Synthesis of coral-like Fe₂N@C nanoparticles and application in sodium ion batteries as a novel anode electrode material

Abstract

Fast development of low-cost sodium ion batteries (SIBs) has boosted research efforts to identify proper anode materials, and now more attention is paid on carbon composites with better performance than simplex compounds. According to the explored different kinds of anode materials for SIBs in the past, only a few works are about transition metal nitrides. Herein a carbon-coated iron nitride was synthesized and applied for the first time in sodium ion batteries (SIBs), and the primary mechanism was detected via in situ XRD confirming Fe₂N activity during discharge in SIBs. Fe₂N nanoparticles as anodes in SIBs showed better stability than most of the materials reported in SIBs. The cycling performance of the synthesized coral-like Fe₂N@C nanoparticles was significantly promoted compared to the pure nitride, and the improved performance of the carbon coated coral-like Fe₂N suggests that more explorations about nitride electrode materials for SIBs are needed.

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1. Introduction

Lithium ion batteries (LIBs) have been regarded as among the most promising energy storage devices for a long time. However, limited availability of Li₂CO₃, high cost of the lithium source, and unsatisfactory performance of the major explored electrode materials serve as the principal obstacles for further LIB development.¹ In recent years, growing demand for new and better energy storage has led to exploring sodium ion batters (SIBs), considering that sodium is much cheaper and has properties similar to lithium.^2–5 At present, a fundamental and predominant way for promoting electrochemical performance in LIBs/SIBs is developing effective and high-performance electrode materials. Although SIBs are very promising in energy storage technology, compared to LIBs the electrode materials explored in SIBs are still incomplete and less than satisfactory.⁶

Thus far, most anode materials applied in SIBs are carbon based,^5,7–9 metal oxide and metal sulphide materials,^10–12 and poor cycle performance or low density problems have been reported for many of these. Since the volume shrinkage which often happens in metal oxide electrode materials always brings disadvantages in the charge–discharge process, exploring for a proper material with little volume expansion should be a good way to improve the cycling stability. It is reported that the transitional metal nitrides used as electrode materials in energy storage have significant advantages over certain oxide materials attributed to good electrical conductivity and high reversible capability.^13–15 According to some reports on metal nitrides in LIBs and SIBs, these hold great promise as suitable metal nitrides used as anode materials in SIBs. For instance, a report about Ni₃N as a negative electrode¹⁶ with a theoretical capacity of 423 mA h g⁻¹ for SIBs exhibited 134 mA h·g⁻¹ capacity at a current density of 0.1 C over 20 circulations. Although cycle stability and capacity are not satisfactory according to the abovementioned sources, investigating other nitride materials suitable for SIBs is worthwhile.

Among these metal nitrides reported both in LIBs and SIBs, iron-based electrodes^15,17–19 have demonstrated good properties in LIBs due to their high capacity and low resistance for ion migration, but these have seldom been applied in SIBs. The reported iron oxides applied in SIBs, such as Fe₂O₃ composited with reduced graphene oxide,²⁰ showed a capability of 30 mA h g⁻¹ and 250 mA h g⁻¹ at 500 mA g⁻¹ and 100 mA g⁻¹ current density, respectively, while pure Fe₂O₃ was unsatisfactory. Poor cycling performance was mainly due to its large volume expansion and contraction, which caused electrical contact loss and severe capacity decline.²¹ Although graphene nanosheets involved obviously improved the rate capability at a low current density of 100 mA g⁻¹, unsatisfying cycling performance at greater current density should not be ignored. Compared to the iron oxides, the reported iron nitride electrodes^17,18 exhibited good cycling performance and stability in LIBs. Carbon coating, a surface modification technique,^8,22–24 can significantly enhance the electronic conductivity of electrode materials. Excellent performance improvements through the carbon-coating method have been reported.^25,26

