A smaller solvent cracks the sodium battery's ion-blocking problem

Lithium-ion batteries rely on four minerals that the U.S. Department of Energy and battery analysts have flagged as economically and strategically vulnerable: lithium, cobalt, nickel, and graphite. Those minerals are concentrated in a few countries. Sodium, the element just below lithium on the periodic table, is roughly 1,000 times more abundant in the Earth's crust and costs about one-hundredth as much per pound.

The trouble is that sodium-metal batteries have died young. Every prior attempt has hit the same wall: when the cell charges and discharges, insoluble side-reaction products build up on the anode and cathode, the surfaces that exchange sodium ions, until the ions can no longer move through the electrolyte, the liquid that carries them. The cell dies long before its theoretical cycle life.

An MIT team now reports in the journal Joule that it has identified an electrolyte that holds up against both electrodes. The molecule, called DMFSA, is small enough to let sodium ions slip through quickly while keeping the liquid stable against the metal surfaces on either end. The work follows a precedent set five years ago, when Ju Li's group at MIT and collaborators identified a solvent called DMTMSA that stayed stable at both electrodes in lithium cells. The new paper extends that family of molecules with the same chemical backbone but different sizes to sodium, and shows that going smaller is what unlocks the speed/stability trade-off. Fifteen MIT researchers co-authored the paper; postdoc Weiyin Chen of the Department of Nuclear Science and Engineering is one of four lead authors.

The team ran most of the search in software. Chia-Wei Hsu, a PhD student in MIT's Department of Materials Science and Engineering, built an algorithm that generated 100,000 candidate solvent molecules in 24 hours, narrowed them to 200 by similarity, shape, and electronic criteria, and then experimentally tested 27 of those head-to-head against the lithium reference. DMFSA, both the smallest molecule in the test set and the best performer, won.

Jinhyuk Lee, an associate professor of materials engineering at McGill University who was not involved in the study, called the result a real advance. "This research addresses one of the most persistent challenges in battery research: improving battery performance at high charging and discharging rates without sacrificing long-term stability," Lee told MIT News.

The caveats matter. A stable electrolyte does not yet make a shippable sodium-metal battery. The group has not disclosed cycle-life numbers, energy density, or any path to manufacturing scale; the Joule paper is a peer-reviewed mechanism result, not a product. Sodium-ion batteries that use a hard-carbon anode instead of metallic sodium are already in early commercial deployment, mainly for stationary grid storage, and the new electrolyte targets a different, harder cell architecture that promises higher energy density if it can be tamed.

The team's next move is to run the same AI-guided search again, this time starting from DMFSA and looking for the next small step beyond it. Funding came from the National Research Foundation of Korea and the U.S. National Science Foundation.