1.1. Function of binder
Binder plays a very important role in the mechanical stability of electrode. When lithium ion is embedded in graphite electrode, the graphite electrode expands by 10% on the c-axis. When lithium ion is de embedded, the graphite electrode will shrink. This step will be repeated with the charging and discharging process of the battery. When this change occurs, the interfacial contact between active substances or conductive agents will be weakened. This will be accompanied by the increase of contact resistance between particles. Finally, the battery performance will deteriorate due to the increase of electrode ohmic resistance. The problems caused by the mechanical instability of the electrode can be solved by using a suitable binder. In the drying process of electrode manufacturing, when the electrode temperature reaches 200 ℃, strong adhesion must be maintained. The structural control of polymers used as binders is very important for the formation of binding force between active substances and fluid collectors.
1.2. Requirements for binder
1) Battery performance and safety
The binder must maintain its adhesion and cannot be dissolved in the organic solvent of high polar carbonate used as lithium-ion battery. They should remain stable in an electrochemical environment. Fluorinated polymers containing TFE (tetrafluoroethylene, – cf2-cf2) or HFP (hexafluoropropylene, – cf2-cf (CF3) -) are easier to reduce than partially fluorinated PVDF (polyvinylidene fluoride) . Therefore, PVDF is the most stable binder known at present. The binder should also exhibit oxidation resistance, because the cathode material is usually metal oxide, which can release reactive oxygen species when overcharged. The oxidation potential values obtained from theoretical calculation (molecular orbital theory) are -12.12 ev (PE), -14.08 ev (PVDF) and -15.47 ev (PTFE), respectively. The oxidation potentials of organic solvents such as EC and PC are -12.46 EV and -12.33 EV, respectively. This shows that fluorinated polymers have strong oxidation resistance. In addition, the binder acts as a buffer for the expansion and contraction of active substances caused by the insertion and de insertion of lithium in the electrode. Considering the bonding form of the binder to the active material, the elastic polymer is still not commercialized due to its poor bonding ability and expansion at high temperature.
In the manufacture of lithium-ion batteries, moisture has a negative impact on battery performance. Considering that the water content in the electrolyte is 10 ppm, the electrode shall basically contain no water. In addition, the binder should have excellent heat resistance, because the temperature during battery manufacturing may be as high as 200 ℃. These conditions limit the types of polymers suitable for use as binders.
2) Manufacturing process
The binder should be uniform and stable in the preparation of active material slurry, and then coated quickly. With the increase of temperature or the progress of charge discharge cycle, the active substance must maintain strong adhesion. Maintaining good adhesion between the active substance and the current collector can prevent the powder from blowing up in the slitting process and contribute to the safety of the battery. High viscosity binder solution is necessary for active substances with high specific gravity.
All binders must be adjusted to appropriate true density and apparent density values. The performance of the battery is mainly determined by the electrode activity and the performance of the material, but it is not affected by the binder. For PVDF, it only shows the influence on the conductivity of lithium ion.
1.3. Suitable polymer binder
Suitable polymers for binder include PVDF, SBR (styrene butadiene rubber) / CMC (carboxylic acid methyl cellulose), chemically stable and heat-resistant PTFE, polyolefin, polyimide, polyurethane and polyester. Polyimide, polyester and other polymers obtained from condensation reaction have strong adhesion and excellent heat resistance. However, amide and ester bonds do not meet the conditions mentioned above. Epoxy resin cannot be used as binder because of the long curing time.
Polymer binder can be regarded as a mixture of polymer and dispersion medium, which usually exists in the form of organic solvent or aqueous solution. The influence of dispersion medium on active substances (solubility, residue, etc.) and the recovery and reuse of solvents should be carefully considered. For the dispersion medium of aqueous solution, the circulation property may be affected by the surfactant remaining in the active substance after drying. Flame retardancy is another important property of binder, which is PTFE > PVDF > polyimide > polyethylene in descending order.
1.4. Ideal binder
In terms of manufacturing process and battery performance, the role of binder can be summarized as follows. There are many technical problems in the coating and drying process of active substances contained in conductive solution. However, due to the high production efficiency of this method, it is still used. If the active material layer can self bond during drying or CVD method can directly and continuously deposit the active material on the collector, the current wet binder will not be needed. However, the mechanism of how the binder adheres to the active materials in the battery is not clear. Atomic force microscopy (AFM) is being used to detect the state of graphite and PVDF.
