Thymol–Menthol Deep Eutectic Solvent as a Functional Additive for PEO-Based Solid Polymer Electrolytes

In the quest for safer and more efficient batteries, scientists have long looked beyond the liquid electrolytes that power today's devices. While liquids work well, they carry risks of leakage, evaporation, and flammability. This has driven a shift toward solid polymer electrolytes, which are flexible sheets of plastic that can conduct electricity without the dangers of a liquid. The most common material for these sheets is a polymer called polyethylene oxide. However, this material has a stubborn weakness: it is too stiff and rigid at room temperature, making it difficult for ions—the tiny charged particles that carry energy—to move through it quickly. To fix this, researchers often try to soften the plastic or add substances that help ions slide more easily, but finding the right additive that is both effective and environmentally friendly remains a significant challenge.

A team of researchers from Malaysia has taken a fresh approach to this problem by turning to nature for a solution. They investigated a specific type of mixture known as a deep eutectic solvent, created by combining two common, naturally occurring compounds: thymol, found in thyme, and menthol, the cooling agent in mint. When mixed in specific proportions, these two solids melt together to form a clear, stable liquid at room temperature, behaving like a solvent without the toxicity of traditional chemicals. The researchers set out to see if this green liquid could be used as an additive to soften the stiff polyethylene oxide sheets and boost their ability to conduct electricity.

The team began by creating several versions of this thymol-menthol liquid, testing different ratios of the two ingredients to find the most stable form. They measured physical properties like density and surface tension to understand how the molecules were interacting. They found that a mixture containing four parts menthol to one part thymol created a liquid with the lowest density and surface tension, suggesting a very organized and fluid molecular structure. Before adding this liquid to the battery material, they confirmed that the polyethylene oxide could dissolve into it, swelling and turning into a translucent gel, which indicated that the two substances were compatible and ready to be combined.

Next, the researchers mixed the liquid additive into the polyethylene oxide to create a new solid film. To understand what was happening at the molecular level, they used a technique called Fourier transform infrared spectroscopy, which acts like a fingerprint scanner for chemicals. The results showed that the hydroxyl groups in the thymol and menthol were forming strong bonds with the oxygen atoms in the polymer chains. This interaction was crucial because it meant the additive was not just sitting on the surface but was deeply integrated into the structure of the plastic, effectively loosening the tight grip the polymer chains had on each other.

The team then tested how well these new films conducted electricity using a method called electrochemical impedance spectroscopy. They compared the pure polymer sheets against sheets containing different amounts of the thymol-menthol additive. The pure polymer was a poor conductor, but as soon as the additive was introduced, the ability to carry charge improved significantly. The best performance came from the mixture containing the highest amount of menthol, specifically the one with a four-to-one ratio of menthol to thymol. This specific blend allowed the material to conduct electricity at a rate of 1.28 × 10⁻⁶ S cm⁻¹, which was a substantial improvement over the unmodified material. The researchers attributed this success to the lower viscosity of the mixture, which gave the ions more room to move, and the unique chemical structure of the thymol, which helped facilitate the flow of charge.

Finally, the researchers checked how well these new materials would hold up under heat. They found that while the pure thymol-menthol liquid broke down at relatively low temperatures, the moment it was locked inside the polymer matrix, it became much more stable. The resulting solid film could withstand temperatures well above what is required for typical battery operation, proving that the additive did not compromise the safety or durability of the final product. The study concludes that using this natural, green liquid as an additive is a promising strategy for making solid polymer electrolytes more efficient. While the researchers note that further work is needed to optimize the material for real-world devices, their findings demonstrate that simple, eco-friendly ingredients can significantly enhance the performance of next-generation battery components.