Engineers at the University of Wisconsin–Madison have developed a versatile new electrolyte that advances the development of an initially anode-free sodium-ion battery—a promising alternative to lithium-ion batteries for electric vehicles and grid energy storage. Led by Assistant Professor Fang Liu and PhD students Qianli Xing and Ziqi Yang, the team is also using this electrolyte as a model system to explore how molecular manipulation can improve compatibility between different battery components, potentially paving the way for more efficient and energy-dense battery technologies.
Containing solvents and dissolved salts, the electrolyte is the liquid medium that touches all parts of the battery’s cells and, in its charging or discharging process, helps ions travel between the electrodes.
In a battery, the anode and cathode are different materials—for example, graphite, hard carbon sodium or lithium for the anode and a transition metal oxide like lithium nickel manganese cobalt oxide or sodium nickel iron manganese oxide for the cathode.
One of the challenges in developing next-generation batteries is that there’s not a one-size-fits-all electrolyte that performs effectively with both electrode material types. Conversely, when an electrolyte contains multiple solvent molecules, controlling their interactions and behavior is challenging.
Tweaking the electrolyte is a balancing act involving multiple factors, including how solvent molecules in the electrolyte form a “shell” around ions that could accelerate or impede the ions’ movement between anode and cathode—which ultimately affects battery charging and discharging, along with overall battery performance. “Using this model system, we are basically trying to understand whether we can present different molecules to different electrode surfaces—for example, an anode-stable solvent to the anode, and then a cathode-stable solvent to the cathode,” says Liu. “In this way, the electrolyte mixture would ideally behave like an anode-stable solvent at the anode, and like a cathode-stable solvent at the cathode.”
To create its new electrolyte, the team mixed two ether-based solvents, 2-methyltetrahydrofuran, or 2-MeTHF, which is more stable at the anode, and tetrahydrofuran, or THF, which is more stable at the cathode. Importantly, they found a way to rationalize electrolyte design: Solvents that dominate the first shell around positively charged ions that travel between electrodes are key to anode stability, while “free” or more weakly bonded solvents are important to the stability of the cathode side. “Through this electrolyte engineering work, we were trying to demystify what determines the stability of the anode and cathode separately, and how to present suitable molecules to both electrodes,” says Liu. “Qianli found out that the key factor is the population of solvents in the first solvation shell versus outside, and their location determines their presentation during the battery formation process.”
Computational testing, conducted by collaborator Reid Van Lehn, an associate professor of chemical and biological engineering at UW-Madison, and his student Jung Min Lee, played a significant role in the research as well. They used all-atom molecular dynamics simulations to predict the composition of solvent molecules near sodium ions and determine whether those ions “preferred” one solvent over the other. “Our results indeed found—in good agreement with experiments from the Liu group—that we could identify a single strongly interacting solvent (2-MeTHF) and a weakly interacting solvent (THF),” says Van Lehn. “We further used these calculations to relate this behavior to the relative strength of interactions of each type of solvent, providing molecular-scale insight that can be extended to even more complex mixtures to continue optimizing electrolyte design.”
The research lays the groundwork for the next steps in developing not simply sodium-metal batteries, but also other new alternatives to lithium-ion batteries. “Through this research, we start to understand that the solvent and anion interactions become really important,” says Liu. “We’re trying to expand our solvent library to manipulate these kinds of interactions, to see whether this kind of working principle can be applied to broader solvent libraries and different battery chemistries.”
