The core of the Li-ion battery is the active material: on the cathode side this is for example a metal oxide, such as NMC (lithium nickel manganese cobalt oxide), and on the anode side e.g., graphite. However, batteries contain a lot of other materials too, and many of them are polymers. All these materials must work as planned and remain stable in the challenging operating conditions inside a battery. The polymeric materials have a major impact on the lifetime and even on the sustainability of a battery. Thus, they require as much attention as the other parts.

Conventional binders

One of the most common polymeric materials in a battery is the binder. This is a material that keeps all active material and conducting additive particles attached (kind of glued) on the current collector and to each other. Without a binder, it would not be possible to prepare an electrode layer as the other materials are powders. Since the binder is electrochemically inactive, it is important to minimize the binder content. Otherwise, the energy density suffers.

On the cathode side, the most common binder is polyvinylidene difluoride, PVDF. It is a stable polymer with an electrochemical window of about 5 V and which can thus withstand the harsh conditions. Note that PVDF belongs in the so called PFAS (per- and polyfluoroalkyl substances) group, which are planned to be banned in the EU and around the world due to their environmental burden. They are very stable and long lasting (this is why they work so well as binders) and if they will leak to the environment, they can pollute the groundwater. It is not yet clear if batteries will be excluded from the PFAS regulations, but it is anyway important to find replacement materials for PVDF.

Another issue with PVDF is the solvent, which it requires for the slurry coating. Again, if we think about the battery operation, it is mandatory that the binder does not dissolve e.g., in the electrolyte. Thus, a binder material with low solubility is needed. In the traditional slurry coating process, the binder anyway needs to be dissolved in some solvent. For PVDF, this solvent is N-methylpyrrolidone, NMP, which is a problematic chemical. NMP is classified as a “substance of very high concern”, and it is known to be toxic to the reproductive system. NMP has also a high boiling point and it requires a lot of energy to dry the electrode layer after coating. Thus, NMP decreases the sustainability of the battery production and affects the safety of the workforce.

Water-based binders are more sustainable and safer. They are already used especially on the anode side where the requirements for the electrochemical stability are not as demanding as for the cathode. One of the most common water-based binders is SBR-CMC, which is a mixture of styrene butadiene rubber and sodium carboxymethyl cellulose. Some of the battery producers already use water-based binders on the cathode side too, but majority are still using PVDF. Using a water-based binder on the cathode side becomes even more challenging when we increase the nickel content in NMC since the active material will become more sensitive to moisture. Special protective layers or other methods are needed to enable using water-based binders and slurries for NMC811 or beyond. This requires still research.

Functional binders

Usually, the binder is an inactive component in the battery. However, some novel binders have also functional properties. One example of a functional binder is a self-healing polymer that can heal the cracks that may be formed in a battery electrode during the charging and discharging cycles. Crack formation is a problem especially in such electrode materials that undergo large volume changes during lithium intercalation and deintercalation. For example, using a silicon anode instead of graphite would increase the energy density of the battery. But silicon suffers from very large volume changes and the electrode will break unless special self-healing methods are used. Self-healing binders are still in the research phase and studied e.g., under the Battery 2030+ initiative. Currently, some commercially available batteries have 5-10% of silicon blended with graphite. But if we would increase the amount of silicon to further increase the energy density, traditional binders won’t be enough.

Another example of functional binders is conducting polymers. They are designed to combine the electronic conductivity of the usually used carbon black and the mechanical properties of the binder. This can increase the energy density as we can reduce the amount of inactive materials in the cells. Conducting polymers consist of a backbone with a conjugated structure. This means that they have alternating single and double bonds, which allows electrons to move along the polymer chain, and also hop from one chain to another. Materials like poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and polyaniline are examples of conducting polymers. Polyaniline is my “old friend” as it was one of the main materials I studied in my doctoral thesis about 15 years ago. I have even made paintings about some of the counter-ions I used for polyaniline, and those are still hanging on our dining room wall:

Back then I did not even realise that these materials could be used also in batteries! Now I’m very happy that we are exploring the use of conducting polymers in solid-state batteries in the SOLiD project. Maybe I should start working with them even more, as I know quite well how their properties and processability can be tuned 😊

Separators

Then, another part where polymers are used in batteries is the separator, which allows ions to flow through it but works as an electrical insulator between the electrodes. The most common separator type is a porous polypropylene (PP) or polyethylene (PE) membrane. PP and PE work relatively well as a separator in standard Li-ion batteries. However, some special/novel electrolytes might require surface treatment for the PP/PE to allow sufficient wetting and thus ionic conductivity.

