Key auxiliary materials for lithium batteries: binders

Polymer and its derivative binders

1 Cellulose binder

Sodium carboxymethyl cellulose (CMC) is a cellulose binder that has been widely studied. It is a carboxymethylated derivative of cellulose. CMC is an ionic chain polymer water-based binder that forms a transparent viscous glue after swelling with water. It has the advantages of being difficult to ferment, good stability, low price, safety and environmental protection.

Compared with cellulose such as methyl cellulose (MC) and ethyl cellulose (EC), when CMC is used as a binder, the graphite negative electrode exhibits better electrochemical performance, with the first reversible capacity reaching 360 mAh/g. Lux et al. compared CMC and PVDF as binders for lithium iron phosphate positive electrodes and found that the use of CMC is conducive to improving the pole piece preparation process and increasing the pole piece's tap density.

At the same time, due to its good electrochemical stability, CMC can be applied to high-voltage positive electrode material systems. Li et al. applied CMC to the Li[Li0.2Mn0.56Ni0.16Co0.08]O2 4.8V positive electrode system. At a rate of 1C, the first reversible capacity of the electrode was 205mAh/g. After 200 cycles, it still had a reversible specific capacity of 169.5mAh/g, which was better than the electrode with PVDF as a binder.

In addition, CMC is also an excellent binder for high-capacity silicon negative electrode materials. The carboxyl functional groups in CMC generate hydrogen bonds or covalent bonds with silicon oxide (SiOx) and silanol (-Si-OH) groups on the silicon surface, which can enhance the bonding between silicon particles and the current collector. CMC, as a polymer, can also form a coating similar to the SEI film on the surface of silicon particles, inhibiting the decomposition of the electrolyte on the surface of the silicon negative electrode, thereby improving the cycle life of the silicon negative electrode. However, CMC has the defects of poor flexibility and high brittleness, which can be improved by blending CMC with high-elastic polymers (such as styrene-butadiene rubber).

CMC-Li polymer material is prepared by replacing Na in CMC with lithium. It is a binder with good ionic conductivity, which 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 improve the charge and discharge capacity and cycle performance of the battery.

2 Polyacrylic acid adhesive

Polyacrylic acid (PAA) is a chain polymer water-based binder. PAA has the following advantages as a binder:

A: Almost no swelling occurs in the carbonate solvent of the electrolyte, and the electrode sheet structure is stable during the charge and discharge process;

B: The carboxyl content in its structure is higher than that of CMC, and it 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;

C: It can form a denser film in the electrode sheet, increasing the electrical contact between the active material and the current collector;

D: Excellent tensile mechanical strength, conducive to machining.

Wening et al. compared the effects of PAA, CMC and PVDF as binders on the performance of lithium titanate electrodes. The study found that the lithium titanate electrode sheet using PAA showed the best uniformity and good electrochemical performance, with a specific capacity of 150mAh/g when charging and discharging at a 1C rate, and still had a specific capacity of 130mAh/g at a 16C rate. PAA is also suitable for lithium iron phosphate positive electrodes and silicon negative electrodes.

The neutralization degree of -COOH in PAA greatly affects its performance as a binder. When PAA is dissolved in water, it is easy to form a molecular agglomeration structure due to the strong carboxyl hydrogen bonding force between molecules. PAA is neutralized with alkali to prepare PAH-M salt. The electrostatic repulsion between carboxyl salt groups increases the stretchability of the molecular chain, which is conducive to reducing the intermolecular agglomeration effect.

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 silicon-graphite electrode using PAH0.2Na0.8 showed the highest first coulombic efficiency (69%), the highest first reversible capacity (1400mAh/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, thereby improving the kinetic parameters of lithium insertion in the material.

Polyacrylonitrile (PAN) contains highly polar nitrile functional groups, which can form hydrogen bonding forces and dipole forces with surrounding materials. As a binder, it is beneficial to improve the stability of the electrode sheet structure and the wettability of the electrolyte. Gong et al. compared the performance of PAN, CMC, and PVDF as binders for graphite, silicon-carbon negative electrodes, and lithium titanate electrodes. The electrode sheets prepared with PAN have improved electrolyte wettability, which is conducive to the effective deintercalation of lithium ions. The solid electrolyte phase interface impedance and charge migration resistance are relatively small, so the battery exhibits good electrochemical performance.

Yoo et al. introduced polar functional groups into polyacrylamide to prepare glyoxalated polyacrylamide, which has the following advantages when used as a binder:

A: The cross-linking reaction forms a stable polymer structure;

B: The covalent bond formed with the active substance is beneficial to inhibit swelling in polar solvents;

C: The introduced polar functional groups promote the wettability of the electrolyte.

When used as a binder for silicon negative electrodes, it 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.

