The study, published in Frontiers in Batteries and Electrochemistry, describes self-supporting composite electrodes made of silicon oxycarbide embedded in a carbon matrix, designated SiOxCy/C. The researchers, led by Oumaima Hatim of Université Clermont Auvergne and Sandrine Berthon-Fabry of MINES Paris, prepared these materials through a sol-gel route that combines resorcinol, formaldehyde, tetraethoxysilane, and a silane catalyst in acetone, then impregnates the resulting sol into kapok fibers at concentrations of just 2 to 2.5 weight percent. After gelation at 60 degrees Celsius, air drying, and pyrolysis under nitrogen at 800 or 1,000 degrees Celsius, the team obtained black monolithic blocks that are astonishingly porous, with densities as low as 0.131 grams per cubic centimeter. For comparison, massive carbon has a density of about 2.2 grams per cubic centimeter and silica about 2.65, meaning these composites are roughly 95 percent empty space by volume.

What makes these monoliths remarkable is that they can be used directly as electrodes by simply cutting slices from the block. No binder, no carbon black, no slurry casting, no current collector coating. The conventional electrode formulation process requires mixing active material with conductive additives and polymer binders such as polyvinylidene fluoride, a PFAS compound now under regulatory scrutiny for environmental persistence. By eliminating these inactive masses entirely, the binder-free design not only simplifies cell assembly but also removes a class of chemicals that battery manufacturers are actively trying to phase out. The monolithic electrode in this study consists solely of the cut composite, with a mass loading of 13.4 milligrams per square centimeter defined entirely by the composite density.

The mechanical story is where the kapok fibers earn their keep. In uniaxial compression tests, the fiber-free monolith behaved like a brittle ceramic: it fractured at strains below 0.1 percent, reaching a maximum stress of only about 0.40 megapascals before catastrophic failure. The chord modulus, a measure of stiffness calculated between nominal strains of 0.04 and 0.06 percent, came in at 30.25 megapascals for the fiber-free material. But when fibers were incorporated at 2 weight percent, the modulus plummeted to 4.86 megapascals, and at 2.5 weight percent it dropped further to 2.84 megapascals. This tenfold reduction in stiffness is not a weakness but a feature. The fiber-containing composites deformed compliantly under load, retaining cohesion where the fiber-free material shattered, and loading-unloading cycles confirmed genuine elastic recovery. The researchers attribute this transformation to the large macropores between fibers, which allow volume variation without damaging the material, and to the inherent flexibility of the pyrolyzed kapok fiber network, which fully recovers its initial volume after compression regardless of orientation.

This mechanical compliance has direct electrochemical consequences. During lithiation and delithiation, silicon-based electrodes swell and contract repeatedly within the fixed volume of the cell, generating internal stresses that crack particles and break conductive networks. A stiffer electrode transmits these stresses more efficiently, accelerating structural failure. A softer, more elastic electrode absorbs them. The fiber-reinforced monolith, with its chord modulus below 5 megapascals, is precisely the kind of compliant architecture that computational models of silicon anode cracking suggest is needed to survive thousands of cycles. The elastic recovery confirmed by the loading-unloading experiments indicates that the conductive network integrity is preserved throughout deformation, which the researchers link directly to the superior capacity retention observed in the electrochemical tests.

Those electrochemical results are impressive. When tested as formulated powder electrodes in coin cells with a voltage window of 0.005 to 3.0 volts at a current density of 168 milliamperes per gram, the best composite, designated RFS-2.5–1,000 for its 2.5 weight percent fiber content and 1,000 degree Celsius pyrolysis, delivered a reversible capacity of 1,091 mAh per gram at the fifth cycle with 91 percent retention. The initial Coulombic efficiency of 94 percent is exceptionally high for a SiOx-based material, where typical literature values hover between 33 and 68 percent. This efficiency matters enormously for full-cell energy density, because every percent of irreversible lithium consumed in the first cycle must be compensated by extra cathode material, adding dead weight to the battery.

