Simulations evaluate new electrolytes for batteries of the future

A computational study published in the Journal of Molecular Liquids makes important contributions to the development of new, safe, high-performance batteries by investigating compounds that can be used as electrolytes in sodium-ion batteries.

The work involved the efforts of a team of researchers linked to the Center for Innovation on New Energies (CINE). Created in 2018, the CINE is based at the State University of Campinas (UNICAMP), the University of São Paulo (USP), and the Federal University of São Carlos (UFSCar), with the participation of eight other Brazilian institutions. Researchers from the University of Bonn in Germany collaborated on this study.

These batteries are based on sodium, an abundant element that is widely distributed across Earth. They are considered to be very promising, especially for storing surplus energy from solar and wind farms. They operate based on the movement of sodium ions between the battery electrodes and across the electrolyte during charging and discharging.

The most widely studied compounds for conducting these ions in the electrolyte are ionic liquids, a class of salts that exist as liquids at room temperature. These compounds are good ion conductors and are nonflammable, offering high safety for batteries. However, adding sodium ions to these liquids increases their viscosity, reducing ion mobility and worsening electrolyte performance.

To overcome this limitation, CINE researchers investigated a series of electrolytes based on two types of ionic liquids: aprotic, which is the most commonly used in electrolyte research, and protic, which is less expensive and easier to produce, though it has received little study. The idea was to add sodium salt to these compounds to improve ion mobility.

“The main point of this work was to evaluate the effect of increasing the concentration of sodium salt in an electrolyte based on a protic ionic liquid and its analog containing an aprotic ionic liquid,” summarizes Tuanan da Costa Lourenço, a postdoctoral researcher at the CINE at the São Carlos Institute of Chemistry (IQSC-USP) and the corresponding author of the article.

To conduct the study, the team performed molecular dynamics simulations, a computational method that describes the interactions of atoms and molecules. Using dozens of networked computers and advanced software, the authors solved the complex mathematical equations involved in the simulations.

In the process, they used computational resources from USP, the National Laboratory for Scientific Computing (LNCC), located in Petrópolis in the state of Rio de Janeiro, and the University of Bonn.

The results showed that increasing the sodium salt concentration changes how the ions in the ionic liquid organize and interact with each other. The study also demonstrated that the magnitude of this change depends on the molecular structure of the ions and whether the ionic liquid is protic or aprotic.

“Additionally, we observed that at high sodium salt concentrations, there’s a decrease in the interaction forces between the sodium ion and the electrolyte anion, which can be beneficial for battery performance,” says Lourenço.

International collaboration

One of the objectives of the CINE’s Computational Materials Design (CMD) program is to select promising electrolytes for the batteries of the future. Juarez Lopes Ferreira da Silva coordinates the program and co-authored the paper.

The project involved members of the program from USP and the research group of Barbara Kirchner at the University of Bonn. Kirchner specializes in modeling large, complex systems involving liquids and their interfaces, and she is a co-author of the article.

“The collaboration not only made it possible to deepen the discussion and conclusions obtained, but also resulted in the development of new tools, improvements to the models used, and new collaborations,” says Lourenço, who spent 15 months working on the study in Kirchner’s group.

The scientific team is continuing the research through a second study that aims to understand how to modulate the interactions between ions in ionic liquids to optimize battery performance.