In-depth Analysis of The Entire Manufacturing Process of Lithium-ion Power Battery Cells

Lithium-ion power batteries play a crucial role in today's new energy sector. Their production process involves numerous complex steps, from cell manufacturing to battery pack assembly, each of which directly impacts the battery's performance, safety, and reliability. Today, let's examine this mysterious yet critical production process in depth.

1. Active Material Slurry Preparation – Mixing Process

Mixing is the first step in battery production, where active materials, binders, conductive additives, and solvents are blended evenly. The mixture is created into a uniform slurry in a vacuum mixer. This process requires precise control over raw material ratios, mixing steps, and stirring time. The production environment, including dust and cleanliness control, must meet medical-grade standards for a dust-free and sterile workshop, ensuring the final quality and product qualification of the batteries.

2. Slurry Coating on Copper Foil – Coating Process

The electrode coating process involves evenly applying the well-mixed slurry onto the current collector and drying the organic solvent in the slurry. This process requires extremely high precision and speed. The consistency of the coating is critical to battery performance; any inconsistencies can lead to a decline in performance. Factors such as coating drying temperature control, coating surface density, coating dimensions, and coating thickness all affect the battery's quality, capacity, and consistency, and may also pose safety risks. Additionally, it is essential to ensure that no particles, debris, or dust are introduced during the coating process to avoid potential safety hazards.

3. Precision Rolling to Form the Core of the Battery – Rolling and Slitting

Rolling and slitting play crucial roles in the manufacturing process of lithium batteries. The core principle of these processes involves applying uniform and controllable pressure to the electrode materials through a precisely designed rolling system. This ensures that the active materials, conductive agents, and other components are evenly distributed and tightly compressed onto the current collector (such as aluminum or copper foil). Simultaneously, the slitting device precisely cuts the electrodes into pre-determined dimensions, ensuring consistency in size and stable quality for each electrode unit.

During the rolling process, precisely controlling the rolling pressure is key to ensuring electrode quality. The quality of the slitting blades directly affects the slitting outcome and the edge quality of the electrodes. The rolling and slitting process generates a small amount of dust and impurities. If these particles are introduced into the electrode materials, they may lead to internal short circuits in the battery, significantly affecting the battery's safety and performance. Therefore, it is essential to equip the equipment with efficient dust extraction systems and implement strict cleaning procedures to maintain a controlled production environment, ensuring the cleanliness of the electrode materials.

4. Formation of the Battery "Embryo" — Winding and Stacking Processes

The winding process involves spirally rolling the positive and negative electrode sheets along with the separator film in a specific order, forming the basic structure of the bare cell under precise control of tension, speed, and other parameters. The advantage of this process is that it creates a continuous current path, which facilitates ion transport and allows for the use of material anisotropy to increase energy density. However, challenges include the potential for edge stress concentration, which may affect cycle life, and uneven winding that can impact performance consistency and safety.

The stacking process alternates the positive and negative electrode sheets with separator films, stacking them layer by layer with high precision to ensure accurate alignment and tight bonding of each layer. The advantage of stacking is that it provides long cycle life and flexibility in shaping and sizing, but it faces challenges in lower production efficiency and multiple tab welding points. Both methods are crucial for forming the core structure of the cell and play key roles in battery performance. Each method has its own characteristics and applications in battery production, with both facing opportunities for technical improvements and optimization to better meet the performance demands of various applications.

Winding Process: In the production chain of lithium-ion batteries, the winding process acts like a skilled artisan, taking on the critical task of cleverly combining the positive electrode, negative electrode, and separator film to shape the embryo of the cell. This step is fundamental in the birth of the battery cell, as it carefully rolls the prepared electrodes and separators in a specific order and manner, essentially weaving a precise "energy cocoon" that sets the foundation for the cell’s charging and discharging process.

Winding equipment utilizes advanced automation technology to precisely control the speed, tension, and alignment during the winding process. Equipped with CCD vision inspection systems, the equipment can monitor key parameters in real time, such as the position of the electrodes, the coverage of the separator, and the diameter of the wound roll.

5. Evacuating Moisture and Injecting Vitality — Baking and Electrolyte Injection Processes

The baking process aims to remove moisture from the battery cells. Using high-temperature ovens, this process precisely controls temperature, time, humidity, and airflow to expel moisture, preventing it from reacting with the electrolyte and affecting the battery's performance. This step creates a stable environment for subsequent processes, reducing self-discharge rates and extending cycle life.

The electrolyte injection process is carried out in a cleanroom environment, where a high-precision metering system controls the amount and speed of electrolyte injection. The organic solvent, containing lithium salts, is injected into the battery cell, turning it into an active unit capable of storing and releasing energy. The electrolyte acts as the medium for ion transport, and its composition significantly influences the electrochemical performance of the battery. After injection, it reacts with the electrode materials to optimize the solid electrolyte interphase (SEI) film, enhancing the battery's performance.

Baking and electrolyte injection work synergistically to ensure the battery's high performance, long lifespan, and safety. These processes are crucial steps in ensuring the overall quality of the battery production.

