[Battery materials] Vol.2 Binder for lithium-ion batteries
Takanori Suzuki
Suzuki Material Technology and Consulting Co., Ltd.
【Serial column : Battery materials】
Vol.02 Binder for lithium-ion batteries1. What is a binder?
- 1-1. Assist in dispersion during kneading and retain dispersed material
- 1-2. Secure viscosity during coating and control the thickness of active material layer immediately after coating
- 1-3. Adhesion and cohesion
- 1-4. Act as a slip aid during press and retain particles at destination
- 1-5. Ionic and electrical conductivity
Fig.1 Roles of a binder in the electrode manufacturing process
1-1. Assist in dispersion during kneading and retain dispersed material
In the kneading process, it is necessary to disperse solid components such as active materials and conductive agents in a solvent, and to retain them until drying is completed. A binder is dissolved in a solvent to give a proper viscosity and contributes to the dispersion and retention of active materials and conductive agents. Polymer materials are suitable for this purpose because they dissolve in solvents and increase viscosity. In addition, the increased viscosity enhances the shear force between the solution (solvent + polymer) and the solid content during kneading, thereby helping to improve the dispersibility of the solid content. Furthermore, the increased viscosity of the solvent reduces the sedimentation rate of the dispersed solid, allowing for a longer pot life of the slurry.
1-2. Secure viscosity during coating and control the thickness of active material layer immediately after coating
In the coating process, the slurry produced in kneading is thinly coated on the current collector foil. The thickness and dispersion state of the slurry in this process determine important conditions such as the thickness, density, electrode resistance, and porosity of the electrodes after drying and pressing, which directly affect the performance of batteries. The role of a binder is to provide viscoelasticity that enables a uniform film thickness. The slurry of batteries must have a "flowing" property when it is left standing and it needs to cause a uniform "flow" until the moment of coating when the shear rate is large and to stop the flow and move almost immediately after coating when the shear rate becomes almost zero. In other words, the slurry is required to show a contradictory flow performance depending on the shear rate. In general, high-concentration solutions of polymers have such performance by nature, and what determines the state of this performance are the shape of molecular chains, molecular weight, and branching state of the elements constituting the molecules.
1-3. Adhesion and cohesion
As mentioned above, a binder is a glue. The role of the glue is to bind the active material, conductive agent, and current collector foil together. The action is the synthetic force of "adhesion", the force acting on the interface, and "cohesion", the force connecting two materials by the strength of the materials itself. If these two forces are not in good balance, the binder does not act properly as a glue. Adhesion force is mainly associated with a covalent bonding force, electrical bonding (hydrogen bonding) force, van der Waals force, and anchoring effect. Cohesion force is largely affected by mechanical strength, hardness, and elongation of a material itself. Common polymers often have such forces, which alone provide essential functions as a binder, but they may be suitable or unsuitable for each material depending on the conditions in which they are used.
1-4. Act as a slip aid during press and retain particles at destination
Pressing imparts smoothness to electrodes, forms a uniform film thickness, and increases density. The major change that occurs in pressing is the migration of active material particles. The density is not easily increased by applying pressure to the powder of active material. This is because the entanglement between particles prevents them from moving. The addition of a binder prevents the particles from getting entangled and allows them to pack (move) to achieve a higher density. A binder must change its form to aid particle movement and retain the particles in place after the pressing pressure is released. When a resin is pulled, the stress increases according to the elongation, and when the stress exceeds the yield point, the resin will not return to its original length and stays there even if the pulling is stopped (plastic deformation). A binder such as rubber does not show this yield point and has a strong tendency to return to the original position when the stress is removed (springs back). Therefore, the ability to hold the particles at the destination is important.
1-5. Ionic and electrical conductivity
There are no great expectations for the electrical conductivity of a binder for the anode because graphite is mainly used there, but it is essential for the cathode. However, since a binder is a resin material and not a conductor, a conductive agent that works together with the binder and forms a network is added to impart electrical conductivity to the cathode electrode. Further, if the electrolyte can be appropriately made to swell in the binder resin, a lithium-ion conductivity is developed through the swollen electrolyte, and so the interaction between the binder and the electrolyte is important. Therefore, a material with a proper electrolyte-swelling property is needed. Of course, if the binder itself has ion conductivity, the electrolyte solution can be eliminated, suggesting the possibility of using the binder as a solid electrolyte.2. Why fluoropolymer (PVdF)?
Fig.2 Properties of fluoropolymer(PVdF) binder
2-1. Mechanical strength
PVdF resin has relatively high strength compared with other fluororesins. As explained earlier, the main contributor to cohesion is the degree of mechanical strength. In addition, PVdF has a molecular weight above a certain level, and it shows a tensile property having a yield point (S-S curve). This feature imparts the performance as a binder of slipping and retaining active materials during pressing.- 2-2. Adhesiveness
- 2-3. Chemical stability
- 2-4. Electrochemical stability
- 2-5. Solubility in organic solvents
- 2-6. Swelling property to electrolyte
3. Daikin’s binder
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