Winding & Lamination Process for Lithium-ion Pouch Cells

Introduction

Why Are Winding & Stacking The Most Critical Cell Assembly Step?

1. Direct control over internal short-circuit risk
    
   All slitting burrs, exposed metal foil and misaligned insulating tape converge here. If winding/stack alignment fails, hard metal fragments will pierce the separator during charge-discharge expansion, triggering micro-shorts, swelling or thermal runaway. Upstream defects from slitting cannot be fully eliminated—only standardized winding/stack rules block these hazards.
2. Determines cell capacity & consistency
    
   Anode-cathode overlapping tolerance directly impacts usable lithium ion storage space. Uneven winding tension or stacking offset creates inconsistent active material utilization across batches, leading to wide capacity grading gaps after formation.
3. Decides rate discharge performance
    
   Current transmission path length differs drastically between wound and laminated cores. Misaligned tabs, loose winding or uneven stacking lengthen ion migration distance, raising internal resistance and limiting fast-charging capability.
4. Shapes long cycle lifespan
    
   Poor forming creates localized lithium plating areas. Separator wrinkles, layer gaps or edge misalignment cause uneven SEI film growth during cycling, accelerating capacity attenuation and shortening service life.

How Upstream Slitting Quality Ruins Winding Outcomes (Cross-Process Topic Cluster Link)

1. Excessive slitting burrs
    
   Oversized metal burrs cannot be fully covered by green tape. During winding coiling, burrs scratch or puncture separator film, leading to latent short circuits. Industry standards limit burr length to less than half the separator thickness to avoid this issue.
2. Width deviation & edge warping
    
   Uneven slitting edges create continuous offset during automatic feeding. The anode fails to fully cover the cathode edge, violating the 1.0–3.0mm dimensional safety margin rule.
3. Edge wrinkling on electrode rolls
    
   Wrinkled pole pieces cause uneven winding tension. Local loose layers form gaps inside the bare cell, resulting in lithium precipitation after liquid injection.
4. Misaligned green protective tape
    
   Off-center taping exposes copper/aluminum foil at pole edges. Winding concentrates exposed foil at overlapping positions, greatly boosting short-circuit probability.

1 Pre-Winding / Pre-Stack Electrode Inspection (What We Need Before Forming)

Mandatory Rejection Defects

• Electrode cracks, surface indentations and deep scratches
• Active material shedding (decarbonization) with exposed copper/aluminum foil
• Green tape failing to fully cover metal foil edges
• Slitting burrs exceeding specification limits

Non-Negotiable Dimensional Matching Standard (Applies to Both Processes)

1. Anode length & width exceed cathode by 1.0–3.0mm
2. Separator length & width exceed anode by 1.0–3.0mm
    
   This margin guarantees all cathode active material is wrapped by anode copper foil and separator, even with minor feeding offset.

2 Winding Process for Wound Bare Cells

2.1 What Is Winding Technology?

2.2 Main Equipment Used in Winding Production Line

1. Semi-automatic / Fully Automatic Winding Machine: Core coiling equipment, controls feeding speed, tension and tab alignment
2. Hot & Cold Press Integrated Machine: Flatten loose wound cores to standard thickness
3. Hi-pot Short-Circuit Tester: Real-time double-tab internal resistance detection to screen short-circuited cells
4. X-Ray Inspection Machine: Automated measurement of anode-cathode overhang offset

1. Thread taped anode, cathode and separator through independent guide rollers
2. Calibrate tab center distance and electrode overhang margin per drawing specifications
3. Maintain constant tension during synchronous spiral coiling
4. Automatically cut separator once target winding layers are reached
5. Transfer finished wound bare cell to hot-cold pressing station for shaping

2.4 Winding Defect Troubleshooting Table

Defect Phenomenon	Root Cause	Targeted Solution	Hidden Risk
Separator wrinkling	Separator tension too low	Adjust feeding roller tension controller	Local lithium plating, capacity fade
Anode-cathode offset	Guide roller misalignment, slitting width deviation	Recalibrate feeding track; filter out width-out-of-tolerance electrodes	Edge short circuit
Loose winding core	Winding torque insufficient	Increase main coiling motor tension	Cell swelling after liquid injection
Tab position deviation	Tab feeding jig shift	Re-fix tab positioning fixture	High internal resistance, poor rate performance

Why Most Battery Manufacturers Still Prioritize Winding

1. Mature, stable equipment supply chain with decades of mass production verification
2. Lower upfront production line investment cost vs stacking equipment
3. Ultra-high production speed, ideal for large-volume mass manufacturing
4. Lower overall defect rate with simple feeding and coiling logic
5. Wide compatibility for small power bank, Bluetooth and consumer electronic pouch cells

3 Zig-Zag Lamination (Stacking) Process for Laminated Bare Cells

3.1 What Is Lamination (Stacking)?

Stacking cuts long slit electrode rolls into independent single sheets, then alternates cathode-separator-anode layers via continuous Z-fold separator to form rectangular bare cells. This technology dominates large-capacity power pouch cells and ultra-high fast-charging products.

