Roll Pressing Process of Lithium-ion Battery Electrodes

【Overview】：
The rolling of battery electrodes involves the friction between the rollers and the electrode, which draws the electrode sheet into the gap between the rotating rollers. During this process, the electrode sheet undergoes compression and deformation. Unlike the rolling of steel, where the rolling process mainly causes the material to extend longitudinally and widen laterally without a change in density, the rolling of battery electrodes focuses on the compaction of active material on both the positive and negative electrode sheets. During this process, the particles undergo migration, rearrangement, and embedding, resulting in reduced voids between particles and a denser arrangement. The purpose is to increase the compaction density of the positive or negative electrode materials. Proper compaction density can increase battery discharge capacity, reduce internal resistance, minimize polarization loss, extend cycle life, and improve the utilization rate of lithium-ion batteries.

Roll Pressing Process of Lithium-ion Battery Electrodes

【Overview】：

The rolling of battery electrodes involves the friction between the rollers and the electrode, which draws the electrode sheet into the gap between the rotating rollers. During this process, the electrode sheet undergoes compression and deformation. Unlike the rolling of steel, where the rolling process mainly causes the material to extend longitudinally and widen laterally without a change in density, the rolling of battery electrodes focuses on the compaction of active material on both the positive and negative electrode sheets. During this process, the particles undergo migration, rearrangement, and embedding, resulting in reduced voids between particles and a denser arrangement. The purpose is to increase the compaction density of the positive or negative electrode materials. Proper compaction density can increase battery discharge capacity, reduce internal resistance, minimize polarization loss, extend cycle life, and improve the utilization rate of lithium-ion batteries.

Battery Cell Section: Introduction to the Manufacturing Process of Lithium-ion Battery Pouch Cells

Introduction

Soft-pack battery cells, referring to cells using aluminum-plastic composite film as packaging material, have seen widespread application in the lithium-ion battery field due to their high energy density, enhanced safety, and flexible design. Below, I will provide a detailed introduction to the manufacturing process of soft-pack battery cells, covering material preparation, critical steps, and post-processing aspects.

Presentation of Wanhua Chemical: Comprehensive Solutions for Lithium Battery Anode Binders

Overview of Lithium - Ion Battery Binders 

Binders, often grouped with conductive agents and additives as "auxiliary materials" in lithium - ion battery production, are crucial and indispensable materials. Their main function is to adhere active materials and conductive agents to current collectors, ensuring electrical contact between powder material particles in the electrode and between the powder materials and the current collector. Binders have a low mass fraction in the electrode and do not participate in electrochemical reactions. Their primary role is to attach active materials and conductive agents to the current collector, maintaining the integrity of the electrode. Binders influence the formation of the solid electrolyte interface (SEI), charge transfer within the electrode and at the electrode - electrolyte interface, the wetting behavior of the electrode, as well as the battery's cycling performance and cost. Therefore, an ideal binder can guarantee the stability of the electrode structure with the minimum possible usage amount. 

Basic Characteristics and Mechanisms of Binders 

1. Working Mechanism

The essence of the adhesive force of binders lies in the intermolecular forces (such as van der Waals forces and surface tension), chemical bond forces (such as hydrogen bonds, covalent bonds, and coordination bonds), and interfacial electrostatic attraction. The following are three theories of binder adhesion, as shown in the schematic diagram:

 (1) Diffusion Theory Polymer binders penetrate the surface of electrode materials and diffuse into the complex voids within the active materials through molecular Brownian motion. This diffusion occurs at the interface between the electrode material surface and the binder, resulting in a firm bonding effect. 

(2) Electrostatic Interaction Theory When the binder and the adhered material system form an electron - acceptor/donor combination, a double - layer can be formed at the interface between the two, generating electrostatic attraction.

 (3) Adsorption Theory After the binder diffuses to the electrode surface, an adsorption force is generated between the molecules of the binder and the adhered material. Non - reactive binders mainly rely on intermolecular interactions (van der Waals forces), while reactive binders mainly rely on hydrogen bonds, covalent bonds, coordination bonds, etc. to form interfacial forces.

2: Failure Mechanisms 

The destruction or failure of the binder structure can disrupt the paths for ions and electrons and cause the loss of active materials, resulting in capacity fade and safety hazards. The failure mechanisms can generally be attributed to three causes: 

(1) Contact Interface Failure

Insufficient adhesive force of the binder prevents it from effectively bonding with the adhered materials, leading to electrode delamination. 

(2) Binder Cracking 

During battery cycling, the binder is affected by strain and stress changes. When the stress exceeds the yield strength, the polymer undergoes plastic deformation, and the strain cannot fully recover after the stress is removed. When the stress exceeds the ultimate strength, the polymer fractures and fails. 

(3) Adherend Fracture

 Even if the adhesive force and mechanical strength of the binder meet the requirements, during battery cycling, the cracking and detachment of electrode materials can also lead to a decrease in battery capacity.

 In recent years, industry professionals have made significant progress in exploring the working mechanisms and failure mechanisms of binders. However, the structure - property relationships of binders are extremely complex, and there are still challenges in establishing an effective correlation between binder structure and performance. Additionally, due to the small proportion of binders in electrodes, characterization is difficult, and the failure mechanisms in practical applications remain unclear. More sophisticated characterization techniques (such as in - situ characterization) and theoretical simulation calculations are needed to further explore the failure mechanisms of binders. 

3: Performance Requirements 

Generally speaking, an ideal binder should have six characteristics:

(1) Stable thermal performance to maintain strong bonding force over a wide operating temperature range. 

(2) Good mechanical properties, including tensile strength, elasticity, flexibility, hardness, and adhesive strength, to withstand the large volume or strain changes in some special systems, such as silicon (Si) anode batteries. 

