A potential pyrrhotite (Fe₇S₈) anode material for lithium storage

Abstract

Fe₇S₈@C nanospheres were prepared by a simple solid–solid reaction and showed a high specific capacity and an excellent high rate performance as the anode material in lithium ion batteries. The core–shell Fe₇S₈@C composites delivered a very high reversible capacity of 695 mA h g⁻¹ at 0.1 A g⁻¹ after 50 cycles between 0.01 and 3.00 V. The Fe₇S₈@C composites also showed a discharge plateau at 1.5 V, cycling between 1.20 and 2.50 V, and exhibited a specific capacity of 397 mA h g⁻¹ at 0.1 A g⁻¹ over 200 cycles, which is higher than the theoretical capacity of Li₄Ti₅O₁₂ (about 175 mA h g⁻¹).

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Transition metal chalcogenides are considered to be promising high-capacity materials for use in lithium ion batteries (LIBs).^1–6 Iron sulfides have been used as the cathode material in lithium/iron sulfide batteries because of their potential plateaus and high specific capacities. For example, FeS has a high capacity of 958 mA h g⁻¹ at 0.05 A g⁻¹ and one potential plateau at 1.6 V vs. Li⁺/Li (0.01–3.00 V).⁶ Fe₃S₄ has also been investigated as an anode material in LIBs and was shown to have a discharge capacity of 1161 mA h g⁻¹ in the first cycle, much higher than the theoretical capacity of 785 mA h g⁻¹, and after 100 cycles at 0.1 A g⁻¹ it had a discharge capacity of 562.9 mA h g⁻¹. Fe₃S₄ showed two voltage plateaus at 1.87 and 1.4 V (0.005–3.00 V).⁷ FeS₂ has been reported to have a high capacity of 495 mA h g⁻¹ after 50 cycles at 0.5 C and two potential plateaus at 2.0 and 1.5 V (1.00–3.00 V).⁸ FeS, FeS₂ and Fe₃S₄ all have high capacities compared with Li₄Ti₅O₁₂ (about 175 mA h g⁻¹) in addition to relatively high voltage plateaus (about 1.5 V versus Li⁺), which avoid the problem of lithium dendrites and the formation of a solid–electrolyte interface (SEI) layer.⁹

However, there are still some problems preventing the use of iron sulfides as electrode materials for LIBs. The discharge products (Li₂S_x, 1 ≤ x ≤ 8) are easily dissolved in liquid electrolytes, leading to a loss of active material and a deterioration in the conductivity of the electrode.¹⁰ Another problem is the change in volume of the electrode during charge/discharge. For example, there is a 200% volume change during the reduction of FeS to Fe and Li₂S, which results in the degradation of the conductivity of the electrode.⁶ Coating with carbon and decreasing the particle size are two ways of solving these problems.^11–13

We report here a new kind of electrode material for LIBs. Fe₇S₈@C nanospheres were prepared by a modified solid–solid reaction.¹⁴ In a typical synthesis, ferrocene (5 mmol) and ammonium persulphate (10 mmol) were mixed and placed in a 55 ml Teflon-lined autoclave and maintained at 230 °C for 1 h. After cooling, the fluffy powder product (Fe₇S₈@C) was washed several times with distilled water and ethanol and dried at 60 °C overnight. The black powder was then calcined at 500 °C for 2 h in a flow of Ar. The structure, morphology and electrochemical properties of the Fe₇S₈@C nanospheres were investigated.

Fig. 1a shows the X-ray diffraction (XRD) patterns of Fe₇S₈@C. The main peaks at 30.03, 33.99, 43.99 and 53.34° can be indexed to (200), (203), (206) and (220) based on the normal XRD data card of the hexagonal phase of Fe₇S₈ (JCPDS no. 76-2308); no carbon diffraction peak was observed, probably as a result of the relatively low degree of crystallinity. The morphology of the Fe₇S₈@C product was further characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 1b is a typical SEM image of Fe₇S₈@C with a homogeneous morphology consisting of spherical nanoparticles. The majority of the particles in the sample had an average diameter <100 nm. Fig. 1c and d show TEM and high-resolution TEM (HRTEM) images of Fe₇S₈@C. The core–shell structures can be easily observed and the carbon shell is usually about 10 nm thick. The disordered carbon shell is seen in the inset of Fig. 1d, which corresponds with the XRD pattern of Fe₇S₈@C in Fig. 1a (no carbon peak). Elemental analysis showed that the carbon shell was about 37 wt% of the composite. The energy-dispersive X-ray spectrum of Fe₇S₈@C is shown in Fig. S1;† the Fe/S molar ratio was close to the Fe₇S₈ stoichiometry.

