For years, battery scientists have operated under a simple assumption: when designing electrolytes for aqueous zinc batteries, more polar is better. The logic seemed sound. Highly polar organic molecules should strongly attract zinc ions and water, disrupting the problematic water shell that surrounds zinc and triggers destructive side reactions. But new research reveals this conventional wisdom is wrong.

A team working on aqueous zinc battery electrolytes has discovered that the relationship between solvent polarity and battery performance follows an unexpected pattern. Instead of improving steadily as polarity increases, performance peaks at a moderate polarity value before declining again. This volcanic curve challenges fundamental assumptions about how these batteries work and opens a pathway to dramatically longer battery life.

The finding matters because aqueous zinc batteries represent one of the most promising alternatives to lithium ion technology. They are safer, cheaper, and more environmentally friendly. Yet they suffer from a critical weakness: the zinc metal electrode degrades rapidly during charging and discharging, limiting cycle life to levels far below what practical applications demand.

The Water Problem

The core issue plaguing aqueous zinc batteries is water itself. When zinc ions plate onto the electrode during charging, water molecules in their surrounding shell get pulled along. This coordinated water has a higher reduction potential than free water, making it prone to decomposition. The result is hydrogen evolution, which consumes electrons and lowers the efficiency of zinc plating. Worse, hydrogen evolution generates hydroxide ions that precipitate as zinc hydroxide on the electrode surface, promoting the growth of destructive zinc dendrites.

The standard strategy for suppressing this water decomposition has been adding organic co-solvents to the aqueous electrolyte. The assumption has been that highly polar organic molecules would muscle their way into the zinc ion coordination shell, displacing water and solving the problem. Researchers have relied on traditional polarity metrics like dielectric constant, dipole moment, and donor number to select these co-solvents.

But when the research team systematically tested a range of organic solvents in zinc battery electrolytes, they found no meaningful correlation between these conventional polarity scales and the actual coulombic efficiency of zinc plating and stripping. Some high polarity solvents performed well. Others did not. The traditional descriptors were failing to predict real battery behavior.

A Different Kind of Polarity

The breakthrough came when the researchers turned to a less commonly used polarity scale known as Et(30), developed by chemists Dimroth and Reichardt. Unlike conventional metrics that measure bulk properties or theoretical interactions with probe molecules in inert solvents, Et(30) captures how a solvent actually behaves when completely surrounding a dissolved substance. It measures both general polarity and hydrogen bonding capability, providing information about how molecules interact as solvents rather than as isolated species.

When the team plotted zinc coulombic efficiency against Et(30) values for different co-solvents, a clear volcanic relationship emerged. Performance increased as Et(30) rose from very low values, peaked sharply around an Et(30) of 45, then declined again at higher values. This was not what existing theories predicted.

The researchers used computational modeling and spectroscopic analysis to understand what was happening at the molecular level. Their calculations revealed that in electrolytes with high Et(30) solvents, both water and the organic solvent occupied the first coordination shell around zinc ions. This meant the solvent molecules themselves were vulnerable to reduction reactions during zinc plating. Since many high Et(30) solvents are protic compounds containing reactive hydrogen, their reduction generates hydrogen gas, destabilizing any protective surface layer that forms and reducing battery efficiency.

Even more problematic, highly polar solvents prevented negatively charged anions from entering the zinc coordination shell. This anion coordination turns out to be crucial for stabilizing the water molecules that do remain around the zinc ion, reducing their reactivity.

At the other extreme, solvents with very low Et(30) values showed poor performance for a different reason. These nonpolar solvents interact weakly with both water and zinc ions. In such electrolytes, water molecules cluster together in water-rich zones, leaving zinc ions in these regions surrounded primarily by water rather than protective anions. The result is the same water decomposition problem the co-solvent was meant to solve.

The Sweet Spot

The optimal performance at mid-range Et(30) values around 45 reflects a delicate balance. Solvents in this range, which are aprotic and moderately polar, can interact with water molecules while allowing water to form concentrated zones. Crucially, this concentration effect occurs without excluding anions from zinc coordination. The result is zinc ions surrounded by more anions and less reactive water, while the organic solvent molecules themselves decompose without generating destabilizing gas.

Among the solvents tested, dimethyl sulfoxide, or DMSO, with an Et(30) of 45.1, performed best as a co-solvent. In an electrolyte containing DMSO, water, and zinc bis(trifluoromethanesulfonyl)imide salt, the average zinc coulombic efficiency reached 99.3 percent over 100 cycles. This was the highest among the co-solvent formulations tested, but still left room for improvement.