1.5. PVDF binder
1) Basic characteristics
Similar to PTFE (4F), PVDF (2f) is a type of fluorinated polymer resin. It is a vinyl copolymer with thermoplastic and solubility in polar solvents. Fluorinated polymers higher than 3F are insoluble, but the copolymers of 2F and 6F (hexafluoropropylene) have strong solubility, which is due to their low crystallinity.
Compared with other polymers, PVDF has a very high relative dielectric constant (9 ~ 10 in the range of 102 ~ 103hz), and the lithium ion conductivity is 10-6 ~ 10-5s / cm after expansion in polar solution. PVDF is a crystalline polymer with a specific gravity of 1.78 g / ml. the glass transition temperature (TG) is close to – 35 ℃, the melting temperature (TM) is 174 ℃, and the crystallization temperature (TE) is 142 ℃. Compared with the emulsion polymerization, the PVDF grown at low melting point and crystallinity has higher melting point and crystallinity, and thus has better anti swelling property.
In order to be used as a binder, PVDF must first be dissolved or dispersed in a suitable medium. The solubility of PVDF varies greatly according to its polymer structure and crystallinity, and the properties of polymer structure and crystallinity are determined by the polymerization method.
3) Characteristics of solvent
The solvent can have a wide range of viscosity, and the viscosity can be appropriately adjusted to the specific gravity of the active substance (real specific gravity and apparent specific gravity). If the cathode material slurry with high real specific gravity (3.5 ~ 4.5 g / ml) cannot be prepared at high viscosity, it will cause precipitation and other problems. The mass ratio of active material to PVDF binder is usually in the range of 96:4 ~ 88:12. If more polymers are added to increase the viscosity of the binder, the binder will be excessive. Therefore, polymers with high degree of polymerization should be used in the preparation of high viscosity slurry. The binder is dissolved and then filtered with the removal of the microgel, and metal impurities including alkali, iron, zinc and copper are checked before use. Once the proper viscosity of the slurry is determined, the slurry will be transferred to the coating device and then coated on the fluid collector. Under a wide range of shear speed, the slurry should maintain uniform viscosity.
4) Expansion and stability of organic solvents
After expansion, the relationship between the weight content of organic solvent in the film and the expansion rate of PVDF binder. The mixed solvent composed of cyclic carbonate with high dielectric constant shows high expansion rate. Except for dimethyl carbonate, most linear carbonates have a low dielectric constant of less than 10. When the expansion rate is lower than 20%, the impregnated electrolyte will not affect the crystal structure, but will remain dispersed in the range of crystalline particles and amorphous regions.
Because PTFE hardly expands, PTFE (polytetrafluoroethylene) is a suitable binder under harsh conditions.
1.6. SBR / CMC binder
Recently, the binder of SBR / CMC (styrene butadiene rubber / carboxymethyl cellulose) has been used to replace PVDF binder on carbon and non carbon based negative electrodes (silicon carbon composite, tin, silicon). In order to maximize the capacity and energy density, it is very important to increase the amount of electrode active material while reducing the binder content. However, PVDF is limited in terms of reducing the use of binder. This problem becomes more serious when the active material particles are nano-sized. Because SBR / CMC has excellent bonding performance, it can make the active material and conductive particles have strong bonding force. SBR / CMC is being considered for application at present. In the non carbon negative electrode, if there is no close combination, the change of large volume will lead to the increase of resistance. Because SBR / CMC binder can be dispersed into aqueous solution, it is more environmentally friendly than PVDF dispersed in NMP solvent. In addition to being difficult to burn, the formation and coating of slurry can be carried out under atmospheric conditions, and the SBR / CMC system reduces the manufacturing cost.
1.7. Basic characteristics
Compared with PVDF, SBR is a diene based synthetic rubber with better heat resistance. In Na CMC (sodium carboxymethylcellulose), the carboxyl functional group of cellulose is partially replaced by sodium carboxylate (- ch2coona +), and the degree of substitution is 0.6 ~ 0.95. It has high solubility in water. SBR and CMC were used separately before, but now they are combined to maximize the adhesion and minimize the binder content. Due to its high anti oxygen stability, PVDF is still widely used as the binder of positive electrode.
SBR in SBR / CMC binder is composed of fine rubber particles uniformly dispersed in water. Compared with PVDF, its content is lower, and it has better mechanical elasticity and stability at high temperature. In addition to strong adhesion, the performance of the binder is also determined by electrochemical stability and different manufacturing conditions.
1.9. Future trend of binder
As lithium secondary batteries become more compact, slender, large capacity and stable, battery production should be further improved, such as efficient preparation of electrode slurry, faster electrode production, rapid liquid injection capacity, high-speed electrode winding, etc. The development of binders is very important because they are very necessary to enhance battery performance and productivity.
