As a researcher focused on advancing energy storage technologies, I have been deeply involved in developing next-generation cathode materials for li-ion batteries. The global push toward carbon neutrality and the volatility of energy markets have intensified the demand for high-performance, cost-effective, and environmentally friendly li-ion batteries. While commercial li-ion batteries currently dominate the market, their traditional intercalation-type cathodes, such as LiCoO2 (LCO) and LiFePO4 (LFP), are constrained by limited theoretical capacities and the high cost of cobalt and nickel resources. This has spurred interest in conversion-type cathode materials, which offer higher theoretical capacities through multi-electron redox reactions. Among these, iron fluoride (FeF2) stands out due to its high theoretical capacity of 571 mAh g−1, low cost, and eco-friendliness. However, FeF2 faces significant challenges, including poor intrinsic conductivity, interfacial side reactions, and structural degradation during cycling, which hinder its practical application in li-ion batteries.
In this study, I aimed to address these issues by designing a composite material that combines FeF2 nanoparticles with conductive carbon fibers using electrospinning technology. The rationale behind this approach is to encapsulate FeF2 within a carbon matrix to enhance electronic conductivity, suppress volume changes, and stabilize the electrode-electrolyte interface. Electrospinning allows for the facile fabrication of one-dimensional nanostructures, which can provide fast electron transport pathways and confine active materials to mitigate pulverization. By optimizing the carbonization temperature, I sought to achieve a balance between sufficient carbonization of the polymer matrix and prevention of FeF2 grain coarsening, thereby maximizing the electrochemical performance of the composite in li-ion batteries.
The importance of li-ion batteries in modern society cannot be overstated; they power everything from portable electronics to electric vehicles and grid storage systems. However, to meet the growing energy demands, improvements in cathode materials are crucial. Conversion-type materials like FeF2 offer a promising avenue due to their high energy density. The theoretical energy density of FeF2 can be calculated using the formula: $$E_d = C \times V$$ where \(E_d\) is the energy density in Wh kg−1, \(C\) is the specific capacity in mAh g−1, and \(V\) is the average voltage in V. For FeF2, with a capacity of 571 mAh g−1 and an average voltage of 2.66 V, the energy density is approximately 1519 Wh kg−1, which is significantly higher than that of conventional cathodes. Despite this potential, the practical implementation of FeF2 in li-ion batteries has been limited by its insulating nature and electrochemical instability.
To overcome these barriers, various strategies have been explored, including nanostructuring, cation doping, composite formation with conductive materials, and electrolyte engineering. In my work, I focused on the composite approach, leveraging electrospinning to create a hierarchical structure. The use of water-soluble polyvinylpyrrolidone (PVP) as a carbon precursor is advantageous because it allows for homogeneous mixing with metal fluoride precursors and forms a nitrogen-doped carbon matrix upon carbonization, which further enhances conductivity and wettability. The confined environment of the carbon fibers can limit the volume expansion during lithiation/delithiation, as described by the strain confinement model: $$\Delta V = \beta \cdot \Delta x$$ where \(\Delta V\) is the volume change, \(\beta\) is a confinement factor, and \(\Delta x\) is the reaction-induced expansion. By minimizing \(\Delta V\), the composite can maintain structural integrity over multiple cycles, which is critical for long-term stability in li-ion batteries.
In the following sections, I will detail the experimental procedures, characterize the materials, and evaluate their electrochemical performance. I will also discuss the influence of carbonization temperature on the morphology, structure, and properties of the composite, using tables and formulas to summarize key findings. The ultimate goal is to demonstrate a viable cathode material for li-ion batteries that combines high capacity with excellent cycling stability.