(3) Good electrical conductivity and ionic conductivity to ensure excellent electrochemical performance. 

(4) Excellent dispersibility in solvents to cover and connect the components of the electrode and prevent non - uniform aggregation of the slurry. 

(5) Excellent chemical and electrochemical stability to meet the application requirements of different chemical solvents and different voltage windows. 

(6) Low cost and environmental friendliness for easy mass production.

Current Status and Research Practices of Lithium - Battery Anode Binders 

In lithium - ion batteries, anode adhesives are similar to those for cathodes. They mainly include oil - based PVDF, and water - based CMC, PAMAC, polyvinyl alcohol (PVA), sodium alginate, etc. 

1. Adhesives for Graphite Anodes 

CMC - based adhesives have currently become the main adhesives for graphite anode materials. Jernei D et al. studied the bonding mechanism of CMC - based adhesives and the failure mechanisms of graphite anodes with different adhesive contents. Four CMC - based adhesives were used, namely methyl cellulose (MC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), and carboxymethyl cellulose (CMC). 

When the mass fractions of the four adhesives in the electrode were 1% and 2% respectively, the test results showed that, compared with the other three adhesives, CMC had the smallest irreversible capacity at both 1% and 2% mass fractions, with a critical mass fraction of 2%. When the CMC mass fraction was 0.25%, when the anode was fully charged, there were brown spots on the surface of the golden - yellow anode (see Figure 3).

Experimental results show that the electrode material at the brown - spotted area is not in a fully - charged state (as indicated by the presence of a LiC₁₂ diffraction peak in the X - ray diffraction spectrum). The reason is that during the electrode charging process, the graphite material on the electrode surface is charged first. Due to the low adhesive content in the electrode, along with the volume expansion of the graphite anode during charging, some of the materials on the electrode surface detach from the main body of the electrode, resulting in a decrease in the reversible capacity of the electrode. The mechanism is that the adhesive forms its own network structure, distributed among the electrode material particles, and the adhesive plays a spatial barrier role. To achieve the bonding effect, a sufficient amount of adhesive is required. 

By using gelatin (Fluka. No48722) and CMC as adhesives respectively with the same graphite material, a comparison shows that to achieve an acceptable specific capacity value for the electrode material, the critical mass fractions of the two adhesives are 0.25% for gelatin and 2.00% - 5.00% for CMC. 

2. Adhesives for Silicon - Based Anodes

 Silicon anode materials are one of the current research focuses in lithium - ion battery materials. Silicon has a high theoretical capacity of up to 4200 mA·h/g and a low lithiation - delithiation potential (0 - 0.4V vs. Li/Li+). Compared with other metal and alloy anode materials, it has a relatively low first - cycle irreversible capacity. Moreover, it is abundant in nature and has a low raw material cost, making it a promising lithiated anode material. 

However, during the lithiation - delithiation process, pure silicon materials have a high expansion - contraction ratio (with an expansion rate of up to 400%), resulting in structural pulverization and poor contact between the active material and the current collector. After several cycles, the electrode capacity decays significantly. 

Currently, to improve the electrochemical performance of silicon - based materials, the following methods are mainly adopted: 

(1) Controlling the electrode porosity. 

(2) Effectively distributing silicon materials in the electrode. 

(3) Developing functional binders for high - specific - energy batteries. 

Currently, the lithium - battery anode binder industry is facing a series of technological innovation challenges, such as how to improve adhesion, reduce production costs, and enhance battery reliability and cycling performance. With the continuous optimization and iteration of technology, the industrialization development of fluorine - free binders, the complexity, cost, and environmental friendliness of PAA and PTFE modification processes are also important considerations and bottlenecks to be broken through in future research.

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Analysis of manufacturing processes of lithium batteries with different packaging methods: square, cylindrical, and soft pack

Automotive Technician

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In the new energy era, lithium batteries are the core power and energy storage unit, and their importance is self-evident. Among the many characteristics of lithium batteries, the external form of packaging shape actually contains complex technical considerations and process logic. The three mainstream packaging shapes of square, cylindrical, and soft pack each correspond to a unique process, like three keys, opening the door to different application scenarios. This article will deeply analyze the technical routes and process secrets behind these three lithium battery packaging shapes.

Comprehensive Solution for Negative Electrode Binders for Lithium Batteries                                                                                                                                                                                            

Overview of Lithium-ion Battery Binders

In the production of lithium-ion batteries, binders are often referred to as "auxiliary materials" together with conductive agents, additives, etc., but they are an indispensable key material. Their main function is to adhere active substances and conductive agents to the current collector to ensure electrical contact between powder material particles in the electrode and between the powder material and the current collector. Binders have a low mass ratio in the electrode and do not participate in electrochemical reactions. Their main function is to adhere active substances and conductive agents to the current collector to keep the electrode intact. Binders affect the formation of the solid electrolyte interface (SEI), charge transfer inside the electrode and between the electrode-electrolyte interface, the wetting behavior of the electrode, and the cycle performance and cost of the battery. Therefore, an ideal binder can ensure the stability of the electrode structure with the least possible usage.

PVDF, CMC, PAA, etc.! Research on Lithium Battery Binders

LIBs electrodes are mainly composed of electrochemically active electrode materials, conductive additives, binders, current collectors, etc. Among them, binders are an important component of LIBs electrodes. Binders can tightly attach active substances and conductive agents to the current collector to form a complete electrode, prevent the active substances from falling off and peeling off during the charge and discharge process , and can evenly disperse the active substances and conductive agents, thereby forming a good electron and ion transmission network and realizing efficient transmission of electrons and lithium ions.