Uniform yolk−shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions

Journal of Alloys and Compounds xxx (xxxx) xxx

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Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions Wangsuo Weng, Jingyi Xu, Chenling Lai, Zhenhua Xu, Yichen Du, Jun Lin**, Xiaosi Zhou* Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210023, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 May 2019 Received in revised form 1 September 2019 Accepted 19 October 2019 Available online xxx

Iron sulfides are potential anode materials for the reversible electrochemical storage of alkali metal ions (Liþ, Naþ, or Kþ) from the perspective of high theoretical capacity and relatively low cost. Nevertheless, their real application is greatly hampered by the inferior conductivity and the huge volume variations in the reaction with alkali metal ions. Few studies have examined desirable candidates that can be used as current host materials for alkali metal ions. In this work, a mild etching approach coupled with further sulfidation has been proposed for the precise design and preparation of uniform yolk
shell Fe7S8@C nanoboxes. The as-prepared yolk
shell Fe7S8@C nanoboxes exhibit excellent storage performance for alkali metal ions in the aspects of high specific capacity, superior cyclic stability, and good rate performance. The excellent electrochemical behaviour is attributed to the rational design of yolk
shell nanostructure. © 2019 Elsevier B.V. All rights reserved.

Keywords: Fe7S8@C nanoboxes Yolk
shell structure Alkali metal ion batteries Anode Energy storage

1. Introduction Alkali metal ions, such as Liþ, Naþ, or Kþ, are crucial energy carriers with potential or practical significance [1e5]. Alkali metal ion batteries have long been considered as an efficient power source for emerging energy storage systems [6e10]. Most notably, lithium ion batteries (LIBs) are widely used in today’s portable electronic products because of their high energy density and long cycle life [11e14]. However, the shortage of naturally stored lithium has led to higher costs of LIBs, and the performance of currently used materials generally has reached their theoretical limits [15e19]. Other alkali metal ion batteries, such as sodium ion batteries (SIBs) and potassium ion batteries (PIBs), have aroused great interest as promising candidates to replace LIBs in the future in light of their abundance in natural resources and the benefit of cost effectiveness [20e24]. Nevertheless, real applications of these three alkali ion batteries are restricted due to the lack of universal anodes, especially for SIBs and PIBs, because the larger ionic radius of Naþ and Kþ will cause a capacity decay and shorten the cycle life with the conventional anode materials used in LIBs [25e28]. Good

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Lin), [email protected] (X. Zhou).

electrode materials used to store one alkali ion are not necessarily favourable hosts for other alkali ions, which is frequently observed. For instance, graphite is generally used as the anode material for LIBs, but it is unfavourable for SIBs and suffers from a poor capacity for PIBs (279 mA h g
1) [29e31]. Considerable efforts have been put forth to find decent electrode materials, such as carbonaceous materials [32,33], metal oxides [34e36], and metal sulfides [37e41]. Iron sulfides have attracted widespread attention due to their high initial theoretical capacity, low cost, earth abundance and environmental friendliness [42e44]. However, the main shortcoming restricting iron sulfides as anode materials is their poor cycling stability. Severe volume expansion, inferior conductivity, and dissolution of polysulfides in the electrolyte will cause the pulverization of host materials during the intercalation/extraction process [45e49]. For example, the pristine FeS2 electrode exhibited fast capacity fading and poor cycle retention [50]. Therefore, it is urgent and necessary to propose valid strategies to overcome these problems. Recent studies demonstrate that adjusting the morphology and structure at the nanoscale and simultaneously introducing carbon coating will improve electrochemical properties. Nanostructured active materials can accelerate the reaction kinetics and a carbon coating will enhance the electrical conductivity and supply buffering to mitigate large volume variations during the repeated discharge/charge cycles [12,51e53]. Fe7S8 has received a great deal of attention as a

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Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732

