Lithium-ion Battery: Structure, Working Principle and Package

Topics Covered in This Article:

Ⅰ. What is a Lithium-Ion Battery?

Ⅱ. How Do Lithium-Ion Batteries Work?

Ⅲ. Lithium-Ion Battery Structure

Ⅳ. Lithium-Ion Battery Packaging Technology

Ⅴ. Main Equipment for Lithium-Ion Battery Production

Ⅰ. What is a Lithium-Ion Battery?

Lithium batteries are divided into lithium metal batteries and lithium-ion batteries. Mobile phones, laptops, and electric vehicles use lithium-ion batteries, commonly referred to as lithium batteries. True lithium metal batteries are rarely used in consumer electronics due to safety concerns, though research continues for specialized applications.

Figure 1: Lithium-Ion Battery

Lithium-ion batteries are rechargeable batteries that primarily rely on lithium ions moving between positive and negative electrodes. During charging and discharging, Li⁺ ions are intercalated and de-intercalated between the two electrodes. When charging, Li⁺ ions are extracted from the positive electrode and inserted into the negative electrode through the electrolyte, creating a lithium-rich state at the anode. During discharge, the process reverses. Modern lithium-ion batteries represent high-performance energy storage technology with applications ranging from portable electronics to electric vehicles and grid storage systems.

Ⅱ. How Do Lithium-Ion Batteries Work?

Lithium-ion batteries use carbon-based materials (typically graphite) as the negative electrode and lithium-containing compounds as the positive electrode. Unlike lithium metal batteries, they contain no metallic lithium, only lithium ions. The charging and discharging process involves the intercalation and de-intercalation of lithium ions, accompanied by equivalent electron transfer. This back-and-forth movement of lithium ions between electrodes has earned these batteries the nickname "rocking chair batteries."

Figure 2: Working Principle of Lithium-Ion Battery

During charging, lithium ions are generated at the positive electrode and migrate through the electrolyte to the negative electrode. The carbon-based anode has a layered structure with micropores where lithium ions are embedded. More embedded ions result in higher charge capacity. During discharge (when the battery powers a device), the embedded lithium ions are released from the carbon layers and return to the positive electrode. The quantity of ions returning determines the discharge capacity.

Charging Considerations: Lithium battery charging current is typically set between 0.2C and 1C (where C represents the battery capacity). Higher currents charge faster but generate more heat. Excessive current prevents complete charging because electrochemical reactions require time—similar to pouring beer too quickly, which creates foam and prevents the glass from filling completely.

Discharge Characteristics

Key discharge considerations:

• Current limits: Excessive discharge current causes internal heating and potential permanent damage. Modern devices include protection circuits to prevent this.

• Over-discharge protection: Lithium-ion batteries are highly sensitive to over-discharge. Voltage below 2.7V can permanently damage the battery. Fortunately, built-in protection circuits in modern batteries prevent discharge below safe thresholds.

• Performance relationship: Higher discharge currents result in reduced discharge capacity and faster voltage drop.

Ⅲ. Lithium-Ion Battery Structure

Figure 3: Battery Structure Components

Positive Electrode (Cathode)

Components: Active material, conductive additives, binder, solvent, and current collector substrate.

Figure 4: Positive Electrode Structure

During discharge, the cathode receives electrons from the external circuit, undergoing reduction. It operates at higher potential. Common cathode materials include lithium cobalt oxide (LiCoO₂), lithium manganese oxide (LiMn₂O₄), lithium nickel manganese cobalt oxide (NMC), and lithium iron phosphate (LiFePO₄).

Negative Electrode (Anode)

Components: Active materials (graphite, MCMB, hard carbon, silicon composites), binder, solvent, and current collector.

Figure 5: Negative Electrode Structure

During discharge, the anode releases electrons to the external circuit through oxidation reactions. Operating at lower potential, graphite remains the dominant anode material, though silicon-based composites are increasingly used to enhance energy density.

Separator

Figure 6: Battery Separator

The separator is a critical safety component positioned between electrodes to prevent short circuits while allowing ion transport. Modern separators often include ceramic coatings for enhanced thermal stability.

