Single-sided pole piece production

Research and application of new binders in lithium batteries

I. Introduction

In the electrode materials of lithium batteries, the proportion of binder is usually between 1% and 10%. Its main function is to bind the active materials, current collectors and conductive agents of the electrode together, thereby enhancing the performance and stability of the electrode. The active materials and conductive agents in the electrode are often nano-scale, and these nano-materials are prone to agglomeration in high-concentration electrode slurries.

In order to improve the dispersibility of these nanopowders in the slurry, the use of chemical dispersants becomes particularly important. Therefore, the research and development and application of new binders and dispersants are gradually becoming a hot topic in the industry.

The application of polymer materials in LIBs faces two problems: 1) Traditional polymers have exposed some drawbacks during use and need to be replaced by new high-performance polymers. For example, traditional polyolefin separators cannot tolerate high temperatures and urgently need to use new high-performance separators. 2) Use new polymer components to improve the performance of lithium-ion batteries. An obvious example is the use of functional polymer coatings to protect key components from the negative effects of high voltage or electrolytes. In addition, new solid polymer electrolytes or polymer active storage materials are also important development directions. In short, new polymer materials are being used more and more widely in LIBs.

2. Functions and characteristics of binders

The main functions of binders in electrode materials include homogenization, stabilizing structure and improving performance.

1.  Homogenization

During the electrode manufacturing process, the binder is first dissolved in a suitable solvent to form a gel-like substance, which is then mixed with the conductive agent and active material and homogenized by ball milling. This process ensures the uniform distribution and stability of the materials.

2.  Stable structure

The volume of the electrodes of lithium batteries changes during charging and discharging. Binders can act as a buffer in this process, preventing the coating containing active substances from falling off or cracking.

3.  Improve performance

Binders improve the overall performance of the battery by reducing the impedance of the electrode.

Key properties of efficient binders

In order to meet the above requirements, an efficient adhesive must have the following characteristics:

1. Stability

In a specific electrode/electrolyte system, the binder should have good stability, be able to withstand the corrosion of the electrolyte, and not undergo redox reactions within the operating voltage range.

2. Solubility

The binder should have a high dissolution rate and solubility in the solvent, and the solvent used needs to be safe, environmentally friendly and non-toxic.

3. Moderate viscosity

The binder should have moderate viscosity to facilitate homogenization and maintain the stability of the slurry. At the same time, its bonding ability should be strong, but the amount should be as small as possible.

4. Good flexibility

The binder needs to be able to withstand the volume changes of the active material particles during the operation of the electrode to ensure the stability of the electrode structure.

3. Classification and application status of mainstream adhesives

After clarifying the role and performance requirements of the adhesive, choosing the right material becomes the key. According to the type of solvent, adhesives can generally be divided into two categories: oil-based adhesives based on organic solvents and water-based adhesives based on deionized water.

1. Application status of new oil-based adhesives

Polyvinylidene fluoride (PVDF) is a traditional oily binder widely used in industry. However, it has a weak interaction with conductive agents and may slowly swell in the electrolyte, thus affecting the ion transport capacity. More importantly, the large amount of PVDF used reduces the overall energy density of the battery. Therefore, it is particularly important to develop new oily binders to solve these problems.

(1) The rise of polyimide

Polyimide (PI) is a material that shows broad prospects in lithium battery applications. It has excellent processing performance, mechanical strength, thermal stability and chemical stability. Studies have shown that when PI is used as a binder in silicon negative electrodes, the battery's capacity retention rate is still not less than 75% after hundreds of charge and discharge cycles, which is about twice that of traditional PVDF.

In addition, fluorinated polyimide (FPI) exhibited excellent stability in electrode materials under high temperature and high pressure conditions, which marked an important technological advancement of PI binders and opened up a new development direction for electrolyte-free systems under high temperature and high pressure environments.

Polyimides (PIs) stand out among many polymers and can be used as coatings, adhesives, diaphragms, solid electrolytes, active storage materials, etc. Although PIs are widely used in LIBs, PIs are still limited to laboratory research. PIs are synthesized by condensation of dianhydrides and diamines and were introduced in the 1955s. At present, there are two methods for synthesizing PIs: 1) hydrothermal method (one-step method); 2) thermal imide or chemical imide method (two-step method). The hydrothermal method refers to the direct polymerization of monomers in a high-boiling point solvent, and the high temperature and high pressure reaction conditions are the main safety considerations. The chemical imide method refers to the dehydration and cyclization polymerization of the PI precursor (polyamic acid, PAA) under the action of a catalyst, and the removal of the catalyst is the main purity consideration. Compared with these two methods, the thermal imide method refers to the cyclization of PAA at high temperature without the introduction of other substances. It has been proven to be an economical and convenient method. PIs can be divided into two categories: aliphatic and aromatic. Generally, aliphatic PIs are flexible and soluble, and are mainly used as coatings and adhesives. Aromatic PIs are rigid and insoluble and are mainly used as films or solid powders. The following table lists the physicochemical properties required of PIs in various applications of LIBs.