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promising anode material for LIBs and SIBs because of its high theoretical capacity, good conductivity, and low cost. When used as an anode electrode material for LIBs and SIBs, it exhibits excellent electrochemical performance. For example, Jin et al. prepared spindle-like Fe7S8/N-doped carbon nanohybrids for highperformance SIB anodes [44]. Shi et al. reported uniform core
shell nanobiscuits of Fe7S8@C for LIBs and SIBs with excellent performance [49]. Choi et al. fabricated extremely small pyrrhotite Fe7S8 nanocrystals with simultaneous carbon-encapsulation for high-performance SIBs [54]. Although great efforts have been made, iron sulfides as universal anode materials for the storage of alkali metal ions are rarely reported. In this study, we exploit a mild etching approach coupled with further sulfidation to prepare uniform yolk
shell Fe7S8@C nanoboxes. These nanoboxes can be used as a decent host material for the rapid and reversible storage of Liþ, Naþ, and Kþ ions. 2. Experimental Section 2.1. Preparation of Fe2O3 nanocubes The Fe2O3 nanocubes were prepared through a simple precipitation method. Briefly, NaOH solution (50 mL, 5.4 M) was added dropwise to the same volume of FeCl3 solution (2.0 M) by a syringe pump over 5 min with continuous stirring at 75  C. The as-formed Fe(OH)3 gel was further stirred at this temperature for another 5 min and then incubated at 100  C for 96 h in a preheated oven. After cooling to room temperature, the dark red product was collected by centrifugation, rinsed and then dried at 60  C overnight.

spectra were measured on an ESCALab250Xi electron spectrometer from VG Scientific with 300 W Al Ka radiation. Nitrogen sorption measurements were carried out on an ASAP 2050 surface area-pore size analyser. TGA was performed on a NETZSCH STA 449 F3 in air. 2.5. Electrochemical measurements Electrochemical tests were executed by assembling coin cells (CR2032) in an Ar-filled glovebox (O2, H2O < 0.1 ppm). The working electrode was made up of 10 wt% sodium alginate, 20 wt% carbon black (Super-P), and 70 wt% active materials. They were thoroughly mixed by grinding in a small amount of water to form a homogeneous slurry. The resulting slurry was adhered uniformly to Cu foil and dried in a vacuum oven at 80  C overnight. The mass of the active material adhered to the copper foil was approximately 0.7e1.0 mg cm
2. For SIBs, the cycling performance of the electrode with a higher mass loading of active materials (1.5 mg cm
2) was also tested. Electrolytes used for LIBs and SIBs were 1.0 M LiPF6 in EC/DEC (1:1 v/v) and 1.0 M NaClO4 in EC/DEC (1:1 v/v) with the addition of 5 vol% FEC, respectively. For PIBs, different electrolytes were prepared as follows: 0.4 M KPF6 in EC/DEC (1:1 v/v) with the addition of 5 vol% FEC (abbreviated as “0.4 M KPF6”), 0.6 M KPF6 in EC/DEC (1:1 v/v) with the addition of 5 vol% FEC (abbreviated as “0.6 M KPF6”), 0.8 M KPF6 in EC/DEC (1:1 v/v) with the addition of 5 vol% FEC (abbreviated as “0.8 M KPF6”), 2 M KFSI in DME (abbreviated as “2 M KFSI”), 3 M KFSI in DME (abbreviated as “3 M KFSI”), and 4 M KFSI in DME (abbreviated as “4 M KFSI”). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed using a PARSTAT 4000 electrochemical workstation. 3. Results and discussion

2.2. Preparation of core
shell Fe3O4@C nanocubes The synthesis of core
shell Fe3O4@C nanocubes is based on previously reported literature [55,56]. Fe2O3 nanocubes (160 mg) and dopamine hydrochloride (80 mg) were dispersed in Tris-buffer solution (200 ml, 10 mM) under stirring. After stirring for 3 h, core
shell Fe2O3@PDA nanocubes were obtained via centrifugation, rinsed and then dried at 60  C overnight. The as-obtained core
shell Fe2O3@PDA nanocubes were annealed in Ar at 500  C for 5 h to transform them into core
shell Fe3O4@C nanocubes. 2.3. Preparation of yolk
shell Fe7S8@C nanoboxes The as-prepared core
shell Fe3O4@C nanocubes were dispersed in HCl solution (4 M) with stable stirring. After an etching time of 60 min, yolk
shell Fe3O4@C nanoboxes were harvested by several centrifugation-rinse cycles and then dried at 60  C overnight. The sample was further sulfurized in the furnace at 700  C for 6 h under an H2S/Ar (10:90 v/v) atmosphere to get the final product. The whole process was carried out in a fume hood, and excess H2S gas was collected using a gas cylinder filled with sodium hydroxide solution. 2.4. Materials characterization TEM and HRTEM images were taken at 200 kV using a JEM2100F transmission electron microscope. Elemental mapping analysis and STEM were performed on the same transmission electron microscope connected with a Thermo Fisher Scientific energy dispersive X-ray spectrometer. SEM images were collected at 10 kV on a JSM-7600F scanning electron microscope. Raman spectra were recorded using a Labram HR800 with a laser wavelength of 514 nm. XRD patterns were acquired with a Rigaku SmartLab diffractometer using Cu Ka irradiation (l ¼ 1.5406 Å). XPS