The team then applied the Et(30) principle to select an additive that could further enhance performance by forming a protective fluoride-rich solid electrolyte interphase on the zinc surface. Such a layer provides multiple benefits: fluoride compounds are hydrophobic, blocking water access to the electrode surface, and they are zinc-phobic, suppressing dendrite growth.

Because Et(30) values for many specialized molecules are not tabulated in databases, the researchers used parent molecule Et(30) values as proxies. They screened fluorine-containing organic additives whose parent molecules had Et(30) values in the successful 38 to 49 range, excluding any with reactive hydroxyl groups that might generate gas. From this rational selection process, they identified trifluoroethyl formate as the optimal additive.

Record Performance

The complete electrolyte formulation, combining DMSO co-solvent with trifluoroethyl formate additive in aqueous zinc salt solution, delivered remarkable results. Average zinc coulombic efficiency reached 99.8 percent over 550 cycles. In symmetric cell tests where zinc was plated and stripped between two zinc electrodes, the electrolyte enabled a cycle life exceeding 5,500 hours, far surpassing previously reported results for electrodes of comparable thickness.

Surface analysis confirmed the formation of a dual-layer protective interphase on cycled zinc electrodes. The inner layer consisted primarily of zinc fluoride, providing hydrophobic and zinc-phobic protection. An outer layer of zinc sulfide, which has high mechanical strength and ionic conductivity, complemented the inner layer's protective function. In situ gas analysis showed dramatically reduced hydrogen evolution compared to electrolytes without the additive.

The researchers tested full battery cells pairing zinc anodes with polyaniline cathodes. Cells using the optimized electrolyte maintained stable capacity for 1,000 cycles, compared to just 300 cycles for a reference electrolyte without the additive. The improvement extended across a wide range of operating conditions, including extreme temperatures from negative 10 to positive 60 degrees Celsius and high electrode loadings designed for practical energy density.

In pouch cell tests with thick cathodes containing 8.4 milligrams of active material per square centimeter, the optimized electrolyte enabled operation for over 150 cycles. Scaling up further to 16.5 milligrams per square centimeter, the cells achieved a specific energy density of 110 watt hours per kilogram based on the combined weight of the cathode and anode, a performance level competitive with previous reports while using a much thinner zinc electrode for improved cycle life.

Beyond Simple Polarity

The volcanic relationship between Et(30) and battery performance reveals that selecting electrolyte components requires considering factors that conventional polarity scales miss entirely. The production of gas during solvent reduction, the partitioning of water between different regions of the electrolyte, the competition between solvents and anions for coordination sites around metal ions—all of these factors influence battery performance but are invisible to metrics like dielectric constant or donor number.

The Et(30) scale captures these effects not through any fundamental theoretical framework but through empirical measurement of how solvents behave in practice. It represents the kind of polarity that matters for real solvation processes, incorporating both electrostatic interactions and hydrogen bonding effects. The scale's strong correlation with hydrogen bonding ability proved particularly important, as it helped predict which solvents would generate problematic gases during reduction.

The research also overturns specific technical assumptions. For instance, DMSO has often been described as entering the coordination shell of zinc ions in aqueous electrolytes based on its high donor number. But the new spectroscopic and computational evidence shows that in water-containing systems, zinc ions preferentially coordinate with water rather than DMSO. The DMSO instead creates regions of varying water concentration, with zinc ions residing in water-rich zones where anions can access the coordination shell. This mechanism is fundamentally different from direct DMSO coordination, yet it produces better battery performance.

The discovery suggests a more general principle for aqueous battery electrolyte design: moderate polarity co-solvents that create favorable concentration gradients and decompose cleanly may outperform highly polar solvents that directly interact with metal ions but introduce new failure modes. This principle could apply beyond zinc to other aqueous metal battery chemistries where water decomposition and dendrite growth pose similar challenges.

The work also demonstrates the value of revisiting established selection criteria. The traditional polarity scales used in battery electrolyte design were developed for other purposes—predicting reaction rates, dissolution behavior, or spectroscopic properties. Their application to battery systems represented an assumption that properties relevant in one context would translate to another. The Et(30) scale, though less commonly used in battery research, proves more predictive precisely because it was designed to capture the comprehensive solvation behavior that batteries depend on.

Looking forward, the research provides both a specific electrolyte formulation that enables significantly improved zinc battery performance and a general framework for discovering better electrolyte components. The Et(30)-guided screening approach could accelerate the identification of promising co-solvents and additives while reducing the trial and error that currently dominates electrolyte development. For a field where small improvements in cycle life and efficiency can determine whether a technology reaches commercial viability, such tools are not merely convenient but essential.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1016/j.joule.2025.101844