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Confined Synthesis of FeS2 Nanoparticles Encapsulated in Carbon Nanotube Hybrids for Ultrastable Lithium-Ion Batteries Lei Xu, Yanjie Hu, Haoxuan Zhang, Hao Jiang,* and Chunzhong Li* Key Laboratory for Ultrafine Materials of the Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: To address the large volume change and polysulfide dissolution of FeS2-based materials for lithium-ion batteries (LIBs), we demonstrate the synthesis of FeS2 nanoparticles encapsulated in carbon nanotubes (CNTs) by a confined reaction. There is sufficient void space between adjacent FeS2 nanoparticles for guaranteeing the highly structural integrity. The resultant FeS2/CNT hybrids, when served as anode materials for LIBs, predictably exhibit a very stable capacity retention of 800 mAh g−1 over 200 cycles at 200 mA g−1. Even at 2000 mA g−1, they still deliver high-rate and long-life performance with a high specific capacity of 525 mAh g−1 after 1000 cycles. Such a kind of encapsulated structure is helpful for enhancing rate capability and cycling stability in LIBs applications. Importantly, the present confined reaction strategy can be extensively applied to synthesize other analogous hybrids for energy storage and conversion. KEYWORDS: FeS2, Confined synthesis, Encapsulated structure, Lithium-ion batteries

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INTRODUCTION To date, metal sulfides have been extensively investigated for energy storage and conversion in view of their similar electrochemical mechanism to metal oxides as well as their abundance in nature and affordable cost.1−4 Among different metal sulfides, pyrite iron sulfide (FeS2) has drawn considerable attention for its high theoretical capacity (894 mAh g−1) and small environmental impact.5−7 Therefore, pyrite FeS2 has been taken as one of the most prominent electrode materials for lithium-ion batteries (LIBs). Nevertheless, the proceeding of FeS2 anode materials has been hindered by the poor conductivity and large volume change during cycling process, resulting in unsatisfied rate ability and low capacity retention. In addition, the polysulfide (Li2Sn, n ≥ 2) generated upon lithiation can be easily dissolved into the electrolytes with shuttling effect like Li-sulfur batteries, which will lead to a continuous reversible capacity fading.8−10 To avoid these issues, the hybridization of FeS2 with carbon has been considered as an effective strategy.11,12 Meanwhile, the introduction of carbon is also favorable to accelerate the electrons transfer and partly buffer the volume change in process of the electrochemical reactions. For example, FeS2 nanoparticles sandwiched in carbon matrix,13 FeS2 cubes attached on graphene oxide sheets,14 FeS2 microspheres mixed with carbon nanotubes,15 etc. These materials were proved to partly improve the electrochemical performance, but the cycle life is still unsatisfactory. Recently, some works focused on realizing the incorporation of active materials into carbon frameworks with large internal void space.16−19 For instance, Wu et al. synthesized nano© XXXX American Chemical Society

architecture CoSnO3/graphene composite with interconnected pores via an assembly method, delivering an excellent specific capacity of 923 mAh g−1 at 500 mA g−1 and superior capacity retention of 566 mAh g−1 at 2000 mA g−1 even after 1500 cycles.20 Our group also fabricated heterostructure MnO/C nanopeads with enough void space by a facile route, which showed a high reversible capacity of 1119 mAh g−1 at 500 mA g−1 and a long cycle life of 525 mAh g−1 at 2000 mA g−1 for 1000 cycles.21 It can be found that such hybrids can not only deliver a high specific capacity, but also show a satisfied cycling stability. However, it is still a great challenge to controllably synthesize the incorporated hybrids. Confined reaction provides the possibility of constructing such smart nanostructures because it can render the chemical reaction and subsequent crystal growth confined in a designated space, and therefore, can precisely control materials’ microstructure. Herein, we demonstrate the confined synthesis of FeS2 nanoparticles encapsulated in carbon nanotubes (CNTs) hybrids with sufficient void space between the adjacent core nanoparticles. When served as LIBs anode materials, our FeS2/ CNT hybrids display several intriguing merits. (a) The sufficient internal void space between the adjacent FeS2 nanoparticles can effectively mitigate the large volumetric expansion of FeS2 nanoparticles upon lithiation, thus keeping the structural integrity to enhance the cycling stability significantly. (b) The CNTs can not only increase overall Received: April 11, 2016 Revised: July 2, 2016