Experimental Methodology
The preparation of FeF2-embedded conductive carbon fiber composites involved several steps, including precursor synthesis, electrospinning, pre-oxidation, and carbonization. All chemicals were of analytical grade and used without further purification. The experimental workflow is summarized in Table 1, which outlines the key parameters and conditions.
| Step | Materials/Parameters | Conditions |
|---|---|---|
| Precursor Preparation | Nanoscale iron powder, H2SiF6 solution | Magnetic stirring for 12 h, centrifugation, evaporation |
| Electrospinning Solution | PVP (K-60), FeSiF6·6H2O, deionized water | Mass ratio PVP:FeSiF6 = 1:2, PVP concentration 20 wt% |
| Electrospinning | Electric field, distance | Voltage 15-20 kV, distance 15 cm, room temperature |
| Pre-oxidation | Air atmosphere, heating rate | 1 °C min−1 to 220 °C, hold for 2 h |
| Carbonization | Argon atmosphere, temperatures | 1 °C min−1 to 600, 700, or 800 °C, hold for 1 h |
First, the FeSiF6·6H2O precursor was synthesized by reacting nanoscale iron powder with H2SiF6 solution. This reaction can be represented as: $$\text{Fe} + \text{H}_2\text{SiF}_6 \rightarrow \text{FeSiF}_6 + \text{H}_2 \uparrow$$ The resulting light green solution was centrifuged to remove unreacted particles, and the supernatant was evaporated to yield FeSiF6·6H2O crystals. This precursor served as the source of iron and fluoride without introducing additional fluorine agents, which simplifies the process and reduces costs for li-ion battery applications.
For electrospinning, PVP and FeSiF6·6H2O were dissolved in deionized water to form a homogeneous spinning solution. The PVP acted as a carbon source and a matrix to encapsulate the precursor. The solution was loaded into a syringe with a metallic needle, and a high voltage was applied to create fibers. The electrospinning process is governed by the Taylor cone formation, where the electrostatic force overcomes the surface tension of the solution. The fiber diameter can be estimated using the following empirical relation: $$d = k \cdot \sqrt{\frac{\gamma}{\rho E^2}}$$ where \(d\) is the fiber diameter, \(k\) is a constant, \(\gamma\) is the surface tension, \(\rho\) is the density, and \(E\) is the electric field strength. By optimizing these parameters, I obtained uniform precursor fibers with diameters around 150-200 nm, as observed under scanning electron microscopy (SEM).
Pre-oxidation was a critical step to stabilize the fibers before carbonization. During pre-oxidation at 220 °C in air, the thermoplastic PVP underwent cyclization and cross-linking, converting into a non-thermoplastic structure that could withstand high temperatures without melting. This process involves the formation of ladder polymers, which enhance thermal stability. The weight loss during pre-oxidation was monitored, and it typically ranged from 10-15%, indicating partial decomposition.
Carbonization was performed in an argon atmosphere at different temperatures (600, 700, and 800 °C) to study the effect on the composite properties. The carbonization process converts PVP into a conductive carbon matrix through pyrolysis, releasing volatile compounds such as H2O, CO, and CH4. The reaction can be simplified as: $$\text{PVP} \xrightarrow{\Delta} \text{C} + \text{N-doped species} + \text{gases}$$ The resulting composites were labeled as FeF2@NFP-600, FeF2@NFP-700, and FeF2@NFP-800, where NFP denotes nitrogen-doped fibrous carbon. For comparison, pure FeF2 was also prepared by carbonizing FeSiF6·6H2O at 700 °C without PVP, labeled as FeF2-700.
Material characterization included X-ray diffraction (XRD) for crystal structure analysis, scanning electron microscopy (SEM) for morphology, X-ray photoelectron spectroscopy (XPS) for surface chemistry, and resistivity measurements for electrical conductivity. The crystallite size of FeF2 was calculated using the Scherrer equation: $$D = \frac{K \lambda}{\beta \cos \theta}$$ where \(D\) is the crystallite size, \(K\) is the shape factor (0.9), \(\lambda\) is the X-ray wavelength (1.5406 Å for Cu Kα), \(\beta\) is the full width at half maximum (FWHM) in radians, and \(\theta\) is the Bragg angle. This formula is essential for understanding the grain growth at different carbonization temperatures.
Electrochemical testing was conducted using coin-type half-cells (CR2032) with lithium metal as the anode. The cathode slurry was prepared by mixing the composite, acetylene black, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 7:2:1, using N-methyl-2-pyrrolidone (NMP) as the solvent. The slurry was coated onto aluminum foil and dried at 70 °C to form electrodes with mass loadings of 1.6-1.8 mg cm−2. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume). Galvanostatic charge-discharge tests were performed at 25 °C in a voltage range of 1.0-4.0 V vs. Li/Li+, using current densities from 0.05 to 3.0 A g−1. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out to investigate reaction mechanisms and kinetics. All electrochemical measurements were focused on evaluating the performance for li-ion batteries.
