A comprehensive academic review published in Frontiers in Chemistry outlines how precise chemical fluorination transforms carbon nanotubes from inert conductive platforms into highly reactive components for advanced electrochemical energy storage. Writing across a framework linking synthesis routes, chemical bonding, structural evolution, and device performance, the researchers show that attaching fluorine atoms to one-dimensional carbon tubular lattices provides an exceptionally direct mechanism for manipulating electronic structure, surface polarity, and interfacial reactivity. The incorporation of fluorine replaces sp2-hybridized carbon networks with fluorinated sp3 local environments, generating a spectrum of semi-ionic and covalent carbon-fluorine bonds. Because pristine carbon nanotubes inherently feature chemically inert surfaces, restricted electrochemically active sites, and poor wettability toward polar ether- and carbonate-based battery electrolytes, chemical modification has become essential for expanding their roles beyond standard passive conductive additives.
The structural characteristics and electrochemical utility of fluorinated carbon nanotubes depend decisively on the synthetic route used to introduce the halogen atoms, as different fluorination methods operate under distinct kinetic, thermal, and mass-transport regimes. Direct gas-phase fluorination utilizing pure fluorine gas or diluted fluorine-nitrogen mixtures at elevated temperatures between 200 °C and 600 °C remains the most common industrial method, enabling high fluorine loadings with fluorine-to-carbon atomic ratios approaching 0.8 to 1.0. However, because molecular fluorine has a kinetic diameter of approximately 0.35 nanometers, diffusion within tightly packed nanotube bundles is restricted, leading to localized over-fluorination, outer-wall exfoliation, and severe lattice disruption. Conversely, atomic fluorination driven by the thermal decomposition of xenon difluoride or terbium tetrafluoride releases atomic fluorine radicals with a van der Waals radius of roughly 0.147 nanometers. These smaller species access nanotube surfaces without mass-transfer barriers, yielding exceptionally homogeneous fluorine distributions dominated by semi-ionic configurations while preserving the structural integrity of the graphitic core.
Alternative low-temperature and liquid-phase fluorination techniques offer further synthetic control by tailoring reactive species and functional penetration depths. Radio-frequency or microwave plasma fluorination using carbon tetrafluoride operates below 150 °C, generating reactive radicals and ionic fragments that are confined to the outermost atomic shells of the carbon nanotubes. This selective surface modification introduces carbon-fluorine, difluoromethylene, and trifluoromethyl groups while keeping the conductive inner graphitic channels intact, yielding modest fluorine contents below a fluorine-to-carbon ratio of 0.3 that enhance interfacial polarity without degrading electrical pathways. Liquid-phase routes, including solvothermal treatments with electrophilic Selectfluor at temperatures from 140 °C to 200 °C or nucleophilic room-temperature reactions with aqueous hydrofluoric acid, provide milder reaction environments that favor semi-ionic bond configurations. Additionally, solid-state pyrolytic techniques employing fluorine-donating precursors such as polytetrafluoroethylene or ammonium fluoride at 400 °C to 700 °C introduce low concentrations of fluorine without requiring corrosive gaseous reagents.
Spectroscopic investigations resolve the intricate evolution of local chemical environments as fluorine accumulates on the carbon nanotube framework. Fourier-transform infrared spectroscopy distinguishes semi-ionic carbon-fluorine vibrations near 1110 reciprocal centimeters from covalent carbon-fluorine stretching bands appearing at 1210 to 1220 reciprocal centimeters, alongside difluoromethylene features emerging near 1340 reciprocal centimeters under deeper fluorination conditions. Complementary X-ray photoelectron spectroscopy identifies semi-ionic species by carbon 1s binding energies between 288.2 and 288.8 electronvolts and fluorine 1s peaks between 685.5 and 687.0 electronvolts, whereas covalent bonds emerge at higher carbon 1s energies of 289.4 to 290.4 electronvolts and fluorine 1s signals between 688.0 and 689.0 electronvolts. Solid-state fluorine-19 magic-angle-spinning nuclear magnetic resonance tracking demonstrates that semi-ionic configurations resonate near minus 152 parts per million, whereas strong covalent bonds appear near minus 190 parts per million and undergo downfield shifts toward minus 188 parts per million when fluorination temperatures increase from 350 °C to 430 °C, directly reflecting a progressive loss of ionic character and a strengthening of covalent bonding.