Cathode materials are developing to avoid the use of diamond, brocade and manganese as much as possible, because it is facing serious resource constraints. Nickel based cathode materials are water-soluble, highly alkaline, and can absorb water at a rate of 2000 ~ 3000ppm. The effect of aqueous binder on cathode material with high alkali content is not good. Even if non-aqueous binder is used, the fluidity of the slurry cannot be increased after mixing. Although the chemical properties of the carbon negative electrode are inactive, the carbon structure and surface show different characteristics (hydrophilic / hydrophobic). For example, due to the low specific gravity, natural graphite needs a large amount of dispersion medium, and further research is needed to improve the fluidity of slurry.
- Conductive agent
Fine toners are added to improve the electronic conductivity between electrode active particles or between electrode active particles and metal collector. These toners are called electronic conductive agents. This is necessary to prevent the adhesive from acting as an insulator and make up for the lack of electronic conductivity of the electrode.
2.1. Type of conductive agent
Carbon based materials are usually used as conductive agents. There are carbon black and acetylene black. The manifestation of toner is that fine particles are intertwined to form a fibrous network. The basic crystal structure is the same as the graphite anode of lithium-ion battery, but the graphite structure of carbon powder is only partially graphitized carbon.
Different manufacturing methods lead to different graphitization degree and different microstructure between B fast black and Keqin carbon black. Generally speaking, due to the high degree of graphitization, the carbon particles of Keqin carbon black show higher conductivity. Although the resistance of both materials is less than 0.5 Ω cm, the mixing of these two types of carbon produces higher resistance than that of Cochin carbon black alone. The toner in B fast black is smaller than that in Cochin carbon black (about 30 nm) and is interconnected in a way similar to Buddha beads. Although Cochin carbon black has larger particles.
Its specific surface area is 800 m2 / g, which is much larger than the specific surface area of 100 m2 / g of B fast black. This is because the particles in Keqin carbon black contain many pores. The two conductive agents have strong adhesion with negative and positive materials.
2.2. Dispersion of conductive agent
When carbon powder is used as conductive agent, carbon, active material and binder shall be evenly mixed,
It is important to mix electrode active material and carbon powder evenly.
2.3. Wettability of conductive agent and battery
As a hydrophobic material, carbon powder is difficult to wet. The commonly used organic solvents have high dielectric constant and better wettability than water. However, depending on the state of carbon, they may not be wetted. Therefore, the surfaces of B fast black and Keqin carbon black should be modified to ensure their wettability. For example, the energy density of electrodes with the same pore size can be adjusted by reducing the affinity of toner to electrolyte. Due to the existence of pore size in carbon particles, carbon can retain electrolyte.
2.4. Modification of conductive agent
Although there is little research on carbon as a conductive agent, boron doping has been studied to change the crystallinity of carbon or control the surface state of carbon powder. If the carbon itself can charge and discharge, the battery performance can be improved. Due to the cost and characteristics of materials, the development of conductive polymer materials as conductive agents has not been paid attention to.
3.3 fluid collector
3.1. Function of fluid collector
The collector acts as a medium for supplying electrons from the external circuit to the electrode active material or transmitting electrons generated by the electrode reaction to the external circuit. It is an important material of electrode. Considering the characteristics of electronic conductivity, electrochemical stability and electrode manufacturing technology, metals are often used as fluid collector materials.
For lithium-ion batteries, aluminum and copper are used as collectors for positive and negative electrodes, respectively. The electrode active material is coated on the surface of the current collector and then dried to obtain the electrode. Although the current collector is very thin foil (thickness: 10 ~ 20 μ m) Composition, but the fluid collector should have sufficient mechanical strength. Its surface state is very important for the preparation of electrode slurry. The actual foil surface has high wettability to the slurry, and it is possible to maintain strong adhesion between the binder and the fluid collector after removing the solvent.
3.3. Negative collector
The negative collector is composed of materials such as copper and nickel. They do not have electrochemical activity in the working voltage range of carbon electrode (0.01 ~ 3.0 V for lithium potential). In particular, copper is relatively stable and not easy to reduce, while nickel is more expensive.
3.4. Positive collector
For the positive electrode, it is very important to prevent the oxidation of metal collector under high voltage potential. Because the oxidation reaction should occur at about 3V, copper cannot be used as positive collector. Considering the cost, electrochemical stability within the working range and other factors, aluminum is the most suitable fluid collector material。