The relentless pursuit of higher energy density and improved safety in energy storage systems has positioned the lithium ion battery as the cornerstone technology for modern applications, from portable electronics to electric vehicles. However, the evolution towards flexible and wearable electronics imposes new challenges, demanding batteries that are not only high-performing but also mechanically robust and integrable into unconventional form factors. A key strategy in this evolution is the development of freestanding electrodes, which eliminate inactive components like binders, conductive additives, and metal current collectors. This architectural shift directly enhances the gravimetric and volumetric energy density of the lithium ion battery and is fundamental to realizing truly flexible energy storage devices.
Conventionally, graphite has served as the dominant anode material due to its stability and low cost, but its theoretical capacity is limited to 372 mAh g-1. While hard carbons derived from precursors like polyacrylonitrile (PAN) offer higher capacity, they often suffer from poor rate capability and electrical conductivity. Conversely, soft carbons provide excellent conductivity and cycling stability but at the expense of specific capacity. This dichotomy presents a clear opportunity: combining the merits of both carbon types within a conductive, freestanding scaffold. To further push the energy boundaries, integrating high-capacity alloying materials is essential. Tin (Sn) stands out with a high theoretical capacity of 994 mAh g-1 and a suitable operating voltage. Its principal drawback, a massive volume change (~260%) during lithiation/delithiation, leads to particle pulverization, loss of electrical contact, and rapid capacity fade. Confining Sn nanoparticles within a resilient, conductive carbon matrix is a proven approach to mitigate these issues.
In this work, we present a rational design and fabrication of a cross-linked, freestanding electrode composed of N-doped carbon nanofibers uniformly embedded with Sn nanoparticles (C-Sn). This architecture is realized via a straightforward electrospinning technique followed by a stabilization and low-temperature carbonization process. The use of a dual carbon source—PAN as a hard carbon precursor and coal tar pitch (CTP) as a soft carbon precursor—creates a synergistic carbon matrix with balanced conductivity and defect-rich active sites. The resulting three-dimensional fibrous network serves as both a mechanical backbone to buffer Sn’s volume changes and a continuous pathway for rapid electron and ion transport. When evaluated directly as an anode in a lithium ion battery, the optimized electrode exhibits exceptional rate capability and outstanding long-term cycling stability. Furthermore, we employ density functional theory (DFT) calculations to elucidate the fundamental role of N-doping in the carbon matrix, revealing its strong affinity for lithium species, which aids in anchoring the alloy phases and maintaining structural integrity. This comprehensive study from material design to mechanistic understanding provides a viable blueprint for next-generation, high-performance lithium ion battery anodes.
Experimental Synthesis and Computational Methodology
The freestanding C-Sn composite films were synthesized via electrospinning. Briefly, a precursor solution was prepared by dissolving polyacrylonitrile (PAN) and tin(II) chloride dihydrate (SnCl2·2H2O) in N,N-Dimethylformamide (DMF). To introduce the soft carbon phase, a specific amount of coal tar pitch (CTP) was added to the solution. This homogeneous solution was then loaded into a syringe and electrospun under controlled voltage and feed rate to generate a non-woven fibrous mat. The as-spun mat was subsequently stabilized in air at 280°C to cross-link the PAN and prevent fusion during carbonization. Finally, the stabilized mat was carbonized at 800°C under an argon atmosphere. During this thermal treatment, PAN and CTP pyrolyzed into N-doped carbon, while SnCl2 was reduced to metallic Sn nanoparticles in situ. By varying the mass ratio of PAN to SnCl2·2H2O in the precursor (9:4.5, 9:5, and 9:7.5), three samples with different Sn contents were obtained and labeled C-Sn-1, C-Sn-2, and C-Sn-3, respectively. For comparison, control samples of bare PAN-derived carbon fibers (P-CF) and PAN/CTP-derived carbon fibers (P/C-CF) were also prepared under identical conditions without the Sn precursor.
Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) to understand the interfacial interactions. The projector-augmented wave (PAW) method and the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) were employed. A model of N-doped amorphous carbon (AMC) was constructed, and the adsorption energies (\(E_{\text{ad}}\)) of Sn and SnxLiy clusters on the AMC surface were calculated using the formula:
$$E_{\text{ad}} = E_{\text{Sn}_x\text{Li}_y} + E_{\text{AMC}} – E_{\text{Sn}_x\text{Li}_y@\text{AMC}}$$
where a positive value indicates a stable adsorption. Bader charge analysis and density of states (DOS) calculations were conducted to examine charge transfer and electronic coupling at the interface.
