The relentless pursuit of sustainable and cost-effective energy storage solutions has propelled the development of sodium-ion battery technology as a promising alternative to the ubiquitous lithium-ion systems. The abundance and low cost of sodium resources offer a significant economic advantage, making sodium-ion batteries particularly attractive for large-scale grid storage and other applications where energy density is not the paramount concern. The performance of a sodium-ion battery is intrinsically linked to the properties of its electrode materials. Among the various cathode candidates, polyanionic compounds have garnered substantial interest due to their robust structural frameworks, tunable operating voltages, and good thermal stability. Sodium iron fluorophosphate, Na2FePO4F, stands out as a particularly compelling candidate. Its crystal structure provides two-dimensional pathways for facile Na+ ion migration, a moderate theoretical capacity of approximately 124 mAh g-1 based on the reversible extraction/insertion of one Na+ ion, an attractive average operating voltage around 3.0 V (vs. Na+/Na), and remarkably small volume change (< 4%) during cycling, which is crucial for long-term structural integrity. However, its practical application is severely hindered by inherently low electronic conductivity, which limits the utilization of the active material, especially at higher charge/discharge rates.
To overcome this fundamental limitation, material nano-structuring and conductive compositing are widely adopted strategies. Reducing particle size shortens the diffusion pathways for both ions and electrons, while coating with a conductive carbon matrix establishes a percolating network for rapid electron transfer throughout the electrode. In this study, we focus on synthesizing a high-performance Na2FePO4F/C composite cathode material. We employ a citrate-assisted spray-drying method, utilizing organic iron and carbon sources, followed by a controlled thermal treatment. This approach is designed to achieve a homogeneous distribution of nanoscale Na2FePO4F crystallites within an in-situ formed carbon matrix. We systematically investigate the effects of critical synthesis parameters—specifically the final calcination temperature and duration—on the phase purity, crystallinity, morphology, and ultimately, the electrochemical properties of the resulting composites. A comprehensive electrochemical analysis, including galvanostatic cycling and electrochemical impedance spectroscopy (EIS), is conducted to elucidate the structure-property-performance relationships and uncover the intrinsic factors governing the cycling stability of this promising cathode material for sodium-ion batteries.
Experimental Methodology: Synthesis and Characterization
Material Synthesis via Spray-Drying
The Na2FePO4F/C composites were synthesized using a precursor solution prepared from analytical-grade reagents. Stoichiometric amounts corresponding to 0.03 mol of target product were used: sodium carbonate (Na2CO3), ferric citrate hydrate (C6H5O7Fe·H2O) as the iron and partial carbon source, ammonium dihydrogen phosphate (NH4H2PO4), sodium fluoride (NaF), and additional citric acid monohydrate (C6H8O7·H2O) as the complexing agent and primary carbon source. These were dissolved in 200 mL of deionized water under vigorous stirring to form a clear, homogeneous solution. This solution was then fed into a spray dryer at a rate of 200 mL h-1 with an inlet temperature of 200 °C and an outlet temperature of 100 °C, yielding a fine precursor powder. The collected powder was dried at 120 °C under vacuum for 12 hours. The thermal treatment involved a two-step process: first, a pre-calcination at 300 °C for 3 hours under flowing argon to decompose nitrates/citrates and initiate crystallization, followed by a final calcination at varying temperatures (600, 650, 700, and 750 °C) for a fixed duration of 6 hours in an argon atmosphere. The obtained black powders were the final Na2FePO4F/C composites.
Material Characterization
The crystalline phase and structure of the synthesized powders were identified by X-ray diffraction (XRD) using a diffractometer with Cu Kα radiation (λ = 1.5405 Å). Data was collected in a 2θ range of 10° to 70°. The morphological features and particle size were examined using scanning electron microscopy (SEM). The carbon content in the composite materials was quantitatively determined by high-frequency infrared carbon-sulfur analysis.
Electrochemical Evaluation
Electrodes were fabricated by mixing the active material (Na2FePO4F/C), conductive carbon (Super P), and polyvinylidene fluoride (PVDF) binder in a weight ratio of 8:1:1. N-methyl-2-pyrrolidone (NMP) was used as the solvent to form a homogeneous slurry, which was then cast onto aluminum foil current collectors and dried at 120 °C under vacuum. CR2032-type coin cells were assembled in an argon-filled glovebox with both moisture and oxygen levels below 1 ppm. The prepared electrode film served as the working electrode, sodium metal foil as the counter/reference electrode, a glass fiber membrane as the separator, and 1.0 M NaClO4 in propylene carbonate (PC) with 5% fluoroethylene carbonate (FEC) additive as the electrolyte. Galvanostatic charge-discharge tests were conducted within a voltage window of 2.0–4.5 V (vs. Na+/Na) at various C-rates (1C = 124 mA g-1). Cyclic voltammetry (CV) was performed at a scan rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) measurements were carried out on cells after the first cycle at a discharge state, over a frequency range from 100 kHz to 10 mHz with a perturbation amplitude of 5 mV.