Separator Requirements:

• High ionic conductivity

• Appropriate mechanical strength

• Electrical insulation

• Chemical stability with electrolyte and electrodes

• Thermal shutdown capability (for safety)

Materials: Single-layer PE (polyethylene), single-layer PP (polypropylene), or tri-layer PP/PE/PP composite structures. Ceramic-coated separators are increasingly common in high-performance applications.

Thickness: Single-layer: 16-20 μm; Tri-layer: 20-25 μm

Electrolyte

The electrolyte facilitates ion transport between electrodes. Modern electrolytes consist of lithium salts (typically LiPF₆) dissolved in organic carbonate solvents. Advanced formulations include additives for improved safety, wider temperature ranges, and enhanced solid-electrolyte interphase (SEI) formation. Solid-state electrolytes represent an emerging technology promising higher energy density and improved safety.

Battery Case and Hardware

Components include steel or aluminum casing, cover plates, tabs, insulators, and insulating tape. The choice of materials affects weight, safety, and manufacturing costs.

Raw Materials Deep Dive

Cathode Materials

Cathode materials represent the largest cost component of lithium batteries (approximately 30-40% of total cost) with gross margins ranging from 15% to over 70%.

Anode Materials

Carbon-based anodes (primarily graphite) dominate commercial applications due to safety, long cycle life, low cost, and non-toxicity. However, they have relatively lower gravimetric energy density.

Advanced anode materials under development include:

• Silicon composites: Offer 10x higher theoretical capacity than graphite but face challenges with volume expansion

• Lithium titanate (LTO): Excellent safety and cycle life, lower energy density

• Hard carbon: Better low-temperature performance

Future development focuses on silicon-graphite composites and other high-capacity materials to improve energy density while maintaining cycle stability.

Separator Materials

Figure 7: Separator Material Comparison

PE vs. PP Characteristics:

• PP offers better high-temperature resistance; PE excels at low temperatures

• PP has lower density than PE

• PP has higher melting point and pore closure temperature

• PP is more rigid than PE

• PE is more sensitive to environmental stress

Modern separator products: Single-layer PP or PE for consumer electronics; ceramic-coated separators, PP/PE/PP tri-layer, and advanced composite structures for electric vehicle batteries. Emerging materials include polyimide (PI), aramid, and other high-performance polymers for specialized applications.

Ⅳ. Lithium-Ion Battery Packaging Technology

Beyond raw materials, packaging technology significantly impacts battery performance, safety, energy density, and cycle life. Even with identical material formulations, different manufacturing processes produce batteries with varying characteristics.

Three Main Packaging Technologies:

1. Prismatic (Square) Batteries

Prismatic batteries feature rectangular single cells with minimal internal gaps and tightly packed materials. The rigid aluminum-magnesium alloy casing provides excellent protection against swelling and enhances safety. These batteries offer flexibility in size customization but face challenges with consistency due to numerous non-standardized models and varying production processes.

Advantages: High safety, good space utilization, customizable dimensions
   Disadvantages: Lower consistency, complex manufacturing, many non-standard sizes

2. Cylindrical Batteries

Cylindrical batteries share the hard-shell packaging approach but in smaller, standardized formats (e.g., 18650, 21700, 4680). They offer excellent cell consistency, high single-cell energy density, flexible pack configurations, mature production processes, and lower costs. However, the cylindrical shape results in lower pack-level energy density due to wasted space, and battery packs require more cells, increasing weight and complexity.

Advantages: Excellent consistency, mature technology, cost-effective, standardized
   Disadvantages: Lower pack energy density, more cells needed, heavier packs

3. Pouch (Soft-Pack) Batteries

Pouch batteries deliver the best performance among the three formats, with flexible sizing, highest energy density, and lightest weight. The aluminum-plastic film packaging allows for efficient space utilization and thermal management. However, lower mechanical strength, more complex production processes, and higher costs limit widespread adoption.

