Just as the heart pumps blood which gives life to humans, likewise batteries pump life into electrical objects or devices. Without batteries, electrical devices cannot function optimally. Basically, a battery is a device that contains one or more electrochemical cells which are dominantly used to power electrical objects such as cars, phones, torchlights, etc.
PVDF (Polyvinylidene Fluoride) plays an important role in powering batteries, particularly, the lithium-ion batteries. This is due to its high level thermal and electrochemical stability as well as its excellent adhesion between electrode films and collectors. It is produced by Vinylidene difluoride and is often utilized in applications demanding the highest level of purity and which are also highly resistant to all forms of solvents, hydrocarbons, and acids. PVDF is also applied as films, piping tools, sheet, tubes, etc. They are commonly applied in other sectors such as defense plants, semiconductors, etc. But essentially, they are most often used as binders in lithium-ion batteries. One of its major roles in these batteries is to act as a connecting agent between two or more electrode species while slowly adhering them to the electricity-based collectors. The essence of the adhesion is to increase its life span and energy density. This longevity is guaranteed by the PVDF's chemical resistance in the extremely harsh surroundings of the lithium-ion batteries, which contains a large amount of lithium salts.
In this article, we will explore the use of PVDF binder in batteries and the relevant aspects.
Properties of PVDF
1. Polyvinylidene Fluoride also called Kynar, polyvinylidene difluoride, and poly (1,1-difluoroethane) appears whitish or crystal and is insoluble. Its chemical formula is —(C2H2F2)n–.
2. It has a low melting point and unlike other fluoropolymers, the PVDF offers low values of permeation, especially in liquids and gases.
3. Compared to other fluoropolymers, in terms of tensile modulus, PVDF has a lower strength effect.
4. At increased temperatures, PVDF becomes soluble in liquids like amines which allow it to be applied on architectural surfaces.
5. PVDF is used in wiring processes due to its possession of a very high dialectic constant and dispersion factor. Its poor electrical makeup permits the formation of piezoelectric based PVDF film. A certain level of voltage is produced when these films are placed under pressure either by compression or stretching.
History and Production of PVDF
PVDF was originally introduced in the year 1965 and has significantly grown to become one of the most recognized extrusion coatings in the world. Steady and consistent improvements in its technology have contributed to its durability and increased lifespan while making room for additional coatings and uses. Furthermore, resin-based coatings are being manufactured on a regular basis and branded with various business names. Over the course of the last few years, there have been improvements in PVDF technology which have helped to strengthen its chemical capacity by 70 percent. PVDF is perhaps the oldest battery binder and the best choice for a perfectly stable resin-based binder. Its choice of use is highly dependent on two main factors which are: proof of high performance and commitment to steady and continuous development. The polyvinylidene fluoride is produced by a process of polymerization of the material, which is highly resistant to heat, electricity, and chemicals. It is molded into bottles in chemical industries and thrust as films for insulation.
Use of PVDF
Due to its high level of thermal and electrochemical stability, as well as its strong adhesion, PVDF is the most used binder in lithium-ion batteries. Binders form a vital part of lithium-ion batteries as they provide a connection between electrodes, thus enabling electronic conductivity. PVDF is generally a resin-based fluoropolymer that can be applied like other polymers. The thermoplastic is heated and used in the extrusion and production of pipes, sheets, films, containers and other resin-based products. PVDF binders are often applied in the following electrochemical industries/sectors: oil and gas; biomedical science; pharmaceutics; food and beverage processing; waste management; construction; batteries and electronic components. Its applications in some of these above-mentioned sectors are discussed below.
- PVDF binder for lithium-ion batteries
The lithium-ion battery is made up of the following components: a separator coating (which goes by the trade name, Kyner); an electrode binder; electrodes (cathode and Anode) and other active materials like Graphite. Electrode binders have high electrical stability; low electrode resistance and strong adhesion.