Considering the predominant properties of Fe₂N and its carbon-based composite materials demonstrated in previous work, and the huge gaps of exploring use of nitrides in SIBs, we chose Fe₂N as the basic electrode material for SIBs in our work. In this article, the carbon-coating method was employed to enhance the performance of iron nitride. The pure Fe₂N electrode without carbon showed a capacity of more than 50 mA h g⁻¹ at 500 mA g⁻¹ current density over 1000 cycles. After being assembled with carbon, the compound exhibited excellent electrochemical performance, which should contribute to the combination nanostructure. Results showed attractive properties of Fe₂N as the electrode anode in SIBs, and the slight promotion of its electrochemical properties after composited with carbon. This work provides support for further applications of iron nitrides, and subsequent research for developing more effective metal nitride nanostructures for SIBs.

2. Experimental section

Synthesis method of coralloid Fe₂N and Fe₂N@C

To obtain the objective products, FeOOH nanospindles were synthesized at the first step as a precursor via hydrothermal route.^17,27–29 First, 0.01 mol FeCl₃ and 0.02 mol NaNO₃ were added to 45 mL deionized water under vigorous stirring at room temperature for 30 min, and then the mixture was transferred to a 50 mL Teflon autoclave sealed at 120 °C for 120 min. After being filtered and dried, FeOOH nanospindles were obtained. The coralloid Fe₂N nanoparticles were prepared by treating previous FeOOH nanospindles with pure ammonia in a tube furnace at 700 °C for 2 h.¹⁷ To obtain the coralloid Fe₂N@C nanoparticles (as shown in Fig. 1), FeOOH@C nanoparticles were prepared beforehand via the hydrothermal route.^18,22,24 During this process, 0.1 g FeOOH powder and 0.001 mol glucose were mixed thoroughly in 45 mL deionized water and sealed in a Teflon autoclave for 12 h at 180 °C. After being filtered, washed with deionized water and ethanol, and dried in a vacuum oven, FeOOH@C was obtained. This powder was transferred into a tube furnace for further nitriding following the same conditions in which the pure Fe₂N was prepared.

Fig. 1 Process schematic of synthesizing coralloid Fe₂N and Fe₂N@C.

Structure and morphology characterization

Morphology and structure of the as-prepared samples were characterized by scanning electron microscope (SEM; FEI Nova Nano SEM 230, 10 kV), transmission electron microscopy (TEM; JEOL JEM-2100F, 200 kV), and X-ray diffraction (XRD; Rigaku D/max2500, Cu Kα radiation). The TEM sample was prepared by inserting a drop of the suspension in ethanol on a lacey support film, while XRD and SEM samples consisted of powder.

Preparation and electrochemical measurements of electrodes

The electrochemical measurements were carried out via CR2016 coin-type cells.^30,31 The fabricated electrodes were prepared by mixing the active material, Super P (conductive agent) and carboxy methyl cellulose (CMC; acted as binder) together in a weight ratio of 8 : 1 : 1, and then magnetically stir the mixture for 24 h. The obtained slurry was homogeneously pasted on copper foil and dried at 100 °C in a vacuum oven for 12 h. The coin cells were laboratory assembled by a CR2016 press, operated in an argon-filled glove box (Mbraun, Garching, Germany). The sodium foil electrode and the composite electrode were isolated with a glass fiber which was ​frequently used in SIBs.¹³ The electrolyte consists of one mole sodium perchlorate dissolved in dimethyl carbonate (DMC) and ethylene carbonate (EC) solution at a volume ratio of 1 : 1; a drop of fluorinated ethylene carbonate (FEC) was added to 100 mL of the electrolyte during the preparation process. The electrochemical performances of the coin-type sodium-ion batteries were tested with discharge–charge measurements in a voltage range of 0.01 to 3.0 V (vs. Na/Na⁺) conducted on a battery test instrument (LANHE CT 2001A, Wuhan, China). Electrochemical impedance spectroscopy (EIS) testing on assembled cells was carried out on an electrochemical workstation (ZAHNER-IM6ex, Kronach, Germany) in the frequency range of 100 kHz–10 mHz. Cyclic voltammetry (CV) measurement was performed on an electrochemical working station (CHI604E, ChenHua Instruments Co., Ltd, Shanghai, China) at a scan rate of 0.1 mV s⁻¹ in the voltage range of 0.01–3.0 V (vs. Na/Na⁺).

