In the quiet pursuit of better batteries and supercapacitors, some of the most promising breakthroughs are happening not in exotic new compounds but in carefully tweaked versions of familiar materials. A research team from St. Joseph’s University and PES University in Bangalore, India, has now shown that a simple chemical substitution—swapping oxygen atoms for fluorine in lithium-doped barium vanadate glass—can be used as a precision tool to steer electrical conductivity and boost energy storage performance. The study, published in the journal Ionics, systematically fluorinated a family of vanadate glasses and mapped, with unusual thoroughness, how each increment of fluorine reshapes the glass structure, the balance of vanadium oxidation states, and ultimately the material’s ability to conduct charge and store energy. Their best-performing composition, a fully fluorinated glass labeled VBOLF, delivered the highest electrical conductivity of the series, a specific capacitance of 242.7 farads per gram at a current density of 0.1 amperes per gram, and good cycling stability—figures that place it firmly on the radar for next-generation energy storage applications.
Vanadate glasses have long attracted attention for energy applications because vanadium is a transition metal that readily adopts multiple oxidation states, chiefly V⁴⁺ and V⁵⁺. This flexibility allows electronic charge carriers—electrons or small polarons—to hop between vanadium sites through the disordered amorphous network, giving the glasses a form of electronic conduction that coexists with the ionic conduction supplied by mobile lithium ions. The result is a mixed ionic-electronic conductor, a class of material that is particularly valuable in electrodes, where both electrons and ions must move efficiently. The starting composition in the new study, 60V₂O₅–20BaO–20Li₂O, was designed in earlier work by the same group, and the team progressively replaced barium oxide and lithium oxide with their fluoride counterparts, barium fluoride and lithium fluoride, ending at 60V₂O₅–20BaF₂–20LiF. Crucially, the vanadium pentoxide content was held constant at 60 mole percent throughout, so any change in properties could be attributed cleanly to the fluorine substitution rather than to shifts in the electroactive component.
The thermal behavior of the glasses told one part of the story. As fluorine content increased, the glass transition temperature rose monotonically, indicating that fluorine was tightening the structural network rather than loosening it. This is somewhat counterintuitive, since fluorine is often introduced into oxide glasses to break bridging bonds and reduce network connectivity; in many fluorophosphate and fluorosilicate systems it acts as a network modifier, lowering working temperatures and softening the glass. Here, however, the steadily increasing transition temperature suggests that fluorine in the vanadate matrix occupies sites that strengthen the overall framework, perhaps by forming strong V–F bonds or by altering the coordination environment of vanadium in ways that stiffen the network against thermal agitation. For device engineers, a higher glass transition temperature is welcome news, since it signals better thermal stability for materials that may need to operate warm.
The electrical conductivity, by contrast, refused to follow a simple trend. Rather than rising or falling smoothly with fluorine content, the conductivity varied non-monotonically, a hallmark of competing transport mechanisms whose relative strengths shift as the structure evolves. The team observed an intriguing phenomenon in the temperature-dependent conductivity plots: at lower temperatures, the oxyfluoride glasses displayed double plateaus in conductivity—two distinct regions where the conductivity changes character—which then merged into single plateaus at temperatures above roughly 190 degrees Celsius. This two-plateau behavior implies that two different conduction or relaxation processes dominate in different temperature windows, plausibly reflecting the interplay between ionic hopping of Li⁺ ions and electronic polaronic hopping between V⁴⁺ and V⁵⁺ sites. To analyze the data quantitatively, the researchers fitted both the single- and double-plateau regions using Jonscher’s power law, the standard empirical description of the universal dielectric response in disordered solids, in which the frequency-dependent conductivity follows a power-law exponent that encodes the nature of the charge carrier interactions with the lattice.
Understanding exactly what fluorine was doing to the glass structure required an arsenal of spectroscopic probes. Fourier-transform infrared spectroscopy revealed that the distorted VO₆ octahedra characteristic of vanadate glasses are actually present in the network as VO₄ tetrahedra and VO₅ square pyramids—shorter, tighter coordination units that form the backbone of the amorphous structure. Raman spectroscopy then provided a window into how these units reorganize as fluorine is introduced, tracking changes in vanadium–oxygen bond lengths and bond orders across the series. Electron paramagnetic resonance spectroscopy complemented the Raman data by sensing the unpaired electrons on V⁴⁺ ions, allowing the team to follow the changing concentrations of V⁴⁺ and V⁵⁺ in the matrix. The two techniques told a mutually consistent story, with the structural features inferred from Raman spectra independently confirmed by the EPR analysis. Together they showed that fluorine substitution does not merely dilute the oxide network—it actively perturbs the vanadium valence balance, which in turn modulates the polaronic electronic conductivity riding on top of the lithium ionic conductivity.