Lattice alterations and morphological reconstructions driven by fluorination fundamentally reshape both physical dimensions and electronic transport within carbon nanotubes. High-resolution transmission electron microscopy reveals that fluorination expands the pristine graphitic interlayer spacing of 0.33 nanometers up to 0.67 nanometers in heavily treated outer walls while inner shells retain their native graphitic dimensions. Raman spectroscopy reflects this structural reorganization through increases in the defect-associated D-band to graphitic G-band intensity ratio, caused by the conversion of sp2 carbon into sp3 hybridization. Because extended covalent carbon-fluorine bonding interrupts electron delocalization across the carbon framework, electrical conductivity drops drastically as fluorine content rises. Systematically evaluated multiwalled carbon nanotubes demonstrate that electrical conductivity decreases from 1.73 siemens per centimeter in pristine samples to 4.8 times 10 to the power of minus 2 siemens per centimeter at a fluorine-to-carbon ratio of 0.09, falling to 1.8 times 10 to the power of minus 8 siemens per centimeter when the ratio reaches 0.75. However, moderate solvothermal fluorination introducing 2.1 weight percent fluorine can marginally increase conductivity from 386 to 402 siemens per meter through subtle lattice distortions and charge-carrier modulation.
Surface physicochemical properties and thermal defluorination pathways display a strong dependence on the specific carbon-fluorine bonding configurations present on the nanotube exterior. At low fluorination degrees where polarized semi-ionic bonds dominate, surface energy increases and polar liquid wettability improves substantially; for example, hydrofluoric acid-treated films exhibit a dramatic reduction in water contact angle from 81.3 degrees down to 10.7 degrees, facilitating electrolyte penetration. In contrast, extensive fluorination that forms nonpolar covalent fluorocarbon groups shifts the surface into a superhydrophobic regime, where polytetrafluoroethylene-treated nanotubes produce water contact angles between 137 and 140 degrees. Thermogravimetric analyses show that gaseous fluorine treatments impart high thermal resilience, with fluorine desorption peaking near 396 °C, whereas plasma-treated and bromine trifluoride-treated samples begin losing fluorine below 150 °C due to weaker bond energies. Pre-oxidizing nanotubes shifts subsequent fluorination pathways toward substitution rather than direct addition, raising decomposition temperatures to approximately 490 °C.
When employed as active cathode hosts in primary and rechargeable lithium and sodium batteries, partially fluorinated carbon nanotubes reconcile the traditional conflict between high theoretical capacity and sluggish electron transport. In primary lithium battery chemistries, retaining intact graphitic pathways alongside fluorinated segments has enabled power densities reaching 8998 watts per kilogram in multiwalled nanotubes and 8057 watts per kilogram at 6C rates in fluorinated nanofibers. Converting single-walled nanotubes into fluorinated graphene nanoribbons via high-temperature gas treatment yields energy densities of 2738.45 watt-hours per kilogram with a discharge plateau near 3.0 volts. In sodium-based systems, fluorinated graphitized carbon nanotubes delivering theoretical capacity levels reach 751 milliampere-hours per gram at 10 milliamperes per gram alongside energy densities of 1733.4 watt-hours per kilogram. Furthermore, room-temperature rechargeable sodium cells utilizing fluorinated multiwalled nanotubes achieve initial discharge capacities of 1368 milliampere-hours per gram and reversible charge capacities of 1102 milliampere-hours per gram by lowering polarization voltages from 2000 millivolts to 1600 millivolts.
