Battery Electrode Winding Bulge (Thick Edge): Causes & Solutions

Electrode winding bulge, also known as thick edge or edge swelling, is one of the most prevalent and destructive defects in lithium-ion battery slot-die coating and winding processes. Even with qualified slurry mixing and formulation, unoptimized coating, drying, tension control, and equipment operation will trigger this defect.

It not only damages electrode roll appearance but also causes inconsistent calendering, slitting defects, poor cell assembly consistency, reduced cycle life, and potential safety hazards. This article systematically sorts out the formation mechanism, core causes, targeted troubleshooting logic, and practical optimization solutions for winding bulge, providing field-verified guidance for battery R&D and manufacturing engineers.

What Is Electrode Winding Bulge?

Electrode winding bulge refers to the abnormal phenomenon that the edge or local area of coated electrodes is significantly thicker than the central area. The thickness deviation and internal stress accumulate continuously during rewinding, forming obvious raised bulges or edge ridges on finished rolls.

This defect is a system-level process problem rather than a single winding failure. Most bulge risks originate from upstream coating and drying stages, and are only amplified in the winding process.

Harms of Winding Bulge Defects

• Uneven calendering pressure and inconsistent electrode compaction
• Edge damage and cracking during slitting and cell stacking
• Deviated areal loading and unstable pore structure of electrodes
• Increased cell-to-cell inconsistency and local internal impedance
• Deteriorated battery cycling stability and reduced production yield

Core Causes of Electrode Winding Bulge

1. Non-Uniform Slot-Die Coating Profile

The primary source of thick-edge bulge is cross-web coating unevenness:

• Die lip dry slurry residue, scratches, and geometric defects cause inconsistent wet film thickness
• Unstable coating gap and misaligned dam bars lead to edge slurry accumulation
• Mismatched slurry delivery speed and line speed destroy coating uniformity
• Poor slurry dispersion generates local particle agglomerates and point bulges

2. Unstable Slurry Rheology & Edge Flow

Slurry properties directly determine dry electrode flatness:

• High surface tension without surfactants triggers capillary edge flow during drying
• Incomplete binder dissolution (PVDF/CMC) and drifted slurry viscosity damage leveling performance
• Low-solid-content slurry carries excessive solvent, aggravating post-coating material migration

3. Improper Multi-Zone Drying Process

Unreasonable drying curves are the most easily overlooked inducement:

• Overly aggressive first-zone drying causes faster edge solvent volatilization than the center
• Uneven cross-web airflow leads to asymmetric shrinkage stress
• Incomplete drying leaves residual solvent, which continues to volatilize after winding and bulge the roll
• Excessive temperature gradient causes edge curl and irreversible internal stress

4. Unstable Web Tension & Winding Mechanics

Tension mismatch amplifies pre-existing coating defects:

• Excessive or fluctuating rewinding tension causes edge stress concentration
• Non-parallel guide rollers, worn bearings, and eccentric air expanding shafts lead to uneven web force
• Unstable nip pressure and speed synchronization errors destroy roll-building uniformity

5. Residual Substrate Stress

Bare copper/aluminum foil is not stress-free:

• Residual internal stress generated during foil production, slitting and transportation
• Foil edge waviness and thickness inconsistency cause asymmetric deformation after coating and drying
• Improper storage leads to foil moisture absorption and dimensional drift

6. Uncontrolled Workshop Environment

• High humidity causes substrate moisture expansion and post-drying shrinkage bulging
• Frequent temperature fluctuations alter slurry rheology and solvent volatilization rate, aggravating stress deviation

Practical Troubleshooting Logic (Engineer Standard Workflow)

1. Locate defect origin: Check thickness uniformity of wet coating and dry coating. Upstream abnormality = coating/slurry/drying failure; qualified dry electrode but bulged roll = tension/equipment failure.
2. Verify cross-web thickness data: Confirm whether edge thickening is measurable.
3. Inspect mechanical status: Check die lip, roller parallelism, bearing wear, and shaft eccentricity.
4. Review process changes: Track slurry batches, line speed, drying parameters, and foil replacement records.

