Lithium Battery Binders: Key Auxiliary Materials, Types, Performance Requirements and Future Trends

 

Binders are one of the essential auxiliary materials for lithium battery manufacturing, accounting for less than 1% of the total battery manufacturing cost but capable of improving battery performance by 5% to 10%. Although the dosage of binders in lithium batteries is relatively small, usually 2% to 5% of the auxiliary materials, their core function is to bond electrode active materials, conductive agents, and electrode current collectors into an integrated structure, ensuring overall connectivity between these components and thereby reducing electrode impedance. As the lithium battery industry moves towards high energy density, the material systems of positive and negative electrode materials, diaphragms, and electrolytes are constantly upgrading, and lithium battery binders, as crucial auxiliary materials, are also undergoing continuous performance improvements to meet the growing demand for battery performance and safety. This article focuses on the types, performance requirements, application characteristics, and future development trends of lithium battery binders, with a focus on key binders such as PVDF, CMC, and SBR, and elaborates on their role in electrode adhesion and battery performance optimization.

 

 

 

Sodium carboxymethyl cellulose (CMC) is a widely studied cellulose binder and a carboxymethylated derivative of natural cellulose. As an ionic chain polymer water-based binder, CMC forms a transparent viscous glue after swelling in water, with the advantages of being difficult to ferment, good stability, low cost, safety, and environmental friendliness. Compared with other cellulose derivatives such as methyl cellulose (MC) and ethyl cellulose (EC), CMC exhibits superior performance when used as a binder for lithium battery electrodes, especially in improving electrode adhesion and electrochemical performance. When CMC is used as a binder for graphite negative electrodes, the graphite negative electrode shows better electrochemical performance, with the first reversible capacity reaching 360 mAh/g, which is significantly higher than that of electrodes using MC or EC as binders. Lux et al. compared the performance of CMC and PVDF as binders for lithium iron phosphate positive electrodes and found that the use of CMC is conducive to optimizing the pole piece preparation process, improving the uniformity of the pole piece, and increasing the pole piece's tap density, thereby enhancing the overall performance of the positive electrode.

 

At the same time, due to its excellent electrochemical stability, CMC can be applied to high-voltage positive electrode material systems, which is crucial for the development of high-energy-density lithium batteries. Li et al. applied CMC to the Li[Li0.2Mn0.56Ni0.16Co0.08]O2 4.8V positive electrode system, and the results showed that at a rate of 1C, the first reversible capacity of the electrode was 205 mAh/g, and after 200 cycles, it still maintained a reversible specific capacity of 169.5 mAh/g, which was better than the electrode using PVDF as a binder. This indicates that CMC has better compatibility with high-voltage positive electrode materials and can effectively improve the cycle stability of high-voltage batteries. In addition, CMC is also an excellent binder for high-capacity silicon negative electrode materials, which are widely used in high-energy-density lithium batteries but suffer from severe volume expansion during charge and discharge.

 

The carboxyl functional groups in CMC can form hydrogen bonds or covalent bonds with silicon oxide (SiOx) and silanol (-Si-OH) groups on the silicon surface, which can significantly enhance the bonding force between silicon particles and the current collector, as well as between silicon particles themselves, thereby improving electrode adhesion. As a polymer, CMC can also form a coating similar to the SEI film on the surface of silicon particles, which can inhibit the decomposition of the electrolyte on the surface of the silicon negative electrode and reduce the side reactions between the silicon negative electrode and the electrolyte, thereby improving the cycle life of the silicon negative electrode. However, CMC has inherent defects of poor flexibility and high brittleness, which can easily lead to electrode cracking and active material shedding during the volume expansion and contraction of silicon negative electrodes. To solve this problem, CMC is usually blended with high-elastic polymers such as styrene-butadiene rubber (SBR) to improve the flexibility and toughness of the electrode, ensuring good electrode adhesion and structural stability during long-term charge and discharge cycles.