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DOI: 10.1021/acssuschemeng.6b00741 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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cavity of CNT with the assistance of capillary forces to form Fe(NO3)3/CNT. When annealing at 400 °C for 8 h in Ar/S atmosphere, the precursor will react with S in the inner space of CNT, which results in the generation of FeS2 nanoparticles only in the CNT inner, forming the FeS2/CNT hybrids. Finally, the resultant FeS2/CNT hybrids were washed and heated to remove external FeS2 nanoparticles and unreacted sulfur powder. The SEM images of as-obtained products (Figure S1) show that no obvious FeS2 nanoparticles and unreacted sulfur can be observed. The TEM image (Figure 2a) further

electrical conductivity but also validly suppress the dissolution of polysulfide into electrolytes. (c) The confined reaction strategy can realize the accurate control of FeS2 nanoparticles size. As a consequence, the as-synthesized FeS2/CNT hybrids exhibit a remarkably enhanced specific capacity with superior rate performance and ultralong lifespan.

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EXPERIMENTAL SECTION

Synthesis of FeS2/CNTs Hybrids. First, 0.1 g of acid-treated CNTs and 0.8 g of Fe(NO3)3 were dispersed in 8 mL concentrated nitric acid solution by ultrasonication for 1 h. Subsequently, the compound was refluxed at 100 °C for 9 h with constantly stirring and cooled down naturally. Then the sediments were gathered by centrifugation and dried at 100 °C for 20 h. The resulting products and sulfur powder were placed in two separate Al2O3 crucibles in pipe furnace and annealed at 400 °C for 8 h in the floating argon. After that, the obtained products were dispersed in deionized water with ultrasound for 1h at room temperature and collected with a membrane filter by washing with deionized water, this process was repeated for 3 times, the collected samples were dried at 70 °C for 12 h, and subsequently heated at 200 °C under vacuum for 12 h to remove the residual sulfur. Characterization. The crystal phases of as-prepared FeS2/CNTS were studied by X-ray diffractometer (XRD) with Cu Kα radiation. Scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, JEOL-2100) were performed at 200 kV with an X-ray energy dispersive spectrometer (EDS) to investigated microscopic morphologies and atomic ratio. The molar ratio of elements the materials were determined by inductively coupled plasma-optical emission spectrometer (ICP-OES, 725ES). Thermogravimetric analysis (NETZSCH STA409PC) was carried out with a temperature ramp of 10 °C min−1 under air flowing. X-ray photoelectron spectroscopy (XPS) spectra were recorded with an AXIS Ultra DLD spectrometer (Al Ka X-ray source). A Raman spectrum was obtained using NEXUS 670 FT-IR Raman spectrometer at environmental temperature. Electrochemical Measurements. Electrochemical measurements were implemented by assembling coin-type 2016 cells in an argonfilled glovebox. The working electrode slurry was prepared by mixing active materials, carbon black, and poly(vinyl difluoride) (PVDF) at a mass ratio of 8:1:1. We use pure lithium foil as counter electrode and a polypropylene membrane (Celgard-2400) as separator. The electrolyte consists of a solution of 1 M LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC) (1:1 in volume). Galvanostatic charge/ discharge measurements were conducted by LANDCT2001A test system in the voltage range of 0.01−3.0 V at different current densities. Cyclic voltammetry (CV) was performed with an Autolab PGSTAT302N electrochemical workstation at a scan rate of 0.2 mV s−1 within a potential window of 0.01−3.0 V. The impedance spectra were recorded on constant voltage mode over the frequency range between 100 kHz and 0.01 Hz.

Figure 2. (a) Low- and (b) high-magnification TEM images of FeS2/ CNT hybrids. (inset) Corresponding HRTEM image. (c) XRD patterns of the as-synthesized FeS2/CNT hybrids and pure CNTs. (d) Raman spectrum of the FeS2/CNT hybrids.

