Small, big role binder how to improve battery performance?
Publish：Sino Energy Corporation Time：2017-08-10
The lithium battery paste is a complex multiphase mixed non-Newtonian fluid. The negative slurry is made of a mixture of active material (graphite), conductive agent (carbon black, CNT, VGCF, etc.), binder, thickener, solvent, deionized water and so on.
The positive slurry is composed of a living substance, a conductive agent, a binder and a solvent. The living material, the conductive agent and the solvent have no adhesion to the metal electrode, so the pole can not be made into a lithium battery. The binder is an important component in the slurry. The binder bonds various particles together to form an adhesive slurry which is tightly bonded to the metal foil. Binders generally use only about 1%, but in lithium batteries, but occupy 5% of the role, it can be described as "small man, great wisdom."".
From the extreme point of view, the performance requirements of binders are as follows:
1. can maintain the viscosity of the slurry for a long time. It will not be lost because of slurry placement.
2., a high concentration solution can be dissolved and the required heat of vaporization is lower.
3. it is easy to shape and not rebound when rolling.
The 4. is flexible and does not form debris when the electrode is broken.
Binder is not only related to the manufacturing process of lithium battery, but also has important influence on the electrochemical performance of lithium battery:
1. keep the living things in good condition.
2. and metal foil has good cohesiveness, will not because of electrolyte and charge discharge use stripping metal foil.
3. good electrochemical stability in a wide range of voltage.
4. has a high melting point and a lower swelling ratio. Even at high temperatures, the composite structure of the binder and the living material needs to remain stable. The binder usually has swelling phenomenon, swelling beyond a certain extent will affect the conductivity between the active material and the collector, and will cause the battery capacity to decay, so it is necessary to control its swelling rate.
5. has good ion transport and electronic conductivity.
Binder can be divided into water system binder and oil system binder, and the common oil system binder has PVDF homopolymer and copolymer, mainly using NMP (methyl pyrrolidone) and DMF (nitrogen two methyl amide) as its solvent. In the battery system of LiFePO4 and NCM three material as cathode material, PVDF is usually used as positive electrode binder.
PVDF has powdery particles and colloidal substances, powdery white particles, colloidal PVDF is light yellow transparent colloid, viscosity is about 5000~8000. Both direct PVDF powder for slurry mixing or to make colloidal into pulping process are mature, the difference is in the solid content and feed ratio under the same circumstances, the process is slightly different, the final viscosity is not the same size.
Water binder mainly has SBR (styrene butadiene emulsion), CMC (hydroxymethyl cellulose), PTFE (polytetrafluoroethylene emulsion), PAA (polyacrylate) and so on. Initially, the negative use of PVDF binder is stirring oil binder, but due to the cell polarization is serious, and water are more environmentally friendly and can replace the bond effect, so the development to the present negative selection of water binder has become the mainstream direction.
Commonly found in negative graphite systems, SBR and CMC are used as negative binders, and SBR and CMC both act as bonding agents, but in practice, SBR is called an adhesive and CMC is called a thickener (stabilized slurry). The reasons for choosing SBR and CMC as synergistic binders are as follows:
1.SBR is more adhesive, but it is easy to break under prolonged stirring, resulting in structural damage and lower its cohesiveness. In general, SBR is chosen to be added at the later stage of mixing. At the same time, the effect of SBR dispersion is not as good as that of CMC. Excessive SBR will cause greater swelling, so SBR can not be used as binder completely.
2.CMC can play a very important role in the dispersion of negative graphite. CMC decomposition of sodium ions and anions in aqueous solution, with the increasing of CMC content, the decomposition products will adhere to the surface of the graphite particles, between graphite particles due to electrostatic interaction and mutual exclusion, achieve good dispersion effect.
However, CMC also has a fatal disadvantage, CMC is brittle, and if all the CMC is used as the binder, the graphite plate will collapse in the process of pressing and cutting, and the serious powder fall will occur.
At present, there is no perfect adhesive that can meet all of the characteristics of the above requirements. Only on this basis, as far as possible to meet the physical processing and electrochemical requirements of the binder. With the development of lithium battery technology, taking into account the cost and environmental problems, lithium battery gradually transferred to the water binder, the new lithium battery technology also appeared not to need the binder.