Beyond active cathode conversion, fluorinated carbon nanotubes function as effective in situ interphase-forming materials that stabilize reactive metal anodes. In lithium-metal batteries, coating metal foils with macroscopic fluorinated nanotube films initiates an in situ chemical conversion during initial cycling, generating an ultrathin 13-nanometer solid electrolyte interphase enriched with lithium fluoride. This artificial interphase possesses an ionic conductivity of 2.59 times 10 to the power of minus 7 siemens per centimeter and a mechanical Young’s modulus of approximately 2.0 gigapascals, suppressing dendritic penetration and supporting 99.3 percent capacity retention after 500 cycles at 0.5C in lithium iron phosphate cells. Similarly, solvothermally fluorinated carbon nanofiber current collectors in sodium-metal batteries lower sodium nucleation overpotential from 32.8 millivolts on copper down to 6.8 millivolts, while continuously releasing fluorine species to generate a 10-nanometer sodium fluoride-rich protective layer that maintains Coulombic efficiencies above 99.9 percent over 850 cycles at 3 milliamperes per square centimeter.
Fluorinated nanotube architectures also serve as multifunction reaction regulators in conversion systems and single-atom electrocatalysis. In lithium-sulfur batteries, self-supporting interlayers constructed from plasma-fluorinated multiwalled nanotubes combine a polar outer shell that chemically anchors soluble lithium polysulfides with a conductive inner core that sustains electron flow, delivering a reversible capacity of 837 milliampere-hours per gram after 200 cycles at 1C compared to 534 milliampere-hours per gram for non-fluorinated analogues. In electrocatalysis, treating single-walled carbon nanotube films with xenon difluoride facilitates the extraction and atomic dispersion of residual iron nanoparticles into catalytically active iron-nitrogen sites upon subsequent thermal ammonia processing. The resulting flexible single-atom catalyst exhibits an oxygen reduction reaction half-wave potential of 0.93 volts and an oxygen evolution potential of 1.60 volts at 10 milliamperes per square centimeter, delivering a small bifunctional voltage gap of 0.67 volts that outperforms conventional platinum-iridium benchmarks in rechargeable zinc-air batteries.
The review concludes by emphasizing critical scientific and engineering barriers that currently impede the wide-scale commercial implementation of fluorinated carbon nanotube materials. Non-uniform fluorination remains an enduring obstacle in scalable gas-phase processes, resulting in local over-fluorinated domains that compromise bulk conductivity alongside inactive under-fluorinated regions. The field also lacks standardized characterization protocols, as disparate X-ray photoelectron deconvolution models and nuclear magnetic resonance assignments prevent rigorous cross-study comparisons. Furthermore, the corrosive toxicity of primary fluorinating agents like fluorine and hydrofluoric acid demands stringent processing safety, while gentler atomic donors such as xenon difluoride remain economically impractical for bulk manufacturing. Overcoming these limitations will require advanced operando synchrotron spectroscopy to resolve dynamic carbon-fluorine bond defluorination under operational battery conditions, predictive machine-learning design frameworks, and scalable precision synthesis to balance atomic-level bonding control with macroscopic transport pathways.
Subject of Research: Fluorinated carbon nanotubes in electrochemical energy storage systems
Article Title: Fluorinated carbon nanotubes for electrochemical energy storage: from bonding chemistry to structure–property relationships
Article References: Wang, Q., Yu, F., Wen, Y., Niu, J., Li, S., Chen, H., Yu, B., & Qu, L. (2026). Fluorinated carbon nanotubes for electrochemical energy storage: from bonding chemistry to structure–property relationships. Frontiers in Chemistry, 14, Article 1982609. https://doi.org/10.3389/fchem.2026.1982609
Image Credits: AI Generated
DOI: 10.3389/fchem.2026.1982609
Keywords: carbon nanotubes, fluorination, electrochemical energy storage, lithium batteries, sodium batteries, solid electrolyte interphase, electrocatalysis, materials science, Fluorinated, carbon, nanotubes, electrochemical