Structural and Compositional Characterization
The phase composition of the synthesized materials was first examined by X-ray diffraction (XRD). The patterns for C-Sn-1 and C-Sn-2 show only a broad peak centered around 25°, corresponding to the (002) plane of disordered carbon, with no discernible crystalline Sn peaks. This suggests that Sn species in these samples are either amorphous or exist as ultrafine nanocrystals below the detection limit. In contrast, the XRD pattern for C-Sn-3 exhibits distinct diffraction peaks matching the tetragonal phase of metallic Sn (JCPDS No. 04-6073), indicating a higher degree of Sn crystallization or larger particle size due to the increased precursor amount. Raman spectroscopy further confirms the carbonaceous nature of all composites, with characteristic D and G bands at approximately 1345 cm-1 and 1566 cm-1, respectively. The intensity ratio ID/IG, which reflects the defect density or disorder in the carbon structure, is highest for the C-Sn-2 sample (1.15), followed by C-Sn-3 (1.11) and C-Sn-1 (1.03). A higher defect density is generally favorable for lithium ion battery anodes as it can provide additional active sites for Li+ storage and facilitate ion diffusion.
Thermogravimetric analysis (TGA) in air was conducted to determine the precise Sn content in the composites. The weight loss below 250°C is attributed to moisture evaporation. The major weight loss between 250°C and 700°C corresponds to the combustion of carbon and the oxidation of Sn to SnO2. The final residual weight represents SnO2. The Sn mass percentage was calculated using the formula:
$$ \text{Sn}(\%) = 100 \times \frac{m_{\text{SnO}_2}}{m_{\text{C-Sn}}} \times \frac{M_{\text{Sn}}}{M_{\text{SnO}_2}} $$
where \(m\) represents mass and \(M\) represents molar weight. The calculated Sn contents are 23.0%, 25.6%, and 32.9% for C-Sn-1, C-Sn-2, and C-Sn-3, respectively.
X-ray photoelectron spectroscopy (XPS) analysis of the C-Sn-2 sample confirms the presence of C, N, O, and Sn. The high-resolution C 1s spectrum can be deconvoluted into peaks for C=C/C-C, C-N, C=O, and O=C-O bonds. The N 1s spectrum reveals the successful incorporation of nitrogen in various configurations: pyridinic-N (398.1 eV), pyrrolic-N (399.9 eV), and graphitic-N (403.1 eV). Pyridinic-N and pyrrolic-N enhance surface wettability and pseudocapacitive behavior, while graphitic-N improves electronic conductivity, all beneficial for lithium ion battery performance. The Sn 3d spectrum shows doublets for Sn0 and Sn4+, the latter likely due to superficial oxidation upon air exposure.
Morphological analysis via scanning electron microscopy (SEM) reveals that all C-Sn composites consist of long, continuous, and interwoven nanofibers with diameters around 200 nm, forming a porous, three-dimensional network. This structure is ideal for a freestanding electrode, providing mechanical flexibility and ample space to accommodate volume changes. Transmission electron microscopy (TEM) of the optimal C-Sn-2 sample shows uniform fibers without obvious large Sn particles on the surface. High-resolution TEM and selected-area electron diffraction (SAED) show no clear lattice fringes or diffraction spots for Sn, corroborating the XRD finding that Sn is amorphous or nanocrystalline within the carbon matrix. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping demonstrates the homogeneous distribution of C, N, and Sn throughout the nanofiber, confirming the successful integration of Sn nanoparticles within the N-doped carbon fibers.
Electrochemical Performance Evaluation
The electrochemical properties of the freestanding C-Sn electrodes were systematically evaluated in half-cell configurations against Li/Li+. Cyclic voltammetry (CV) curves during the initial cycles exhibit cathodic peaks around 0.6–0.8 V in the first cycle, associated with the formation of a solid electrolyte interphase (SEI) layer, which disappear in subsequent cycles. Prominent redox pairs are observed at approximately 0.4/0.55 V and 0.01/1.2 V. The former is attributed to the reversible alloying/dealloying reaction of Sn with lithium (Sn + xLi+ + xe– ↔ LixSn), while the latter corresponds to Li+ insertion/extraction into/from the defect sites and graphitic domains of the N-doped carbon matrix. The overlapping CV curves after the first cycle indicate excellent electrochemical reversibility and structural stability of the electrodes.
