Vanillin Batteries – Sustainability Research at the Max Planck Institute of Colloids and Interfaces

After all, the energy supply of the future depends on this in order to save surplus electricity from wind turbines and solar plants for times when there is too little. In the best case, the basic material for the batteries comes from renewable raw materials and is available in almost unlimited quantities. This is exactly not the case for today’s lithium batteries. Lithium is a rather rare metal of which there are not many deposits worldwide. About two-thirds of all lithium batteries also contain cobalt, the mining of which is often socially and ecologically problematic. In addition, there are electrolytes, which are sometimes toxic or flammable. In view of the current strong growth in demand for storage capacity, these are not optimal conditions.

Organic molecules store electricity from wind power

Clemens Liedel sees the vanillin approach as a much more sustainable alternative. This is because the substance can be made from one of the most common substances of all – lignin, a main component of wood. Year after year, nature produces many billions of tons of it. But how can an organic molecule serve as a material for an electricity storage device? After all, when it comes to batteries, people tend to think of metals such as lead, cadmium or lithium. “What you need are substances that release electrons and can then take them up again,” explains Liedel. “These can be metals, which then become metal ions, but also organic substances.”

Instead of substances containing heavy metals, the researchers are therefore relying on vanillin at the cathode, the positive terminal of lithium batteries. But the brittle powder still needs to be optimized. “Normally, the substance is mixed with a binder to form a compact mass and made conductive by adding carbon,” explains Liedel. After several experiments with various substances, the researchers have now made quite a bit of progress and get by with a pure vanillin-carbon mixture without any binders.

For the test as an electrode material, an employee applies the mixture wafer-thin to carbon paper. He punches a small circular piece from it, which he maneuvers into a plastic housing. Then a separating membrane, an electrolyte and a counter electrode are added to the chamber. This completes the battery. The battery voltage can now be measured on two stainless steel cylinders that protrude from the plastic housing. This latest vanillin approach also had to pass long-term tests with many charging and discharging processes – and was convincing. Clemens Liedel’s group is also thinking about the future electrolyte – the component in a battery that ensures the electrical charge balance between the two pole regions with its movable charge carriers, the ions. It usually consists of a conductive salt adapted to the electrodes and a solvent. “Currently, solutions of toxic lithium salts in flammable organic carbonates are common here,” said Liedel. However, the chemist has a more sustainable and safer solution in mind.

There are also initial successes here. Clemens Liedel reaches for a round flask: “It’s an ionic liquid,” he says. This is the name given to salts that are liquid at temperatures below 100 degrees Celsius. As characteristic of salts, they consist of positively charged cations and negatively charged anions. “Ionic liquids are highly conductive for other ions, are hardly volatile and therefore also flame retardant,” he says, explaining why this group of substances is recommended as a solvent for electrolytes.

There are also alternatives for membranes made of petroleum

Ionic liquids are also available on a purely organic basis; in the best case, they can therefore be obtained from renewable raw materials. Another important thing for Liedel: “They are chemically easy to design.” This makes it easy to adjust the ion conductivity of the molecules and also the temperature range in which they are liquid. In the meantime, his group has found a suitable ionic liquid, half of which can be produced from renewable raw materials. In commercial batteries, the two poles are usually close together. To avoid an electrical short circuit, there is a fine-pored partition between them. This separator is permeable to the ions of the electrolyte. This allows the charge balancing that is necessary as soon as electrons migrate from the negative pole via a consumer to the positive pole or when the battery is charged. Usually, petroleum-based plastic membranes serve as separators. Here, too, Liedel’s group has an Alternative. This has opened up quite incidentally during the work. The researchers linked chains of the bio-based polymer chitosan by adding other substances to form a network of macromolecules with small pores. Trials with this have already been successful.

After so much basic research on the battery components, the group is now planning the next step. “Now we want to try to marry all approaches with each other,” says Liedel. So the organic cathode material with the ionic liquid and the chitosan membrane. Instead of the currently widespread lithium-graphite combination, the researchers envisage an electrode based on the more readily available metals sodium or magnesium as a negative pole. “But we still have to clarify some detailed questions,” says Liedel. For example, how well the ionic liquid actually gets along chemically with a bio-based electrode material such as oxidized vanillin.

Text: Karl Hübner, Foto: Bettina Ausserhofer

more articles in the PNN special supplement on the Potsdam Science Park from 21.09.2019