Iron Atoms in Porous Carbon Push Lithium-Ion Battery Anodes to New Heights

A team of researchers at Linyi University in China has unveiled a remarkably simple recipe for one of the most capable carbon anodes ever reported for lithium-ion batteries. By heating a single, carefully designed powder in a furnace just once, the group produced a porous carbon material studded with isolated iron atoms locked into four nitrogen neighbors, the so-called Fe-N4 configuration. The resulting anode, designated Fe40-N4C-900, delivered a reversible capacity of 1180 milliampere-hours per gram after 1000 cycles at a current density of 0.1 amperes per gram, and it still managed 235 milliampere-hours per gram when the current was pushed to a punishing 10 amperes per gram. Those numbers dwarf the theoretical capacity of conventional graphite anodes, which tops out at 372 milliampere-hours per gram, and they outperform most previously reported metal-nitrogen-carbon anodes built around cobalt, nickel, or manganese active sites.

The achievement matters because carbon anodes, despite their abundance, sustainability, and low cost, have long been held back by two stubborn problems. The first is limited lithium storage capacity, capped by the intercalation chemistry of graphite itself. The second is poor initial Coulombic efficiency, a consequence of the solid-electrolyte interphase that forms irreversibly during the first charge-discharge cycle and consumes lithium that can never be recovered. Both problems translate directly into shorter driving ranges for electric vehicles and lower energy density for grid storage systems. The new study, published in the Journal of the Saudi Chemical Society, attacks both limitations at once through a combination of atomic-scale doping, defect engineering, and hierarchical porosity, all achieved in a single calcination step that the authors argue is scalable for industrial production.

The synthesis begins with a metal-organic framework, a class of crystalline materials in which metal nodes are connected by organic linkers into periodic, sponge-like lattices. The researchers chose ZIF-8, a well-known framework built from zinc ions and 2-methylimidazole, and modified it by adding a small, precisely controlled amount of iron nitrate to the precursor solution. When the zinc-to-iron molar ratio in the feed was set to 40:1, the resulting Fe-ZIF-8 crystals retained the pristine rhombic dodecahedral morphology of the parent framework, with no iron oxide phases detectable by X-ray diffraction and no metallic iron nanoparticles visible in any transmission electron microscopy field of view. The iron, in other words, was incorporated into the framework by lattice substitution and remained atomically dispersed, a prerequisite for forming discrete Fe-N4 sites rather than clumps of electrochemically dead iron compounds.

Pyrolysis of this precursor under high-purity argon at 900 degrees Celsius, with a slow heating ramp of one degree per minute and a two-hour hold, accomplished several transformations simultaneously. Thermogravimetric analysis showed that carbon-nitrogen bond cleavage begins near 640 degrees Celsius, while zinc evaporation occurs near 750 degrees. By the time the furnace reached 800 degrees, X-ray diffraction revealed only amorphous carbon, with no residual zinc or iron crystalline phases. The evaporation of zinc is the key to the porosity: as the volatile metal leaves the carbonizing framework, it leaves behind an open network of pores. Acid washing with dilute hydrochloric acid then removed any remaining zinc species and uncoordinated iron particles, opening the pore structure further, reducing ion diffusion resistance, and unblocking the Fe-N4 active sites.

Characterization confirmed that the iron did far more than simply occupy space. Raman spectroscopy showed a high ratio of the disorder-induced D band to the graphitic G band, 1.14, signaling a defect-rich carbon structure. High-resolution transmission electron microscopy revealed a predominantly amorphous matrix with lattice distortions, edge defects, and irregular pore distributions. The researchers attribute this wrinkled, defective texture to the iron itself: iron-group elements catalyze the conversion of pyridinic nitrogen into the more electrochemically active pyrrolic configuration and distort the graphene lattice, increasing the accessible surface area. The numbers bear this out. Brunauer-Emmett-Teller measurements gave a specific surface area of 122.64 square meters per gram for the iron-doped material, nearly double the 61.84 square meters per gram of the undoped NC-900 reference, with mesopores concentrated in the 2 to 6 nanometer range.

X-ray photoelectron spectroscopy provided the chemical fingerprint of the active sites. The nitrogen 1s spectrum deconvoluted into pyridinic, pyrrolic, graphitic, and iron-nitrogen-bonded components, and the iron-doped sample contained markedly higher levels of pyridinic nitrogen and Fe-N species than the undoped control, consistent with the metal-nitrogen coordination geometry familiar from single-atom catalysts. The iron 2p spectrum showed peaks attributable to Fe-N4 species at 710.5 electron volts, alongside satellite features confirming a mixture of Fe2+ and Fe3+ states. The absence of zinc 2p signals confirmed complete metal evaporation during carbonization. Nitrogen doping itself is electrochemically meaningful: it creates an asymmetric charge distribution in the carbon lattice, generating strong lithium-ion binding sites and lowering the work function through defect generation.