3. Results and discussion

The synthesized FeOOH nanospindles as precursor were characterized by SEM and XRD measurements to confirm uniform morphology and pure phase, shown in Fig. 2 with an average length of about 800 nm and width of about 100 nm. The XRD pattern (Fig. 2(b)) can be referred to the standard data PDF#34-1266 that corresponds to tetragonal structure, with cell parameters of 10.535 Å, 10.535 Å and 3.03 Å. Based on the homogeneous sample obtained above we synthesized the coralloid Fe₂N and Fe₂N@C by nitriding and the hydrothermal coating method; the morphologies of these two products were also characterized by SEM and XRD methods.

Fig. 2 SEM images (a) and XRD pattern (b) of FeOOH nanospindles.

Fig. 3(a) shows XRD patterns of the Fe₂N and Fe₂N@C products. Results indicate that the amorphous carbon peak of Fe₂N@C can be clearly observed around 25 degrees, which proved the existence of the carbon layer. The phase in Fig. 3(a) can be referred to standard data PDF#50-0958 with orthorhombic crystalline structure. SEM images in Fig. 3(b) correspond to the nitrided precursor showing the uniform nanoparticles morphology, which is completely changed compared to the precursor due to the high-temperature nitride process, while the SEM images in Fig. 3(c) and (d) show the morphology of the carbon-coated nitride at low and high magnification, respectively; the previous spindle shape was partly maintained after coating with the carbon layer. The distinct morphology may lead to electrochemical performance differences, and the TEM test method was applied to confirm the structure of the carbon-coated precursor and target product.

Fig. 3 XRD patterns (a) of Fe₂N and Fe₂N@C, SEM images of Fe₂N (b) and Fe₂N@C (c, d).

Before the nitride treating process, FeOOH nanospindles were uniformly coated with carbon by the hydrothermal method, using glucose as the carbon source. TEM testing was carried out to confirm that the precursor was coated with amorphous carbon. As indicated in Fig. 4(a), FeOOH@C well maintained the nanospindle shape, and the surface of the FeOOH nanospindles was uniformly covered with carbon after the hydrothermal treatment, as shown in Fig. 4(b and c) under higher magnification. In the HRTEM view (Fig. 4(c)), the amorphous carbon separation was easily recognized as it is marked with white. The fully coated FeOOH sample provides a possibility for preparing the next carbon coating Fe₂N. Fig. 4(d–f) indicate TEM images of the carbon-coating nitride at different magnifications. In the HRTEM image (Fig. 4(f)), the carbon layer was marked with a white line, and fringes were also measured and calibrated. Results in Fig. 4 show the carbon layer was still maintained after the FeOOH@C nanospindles were treated in the ammonia atmosphere at high temperature, and this carbon composite structure may contribute to the electrochemical properties of the nitride.

Fig. 4 TEM and HRTEM images of FeOOH@C (a–c) and Fe₂N@C (d–f).

To further confirm carbon layer uniformity, which should be decorated on the surface of Fe₂N nanoparticles, energy-dispersive spectroscopy (EDS) was carried out. EDS results shown in Fig. 5 indicate the homogeneity of the elements in the Fe₂N@C nanoparticles. As the original image shown in Fig. 5(a), the selected area chosen for sweeping was displayed in Fig. 5(b). Iron was shown in Fig. 5(c and d) corresponding to the different valences of trivalent and monovalent, respectively. Nitrogen and carbon are shown in Fig. 5(e and f), and based on the previous EDS results, carbon homogeneously covered the Fe₂N nanoparticle surfaces. These characterization investigations elaborate the purity and carbon-coated coralloid morphology of the objective products.

