In the development process of advanced functional binders, there are many optimization directions for binders, such as polarity, electrochemical stability, mechanical properties, and the interface constructed between the binder network and other components. At the same time, each cathode material has its own characteristics, and the design of the binder should conform to the characteristics of each specific type of positive electrode material.

The Importance of Binders

Binders are one of the essential components in lithium-ion battery electrodes. Their main role is to adhere the active materials and conductive agents in the electrode sheet to the current collector, while maintaining a close solid-solid interface contact between the active materials, conductive carbon, and current collector, thus stabilizing the structure of the electrode sheet. They are high-tech additional materials in lithium-ion battery materials.

Research shows that although the amount of binder used in the electrode sheet is small, the performance of the binder directly affects the battery's capacity, lifespan, and safety. It must be evenly distributed among various solid particles in the entire electrode. If the contact between the network formed by the polymer chains of the binder and different solid particles is unbalanced, it will significantly affect the ion and electron transport within the electrode.

Limitations of Polyvinylidene Fluoride

Polyvinylidene fluoride (PVDF) has been widely used as a non-aqueous binder in commercial battery cathodes due to its wide electrochemical window and high stability. However, it has certain limitations when applied in high-voltage TMOC (transition metal oxide cathode material) systems.

1. Weak Binding Ability

As a linear polymer, PVDF's structure limits it to relying on weak van der Waals forces and molecular chain entanglement to fix various components in the cathode electrode. Therefore, PVDF has poor binding ability in the electrode, and the mechanical properties of the PVDF binder, its adhesion to the current collector, and the cohesion of the electrode film are not high, making it unable to avoid intragranular cracks caused by high internal mechanical stress. Additionally, it is prone to swelling in the electrolyte under high-temperature conditions, leading to poorer adhesion of active materials to the current collector.

2. Gelation

During the slurry preparation process of PVDF binders in high-voltage TMOC systems, due to the alkaline environment, PVDF releases HF to form C-C bonds. The formation of C-C bonds between the binder molecular chains can lead to the gelation of PVDF, which poses significant difficulties for processing.

3. Side Reactions

PVDF binders can produce trace amounts of HF at high temperatures, which will irreversibly corrode TMOC, further leading to the fragmentation of active particles and accelerating the leakage of transition metal ions from the active material into the electrolyte, making the interface more unstable and causing rapid capacity decay of the battery.

4. Difficulty in Modification

Considering the important role of binders in stabilizing interfaces and reducing side reactions, there is a desire to modify and functionalize PVDF cathode binders to meet the stringent requirements of advanced lithium-ion batteries. However, the copolymer modification of vinylidene fluoride (VDF) is quite challenging, with certain technical barriers in selecting suitable copolymer monomers and their proportions. Additionally, due to the significant difference in the copolymerization rate between VDF and olefin monomers, only a small proportion of olefin monomers can be copolymerized with VDF (generally not exceeding 5%), making it difficult to achieve significant performance improvements.

Exploration of New Binders

With the continuous development of the lithium-ion battery industry, the performance requirements for binders are also increasing. Traditional PVDF binders cannot meet the development needs of the lithium-ion battery industry. New binder materials have become one of the current research hotspots. Theoretically, by precisely controlling the types and proportions of various monomers in the binder, the polarity and functionality of the binder can be flexibly adjusted to meet the needs of different positive electrode materials and electrolytes. This method can adapt to a wide range of materials and achieve various functions that PVDF cannot.

Currently, new oil-based positive electrode binders are mainly divided into four categories: polyimide, polyacrylonitrile, polyacrylic acid, and conductive polymers.

1. Polyimide (PI)

Polyimide (PI) is a type of macromolecule that has an imide ring structure in its molecular framework. Its structure is diverse, and it can be designed to have ideal structures and properties by changing the types of diamines and dianhydrides. With excellent mechanical properties, heat resistance, and chemical stability, PI materials are receiving increasing research attention.

2. Polyacrylonitrile (PAN)

Polyacrylonitrile (PAN) exhibits a wide electrochemical window and high melting point. At the same time, its surface ligand bonding reduces the irreversible migration of TM ions, improving cycling stability. The -C≡N of PAN interacts with unstable TM ions through coordination bonds. These interactions suppress the irreversible migration of TM ions between layers by increasing the energy barrier, enhancing cycling stability.

3. Polyacrylic Acid (PAA)

Copolymers based on acrylic acid (AA) can prevent the diffusion concentration gradient during the lithiation process and promote the transport of Li+ from the electrolyte to the center of the active particles. They also exhibit high adhesion, low resistance, low polarization, and long-term cycling stability.

4. Conductive Polymers

Conductive binders that are electronic or mixed (ionic and electronic) conductors are an important direction in the development of new binders. When used in cathodes, they can provide effective channels for electron and ion transport, thereby improving the conductivity and charge-discharge efficiency of the battery. However, the synthesis and preparation process of conductive polymers is more complex than that of general binders, requiring further exploration.

5. Composite Binders

If a binder has specific disadvantages when used alone, such as high cost, polarity imbalance, or unsatisfactory mechanical properties, it can be chosen to use that binder in combination with another complementary binder. By selecting suitable matches or combinations, two or more binders can be uniformly mixed in different proportions to flexibly adjust the mechanical and electrical properties of the mixture.

In response to the performance bottlenecks of existing oil-based binders and the rapid development of future technologies, High Rui Power has now formed a professional team to continuously increase research and development investment in the exploration and innovation of new lithium battery positive electrode oil-based binders, and is committed to developing new positive electrode binders.

Through in-depth basic research and application development, we continuously explore new materials and new processes; precise control is conducted from three aspects: the molecular structure of the binder, cross-linking methods, and functional properties, with the hope of achieving breakthroughs in bonding strength, ionic conductivity, thermal stability, environmental adaptability, and functionality.

In the future, High Rui Power will provide customized solutions for new lithium battery positive electrode oil-based binders to meet the diverse needs of the market, targeting different application scenarios (such as electric vehicles, energy storage stations, portable devices, etc.) and their varying requirements for battery performance.