In recent years, the escalating environmental crises and energy shortages driven by fossil fuel consumption have necessitated the development of green and efficient energy storage systems. As a promising alternative to lithium-ion batteries, sodium-ion batteries have garnered significant attention due to the abundant natural reserves of sodium, lower cost, and comparable electrochemical mechanisms. However, the practical application of sodium-ion batteries is still hindered by the lack of cathode materials that combine high performance, long cycle life, and low cost. Among various candidates, polyanionic compounds, particularly iron-based phosphates, stand out for their structural stability, moderate operating voltage, and environmental friendliness. In this study, we focus on the Na4FexP4O12+x/C system, exploring how compositional tuning influences phase evolution and, consequently, the electrochemical properties in sodium-ion batteries.
The quest for sustainable energy storage has led us to investigate polyanionic cathode materials for sodium-ion batteries. These materials often exhibit robust frameworks that facilitate reversible sodium insertion and extraction. Specifically, the Na4FexP4O12+x series (where 2 ≤ x ≤ 4) encompasses several phases: Na2FeP2O7 (NFPO), Na4Fe3(PO4)2P2O7 (NFPP), and NaFePO4 (NFP). Each phase has distinct structural and electrochemical characteristics. NFPO offers a three-dimensional ion diffusion pathway but suffers from low specific capacity. NFP exists in electrochemically active olivine and inactive maricite forms, with the latter being thermodynamically stable. NFPP, with an open three-dimensional framework, provides higher theoretical capacity and faster sodium-ion diffusion, making it an attractive candidate for high-performance sodium-ion batteries. However, synthesizing pure-phase Na4FexP4O12+x is challenging due to complex phase equilibria, often resulting in mixed-phase materials. Understanding the correlation between phase composition and electrochemical behavior is crucial for optimizing these cathode materials. Moreover, the inherent low electronic conductivity of phosphates necessitates conductive modifications, such as carbon coating, to enhance rate capability and cycle stability.
In our work, we employed a sol-gel method followed by thermal treatment to prepare a series of carbon-coated Na4FexP4O12+x/C materials with x ranging from 2.6 to 3.3. The sol-gel process ensures homogeneous mixing of precursors, while the carbon derived from citric acid pyrolysis improves electronic conductivity. We systematically varied the sodium-to-iron ratio to manipulate the phase composition, aiming to maximize the content of the NFPP phase. The structural, morphological, and electrochemical properties were thoroughly characterized to establish structure-performance relationships. Our findings reveal that Na4Fe3.1P4O15.1/C, with the highest NFPP content, delivers superior electrochemical performance in sodium-ion batteries, including high reversible capacity, excellent rate capability, and long-term cycling stability.
The synthesis of Na4FexP4O12+x/C materials involves a straightforward sol-gel technique. We dissolve stoichiometric amounts of NaH2PO4, Fe(NO3)3·9H2O, and citric acid in deionized water. The molar ratio of citric acid to iron nitrate is maintained at 1.5:1 to ensure sufficient carbon source for in-situ coating. After adjusting the pH to 7–8 with ammonia, the solution is stirred at 80°C to form a viscous gel. The gel is then freeze-dried to obtain a porous precursor, which is subsequently ground and annealed in a nitrogen atmosphere. The thermal treatment consists of two stages: pre-calcination at 300°C for 2 hours to decompose organic components, followed by crystallization at 470°C for 10 hours to form the desired phosphate phases. This process yields fine particles coated with amorphous carbon, enhancing conductivity for sodium-ion battery applications.
