Sheet-like structure FeF₃/graphene composite as novel cathode material for Na ion batteries

Abstract

A sheet-like structure FeF₃/graphene composite is successfully synthesized by a novel and facile sol–gel method. The structure and electrochemical performance of the as-synthesized FeF₃/graphene composite are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and electrochemical measurement. The results indicate that the FeF₃ nanosheets are loaded on the surface of the graphene sheets to form the sheet-like structure hybrid. Fourier transform infrared (FTIR) spectrum confirms that C–F bonds exist in FeF₃/graphene composite, and it further indicates that a chemical bond between FeF₃ and graphene has been formed and FeF₃ can preferably stick to the surface of the graphene. The FeF₃/graphene composite as cathode material of rechargeable Na ion batteries (NIB) exhibits a fairly high initial discharge capacity of 550 mA h g⁻¹ at 0.1 C, and it still keeps a capacity of 115 mA h g⁻¹ after 50 cycles at 0.3 C at a range of 1.0–4.0 V for NIB.

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1. Introduction

For the past decade, Li ion batteries (LIB) have been extensively used in the consumer electronics, military, electric vehicle and aerospace fields due to their excellent energy density, good power capability, and long cycle life. However, along with the development of low cost and high efficient recycling technology, the low abundance of lithium in the earth's crust and large-scale applications of LIB become controversial. Alternatively to the widely studied LIB, the NIB can be a suitable choice for smart grid, energy conversion and storage applications in terms of the low cost, safety and natural abundance of sodium resources.¹ For the past decade, many efforts have been devoted to the development of novel cathode active materials. Transition metal oxide compounds and polyanion compounds with layered structure have been investigated as the cathode materials for NIB, such as Na_0.44MnO₂,^2,3 Na₄Mn₉O₁₈,⁴ Na_0.71CoO₂,⁵ NaNi_1/3Mn_1/3Co_1/3O₂,⁶ Na[Ni_1/3Fe_1/3Mn_1/3]O₂,⁷ Na_2/3Ni_1/3Mn_2/3−xTi_xO₂,⁸ Na₂FeP₂O₇,⁹ Na₂CoP₂O₇,¹⁰ Na[Fe,Mn]PO₄,¹¹ Na₃V₂(PO₄)₃,¹² Na[Mn_1−xM_x]PO₄ (M = Fe, Ca, Mg).¹³ Recently, transition metal fluorides have been considered as a promising new class of cathode materials for LIB, which exhibit large theoretical capacities and high discharge voltages due to their highly ionic metal–ligand bonds and small atomic weight. Unlike conventional intercalation reaction, transition metal fluorides, based on reversible conversion reaction, enable the full utilization of redox during the charge–discharge process and thus possess high theoretical specific capacity. Especially, due to high theoretical capacity and theoretical energy density, FeF₃ have been researched as new and promising cathode materials for LIB.^14–20 Besides, iron based fluorides have attracted interest as a promising positive electrode for rechargeable Na batteries. The important fluoride materials are currently reported as Na-ion batteries cathodes, such as FeF₃ (ref. 14–17) and NaFeF₃.^21,22 Li and his coworkers reported FeF₃·0.33H₂O and FeF₃·0.5H₂O wired by carbon nanotubes through ionic-liquid-based synthesis method for Na-storage, and which exhibited a considerable capacity and rate performances as cathode materials for NIB.^15,16 Because of the dense structure and high insulating character of pure FeF₃, the electrochemical performance deteriorates promptly as the cathode material of LIB, let alone NIB.¹⁵

Usually, FeF₃ used as cathode materials of LIB or NIB is its hydrated compounds. Besides, it has also been reported that lithium aluminate nanosheet and α-Fe₂O₃ nanoplates well-dispersed on the graphene can enhance their ionic conductivity.^23,24 In order to overcome the intrinsic drawback of the FeF₃, a sheet-like structure FeF₃ loaded by graphene is first designed and synthesized via a novel and facile sol–gel route in this work. The FeF₃/graphene nanosheets are obtained by controlling the dry temperature and the amount of graphene in a bottom-up synthesis. Herein, graphene is both used as the conductive agent to further improve the electrical conductivity of FeF₃ and a supporter for the stabilization of iron fluoride nanosheets. The FeF₃ nanosheets and graphene stack each other to form a hierarchical electron/ion conducting network, arising from the bilateral interaction of iron fluoride nanosheets and graphene sheet. The morphology and electrochemical performances of the sheet-like FeF₃/graphene composites are subsequently investigated as cathode material for NIB.

