Coaxial electrospinning provides both a controllable core–shell architecture and a continuous electrode network. By feeding two precursor fluids through a dual-capillary spinneret, it can produce nanofibers with a hard-carbon shell and soft-carbon-rich core after stabilization and carbonization. This structure combines the higher lithium-storage capacity of hard carbon with the conductivity, rate capability, and structural compliance of soft carbon, enabling reversible capacities of about 390 mAh g⁻¹ at 100 mA g⁻¹ in the described system.
Coaxial electrospinning is valuable because it integrates two carbon phases within one continuous nanofiber rather than mixing separate particles. The resulting core–shell design can improve lithium-ion transport, electrical connectivity, mechanical stability, and overall anode utilization.
How Coaxial Electrospinning Creates the Core–Shell Structure
Independent control of the two precursor regions
A dual-capillary spinneret delivers the shell and core precursors simultaneously while keeping them spatially separated. For example, a PAN-based outer solution can form the shell, while a mineral-oil-based inner precursor occupies the core.
During stabilization and carbonization, the PAN-derived outer region converts into hard carbon, while the inner precursor produces a soft-carbon-rich core under suitable processing conditions.
Continuous fiber formation
Unlike a conventional powder blend, coaxial electrospinning generates a continuous one-dimensional fiber. This continuity reduces the number of poorly connected particle–particle interfaces within the electrode.
The fibers can also form an interconnected mat, helping create pathways for electron transport throughout the active material.
Tunable shell and core dimensions
The relative flow rates, precursor concentrations, solution properties, and processing conditions influence the core diameter and shell thickness. This provides a way to tune the balance between:
- Hard-carbon storage capacity.
- Soft-carbon conductivity and kinetics.
- Lithium-ion diffusion distance.
- Mechanical protection of the inner phase.
Structural Advantages for Lithium-Ion Anodes
Combines complementary carbon properties
The hard-carbon shell offers a higher theoretical storage contribution in the described system, with a reported capacity range of approximately 400–500 mAh g⁻¹. The soft-carbon core contributes different advantages, including more favorable electronic transport and potentially faster lithium insertion and extraction.
The core–shell configuration therefore uses each carbon phase where it is most useful instead of requiring one material to provide every function.
Creates short lithium-ion transport paths
Nanofiber dimensions reduce the distance lithium ions must travel to access active carbon. The shell is also relatively thin compared with a bulk particle, allowing electrolyte access to the active structure more efficiently.
The core–shell geometry can therefore improve utilization of the carbon phases, particularly when the shell thickness is optimized rather than made excessively dense.
Improves electrical connectivity
The carbonized fibers form a continuous conductive framework. This can reduce dependence on large quantities of separate conductive additives and improve contact between active material regions and the current collector.
The soft-carbon core can further support charge transport along the fiber, while the hard-carbon shell provides the principal high-capacity storage region.
Provides internal structural support
The shell surrounds the inner carbon domain and helps preserve the fiber geometry during repeated lithiation and delithiation. This confinement can reduce structural degradation compared with unsupported, irregular carbon particles.
The benefit is not that volume change disappears, but that the architecture can distribute mechanical stresses more effectively.
Reduces phase separation
In a physical mixture, hard-carbon and soft-carbon particles may have different sizes, surfaces, and electrochemical behavior. Coaxial electrospinning places the two phases into a single fiber with a defined spatial relationship.
This reduces macroscopic phase segregation and can make the electrode structure more uniform.
Performance Advantages in Battery Operation
Higher reversible capacity than a soft-carbon-only design
The hard-carbon shell supplies the higher-capacity component of the composite. The reported core–shell carbon nanofiber electrode reaches approximately 390 mAh g⁻¹ at 100 mA g⁻¹, indicating that a substantial fraction of the active carbon can participate reversibly.
The composite capacity should not be interpreted as a simple arithmetic sum of the individual capacity ranges. Actual performance depends on phase proportions, porosity, carbonization conditions, electrode composition, and test protocol.
Improved charge-transfer and rate behavior
Soft carbon generally has a more disordered and readily conductive structure than hard carbon. Incorporating it into the fiber core can provide a lower-resistance pathway for electron movement and support faster electrochemical reactions.
The nanofiber geometry also reduces transport distances, which can help maintain capacity as current density increases.
Better active-material utilization
The combination of nanoscale dimensions, continuous conductivity, and an accessible shell can expose more electrochemically useful carbon than a poorly connected bulk electrode.
This is particularly important for hard carbon, whose measured capacity can be strongly affected by particle size, porosity, surface chemistry, and diffusion limitations.