Advantages: Highest energy density, lightweight, flexible form factor
   Disadvantages: Lower mechanical strength, complex manufacturing, higher cost

Advanced Packaging: Cell-to-Pack (CTP) Technology

Cell-to-Pack (CTP) technology represents a significant advancement in battery packaging, eliminating the intermediate module step and integrating cells directly into the battery pack. This approach offers:

• Improved space utilization (up to 15-20% increase)

• Reduced weight and component count

• Dramatically increased pack-level energy density

• Simplified manufacturing processes

• Lower production costs

Two main CTP approaches:

1. Blade Battery (BYD): Completely eliminates modules, using long, thin prismatic cells arranged like blades. Offers exceptional safety and space efficiency.

2. CTP Battery (CATL): Integrates small modules into large modules, maintaining some modularity for easier manufacturing and service.

Note: As of 2025, CTP technology has matured significantly and is widely adopted in electric vehicles. Both approaches have proven successful, with blade batteries excelling in safety and CTP batteries offering manufacturing flexibility. Cell-to-Body (CTB) and Cell-to-Chassis (CTC) technologies are emerging as next-generation solutions.

Ⅴ. Main Equipment for Lithium-Ion Battery Production

1. Vacuum Planetary Mixer

Figure 9: Vacuum Planetary Mixer

Purpose: Uniformly mixes battery materials into a homogeneous slurry under vacuum conditions to eliminate air bubbles and ensure consistent electrode coating.

2. Electrode Coating Machine

Figure 10: Electrode Coating Machine

Purpose: Precisely coats mixed slurry onto metal foil (aluminum for cathode, copper for anode) with thickness accuracy within ±3 micrometers. Modern systems feature real-time thickness monitoring and automatic adjustment.

3. Roller Press (Calendering Machine)

Figure 11: Roller Press

Purpose: Compresses coated electrodes to target density, improving energy density, electrode adhesion, and electrical conductivity. Precise control of compression force and electrode density is critical for battery performance.

4. Electrode Slitting Equipment

Figure 12: Electrode Slitting Equipment

Purpose: Cuts large electrode sheets into precise widths required for specific battery designs. High-precision cutting ensures consistent cell performance.

5. Ultrasonic Tab Welding Equipment

Figure 13: Ultrasonic Tab Welding Equipment

Purpose: Attaches conductive tabs to electrodes using ultrasonic welding, creating low-resistance electrical connections essential for efficient current collection.

6. Winding Machine

Figure 14: Winding Machine

Purpose: Winds cathode, anode, and separator layers together to form the battery cell core. Advanced machines achieve high-speed winding (up to 60-80 meters/minute) while maintaining precise alignment and tension control.

7. Glove Box (Dry Room)

Figure 15: Glove Box/Dry Room

Purpose: Provides ultra-low humidity environment (dew point below -40°C) for electrolyte filling and cell assembly. Moisture control is critical as water reacts with lithium salts, degrading battery performance and safety.

8. Electrolyte Filling Machine

Figure 16: Electrolyte Filling Machine

Purpose: Precisely injects electrolyte into battery cells under vacuum conditions, ensuring complete wetting of electrodes and separator. Advanced systems feature automated volume control and leak detection.

9. Formation and Testing Equipment

Figure 17: Formation and Testing Equipment

Purpose: Performs initial charging cycles (formation) to activate the battery and establish the solid-electrolyte interphase (SEI) layer. Subsequently tests battery capacity, internal resistance, and other performance parameters. Modern systems include automated sorting based on performance characteristics.

Additional Production Equipment (2025):

• Laser welding systems: For precise tab and case welding

• X-ray inspection: Non-destructive quality control

• Automated optical inspection (AOI): Defect detection

• Battery management system (BMS) integration: Smart battery assembly

• End-of-line testing: Comprehensive performance validation

Conclusion

Lithium-ion battery technology continues to evolve rapidly, with ongoing improvements in energy density, safety, cycle life, and manufacturing efficiency. As of 2025, the industry has achieved significant milestones including widespread adoption of high-nickel cathodes, silicon-composite anodes, and advanced packaging technologies like CTP. Future developments focus on solid-state batteries, sustainable materials, and enhanced recycling processes to support the global transition to electric mobility and renewable energy storage.

Last Updated: November 2025
       Note: Battery technology advances rapidly. Specifications and best practices may evolve. Always consult current manufacturer guidelines and safety standards.