2. Experimental

Fe(NO₃)₃·9H₂O (10 mmol, 4.04 g) and conduction type graphene 0.1 g (8% of total weight) (The Sixth Element Inc) were dissolved or suspended in 50 mL methanol, then 1.5 mL (30 mmol) of 40% HF acid was added with stirring to gain a solution. The obtained FeF₃·3H₂O sol was aged for 24 h before dried at 60 °C. The product xerogel was further dried at 180 °C for 8 h in a vacuum drying oven and ground into fine powders.

X-ray powder diffraction was performed using Rigaku D/MAX-2500/PC equipped with Cu-Kα source (40 kV, 250 mA) to get the crystal structure. The sizes and morphologies of compound particles were characterized by a field emission scanning electron microscope using JEOL JSM-3500N. Transmission electron microscopy (TEM), selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) images were taken on a JEOL-2100 microscope instrument at an acceleration voltage of 200 kV.

The electrochemical performance of the as-synthesized material was characterized on 2025 type coin cells as a cathode and a sodium disk as anode for Na-ion batteries. The cathodes for Na cells were fabricated by mixing the cathode material, super carbon (SP), and polyvinylidene fluoride (PVDF) binder with a weight ratio of 80 : 15 : 5 in N-methyl pyrrolidinone, which were then pasted on aluminum foil followed by drying under vacuum at 110 °C for 24 h. The average mass loading of materials on aluminum foil is about 1.83 mg cm⁻². The test Na cells were assembled with the cathodes thus fabricated, metallic sodium anode, Glass fiber (GF/D) from Whatman was employed as the separator, and the electrolyte was 1 mol L⁻¹ NaClO₄ in a solvent of propylene carbonate (PC). The assembly of the testing cells was carried out in an argon-filled glove box, where water and oxygen concentration were kept less than 5 ppm. The charge–discharge experiments were conducted on a battery cycler (Newell Battery Test) at 25 °C. Charge–discharge measurements of fluoride-based cathodes versus Na/Na⁺ were performed at room temperature under various rates of 0.1–5 C in a voltage range of 1.2–4.0 V and 1.0–4.0 V for Na-storage. All of the specific capacities were calculated based on the mass of as-synthesized material (including graphene). Specifically, 0.1 C represents 23.7 mA g⁻¹, 0.3 C represents 71.1 mA g⁻¹, 1 C represents 237 mA g⁻¹ and so on.

The galvanostatic intermittent titration technique (GITT) was performed at room temperature at a voltage range of 1.0–4.0 V. The GITT measurements were performed on the second cycle to determine the diffusion coefficient of Na ions (D_Na) in electrode active materials. To achieve nearly equilibrium state (Es), relaxing 60 min after an interval of 10 min at a current density of 0.1 C have been combined with the GITT test. The procedure continued until the cutoff voltage was reached. The Na ion diffusion coefficient can also be determined by GITT by Fick's second laws of diffusion and calculated according to the following equation:

where I₀ (A) is the applied current in the charge–discharge process, V_m (cm³ mol⁻¹) is the molar volume of active materials, F (96 485 C mol⁻¹) is the Faraday constant, S (cm²) is the total contact area between the electrolyte and electrodes and L (cm) is the thickness of the electrode.

3. Results and discussion

Fig. 1a illustrates FeF₃ nanosheets anchored on the surface of graphene sheet to form the FeF₃/graphene composite with sheet-like structure. As schematically shown in Fig. 1a, firstly, FeF₃·3H₂O, which is quickly formed after Fe(NO₃)₃·9H₂O and HF added in methanol solution, generates C–F bond with graphene for nucleation sites to induce surface nucleation of iron fluoride during the aged process. The new produced C–F bonds are expected to act as nucleation sites for FeF₃·3H₂O due to intermolecular interactions. Besides, controlling the graphene amount, the graphene sheets, which can anchor FeF₃·3H₂O crystal, can provide more growing spots for FeF₃·3H₂O crystals and can also prevent the growth of big fluoride nanocrystals, thus FeF₃·3H₂O nanocrystals distribute homogeneously on the surface of the graphene sheet, a FeF₃·3H₂O/graphene composite precursor is obtained. Finally, the precursor was heated in a vacuum drying oven at the temperature of 180 °C to remove H₂O molecules and the sheet-like FeF₃/graphene composite was obtained.

Fig. 1 (a) Schematic illustration of fluoride nanosheet with graphene to form FeF₃/graphene, (b) SEM image, (c and d) TEM images, SAED image (the inset of (c)), and (e) HRTEM images of nanosheets FeF₃/graphene.