To elucidate the phase evolution, we performed X-ray diffraction (XRD) analysis on all samples. The diffraction patterns clearly indicate the presence of multiple phases depending on the x value. As x increases from 2.6 to 3.3, the intensity of characteristic peaks corresponding to NFPP (e.g., at 2θ ≈ 16.8° and 33.8°) gradually strengthens, while those of NFPO (e.g., at 2θ ≈ 10.8°) diminish. At x = 3.1, the NFPP phase dominates, suggesting optimal compositional tuning. Further increasing x leads to a reduction in NFPP content and the emergence of NFP phases. The phase fractions can be estimated using the Rietveld refinement method, but for simplicity, we summarize the trend in Table 1.
| x Value | Dominant Phases | NFPP Content (Relative) | NFPO Content (Relative) | NFP Content (Relative) |
|---|---|---|---|---|
| 2.6 | NFPO > NFPP > NFP | Low | High | Trace |
| 2.7–3.0 | NFPP increases, NFPO decreases | Moderate to High | Moderate to Low | Trace to Low |
| 3.1 | NFPP >> NFPO ≈ NFP | Highest | Low | Low |
| 3.2–3.3 | NFPP decreases, NFP increases | High to Moderate | Very Low | Moderate |
The structural details are further confirmed by Fourier-transform infrared (FT-IR) spectroscopy and Raman spectroscopy. FT-IR spectra show characteristic bands for P2O7 and PO4 groups, confirming the polyanionic nature. Raman spectra exhibit D and G bands around 1335 cm−1 and 1576 cm−1, respectively, indicating the presence of disordered carbon. The intensity ratio ID/IG is approximately 1.3, suggesting a moderately graphitic carbon coating that facilitates electron transport in sodium-ion batteries.
Thermogravimetric analysis (TGA) under air atmosphere reveals the carbon content in Na4Fe3.1P4O15.1/C to be about 23.8 wt.%, which is beneficial for enhancing conductivity. X-ray photoelectron spectroscopy (XPS) confirms the oxidation state of iron as predominantly Fe2+, consistent with the expected chemistry in these phosphates.
Morphological examination using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) shows that the materials consist of irregularly shaped particles with sizes ranging from submicron to a few microns. The carbon coating is uniformly distributed, as evidenced by energy-dispersive X-ray spectroscopy (EDS) mapping. This porous and carbon-wrapped morphology promotes electrolyte penetration and shortens sodium-ion diffusion paths, which is advantageous for high-rate performance in sodium-ion batteries.
We evaluate the electrochemical performance by assembling coin cells with sodium metal as the anode and Na4FexP4O12+x/C as the cathode. The electrolyte is 1 M NaClO4 in EC/DEC with 5% FEC additive. Cyclic voltammetry (CV) curves measured between 1.5 and 4.3 V vs. Na/Na+ display redox couples corresponding to sodium insertion/extraction. For samples with high NFPP content, two distinct redox pairs are observed at around 2.72/3.05 V and 3.15/3.27 V, indicating multi-step sodium storage mechanisms. The CV profiles evolve with x, reflecting changes in phase composition. The electrochemical reactions can be represented by the following general equation:
$$ \text{Na}_4\text{Fe}_x\text{P}_4\text{O}_{12+x} + y\text{Na}^+ + y e^- \leftrightarrow \text{Na}_{4+y}\text{Fe}_x\text{P}_4\text{O}_{12+x} $$
where y denotes the number of sodium ions involved in the reversible process. The theoretical capacity (Ctheo) of the NFPP phase is calculated as:
$$ C_{\text{theo}} = \frac{nF}{3.6M} $$
where n is the number of electrons transferred per formula unit (for NFPP, n ≈ 2 for full sodium extraction), F is Faraday’s constant (96485 C mol−1), and M is the molar mass (g mol−1). For Na4Fe3(PO4)2P2O7, Ctheo ≈ 129 mAh g−1, which sets the upper limit for capacity in these sodium-ion battery cathodes.