indicates that most FeS2 nanoparticles are well-confined in cavities of the CNTs with negligible FeS2 nanoparticle outside. The microstructure of as-obtained materials was further characterized by high-magnification TEM (Figure 2b), observing that FeS2 nanoparticles with diameters of 5−10 nm uniformly dispersed in the CNT channel with enough void space between adjacent FeS2 nanoparticles. It is expected that such structural arrangement can effectively accommodate the distinct volume expansion to keep structural integrity. So during the lithiation/delithiation process, the FeS2 nanoparticles expand/shrink along the CNT axis without breaking the outer carbon layer, thus avoiding the structure pulverization and polysulfide dissolution. From the high resolution TEM image (the inset of Figure 2b), an obvious lattice fringe distance of 0.22 nm can be well verified, indicating the agreement with the distance of (211) plane belonging to pure FeS2. Figure 2c shows XRD patterns of the FeS2/CNT hybrids and pure CNTs. The peaks around 26° and 43° of the FeS2/CNT hybrids can be indexed to the pure CNTs, and the surplus peaks correspond to the standard patterns of the cubic iron sulfide (JCPDS No. 42-1340). No other visible impurity peaks exist, proving the successful synthesis of FeS2/CNT hybrids. Meanwhile, the Raman spectrum was used to evaluate the composition of the hybrid material. Two sharp peaks appearing at 340 and 376 cm−1 as well as a slight peak at 424 cm−1 are shown in Figure 2d, which fit well with the reported values of the Eg, Ag, and Tg modes of pyrite FeS2 (Figure S2).22,23 The absence peaks in the range of 200−300 cm−1 indicates the nonexistence of FexS or FexO in the synthesized sample, further

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RESULTS AND DISCUSSION The synthesis process for FeS2/CNT hybrids and their volume change during charge/discharge process are schematically illustrated in Figure 1. First, according to a wet chemical method, a Fe(NO3)3 aqueous solution was introduced into the

Figure 1. Schematic illustration for the confined synthesis of FeS2/ CNT hybrids and their volume change during the charge/discharge process. B

DOI: 10.1021/acssuschemeng.6b00741 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering evidencing phase purity of the target products.24 Moreover, two strong peaks at 1351 and 1582 cm−1 can be discovered, representing the disordered carbon (D-bond) and ordered graphitic carbon (G-bond) of CNTs, respectively. The ID/IG intensity ratio is figured up to be 0.85 with an estimated carbon content of ∼63.9% (Figure S3), suggesting a fairly high graphitization grade,25 which is beneficial for boosting the conductivity. To evaluate the state of Fe and S, the XPS analysis was conducted in Figure S4. The Fe 2p spectrum reveals a Fe 2p3/2 peak at 706.9 eV and a Fe 2p1/2 peak at 720.1 eV, being in good agreement with those reported for FeS2. The S 2p signal displays two peaks corresponding to S 2p3/2 (162.6 eV) and S 2p1/2 (163.9 eV), which matches well with the sulfur binding energy in pyrite.22 Meanwhile, both of the ICP and EDS results reveal that the atomic ratio of Fe/S is almost consistent with that of stoichiometric FeS2 (Figure S5). The electrochemical performance of FeS2/CNT hybrids was evaluated as anode materials for LIBs. The initial three cycles of cyclic voltammetry (CV) curves were investigated in the potential window of 0.01−3.0 V at 0.2 mV s−1, as shown in Figure 3a. In the first cathodic scan, two prominent reduction