PI improves the integrity of the electrode structure

Silicon materials have the following disadvantages as negative electrodes: 1) large volume change (~300%); 2) easy to pulverize; 3) short cycle life. The current solution strategy is to use nano-Si, Si/C composite materials, and new binders. Among them, the use of new binders can effectively prevent silicon particles from pulverizing, that is, even if the amount used is small, it can greatly improve the stability of Si negative electrodes. Polyimide binders have high adhesion, good mechanical properties, excellent thermal stability and outstanding electrolyte wettability. Many research works have reported PI binders from the perspective of molecular structure design, aiming to improve their specific properties. The introduction of ether oxygen functional groups can improve the lithium ion conductivity of PI binders; the synthesis of soft and hard segment co-embedded polymers can give PI binders better elasticity; functional groups with strong adhesion can be introduced into the molecular structure of PI to enhance the bonding properties of PI binders. The figure below shows the design ideas of 4 PI binders.

Figure: a Schematic diagram of the structure of PI binder and CMC binder obtained by polymerization of PEG-200, trimellitic anhydride chloride (TMAC) and 4,4'-diaminodiphenyl ether (MDA), schematic diagram of volume expansion during the cycle, cycle performance curve and SEM photo of the electrode after cycling; b PI binder with co-embedded soft and hard segments; c Schematic diagram of the peeling effect of P84 vs PVDF binder; d Structure, electrochemical properties and SEI formation mechanism of PI-COOH binder.

PI binders such as 3D cross-linked network structures can maintain the integrity of the silicon anode structure. Introducing hydrogen bonds or metal-ligand coordination into the molecular structure of the binder provides a way to enhance the adhesion. In addition to these traditional methods, multifunctionalization of PI binders is also a promising research direction.

(2) Modification based on PVDF

Common modification methods include copolymerization and blending. Studies have shown that the copolymer formed by copolymerizing polyvinylidene fluoride (PVDF) with polytetrafluoroethylene (TFE) and polypropylene (PP) has significantly improved elongation at break (i.e., degree of elasticity) compared to pure PVDF.

Specifically, the elongation at break of this copolymer reached 100%, while that of pure PVDF was less than 10%. This shows that the copolymer can better maintain elasticity during the charge and discharge process of the electrode material, maintain the aggregation state of the electrode material, and ensure effective electron transfer between the active material and the current collector. Therefore, this copolymer can be used as an alternative to PVDF in application scenarios that require large volume changes.

2. Application status of new water-based adhesives

Water-based binders are favored for their environmentally friendly properties, but despite their excellent performance, they still face some challenges. For example, water-based binders may lead to poor slurry dispersion and easy agglomeration. In addition, factors such as poor wettability of water-based binders to the substrate and the higher heat capacity of water also limit their application to a certain extent.

(1) Application of medium molecular weight polyacrylic acid

The study found that polyacrylic acid (PAA) with a molecular weight in the range of 10,000 to 1,000,000 has significant advantages as a water-based binder. PAA can effectively promote the contact between lithium ions and active substances and reduce the side reactions between the electrolyte and the active substances, thereby improving the overall cycle efficiency. At the same time, PAA can form a mesh structure on the electrode surface, allowing lithium ions to pass quickly, thereby improving the ion transmission efficiency and improving the battery's rate performance.

(a) PVDF’s barrier to lithium ion flow and its effect on the structure of active materials

Although polyvinylidene fluoride (PVDF) is widely used as a traditional oily binder in lithium batteries, it has a certain hindering effect on the flow of lithium ions. In addition, PVDF may cause changes in the structure of active materials, thus affecting the overall performance of the battery.

(b) The mesh structure formed by PAA promotes lithium ion transport

In contrast, polyacrylic acid (PAA) as a water-based binder forms a mesh structure in the electrode that can effectively promote the transmission of lithium ions. This structure not only increases the speed at which ions pass through the electrode, but also improves the rate performance of the battery.

The function of polyacrylic acid (PAA) is closely related to its molecular weight. As mentioned above, medium molecular weight PAA can be used as a binder, while low molecular weight PAA with a molecular weight of less than 10,000 is an excellent dispersant. On this basis, some modified polymers also show excellent dispersing properties.

For example, sodium polyacrylate (PAANa) is the ionic form of PAA and is highly water-soluble. PAANa can effectively adsorb on the surface of powder particles, reduce the surface energy of the particles, and achieve dispersion between particles through the charge repulsion on the surface of the particles. In addition, PAANa further improves the dispersion performance of particles by reducing the surface tension of the medium.

3. Combined application of traditional water-based binders

The combination of sodium carboxymethyl cellulose (CMC-Na) and sodium alginate (SA) has been shown to significantly improve electrode performance. Test results show that the capacity of the electrode with CMC-Na is still higher than that of the electrode using PVDF after 80 cycles under 0.1C charge and discharge conditions.

At the same time, both CMC-Na and SA can provide abundant sodium ions. Since the radius of sodium ions is larger than that of lithium ions, these sodium ions can be inserted into the vacancies formed by the migration of lithium ions during the discharge process, thereby helping to stabilize the crystal structure.

-End-

Canrd Brief Introduce

Canrd use high battery R&D technology(core members are from CATL) and strong Chinese supply chain to help many foreign companies with fast R&D. We provide lab materials,electrodes, custom dry cells, material evaluation, perfomance and test, coin/pouch/cylindrical cell equipment line, and other R&D services.

Email:janice@canrd.com    

Phone/Wechat/WhatsApp/Skype:+86 18928276992

Website : www.canrud.com