Uniform yolk
shell Fe7S8@C nanoboxes were prepared via a mild etching approach coupled with subsequent sulfidation (see Experimental Section for details). The fabrication process for the synthesis of uniform yolk
shell Fe7S8@C nanoboxes is schematically shown in Fig. 1. First, the homogeneous Fe2O3 nanocubes were prepared through a hydrothermal method. Then, polydopamine (PDA) was uniformly wrapped on the surface of the Fe2O3 precursor to form core
shell Fe2O3@PDA nanocubes, and the obtained product was annealed in an Ar atmosphere to transform it into carbon-coated Fe3O4 nanocubes (Fe3O4@C). Next, the yolk
shell Fe3O4@C nanoboxes were fabricated by partially removing the Fe3O4 core in hydrochloric acid (HCl) solution. Finally, the yolk
shell Fe3O4@C nanoboxes were converted to yolk
shell Fe7S8@C nanoboxes via annealing under a H2S/Ar atmosphere in the tube furnace. With the advantages of the outer carbon shell to strengthen the conductivity, the inner Fe7S8 core to exhibit high specific capacity and the void space to buffer the huge volume variations, the uniform yolk
shell Fe7S8@C nanoboxes deliver high specific capacity, good rate performance, and excellent cycling stability when serving as anodes for LIBs, SIBs and PIBs. Uniform Fe2O3 nanocubes prepared by a mild hydrothermal method were employed as the initial template. The XRD pattern proves that a-Fe2O3 (Fig. S1a) with high crystallinity was successfully fabricated. The FESEM and TEM images show that these Fe2O3 nanocubes are comparatively uniform with an average side length of approximately 650 nm (Figs. S1bed). Then, thin and smooth PDA shells were wrapped over the whole surface of the Fe2O3 nanocubes (Figs. S2a and 2b). The core
shell Fe2O3@PDA structure is further affirmed through the low-magnification TEM images, from which it is apparent that the thickness of the PDA shell is approximately 23 nm (Figs. S2c and 2d). After annealing at 500  C in an argon atmosphere, the core
shell Fe2O3@PDA nanocubes were totally transformed into core
shell Fe3O4@C nanocubes, which is

Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732

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Fig. 1. Schematic diagram of preparing uniform yolk
shell Fe7S8@C nanoboxes.

verified by the XRD measurement (Fig. S3a). The results indicate that all of the diffraction peaks are well matched to the standard card of magnetic Fe3O4 without obvious Fe2O3 residues. FESEM and TEM images (Figs. S3bed) illustrate the core
shell structure of Fe3O4@C nanocubes with a smooth carbon shell covering over the surface of the Fe3O4 core. After an etching time of 60 min, yolk
shell Fe3O4@C nanocubes are achieved, which were characterized by XRD (Fig. S4). After etching, the composition of the sample was not changed. As shown in the FESEM images (Fig. 2a and b), the yolk
shell Fe3O4@C nanocubes have an average side length of approximately 600 nm. The high-magnification FESEM image clearly illustrates that the nanocubes with encapsulated Fe3O4 nanoparticles have a smooth surface (Fig. 2c). A visible void space between the Fe3O4 core and the carbon shell can be observed from the TEM images (Fig. 2d and e). The thickness of the carbon shell is measured to be approximately 20 nm from the magnified TEM image (Fig. 2f). We also performed experiments with different etching times for comparison. With a shorter etching time (40 min), the void space in the nanobox is not enough to accommodate the volume expansion during sulfidation (Fig. S5a). When the etching time is extended to 80 min, Fe3O4 core is basically completely etched (Fig. S5b). Finally, the yolk
shell Fe3O4@C nanoboxes are further sulfurized at 700  C in a H2S/Ar atmosphere to produce yolk
shell Fe7S8@C nanoboxes. The XRD pattern (Fig. 3a) preliminarily confirms the success of our synthetic strategy. The final product is pyrrhotite Fe7S8 without any impurities. The nitrogen adsorption-desorption test (Fig. 3b) demonstrates that the yolk
shell Fe7S8@C nanoboxes possess a