Galvanostatic charge-discharge tests were performed to assess capacity and cycling performance. The initial discharge (lithiation) and charge (delithiation) capacities, along with the corresponding coulombic efficiency (CE), are summarized in Table 1. The optimized C-Sn-2 electrode delivers a high initial charge capacity of 965.9 mAh g-1 at 50 mA g-1 with an initial CE of 78.26%. The irreversible capacity loss in the first cycle is common for carbon and alloy-based anodes and is primarily due to SEI formation and irreversible lithium trapping at defect sites.
| Electrode | Sn Content (%) | 1st Discharge Capacity (mAh g-1) | 1st Charge Capacity (mAh g-1) | Initial Coulombic Efficiency (%) |
|---|---|---|---|---|
| C-Sn-1 | 23.0 | 1257.5 | 974.5 | 77.49 |
| C-Sn-2 | 25.6 | 1234.2 | 965.9 | 78.26 |
| C-Sn-3 | 32.9 | 1219.3 | 945.8 | 77.47 |
The rate capability, a critical metric for high-power lithium ion battery applications, was tested at increasing current densities from 0.05 to 2 A g-1. As shown in Table 2, the C-Sn-2 electrode consistently delivers the highest capacities at all rates. Impressively, it retains a charge capacity of 475.7 mAh g-1 even at a high current of 2 A g-1. When the current density is returned to 0.05 A g-1, the capacity recovers to 819.1 mAh g-1, demonstrating remarkable structural resilience and electrochemical reversibility. This superior rate performance is attributed to the optimal combination of conductive soft carbon, N-doping, and the finely dispersed Sn nanoparticles, which collectively facilitate fast charge transfer and Li+ diffusion.
| Current Density (A g-1) | Discharge Capacity (mAh g-1) | Charge Capacity (mAh g-1) |
|---|---|---|
| 0.05 | 889.8 | 885.0 |
| 0.1 | 822.4 | 820.1 |
| 0.2 | 757.6 | 756.1 |
| 0.3 | 712.8 | 711.6 |
| 0.5 | 657.1 | 656.3 |
| 1.0 | 574.4 | 573.7 |
| 2.0 | 476.2 | 475.7 |
| Return to 0.05 | 820.5 | 819.1 |
Long-term cycling stability is paramount for the practical deployment of a lithium ion battery. The cycling performance of the electrodes at 0.5, 1, and 2 A g-1 is illustrated in Figure 1 (conceptual description). The C-Sn-2 electrode exhibits the best capacity retention. At a current density of 1 A g-1, it maintains a stable charge capacity of 644.2 mAh g-1 after 800 cycles, with the CE stabilizing above 99.5% after the first few cycles. Even under a more strenuous regime of 2 A g-1, it delivers a charge capacity of 412.7 mAh g-1 after 1000 cycles. In contrast, the C-Sn-3 electrode with higher Sn content shows faster capacity decay, particularly at higher currents, due to the exacerbated volume changes that the carbon matrix cannot fully accommodate. The C-Sn-1 electrode, with lower Sn content, shows good stability but lower overall capacity. Post-cycling SEM analysis of the C-Sn-2 electrode confirms that the fibrous network remains largely intact without visible cracks or pulverization, validating the effectiveness of the design in buffering mechanical stress.
Electrochemical Kinetics and Storage Mechanism Analysis
To deconvolute the charge storage mechanism, we analyzed the CV data at various scan rates. The current response (\(i\)) obeys a power-law relationship with the scan rate (\(v\)): \(i = av^b\), where \(b\) is an exponent determined from the slope of log(\(i\)) versus log(\(v\)). A \(b\)-value of 0.5 indicates a diffusion-controlled process (battery-like behavior), while a value of 1.0 signifies a capacitive-controlled process (capacitor-like behavior). For the C-Sn-2 electrode, the calculated \(b\)-values for the major anodic and cathodic peaks range between 0.657 and 0.771, indicating a mixed storage mechanism with a significant contribution from surface-controlled capacitive processes. This pseudocapacitive behavior, enhanced by N-doping and the nanoconfined Sn, is responsible for the excellent rate performance of the electrode.