Fig. 5 TEM (a), magnification (b) image of the selected area in (a); iron-K (c), iron-L (d), nitrogen (e), and carbon (f) corresponding to the EDS results of Fe₂N@C in (b).

The coral-like Fe₂N and Fe₂N@C nanoparticles synthesized before were applied in sodium ion batteries to investigate their electrochemical properties. In order to evaluate the long-term cycle stability of these two samples, the two electrodes were charged and discharged between 0.01 V and 3.0 V, at a current density of 500 mA g⁻¹ for 1000 cycles (Fig. 6(a)). As revealed in Fig. 6(a), the Fe₂N nanoparticles electrode showed a capacity of 129 mA h g⁻¹ at the first discharge, and in subsequent cycle the capacity rapidly decreased to 50 mA h g⁻¹. After 1000 cycles, the capacity of Fe₂N nanoparticles held at over 20 mA h g⁻¹, which is 40% of the second charge and discharge capacity. Although the capacity of this iron nitride is lower than the carbon-based materials in SIBs, its long-cycle life is superior to most oxides because of its stability on structure during the charge-discharge process. Since structural design could bring about property promotion, and carbon plays an important role in many reports on SIBs for its stability and good performance, many researchers have chosen carbon as a main component in designing electrode materials. Herein, the carbon coating of structured Fe₂N nanoparticles shows an improvement of stability and capacity retention rate (Fig. 6(a)). At the same current density, the Fe₂N@C electrode exhibited a capacity of 142 mA h g⁻¹ that is very close to the single nitride at the first charge process, and then reduced to 75 mA h g⁻¹, which is more than the pure Fe₂N. Over 1000 cycles, the capacity of this composite preserved more than 60 mA h g⁻¹, which is 80% of its second charge and discharge performance. Poor results of pure Super P in the charge and discharge process (Fig. 6(a)) confirmed that most of the capacities were from the active materials themselves. This long-term cycling result indicates that the Fe₂N as electrode holds promise in further applications for SIBs, for its good stability at high current density, and the carbon coating significantly improves the single nitride properties, especially capacity retention. In order to investigate the capacity rate capability of these two electrodes, current density was increased gradually from 100 mA g⁻¹ to 3 A g⁻¹, and then resumed to 100 mA g⁻¹ for another 150 cycles; cycling results are presented in Fig. 6(b). It is also clear that the Super P contributes little to the rate performance of Fe₂N and Fe₂N@C; when the current density was 100 mA g⁻¹, specific capacity of Fe₂N and Fe₂N@C were around 70 mA h g⁻¹ and 100 mA h g⁻¹, respectively. When the current was gradually increased from 100 mA g⁻¹ to 3 A·g⁻¹, performance of the pure iron nitride was decreased to 36 mA h g⁻¹, while the carbon-coated nitride decreased to 75 mA h·g⁻¹, which was much slower than the pure nitride. The current was then increased to 100 mA·g⁻¹, capacities of Fe₂N and Fe₂N@C electrodes were recovered to their previous charge and discharge abilities and cycled for another 90 stable circulations, confirming the good high-rate discharge/charge capability of these two electrodes. Moreover, the rate cycling performance results in Fig. 6 indicate the good rate capabilities and-long term cycling abilities of both materials. Compared to the pure nitride the carbon-coating method improved capacity and stability of Fe₂N nanoparticles remarkably, showing much higher specific capacity and smaller fluctuations due to current change.

Fig. 6 Long-term cycling stability at current density of 500 mA g⁻¹ (a), rate capability and cycling performance at variant current densities (b) of Fe₂N, Fe₂N@C and Super P electrodes in voltage range of 0.01–3.0 V (vs. Na/Na⁺).