specific surface area of 64.2 m2 g
1 and a pore volume of 0.09 cm3 g
1. These values are much larger than those of Fe7S8@C nanocubes (13.49 m2 g
1 and 0.019 cm3 g
1, respectively). Properly large specific surface areas and pore volumes can effectively shorten and simplify the diffusion pathway of electrolyte ions, thereby improving their electrochemical performance. To determine the carbon content of yolk
shell Fe7S8@C nanoboxes, we operated TGA analysis under flowing air (Fig. 3c). When the temperature was higher than 800  C, the weight remained almost unchanged. The carbon content of yolk
shell Fe7S8@C nanoboxes can be measured by virtue of the pure Fe7S8 thermogravimetric data showing that Fe7S8 is converted into Fe2O3 and SO2 after calcination. The related chemical reaction can be described as: 4Fe7S8 þ 53O2 / 14Fe2O3 þ 32SO2. By calculation, the carbon content in the yolk
shell Fe7S8@C nanoboxes is estimated to be approximately 13.7 wt%. The above results suggest that the uniform yolk
shell Fe7S8@C nanoboxes were successfully prepared. The carbon shells wrapped on the surface of yolk
shell Fe7S8@C nanoboxes were further characterized by Raman spectra (Fig. 3d). The sharp peaks centred at 1580 and 1350 cm
1 correspond to the sp2-hybridized graphitic carbon band (G-band) and the disordered carbon band (D-band), respectively. The peak intensity of the G-band is higher than that of D-band, suggesting that the carbon in the composite is partially graphitized. In addition, the surface chemistry composition of the yolk
shell Fe7S8@C nanoboxes (Fig. 3e and f) is identified by the XPS measurement. The peaks centred at 727.7 and 713.7 eV are derived from the Fe3þ state, and the Fe 2p peaks centred at 724.1 and 710.7 eV are ascribed to the Fe2þ state. The

Fig. 2. (aec) SEM images and (def) TEM images of the yolk
shell Fe3O4@C nanoboxes.

Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732

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Fig. 3. (a) XRD pattern of the yolk
shell Fe7S8@C nanocubes, (b) nitrogen adsorption-desorption isotherms of the Fe7S8@C nanocubes and yolk
shell Fe7S8@C nanoboxes, (c) TGA analysis of the Fe7S8 nanocubes and yolk
shell Fe7S8@C nanoboxes, (d) Raman spectra and (eef) XPS spectra of the yolk
shell Fe7S8@C nanoboxes.

peaks centred at 284.8, 286.3, and 289.0 eV are attributed to the carbon atoms in C]C, C]N, and CeN groups of the carbon shell of the yolk
shell Fe7S8@C nanoboxes, respectively. N-doping can increase the electronic conductivity of the yolk
shell Fe7S8@C nanoboxes. The results mentioned above are consistent with the

composition of yolk
shell Fe7S8@C nanoboxes. As depicted in the FESEM images (Fig. 4a and b) and TEM images (Fig. 4c and d), the yolk
shell nanostructure is well preserved after sulfidation as a result of the sufficiently large void space between the Fe3O4 core and the carbon shell. To get detailed information on

Fig. 4. (a,b) FESEM images, (cee) TEM and HRTEM images, and (f) STEM image and corresponding elemental mapping images of the yolk
shell Fe7S8@C nanoboxes.

Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732

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the yolk
shell Fe7S8@C nanoboxes, HRTEM was used to study their microstructure. Apparently, HRTEM image (Fig. 4e) displays clear lattice fringes with a lattice spacing of 0.264 nm, corresponding to the (203) planes of a highly ordered nanocrystalline Fe7S8. Furthermore, elemental mapping images (Fig. 4f) display the uniform distribution of C (red), Fe (green), and S (purple) elements in the yolk
shell Fe7S8@C nanoboxes. As a comparison, Fe7S8@C nanocubes were successfully synthesized, as demonstrated by XRD (Fig. S6a). FESEM and TEM images (Figs. S6bed) indicate that the carbon shells of Fe7S8@C nanoboxes are seriously broken because of the insufficient void space to withstand the huge volume expansion during the sulfidation process. Fig. 5 displays the excellent electrochemical properties of the uniform yolk
shell Fe7S8@C nanoboxes serving as an anode in LIBs, which are in full compliance with our expectations. CV (Fig. 5a) was first used to evaluate the electrochemical performance. The pronounced cathodic peaks at 1.68, 0.77 and 1.26 V in the first cycle can attributed to the initial reduction of Fe7S8 to Fe (Eq. (1)), the generation of SEI film, and the conversion reaction to produce Li2S (Eq. (2)), respectively. The strong anodic peak centred at 1.86 V is related to the oxidation of Fe to create Li2FeS2 (Eq. (3)) [57]. The peak centred at 2.35 V can be ascribed to the formation of Li2-xFeS2 (Eq. (4)), and it gradually disappears in the subsequent cycles [58]. There is no significant loss of area in the next cycles, revealing the superior cycling stability of the uniform yolk
shell Fe7S8@C nanoboxes. It is noteworthy that all the redox peaks in the CV curves exactly match the charge
discharge plateaus in the following galvanostatic measurements (Fig. 5b). The related electrochemical reactions could be described as [49]: Discharge process: Fe7S8 þ 8Liþ þ 8e
/ 4Li2FeS2 þ 3Fe0