Galvanostatic charge-discharge tests at 0.1C rate (1C = 129 mA g−1) demonstrate that the initial discharge capacity varies with x. Na4Fe3.1P4O15.1/C delivers the highest capacity of 102.8 mAh g−1, corresponding to about 79.7% of the theoretical value. The capacity trends align with phase composition: samples richer in NFPP exhibit higher capacities. The Coulombic efficiency in the first cycle exceeds 95%, indicating minimal irreversible side reactions. Long-term cycling stability is excellent for x = 3.1, with capacity retention of 96.5% after 300 cycles and 88.7% after 700 cycles. This outstanding cyclability is attributed to the stable NFPP framework and conductive carbon network, which mitigate volume changes and maintain electrical contact during repeated sodiation/desodiation in sodium-ion batteries.
Rate capability tests show that Na4Fe3.1P4O15.1/C retains a discharge capacity of 61.5 mAh g−1 at 5C, demonstrating good kinetics. Even after high-rate cycling, the capacity recovers to nearly initial values when the rate is returned to 0.1C, highlighting the structural resilience. The electrochemical impedance spectra (EIS) fitted with an equivalent circuit model reveal low charge-transfer resistance (Rct ≈ 167.8 Ω) and series resistance (Rs ≈ 9.81 Ω) for the optimal composition, confirming facilitated ion and electron transport.
To quantify the impact of phase composition on performance, we derive a simple linear relationship between NFPP content and discharge capacity. Let φ be the fraction of NFPP phase in the material, and C be the measured discharge capacity at 0.1C. Empirical data suggest:
$$ C \approx C_{\text{NFPP}} \cdot \phi + C_{\text{other}} \cdot (1 – \phi) $$
where CNFPP is the intrinsic capacity of pure NFPP (~102.8 mAh g−1 in our case) and Cother is the average capacity contribution from other phases (lower). Thus, maximizing φ is key to enhancing capacity. Moreover, the sodium-ion diffusion coefficient (DNa+) can be estimated from CV data using the Randles-Sevcik equation:
$$ I_p = 0.4463 \cdot n F A C \left( \frac{n F v D_{\text{Na}^+}}{R T} \right)^{1/2} $$
where Ip is peak current, A is electrode area, C is sodium concentration, v is scan rate, R is gas constant, and T is temperature. For Na4Fe3.1P4O15.1/C, DNa+ is on the order of 10−12–10−11 cm2 s−1, comparable to other polyanionic cathodes for sodium-ion batteries.
We also compare our materials with other reported cathodes for sodium-ion batteries in terms of cycling performance. Table 2 summarizes key data, illustrating the competitiveness of our optimized Na4Fe3.1P4O15.1/C.
| Material | Initial Discharge Capacity (mAh g−1) | Cycle Number | Capacity Retention | Reference |
|---|---|---|---|---|
| Na4Fe3.1P4O15.1/C | 102.8 | 700 | 88.7% | This work |
| Na4Fe3(PO4)2P2O7/C | 95.3 | 100 | 94.7% | Literature |
| Na3V2(PO4)3/C | 102 | 500 | 92.1% | Literature |
| Na0.44MnO2/C | 114 | 1000 | 65% | Literature |
The superior performance of our material stems from the synergistic effects of phase engineering and carbon modification. The NFPP phase provides a stable host for sodium ions with low strain during cycling, while the carbon coating enhances electronic conductivity and mitigates particle aggregation. Furthermore, the mixed-phase nature at optimal composition may create beneficial interfaces that facilitate sodium-ion migration. These attributes make Na4FexP4O12+x/C materials promising for practical sodium-ion battery applications, especially in large-scale energy storage where cost and longevity are paramount.
In conclusion, we have demonstrated that tailoring the sodium-to-iron ratio in Na4FexP4O12+x/C materials effectively controls phase composition, with x = 3.1 yielding the highest proportion of the desirable NFPP phase. This optimized cathode exhibits excellent electrochemical properties in sodium-ion batteries, including high capacity, outstanding rate capability, and remarkable cycling stability. Our work underscores the importance of phase manipulation in designing advanced polyanionic cathodes for sodium-ion batteries. Future studies could explore doping strategies or nanostructuring to further improve performance, paving the way for commercially viable sodium-ion battery technologies.