Electrochemical impedance spectroscopy (EIS) was employed to investigate charge transfer kinetics. The Nyquist plots consist of a depressed semicircle in the high-to-medium frequency region, representing the charge transfer resistance (\(R_{ct}\)), and a sloping line in the low-frequency region, associated with Li+ diffusion. The C-Sn-2 electrode exhibits the smallest \(R_{ct}\) value among the three composites, indicating the most efficient charge transfer at the electrode/electrolyte interface. The Li+ diffusion coefficient (\(D_{Li^+}\)) can be estimated from the low-frequency Warburg region using the following equation:
$$ D_{Li^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C_{Li}^2 \sigma^2} $$
where \(R\) is the gas constant, \(T\) is the absolute temperature, \(A\) is the electrode area, \(n\) is the number of electrons transferred, \(F\) is Faraday’s constant, \(C_{Li}\) is the concentration of Li+ in the electrode, and \(\sigma\) is the Warburg factor obtained from the slope of \(Z_{re}\) vs. \(\omega^{-1/2}\). The calculated \(D_{Li^+}\) for C-Sn-2 is \(2.49 \times 10^{-14}\) cm2 s-1, which is higher than that of C-Sn-1 (\(6.76 \times 10^{-15}\) cm2 s-1) and C-Sn-3 (\(9.15 \times 10^{-15}\) cm2 s-1). This confirms that the optimized composition in C-Sn-2 facilitates faster ionic transport, a key factor for its superior rate capability.
Theoretical Insights from DFT Calculations
To gain a fundamental understanding of the enhanced stability, we performed DFT calculations. The adsorption energy (\(E_{ad}\)) of Sn and lithium-tin alloy clusters (e.g., Sn3Li2) on the model N-doped amorphous carbon (AMC) surface was computed. The results are profound: while metallic Sn shows a stable adsorption (\(E_{ad}\) = -0.81 eV), the adsorption becomes significantly stronger for Sn3Li2 (\(E_{ad}\) = -1.68 eV) and strongest for a Li atom (\(E_{ad}\) = -2.91 eV). This trend reveals that the N-doped carbon matrix has an exceptionally high affinity for lithium species. During the operation of the lithium ion battery, as Sn alloys with Li to form LixSn, this strong interfacial interaction effectively “anchors” the expanding alloy particles to the carbon fiber surface. This anchoring effect mitigates particle detachment and agglomeration, thereby preserving electrical contact and structural integrity throughout cycling.
Bader charge analysis indicates substantial electron transfer from the SnxLiy clusters to the AMC substrate, increasing with lithium content (e.g., 1.28e for Sn3Li2). The density of states (DOS) analysis shows strong orbital hybridization between Li states and the C/N states of AMC in the alloy@AMC systems. This chemical interaction, beyond mere physical confinement, is the electronic-structure origin of the robust adhesion and improved cycling performance. It ensures that the active material remains integrated with the conductive matrix despite large volume strains.
Conclusion and Perspective
In summary, we have successfully designed and fabricated a high-performance freestanding anode for lithium ion battery applications through the rational integration of N-doped carbon nanofibers and Sn nanoparticles. The electrospinning technique, coupled with a dual carbon source strategy (PAN and CTP), yields a flexible, binder-free electrode with an ideal architecture. The cross-linked 3D fibrous network provides mechanical robustness, continuous electron pathways, and abundant pores for electrolyte infiltration. The optimized C-Sn-2 composite, with a Sn content of ~25.6%, achieves an outstanding balance between high capacity (965.9 mAh g-1 at 50 mA g-1), exceptional rate capability (475.7 mAh g-1 at 2 A g-1), and ultralong cycling stability (412.7 mAh g-1 after 1000 cycles at 2 A g-1).
The synergistic effects contributing to this performance are multi-faceted: (1) The soft carbon from CTP enhances the overall electronic conductivity of the fiber. (2) Nitrogen doping improves surface wettability, introduces pseudocapacitive sites, and strengthens the interaction with lithium. (3) The carbon matrix uniformly confines Sn nanoparticles, buffering their volume expansion and preventing aggregation. (4) DFT calculations provide a crucial mechanistic insight, revealing that the N-doped carbon has a strong affinity for lithium, which helps anchor the Li-Sn alloy phases during cycling, a key factor for stability.
This work demonstrates a feasible and scalable strategy for developing advanced freestanding electrodes. The principles elucidated here—combining conductive carbon matrices with nanoscale alloying materials and employing heteroatom doping to tune interfacial properties—are broadly applicable to other high-capacity electrode materials (e.g., Si, Ge, P) that suffer from volume change issues. Future work may focus on further optimizing the porosity of the fibers, engineering the carbon crystallinity, or exploring full-cell integration with high-voltage cathodes to assess practical energy density. This study marks a significant step toward the realization of high-energy, long-lasting, and flexible lithium ion battery systems for the next generation of portable and wearable electronics.