From the SEM images in Fig. 1b, the FeF₃/graphene composite is actually the hierarchical sheet-like structure. A rough wavy structure could originate from the intrinsic wrinkles and ripples of graphene. The TEM images (Fig. 1c and d) confirm further that the FeF₃ nanosheets (red arrows) anchor on the surface of the graphene sheet (blue arrows) and form the hybrid with both sheet-like structure. The sizes of the nanosheets FeF₃/graphene range from several hundred nanometers to a few micrometers. As being seen from the inset of Fig. 1c, the SAED pattern shows a set of broad diffuse rings instead of spots due to the random orientation of the crystallites, corresponding to diffraction from different planes of the nanocrystallites, which indicate the FeF₃ nanosheets are formed by tiny the FeF₃ nanocrystallites instead of growing along a certain direction. The SAED patterns are consistent with a hexagonal phase structure of FeF₃ with strong ring patterns due to (012) and (024) planes. Moreover, it can also be found from Fig. 1d that the FeF₃ nanosheets are formed by self-assembly of numerous nanoparticles with various sizes from 10 nm to 100 nm, which stretch outwards from the aggregate core, thus presenting the nanosheets morphology. HRTEM image (Fig. 1e) also reveals that FeF₃ tiny nanocrystals were formed. Lattice fringes can be discerned from the HRTEM image, suggesting that the FeF₃ nanoparticles are well-crystallized. Fig. 1e show that the interplanar spacing is about 0.381 nm and 0.265 nm, which also corresponds to the distance of (012) and (104) planes of FeF₃, respectively, indicating that the nanoparticles are the iron based fluoride.

Fig. 2a–c represent the typical XRD patterns of the FeF₃·3H₂O precursor, graphene, and the synthesized FeF₃/graphene by sol–gel method, respectively. According to the Scherer formula: D = Rλ/(β cos θ), when 2θ is 15.940°, 22.641° and 25.580° in XRD pattern of FeF₃·3H₂O precursor, it can be calculated that the average particle sizes are 53.8 nm, 46 nm and 44.8 nm, respectively. The results indicate that the precursor consists of nanosized particles. The peak locations in Fig. 2c indicate the formation of FeF₃/graphene composite. Moreover, a very small intensity reflection of graphene in Fig. 2c due to the presence of graphene can also be observed for the FeF₃/graphene composite. The broad reflections indicate its low crystallisation degree, which is one common feature shared by many nanoscopic metal fluorides.²⁵ In order to analyze the interaction of FeF₃ nanosheets with graphene sheets, Fourier transform infrared (FTIR) spectrum was used. The samples prepared as KBr pellets were recorded on a Perkin-Elmer Spectrum One FTIR spectrophotometer in a scan range of 400–4000 cm ⁻¹, and the results are shown in Fig. 2d. The characteristic peak at approximately 530 cm⁻¹ is an associated with the typical vibration band of Fe–F bond of FeF₃. Meanwhile, the C–F stretching vibrations are generally found at around 1070 cm⁻¹,¹⁷ which confirms that FeF₃ nanosheets tightly anchor on the surface of graphene sheets (as Fig. 1a) due to interaction of F atom with C atom each other, then the FeF₃/graphene composite with hybrid sheet-structure is formed.

Fig. 2 XRD patterns of (a) the FeF₃·3H₂O precursor, (b) graphene and (c) FeF₃/graphene, (d) IR spectra of nanosheets FeF₃/graphene.

Fig. 3a indicates the comparison of the rate capability for the FeF₃/graphene composite at a range of 1.0–4.0 V. The FeF₃/graphene cathode presents apparently a high irreversible discharge capacity of 550 mA h g⁻¹ at the first cycle at 0.1 C. The discharge capacity is close to the theoretical capacity of FeF₃. It can also find from Fig. 3a that there is two reduction peaks at about 2.6 V and 1.2 V, which can correspond to the insertion/deinsertion reaction and the reversible conversion reaction, respectively. Usually, the electrode reaction mechanism of FeF₃ as the cathode material of Na ion battery can be expressed as following:²⁶

FeF₃ + Na → NaFeF₃ (4.0–1.2 V) (1)

NaFeF₃ + 2Na → Fe + 3NaF (1.2–1.0 V) (2)

Fig. 3 Electrochemical behavior for FeF₃/graphene: (a) prolonged cycling behavior at different C rates, respectively at the range of 1.0–4.0 V, (b) rate performance at different rates at the range of 1.0–4.0 V, (c) rate performance at different rates at the range of 1.5–4.0 V, (d) the discharge–charge GITT curves of FeF₃/graphene electrode as a function of time in the potential range of 1.0–4.0 V, and (e) calculated D_Na for FeF₃/graphene as a function of x (the total Na insertion and conversion number) during charge–discharge process, respectively.

