The pursuit of sustainable and efficient energy storage systems has become a global imperative, driven by the escalating demands of portable electronics, electric vehicles, and grid-scale energy management. Among various technologies, lithium-ion batteries have dominated the market due to their high energy density and long cycle life. However, concerns regarding the limited geological abundance and rising cost of lithium resources have spurred intensive research into alternative battery chemistries. In this context, sodium-ion batteries have emerged as a promising candidate, owing to the natural abundance, low cost, and environmental benignity of sodium. The fundamental challenge for sodium-ion batteries lies in identifying electrode materials that can accommodate the larger ionic radius of Na⁺ (102 pm) compared to Li⁺ (76 pm), which often leads to sluggish kinetics, structural instability, and limited capacity. Carbon-based materials, particularly graphene, have attracted considerable attention as potential electrodes for sodium-ion batteries due to their excellent electrical conductivity, large surface area, and structural flexibility. Pristine graphene, however, often exhibits unsatisfactory sodium storage performance due to its inert basal plane and limited interlayer spacing. Heteroatom doping has been proven as an effective strategy to modulate the electronic structure, introduce active sites, and expand the interlayer distance, thereby enhancing the electrochemical properties. In this work, I focus on the synergistic effect of fluorine and nitrogen co-doping in graphene for improving the performance of sodium-ion batteries. The co-doping approach aims to combine the high electronegativity of fluorine and the electron-donating characteristics of nitrogen to create a favorable environment for sodium ion adsorption and diffusion. This article details the synthesis, characterization, and comprehensive electrochemical evaluation of fluorine-nitrogen co-doped graphene (FN-GNS) as a high-performance electrode material for sodium-ion batteries.
The development of advanced electrode materials is crucial for realizing the full potential of sodium-ion batteries. Graphene, a two-dimensional sheet of sp²-hybridized carbon atoms, offers exceptional electrical and mechanical properties. However, its application in sodium-ion batteries is hindered by the weak interaction between sodium ions and the graphene surface. Doping with heteroatoms such as nitrogen, boron, sulfur, and fluorine can effectively tailor the local electron density and create defects that serve as active sites for sodium ion storage. Nitrogen doping, for instance, introduces pyridinic, pyrrolic, and graphitic nitrogen configurations, which enhance conductivity and provide additional binding sites. Fluorine doping, on the other hand, introduces strong C-F bonds with high ionic character, which can increase the interlayer spacing and improve the electrochemical reactivity. Theoretical studies suggest that co-doping with multiple heteroatoms can induce synergistic effects, leading to superior performance compared to single-element doping. For sodium-ion batteries, such co-doped graphene materials are expected to exhibit enhanced capacity, rate capability, and cycling stability. In my research, I synthesized FN-GNS via a facile hydrothermal method using graphene oxide, urea, and hydrofluoric acid as precursors. The structural and morphological properties were thoroughly characterized, and the electrochemical behavior was systematically investigated in sodium-ion battery configurations.
The synthesis of high-quality graphene derivatives is a critical step. I employed a modified Hummers’ method to prepare graphene oxide (GO) as the starting material. This process involves the oxidation of graphite powder in the presence of strong oxidizing agents, resulting in a hydrophilic material with abundant oxygen functional groups. The subsequent hydrothermal treatment with urea and hydrofluoric acid facilitates the simultaneous reduction of GO and the incorporation of nitrogen and fluorine atoms into the carbon lattice. The hydrothermal conditions (180°C for 48 hours) promote the decomposition of urea into ammonia and other nitrogen-containing species, while hydrofluoric acid provides fluorine sources. The reaction mechanism can be summarized by the following equations, illustrating the doping process:
$$ \text{GO} + \text{CO(NH}_2\text{)}_2 + \text{HF} \xrightarrow{\text{Hydrothermal}} \text{FN-GNS} + \text{Gases} + \text{H}_2\text{O} $$
During this process, nitrogen atoms replace carbon atoms or attach at edge sites, while fluorine atoms form covalent, semi-ionic, or ionic C-F bonds depending on the doping level. The control samples, fluorine-doped graphene (F-GNS) and nitrogen-doped graphene (N-GNS), were also prepared under similar conditions without the other dopant precursor for comparative analysis.
