Self-adaptive electrolytes expand stability for fast charging and high-energy batteries

To support the ongoing transition to electric vehicles and reduce greenhouse emissions, engineers have been trying to develop batteries that can store more energy, while also operating safely and lasting for long periods of time. Typically, however, high-energy batteries entail longer charging times, which is not ideal for most real-world applications.

Researchers at the University of Maryland recently introduced new electrolytes with an electrochemical stability window that dynamically expands while a battery is charging. These electrolytes, introduced in a paper published in Nature Energy, proved promising for the development of fast-charging high-energy batteries with diverse compositions.

“We wanted to address a longstanding challenge in battery technology: the trade-off between fast charging and high energy density,” Chang-Xin Zhao, first author of the paper, told Tech Xplore.

“During fast charging, the electrode potential can exceed the electrochemical stability window of the electrolyte, leading to undesirable side reactions. We wondered—what if the electrolyte could dynamically respond to the charging process and expand its stable potential window in real time? That could be a promising way to overcome this limitation.”

The newly designed electrolytes draw inspiration from the so-called “salting-out” effect, which is rooted in phase equilibrium theory. This is a phase-separation that occurs when the addition of salt to a solution prompts some components to become less soluble (i.e., separating out of the solution).

“Interestingly, the charging process in a battery inherently generates salt concentration gradients in the electrolyte, which provides the necessary conditions for this effect to occur,” explained Zhao. “Building on this idea, we developed an electrolyte system that leverages such concentration-driven phase behavior to adaptively expand its stability window during operation.”

The self-adaptive electrolytes developed by the researchers have two characterizing features. The first is their ternary composition and associated “salting-out” behavior.

Each electrolyte is comprised of two solvents and a salt, all of which are carefully selected to successfully produce the salting-out effect. As a result of their composition, changes in salt concentration will prompt a phase separation, which in turn expands a battery’s electrochemical stability window during fast-charging.

The second defining feature of our electrolytes is that they are formulated at the cloud point. This means that they are engineered to sit precisely at the cloud point—the critical composition just before phase separation begins.

This positioning makes the system highly sensitive to concentration gradients during charging, allowing it to respond adaptively by undergoing localized phase separation. This, in turn, enables real-time expansion of the electrolyte’s electrochemical stability window as the battery charges.

This recent work opens new exciting possibilities for the development of batteries that can store more energy, while also charging faster. The researchers have already tested the electrolytes they created in both aqueous zinc-metal and non-aqueous lithium-metal batteries, achieving remarkable Coulombic efficiencies and improved stability.

“Traditionally, electrolyte development has focused on molecular-level modifications—tuning the structure of individual solvents or salts,” said Zhao.

“In contrast, our work takes a more macroscopic approach by leveraging phase equilibrium principles. By considering how the overall electrolyte system behaves under dynamic conditions, rather than focusing solely on the molecules themselves, we demonstrate that it’s possible to engineer electrolytes that adapt during operation.”

The researchers hope that their paper will pave the way for a new line of research aimed at overcoming common challenges associated with the advancement of battery technologies leveraging concepts rooted in phase equilibrium theory.

In the future, the approach employed by these researchers could be used to design other promising self-adaptive electrolytes. Meanwhile, they plan to use their proposed strategy to identify other promising electrolytes, while also integrating and testing them in different types of batteries.

“Our future work will focus on operando characterization of interfacial processes in self-adaptive electrolytes, as well as extending this strategy to gel-like systems,” added Wang.

“Scaling up the formulation for pouch-cell validation under practical charging protocols is also an important next step.”

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