Polymers: The Future of Lithium-Ion Batteries

I. Introduction

With the rise of innovative electrical technologies such as smart homes and electric cars, efficient and safe batteries are becoming increasingly sought after on a global scale. Polymers are already an essential component of all batteries, but modern research has shown that their usage can be pushed to improve the capabilities of the battery further. Not only can they significantly improve current lithium-ion batteries, but they also offer a sustainable and low-cost energy source; lithium mining is toxic and detrimental to the environment. In this article, we will discuss how lithium-ion batteries work, how polymers are currently being used in lithium-ion batteries, and how polymers are a more sustainable material for the future.

II. What are Polymers?

Polymers—coming from the Greek words poly (many) and meros (a share)—are large molecules made of long chains of smaller molecules known as monomers. Each of these monomers can be just a few atoms long or a complicated ring composed of dozens of atoms which are bonded together to form a macromolecule. Polymers are classified as either synthetic or natural. Synthetic polymers are artificially produced by combining several copies of a single molecule, such as hydrocarbons. This process of chemically merging molecules to form a polymer is called polymerization, and it cannot happen without external intervention. Natural polymers are omnipresent in living organisms; proteins and DNA are instances of natural polymers in our bodies. Inorganic polymers, especially chains of carbon atoms, also form naturally, such as graphite and diamond. The polymers currently used in lithium-ion batteries are all synthetic, the most common being polyvinylidene fluoride (PVdF).

III. Structure of a Lithium-Ion Battery

Lithium-ion batteries are by far the most common type of battery. They consist of four main components: a cathode, an anode, a separator, and an electrolyte. The lithium is stored in the cathode and the anode, and the lithium ions move from one to the other when charging or discharging the battery. The separator is a porous physical barrier that prevents the cathode from interacting with the anode, which would cause a short-circuit. An electrolyte is a conductive liquid that carries charged ions back and forth between the cathode and the anode through the separator. We use lithium in batteries because of its high energy density, low self-discharge rate, and long cycle life.

IV. Polymers as Binders of Electrodes

Binders of electrodes are located in the cathode and anode, and they serve a dual function within the battery. They ensure adhesion between the redox active materials and the current collector, and facilitate electron/ion movement. Polymers are used as binders in lithium-ion batteries because of their electrochemical stability and adhesion. Binders also need to be flexible and insoluble in the electrolyte. Historically, the most common polymer used is PVdF. However, its electrode fabrication process requires a toxic and expensive solvent, N-methyl-2-pyrrolidone (NMP). Ongoing research is being conducted to find a more eco-friendly and inexpensive binder. In recent years, biopolymers and water-processable polymeric binders (aqueous binders) have been investigated, but their use comes with challenges: water-processable binders can cause cracks as well as degradation of the active materials. Researchers are also developing multifunctional polymer binders, which have electronic and ionic conductivity. Essentially, they are fulfilling the role of binder while contributing to the transport of electrons and ions. Conducting polymers such as PEDOT:PSS, PPy, and PANI are very promising materials as they reduce the carbon content of the cathode while improving the performance of the battery.

V. Polymers as Separators

Polymers are also used as separators, namely polyolefin, because of their low production cost. However, several other polymers (namely PVdF) have additional properties such as wettability and varying pore size, which greatly improve the effectiveness of the separator. Biopolymers and natural minerals are the most popular alternatives, as they are bio-based and relatively inexpensive, especially cellulose-based biopolymers, due to their remarkable flexibility. However, at this moment, the production costs are too high for large-scale production of these organic materials. Other types of polymers are being investigated to improve thermal stability and flame retardancy, the latter a major security concern, especially in batteries for electric cars. In particular, polyethylene oxide (PEO) is one of the most analyzed polymers because of its extremely high flame retardant effects in conjunction with its compatibility with lithium salts.

VI. Redox Polymers

Most lithium-ion batteries use inorganic redox materials to transfer electrons and produce electricity. However, redox polymers maintain a better performance at higher currents. For instance, polymer group TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy)-based electrodes have been reported to display a 3.5 V redox potential, while usual inorganic materials have potentials around 3.0 V. Due to their bio-based nature, redox polymers present several drawbacks, such as low electrode loading, high carbon content, shorter lifetime, and high self-discharge rate. Once these shortcomings are solved, redox polymers would enable all sorts of new technological advances in battery development.

Indeed, having polymers as the active materials enables the possibility of using other ions such as aluminum, calcium or magnesium in place of lithium. These multivalent ions used in conjunction with aqueous electrolytes could produce significantly more powerful and efficient batteries. Redox polymer batteries will also propel technologies like redox flow batteries (RFBs) and all-organic/polymer-air printable batteries. Current RFBs — which have a very high longevity — use vanadium as the active material, which has a high energy cost because of its low energy density. Polymer RBFs would not have this setback. Organic printable batteries have the advantage of having extremely precise designs that are thin, flexible, and light.

VII. Conclusion

As the global demand for clean energy grows, batteries will only increase in importance in the upcoming years. Polymers will play a crucial role in the development of a sustainable source of power. They are already used in lithium-ion batteries, but research has shown that they are also extremely useful in other parts of the battery because of their unique properties. In addition, they are eco-friendly and inexpensive to produce, which is the main drawback of many current inorganic materials used in batteries, namely lithium and graphite. Polymer batteries, alongside new energetic technologies, will substantially improve the current lithium-ion batteries while providing a green sustainable energy source to the world.