (e.g., controlling roughness Ra≤0.2μm). Surface quality is far more about prevention than treatment.
4. Edge and Shape Defects
These defects are closely related to local stress concentration and material ductility of the electrode.
Wavy Edges: Classic mechanism. The coated area stretches under pressure, while the unpressurized blank area remains unchanged in length. This perimeter difference forces the blank area to "wrinkle" to absorb the excess length. The smaller the roller diameter and the greater the compression ratio, the more severe the wavy edges.
Edge Chipping: Cracks and powder loss at the edge of the electrode. The main cause is excessive rolling pressure and a right-angled coating edge, creating stress concentration points. Changing the coating edge to a trapezoidal transition and using gradient pressure can effectively alleviate this.
Warping (Sickle Bend): Primarily caused by the "thick edge" effect of the coating or non-parallel rolling, resulting in uneven stress on the electrode sheet in the width direction, causing it to bend to one side.
Wrinkles (Positive Electrode Blank Area): Due to the high hardness and poor ductility of the positive electrode aluminum foil, the difference in elongation between the coated area and the blank area during rolling can reach 0.5%-1.2%, leading to wrinkles in the blank area. Using electromagnetic pulse (IH) preheating or hot rolling at 60-100℃ can improve the aluminum foil's ductility and is an effective solution.
Key Control Measures: Optimize the coating edge design and use differentiated preheating or rolling temperature processes based on the different mechanical properties of the positive and negative electrode foils (aluminum foil is hard and brittle, copper foil is softer).
5. Process Defects
These defects directly affect whether the process objectives are achieved.
Incomplete Compaction: The compaction density cannot reach the design value. This may be due to insufficient roller pressure or excessively hard active particles (e.g., the D50 of the cathode material should be controlled within 8-12μm). A reasonable linear pressure range needs to be matched according to the material system (e.g., ternary, lithium iron phosphate).
Slippage: The electrode slips on the roller surface, causing skewed indentations. The root cause is roller surface wear, excessive bearing clearance leading to unstable biting, or excessively large differences in incoming material thickness.
Key Control Measures: Establish a scientific first-piece inspection system (mandatory testing of compaction density and springback rate), and strictly implement preventative equipment maintenance, regularly inspecting and replacing rollers and bearings.
III. Building a Robust Rolling Process: Core Logic Chain
In summary, the prevention and control of rolling defects can be summarized into a clear logic chain, which is also the key control point for high-level manufacturing:
Strictly control incoming materials → Adhere to equipment discipline → Strictly adhere to process boundaries → Implement closed-loop monitoring
Incoming Material Access Control: Ensure that the uniformity, edge morphology, cleanliness, and solvent residue of the coated electrode meet the standards. Equipment Discipline: Regularly calibrate the parallelism and coaxiality of the rolls, maintain a smooth roll surface and specified roughness, and ensure the stability of bearings and hydraulic systems.
Process Boundaries: Set reasonable compaction density, linear pressure, roll temperature (hot rolling), and tension ranges based on material characteristics to avoid excessive rolling in pursuit of a single performance indicator.
Closed-Loop Monitoring: Effectively utilize online thickness measurement, areal density detection, and CCD data to create a linkage alarm and adjustment system with pressure and tension systems, achieving proactive prevention and control.
The stability of the rolling process is the cornerstone of consistent lithium battery manufacturing. We hope this systematic overview will help you quickly pinpoint the root cause of problems and achieve a leap from reactive response to proactive prevention.
What is the most challenging rolling problem you encounter in production? We welcome your feedback.
ACEY New Energy focuses on providing process insights and solutions for the entire lithium battery manufacturing process.