To investigate variation tendencies between capacities and voltages, electrochemical performances of the as-made iron nitride and carbon-coated iron nitride were evaluated by galvanostatic charge/discharge measurements between 0.01 V and 3 V at a current density of 500 mA g⁻¹, as shown in Fig. 7. The nitride delivered a capacity of 142 mA h g⁻¹, and then fast faded to 45 mA h g⁻¹ in Fig. 7(a). Two voltage plateaus can be observed at about 0.5 V and 1.5 V, which can well match the peaks of its cyclic voltammogram curves in the first discharge process. The carbon-coated sample shown in Fig. 7(b) exhibited significantly improved capacity of about 154 mA h g⁻¹, and then faded to 93 mA h g⁻¹ with a high initial coulombic efficiency of about 60%, much higher than the pure nitride. Subsequent cycles of Fe₂N@C were concentrated and more stable than the pure nitride, and this matches the cycling performance of these two electrodes in Fig. 6(a), certifying that the carbon layer can contribute to the stability and capability of the nitride.

Fig. 7 Voltage-capacity profile of Fe₂N (a) and Fe₂N@C (b) electrodes at current density of 500 mA g⁻¹.

Cyclic voltammogram curves of both materials are presented in Fig. 8(a). To further investigate the basic reaction mechanism during charge and discharge, in situ XRD was carried out by characterizing the cycled electrode materials (Fig. 8(b)). According to standard patterns, during the discharge process two new phases identified as NaN₃ and Fe₃N appeared, indicating that part of the active negative material had reacted with sodium. Then in the charge process the reacted material recovered to the initial phase. In the reduction scan, reduction peaks can be observed near 0.5 V in both materials, which may be ascribed to reduction of iron to lower valences, corresponding to the discharge XRD patterns of Fe₂N in Fig. 8(b). During the oxidation process, a mild peak can be recognized at about 1.5 V, and this reversible oxidation peak may be caused by the escape of sodium ions that bring about oxidation of the lower valence iron. For Fe₂N@C, an oxidation peak occurred at 0.01 V in the anodic scan, and the corresponding reduction peak was also obvious, which is ascribed to the introduction of carbon. The peaks reflect the activity for both nanoparticles, and the two peaks near 0.01 V in the reduction and anodic scans for carbon-coated nitride also indicate the good reversibility for its superior carbon-coated nanostructure. Moreover, the circulation area of Fe₂N@C curves is much larger than the pure nitride and most of the dedicated capacity is near 0.01 V. Carbon contributed a lot to improved capability, which confirms the greater capability of Fe₂N@C.

Fig. 8 Cyclic voltammograms (a) of Fe₂N and Fe₂N@C electrodes at scan rate of 0.1 mV s⁻¹ between 0.01 V and 3.0 V (vs. Na/Na⁺); in situ XRD patterns (b) of Fe₂N electrode after being charged and discharged for 10 cycles.

4. Conclusions

In summary, the coral-like Fe₂N and Fe₂N@C nanoparticles were synthesized through a facile strategy, and these two samples were applied in SIBs for the first time. The in situ XRD result proved that the sodium inserted in Fe₂N by reducing the iron to lower valence and forming a new sodium nitride compound. Moreover, the results indicate both the iron nitride and carbon-coated nitride discharge/charge process showed good stability, and the carbon-coating method significantly improved the capability of the iron nitride with retention of about 80% over 1000 cycles. Considering of the good stability of Fe₂N@C nanoparticles and the demands for exploring proper new materials for SIBs, this work may contribute to the application of transition metal nitrides in SIBs.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (nos. 51472271, 61376018, and 51174233), Project for Innovation-Driven Planning in Central South University (2016CX002), Program for New Century Excellent Talents in University (NCET-12-0554), and National Basic Research Program of China (973 Program, no. 2013CB932901 and 2011 Program).

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