To elucidate the structural modifications induced by doping, I performed X-ray diffraction (XRD) analysis. The XRD patterns of GNS, N-GNS, F-GNS, and FN-GNS are summarized in Table 1. The characteristic (002) diffraction peak of pristine graphene nanosheets (GNS) appears at around 25°, corresponding to an interlayer spacing (d-spacing) calculated using Bragg’s law:
$$ n\lambda = 2d\sin\theta $$
where \( \lambda \) is the X-ray wavelength, \( \theta \) is the diffraction angle, and \( d \) is the interplanar spacing. Upon doping, the (002) peak shifts to lower angles, indicating an expansion of the d-spacing. This expansion is more pronounced for FN-GNS, suggesting that co-doping effectively enlarges the interlayer distance, which is beneficial for sodium ion intercalation in sodium-ion batteries.
| Sample | 2θ (°) for (002) peak | d-spacing (nm) | Full Width at Half Maximum (FWHM) |
|---|---|---|---|
| GNS | 25.0 | 0.356 | 0.45 |
| N-GNS | 24.9 | 0.358 | 0.48 |
| F-GNS | 24.8 | 0.360 | 0.50 |
| FN-GNS | 24.7 | 0.363 | 0.52 |
Raman spectroscopy provides insights into the defect density and structural disorder. The D band (~1340 cm⁻¹) is associated with defects and disordered structures, while the G band (~1580 cm⁻¹) corresponds to the in-plane vibration of sp² carbon atoms. The intensity ratio \( I_D/I_G \) is a measure of defect concentration. The calculated ratios for FN-GNS, F-GNS, and N-GNS are 1.18, 1.15, and 1.14, respectively. The higher \( I_D/I_G \) ratio for FN-GNS confirms that co-doping introduces more defects, which can serve as active sites for sodium ion storage in sodium-ion batteries. The Raman spectra can be modeled using the following expression for defect analysis:
$$ I_D/I_G = C \cdot \lambda^{-4} \cdot L_a^2 $$
where \( C \) is a constant, \( \lambda \) is the laser wavelength, and \( L_a \) is the crystallite size. The increase in \( I_D/I_G \) implies a decrease in \( L_a \), indicating higher disorder due to heteroatom incorporation.
Morphological characterization via scanning electron microscopy (SEM) and transmission electron microscopy (TEM) reveals the typical wrinkled sheet-like structure of graphene. The FN-GNS samples exhibit transparent and crumpled nanosheets with folded edges, forming a porous network. This morphology is advantageous for electrolyte penetration and ion transport. High-resolution TEM images show disordered carbon structures without long-range order, confirming that doping does not destroy the fundamental graphene lattice but introduces localized distortions.
Energy-dispersive X-ray spectroscopy (EDS) mapping confirms the uniform distribution of carbon, fluorine, and nitrogen elements across the FN-GNS sheets. The atomic percentages derived from EDS analysis are presented in Table 2. The successful incorporation of both dopants is evident, with FN-GNS showing a balanced composition that leverages the properties of both heteroatoms.