Electrochemical testing in half-cell coin cells against lithium metal told a striking story. Cyclic voltammetry showed an irreversible cathodic feature between 0.6 and 0.9 volts in the first sweep, the signature of electrolyte decomposition and solid-electrolyte interphase formation, which vanished in subsequent cycles. The overlapping curves of the tenth, fiftieth, and hundredth cycles confirmed excellent reversibility. In galvanostatic tests, the Fe40-N4C-900 electrode delivered an initial discharge capacity of 1463.01 milliampere-hours per gram and an initial charge capacity of 1085.82 milliampere-hours per gram, an initial Coulombic efficiency of 73.18 percent, compared with just 61.7 percent for the undoped NC-900. Rate capability was equally impressive: stepping the current from 0.1 up to 10 amperes per gram yielded reversible capacities of 1005, 853, 736, 608, 398, and finally 235 milliampere-hours per gram, and when the current dropped back to 0.1 amperes per gram the capacity snapped back to 1050 milliampere-hours per gram almost instantly.

Long-term cycling revealed another unusual behavior: the capacity actually rose during the first fifty cycles before stabilizing. The researchers attribute this activation to a gradual reduction in charge-transfer resistance, the progressive formation of a stable, ionically conductive interphase, and the opening of micropores that progressively expose more Fe-N4 sites to the electrolyte. Electrochemical impedance spectroscopy supported this picture, showing an initial charge-transfer resistance of 54.18 ohms for the iron-doped anode versus 60.05 ohms for NC-900. After 1000 cycles at 0.1 amperes per gram, the electrode still held 1180 milliampere-hours per gram, a loss of only 5.6 percent from its maximum stable capacity, while the undoped reference faded to 726 milliampere-hours per gram under identical conditions.

Density functional theory calculations added a mechanistic layer to the experimental findings. The computations showed that Fe-Nx sites exhibit moderate adsorption energies toward lithium ions compared with the Co-N4, Ni-N4, and Mn-N4 analogues, striking a balance the authors describe as strong adsorption with fast diffusion. The calculated adsorption energy on Fe-N4 sites was minus 1.74 electron volts, strong enough to anchor lithium reversibly but not so strong that the ions become trapped. The calculations also showed that the nitrogen-carbon bond lengths within the Fe-N4C structure change minimally during lithiation and delithiation, which helps explain the exceptional structural stability over a thousand cycles. The optimal iron content of roughly two weight percent proved to be a genuine sweet spot: too little iron fails to generate enough catalytically active sites during carbonization, while too much causes aggregation into inactive iron nitrides and carbides and can collapse the porous framework entirely.

The broader significance of the work lies in its synthesis philosophy. Conventional routes to metal-nitrogen-carbon materials typically rely on physically mixing metal precursors with carbon matrices, an approach plagued by metal aggregation and phase segregation during pyrolysis. By encoding the metal directly into a metal-organic framework whose periodic node-linker architecture guarantees atomic-level dispersion, and then removing the sacrificial zinc in the same heat treatment that forms the carbon, the researchers collapsed a multi-step process into one. The study also fills a conceptual gap: while M-N4C materials have been extensively explored as electrocatalysts and cathode hosts for metal-air and lithium-sulfur batteries, their use as lithium-ion anodes, and the specific influence of the central metal atom on lithium-ion binding, had remained underexplored. With electric vehicle and grid storage markets demanding anodes that combine graphite-level stability with far higher capacity, a one-step, MOF-derived route to defect-rich, single-atom iron carbon may prove to be exactly the kind of practical materials chemistry the battery industry has been waiting for.

Subject of Research: MOF-derived Fe-N4-doped porous carbon anodes for high-performance lithium-ion batteries

Article Title: One-step synthesis of MOF-derived Fe-N4 -doped porous carbon for high-performance lithium-ion battery anodes

Article References: Wang, G., Zhao, K., Guo, H., Hu, H., Yang, Y., Gao, J., & Wang, S. (2026). One-step synthesis of MOF-derived Fe-N4 -doped porous carbon for high-performance lithium-ion battery anodes. Journal of Saudi Chemical Society, 30(2), Article 23. https://doi.org/10.1007/s44442-026-00072-y

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00072-y

Keywords: lithium-ion batteries, anode materials, metal-organic frameworks, ZIF-8, Fe-N4 active sites, porous carbon, nitrogen doping, single-atom catalysis, pyrolysis, reversible capacity, solid-electrolyte interphase, DFT calculations