6. Awakening the Potential of the Battery Cell, Starting the Energy Journey — Formation and Capacity Sorting Processes

Formation Process: Activating the Battery Cell and Building a Stable Interface

·Core Objective of Formation The formation process is a critical step in lithium-ion battery production, with its primary goal being to activate the battery cell through the first charge-discharge cycle. During this cycle, a solid electrolyte interphase (SEI) film forms on the negative electrode surface. This SEI film plays a crucial role as a protective barrier, preventing further reactions between the electrolyte and the negative electrode material, thus stabilizing the electrochemical performance of the electrodes. Additionally, the SEI film provides a stable channel for lithium ions to insert and extract, ensuring that the battery operates efficiently and stably during subsequent charge-discharge cycles.

·Implementation Process and Key Points of Formation The formation process is typically carried out in dedicated formation equipment, which can precisely control parameters such as charging current, voltage, and time. During the initial stage of formation, a small current is used to charge the battery cell slowly, allowing lithium ions to gradually move from the positive electrode to the negative electrode. In this process, solvents and lithium salts from the electrolyte undergo a reduction reaction on the negative electrode surface, gradually forming the SEI film. The charging voltage must be strictly controlled to avoid overcharging, which could damage the internal structure of the battery or cause safety issues. For example, for typical lithium-ion batteries, the charging voltage limit is usually controlled between 4.2V to 4.35V.

Once charging is complete, the battery cell undergoes a discharging process, with both the discharge current and cutoff voltage being precisely controlled. The discharge process helps further stabilize the structure of the SEI film and evaluates the initial performance of the cell. The entire formation process may involve multiple charge-discharge cycles to ensure that the quality and performance of the SEI film are optimal. Temperature control during formation is also crucial, typically maintained between 25°C and 45°C, as the appropriate temperature helps facilitate the chemical reactions and improves the quality and uniformity of the SEI film.

·Impact of Formation on Battery Cell Performance A high-quality SEI film is essential for the battery cell's high performance and long lifespan. It reduces irreversible capacity loss during charge-discharge cycles and improves the initial Coulombic efficiency of the battery. For example, a well-formed cell may achieve an initial Coulombic efficiency of over 80%, while a cell with poor formation or a poorly formed SEI film may have an initial Coulombic efficiency of only 70% or even lower. Furthermore, a stable SEI film reduces self-discharge during cycling, extending the shelf life of the battery and ensuring stable performance during both storage and use.

Capacity Sorting Process: Screening and Classification to Ensure Consistent Performance

·Importance of Capacity Sorting The capacity sorting process takes place after the formation process and involves sorting cells based on their capacity. Due to process variations during production, even under the same conditions, each cell may have slight differences in actual capacity. The purpose of capacity sorting is to accurately measure each cell's capacity and group cells with similar performance. This step is crucial for battery pack assembly because in a battery pack, multiple cells must work in tandem. If the capacity of the cells varies significantly, it can lead to overcharging or deep discharging of individual cells during charge-discharge cycles, accelerating the degradation of the entire pack, reducing its lifespan, and potentially causing safety issues.

·Operation and Equipment Characteristics of Capacity Sorting Capacity sorting equipment typically features high-precision measurement systems capable of accurately measuring key parameters such as capacity, voltage, and internal resistance of the cells during charge-discharge cycles. During the sorting process, each cell is connected to the sorting equipment and charged and discharged according to a predefined program. Typically, the cells are first charged to full capacity, and then discharged at a constant current, with the discharge capacity measured. Based on the measured data, the sorting equipment automatically classifies the cells into different groups. For example, in electric vehicle batteries, cells with capacity differences within ±2% may be grouped into the same level.

Capacity sorting equipment is designed to handle large volumes, enabling the simultaneous processing of multiple cells to improve production efficiency. To ensure measurement accuracy and reliability, regular calibration and maintenance of the sorting equipment are essential. Additionally, environmental factors such as temperature and humidity must be strictly controlled during operation, as these factors can affect the measurement parameters of the cells.

·Key Role of Capacity Sorting in Battery Pack Performance Capacity sorting is critical to ensuring the overall performance of a battery pack. Cells with good consistency, as identified during the sorting process, can work more efficiently when assembled into a battery pack. During charge-discharge cycles, the voltage and current of each cell in the pack remain more synchronized, reducing internal imbalance. This not only improves the overall capacity and energy efficiency of the battery pack but also extends its cycle life. For example, in some high-end electric vehicle battery packs, those assembled with cells that have undergone rigorous capacity sorting can achieve cycle life exceeding 2000 cycles. In contrast, packs assembled with cells that have not been sorted or have undergone inadequate sorting may only last fewer than 1500 cycles.

Moreover, capacity sorting helps reduce the complexity of thermal management within the battery pack during use, improving the pack's safety and reliability.

Additional Processes in Formation and Capacity Sorting The formation and capacity sorting processes also include secondary electrolyte injection after cell activation, weighing, liquid injection port welding, and leak testing. Additional tests such as self-discharge tests, high-temperature aging, storage, and further sorting are carried out to ensure optimal product performance.

Each battery cell has a separate QR code after it is manufactured, which records the date of birth, manufacturing environment, performance parameters, etc. The powerful traceability system can record any information. If an abnormality occurs, production information can be retrieved at any time; at the same time, these big data can provide data support for subsequent improved designs.

A single battery cell cannot be used. Only when many batteries are combined together, and a protective circuit and a protective shell are added, can they be used directly. This is the so-called battery module.