 

CMC-Li polymer material is a modified product prepared by replacing Na in CMC with Li . As a binder with good ionic conductivity, CMC-Li can effectively increase the number of freely moving lithium ions in the battery, reduce the diffusion distance of lithium ions to the surface of active materials, improve the efficiency of lithium extraction and insertion of positive and negative electrode materials, and thus enhance the charge and discharge capacity and cycle performance of the battery. This modification method further expands the application scope of CMC in high-performance lithium batteries, especially in batteries that require high ionic conductivity and cycle stability.

 

 

Polyacrylic acid (PAA) is a chain polymer water-based binder that has attracted extensive attention in the field of lithium battery binders due to its unique performance advantages. PAA has four key advantages when used as a lithium battery binder: first, it almost does not swell in the carbonate solvent of the electrolyte, which can ensure the stability of the electrode sheet structure during the charge and discharge process, avoiding electrode deformation and active material shedding caused by binder swelling; second, the carboxyl content in its structure is higher than that of CMC, which can form stronger hydrogen bonds with active materials containing hydroxyl groups on the surface, promoting a more uniform coating on the electrode surface than CMC, thereby improving the uniformity and stability of the electrode; third, it can form a denser film in the electrode sheet, increasing the electrical contact between the active material and the current collector, reducing electrode impedance, and improving the rate performance of the battery; fourth, it has excellent tensile mechanical strength, which is conducive to the machining of electrode sheets and ensures the structural integrity of the electrode during the preparation and use process.

 

Wening et al. compared the effects of PAA, CMC, and PVDF as binders on the performance of lithium titanate electrodes, and the study found that the lithium titanate electrode sheet using PAA showed the best uniformity and good electrochemical performance. At a 1C rate of charge and discharge, the specific capacity of the electrode reached 150 mAh/g, and even at a high rate of 16C, it still maintained a specific capacity of 130 mAh/g, which was significantly higher than that of electrodes using CMC or PVDF as binders. In addition to lithium titanate electrodes, PAA is also suitable for lithium iron phosphate positive electrodes and silicon negative electrodes, showing good versatility and compatibility.

 

The neutralization degree of -COOH in PAA greatly affects its performance as a binder. When PAA is dissolved in water, due to the strong carboxyl hydrogen bonding force between molecules, it is easy to form a molecular agglomeration structure, which affects the dispersibility of PAA in the slurry and the bonding effect of the electrode. To solve this problem, PAA is usually neutralized with alkali to prepare PAH-M salt, where M is Li, Na, K, NH4, etc. The electrostatic repulsion between carboxyl salt groups can increase the stretchability of the molecular chain, which is conducive to reducing the intermolecular agglomeration effect, improving the dispersibility of PAA in the slurry, and enhancing the electrode adhesion and electrochemical performance.

 

Han et al. used different types of PAH-M (M = Li, Na, K, NH4) as binders for silicon/graphite composite negative electrode materials and studied the effects of salt types and their neutralization degree on electrode performance. The results showed that the silicon-graphite electrode using PAH0.2Na0.8 showed the highest first coulombic efficiency (69%), the highest first reversible capacity (1400 mAh/g), and the best cycle performance. This may be because Na+ is beneficial to improving the performance of the SEI film on the surface of the material, optimizing the kinetic parameters of lithium insertion in the material, and reducing the side reactions between the electrode and the electrolyte. This study provides an important reference for the optimization of PAA-based binders and their application in high-capacity silicon-based negative electrodes.

 

 

Polyacrylonitrile (PAN) is a polymeric binder containing highly polar nitrile functional groups, which can form hydrogen bonding forces and dipole forces with surrounding materials such as active materials and current collectors. As a binder, PAN is conducive to improving the stability of the electrode sheet structure and the wettability of the electrolyte, thereby enhancing the electrochemical performance of the battery. Gong et al. compared the performance of PAN, CMC, and PVDF as binders for graphite, silicon-carbon negative electrodes, and lithium titanate electrodes, and found that the electrode sheets prepared with PAN have improved electrolyte wettability, which is conducive to the effective deintercalation of lithium ions. At the same time, the solid electrolyte phase interface impedance and charge migration resistance of the electrode are relatively small, so the battery exhibits good electrochemical performance, especially in terms of cycle stability and rate performance.