Potentially improved cycling stability
The core–shell arrangement can help maintain contact between the carbon phases and the conductive network during cycling. The outer shell also acts as a structural envelope around the inner region.
Long-term stability still depends on the shell integrity, pore structure, surface area, electrolyte compatibility, and formation of the solid-electrolyte interphase.
Why the Architecture Matters More Than the Materials Alone
A mixture and a core–shell fiber are not equivalent
Simply blending hard-carbon and soft-carbon powders does not guarantee the same performance. A powder mixture can contain isolated particles, inefficient interfaces, and nonuniform current distribution.
Coaxial electrospinning builds the relationship between the two phases directly into each fiber, giving control over where storage, transport, and mechanical functions occur.
The shell acts as both active material and structural layer
The hard-carbon shell is not merely a coating. It contributes to lithium storage while also defining the outer surface exposed to the electrolyte and helping contain the inner carbon.
Its thickness must therefore be optimized: an overly thick or dense shell may restrict ion transport, while an excessively thin shell may provide insufficient structural protection.
The core provides complementary rather than redundant functionality
The soft-carbon-rich core is intended to support conductivity, kinetics, and structural compliance rather than duplicate the full role of the hard-carbon shell. This division of functions is the central design advantage of the coaxial configuration.
Understanding the Trade-offs
More processing complexity
Coaxial electrospinning requires a specialized spinneret and careful control of two precursor streams. Stable core–shell formation can be affected by viscosity, conductivity, flow-rate ratio, solvent compatibility, and interfacial stability.
This makes the process more complex than single-fluid electrospinning or ordinary carbon-powder blending.
Carbonization can alter the intended geometry
Stabilization and carbonization cause shrinkage, mass loss, and chemical transformation. The final core–shell dimensions may differ substantially from the as-spun dimensions.
The claim that the inner precursor forms soft carbon should therefore be understood as process-dependent; the final composition and structure must be verified experimentally.
Capacity may be limited by inactive mass and composition
If the shell, core, binder, current collector, or residual non-carbon species contribute differently to the mass calculation, the reported gravimetric capacity can vary. The hard-carbon fraction, soft-carbon fraction, and accessible porosity must be considered together.
A higher nominal capacity does not automatically mean better practical energy density if the electrode has low active-material loading or poor first-cycle efficiency.
Surface area can increase irreversible reactions
Nanofibers provide short transport distances but may also expose substantial surface area to the electrolyte. This can increase solid-electrolyte interphase formation and lead to irreversible lithium consumption during the first cycle.
Surface chemistry and pore structure must therefore be controlled alongside the core–shell geometry.
Performance is not determined by architecture alone
The reported reversible capacity near 390 mAh g⁻¹ at 100 mA g⁻¹ demonstrates the potential of the design, but it does not by itself establish superior performance under every condition. Rate capability, cycling retention, initial Coulombic efficiency, electrode loading, and full-cell behavior are also required for a complete evaluation.
Making the Right Choice for Your Goal
Coaxial electrospinning is most useful when the objective is to engineer the relationship between carbon phases, not merely combine them in a powder.
- If your primary focus is high reversible capacity: Use a hard-carbon-rich shell with sufficient accessibility and porosity, while preserving enough soft-carbon core to maintain transport and structural support.
- If your primary focus is rate capability: Emphasize thin shells, continuous fiber conductivity, and an accessible core–shell interface to shorten lithium-ion and electron transport paths.
- If your primary focus is cycling stability: Optimize the shell thickness and carbonization conditions so the fiber can accommodate mechanical stresses without losing electrolyte access.
- If your primary focus is scalable manufacturing: Treat precursor compatibility, spinneret stability, fiber collection, and thermal-processing uniformity as critical design constraints rather than secondary process details.
The central advantage of coaxial electrospinning is its ability to turn complementary carbon materials into one integrated, transport-efficient, and mechanically coherent anode architecture.
Summary Table:
| Advantage | Description |
|---|---|
| Core-Shell Architecture | Integrates hard carbon shell and soft carbon core for balanced performance. |
| Short Li+ Transport Paths | Nanofiber dimensions reduce ion diffusion distances, improving utilization. |
| Continuous Conductive Network | Fibers form interconnected mat, reducing reliance on conductive additives. |
| Structural Support | Shell confines core, mitigating degradation during cycling. |
| Tunable Dimensions | Core and shell sizes adjustable via processing parameters. |
| Improved Rate Capability | Soft carbon core enhances electron transport and kinetics. |
| Higher Reversible Capacity | Achieves ~390 mAh/g at 100 mA/g, superior to soft carbon alone. |
| Reduced Phase Segregation | Uniform distribution of phases within individual fibers. |
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