Cell Section: Detailed Process Introduction of Cell Formation and Capacity Testing

I.Introduction

As a representative of modern high-performance secondary batteries, each stage of the manufacturing process of lithium-ion batteries directly impacts the performance, lifespan, and safety of the final product. Among these, Formation and Aging & Grading are crucial post-processing techniques in cell manufacturing. This article will systematically analyze the core aspects of these processes, covering their principles, key parameters, influencing factors, and directions for technological development.

II.Formation Process

1. Definition and Purpose of Formation Process

The formation process refers to the initial charging activation of a cell, during which solid electrolyte interface (SEI) films are formed on the surface of the anode through electrochemical reactions. The main functions include:

• SEI Film Formation: Passivating the active surface of the anode to prevent continuous decomposition of the electrolyte.
• Removal of Residual Moisture: Reducing moisture content through electrochemical decomposition.
• Activation of Electrode Materials: Optimizing the pathways for lithium-ion intercalation and deintercalation.

2. Formation Process Flow

A typical formation process is divided into three stages:

• Pre-Charging Stage: Cells are initially charged at a low current (0.02C-0.1C) up to a voltage of 3.0-3.5V.
• High-Temperature Aging: Cells are left to rest in an environment of 45-60°C for 12-48 hours to promote the stabilization of the SEI layer.
• Second Charging: Cells are charged to full capacity using a multi-stage constant current-constant voltage (CC-CV) method, typically up to 4.2V.

3. Key Parameter Control

• Current Density: The initial current should be kept below 0.1C to avoid the formation of lithium dendrites.
• Temperature Control: An ideal formation temperature is around 25±2°C. While higher temperatures can accelerate reactions, they may lead to uneven SEI formation.
• Voltage Window: The cutoff voltage for the initial charge should be adjusted according to the cathode material (e.g., 4.0V for LCO, 3.8V for NCM, and 3.5V for LFP).
• Pressure Holding Time: Cells need to dwell at a specific voltage point for 1-2 hours to stabilize polarization.

4. Process Focus Areas

• Pouch Cells: Pay attention to fixture pressure (5-10kPa) to prevent swelling and deformation.
• Cylindrical Cells: Focus on controlling stress distribution within the wound core to avoid internal short circuits.
• Prismatic Cells: Optimize the electrolyte filling amount to balance SEI formation and gas generation.

5. Analysis of Influencing Factors

 

Factor	Influence Mechanism
Electrolyte Formulation	Inclusion of FEC additive can enhance the stability of the SEI film and reduce resistance.
Anode Material	Silicon-based materials require longer formation times compared to graphite (with silicon expansion rates reaching up to 300%).
Electrolyte Wettability	Poor wetting can lead to localized electrode overreaction, triggering lithium plating risks.
Environmental Humidity	Dew point requirement is ≤ -40°C; excessive moisture can lead to the formation of LiOH/Li2CO3, degrading the SEI film.

 

III.Aging & Grading Process

 

1. Definition and Purpose of Aging & Grading

Aging & grading is a process of calibrating cell capacity and screening performance through charge-discharge testing. Its core objectives include:

• Capacity Sorting: Eliminate cells with capacity deviation >2% to ensure pack consistency;
• Internal Resistance Screening: Identify cells with high internal resistance (typically required ≤30mΩ);
• Self-Discharge Detection: Voltage drop must be <5mV/day after 72 hours of standing.

2. Aging & Grading Process Flow

• First Charge-Discharge: Complete a charge-discharge cycle at 1C current and record actual capacity;
• High-Temperature Aging: Store at 55℃ for 7 days to accelerate unstable side reactions;
• Secondary Grading: Retest capacity and calculate capacity decay rate (required ≤3%);
• Voltage Recovery Test: Detect voltage rebound amplitude after standing (reflecting SEI stability).

 

IV. Process Challenges and Solutions

 

1. Gas Generation Control During Formation

• Issue: CO₂, CH₄, and other gases are generated during the first charge (gas production for pouch cells is approximately 3–5 mL/Ah).
• Countermeasure: Use a gradient pressure fixture (gradually increasing from 0 to 5 kPa) combined with vacuum electrolyte injection to reduce residual bubbles.

 

2. Improvement of Charging/Discharging Efficiency

• Traditional Bottleneck: Charging/discharging accounts for over 40% of the total production time.
• Innovative Solution: Introduce multi-channel parallel testing (a single cabinet supports 512 channels) and use AI prediction to shorten the standing time.

 

3. Optimization of Consistency

• Material Aspect: Require the particle size D50 deviation of cathode materials to be ≤1 μm and the graphitization degree of graphite to be ≥85%.
• Process Aspect: Add an OCV matching process after formation (voltage difference ≤10 mV).

 

V.Development of Cutting-Edge Technologies

 

1.Innovation in Solid-State Battery

• Formation Liquid-Free Formation: Activate the electrode interface by laser to reduce the penetration time of traditional electrolytes.
• High-Voltage Formation: Use a voltage above 5 V to accelerate interface passivation, shortening the process cycle by 30%.

2.Intelligent Charging/Discharging System

• Digital Twin Technology: Establish a performance prediction model for battery cells to achieve virtual charging/discharging.
• Blockchain Traceability: Record the process parameters of each battery cell to support full lifecycle management.

VI. Conclusion

 

As the "final mile" in lithium-ion battery manufacturing, the formation and grading processes directly determine a product's market competitiveness. With the adoption of new materials such as high-nickel systems and silicon-carbon anodes, there are higher requirements for process control. Future development directions will focus on process digitization (e.g., Industry 4.0 integration), non-destructive testing (ultrasonic/infrared online monitoring), and high-precision equipment (nanometer-level pressure control) to meet the industrialization needs of power batteries with an energy density of 300 Wh/kg and above.

 

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