 

Yoo et al. introduced polar functional groups into polyacrylamide to prepare glyoxalated polyacrylamide, which has significant advantages when used as a lithium battery binder. First, it can undergo a cross-linking reaction to form a stable polymer structure, which can improve the structural stability of the electrode and avoid electrode deformation during charge and discharge; second, the covalent bond formed with the active substance is conducive to inhibiting swelling in polar solvents, ensuring the integrity of the electrode structure; third, the introduced polar functional groups can promote the wettability of the electrolyte, accelerating the diffusion of lithium ions and improving the rate performance of the battery. When used as a binder for silicon negative electrodes, glyoxalated polyacrylamide can effectively improve the reversibility of the battery's first lithium desorption and form a stable solid electrolyte interface film, thereby effectively improving the battery's first coulombic efficiency and cycle life.

 

 

Lithium battery binders not only need to effectively bond electrode active materials, conductive agents, and electrode current collectors to ensure the structural integrity of the electrode, but also need to have the ability to resist the influence of various external factors because they are in a very special working environment for a long time. These special environmental factors mainly include three aspects: first, the binder and the electrode material are immersed in the electrolyte for a long time, so the binder needs to be able to maintain the stability of its shape, structure, and properties in the electrolyte, without swelling, decomposition, or deterioration; second, the binder is in a high potential environment (for positive electrode binders) or a low potential environment (for negative electrode binders) for a long time, so the positive electrode binder needs to have good oxidation resistance and not be oxidized under high pressure conditions, while the negative electrode binder needs to have good reduction resistance and not be reduced under low pressure conditions; third, many lithium storage active materials will undergo continuous volume changes during battery operation, with their volume increasing with the embedding of lithium ions and decreasing with the removal of lithium ions, so the binder must have sufficient flexibility and toughness to ensure that the active material does not fall off during repeated expansion and contraction, and the bonding between the electrode particles is not destroyed.

 

Therefore, lithium battery binders usually need to meet the following key performance requirements: first, good bonding performance, high tensile strength, good flexibility, and low Young's modulus, which can effectively bond various components of the electrode and adapt to the volume change of the active material; second, good chemical stability and electrochemical stability, no reaction or deterioration during storage and charge-discharge cycles, and no adverse effects on the battery's electrochemical performance; third, no swelling or a small swelling coefficient in the electrolyte, ensuring the structural stability of the electrode sheet during long-term use; fourth, good dispersibility in the slurry medium, which is conducive to uniformly bonding the active material to the current collector and improving the uniformity of the electrode; fifth, little effect on the conduction of electrons and ions in the electrode, avoiding increasing the internal resistance of the battery; sixth, environmentally friendly, safe to use, and low cost, which is conducive to large-scale industrial production and meets the requirements of environmental protection policies.

 

 

 

During the charge and discharge process of lithium batteries, the insertion and extraction of lithium ions will cause significant volume changes in the electrode active materials, which is called the volume effect of the electrode. For example, the volume expansion rate of silicon negative electrodes can reach 300% to 400% during lithium insertion, while the volume expansion rate of graphite negative electrodes is about 10% to 15%, and the volume change of positive electrode materials such as lithium iron phosphate and ternary materials is relatively small, usually between 5% and 10%. The volume effect of the electrode will cause repeated expansion and contraction of the electrode during long-term charge and discharge cycles, which will easily lead to the cracking of the electrode sheet, the shedding of active materials, and the breakage of the conductive network, thereby reducing the cycle life and safety of the battery. Therefore, the binder plays a crucial role in alleviating the volume effect of the electrode, and its flexibility and toughness directly determine the ability of the electrode to resist volume changes.