Criteria for battery binder
Given the complex nature of the production process, as outlined earlier, the properties of the binder are so important and must conform to the stipulated criterion. The major characteristics include water absorption; ductility; adhesion; swelling in electrodes (cathode and anode); purity; crystallinity; melting point and distribution of molecular weight. As we know, PVDF is perhaps the only polymer material with high voltage stability and bonding capacity in an NMP solution. Where the molecular weight is low, chances are that it will dissolve more easily, with the battery performance being unstable. However, where the molecular weight is high, the PVDF material will only swell but not dissolve completely. For instance, if a PVDF of three different molecular weights were to experiment, with the first containing about 3 million polymers, the 2nd containing 1 million and the third, 500, 000 respectively, it would be observed that the 2nd and 3rd PVDFs, which have a lower molecular weight would have the best battery performance. However, the first having a larger molecular weight would not perform as good. Thus, it can be confirmed that the magnitude of molecular weight will affect battery performance. The CuO anodes, using the three different molecular sizes would demonstrate a poor cycle performance and discharge capacitance. Using the 2nd PVDF, the CuO would show the best cycle performance and discharge capacitance. The reason for the weak cycle capacity of lithium batteries can be explained as: overcharge and discharge on the surface of the electrode materials; disintegration of electrolyte solvent during the process of discharge; the production of SEI films during the cycle process on the same surface; an irreversible side effect also during the process.
PVDF’s use in Electronics
PVDF is often used as an insulator in electrical wires due to its high thermal conductivity; its resistance to corrosion and heat; low weight and flexibility. Majority of the wires used in wrap circuits assembly is insulated by PVDF. The wire in this context of use is commonly referred to as "Kyner wires" (also a trading name). In lithium-ion batteries, PVDF is the basic binder material used in the production and sustenance of composite electrodes. In preparation, a solution of 1 to 2 percent of PVDF is mixed with a lithium storage material like silicon, tin or graphite. A conductive additive is also added, such as carbon fibers. This solution is then thrown into a current collector with the NMP (N-methyl-2-pyrrolidone), transitioning into a gaseous state to form a compound. PVDF is a preferred choice here due to its high electrochemical stability and resistance to lithium.
PVDF in Biomedical Science
Here, PVDF is used in protein immunoblotting as a synthetic membrane, whereby proteins are conveyed using electrical conduit. Since PVDF is highly resistant to solvents, the membranes above can, therefore, be easily removed and used to study other forms of protein. These membranes are used as filtration devices in other biomedical applications in the form of injections or wheel filters. The outstanding characteristics of this material make it essential in this field.
Other applications of PVDF
The flexible tubes produced from the resin-based polymer is commonly used in the production of fuel lines, pure water, and also used in the large scale production of pipes and valves liner. The material is equally used in separation and filtration facilities in production industries. It is also used in making pyroelectric and laser beam profile sensors in other advanced applications.
Energy industries, automotive and aircraft industries also depend on PVDF materials since they are resistant to corrosion and heat.
Application in high-temperature activities: PVDF is applied in sheet, coatings, and piping in high temperatures due to its resistance to heat and corrosion. Examples of such situations include chemical production, air plenums, etc.
Other forms of PVDF:
Copolymers: Copolymers are an integral part of PVDF which are utilized in such applications as piezoelectric and electrostrictive. One widely used copolymer is p (VDF-trifluoro ethylene) which is most often available in proportions of 50:50, 70:30. Another commonly used copolymer is P (VDF-tetrafluoroethylene). They play an essential role in improving the crystalline appearance of the material. Compared to the pure PVDF, copolymers have a higher crystalline appearance resulting in a wider piezoelectric response.
This also forms part of PVDF and are also commonly used due to the high electromechanical capacity. The most widely used terpolymers are P(VDF-TrFE-CTFE) and TIFE-CFE. This polymer is produced by the introduction of chlorotrifluoroethylene-CTFE into the P (VDF-TiFE) chains. This incorporation is used to disturb the polar phase ordering leading to the assemblage of nano-polar areas. When electricity is applied, the deformed areas translate to an all-trans state.
In conclusion, it can be noticed from the above discussion that PVDF plays an important role in the powering of lithium-ion batteries, and the reasons for its wide usage have been explained as well. The presence of strong piezoelectricity was first discovered in the material in 1969. Since then, several technologies have been developed using the material as a strong base, and these technologies have continued to evolve to date. The properties include its resistance to heat and corrosion, insolubility, and crystalline nature. Apart from its application in the lithium-ion batteries, PVDF is also applied in other areas such as pharmaceutics, energy firms, construction industries, biochemical science/research, food and beverage processing, and other electronic components. It should be noted that lithium-ion batteries are commonly found in mobile phones and is sometimes written boldly on them. Lithium-ion batteries are one of the world's most sought after technologies, powering almost everything from cars to mobile phones. Therefore, to have a battery that functions at its best, in terms of quality and safety, the quality of materials used have to be topnotch. As stated in the introductory paragraph of this article, it is almost impossible for a lithium-ion battery to function without PVDF as it’s the main binding agent.