Results and Discussion: The Impact of Synthesis Temperature
Crystalline Structure and Phase Purity
The XRD patterns of the samples calcined at different temperatures for 6 hours are pivotal in assessing phase formation. Na2FePO4F crystallizes in a structure analogous to Na2FePO4OH. The diffraction patterns were therefore referenced against the standard card for Na2FePO4OH (PDF# 72-1829). The sample synthesized at 600 °C exhibited sharp and well-defined diffraction peaks that matched perfectly with the reference pattern, indicating the formation of a highly pure and well-crystallized Na2FePO4F phase. No detectable impurity peaks were observed.
Upon increasing the temperature to 650 °C, a slight attenuation in peak intensities suggested a marginal decrease in crystallinity. More importantly, the emergence of weak but discernible impurity peaks, identifiable as Na3PO4 and Na3Fe(PO4)2, indicated the onset of side reactions. This trend became pronounced at 700 °C and 750 °C. The patterns for these higher-temperature samples showed significant alterations: the characteristic peaks of Na2FePO4F weakened considerably, while the peaks corresponding to impurity phases (Na3PO4, Na3Fe(PO4)2, and even Fe3O4 at 750 °C) grew stronger and more numerous. This evolution confirms the thermal instability of Na2FePO4F at elevated temperatures, leading to its partial decomposition. The carbon in all composites remained amorphous, as evidenced by the absence of graphite diffraction peaks. The carbon content, as determined by elemental analysis, systematically decreased with increasing calcination temperature due to more complete carbonization and loss, as shown in Table 1.
| Synthesis Temperature (°C) | Phase Purity (XRD) | Carbon Content (wt.%) | Primary Particle Feature (SEM) |
|---|---|---|---|
| 600 | High, pure Na2FePO4F | 8.2 | Irregular micro-blocks with nano-particles |
| 650 | Moderate, minor impurities | 7.9 | Larger, fused blocks |
| 700 | Low, significant impurities | 7.1 | Severely aggregated, fused morphology |
| 750 | Very Low, dominant impurities | 6.5 | Highly sintered, dense aggregates |
Morphological Evolution
SEM imaging revealed a clear correlation between synthesis temperature and particle morphology. The composite synthesized at 600 °C consisted primarily of irregular micro-sized blocks. Crucially, the surfaces of these blocks were decorated with finer, nano-sized particles (tens to hundreds of nanometers). This hierarchical structure is beneficial as the nano-particles provide short diffusion paths, while the carbon matrix from citric acid pyrolysis helps interconnect them and inhibits excessive growth. At 650 °C, particle coarsening and fusion became evident, leading to larger blocks and increased agglomeration. This trend accelerated at 700 °C and 750 °C, where the distinct nano-features disappeared entirely, and the microstructure transformed into heavily sintered, dense aggregates with poorly defined grain boundaries. This morphological degradation is attributed to enhanced atomic diffusion and sintering at higher temperatures, coupled with the diminishing content and hence the diminishing “barrier” effect of the amorphous carbon network, which otherwise suppresses particle growth and agglomeration. The optimal balance between crystallinity, purity, and desirable nano/micro structure was achieved at 600 °C.
Electrochemical Performance and Kinetic Analysis
Rate Capability and Cycle Stability
The galvanostatic charge-discharge profiles at 1C rate (Fig. 3a-d in original work, described here) clearly demonstrate the superiority of the 600 °C-synthesized material. It delivered the highest initial discharge capacity of 67.4 mAh g-1 with a low polarization gap (~0.2 V) between charge and discharge plateaus. The capacity retention was 85.2% after 50 cycles. In contrast, materials from higher temperatures suffered from increased polarization (up to 0.78 V at 750 °C) and significantly lower capacities (e.g., 32.0 mAh g-1 at 750 °C), along with poorer capacity retention. This performance hierarchy directly mirrors the trends in phase purity, crystallinity, and morphology. Impurities and poor crystallinity degrade electrochemical activity, while excessive particle growth and agglomeration lengthen ion/electron transport paths.