 

 

The proportion of binder in the electrode must be an appropriate value, as too little or too much binder will have an adverse impact on the performance of the battery. If the amount of binder is too small, the bonding effect between the active material, conductive agent, and current collector will be poor, leading to problems such as electrode powdering, peeling, and active material shedding. At the same time, the poor bonding effect will also increase the ohmic resistance of the electrode, affecting the charge and discharge capacity, rate performance, and cycle performance of the battery. Although too much binder can ensure the bonding performance of the electrode and improve the structural stability of the electrode, the electrochemical inertness of the binder itself will inevitably increase the ohmic resistance of the electrode. The increase in the internal resistance of the battery will lead to a decrease in energy conversion efficiency, an increase in heat generation during charge and discharge, and thus affect the overall performance and safety of the battery. Therefore, in practical production, the dosage of the binder is usually controlled between 2% and 5%, and the optimal dosage is determined according to the type of active material, the preparation process of the electrode, and the performance requirements of the battery.

 

 

According to the different solvents used, lithium battery binders can be divided into oil-based binders and water-based binders. Of course, some binders can be dissolved in both organic solvents and deionized water, such as polyacrylic acid (PAA), which has good versatility and can be applied to different electrode preparation processes. The choice of binder type is mainly determined by the type of electrode material, the preparation process, environmental protection requirements, and cost factors, and different types of binders have their own unique advantages and application scenarios.

 

 

Oil-based binders refer to binders that use organic matter as a solvent, and the corresponding slurry formed is an oil-based slurry. The slurry formed by this system has good dispersibility, is not easy to settle, and the electrode prepared has good bonding performance, which is widely used in the early lithium battery production. At present, the most commonly used oil-based binder in industrial lithium-ion batteries is polyvinylidene fluoride (PVDF), and the oily solvent used in combination with it is N-methylpyrrolidone (NMP), which is a high-boiling point, strong polar aprotic solvent that can effectively dissolve PVDF and form a uniform slurry.

 

PVDF is a chain polymer with a molecular weight generally greater than 300,000 and is an insulator. Its bonding mechanism is to form hydrogen bonds between the F atoms on the long chain and the particles of other components in the pole piece, and the action of hydrogen bonds makes the particles of each component string together to form a stable electrode structure. PVDF has many advantages: it has a wide electrochemical stability window and stable electrochemical performance in the range of 0-5V (Li/Li ), which can adapt to the working environment of most lithium battery positive and negative electrodes; at the same time, PVDF has good antioxidant resistance and chemical reaction inertness, is not easy to deteriorate, and can maintain stable performance during long-term storage and use; in addition, PVDF has good swelling properties, and the pole piece using PVDF as a binder has good electrolyte wettability, which is conducive to the diffusion of lithium ions and the improvement of battery performance.

 

In most domestic and foreign papers on binders, PVDF appears frequently. On the one hand, it is because PVDF binders have long been commercialized, are widely available, and have excellent comprehensive performance; on the other hand, the battery preparation process using PVDF as a binder has become a mature system, which is easy to realize large-scale industrial production. However, with the development of new energy and the increasing demand for high-energy-density, environmentally friendly lithium batteries, the shortcomings of PVDF binders have gradually emerged. PVDF is a semi-crystalline polymer. Although it has excellent electrochemical and chemical stability, its own electronic and ionic conductivity is weak, which will increase the internal resistance of the electrode to a certain extent. At the same time, the bonding effect of PVDF binders generally comes from the van der Waals force between molecules and the hydrogen bonds formed by the CF bonds on the main chain and other electrode substances. Especially when PVDF binders are used in silicon negative electrodes with large volume expansion, they are easy to crack due to poor flexibility, leading to capacity loss and conductive network breakage.

 

In addition, the molecular weight of PVDF decreases and the viscosity becomes worse after absorbing water, so the humidity requirements of the production environment are relatively high, which increases the production cost; and PVDF reacts exothermically with metallic lithium and LixC6 at higher temperatures, which is not conducive to the safety of the battery. For this reason, researchers have improved the PVDF structure from various angles in recent years, mainly including grafting, blending, and copolymerization, in order to obtain better performance and ensure that it is more suitable for use in high-performance lithium batteries.

 

 

The grafting modification of PVDF binders is generally based on PVDF, in which small molecules or inorganic particles