difficult dissolution of the SEI layer.30 In the following two cycles, the discharge capacities of 797 and 775 mAh g−1 are obtained with high Coulombic efficiency of 93% and 95%, declaring the FeS2/CNT hybrids have high reversible capacity and great cycling stability. Figure 3c shows the rate performance of the FeS2/CNT hybrids at different current densities from 100 to 5000 mA g−1. The as-obtained FeS2/CNT hybrids deliver an average reversible charge capacity of 735, 650, 572, 527, 454, and 345 mAh g−1 at various current rates from 100, 200, 500, 1000, 2000, to 5000 mA g−1, respectively. More importantly, even after deep cycling at 5000 mA g−1, a high stable reversible capacity of 750 mAh g−1 can be recovered when the current density returned to 100 mA g−1, implying the superior Li+ storage reversibility. For comparison, we synthesized FeS2− CNTs mixture by a simple mechanical mixing. The FeS2 nanoparticles (Figure S6) were prepared according to the previous report31 and confirmed by XRD (Figure S7). It can be found that FeS2−CNTs mixture deliver a lower charge capacity of 572 mAh g−1 at 100 mA g−1. Particularly at a high rate of 5000 mA g−1, only 163 mAh g−1 can be retained. The remarkably enhanced capacity retention can be mainly attributed to the encapsulated structure of FeS2/CNT hybrids with enough void space for accommodating the volume expansion and the avoidance of polysulfide dissolution. To better understand the electrochemical performance of FeS2/ CNT hybrids, we particularly investigate the capacity contributions of net FeS2 based on the values of CNTs (Figure S8) and FeS2/CNT hybrids at different current rates, the results are shown in Figure 3d. The blue and red histograms are corresponding to the tested electrochemical performances of pure CNTs and the FeS2/CNT hybrids. The green histogram is the calculated values only from FeS2 in the FeS2/CNT hybrids. Notably, the calculated charge capacities contributed from FeS2 almost all much higher than its theoretical value (894 mAh g−1). This phenomenon may be ascribed to some reasons as follows: first, the strong synergistic effect between FeS2 nanoparticles and CNTs, second, the invertible formation/ dissolution of a polymeric gel-like layer resulted from electrolyte decomposition, which is common for transition metal compounds, and third, an extra capacity connected with the charge interfacial storage.21,32−34 Impressively, our FeS2/CNT hybrids also own outstanding cycling performance. Figure 4a displays the charge−discharge capacity and the relevant Coulombic efficiency at 200 mA g−1. The reversible capacity of our FeS2/CNT hybrids reaches to 800 mAh g−1 after 200 cycles with a Coulombic efficiency of nearly 100%. The gradually increased capacity in the initial 40 cycles can be mainly attributed to an activation process with full infiltration by the electrolyte.35,36 It is worthwhile that no obvious structural crack can be seen in the TEM image (the inset of Figure 4a) after 200 cycles, which manifests the structural stability and integrity of the material directly. However, FeS2−CNTs mixture reveals evident capacity decay at the same current density for 100 cycles (Figure S9). The electrochemical impedance spectrum (EIS) of the FeS2/CNT hybrids before and after 200 cycles are provided in Figure 4b. The charge transfer resistance (∼133 Ω) is a little bit smaller than that before cycling (∼145 Ω), further demonstrating the structural stability. Meanwhile, the FeS2 nanoparticles (∼320 Ω) and FeS2/CNTs mixture (∼195 Ω) (Figure S10) deliver bigger charge transfer resistances than that of FeS2/CNT hybrids, which is attributed to the hybridization with high

Figure 3. (a) First three CVs at 0.2 mV s−1. (b) Charge−discharge curves at 100 mA g−1 for the FeS2/CNT hybrids. (c) Capacity retention of the FeS2/CNT hybrids and FeS2−CNTs mixture at different current densities. (d) Capacity contributions of FeS2 based on the values of CNTs and FeS2/CNT hybrids.

peaks appear at 1.40 and 0.65 V, corresponding to insertion of Li ions into FeS2 nanoparticles (FeS2 + 4Li+ + 4e− → Fe + 2Li2S) and the irreversible formation of a solid electrolyte interlayer (SEI) membrane, respectively.26,27 In reverse anodic scan, the peak around 1.9 V is due to the formation of Li2−xFeS2, which can vest in the next two reactions: Fe + 2Li2S → Li2FeS2 + 2Li+ + 2e− and Li2FeS2 → Li2−xFeS2 + xLi+ + xe−, and the peak at 2.5 V is attributed to generation of FeSy and S.28 In the subsequent two cycles, reduction peaks appear at 2.0 and 1.4 V, which are related to two reactions: FeSy + (2 − y)S + 2Li+ + 2e− ↔ Li2FeS2 (2.0 V) and Li2FeS2 + 2Li+ + 2e− ↔ Fe + 2Li2S (1.4 V).26,29 Figure 3b shows the first three charge− discharge curves at 100 mA g−1 in the voltage range 0.01−3.0 V, the potential plateaus in charge−discharge process match well with the CV curves. The initial discharge and charge capacities of FeS2/CNT hybrids can reach 1193 and 758 mAh g−1 with a Coulombic efficiency (CE) of 63.5%. The irreversible capacity loss occurring during the first cycle may be attributed to the inadequate decomposition of Li2S and the C

DOI: 10.1021/acssuschemeng.6b00741 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Figures S1−S11 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-21-64250949. Fax: +86-21-64250624. E-mail: [email protected] (H.J.). *E-mail: [email protected] (C.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21236003, 21522602, 91534202), the Shanghai Rising-Star Program (15QA1401200), the International Science and Technology Cooperation Program of China (2015DFA51220), Program for New Century Excellent Talents in University (NCET-13-0796), the 111 Project (B14018), and the Fundamental Research Funds for the Central Universities.