The electrochemical reaction of Na2FePO4F involves a two-step process, which is revealed by both cyclic voltammetry (CV) and low-rate galvanostatic tests. The CV curve for the optimal sample (600 °C) shows two distinct redox couples: anodic peaks at ~3.01 V and ~3.17 V, and corresponding cathodic peaks at ~3.01 V and ~2.82 V. This corresponds to the sequential phase transformations during Na+ extraction/insertion:
$$
\text{Na}_2\text{FePO}_4\text{F} \rightleftharpoons \text{Na}_{1.5}\text{FePO}_4\text{F} + 0.5\text{Na}^+ + 0.5e^-
$$
$$
\text{Na}_{1.5}\text{FePO}_4\text{F} \rightleftharpoons \text{NaFePO}_4\text{F} + 0.5\text{Na}^+ + 0.5e^-
$$
At a low current rate of 0.1C, the optimal material exhibited a high initial discharge capacity of 95.5 mAh g-1, approaching its theoretical limit for one Na+ transfer. More importantly, it demonstrated excellent long-term cycling stability, retaining 78.3% of its initial capacity after 150 cycles with coulombic efficiency consistently above 99% after the first cycle.
Electrochemical Impedance and Ion Diffusion Kinetics
EIS is a powerful tool to probe the interfacial reactions and mass transport within the electrode. The Nyquist plots for all samples after one cycle typically consist of a high-frequency intercept (Rs, solution/contact resistance), a depressed semicircle in the mid-to-high frequency region (Rct, charge-transfer resistance), and an inclined line in the low-frequency region (Warburg impedance, Zw, related to solid-state Na+ diffusion). The fitted parameters using an equivalent circuit model are summarized below.
| Sample (Synthesis T) | Rs (Ω) | Rct (Ω) | Relative Kinetics |
|---|---|---|---|
| 600 °C | 8.27 | 221.8 | Fastest |
| 650 °C | 8.42 | 280.6 | Moderate |
| 700 °C | 9.19 | 372.5 | Slow |
| 750 °C | 10.00 | 498.1 | Slowest |
The data shows a systematic increase in both Rs and Rct with rising synthesis temperature. The increase in Rs suggests slightly poorer electronic wiring or contact in electrodes made from higher-temperature samples, likely due to their more aggregated morphology and lower carbon content. The substantial growth in Rct is more critical; it indicates a progressively more sluggish charge-transfer reaction at the electrode/electrolyte interface. This is a direct consequence of the lower electrochemical activity caused by impurities, poorer crystallinity, and reduced effective surface area in samples calcined at higher temperatures. The sodium ion diffusion coefficient (DNa+) can be estimated from the low-frequency Warburg region using the formula:
$$
D_{Na^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2}
$$
Where R is the gas constant, T is the absolute temperature, A is the electrode area, n is the number of electrons per reaction, F is Faraday’s constant, C is the concentration of Na+ ions in the solid, and σ is the Warburg coefficient obtained from the slope of Z’ vs. ω-1/2. While a full calculation is sample-specific, the increasingly steeper low-frequency line for the 600 °C sample compared to others in the original study suggests a higher DNa+, corroborating its superior rate performance. The EIS analysis therefore reveals the kinetic origins of the cycling performance: the optimal 600 °C sample possesses the lowest interfacial resistance and most favorable ion diffusion kinetics, which are essential for high-performance sodium-ion battery cathodes.
Conclusion and Perspective
In this work, a citrate-assisted spray-drying method was successfully employed to synthesize carbon-coated Na2FePO4F composites for use as a cathode in sodium-ion batteries. A systematic investigation established 600 °C as the optimal calcination temperature for a duration of 6 hours. Under these conditions, the synthesized Na2FePO4F/C composite exhibited high phase purity, good crystallinity, and a favorable hierarchical morphology consisting of micro-blocks adorned with nano-particles, all embedded within a conductive carbon matrix. This optimal structure translated into outstanding electrochemical properties: a high reversible capacity of 95.5 mAh g-1 at 0.1C, excellent cycling stability with 78.3% capacity retention after 150 cycles, and good rate capability. EIS analysis provided direct evidence that this sample had the lowest charge-transfer resistance, facilitating faster reaction kinetics.
This study underscores the critical importance of precise control over synthesis parameters in determining the structural and electrochemical properties of polyanionic cathode materials. The spray-drying technique proves effective in producing homogenously carbon-composited materials. The promising results confirm Na2FePO4F as a viable and attractive cathode candidate for next-generation, cost-effective sodium-ion batteries. Future work could focus on further optimizing the carbon coating quality and porosity, exploring cation doping to potentially enhance intrinsic conductivity and Na+ diffusion rates, and scaling up the synthesis process for practical application assessments.