Fig. 1 (a) Power X-ray diffraction patterns of the Fe₇S₈@C composite. (b) SEM, (c) TEM and (d) HRTEM images of the Fe₇S₈@C composite.

Fig. 2 shows the thermogravimetric analysis (TGA) curves of Fe₇S₈@C. The weak baseline drift is attributed to ferrous oxidation¹⁵ and the strong exothermal peak between 340.1 and 535.94 °C accompanied by a severe weight loss can be ascribed to the burning off of the carbon shell and the oxidation of Fe₇S₈. In another experiment, the Fe₇S₈@C composite was oxidized in air from room temperature to 400 °C (500 °C, 700 °C) for 5 h to determine the intermediate oxidation products, which were identified by their XRD profiles (Fig. S2†). The final step of the weight loss between 535.94 and 703 °C is a result of the decomposition of Fe₂(SO₄)₃ into Fe₂O (Fig. S2†).¹⁶ The TGA results show a 51.6% weight loss, implying that 46.3% of the mass was carbon.

Fig. 2 TGA curves of Fe₇S₈@C composite heated in air from room temperature to 750 °C.

X-ray photoelectron spectrometry (XPS) and Raman shifts were used to determine the surface chemical composition and the valence states of the Fe₇S₈@C nanostructures. Fig. 3a shows the weak XPS spectrum of Fe 2p. Peaks corresponding to 707.1 and 719.6 eV were attributed to the Fe²⁺ state, whereas peaks at 711 and 723.4 eV were ascribed to the Fe³⁺ state.^17–20 The spectra of S 2p are shown in Fig. 3b. Two peaks at 161 and 163.1 eV were attributed to S 2p_3/2, characteristic peaks of FeS and FeS₂, respectively; the one peak at 165.1 eV and the weak peak at 167.5 eV represent the spin–orbit contributions from S 2p_1/2.^18–22 A very strong signal at 284.6 eV corresponds to the main carbon shell (Fig. 3c).²³ The spectrum for the Fe₇S₈@C sample is shown in Fig. S3.† Over the whole spectrum, the signal of the carbon shell (about 40 wt%) was too strong and XPS could not be used to explore the core as the other spectra were relatively weak. Fig. 3d shows two Raman shifts of the carbon shell (I_D/I_G ≫ 1); the D-band located at 1348 cm⁻¹ is associated with disordered carbon, vacancy defects, edge defects and topological defects, whereas the G-band at 1574 cm⁻¹ corresponds to the Raman-allowed optical mode E_2g, which is the high-frequency E_2g first-order mode. The high I_D/I_G (the integral curve area) is about 3.3, which means that the main carbon shell is disordered.²⁴

Fig. 3 XPS spectra for the as-prepared Fe₇S₈@C nanostructures. (a) Fe 2p, (b) S 2p and (c) C 1s. (d) Raman shifts for the carbon shell of the Fe₇S₈@C composite.

To demonstrate the role of the core–shell structure of Fe₇S₈@C in improving the lithium storage performance, we investigated the electrochemical properties of the composite in a number of different ways. Fig. 4a shows the charge/discharge curves of the Fe₇S₈@C electrode at 0.1 A g⁻¹ (the active material, including the 37 wt% carbon shell). Pyrrhotite (Fe₇S₈) is common phase with a variable stoichiometry of Fe_1−xS (x = 0–0.2); its end-member FeS is also known as troilite. In the initial discharge process, an inconspicuous plateau at 1.3 V was observed, which may be attributed to the reaction between Fe₇S₈ and Li with the formation of Fe, Li₂S and Li-rich stages depending on the number of Li transfers per pyrrhotite molecule.²⁵ Another fairly weak plateau between 0.8 and 0.6 V disappears after the first cycle. This is mainly caused by the formation of an SEI layer on the electrode.⁶ During the charge process, only one voltage slope was observed at 1.9–2.0 V; this is associated with the oxidation of Fe to Li₂FeS₂.²⁶ The Li-rich phases can be seen more clearly in Fig. 4b. In the first discharge cycle of the cyclic voltammetry curves, two mild reduction peaks at 1.2 and 0.75 V and one oxidation peak at 1.9 V can be observed. The broad peak at about 0.75 V suggested the formation of an SEI on the carbon shell. The peaks at 1.9 V can be ascribed to the formation of Li₂FeS₂.²⁷ There was a slight change in the position of the peaks after the first cycle. One additional small reduction peak appeared at 1.9 V, which indicated a change in the Li⁺/FeS₂ reaction between the first and subsequent cycles.^7,27 The 1.4 V peak represents the reaction of Li with Li_2−xFeS₂ after the first cycle.^6,28