Results and Discussion
Morphological and Structural Analysis
The morphology of the composites was significantly influenced by the carbonization temperature, as revealed by SEM observations. For FeF2@NFP-600, the fibers appeared twisted and non-uniform, with diameters around 150 nm. Small FeF2 nanoparticles were dispersed on the surface, but the carbon matrix seemed incomplete due to insufficient carbonization. In contrast, FeF2@NFP-700 exhibited straight, continuous fibers with diameters of approximately 100 nm, and FeF2 nanoparticles were uniformly embedded within the carbon matrix. This indicates that 700 °C is optimal for achieving a well-carbonized structure without excessive grain growth. At 800 °C, FeF2@NFP-800 showed thinner fibers (about 65 nm in diameter) but with severe coarsening of FeF2 particles, which grew to micron-sized aggregates. This coarsening is attributed to the high temperature promoting particle migration and coalescence, which can detrimentally affect the electrochemical activity in li-ion batteries.
XRD patterns confirmed the formation of crystalline FeF2 (PDF No. 74-0915) in all composites, with no impurity phases. The peak intensities increased with carbonization temperature, reflecting enhanced crystallinity. Using the Scherrer equation, the crystallite sizes were calculated as 46.4 nm for FeF2@NFP-600, 94.3 nm for FeF2@NFP-700, and over 100 nm for FeF2@NFP-800. This trend aligns with the SEM observations and underscores the trade-off between carbonization and grain growth. The larger crystallites at higher temperatures may reduce the active surface area, impacting the conversion reaction kinetics in li-ion batteries.
XPS analysis provided insights into the surface chemistry of FeF2@NFP-700. The C 1s spectrum showed peaks at 284.8 eV (C-C), 286.2 eV (C-N), and 289.3 eV (C-F), indicating the presence of carbon matrix, nitrogen doping, and interaction between carbon and fluoride. The N 1s spectrum revealed pyridinic N (398.9 eV), pyrrolic N (400.1 eV), and graphitic N (401.8 eV), with a nitrogen content of 4.19 at%. Nitrogen doping is known to improve electronic conductivity and electrolyte wettability, which are beneficial for li-ion battery cathodes. The F 1s spectrum displayed peaks at 684.9 eV (Fe-F) and 686.2 eV (C-F), confirming the formation of FeF2 and its bonding with carbon. These results suggest strong interfacial interactions that can enhance structural stability during cycling.
Electrical resistivity measurements were performed under varying pressures to assess the conductivity of the composites. The average resistivity values are summarized in Table 2, along with the carbonization temperatures. The resistivity decreased with increasing temperature, demonstrating improved carbonization. However, the electrochemical performance did not correlate linearly with resistivity due to the influence of FeF2 particle size.
| Sample | Carbonization Temperature (°C) | Average Resistivity (Ω cm) | Initial Discharge Capacity (mAh g−1) | Capacity Retention after 100 Cycles (%) |
|---|---|---|---|---|
| FeF2@NFP-600 | 600 | 437.6 | 125.9 | 45.01 |
| FeF2@NFP-700 | 700 | 3.9 | 301.5 | 92.98 |
| FeF2@NFP-800 | 800 | 0.2 | 116.1 | 46.91 |
| FeF2-700 (pure) | 700 | N/A | 390.8 | 9.03 |
The resistivity data can be modeled using the percolation theory for conductive composites: $$\rho = \rho_0 (p – p_c)^{-t}$$ where \(\rho\) is the resistivity, \(\rho_0\) is a constant, \(p\) is the volume fraction of conductive phase, \(p_c\) is the percolation threshold, and \(t\) is a critical exponent. For FeF2@NFP-700, the low resistivity indicates that the carbon matrix forms a continuous network, facilitating electron transport in li-ion battery electrodes.