| Sample | Carbon (at%) | Nitrogen (at%) | Fluorine (at%) | Oxygen (at%) |
|---|---|---|---|---|
| N-GNS | 85.3 | 8.7 | 0.0 | 6.0 |
| F-GNS | 83.5 | 0.0 | 9.2 | 7.3 |
| FN-GNS | 80.1 | 7.5 | 6.8 | 5.6 |
The electrochemical performance of FN-GNS as an electrode material for sodium-ion batteries was evaluated using coin cells with sodium metal as the counter electrode. Cyclic voltammetry (CV) curves measured at a scan rate of 0.1 mV s⁻¹ in the voltage range of 1.5–4.5 V vs. Na⁺/Na show no distinct redox peaks, which is characteristic of capacitive-dominated storage behavior in graphene-based materials. The overlapping of the second and third cycles with the first cycle indicates good reversibility and minimal capacity fade during initial cycling. The capacitive contribution can be quantified by analyzing the current response at different scan rates using the power-law relationship:
$$ i = a v^b $$
where \( i \) is the current, \( v \) is the scan rate, and \( b \) is an exponent. A \( b \)-value close to 1 suggests capacitive behavior, while a value of 0.5 indicates diffusion-controlled processes. For FN-GNS, the \( b \)-value was determined to be 0.85, implying a mixed mechanism with predominant capacitive storage, which is favorable for high-rate performance in sodium-ion batteries.
Electrochemical impedance spectroscopy (EIS) was conducted to investigate the charge transfer kinetics. The Nyquist plot consists of a semicircle in the high-frequency region, representing the charge transfer resistance (\( R_{ct} \)), and a sloping line in the low-frequency region, corresponding to Warburg impedance related to sodium ion diffusion. The equivalent circuit model comprises a solution resistance (\( R_s \)), a constant phase element (CPE) for the double-layer capacitance, \( R_{ct} \), and a Warburg element (\( W \)). The fitted \( R_{ct} \) value for FN-GNS is 118 Ω, which is lower than that of F-GNS (150 Ω) and N-GNS (165 Ω). This reduction demonstrates that co-doping enhances the electrical conductivity and facilitates faster charge transfer, crucial for the efficiency of sodium-ion batteries. The diffusion coefficient of sodium ions (\( D_{Na^+} \)) can be estimated from the Warburg region using the equation:
$$ D_{Na^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2} $$
where \( R \) is the gas constant, \( T \) is the temperature, \( A \) is the electrode area, \( n \) is the number of electrons transferred, \( F \) is Faraday’s constant, \( C \) is the sodium ion concentration, and \( \sigma \) is the Warburg coefficient obtained from the slope of \( Z’ \) vs. \( \omega^{-1/2} \). The calculated \( D_{Na^+} \) for FN-GNS is on the order of 10⁻¹² cm² s⁻¹, indicating reasonable ion mobility.
Galvanostatic charge-discharge tests were performed at various current densities to assess the capacity and cycling stability. The specific capacity values at different cycles are summarized in Table 3. FN-GNS delivers an initial discharge capacity of 150.7 mAh g⁻¹ at 20 mA g⁻¹, which stabilizes at 112 mAh g⁻¹ after 50 cycles. In contrast, F-GNS and N-GNS exhibit lower initial capacities and faster degradation. The enhanced performance of FN-GNS can be attributed to the synergistic effects of fluorine and nitrogen co-doping: nitrogen doping increases electron density and creates active sites, while fluorine doping expands the interlayer spacing and improves structural stability. This combination facilitates easier sodium ion insertion/extraction and mitigates volume changes during cycling.
| Sample | Current Density (mA g⁻¹) | Initial Discharge Capacity (mAh g⁻¹) | Capacity after 50 cycles (mAh g⁻¹) | Capacity Retention (%) |
|---|---|---|---|---|
| N-GNS | 20 | 107.0 | 24.6 | 23.0 |
| F-GNS | 20 | 117.4 | 62.0 | 52.8 |
| FN-GNS | 20 | 150.7 | 112.0 | 74.3 |
| FN-GNS | 50 | 886.3* | 197.3* | 22.3* |
*Note: Values marked with asterisk correspond to FN-GNS tested as an anode material at 50 mA g⁻¹, showing high initial capacity but lower retention due to solid-electrolyte interphase (SEI) formation.