1.1. What requirements should the adhesive meet? Electrode binders not only need to effectively bond electrode active materials, conductive agents and electrode current collectors, but also have the ability to resist the influence of various external factors because they are in a very special environment for a long time. These special environmental factors are: ① The binder and the electrode material are immersed in the electrolyte for a long time, and the binder needs to be able to maintain the stability of shape, structure and properties in the electrolyte; ② It is in high potential (positive electrode binder) or low potential (negative electrode binder) conditions for a long time. Therefore, the positive electrode binder needs not to be oxidized under high pressure conditions, and the negative electrode binder needs not to be reduced under low pressure conditions; ③ Many lithium storage active materials will continue to change in volume during battery operation. Their volume increases with the embedding of lithium ions and decreases with the removal of lithium ions. Therefore, the binder must have sufficient flexibility 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, battery adhesives usually need to have the following properties: ① good bonding performance, high tensile strength, good flexibility, and low Young's modulus; ② good chemical stability and electrochemical stability, no reaction or deterioration during storage and circulation; ③ no swelling or small swelling coefficient in the electrolyte; ④ good dispersibility in the slurry medium, which is conducive to uniformly bonding the active material to the current collector; ⑤ little effect on the conduction of electrons and ions in the electrode; ⑥ environmentally friendly, safe to use, and low cost.

Volume effect of electrode during lithium insertion and extraction 

1.2. Is it better to use more adhesive? The proportion of adhesive must be an appropriate value. The electrode with too little adhesive is prone to powdering, peeling and active material shedding, and the ohmic resistance of the electrode increases due to poor bonding effect, which will affect the battery's electrical performance. Although too much adhesive ensures the bonding performance of the electrode, the electrochemical inertness of the adhesive will inevitably increase the ohmic resistance of the electrode. The increase in the internal resistance of the battery will also affect its electrical performance. 

2. Classification of adhesives Adhesives can be divided into oil-based adhesives and water-based adhesives according to the different solvents used. Of course, some adhesives can be dissolved in both organic solvents and deionized water, such as polyacrylic acid -PAA. 

2.1. Oil-based adhesives Oil-based adhesives refer to the use of organic matter as a solvent as a binder, and the corresponding slurry formed is an oil-based slurry. The slurry formed by this system has good dispersibility and is not easy to settle, and the electrode bonding performance is good. 

① Polyvinylidene fluoride (PVDF)

The oil-based binder commonly used in industrial lithium-ion batteries is polyvinylidene fluoride ( PVDF), and the oily solvent used in combination is N-methylpyrrolidone (NMP). 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 other component particles in the pole piece, and the action of hydrogen bonds makes the particles of each component string together. PVDF has many advantages: it has a wide electrochemical stability window and stable electrochemical performance at 0-5V (Li/Li+); at the same time, PVDF has good antioxidant resistance and chemical reaction inertness and is not easy to deteriorate; in addition, PVDF has good swelling properties, and the pole piece electrolyte wettability using PVDF as a binder is good. In most papers on binders at home and abroad, PVDF appears frequently. Among them, on the one hand, it is because PVDF binder has long been commercialized, widely available and has excellent performance; on the other hand, the battery preparation process using PVDF as a binder has become a system. With the development of new energy, the shortcomings of PVDF binder 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. At the same time, the bonding effect of PVDF binder 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 binder is used in silicon negative electrodes with large volume expansion, it is easy to cause 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 environment are relatively high; and it 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 lithium batteries. PVDF modification method The grafting modification of PVDF binder is generally based on PVDF, and small molecules or inorganic particles are dispersed in PVDF, and chemical bonds often occur between them. In the modification of PVDF binder, blending modification is simple and effective. Generally, another polymer is mixed with PVDF to obtain a binder with better performance. Polymers that are often blended with PVDF include polyethylene glycol (PEG), polymethyl methacrylate (PMMA) and polyvinyl acetate (PVAc). The modified binder after blending generally shows that the PVDF crystallization is appropriately reduced, the composite electrode is more affinity with the electrolyte, the lithium ion transmission is accelerated, and the battery performance is improved. PVDF is also often copolymerized with fluorine-containing molecular segments, such as hexafluoropropylene, tetrafluoroethylene, etc., to obtain functional binders. The researchers used PVDF-HFP (polyvinylidene chloride-hexafluoropropylene) as a thermosensitive binder (TSB) and cross-linked it. It was found that TSB can effectively reduce the peak temperature of the internal short circuit without affecting the cycle performance of LIB, which has a significant effect on alleviating the thermal runaway of lithium-ion batteries. ② Other oily binders (non-PVDF) In addition to the commonly used PVDF binders, the advantages of other oil-soluble binders have also received a lot of attention. Non-PVDF oily binders mainly include: polyacrylonitrile (PAN), polyimide (PI), and perfluorosulfonic acid ionomer (Nafion). PAN is a semi-crystalline polymer whose main functional group, the nitrile group, has strong polarity. It generally connects the active material and the current collector through hydrogen bonds, van der Waals forces, and dipole-dipole interactions. At the same time, PAN can well infiltrate the electrolyte, and the strong polarity of the nitrile group can also promote the movement of lithium ions in the electrode. PI binders generally have good mechanical properties and heat resistance, and are often used in silicon negative electrodes with large volume expansion and high-voltage layered positive electrode materials. Nafion has excellent ionic conductivity and suitable bonding strength. The sulfonic acid group on Nafion can generate electrostatic effects with Li+ in the electrolyte, causing Li+ to creep and migrate on the polymer main chain, thereby improving the ionic conductivity of the electrode. 2.2. Aqueous binders In order to overcome the problems of environmental pollution and high cost of oily binders, water-soluble binders have gradually developed and have become a direction that battery workers have generally paid attention to in recent years. The widely used aqueous binders in industrial lithium-ion batteries include sodium carboxymethyl cellulose ( CMC) combined with styrene-butadiene rubber (SBR), polyacrylate (LA series) binders, etc. ① Sodium carboxymethyl cellulose CMC The preparation of sodium carboxymethyl cellulose CMC is formed by embedding carboxymethyl functional groups into natural cellulose. Due to the presence of carboxymethyl functional groups in CMC, it is soluble in water. When CMC is used as a binder for carbon negative electrodes of lithium-ion batteries, the amount used is relatively small, generally between 2% and 5%. The electrode made with this binder has a small loss in irreversible capacity and a high reversible capacity. Some companies have applied CMC in the manufacture of lithium-ion negative electrodes. However, CMC is very brittle and has poor compliance. There are three reasons for this: 1) The molecules are polar and the interaction between molecular chains is very strong; 2) The six-membered pyran ring structure in cellulose makes internal rotation difficult; 3) It can form hydrogen bonds both within and between molecules, especially intramolecular hydrogen bonds, which make the glycosidic bonds unable to rotate. In order to increase the flexibility of the electrode sheet, CMC is usually mixed with highly elastic SBR (water-based adhesive represents CMC+SBR). In addition to good adhesion, CMC also has the function of dispersing SBR. CMC contains only four elements: C, H, O, and Na, which makes CMC easier to decompose than PVDF. When the battery life is over, the electrode sheet can be taken out for pyrolysis. In addition, another advantage of CMC is that its price is cheaper than PVDF, which has great research significance for reducing the cost of battery production. ②Styrene-butadiene rubber SBR SBR is an elastomer made by copolymerization of 1,3-butadiene and styrene. Its emulsion has good elasticity and high elongation when used as a binder. It is mainly used to increase the flexibility of the electrode sheet. In industry, SBR is often mixed with CMC. The mechanical properties of polymer films with different CMC/SBR ratios are shown in the figure below. It can be seen that SBR effectively reduces the Young's modulus of the film and increases the flexibility of the film.

