The Grainger College of Engineering
Civil & Environmental Engineering

A drop of water makes all the difference: Rethinking sodium battery electrolytes

6/22/2026 Jackson Brunner

Written by Jackson Brunner

Lithium-ion batteries have powered the portable electronics revolution and much of the world's growing electric vehicle infrastructure, but they are approaching a ceiling. Over the past decade their energy density has plateaued near its theoretical limits, and mounting concerns about the availability of raw materials like cobalt and lithium have made finding alternatives increasingly urgent. Sodium-ion batteries are among the most promising candidates, but key aspects of how they behave at the molecular level remain poorly understood. This gap is a significant obstacle to building batteries that can genuinely compete. 

Schematics showing the aggregation of clusters near the charged surface in the dry salt-in-ionic liquid, and disruption of aggregation upon addition of a small amount of water.
Schematics showing the aggregation of clusters near the charged surface in the dry salt-in-ionic liquid, and disruption of aggregation upon addition of a small amount of water.
Professors from The Grainger College of Engineering, University of Illinois Urbana-Champaign Rosa Espinosa Marzal, in the Departments of Civil and Environmental Engineering and Materials Science and Engineering, and Cecilia Leal, also in the Department of Materials Science and Engineering, have been working to find solutions. Their paper, "Water doping sodium battery electrolyte controls nanostructure, interactions and electrochemical properties," published in Science Advances, focuses on electrolytes called salt-in-ionic liquids, in which a sodium salt is dissolved in an ionic liquid — a molten salt that remains liquid at room temperature. These electrolytes are attractive because they can accommodate very high ion concentrations and remain stable across a wide voltage range. Their critical weakness is that ions within them cluster into aggregates, making the electrolyte extraordinarily viscous, restricting ion movement, and limiting battery power. 

The researchers found that introducing very small amounts of water into this system disrupts those aggregates, reducing their size and complexity, and improving ionic conductivity. Water molecules do not simply push ions apart — they insert themselves directly into the structure surrounding each sodium ion, actively participating in its solvation shell alongside the ionic liquid's own anions, as found from the molecular simulations performed by their colleague Dr. Zac Goodwin at the University of Oxford. 

Pictured: Typical bulk structure in dry SiIL (left) and water-in-SiIL (right)
Pictured: Typical bulk structure in dry SiIL (left) and water-in-SiIL (right)

 

"What makes this behavior so interesting is that it arises from a balance between water–ion and ion–ion interactions at the molecular scale — water doesn't just dilute the system, it actively reorganizes it,” said Espinosa Marzal.  

The effects extend to the electrode surface as well, where water modifies the arrangement of ions at the electrode interface and influences the formation of the solid electrolyte interphase, a thin protective film whose stability is critical to a battery's long-term performance. The water-doped electrolyte promotes a more compact, uniform version of this film, suppressing the dendritic growth that can short-circuit a battery and limit its usable life. 

By connecting nanoscale ion interactions to a concrete improvement in battery behavior, Espinosa Marzal and Leal offer a new and practical lever for researchers working to make sodium-ion batteries a viablealternative to the lithium-ion technology that has defined energy storage for a generation.

 Illinois Grainger Engineering Affiliations 

Rosa Espinosa Marzal is an Illinois Grainger Engineering professor in the Department of Materials Science and Engineering and the Department of Civil and Environmental Engineering. She is affiliated with the Materials Research Laboratory and the Institute for Genomic Biology. Espinosa Marzal holds the Ivan Racheff Professorship and the Donald Biggar Willett faculty scholar appointment.  

Cecilia Leal is an Illinois Grainger Engineering professor of materials science and engineering and is affiliated with the Department of Bioengineering and the Carle Illinois College of Medicine. She holds the Racheff Faculty Scholar appointment.

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This story was published June 22, 2026.