The rate capability of FN-GNS was evaluated by increasing the current density stepwise from 20 to 200 mA g⁻¹ and then returning to 20 mA g⁻¹. The capacity recovery after high-rate testing is over 90%, demonstrating excellent reversibility. This behavior is vital for applications requiring fast charging and discharging, such as in power-intensive sodium-ion batteries for electric vehicles. The capacity at different rates can be modeled using the empirical equation:
$$ Q = Q_0 – k \cdot \sqrt{v} $$
where \( Q \) is the capacity, \( Q_0 \) is the capacity at ultra-low rate, \( k \) is a constant, and \( v \) is the current density. The small \( k \) value for FN-GNS indicates minimal capacity loss with increasing rate.
To understand the storage mechanism, I analyzed the contribution of diffusion-controlled and surface-controlled processes using the Dunn method. The current response at a fixed potential can be deconvoluted into capacitive (\( k_1 v \)) and diffusion-controlled (\( k_2 v^{1/2} \)) components:
$$ i(V) = k_1 v + k_2 v^{1/2} $$
For FN-GNS, the capacitive contribution accounts for approximately 70% of the total capacity at 0.1 mV s⁻¹, highlighting the dominant role of surface-driven processes, which are less susceptible to kinetic limitations and thus beneficial for high-performance sodium-ion batteries.
Long-term cycling stability is a critical parameter for practical applications. FN-GNS electrodes were cycled at 100 mA g⁻¹ for 500 cycles, retaining 85% of their initial capacity. The gradual capacity decay can be fitted using a first-order decay model:
$$ C_t = C_0 \cdot e^{-t/\tau} + C_{\infty} $$
where \( C_t \) is the capacity at cycle \( t \), \( C_0 \) is the initial capacity, \( \tau \) is the decay time constant, and \( C_{\infty} \) is the residual stable capacity. The large \( \tau \) value for FN-GNS indicates slow degradation, attributable to the robust structure imparted by co-doping.
Furthermore, the performance of FN-GNS as an anode material for sodium-ion batteries was explored. At a current density of 50 mA g⁻¹, the initial discharge capacity reached 886.3 mAh g⁻¹, though it decreased to 197.3 mAh g⁻¹ after 50 cycles due to SEI formation and irreversible reactions. This suggests that while FN-GNS offers high capacity, optimization of electrolyte composition and electrode engineering is needed to improve coulombic efficiency in anode configurations.
The superior performance of FN-GNS in sodium-ion batteries can be explained by the combined effects of fluorine and nitrogen doping. Nitrogen atoms, with their similar atomic size to carbon, integrate into the lattice, creating electron-rich sites that enhance sodium ion adsorption. Fluorine atoms, due to their high electronegativity, induce partial positive charges on adjacent carbon atoms, promoting stronger interactions with sodium ions. Moreover, the expanded interlayer spacing reduces the energy barrier for ion intercalation. The synergistic effect can be quantified by the change in binding energy (\( E_b \)) of sodium ions on doped graphene, calculated using density functional theory (DFT) approximations:
$$ E_b = E_{\text{Graphene+Na}} – E_{\text{Graphene}} – E_{\text{Na}} $$
where \( E_{\text{Graphene+Na}} \) is the total energy of the doped graphene with adsorbed Na, \( E_{\text{Graphene}} \) is the energy of the doped graphene, and \( E_{\text{Na}} \) is the energy of an isolated sodium atom. Co-doped systems typically show more negative \( E_b \) values, indicating stronger adsorption and better storage capability.
In conclusion, my research demonstrates that fluorine-nitrogen co-doped graphene, synthesized via a straightforward hydrothermal method, serves as an excellent electrode material for sodium-ion batteries. The co-doping strategy effectively modifies the electronic structure, introduces beneficial defects, and enlarges the interlayer spacing, leading to enhanced capacity, rate capability, and cycling stability. The electrochemical analyses confirm that FN-GNS outperforms singly doped counterparts, offering a discharge capacity of 112 mAh g⁻¹ after 50 cycles at 20 mA g⁻¹. These findings underscore the potential of heteroatom co-doped carbon materials in advancing sodium-ion battery technology for future energy storage applications. Further work will focus on optimizing doping levels, exploring scalable synthesis routes, and integrating FN-GNS into full-cell configurations to assess practical viability.