3.2 Types of Battery Stacking Technology

1. Z-fold continuous separator stacking: Single unbroken separator zigzags between electrode sheets, highest alignment precision
2. Single-sheet separator stacking: Independent separator cut for each electrode layer, low equipment cost but slower speed
3. High-speed automatic stacking: Fully robotic feeding for automotive power cell mass production

3.3 Complete Stacking Production Flow (How to Complete Lamination)

1. Slit long electrode rolls into fixed-size single cathode/anode sheets
2. Ultrasonic weld aluminum tabs to cathode pieces, nickel tabs to anode pieces
3. Apply four-corner green insulating tape to cover sheet edge metal burrs
4. Alternately stack electrodes with continuous Z-fold separator isolation
5. Fix stacked core edges with adhesive tape to prevent layer dislocation during packaging

3.4 Stacking Core Control Points

1. Single electrode cutting dimensional tolerance and burr limits consistent with slitting standards
2. Tab welding tension and green tape full coverage requirement
3. Layer-by-layer stacking alignment offset control
4. Z-fold separator tension to eliminate folding defects

3.5 Stacking Defect Troubleshooting Table

Defect Phenomenon	Root Cause	Targeted Solution	Hidden Risk
Layer dislocation	Robotic feeding positioning error	Calibrate stacking machine visual alignment system	Uneven capacity distribution
Separator tearing	Excessive Z-fold tension	Reduce separator traction force	Direct internal short circuit
Corner tape peeling	Insufficient tape pressing pressure	Adjust taping fixture compression	Edge burr piercing separator

Why Power Cell Brands Like CATL Are Expanding Stacking Lines

1. Uniform layer current distribution delivers far lower internal resistance for fast charging
2. Higher energy density from tighter electrode stacking with minimal dead space
3. Better cycle stability for large automotive pouch cells
4. Flexible thickness design for ultra-thick energy storage batteries

4 Winding vs Stacking: Full In-Depth Comparison & Selection Guide

Comparison Item	Winding Process	Z-Fold Stacking Process	Recommendation Scenario
Mass Production Efficiency	⭐⭐⭐⭐⭐ Fast continuous coiling	⭐⭐⭐Discrete single-sheet feeding	Small consumer cells: Winding; Power cells: Stacking
Cell Internal Resistance	⭐⭐⭐ Longer ion path	⭐⭐⭐⭐⭐ Uniform short current path	Fast-charging products: Stacking
Achievable Energy Density	Medium	Higher (less internal gap waste)	High-capacity energy storage: Stacking
Equipment & Labor Cost	Low upfront investment	High automation cost	Small-batch low-budget factories: Winding
Large Pouch Cell Adaptability	Medium layer alignment error	Excellent uniform stacking	EV power batteries: Stacking
Fast Charging Performance	Good	Superior even under high C-rate	3C fast-charge / EV batteries: Stacking
Automation Difficulty	Simple feeding logic	Complex robotic vision alignment	Entry-level factories: Winding

5 Aluminum-Plastic Film Punch & Forming Matching Process

Why Aluminum-Plastic Laminate Film Is Used for Pouch Cells

Compared with rigid steel or aluminum cylindrical cans, aluminum-plastic composite film delivers irreplaceable advantages:

1. Ultra-lightweight structure boosts overall cell energy density
2. Flexible forming supports customized ultra-thin, irregular battery shapes
3. Superior heat dissipation to reduce thermal runaway risk
4. Soft buffer space for gas expansion during formation
5. Low packaging cost for consumer electronic mass production

5.1 What Is Aluminum-Plastic Forming?

After winding/stacking and X-ray inspection, bare cells enter pre-packaging: the aluminum plastic film punching machine stamps fixed-depth pockets to hold the bare core, followed by top-side heat sealing to temporarily seal the cell cavity.

5.2 Core Forming Equipment

Automatic aluminum-plastic film punching & shell forming machine

5.3 Critical Control Standards & Defect Hazards

1. Pocket forming depth tolerance, four-corner R-angle smoothness
    
   Sharp R angles crack the aluminum barrier layer, creating pinholes
2. Zero pinhole / stretch crack requirement on composite film
    
   Pinholes allow ambient moisture to penetrate before baking, reacting with LiPF6 electrolyte to generate corrosive HF, accelerating capacity decay. Cross reference: Vacuum Baking & Electrolyte Injection Guide

 

6 FAQ

Q1: Why must anode sheets be larger than cathode sheets in winding and stacking?

If the cathode edge extends past the anode boundary, cathode active material directly contacts the separator without copper foil shielding. Slitting metal burrs will puncture the separator during cell expansion, causing permanent internal short circuits.

Q2: What damage does separator folding bring to finished cells?

Folded separators create uneven lithium ion migration paths. Lithium metal precipitates on the anode surface, triggering cell swelling, rapid capacity attenuation and thermal runaway under heavy discharge loads.

Q3: How do aluminum plastic film pinholes affect subsequent baking and liquid injection?

Pinholes break the moisture barrier of the packaging film. Uncontrolled moisture inside the cell decomposes electrolyte to produce HF acid, corroding the SEI protective film and drastically shortening cycle life. Detailed chemical reactions are explained in our baking & injection article.

Q4: What is the core tab difference between winding and stacking?

Winding uses one pair of continuous long tabs per bare cell; stacking welds independent tabs to every single electrode sheet. Multi-tab stacking design greatly reduces internal resistance for high-rate fast-charging batteries.

Conclusion

Winding and Z-fold stacking form the structural backbone of soft-pack lithium-ion cells, bridging front-end electrode processing and post-stage packaging, baking and formation. Understanding why each forming standard exists, comparing process pros and cons, and mastering defect troubleshooting logic allows manufacturers to optimize yield, boost cell consistency and extend cycle life. Select winding for low-cost mass consumer electronics production, and adopt Z-fold stacking for high-energy-density, fast-charging automotive and energy storage pouch cells.