Electrochemical Performance
The galvanostatic charge-discharge profiles of the composites at 0.1 A g−1 are shown in Figure 1 (not referenced numerically, as per instructions). FeF2@NFP-700 exhibited the highest initial discharge capacity of 301.5 mAh g−1 with distinct voltage plateaus around 2.0-2.2 V and 3.0-3.5 V, corresponding to the conversion and intercalation-like reactions, respectively. The reaction mechanism for FeF2 in li-ion batteries can be described as: $$\text{FeF}_2 + 2\text{Li}^+ + 2\text{e}^- \leftrightarrow \text{Fe} + 2\text{LiF}$$ This conversion reaction involves the breakdown and reformation of FeF2, which is often associated with large volume changes. The carbon matrix in FeF2@NFP-700 effectively confines these changes, as evidenced by the stable cycling performance.
In contrast, FeF2@NFP-600 showed a lower capacity of 125.9 mAh g−1 and poorly defined plateaus, indicating incomplete conversion due to insufficient conductivity. FeF2@NFP-800 had a capacity of 116.1 mAh g−1 with slight plateaus, but the large particle size limited the active material utilization. Pure FeF2-700 delivered a high initial capacity of 390.8 mAh g−1 but suffered rapid decay, retaining only 9.03% after 100 cycles. This highlights the role of the carbon matrix in stabilizing the electrode for li-ion batteries.
Cycling stability tests at 0.1 A g−1 revealed that FeF2@NFP-700 maintained a capacity of 243.2 mAh g−1 after 100 cycles, with a capacity retention of 92.98%. The capacity fade rate can be quantified using the formula: $$R_f = \frac{C_0 – C_n}{C_0 \cdot n} \times 100\%$$ where \(R_f\) is the fade rate per cycle, \(C_0\) is the initial capacity, \(C_n\) is the capacity after \(n\) cycles, and \(n\) is the cycle number. For FeF2@NFP-700, \(R_f\) was approximately 0.07% per cycle, demonstrating excellent stability. In comparison, FeF2@NFP-600 and FeF2@NFP-800 had fade rates of 0.55% and 0.53% per cycle, respectively, while pure FeF2-700 degraded at 0.91% per cycle. These results underscore the importance of optimized carbonization for durable li-ion battery cathodes.
Rate capability tests were conducted from 0.05 to 3.0 A g−1, as summarized in Table 3. FeF2@NFP-700 exhibited superior rate performance, delivering capacities of 303.96, 244.59, 212.98, 195.83, 177.61, 163.15, 121.57, 85.21, and 67.46 mAh g−1 at current densities of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 2.0, and 3.0 A g−1, respectively. When the current density was returned to 0.1 A g−1, the capacity recovered to 268.68 mAh g−1, indicating good reversibility. The rate performance can be analyzed using the power-law relationship: $$C = C_0 – k \cdot i$$ where \(C\) is the capacity at current density \(i\), \(C_0\) is the capacity at near-zero current, and \(k\) is a kinetic parameter. The low \(k\) value for FeF2@NFP-700 suggests fast reaction kinetics, attributed to the conductive carbon network and nanosized FeF2 particles.
| Current Density (A g−1) | Discharge Capacity (mAh g−1) | Capacity Retention (%) |
|---|---|---|
| 0.05 | 303.96 | 100.00 |
| 0.1 | 244.59 | 80.47 |
| 0.2 | 212.98 | 70.08 |
| 0.3 | 195.83 | 64.44 |
| 0.4 | 177.61 | 58.44 |
| 0.5 | 163.15 | 53.68 |
| 1.0 | 121.57 | 40.00 |
| 2.0 | 85.21 | 28.04 |
| 3.0 | 67.46 | 22.20 |
Cyclic voltammetry (CV) curves at a scan rate of 0.1 mV s−1 provided insights into the redox behavior. For FeF2@NFP-700, well-defined reduction peaks at 1.5 V and 3.0 V, and oxidation peaks at 3.0 V and 3.5 V were observed, corresponding to the conversion and intercalation-like processes, respectively. The peak separation (\(\Delta E\)) for the conversion reaction was about 1.0 V, indicating low polarization. In contrast, FeF2@NFP-600 and FeF2@NFP-800 showed larger \(\Delta E\) values of 1.5 V and 1.6 V, respectively, suggesting higher kinetic barriers. The CV data can be used to calculate the diffusion coefficient of lithium ions using the Randles-Sevcik equation: $$I_p = 0.4463 n F A C \sqrt{\frac{n F v D}{R T}}$$ where \(I_p\) is the peak current, \(n\) is the number of electrons, \(F\) is Faraday’s constant, \(A\) is the electrode area, \(C\) is the concentration, \(v\) is the scan rate, \(D\) is the diffusion coefficient, \(R\) is the gas constant, and \(T\) is the temperature. For FeF2@NFP-700, the estimated \(D\) was on the order of 10−12 cm2 s−1, which is comparable to other conversion materials and supports efficient ion transport in li-ion batteries.