Another separator material is cellulose. It is also a polymer, but a natural one. Cellulose is not (yet) used in commercial batteries that much, but I see high potential for it. Its wetting properties are superior, and it has also very good thermal stability. Being also a sustainable option, I hope and believe that we will see much more cellulose separators in batteries in future.

A separator can have self-healing properties too. We just finished a European project called HIDDEN, where we developed a self-healing piezoelectric separator that slowed down the dendrite growth in batteries with a metallic lithium anode. This helped to increase the lifetime by 65 % in the best case. The best results were achieved in coin cells, but we saw also some improvement in pouch cells. This means that up-scaling of the processing requires still more work, but in principle the idea seems to work.

Electrolytes

The electrolyte in a conventional Li-ion battery is a lithium salt, dissolved in an organic solvent. However, solid-state batteries have a solid electrolyte instead, and often it is an ionically conducting polymer or a blend of polymer and ceramic particles. A solid electrolyte allows using a metallic lithium anode, which enables increased energy density. Fully ceramic solid electrolytes do also exist, but here I focus only on the polymeric ones.

The main benefits of using a polymeric electrolyte include its intrinsic safety (no flammable solvents), relatively easy processing, mechanical roughness, and flexibility. However, there are still challenges, as most of the polymers have modest ionic conductivity especially at room temperature and they might thus require heating the battery to 50-60 degrees Celsius to work properly. Better and more ionically conducting polymers are being actively developed.

Usually, the ionically conducting polymer is mixed with a Li-salt. Here the polymer functions as a matrix for the Li-salt. In the matrix, the mobility of the Li+ cation is much lower compared to the anion, which leads to a so-called concentration polarization. This means that the anions accumulate on the cathode side during charging, and the lithium electrode interface thus lacks anions, which will generate a strong electric field, eventually leading to growth of dendrites due to uneven deposition of lithium on the negative electrode. And dendrites, as we know, are detrimental to the battery. But it is also possible to synthetize polymers, which have the anionic counterpart covalently bound to the polymer backbone and only the Li+ cation is moving. These are called single-ion polymer electrolytes. They allow better dendrite prevention as the anion cannot move, and thus there won’t be a depletion of anions at the lithium anode interface.

Active materials

Finally, one very interesting option of using polymers in batteries is to use them as the active material. This is important especially due to sustainability aspects, as we would not need that much mining if we could use polymers to replace critical battery raw materials – at least in some applications. These materials can be even synthetized from bio-based sources and in the best case from waste/side streams.

There are still challenges with organic active materials. Most of them are related to their stability, as they may e.g., dissolve into the electrolyte. This is of course a problem and would decrease the lifetime significantly. They also do not have as high volumetric energy density as the inorganic ones, but they could be very much suitable for applications where this is not a problem.

We are studying organic cathode active materials in batteries in one Finnish project, FinnCERES. The research is still mainly focused on lab-scale studies, both in our work and globally, and up-scaling needs to be still demonstrated. However, there is high potential for organic materials, as also summarized in recent publications e.g., by Alexandru Vlad et al. and Jan Bitenc et al.

Conclusions

As we can see, polymers are already used in several parts in a battery. I mentioned here only such polymers, which are used inside a battery cell. But there are much more examples for the use of polymeric materials also at the module and pack level, such as in casings.

Even though the polymers are working relatively well already, there is still more work and research to be done to improve their properties. This can help to improve the sustainability and lifetime of both currently used and future battery chemistries. With the help of functional polymers, we can also overcome some of the challenges in increasing the energy density.

Thus, if you are familiar with polymers, why not consider using this knowledge in the battery field. There is still a lot of room and a clear need for more people to work with battery development.