Eqn (1) corresponds to the Na ion intercalation/unintercalation reactions between phases containing Fe³⁺ and Fe²⁺. Eqn (2) should be attributed to the redox reactions between phases containing Fe²⁺ and metallic Fe⁰ based on chemical conversion mechanism. It can be found from Fig. 3b that although the first discharge capacity is high, the second discharge capacity drops down to 396 mA h g⁻¹ and charge capacity dives to 388 mA h g⁻¹. The reasons of the sudden descent of capacity are likely to originate from not only the formation of insulating phases during conversion reaction but also the formation of solid electrolyte interphase (SEI) layers. The presence of a relatively stable conversion reaction for the FeF₃/graphene composite provides extra capacity for the first few cycles. However, the Fe⁰ originated from eqn (2) will probably aggregate to form big particles during the charge–discharge cycle process, thus it will result in the slow kinetics of the conversion reaction and the electrode process exhibits mainly the Na⁺ insertion reaction after the 50th cycle. Besides, GITT test results in Fig. 3d and e show that the Na ion (D_Na) diffusion coefficient during insertion reaction is higher than during conversion reaction. Fig. 3c indicates the rate performance at different rates at the range of 1.5–4.0 V, in this potential window the electrode reaction only behaves the insertion/deinsertion reaction. To our surprise, the nanosheet FeF₃/graphene composite delivers 234 mA h g⁻¹ in the insertion process at the first cycle at 0.1 C, which is nearly accordance with the theoretical capacity of 1 Na insertion. This high capacity is maybe ascribed to the special sheet-like structure of the FeF₃/graphene composite. Especially, the FeF₃/graphene electrode can deliver 90 mA h g⁻¹ at 1 C rate at the range of 1.0–4.0 V (Fig. 3b), while it can only deliver about 60 mA h g⁻¹ at the same rate at the range of 1.5–4.0 V (Fig. 3c). Compared Fig. 3b with Fig. 3c, it can be found that the capacity provide by the insertion reaction is about two-thirds of the total capacity. Therefore, the sheet-like structure of FeF₃/graphene composite plays likely an important role for improving its capacity.

Fig. 4 shows the cycling performances of FeF₃/graphene and FeF₃ as the cathode material of NIB at 0.3 C at a range of 1.0–4.0 V. The capacities of FeF₃ and FeF₃/graphene composite during the first discharge are 91 and 344 mA h g⁻¹, respectively. The capacities of discharge decay faster in the first few cycles, but capacities during the later cycles decline slowly, and the discharge capacity of the FeF₃/graphene composite can still keep 115.8 mA h g⁻¹ after 50 cycles. However, for the FeF₃ electrode the discharge capacity is only 10 mA h g⁻¹ after the 50th cycle. Apparently, the addition of graphene and formation of sheet-like structure between FeF₃ and graphene can well improve the electrochemical performance.

Fig. 4 The discharge curves of FeF₃ cathode and FeF₃/graphene cathode at 0.3 C at the range of 1.0–4.0 V for NIB.

4. Conclusions

In conclusion, the sheet-like structure FeF₃/graphene composite for the application of NIB was successfully synthesized by a facile sol–gel route. The sheet-structural formation between FeF₃ and graphene for FeF₃/graphene composite can contribute positively to the large reversible Na-storage capacity compared with FeF₃ during insertion/deinsertion process as well as conversion reaction. In the aspect of electrochemical behavior, FeF₃/graphene exhibits a fairly high initial discharge capacity of 550 mA h g⁻¹ at 0.1 C, and it can still retain 115.8 mA h g⁻¹ after 50 cycles at 0.3 C at a range of 1.0–4.0 V. Although the cycling performance and specific capacity of FeF₃/graphene need to be improved further, the design and preparation for the hybrid sheet-like structure FeF₃/graphene composite as the cathode material of NIB is still a valuable exploration.

Acknowledgements

This work is supported financially by the National Natural Science Foundation of China under project no. 51472211, Scientific and Technical Achievement Transformation Fund of Hunan Province under project no. 2012CK1006, Key Project of Strategic New Industry of Hunan Province under project no. 2013GK4018, and Science and Technology plan Foundation of Hunan Province under project no. 2013FJ4062.

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