Binder is one of the essential materials for battery manufacturing, and its cost accounts for less than 1% of the battery manufacturing cost, but it can improve the battery performance by 5%-10%. Although the amount of binder used in lithium batteries is very small, the amount of auxiliary materials used is generally 2%-5%, and its main function is to connect the electrode active material, conductive agent and electrode current collector, so that the electrode active material, conductive agent and current collector have overall connectivity, thereby reducing the impedance of the electrode.

 

    Under the development trend of high energy density, the current lithium-ion power battery's positive and negative electrode materials, diaphragms, electrolytes and other material systems are constantly upgrading. In addition to these main raw materials, some extremely small but crucial auxiliary materials have also ushered in changes in product performance, such as lithium battery adhesives.

 

    At present, the power battery market has become highly concentrated, with the top 10 companies accounting for more than 70% of the market share. This also means that only water-based adhesive manufacturers with a high proportion of high-end power battery companies in their customer structure can fully enjoy the dividends brought by the development of power batteries.

 

    The localization rate of lithium battery materials is generally high, but the domestic high-end water-based adhesives used as auxiliary materials are almost blank, which is thought-provoking.

 

    At present, the binder used for positive electrode stirring is mainly oily binder PVDF, which needs to be used with NMP solvent. However, NMP solvent contains some undesirable substances that are harmful to human health, and it needs to be recycled when used. Initially, the binder used for negative electrode stirring was also oily binder such as PVDF, but considering the serious polarization in the battery, and the water system is more environmentally friendly and can replace its bonding effect, the use of water-based binders for negative electrodes has become the mainstream direction.

 

    The main water-based binders include SBR (styrene butadiene emulsion), CMC (hydroxymethyl cellulose), PTFE (polytetrafluoroethylene emulsion), PAA (polyacrylate), etc. They can be used with lithium iron phosphate in the positive electrode material, and can also be used with some lithium manganese oxide (the lithium manganese oxide of each company is different), but they are incompatible with ternary materials and there are technical difficulties that are difficult to overcome. Ternary materials, especially high-nickel ternary materials, are the general trend of the industry, making the expansion of water-based binders in the positive electrode field more difficult.

 

    However, some senior experts believe that as the country's environmental protection policies become stricter (the EU has issued a policy to ban the sale of oil-based system binders, and China may follow suit), and internationally renowned water-based binder manufacturers such as Japan's Zeon and JSR are increasing their research and development efforts to overcome the technical difficulties of using water-based binders for positive electrodes, water-based binders are expected to gradually replace PVDF in the future. Of course, replacing PVDF will win some market space, but the key to future sales growth still lies in the expansion of the downstream power battery market.

-End-

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