Figure 4. (a) Cycling performance of the FeS2/CNT hybrids at 200 mA g−1 for 200 cycles. (inset) TEM image of the FeS2/CNT hybrids after cycles. (b) Electrochemical impedance spectra of the FeS2/CNT hybrids before and after cycling. (c) Cycling performance of the FeS2/ CNT hybrids at 2000 mA g−1 for 1000 cycles.

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conductive CNTs (Figure S11). Even at a higher constant rate of 2000 mA g−1, the hybrids still deliver long-term cycling stability with a high specific capacity of 525 mAh g−1 over 1000 cycles, as shown in Figure 4c. It is noted that the charge capacity of the hybrids shows a decreasing trend at the beginning, which may be closely related to the formation of the SEI membrane and irreversible reaction among Li, FeS2, and CNTs.37 During the subsequent cycles, the capacity rises up gradually to a maximum value of 530 mAh g−1 at the 500th cycle and presents a relatively mild change for the next 500 cycles. This phenomenon is associated with invertible growth of the polymeric gel-like layer deriving from kinetic activation38,39 and the long activation process with full infiltration by the electrolyte. Additionally, the Coulombic efficiency is nearly 100% throughout the overall cycling, also indicating a good reversibility. The excellent cycling capability should be ascribed to the embedded structure with sufficient void space, which is favor for maintaining structural integrity during cycling process and be the key for ultrastable lithium-ion batteries.

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CONCLUSIONS In summary, the FeS2 nanoparticles encapsulated in CNTs with sufficient void space has been successfully fabricated via a confined reaction strategy. The FeS2/CNT hybrids possess good conductivity, structural integrity and polysulfide preservation, which is vital for obtaining high specific capacity and great cycling stability as LIBs anodes. When compared with FeS2−CNTs mixture, the hybrids deliver higher capacity retention of 800 mAh g−1 at 200 mA g−1 for 200 cycles without any structural damage. More significantly, our hybrids also exhibits excellent rate capability and long lifespan with high specific capacity of 525 mAh g−1 even at 2000 mA g−1 after 1000 cycles. As a result, the present work sets a feasible and scalable case for the confined synthesis of encapsulated structure hybrids in energy storage and conversion.

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REFERENCES

(1) Gao, Z.; Song, N.; Zhang, Y.; Li, X. Cotton-textile-enabled, flexible lithium-ion batteries with enhanced capacity and extended lifespan. Nano Lett. 2015, 15, 8194−8203. (2) Mahmood, N.; Zhang, C.; Hou, Y. Nickel sulfide/nitrogen-doped graphene composites: phase-controlled synthesis and high performance anode materials for lithium ion batteries. Small 2013, 9, 1321− 1328. (3) Lai, C. H.; Lu, M. Y.; Chen, L. J. Metal sulfide nanostructures: synthesis, properties and applications in energy conversion and storage. J. Mater. Chem. 2012, 22, 19−30. (4) Xia, X.; Zhu, C.; Luo, J.; Zeng, Z.; Guan, C.; Ng, C. F.; Zhang, H.; Fan, H. J. Synthesis of free-standing metal sulfide nanoarrays via anion exchange reaction and their electrochemical energy storage application. Small 2014, 10, 766−773. (5) Zhang, S. S. The redox mechanism of FeS2 in non-aqueous electrolytes for lithium and sodium batteries. J. Mater. Chem. A 2015, 3, 7689−7694. (6) Hu, Z.; Zhu, Z.; Cheng, F.; Zhang, K.; Wang, J.; Chen, C.; Chen, J. Pyrite FeS2 for high-rate and long-life rechargeable sodium batteries. Energy Environ. Sci. 2015, 8, 1309−1316. (7) Yoder, T. S.; Tussing, M.; Cloud, J. E.; Yang, Y. Resilient carbon encapsulation of iron pyrite (FeS2) cathodes in lithium ion batteries. J. Power Sources 2015, 274, 685−692. (8) Liu, W. L.; Rui, X. H.; Tan, H. T.; Xu, C.; Yan, Q. Y.; Hng, H. H. Solvothermal synthesis of pyrite FeS2 nanocubes and their superior high rate lithium storage properties. RSC Adv. 2014, 4, 48770−48776. (9) Wu, B.; Song, H.; Zhou, J.; Chen, X. Iron sulfide-embedded carbon microsphere anode material with high-rate performance for lithium-ion batteries. Chem. Commun. 2011, 47, 8653−8655. (10) Rui, X.; Tan, H.; Yan, Q. Nanostructured metal sulfides for energy storage. Nanoscale 2014, 6, 9889−9924. (11) Hu, Z.; Zhang, K.; Zhu, Z.; Tao, Z.; Chen, J. FeS2 microspheres with an ether-based electrolyte for high-performance rechargeable lithium batteries. J. Mater. Chem. A 2015, 3, 12898−12904. (12) Zhang, D.; Mai, Y. J.; Xiang, J. Y.; Xia, X. H.; Qiao, Y. Q.; Tu, J. P. FeS2/C composite as an anode for lithium ion batteries with enhanced reversible capacity. J. Power Sources 2012, 217, 229−235. (13) Fei, L.; Jiang, Y.; Xu, Y.; Chen, G.; Li, Y.; Xu, X.; Deng, S.; Luo, H. A novel solvent-free thermal reaction of ferrocene and sulfur for one-step synthesis of iron sulfide and carbon nanocomposites and their electrochemical performance. J. Power Sources 2014, 265, 1−5. (14) Wen, X.; Wei, X.; Yang, L.; Shen, P. K. Self-assembled FeS2 cubes anchored on reduced graphene oxide as an anode material for lithium ion batteries. J. Mater. Chem. A 2015, 3, 2090−2096.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00741. D