The electrical characterization went beyond simple conductivity measurements. The team employed impedance spectroscopy analyzed through Cole-Cole plots, the classical complex-plane representation that separates bulk, grain-boundary, and electrode contributions to the measured impedance, and they applied the electric modulus formalism, which suppresses electrode polarization effects and isolates the bulk relaxation dynamics of the mobile ions. These approaches allowed a detailed interpretation of the relaxation mechanism—how charge carriers in the glass respond to alternating electric fields across a range of frequencies and temperatures, and how the characteristic relaxation times shift with composition. Such analyses are essential for distinguishing genuine bulk transport from interfacial artifacts, and they lent confidence to the composition-property trends extracted from the study.
The electrochemical tests were where the practical payoff became apparent. Using cyclic voltammetry, the researchers probed the reversibility of the redox processes at the glass electrodes and evaluated their suitability for charge storage. Galvanostatic charge-discharge measurements then provided direct determinations of specific capacitance at controlled current densities, the key metric for supercapacitor performance. The fully fluorinated VBOLF glass emerged as the standout: it combined the best electrical conductivity of the series with the highest specific capacitance, 242.7 F/g at 0.1 A/g, and it maintained good stability over repeated charge-discharge cycling. The synergy makes physical sense—a more conductive glass delivers electrons and ions to the electrochemical interface more efficiently, while the fluorine-modified vanadium environment appears to support favorable redox activity and structural resilience during cycling.
The findings arrive amid a broader resurgence of interest in vanadium-based amorphous and glassy materials for energy storage. Vanadate-borate glasses have been proposed as high-capacity cathodes for rechargeable lithium-ion batteries, and glass-ceramic-like vanadate cathodes have demonstrated high-rate performance, exploiting the multi-electron redox chemistry of vanadium. Fluoride-containing electrode materials, meanwhile, are prized for their high working voltages and the improved cycling stability that fluorine can impart to disordered rock-salt oxyfluoride cathodes. The Bangalore study ties these threads together at the level of fundamental glass science, providing a coherent mechanistic picture of why fluorine helps: it tunes the balance between the two dominant charge carriers, stiffens the network thermally, and reorganizes the vanadium coordination units in ways that benefit both conduction and capacitive storage.
What makes the work particularly valuable methodologically is the disciplined one-variable-at-a-time design combined with cross-validating spectroscopy. Because the vanadium oxide content was fixed, and barium and lithium components were converted one-for-one from oxides to fluorides, the study isolates the role of the anion sublattice with unusual clarity. The agreement between Raman-derived structural models and EPR-derived valence data gives the conclusions a robustness that single-technique studies often lack. And the observation of double conductivity plateaus—with their eventual merger above 190 degrees Celsius—offers a rich phenomenological fingerprint that future theoretical models of mixed conduction in oxyfluoride glasses will need to reproduce.
For now, the message is straightforward: fluorine substitution is not merely a compositional tweak but a genuine steering mechanism for the functional properties of vanadate glasses. By dialing in the right amount of fluorine, materials scientists can tilt a glass toward higher ionic mobility, stronger electronic conduction, or superior electrochemical storage—and the fully fluorinated composition examined here suggests that, for supercapacitor applications at least, the dial has room yet to turn. As demand grows for cheap, stable, and easily processed electrode materials, amorphous oxyfluoride conductors like VBOLF may find themselves moving from the impedance spectrometer to the prototype cell.
Subject of Research: Fluorine substitution in lithium-doped barium vanadate oxyfluoride glasses to control electrical conductivity and enhance electrochemical energy storage performance
Subject of Research: Technology and Engineering
Article Title: Fluorine substitution as a tool to steer conductivity in oxyfluoride glasses
Article References: Goel, P., B.R., H., Sharma, O., & Honnavar, G. V. (2026). Fluorine substitution as a tool to steer conductivity in oxyfluoride glasses. Ionics. https://doi.org/10.1007/s11581-026-07491-2
Image Credits: AI Generated
DOI: 10.1007/s11581-026-07491-2
Keywords: oxyfluoride glasses, vanadate glass, fluorine substitution, lithium ion conductivity, Raman spectroscopy, EPR, impedance spectroscopy, Cole-Cole plots, cyclic voltammetry, specific capacitance, energy storage
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Faith Mcneil. (September 4, 2026). Fluorine doping tunes conductivity in oxyfluoride glasses. Scienmag. https://scienmag.com/fluorine-doping-tunes-conductivity-in-oxyfluoride-glasses/
Faith Mcneil. “Fluorine doping tunes conductivity in oxyfluoride glasses.” Scienmag, 4 September 2026, https://scienmag.com/fluorine-doping-tunes-conductivity-in-oxyfluoride-glasses/. Accessed 4 September 2026.
Faith Mcneil. “Fluorine doping tunes conductivity in oxyfluoride glasses.” Scienmag. September 4, 2026. https://scienmag.com/fluorine-doping-tunes-conductivity-in-oxyfluoride-glasses/
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