Targeted Process Optimization Solutions

1. Optimize Slurry Formulation & Coating Parameters

• Add surfactants to reduce slurry surface tension and suppress edge capillary flow
• Optimize stirring process to improve dispersion, eliminate agglomerates, and stabilize rheology
• Reduce coating speed and narrow die-to-roll gaps to weaken edge thickening effect
• Properly increase slurry solid content to lower solvent volatilization pressure

2. Refine Multi-Zone Drying Profile

• Adopt gentle low-temperature pre-drying in the first zone to avoid rapid edge dehydration
• Balance cross-web airflow to ensure uniform solvent volatilization
• Set graded drying temperature, strictly avoid baking temperature exceeding foil softening temperature
• Ensure thorough drying to eliminate residual solvent

3. Stabilize Web Tension & Winding System

• Calibrate closed-loop tension control to maintain stable and low-fluctuation tension
• Regularly adjust guide roller parallelism and lubricate bearings
• Replace eccentric air shafts and clean roller surface dirt with alcohol
• Match tension parameters with line speed and electrode rigidity

4. Pre-Treat Substrate & Control Quality

• Pre-heat (80–100℃) or pre-calender bare foil before coating to release residual stress
• Strictly screen incoming foil flatness and thickness consistency
• Standardize foil storage to prevent moisture absorption and deformation

5. Stabilize Workshop Environment

• Constantly control workshop temperature at The answer is 20 25 degrees. and humidity at 40%-60%
• Regular ventilation to reduce water vapor and corrosive media in the air

6. On-Site Emergency Mitigation (For Mild Bulges)

• Temporarily disable automatic tension control and manually wind 5–10 circles to release concentrated stress
• Clip thin buffer paper strips at thick edges to balance thickness difference and prevent electrode cracking

Symptom-to-Cause Quick Reference Table

Observed Symptom	Priority Root Cause
Thick edge visible after coating	Die lip defect, uneven coating gap, poor slurry distribution
Uniform wet coating but raised dry edge	Drying gradient, edge solvent migration, binder uneven distribution
Qualified dry electrode but post-winding bulge	Tension fluctuation, shaft eccentricity, roll-building instability
One-sided bulge deviation	Asymmetric roller alignment, uneven airflow, defective foil
Worsened bulge with increased line speed	Insufficient drying capacity, rheology mismatch, tension response lag

Common Process Misunderstandings

1. ❌ Winding bulge is only a winding process problem: Most defects originate from coating, slurry or drying, and winding only exposes problems.
2. ❌ Higher drying temperature eliminates bulges: Aggressive drying intensifies edge shrinkage stress and worsens defects.
3. ❌ Single parameter adjustment solves fundamental problems: Bulge is a systemic defect requiring multi-process collaborative optimization.
4. ❌ Emergency operation replaces root-cause optimization: Temporary mitigation cannot fix process instability.

Conclusion

Electrode winding bulge is a typical multi-factor composite defect in lithium-ion battery manufacturing, mainly driven by coating non-uniformity, unstable slurry rheology, unbalanced drying gradient, abnormal tension, residual substrate stress, and equipment deviation.

Eliminating bulges cannot rely on single parameter tuning. Manufacturers need to adopt systematic troubleshooting: locate defect links first, then optimize slurry formula, coating parameters, drying curves, tension control and equipment maintenance in a targeted manner.

Stable winding bulge control is the core premise of improving electrode consistency, reducing production scrap, and upgrading final battery electrochemical performance and safety.

FAQ

Q1: What is the main cause of electrode winding bulge?

A: It is a systemic problem. The core inducements are edge thickening during slot-die coating and uneven drying shrinkage, while unstable tension and equipment defects further amplify the bulge.

Q2: Can drying process adjustment solve winding bulge?

A: Yes. Optimizing graded and uniform drying is one of the key solutions, but it needs to cooperate with slurry and coating optimization for best results.

Q3: Why do bulges occur with qualified slurry mixing?

A: Slurry quality is only one link. Coating gap, drying airflow, tension stability and substrate stress will all independently cause bulge defects.

Q4: How to quickly eliminate mild winding bulges on the production line?

A: Appropriately reduce winding tension and use edge buffer paper strips for emergency mitigation, and complete root-cause process optimization in follow-up production.