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of electrode materials reacting through conversion reactions. Adv. Mater. 2010, 22, E170−E192. (35) Xue, H.; Yu, D. Y. W.; Qing, J.; Yang, X.; Xu, J.; Li, Z.; Sun, M.; Kang, W.; Tang, Y.; Lee, C. S. Pyrite FeS2 microspheres wrapped by reduced graphene oxide as high-performance lithium-ion battery anodes. J. Mater. Chem. A 2015, 3, 7945−7949. (36) Luo, J.; Liu, J.; Zeng, Z.; Ng, C. F.; Ma, L.; Zhang, H.; Lin, J.; Shen, Z.; Fan, H. J. Three-dimensional graphene foam supported Fe3O4 lithium battery anodes with long cycle life and high rate capability. Nano Lett. 2013, 13, 6136−43. (37) Chen, Y.; Song, B.; Tang, X.; Lu, L.; Xue, J. Ultrasmall Fe3O4 nanoparticle/MoS2 nanosheet composites with superior performances for lithium ion batteries. Small 2014, 10, 1536−1543. (38) Cao, K.; Jiao, L.; Liu, H.; Liu, Y.; Wang, Y.; Guo, Z.; Yuan, H. 3D hierarchical porous α-Fe2O3 nanosheets for high-performance lithium-ion batteries. Adv. Energy Mater. 2015, 5 (1−9), 1401421. (39) Liu, Z.; Yu, X. Y.; Paik, U. Etching-in-box: a novel stragtegy to synthesize unique yolk-shelled Fe3O4@Carbon with an ultralong cycling life for lithium storage. Adv. Energy Mater. 2016, 6 (1−5), 1502318.

