Molecular engineering of a zinc battery co-solvent improves plating stability, suppresses side reactions, and delivers near-perfect efficiency through a single functional group substitution.
(Nanowerk Spotlight) Rechargeable batteries built around zinc and water have obvious appeal. They’re safer than lithium systems, cheaper to make, and based on widely available materials. But at the interface between electrolyte and electrode, their chemistry fails to deliver. Zinc deposits unevenly. Water decomposes. Performance collapses.
These failures are rooted in a molecular structure that seems benign: the solvation shell, a cluster of molecules that surround each zinc ion. Get that structure wrong, and zinc forms dendrites and parasitic reactions dominate. Hybrid electrolytes, which blend water with organic co-solvents, aim to fix this by modifying ion behavior. But progress has been slow, and current solvent options introduce their own tradeoffs.
Now, researchers at Seoul National University report that precise molecular engineering of a common co-solvent, changing just one functional group, can shift this balance. Their study, published in Advanced Energy Materials (“Molecular Engineering of Hybrid Electrolytes for Aqueous Zinc Ion Batteries”), describes how a subtle structural change unlocks faster ion transport, more stable interfaces, and a major improvement in long-term battery performance.
The research centers on a well-known molecule called triethyl phosphate (TEP). This compound has attracted attention as a co-solvent in aqueous zinc-ion battery systems because of its non-flammable nature and ability to bind strongly to zinc ions. That binding helps displace water from the solvation shell and suppress hydrogen gas formation, one of the most damaging side reactions in these systems.
However, triethyl phosphate has its own limitations. Its strong interaction with zinc ions slows the de-solvation process, making it harder for zinc to deposit cleanly on the electrode. It also attracts water molecules into the coordination shell, which reintroduces the very reaction it was meant to suppress.
To overcome these limitations, the team designed a new molecule by replacing just one ethoxy group in triethyl phosphate with a difluoromethyl group. The result is diethyl(difluoromethyl)phosphonate (DEDFP). This small change reshapes the molecule’s electronic structure and physical behavior. It reduces the strength of the zinc-co-solvent interaction while still coordinating more strongly than water. It also increases the molecule’s hydrophobicity, meaning it resists contact with water and keeps water molecules away from the zinc surface.
Structural characterization of TEP and DEDFP co-solvents. a) Schematic representations illustrating the molecular characteristics of co-solvents and their influence on interfacial phenomena. b) Electrostatic potential minimum of the co-solvents. c) Binding energy of the co-solvents with Zn2+. d) Comparison of the miscibility of co-solvent mixtures (before Zn salt addition) and corresponding electrolytes (after Zn salt addition). (Image: Reprinted from DOI:10.1002/aenm.202504692, CC BY) (click on image to enlarge)
Density functional theory calculations supported these effects. The modified molecule showed a slightly weaker binding energy with zinc ions than the original version, which reduces the energy barrier during de-solvation. Its lowest unoccupied molecular orbital, or LUMO, was also lower in energy and shifted in spatial distribution.
This indicates that DEDFP is more likely to undergo controlled reduction during battery operation, which leads to the formation of a thin, stable passivation layer at the metal interface. That layer contains zinc fluoride, a compound that allows zinc ions to move while blocking water and other reactive species.
Experimental evidence confirmed these predictions. Spectroscopic measurements showed that the DEDFP-based electrolyte disrupted the hydrogen-bonding network of water more effectively than the TEP-based system. Raman spectroscopy and X-ray scattering revealed increased ion-pair formation and altered solvation structures. These changes support better control over how zinc ions behave in the electrolyte and how they deposit onto the electrode.
The authors also studied the electrochemical effects of this change. Using techniques such as linear sweep voltammetry and chronoamperometry, they measured the behavior of zinc electrodes across a range of voltages. The DEDFP-based electrolyte reduced the onset and severity of hydrogen evolution. It also promoted the early formation of a protective interphase, which reduced further side reactions at lower potentials.
Surface analysis confirmed the presence of a ZnF₂-rich layer on electrodes cycled in the DEDFP system. This protective film was absent or much weaker in the TEP-based and pure water systems.
These changes had clear effects on cycling performance. Zinc electrodes in symmetric cells using the DEDFP electrolyte maintained low overpotentials and stable morphology across hundreds of cycles, even at high current densities. Scanning electron microscopy showed that zinc deposited in flat, laterally aligned layers, rather than forming vertical or misaligned structures that can lead to short circuits. In contrast, the TEP-based system showed more irregular growth, and the pure water electrolyte resulted in dendritic structures and early failure.
The improvement in Coulombic efficiency was equally significant. Coulombic efficiency measures how completely zinc can be stripped and replated during each cycle without losses. Cells using the DEDFP electrolyte reached an average Coulombic efficiency of 99.96 percent and maintained that performance over 600 cycles. The TEP system reached a maximum of 99.79 percent but failed by the 91st cycle under the same conditions. The water-only system failed before 50 cycles and never exceeded 97.2 percent efficiency. These results place the DEDFP-based electrolyte among the top tier of published zinc-ion battery systems.
The study also included full-cell tests with V₆O₁₃ cathodes, using a realistic anode-to-cathode capacity ratio of 1.7. This configuration mimics the material balance required for commercial viability. The full cell with the DEDFP electrolyte retained 70 percent of its capacity after 486 cycles and remained functional after 700. The same configuration using the TEP electrolyte failed before 400 cycles.
When tested at higher charge and discharge rates, the DEDFP system continued to show stable capacity retention, while the others degraded quickly. A pouch cell version using DEDFP also performed well, retaining more than 65 percent capacity after 300 cycles.
These results show how control at the level of molecular structure can determine system-level performance. By replacing a single chemical group in a known co-solvent, the researchers altered the solvation environment, suppressed unwanted reactions, and enabled the formation of a stable interface. The study demonstrates how electrolyte design can move beyond empirical screening and toward rational, targeted molecular engineering.
The use of a difluoromethyl group in place of an ethoxy group is not an exotic choice, but it reflects a strategic understanding of electron distribution, coordination chemistry, and solvent dynamics. It also sets a practical precedent. Rather than searching for entirely new materials, researchers can refine and adapt existing solvent frameworks to meet specific electrochemical needs. In the case of aqueous zinc-ion batteries, the impact of that approach is now clear: more reversible zinc cycling, longer cycle life, and fewer compromises between stability and kinetics.
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