Electrochemical impedance spectroscopy (EIS) revealed the interfacial resistance of the electrodes. The Nyquist plots consisted of a semicircle in the medium-frequency region, representing the charge transfer resistance (\(R_{ct}\)), and a slope in the low-frequency region, associated with Li+ diffusion. The equivalent circuit fitting results are shown in Table 4. FeF2@NFP-700 had the lowest \(R_{ct}\) value of 16.61 Ω, indicating facilitated charge transfer due to the conductive carbon matrix. In comparison, FeF2@NFP-600 and FeF2@NFP-800 had higher \(R_{ct}\) values of 47.26 Ω and 4.32 Ω, respectively, but the latter’s low resistance did not translate to good performance due to particle coarsening. The impedance data underscore the importance of both conductivity and nanoscale morphology for optimal li-ion battery cathodes.
| Sample | \(R_s\) (Ω) | \(R_{CEI}\) (Ω) | \(R_{ct}\) (Ω) |
|---|---|---|---|
| FeF2@NFP-600 | 3.26 | 41.04 | 47.26 |
| FeF2@NFP-700 | 3.94 | 7.64 | 16.61 |
| FeF2@NFP-800 | 4.44 | 5.81 | 4.32 |
The enhancement in electrochemical performance for FeF2@NFP-700 can be attributed to several factors: (1) the conductive carbon fibers provide continuous electron pathways, reducing the internal resistance of the li-ion battery cathode; (2) the embedded FeF2 nanoparticles shorten the Li+ diffusion paths, improving reaction kinetics; (3) the carbon matrix confines volume changes and prevents particle aggregation, enhancing structural integrity; and (4) nitrogen doping improves wettability and conductivity. These synergies make the composite a promising candidate for high-energy li-ion batteries.
Conclusion
In this study, I successfully fabricated FeF2-embedded conductive carbon fiber composites via electrospinning and investigated the effect of carbonization temperature on their properties for li-ion battery cathodes. The results demonstrate that 700 °C is the optimal temperature, yielding a composite with well-carbonized fibers, uniformly dispersed FeF2 nanoparticles, and excellent electrochemical performance. The FeF2@NFP-700 composite delivered a high initial discharge capacity of 301.5 mAh g−1 at 0.1 A g−1 and maintained 243.2 mAh g−1 after 100 cycles, with a capacity retention of 92.98%. It also exhibited superior rate capability and low charge transfer resistance, underscoring its potential for practical li-ion battery applications.
The key findings can be summarized as follows: first, carbonization temperature critically influences both the conductivity of the carbon matrix and the size of FeF2 particles; second, a balance between these factors is essential to maximize capacity and stability; third, the confined environment of the carbon fibers mitigates degradation mechanisms common in conversion materials. Future work could explore other polymer precursors, doping strategies, or hybrid composites to further enhance performance. Additionally, scaling up the electrospinning process and integrating the composite into full-cell li-ion batteries would be valuable steps toward commercialization.
This research contributes to the ongoing efforts to develop advanced cathode materials for li-ion batteries, addressing the need for higher energy density and longer cycle life. By leveraging nanostructuring and carbon composites, conversion-type materials like FeF2 can overcome their inherent limitations and play a significant role in the next generation of energy storage systems. The insights gained from this study may also apply to other metal fluorides or conversion materials, broadening the impact on li-ion battery technology.