(15) Liu, L.; Yuan, Z.; Qiu, C.; Liu, J. A novel FeS2/CNT microspherical cathode material with enhanced electrochemical characteristics for lithium-ion batteries. Solid State Ionics 2013, 241, 25−29. (16) Wang, H.; Ren, D.; Zhu, Z.; Saha, P.; Jiang, H.; Li, C. Few-layer MoS2 nanosheets incorporated into hierarchical porous carbon for lithium-ion batteries. Chem. Eng. J. 2016, 288, 179−184. (17) Choi, S. H.; Ko, Y. N.; Lee, J. K.; Kang, Y. C. 3D MoS2graphene microspheres consisting of multiple nanospheres with superior sodium ion storage properties. Adv. Funct. Mater. 2015, 25, 1780−1788. (18) Zhu, Y.; Fan, X.; Suo, L.; Luo, C.; Gao, T.; Wang, C. Electrospun FeS2@carbon fiber electrode as a high energy density cathode for rechargeable lithium batteries. ACS Nano 2016, 10, 1529− 1538. (19) Liu, J.; Wen, Y.; Wang, Y.; van Aken, P. A.; Maier, J.; Yu, Y. Carbon-encapsulated pyrite as stable and earth-abundant high energy cathode material for rechargeable lithium batteries. Adv. Mater. 2014, 26, 6025−6030. (20) Wu, C.; Maier, J.; Yu, Y. Sn-based nanoparticles encapsulated in a porous 3D graphene network: advanced anodes for high-rate and long life li-ion batteries. Adv. Funct. Mater. 2015, 25, 3488−3496. (21) Jiang, H.; Hu, Y.; Guo, S.; Yan, C.; Lee, P. S.; Li, C. Rational design of MnO/carbon nanopeapods with internal void space for highrate and long-life li-ion batteries. ACS Nano 2014, 8, 6038−6046. (22) Xu, J.; Xue, H.; Yang, X.; Wei, H.; Li, W.; Li, Z.; Zhang, W.; Lee, C. S. Synthesis of honeycomb-like mesoporous pyrite FeS2 microspheres as efficient counter electrode in quantum dots sensitized solar cells. Small 2014, 10, 4754−4759. (23) Li, L.; Caban-Acevedo, M.; Girard, S. N.; Jin, S. High-purity iron pyrite (FeS2) nanowires as high-capacity nanostructured cathodes for lithium-ion batteries. Nanoscale 2014, 6, 2112−2118. (24) Morrish, R.; Silverstein, R.; Wolden, C. A. Synthesis of stoichiometric FeS2 through plasma-assisted sulfurization of Fe2O3 nanorods. J. Am. Chem. Soc. 2012, 134, 17854−7. (25) Wang, H.; Yan, N.; Li, Y.; Zhou, X.; Chen, J.; Yu, B.; Gong, M.; Chen, Q. Fe nanoparticle-functionalized multi-walled carbon nanotubes: one-pot synthesis and their applications in magnetic removal of heavy metal ions. J. Mater. Chem. 2012, 22, 9230. (26) Shao-Horn, Y.; Osmialowski, S.; Horn, Q. C. Reinvestigation of lithium reaction mechanisms in FeS2 pyrite at ambient temperature. J. Electrochem. Soc. 2002, 149, A1547−A1555. (27) Yan, N.; Zhou, X.; Li, Y.; Wang, F.; Zhong, H.; Wang, H.; Chen, Q. Fe2O3 nanoparticles wrapped in multi-walled carbon nanotubes with enhanced lithium storage capability. Sci. Rep. 2013, 3, 3392− 3397. (28) Evans, T.; Piper, D. M.; Kim, S. C.; Han, S. S.; Bhat, V.; Oh, K. H.; Lee, S. H. Ionic liquid enabled FeS2 for high-energy-density lithium-ion batteries. Adv. Mater. 2014, 26, 7386−7392. (29) Golodnitsky, D.; Peled, E. Pyrite as cathode insertion materials in rechargeable lithium composite polymer electrolyte batteries. Electrochim. Acta 1999, 45, 335−350. (30) Jiang, F.; Yang, L. W.; Tian, Y.; Yang, P.; Hu, S. W.; Huang, K.; Wei, X. L.; Zhong, J. X. Bi-component MnO/ZnO hollow microspheres embedded in reduced graphene oxide as electrode materials for enhanced lithium storage. Ceram. Int. 2014, 40, 4297−4304. (31) Xia, J.; Jiao, J.; Dai, B.; Qiu, W.; He, S.; Qiu, W.; Shen, P.; Chen, L. Facile synthesis of FeS2 nanocrystals and their magnetic and electrochemical properties. RSC Adv. 2013, 3, 6132−6140. (32) Yu, W. J.; Zhang, L.; Hou, P. X.; Li, F.; Liu, C.; Cheng, H. M. High reversible lithium storage capacity and structural changes of Fe2O3 Nanoparticles Confined inside Carbon Nanotubes. Adv. Energy Mater. 2016, 6, 1501755−1501764. (33) Pan, L.; Zhu, X. D.; Xie, X. M.; Liu, Y. T. Smart hybridization of TiO2 nanorods and Fe3O4 nanoparticles with pristine graphene nanosheets: hierarchically nanoengineered ternary heterostructures for high-rate lithium storage. Adv. Funct. Mater. 2015, 25, 3341−3350. (34) Cabana, J.; Monconduit, L.; Larcher, D.; Palacin, M. R. Beyond intercalation-based Li-ion batteries: the state of the art and challenges E

DOI: 10.1021/acssuschemeng.6b00741 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX