Learn more about battery adhesives

1. Battery Adhesive

Lithium battery pole pieces are mainly composed of active materials, conductive agents, and binders. Usually, binders are inactive materials and account for a small proportion of the electrode. However, binders are an indispensable and important component in the preparation of lithium batteries, and play an important role in maintaining the integrity of the electrode structure and improving the electrochemical performance of the electrode.

2. Classification of Binders

Normally, battery binders are polymer compounds, and commonly used binders are:

(1) Polyvinyl alcohol (PVA)

PVA is polymerized from vinyl alcohol, and the degree of polymerization is usually 700-2000. PVA is a hydrophilic white powder with a density of 1.24-1.34 g/cm3. Other water-soluble polymers can be miscible with PVA, such as CMC, starch, sodium alginate, etc.

(2) TFE (polytetrafluoroethylene)

Polytetrafluoroethylene, commonly known as the "King of Plastics", is a white powder with a density of 2.1-2.3 g/cm3 and a thermal decomposition temperature of 415°C. PTFE has the advantages of good electrical insulation, oxidation resistance, acid resistance and alkali resistance. PTFE is made by polymerizing tetrafluoroethylene. PTFE with a concentration of 60% is often used as a binder in lithium ions.

(3) MC (sodium carboxymethyl cellulose)

CMC is a white powder that forms a transparent solution after dissolving in water. It has good dispersibility and binding ability, and has the ability to absorb and retain water.

(4) Polyolefins (PP, PE and other polymers); (5) Modified SBR rubber with good adhesion; (6) PVDF/NMP and other solvent systems; (7) Fluorinated rubber; (8) Polyurethane

3. Binders suitable for lithium ions

Since the use of non-aqueous solvents in lithium-ion batteries results in low lithium ion conductivity, the electrode area is required to be as large as possible, and a roll structure is used in the battery assembly process. Therefore, the performance of the battery not only depends on the electrode itself, but also has certain requirements for the binder used in the battery manufacturing process.

 1. Performance and function of adhesive

1) Ensure the uniformity and safety of active materials during electrode manufacturing; 2) Effectively bond active materials; 3) Effectively bond active materials to current collectors; 4) Facilitate the formation of a protective SEI film on the graphite negative electrode.

 2. Requirements for adhesive properties

1) Maintain sufficient thermal stability during the drying process; 2) Can be effectively wetted by the electrolyte; 3) Easy to process; 4) Not easy to burn; 5) Remain stable to the lithium salts LiClO4, LiPF6 and the by-product Li2CO4 in the electrolyte; 6) Have high electronic conductivity and ionic conductivity at the same time; 7) Low cost and small usage.

In the past, nickel-cadmium and nickel-metal hydride batteries, which were widely used, used water-soluble electrolyte systems as electrolytes. Water-soluble materials such as PVA and CMC can be used as binders, and water-dispersible emulsion PTFE can also be used. However, the electrolyte of lithium-ion batteries is carbonate, which has a large polarity and high solubility and swelling capacity. Therefore, the binder used cannot be dissolved by carbonate, and the above requirements must be met at the same time. The electrochemical stability is particularly important. When the positive electrode is overcharged to produce oxygen, it will not be oxidized, and when the negative electrode is at a negative potential, no reduction reaction will occur. In addition, during the battery charging and discharging process, lithium ions will be embedded/de-embedded in the active material, thus causing the active material to expand/contract. This process requires the binder to play a certain buffering role. The electrode of a dry lithium battery can reach a maximum temperature of 200°C, so the binder is required to remain stable at this temperature.

3. Binder affects battery performance

It can be seen that the performance of the binder directly affects the performance of the lithium battery. The process of manufacturing lithium-ion batteries is usually coating, using roller coating or scraper methods, and using the gap between the blades to make the thickness of the binder. The binder layer required for lithium batteries is very small, so the gap between the blades is also very small, so there cannot be large particle agglomerates in the slurry. The electrodes are made by rolling, cutting, and winding, and then placed in the battery housing. No active material powder or sheets can fall off during this process.

The density of positive and negative electrode materials of lithium batteries varies greatly. The density of positive electrode materials is generally around 4g/cm3, while that of negative electrode materials does not exceed 2g/cm3. The density of different active substances is matched by diluting or thickening the binder to ensure that the active substances in the slurry are stably suspended.

The oxidation index reflects the non-flammability of the binder, thereby reflecting the safety performance of the binder. For example, the oxidation index of the following binders is: polyethylene binder 2.8% ~ 5.7%, polyamide binder 24% ~ 29%, PTFE binder> 95%. In terms of non-flammability, fluorinated resin is the safest.

4. Polyvinylidene fluoride binder

Commonly used binders for lithium batteries Due to the high fluorine content of PVDF, it has good chemical stability and good thermal stability. In addition, compared with PTFE, it has high mechanical strength and good thermoplasticity, and is easy to process. Therefore, PVDF binders are widely used in lithium batteries, especially in the process of making thin electrodes, using its soluble properties to achieve casting or sauce coating processes, greatly improving production efficiency.

1. Performance of PVDF binder

VF2 monomers undergo polymerization to become PVDF polymers, which have a structure of interlinked CF2 bonds and CH2, and have the stability characteristics of general fluorinated polymers. There are interactive groups on the PVDF chain, resulting in a unique polarity. This polarity not only affects the solubility of the polymer, but also affects the interaction between lithium ions, metal current collectors and active substances.

The polymer VF2/HFP and PVDF are highly crystalline copolymers with a crystallinity of up to 60%. The properties of the polymer are directly affected by its degree of crystallinity, such as the insulation properties, brittleness, melting point, permeability and tensile properties of the polymer.

HFP will prevent the orderly arrangement of VF2/HFP polymers, thus reducing the crystallinity of VF2/HFP polymers. Generally, the crystallinity decreases with the increase of HFP amount.

The dielectric constant of PVDF polymer is relatively high. The physical properties of monomer polymer are: when its average molar mass increases, its melting point is not affected, but the crystallinity decreases accordingly.

The melting point of VF2/HFP copolymer decreases with the increase of HFP content, but the HFP content has little effect on the crystallinity of the copolymer. In VF2/CFFE copolymer, the melting point of the copolymer is not greatly affected by the CFFE content, but its crystallinity decreases with the increase of CFFE content.

When using PVDF as a battery binder, an organic solvent that can dissolve the PVDF binder is required. Such solvents can be classified into active, intermediate, and co-solvents. Active solvents that can dissolve or swell PVDF binders at room temperature include N-methylpyrrolidone NMP, dimethylacetamide DMAc, dimethylformamide DMF, acetone, ethyl acetate EtAc, propylene carbonate PC, isophorone, cyclohexanone, dimethyl sulfoxide, methyl ethyl ketone, etc.

The intermediate solvent cannot dissolve or swell the PVDF binder at room temperature, but it can dissolve PVDF when the temperature rises. After the temperature cools to a certain degree, PVDF can still be retained in the solution. The intermediate solvents include carbitol acetate, butyrolactone 65℃, and isophorone 75℃.

The co-solvent cannot dissolve or swell the PVDF binder at room temperature, but it can dissolve PVDF when the temperature rises. When the temperature cools to a certain degree, PVDF will crystallize and precipitate. The co-solvents are: hexanediol ether ester, hexanediol ether, n-butyl acetate, diisobutyl ketone, hexyl acetoacetate, triethyl phosphate 100℃, triacetin 100℃, diacetone alcohol, PC80℃, hexanediol methyl propyl ether 115℃, dibutyl phthalate 110℃, cyclohexanone 70℃, methyl isobutyl ketone 102℃.

The most suitable PVDF binder in lithium batteries is NMP solvent. When the temperature is 35°C, the solubility of PVDF in NMP is higher than 100%.

The viscosity of PVDF-NMP directly affects the performance of lithium-ion batteries. Under the condition of quantitative NMP, the viscosity of PVDF-NMP increases with the increase of PVDF content.

In the process of lithium battery manufacturing, the viscosity of the binder can be selected based on the density of the active material on the electrode. If the density of the positive electrode active material is high, a high-viscosity binder is used to avoid the problem of slurry instability. In the actual manufacturing process, the ratio of active material to binder ranges from 96:4 to 88:12.

The concentration of PVDF in the binder solution needs to be controlled at (12.0±0.1)%, and the viscosity is (550±100) mPa·s.

In the slurry making process, the binder is first completely dissolved in NMP, the microgel in the solvent is filtered out, the active material is evenly mixed in the binder solution, and then its viscosity is adjusted to a certain range. The coating process is used on the current collector. Generally, the effect of low polymer binder is stronger than that of high polymer binder.

2. The swelling properties of PVDF polymer are affected by the electrolyte

Different electrolytes have different effects on the expansion rate of PVDF polymers. The expansion rate of PVDF polymers in linear structure solvent systems is less affected by temperature, while the expansion rate of PVDF polymers in cyclic structure solvent systems is more affected by temperature.

Method for testing the heat resistance and durability of battery adhesives: Store at 80°C for three days and test its expansion rate at the same interval.

The cycle life of lithium-ion batteries was tested at 25°C, 40°C and 60°C respectively. The results showed that, under the same other conditions, the higher the temperature, the fewer cycles of the lithium-ion battery. This is due to the excessive expansion of the PVDF binder at 60°C.

PVDF has temperature-sensitive properties and will not expand in common electrolytes, so it is suitable as a binder.

The stability of PVDF was studied by immersing it in LiPF6 and LiClO4 propylene carbonate electrolyte. No obvious changes were found by FTIR observation. However, after repeated cycles, PVDF may be affected by the decomposition products of conductive salts and organic solvents.

3. Practical applications of PVDF

In the actual processing technology of lithium batteries, the main use of PVDF is as a binder for the positive or negative electrode and as a separator for lithium-ion polymers.

(1) Positive and negative electrode binders

In the process of preparing the positive and negative electrode slurries, PVDF is added to the NMP electrolyte to dissolve, and then the lithium salt (such as LiClO4, LiBF4, LiPF6, etc.) is added to the organic solvent (such as EC/PC, etc.), and the two are mixed to prepare the positive electrode slurry; or PVDF is added to the NMP electrolyte to dissolve, and then mixed with negative electrode active materials such as graphite or MCMB to prepare the negative electrode slurry. The slurry is evenly applied to the metal current collector to form an electrode film.

For example, the method for preparing the positive electrode film is as follows: 3g PVDF and 30g NMP (ratio 1:10) are mixed into a binder solution, and after being stored at a constant temperature of 50°C for two hours, 45.5g LiClO2, 1.5g acetylene black, and 32g MNP are added, and stirred with a magnetic bar at room temperature for 15min, and then stirred at a speed of 2000r/min for 3min. After the obtained slurry is coated on aluminum foil, the drying oven is adjusted to 120°C to dry the slurry until the thickness of the film is gradually reduced to about 120μm.

The method for preparing the negative electrode film is as follows: 3 g of PVDF and 30 g of NMP (ratio 1:10) are mixed into a solution. After being stored at a constant temperature of 60°C for two hours, 30 g of the negative electrode active material MCMB (ratio with PVDF is 10:1) is mixed into the PVDF-NMP solution. The slurry is prepared and evenly coated on the copper foil. The film thickness needs to be controlled within 200 μm. The drying oven is adjusted to 150°C and then dried for 30 minutes.

The performance of electrode slurry is directly affected by the viscosity of the slurry. If the slurry viscosity is too high, the electrode active material cannot be dispersed, affecting the quality of the electrode; on the contrary, if the slurry viscosity is too low, the active material will precipitate. Time changes will also affect the slurry viscosity, resulting in unstable slurry performance, which will also affect the electrode performance.

In summary, the slurry viscosity is mainly affected by the internal ratio of the slurry and the different properties of the electrode active materials (such as surface area, morphology, particle size, etc.).

(2) Diaphragm

In the actual processing of lithium batteries, the foaming technology can be used to produce a colloidal microporous membrane with a thickness of about 110μm and a pore size range of 50% to 60%. This colloidal microporous membrane can use fumed silica, PVDF, DDP, etc. as the foaming medium.

     

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