(1)

Li2FeS2 þ 2Liþ þ 2e
/ 2Li2S þ Fe0

(2)

5

Charge process: 2Li2S þ Fe0 / Li2FeS2 þ 2Liþ þ 2e


(3)

Li2FeS2 / Li2-xFeS2 þ xLiþ þ xe


(4)

Remarkably, the yolk
shell Fe7S8@C nanobox electrode delivers first charge and discharge capacities of 1186.4 and 1553.1 mA h g
1, respectively, with an initial Coulombic efficiency of 76.4%. The formation of the SEI film, as well as the decomposition of the electrolyte, results in the irreversible capacity loss of 23.6%. The capacity of the yolk
shell Fe7S8@C nanobox electrode gradually stabilizes after the first cycle, demonstrating that the redox reactions in the composite electrode have good stability and reversibility. The rate performance of the yolk
shell Fe7S8@C nanobox electrode is measured at different current densities ranging from 0.1 to 5 A g
1 (Fig. 5c). The reversible specific capacities of the yolk
shell Fe7S8@C nanobox electrode are 831.0, 706.2, 596.5, 533.6, 468.6, and 354.8 mA h g
1 at current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A g
1, respectively. More significantly, the reversible specific capacity fully recovers to 831.5 mA h g
1 with the current density returning to 100 mA g
1 after 60 cycles, confirming an outstanding electrochemical stability. The good rate performance of the LIBs suggests that the uniform yolk
shell Fe7S8@C nanoboxes facilitate the continuous intercalation/extraction of Liþ during the discharge and charge processes. To demonstrate the effect of the yolk
shell nanostructure, we investigated the long-term cycling stability of the yolk
shell Fe7S8@C nanobox electrode and the Fe7S8@C nanocube electrode at the same current rate. As Fig. 5d shows, the Fe7S8@C nanocube electrode has a much lower capacity value and a more serious capacity decay compared with the yolk
shell Fe7S8@C nanobox electrode. The yolk
shell Fe7S8@C nanobox electrode can provide a specific capacity of 801.3 mA h g
1 after 100 cycles and 765.5 mA h g
1 after 200 cycles, while the capacity of the Fe7S8@C nanocube

Fig. 5. Electrochemical properties of the uniform yolk
shell Fe7S8@C nanobox electrode towards Liþ ion storage. (a) CV curves of the yolk
shell Fe7S8@C nanobox electrode at a scan rate of 0.1 mV s
1. (b) Galvanostatic charge-discharge curves of the yolk
shell Fe7S8@C nanobox electrode at a current density of 0.1 A g
1. (c) Rate capability of the yolk
shell Fe7S8@C nanobox electrode. (d) Cycling stability and the corresponding Coulombic efficiency of the Fe7S8@C nanocube electrode and the yolk
shell Fe7S8@C nanobox electrode at 0.1 A g
1.

Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732

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electrode fades rapidly, with only 188.2 mA h g
1 left after 100 cycles under the same conditions. EIS analysis demonstrates that the yolk
shell Fe7S8@C nanobox electrode shows a smaller highfrequency semi-circle diameter than the Fe7S8@C nanocube electrode, indicating a lower solid-state interface resistance (Fig. S7). The uniform yolk
shell Fe7S8@C nanoboxes have better lithium storage properties in terms of cycling stability and rate performance than many other metal sulfide anode materials (Table S1) reported in the literature, indicating the success of our synthetic strategy in improving the electrochemical performance. Although the electrochemical test results mentioned above are very attractive, they do not make our uniform yolk
shell Fe7S8@C nanoboxes stand out from the vast majority of existing anode materials for LIBs. For fully unveiling the structural advantages of the yolk
shell Fe7S8@C nanoboxes, we further systematically evaluated their electrochemical performance towards Naþ ion storage and Kþ ion storage, which is more challenging than Liþ ion storage. Fig. 6 demonstrates the extraordinary electrochemical behaviour of the uniform yolk
shell Fe7S8@C nanoboxes serving as an anode in SIBs. Fig. 6a shows the CV curves of the yolk
shell Fe7S8@C nanobox electrode. During the initial cathodic scan, a broad peak centred at 0.72 V was caused by the formation of the SEI layer and Na2FeS2 (Eq. (5)). A strong peak at 0.56 V can be attributed to the conversion reaction of Fe7S8 to the mixture of Na2S and Fe (Eq. (6)). The peaks shown at 1.42 V, 1.61 V and 2.17 V are ascribed to the formation of Na2FeS2 (Eq. (7)) and Fe7S8 (Eq. (9)) in the anodic scan, respectively. The CV curves are fully consistent in the ensuing cycles, showing a highly reversible and stable desodiation/sodiation process. The related electrochemical reactions could be described as [49]: Discharge process: Fe7S8 þ 8Naþ þ 8e
/ 4Na2FeS2 þ 3Fe0

(5)

Na2FeS2 þ 2Naþ þ 2e
/ 2Na2S þ Fe0

(6)

Charge process: 2Na2S þ Fe0 / Na2FeS2 þ 2Naþ þ 2e


(7)

Na2FeS2 / Na2-x FeS2 þ xNaþ þ xe


(8)

4Na2FeS2 þ 3Fe0 / Fe7S8 þ 8Naþ þ 8e


(9)

The galvanostatic charge-discharge profiles of the uniform yolk
shell Fe7S8@C nanobox electrode for SIBs are depicted in Fig. 6b. The first charge and discharge capacities are 789.3 and 1076.2 mA h g
1, respectively. An irreversible capacity loss of 26.7% stems from the generation of the SEI film and the decomposition of the electrolyte. During the entire measurement after the first cycle, the charge
discharge voltage profiles gradually overlap, proving the excellent cycling reversibility of the electrochemical properties in the yolk
shell Fe7S8@C nanobox electrode, which is mainly ascribed to the synergetic effect between the inner Fe7S8 core and the outer carbon layer framework. In addition, the charge
discharge plateaus are highly consistent with the redox peaks in the CV curves mentioned above. FESEM images and TEM images (Fig. S8) show that the yolk
shell structure of Fe7S8@C nanobox can still be well maintained after the fifth cycle, which can account for the good electrochemical performance. The rate performance of the yolk
shell Fe7S8@C nanoboxes was further investigated at various current densities (Fig. 6c). The average specific capacities were measured to be 750.4, 667.2, 545.5, 481.4, 406.9, and 322.8 mA h g
1 at the current densities of 0.1, 0.2, 0.5, 1, 2, and 5 A g
1, respectively. Impressively, a stable capacity of 619.9 mA h g
1 was restored when the current density returned to 0.1 A g
1, retaining 83% of the initial capacity before the following higher rate test and indicating extraordinarily high cycling stability. Fig. 6d compares the cycling performance of the yolk
shell Fe7S8@C nanobox electrode and the Fe7S8@C nanocube electrode at 0.1 A g
1. The yolk
shell Fe7S8@C nanobox electrode undergoes relatively less capacity decay and maintains a stable capacity of

Fig. 6. Electrochemical properties of the uniform yolk
shell Fe7S8@C nanobox electrode towards Naþ ion storage. (a) CV curves of the yolk
shell Fe7S8@C nanobox electrode at a scan rate of 0.1 mV s
1. (b) Galvanostatic charge
discharge curves of the yolk
shell Fe7S8@C nanobox electrode at 0.1 A g
1. (c) Rate capability of the yolk
shell Fe7S8@C nanobox electrode. (d) Cycling stability and the corresponding Coulombic efficiency of the Fe7S8@C nanocube electrode and the yolk
shell Fe7S8@C nanobox electrode at 0.1 A g
1.

Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732

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547.6 mA h g
1 after 100 cycles and 491.6 mA h g
1 after 200 cycles (Fig. 6d). Compared to the yolk
shell Fe7S8@C nanobox electrode, the Fe7S8@C nanocube electrode shows a rapid capacity loss, with a charge capacity of 409.3 mA h g
1 after the 2nd cycle but only 179.5 mA h g
1 after 100 cycles. This consequence can be attributed to the collapse of carbon shells during sulfidation, resulting in agglomeration of the unencapsulated Fe7S8 particles and continuous formation of unstable SEI films in the Fe7S8@C nanocube electrode. EIS measurements demonstrate that the yolk
shell Fe7S8@C nanobox electrode shows a smaller diameter of the highfrequency semi-circle than the Fe7S8@C nanocube electrode, indicating enhanced electrode kinetics (Fig. S9). Fig. S10 shows the cycling performance of the yolk
shell Fe7S8@C nanobox electrode with a higher mass loading of active materials (1.5 mg cm
2). Notably, the SIBs can still exhibit a large reversible capacity of 532.1 mA h g
1 and a high average Coulombic efficiency over 98% after 100 cycles at 0.1 A g
1. The excellent electrochemical properties of the yolk
shell Fe7S8@C nanobox electrode are comparable or even superior to other metal sulfide anode materials (Table S2) and other energy storage materials (Table S4) previously reported. Fig. 7a demonstrates the galvanostatic charge-discharge profiles of the uniform yolk
shell Fe7S8@C nanobox electrode for PIBs. The yolk
shell Fe7S8@C nanobox electrode delivers the first charge and discharge capacities of 373.1 and 664.7 mA h g
1, respectively, with an initial Coulombic efficiency of 56.1%. The Coulombic efficiency in the second and third circles rise to 86.9% and 90.3%, respectively, indicating a gradual stable charge and discharge process. The rate performance of the yolk
shell Fe7S8@C nanobox electrode is depicted in Fig. 7b. The average specific capacities were measured to be 224.3, 169.9, 118.8, and 83.6 mA h g
1 at the current densities of 0.1, 0.2, 0.5, and 1 A g
1, respectively (Table S3). A stable capacity

7

of 169.1 mA h g
1 was retained when the current density decreased back to 0.1 A g
1. Irreversible capacity loss was caused by the excessive radius of Kþ ions, making it difficult to achieve depotassiation/potassiation in the electrode. The yolk
shell Fe7S8@C nanobox electrode delivers a reversible capacity of 110.3 mA h g
1 after 50 cycles (red solid sphere in Fig. 7c) and it falls to 86.9 mA h g
1 after 100 cycles, showing a moderate capacity for Kþ storage. The Coulombic efficiency can reach 98% or more. The high Coulombic efficiency of this composite provides a reliable prospect for further modification and enhancement of its storage properties for Kþ ions. To study the effect of different concentrations of the electrolyte on the capacity of the yolk
shell Fe7S8@C nanobox electrode, we also tested it in 0.4 M and 0.6 M KPF6 electrolytes. The electrode exhibited capacities of 38.8 mA h g
1 and 62.9 mA h g
1 after 100 cycles, respectively (Fig. 7c). Compared to the electrode in 0.8 M KPF6 electrolyte, the electrode has low capacity and bad cyclic stability in the 0.4 M and 0.6 M KPF6 electrolytes. In the highly concentrated electrolyte, it is easier to form a denser and thinner SEI layer. The robust SEI layer can protect the active material from further reaction with the electrolyte and buffer the volume expansion during charge and discharge processes, resulting in good K-storage properties for the yolk
shell Fe7S8@C nanoboxes [59]. Furthermore, the type of electrolyte is also critical for stabilizing PIB anodes [60]. To achieve a high capacity and excellent cycling stability, we also tested the electrochemical performance of the yolk
shell Fe7S8@C nanobox electrode in the KFSI electrolytes. The results show that 3 M KFSI outperformed 2 M and 4 M KFSI for potassium storage. Specifically, the yolk
shell Fe7S8@C nanobox electrode retained reversible capacities of 77.8, 97.8, and 87.5 mA h g
1 after 100 cycles in 2 M, 3 M, and 4 M KFSI, respectively

Fig. 7. Electrochemical performance of the uniform yolk
shell Fe7S8@C nanobox electrode towards Kþ ion storage. (a) Galvanostatic charge-discharge curves of the yolk
shell Fe7S8@C nanobox electrode at 0.1 A g
1. (b) Rate capability of the yolk
shell Fe7S8@C nanobox electrode. (c) Cycling stability and the corresponding Coulombic efficiency of the yolk
shell Fe7S8@C nanobox electrode in 0.4, 0.6, and 0.8 M KPF6 electrolytes at 0.1 A g
1, respectively.

Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732

8

W. Weng et al. / Journal of Alloys and Compounds xxx (xxxx) xxx

(Fig. S11). In addition, compared to the 0.8 M KPF6 electrolyte, the 3 M KFSI electrolyte enhanced the cyclic stability of the yolk
shell Fe7S8@C nanobox electrode. This result is consistent with that reported in the previous literature [61,62]. 4. Conclusion In summary, we have reported uniform yolk
shell Fe7S8@C nanoboxes as the general host material for alkali metal ions for the first time. The as-prepared uniform yolk
shell Fe7S8@C nanoboxes enable the durable and rapid storage of Liþ, Naþ, and Kþ ions. For LIBs tests, reversible specific capacities can reach up to 801.3 mA h g
1 after 100 cycles and 765.5 mA h g
1 after 200 cycles. For SIBs tests, the electrode can maintain a stable capacity of 547.6 mA h g
1 after 100 cycles and 491.6 mA h g
1 after 200 cycles. Furthermore, the composite also exhibits satisfactory Kþ ion storage. It retains a high capacity of 110.3 mA h g
1 after 50 cycles in 0.8 M KPF6 as the electrolyte. Inspiringly, it can retain a stable capacity of 97.8 mA h g
1 even after 100 cycles in the 3 M KFSI electrolyte. The remarkable electrochemical behaviour of the composite is ascribed to several factors, including the high theoretical specific capacity of Fe7S8, nanosized Fe7S8 active particles, the yolk
shell structure that buffers the large volume expansion, and the carbon shell that enhances both the conductivity and structural integrity. The gradual decrease in the capacity of Liþ, Naþ and Kþ is due to the increase in their ionic sizes. Such electrochemical performances for alkali metal ion storage are extraordinary and superior to other metal sulfides, making this material very suitable for novel energy storage. Acknowledgements This work was supported by the National Natural Science Foundation of China (51577094), the Natural Science Foundation of Jiangsu Province of China (BK20180086), and the 100 Talents Program of Nanjing Normal University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152732. References [1] Y. Xu, C. Zhang, M. Zhou, Q. Fu, C. Zhao, M. Wu, Y. Lei, Highly nitrogen doped carbon nanofibers with superior rate capability and cyclability for potassium ion batteries, Nat. Commun. 9 (2018) 1720. [2] J.H. Zhou, L. Wang, M.Y. Yang, J.H. Wu, F.J. Chen, W.J. Huang, N. Han, H.L. Ye, F.P. Zhao, Y.Y. Li, Y.G. Li, Hierarchical VS2 nanosheet assemblies: a universal host material for the reversible storage of alkali metal ions, Adv. Mater. 29 (2017), 1702061. [3] H. Kim, J.C. Kim, M. Bianchini, D.H. Seo, J. Rodriguez-Garcia, G. Ceder, Recent progress and perspective in electrode materials for K-ion batteries, Adv. Energy Mater. 8 (2018), 1702384. [4] Z. Li, Y.J. Fang, J.T. Zhang, X.W. Lou, Necklace-like structures composed of Fe3N@C yolk-shell particles as an advanced anode for sodium-ion batteries, Adv. Mater. 30 (2018), 1800525. [5] Y. Zhao, J.J. Zhu, S.J.H. Ong, Q.Q. Yao, X.L. Shi, K. Hou, Z.C.J. Xu, L.H. Guan, Highrate and ultralong cycle-life potassium ion batteries enabled by in situ engineering of yolk-shell FeS2@C structure on graphene matrix, Adv. Energy Mater. 8 (2018), 1802565. [6] M.L. Mao, C.Y. Cui, M.G. Wu, M. Zhang, T. Gao, X.L. Fan, J. Chen, T.H. Wang, J.M. Ma, C.S. Wang, Flexible ReS2 nanosheets/N-doped carbon nanofibersbased paper as a universal anode for alkali (Li, Na, K) ion battery, Nano Energy 45 (2018) 346e352. [7] W.F. Miao, Y. Zhang, H.T. Li, Z.H. Zhang, L. Li, Z. Yu, W.M. Zhang, ZIF-8/ZIF-67derived 3D amorphous carbon-encapsulated CoS/NCNTs supported on CoScoated carbon nanofibers as an advanced potassium-ion battery anode, J. Mater. Chem. 7 (2019) 5504e5512. [8] S. Chu, Y. Cui, N. Liu, The path towards sustainable energy, Nat. Mater. 16 (2017) 16e22.

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Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732

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Please cite this article as: W. Weng et al., Uniform yolk
shell Fe7S8@C nanoboxes as a general host material for the efficient storage of alkali metal ions, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152732