A free-standing sodium-ion battery anode combines bismuth, molybdenum disulfide, and carbon nanofibers to deliver strong long-term cycling performance in lab-based half-cell tests.
Study: Construction of a Free-Standing Bismuth Carbon Nanofiber-Based Composite Anode Integrated with Molybdenum Disulfide for High-Performance Sodium-Ion Batteries. Image Credit: icestylecg/Shutterstock.com
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Sodium-ion batteries show potential as a lower-cost, more sustainable alternative to lithium-based systems because sodium is far more abundant and easier to access. But many anode materials still come in powder form and depend on metal current collectors, binders, and conductive additives.
Those extra components add inactive mass and reduce overall energy density.
Free-standing electrodes are different. By removing those inactive materials, they can form integrated conductive networks with better mechanical strength. In the study published in nanomaterials, the researchers focused on combining three materials with complementary strengths and weaknesses.
Bismuth (Bi), an alloy-type anode, has a theoretical specific capacity of 386 mAh g-1 and a high volumetric capacity. Its drawback is severe volume expansion of about 250 % during sodium alloying and dealloying, which can quickly degrade performance.
MoS2, with its layered structure and 0.62 nm interlayer spacing, is also a promising sodium-storage material, but it suffers from low intrinsic conductivity and marked volume change during cycling. Carbon coatings are commonly used to improve conductivity and help buffer those structural stresses.
The idea behind the composite is to combine Bi, MoS2, and carbon within a free-standing architecture to improve electron transport, maintain structural integrity, and provide sodium ions with easier access to active sites.
The Free-Standing Study
The team produced the Bi@MoS2@C carbon nanofiber (CNF) composite through a multi-step synthesis route. Bi nanoparticles were first prepared hydrothermally, then incorporated into carbon nanofibers by electrospinning a precursor solution containing Bi nanopowder and polyacrylonitrile (PAN). After pre-oxidation and carbonization, this yielded Bi CNFs.
MoS2 nanospheres were then grown on the Bi CNFs in a second hydrothermal step with thiourea and ammonium molybdate tetrahydrate.
The embedded Bi nanoparticles were described as anchoring sites for the growth of MoS2 nanospheres. The resulting Bi@MoS2 CNFs were then annealed to improve crystallinity.
To further enhance conductivity and accommodate volume change, the researchers added a thin glucose-derived carbon coating via hydrothermal treatment followed by annealing, producing the final free-standing Bi@MoS2@C CNF electrodes.
The material was characterized with a range of techniques, including X-ray diffraction (XRD), Raman spectroscopy, field emission scanning and transmission electron microscopy (FE-SEM and TEM), and thermogravimetric analysis (TGA).
Electrochemical performance was tested in sodium half-cells without binders or current collectors.
Results And Future Potential
Microscopy analysis showed a well-integrated three-dimensional structure, with Bi nanoparticles uniformly dispersed inside the carbon nanofibers and MoS2 nanospheres distributed across the composite.
TEM analysis also identified the characteristic 0.62 nm lattice spacing of the MoS2 (002) plane. A layered outer carbon coating surrounded the structure, helping improve conductivity and mechanical stability.
Electrochemically, the Bi@MoS2@C composite delivered a reversible specific capacity of about 275.31 mAh g-1 at 0.5 A g-1. That was markedly higher than Bi CNFs alone, which reached 150.6 mAh g-1 under the reported comparison conditions.
The composite also retained 96.07 % of its capacity after 800 cycles, while pure MoS2 retained only about 72-74 mAh g-1 after extended cycling.
The paper argues that this improvement comes from the combined effects of the composite design. The carbon nanofiber network provides efficient electron pathways, the layered MoS2 structure supports sodium-ion diffusion, and the integrated architecture helps buffer the large volume changes associated with alloying and conversion reactions. The carbon coating also helps preserve the framework during repeated cycling.
The initial Coulombic efficiency of the Bi@MoS2@C electrode was 68.49 %, with irreversible capacity loss linked to solid electrolyte interphase formation and sodium trapping in defects and voids.
Rate testing over current densities from 0.1 to 10 A g-1 showed that the composite outperformed its individual components, indicating a clear synergistic effect. Electrochemical impedance spectroscopy further indicated improved conductivity and charge-transfer behavior.
Kinetic analysis indicated a mixed sodium-storage process, with a predominantly diffusion-controlled component, suggesting that the electrode operates through a more complex multiphase mechanism rather than a single storage pathway.
What Comes Next for Free-Standing Bi@MoS2@C?
The study clearly presented the free-standing Bi@MoS2@C carbon nanofiber anode that combines alloy-type and conversion-type sodium storage within a conductive carbon framework. By integrating Bi nanoparticles, MoS2 nanospheres, and a glucose-derived carbon coating, the design improves conductivity, supports ion transport, and helps manage volume expansion.
In sodium half-cell testing, this translated into strong specific capacity, good rate performance, and excellent long-term cycling stability.
The work indicates a promising direction for sodium-ion anode design, although further device-level testing will be needed to judge performance beyond laboratory half-cell conditions.
Journal Reference
Mai G., et al. (2026). Construction of a Free-Standing Bismuth Carbon Nanofiber-Based Composite Anode Integrated with Molybdenum Disulfide for High-Performance Sodium-Ion Batteries. Nanomaterials 16(5):327. DOI: 10.3390/nano16050327