Fig. 4 Electrochemical performance of electrodes at room temperature. (a) Charge/discharge curves of the Fe₇S₈@C electrode at 0.1 A g⁻¹ from 0.01 to 3.00 V. (b) Cyclic voltammetry curves of the Fe₇S₈@C electrode at a scan rate of 0.1 mV s⁻¹ in the voltage range 0.01–3.00 V. (c) Charge/discharge curves of the Fe₇S₈@C electrode at 0.1 A g⁻¹ between 1.20 and 2.50 V. (d) Cycle performance of the composite at a current density of 0.1 A g⁻¹ between 1.20 and 2.50 V. (e) Voltage–capacity curves at different rates (increased from 0.05 to 1 A g⁻¹). (e) Rate capability at different rates (increased from 0.05 to 1 A g⁻¹).

Fig. 4c shows the discharge and charge curves for the 2^nd, 3^rd and 200^th cycles at 0.1 A g⁻¹ (the cutoff voltage of the first cycle is between 0.01 and 2.50 V). Consistent with these discharge and charge curves, the Fe₇S₈@C electrode (the active material not including the carbon shell) also maintains a stable cycle performance between 1.20 and 2.50 V. In Fig. 4d, the Fe₇S₈@C electrode shows a high capacity of 397 mA h g⁻¹ at 0.1 A g⁻¹ after 200 cycles, which is higher than the Li₄Ti₅O₁₂. It also shows a high and stable capacity of 695 mA h g⁻¹ at a constant current density of 0.1 A g⁻¹ after 50 cycles between 0.01 and 3.00 V (Fig. S4†). The Fe₇S₈@C nanospheres were subjected to cycling at various charge/discharge rates to study their capacity–rate relationship. Fig. 4e shows the voltage–capacity profiles at different current rates increasing from 0.05 to 1 A g⁻¹ between 1.20 and 2.50 V. The electrode presented stable reversible capacities of 448, 401, 363, 319, 241 and finally back to 404 mA h g⁻¹ at 0.05, 0.1, 0.2, 0.5, 1 and 0.05 A g⁻¹, respectively. If the stoichiometric molecular formula of pyrrhotite is Fe₇S₈, which corresponds to the theoretical capacity of 663 mA h g⁻¹, then, by calculation from the following reaction:

16Li + Fe₇S₈ → 7Fe + 8Li₂S

Provided that the capacity of carbon in Fe₇S₈@C is equivalent to that of graphite (372 mA h g⁻¹), then, by combining the mass contents of carbon and pyrrhotite in Fe₇S₈@C, we can obtain the theoretical specific capacity of the Fe₇S₈@C electrode as about 666 mA h g⁻¹, which is very close to the measured reversible capacity of 695 mA h g⁻¹ at 0.1 A g⁻¹ after 50 cycles.

The stable and high electrochemical performance of the Fe₇S₈@C composites may be attributed to their structure and morphology. The relatively uniform average size of the pyrrhotite crystals is so small (30 nm) that a high rate of conduction of the Li ions can easily be realized between the materials. The uniform carbon shells effectively control the volume change of pyrrhotite and provide a good electronic conductive route. The thickness of the carbon shell reduces the contact between electrolyte and pyrrhotite and therefore the products (i.e. Li₂S and Li_2−xFeS₂) do not easily dissolve in the electrolyte.

Composites of Fe₇S₈@C with diameters of 30–40 nm have been synthesized by a one-step solid–solid reaction at low temperatures. As an anode for LIBs, the Fe₇S₈@C composites show a high capacity and high rate performance, even at 1.20 and 2.50 V.

Acknowledgements

This study was supported by the 973 Project of China (no. 2011CB935901), and the National Natural Science Fund of China (no. 91022033, 21201158).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14819c

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