Sodium-metal batteries are drawing attention because they could offer a more resource-abundant alternative for future energy storage. A team based at MIT has now shown that the right electrolyte solvent may help these batteries move closer to the combination researchers want: stability, fast charging, and strong power delivery.
Why sodium-metal batteries matter
Lithium-ion batteries remain the leading option in electric vehicles and battery energy storage systems. But they depend on critical minerals including lithium, cobalt, nickel, and graphite. These materials are considered important for economic and national security reasons, which makes them vulnerable to supply chain disruptions.
That concern is growing as renewable energy, electrified infrastructure, and high-power digital technologies expand. The source article describes a need for energy storage systems that are low-cost, resource-abundant, and able to charge and discharge quickly.
The MIT team, led by Ju Li, the Carl Richard Soderberg Professor of Power Engineering in the departments of Nuclear Science and Engineering and Materials Science and Engineering, is exploring complementary energy storage solutions. Sodium-metal batteries are one focus because sodium is about 1,000 times more abundant than lithium and, pound for pound, about one-hundredth the cost.
The appeal is clear, but so is the problem. Sodium metal is highly reactive. That makes it hard for sodium-metal batteries to achieve both long-term stability and fast cycling.
The electrolyte problem
A battery has three main components: a negative electrode, a positive electrode, and an electrolyte. The electrolyte allows electrically charged ions to move between the electrodes. In principle, it should serve mainly as an ion conductor.
In practice, many electrolytes do more than transport ions. They can react with the electrodes, creating unwanted side reactions that weaken battery stability. Weiyin Chen, a postdoc in Nuclear Science and Engineering and one of four lead authors of the Joule paper, explains that insoluble compounds formed by these reactions can build up on electrodes. That buildup can block ion transport and eventually cause the battery to fail.
Until recently, Chen says, no electrolyte used in sodium-metal batteries was fully stable against unwanted reactions at both the anode and cathode. That matters because rechargeable batteries need this kind of stability to reach a long cycle life.
The team’s earlier reference point came in 2021, when Li’s group and collaborators identified a sulfonamide molecule made of sulfur, oxygen, and nitrogen atoms. Used as a solvent, that molecule, known as DMTMSA, was stable at both electrodes in lithium batteries.
How smaller molecules can help
Building from DMTMSA, Li and colleagues looked for related molecules that could improve sodium batteries. The goal was not just stability. The solvent also needed to support fast charging and fast discharging.
The source article uses a simple analogy from Chen: moving through a crowded street is easier with a small backpack than with a bulky suitcase. In batteries, sodium ions surrounded by smaller solvents can move faster than ions surrounded by larger solvents. Faster ion transport supports quicker charging and discharging.
That creates a design challenge. Highly conductive electrolytes can react more easily with electrodes, shortening battery life. The team’s idea was that reducing solvent size could offer a path through this trade-off.
To search for candidates, the researchers focused on molecules that were congeneric, meaning they were in a similar molecular family. They wanted molecules related to DMTMSA that were smaller while retaining the stability that made DMTMSA promising.
AI narrowed the search
Chia-Wei Hsu, an MIT PhD student in materials science and engineering, created an AI-guided algorithm to generate possible solvent molecules. The algorithm designed 100,000 candidate molecules on his computer within 24 hours.
Hsu then reduced the pool to 200 candidates using technical criteria, including similarity in shape to DMTMSA and comparable electronic properties. From there, 27 representative candidates were chosen for experimental testing.
The tests were run under the same conditions so the comparison would be direct. One solvent stood out: DMFSA. It was both the smallest and the best performer among the tested candidates.
A new paper in Joule, written by 15 members of the MIT team and published online this week, presents the result. The central finding is not simply that one solvent performed well. It is that solvent size and molecular similarity can guide electrolyte design for a difficult battery chemistry.
What the finding could mean
Jinhyuk Lee, an associate professor of materials engineering at McGill University who was not part of the study, described the work as addressing a persistent battery research challenge: improving performance at high charging and discharging rates without giving up long-term stability.
The MIT group is continuing the search. This time, DMFSA rather than DMTMSA is serving as the starting point. Chen believes the new solvents being uncovered could eventually help rechargeable sodium-metal batteries combine low-cost, abundant materials with fast charging and high-power performance.
The broader point is that sodium-metal batteries are also serving as a model system. The researchers see the work as a way to demonstrate a more general design principle for electrolytes, using solvent size and molecular similarity as guideposts.
The project was supported in part by a National Research Foundation of Korea grant funded by the government of Korea government, as well as U.S. National Science Foundation graduate research fellowship. Characterization equipment used in the project is partly from the MIT.nano Characterization Facilities.