Small and beautiful fluorine material in industrial lithium battery - PVDF binder
PVDF is one of the fastest growing fluoropolymers in recent years, and PVDF for lithium battery binders is one of the main varieties.
What is a lithium battery binder?
The binder is an inactive component in the electrode sheet of a lithium ion battery, and is one of the important materials that must be used in the preparation of the electrode sheet of the lithium battery. The main function of the binder is to connect the electrode active material, the conductive agent and the electrode current collector so as to have an overall connection between them, thereby reducing the impedance of the electrode, and at the same time, the electrode sheet has good mechanical properties and processability. Meet the needs of actual production. The binder is generally a polymer compound, and can be classified into a water-based binder and an oil-based binder depending on the solvent to be dissolved. The oil-based binder commonly used in industrial lithium batteries is a blended solution of PVDF and solvent N-methylpyrrolidone (NMP).
Why is PVDF?
PVDF is a chain polymer which is synthesized by addition polymerization of VF2 monomer. Its molecular weight is generally above 300,000, and it has typical fluoropolymer characteristics - excellent stability and corrosion resistance.
As a crystalline polymer, PVDF crystallinity is generally about 50%, and the crystal melting temperature is about 170 °C. Compared with other polymers of perfluorinated, thermoplastic PVDF has excellent mechanical properties and processing properties, so PVDF is widely used in lithium batteries, especially in the preparation of thin electrodes, using soluble binders. Processes such as slurry coating or casting can be realized, which is of great significance for improving production efficiency.
From the effect of the binder on the performance of the electrode, the molecular weight, crystallinity and content of the PVDF itself affect the performance of the electrode. In general, the larger the molecular weight of PVDF, the better the bonding performance, and the proportion of binder required to prepare the electrode is reduced. However, the excessive molecular weight will cause the solubility of PVDF in NMP and the dispersion in the electrode. decline. The crystallinity of PVDF directly affects its mechanical properties, high crystallinity, denser molecular chains, and stronger inter-molecular forces, which makes it have better bonding properties. However, if PVDF crystallinity is too high, The migration resistance of electrons and protons in the electrode is large, which also causes a problem that the impedance of the pole piece is high and the charge and discharge overpotential is large. In addition, the content of PVDF in the electrode significantly affects the conductive and mechanical properties of the pole piece, which affects the capacity properties, rate characteristics and long-term cycle performance of the battery.
There are some obvious deficiencies in the use of PVDF binders in lithium batteries, including the relatively high Young's modulus. Between 1 and 4 GPa, the flexibility of the pole pieces is not good enough; the molecular weight of PVDF decreases after water absorption, and the viscosity is poor. The requirements for environmental humidity are relatively high; for ion and electronic insulation, there is a certain degree of swelling in the electrolyte, and an exothermic reaction occurs with metal lithium, LixC6, etc. at a relatively high temperature, which is disadvantageous to the safety of the battery. At present, people mainly focus on the development of some PVDF copolymers to improve its properties.
In addition, the use of PVDF as a binder for lithium batteries requires the use of NMP as a solvent, which is expensive and has some environmental pollution. Therefore, many technicians in recent years have paid more attention to the development of aqueous binders.