But the real breakthrough came when the researchers compared the powder form against the monolithic form of the same composite. The monolithic electrode, cut directly from the block and implemented without any formulation, delivered discharge capacities of 1,064, 1,068, and 1,070 mAh per gram at the 10th, 50th, and 100th cycles respectively, with retention exceeding 100 percent from the 10th cycle onward. This progressive increase beyond the initial capacity reflects gradual activation of previously inaccessible active mass within the tortuous porous network, a phenomenon the researchers confirmed through nitrogen sorption isotherms showing a mix of macropores, mesopores centered around 25 nanometers, and micropores. The powder form, by contrast, delivered only 943 mAh per gram after 100 cycles. The monolithic configuration, with zero carbon additive in the electrode formulation, outperformed the conventional powder electrode that required 10 weight percent acetylene black and 10 weight percent PVDF binder.

Rate capability tests reinforced the monolith’s advantages. At the slowest rate of C/25, the monolithic anode delivered an initial specific capacity of 1,151 mAh per gram. As the current increased through C/12, C/6, C/3, and 2C/3, capacities declined to approximately 946, 850, 711, and 498 mAh per gram respectively, but upon returning to C/25 the capacity partially recovered to 977 mAh per gram. The powder form started at 957 mAh per gram and fell to 541 mAh per gram at 2C/3, recovering only to 960 mAh per gram. The partial capacity restoration in both forms suggests the electrodes did not suffer permanent degradation from high-rate cycling, but the monolith consistently held an edge across every rate tested.

The final piece of the puzzle involved surface chemistry. The researchers treated the best monolith with xenon difluoride, a mild fluorinating agent that reacts selectively with silicon-oxygen-rich domains rather than the carbon framework. Solid-state NMR confirmed the formation of fluorosilicate species such as O2/2SiF3 and O4/2SiF, with no carbon-fluorine bonds detected, consistent with the stronger thermodynamic affinity of silicon for fluorine. The effect on electrochemistry was dramatic: the fluorinated monolith delivered an initial discharge capacity of approximately 2,381 mAh per gram, more than double the non-fluorinated version. The researchers attribute this surge to modified surface chemistry that promotes faster electrolyte wetting of the smallest pores from the very first cycle, whereas non-fluorinated composites require progressive activation over multiple cycles to access comparable surface areas. The fluorosilicate species may also partially passivate reactive surface defects such as under-coordinated silicon atoms and residual silanol groups, which normally promote electrolyte decomposition and irreversible lithium consumption.

The trade-off, however, is durability. While the fluorinated monolith maintained discharge capacities of 2,124, 2,049, and 1,786 mAh per gram at the 10th, 50th, and 100th cycles, its retention at 100 cycles stood at 75 percent, notably lower than the non-fluorinated monolith’s retention above 100 percent. The researchers identify the xenon difluoride dose, reaction temperature, and exposure time as the key parameters to optimize, suggesting that a carefully tuned fluorination protocol could preserve the initial capacity gain while eliminating the accelerated capacity fading. Even in its current form, the work establishes a compelling proof of concept: biomass-derived fibers can transform brittle silicon-based anodes into flexible, self-supporting electrodes that outperform conventional formulations, and controlled surface fluorination can unlock capacities among the highest reported for binder-free SiOx anodes. As the demand for fast-charging, high-energy-density batteries intensifies, the humble kapok fiber may find itself at the center of a battery revolution nobody saw coming.

Subject of Research: Binder-free self-supporting SiOxCy/C anodes reinforced by biomass kapok fibers and surface fluorination for lithium-ion batteries

Article Title: Binder-free self supporting SiOxCy/C anodes: mechanical reinforcement by biomass fibers and surface fluorination effects on lithium-ion battery performance

Article References: Hatim, O., Ramaswamy, A. P., Pradille, C., Boisard, A., Berthon-Fabry, S., & Guérin, K. (2026). Binder-free self supporting SiOxCy/C anodes: mechanical reinforcement by biomass fibers and surface fluorination effects on lithium-ion battery performance. Frontiers in Batteries and Electrochemistry, 5, Article 1838088. https://doi.org/10.3389/fbael.2026.1838088

Image Credits: AI Generated

DOI: 10.3389/fbael.2026.1838088

Keywords: lithium-ion batteries, silicon anodes, kapok fibers, binder-free electrodes, surface fluorination, sol-gel synthesis, SiOxCy composites, energy storage, Binder-free, self, supporting, SiOxCy