Robust Pitaya-Structured Pyrite as High Energy Density Cathode for

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Robust Pitaya-Structured Pyrite as High Energy Density Cathode for High Rate Lithium Batteries Xijun Xu, Jun Liu, Zhengbo Liu, Jiadong Shen, Renzong Hu, Jiangwen Liu, Liuzhang Ouyang, Lei Zhang, and Min Zhu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b03530 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Robust Pitaya-Structured Pyrite as High Energy Density Cathode for High Rate Lithium Batteries Xijun Xu,†,‡ Jun Liu,†,‡,*Zhengbo Liu,†,‡ Jiadong Shen,†,‡ Renzong Hu,†,‡ Jiangwen Liu,†,‡ Liuzhang Ouyang,†,‡ Lei Zhang,§ and Min Zhu†,‡,* †

School of Materials Science and Engineering and Guangdong Provincial Key

Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou, 510641, PR China ‡

China-Australia Joint Laboratory for Energy & Environmental Materials, South

China University of Technology, Guangzhou, 510641, PR China Email: [email protected]; [email protected] §

School of Chemistry and Chemical Engineering, South China University of

Technology, Guangzhou, 510640, China

1

ACS Paragon Plus Environment

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ABSTRACT To solve the serious problems (the agglomeration of nano-Fe0, dissolution of polysulfide and low electronic conductivity of Li2S) of earth-abundant pyrite (FeS2) cathode for lithium batteries, a simple in-situ encapsulation and transformation route has been successfully developed to synthesis pitaya-structured porous carbon embedded with FeS2 nanoparticles. Due to such hierarchical architecture design, this cathode of pitaya-structured FeS2@C can effectively avoid the serious agglomeration and coarsening of small Fe nanoparticles, reduce the dissolution of polysulfide and provide superior kinetics toward lithium storage, resulting in enhanced reversibility and rate capability. Cycling in the voltage region of 1.0−3.0 V at 0.3 A g−1, the current conversion-based FeS2@C cathode displays a high and stable energy density (about 1100 Wh kg−1), ultrahigh rate capability (a reversible capability of 660, 609, 554, 499, 449, and 400 mA h g−1 at 0.2, 0.5, 1.0, 2.0, 5.0, and 10 A g−1, respectively), and stable cycling performance.

KEYWORDS: FeS2 cathode; biomimetic structure; Li-ion batteries; rate capability; conversion reaction

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Until now, the energy and power densities of commercial Li-ion batteries (LIBs) with conventional graphite anode and LiTMO2 (TM = transition metal) cathodes have approached practical upper limits.1-3 The past two decades have witnessed much progress in searching various kind of anode materials for LIBs,4-13 such as alloying type Si, Ge, Sb, Sn, and conversion type metal oxide.9-13 While for the cathode candidates, much less achievements have been obtained, which may be the nonflexible structure control of such complex LiMO2 oxides.14 Generally speaking, the suitable cathode materials for LIBs should combine the merits of high energy density, long cycling life and low cost. Over the past decade, some kind of low-cost Fe-based conversion cathode materials (e.g. the fluoride FeF3 and sulfide FeS2) have attracted researchers' intensive attention, and such king of simple compounds are possible candidates to replace the typical LiMO2 intercalation cathodes because they possess high theoretical energy densities and low cost feature.15-20 Specially, the earth-abundant pyrite FeS2 is approved with a high theoretical capacity of 894 mA h g-1, corresponding to the generation of metallic Fe and insulate Li2S (FeS2 + 4Li+ + 4e- → Fe + 2Li2S),17-20 which is much higher than that of very best intercalation LiNixCoyMnzO2 cathodes (less than 300 mA h g-1).14 During the discharging process, highly reactive nano-Fe0 and highly resistive Li2S will be formed through the conversion reaction of FeS2. As a consequence of the insulate feature of Li2S, the serious agglomeration of nano-Fe0, and the dissolution of polysulfide converted from the in-situ generated sulfur, a rapid capacity loss is normally observed.21 Modifications (e.g. microstructure engineering, carbon/conductive polymer coating) 3


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of electroactive FeS2 are necessary to improve the reversibility of this kind of cathode.22-31 Specially, fully embedding of electrode particles with uniform small size in porous conductive matrix not only promotes the facile access of Li+/e-, but also supplies sufficient space to accommodate the accumulated strain resulted from Li+ insertion/extraction.22-24 Besides, such hybrid structures can effectively avoid the serious agglomeration and coarsening of small metal nanoparticles, resulting in enhanced reversibility and rate capability.23 Herein, we demonstrate that such kind of pitaya-structured nanospheres (FeS2@C) can well improve the electrochemical performance of conversion-type FeS2 cathode. A simple organometallic chemical compound ferrocene was chosen as the sole reactant (both as iron and carbon sources), which can form homogeneously distributed Fe3O4 nanoparticles in the spherical matrix of carbon. After a convenient sulfuration treatment, these uniform pitaya-structured Fe3O4@C nanocomposites can be completely transformed into FeS2@C, with pitaya-structured framework well retained. Such 3D porous vesica-like carbon matrix can alleviate the large volume change, inhibit the coarsening of Fe nanoparticles, and reduce the dissolution of polysulfide. As a consequence, these pitaya-structured FeS2@C nanospheres exhibit high energy density, long cycling life, and rate capability for lithium batteries.

RESULTS AND DISCUSSION Figure 1a exhibits the detailed synthesis process of the pitaya-structured FeS2@C uniform nanospheres. Firstly, the precursor of uniform Fe3O4@C nanospheres was 4


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facilely obtained via a one-pot solvothermal decomposition route with a simple organometallic chemical compound ferrocene acted as both iron and carbon sources. During this ferrocene decomposition process, iron atoms were oxidized into magnetite and grew as nanoparticles. Accompany with the crystallization of Fe3O4, carbon-rich macromolecular organic matrix derived from cyclopentadienyl rings in ferrocene was full coated on these nanoparticles via in-situ pyrolysis of cyclopentadienyl rings. Subsequently, the carbon-rich organic matrix was transformed into inorganic carbon via a simple annealing treatment, resulting in uniform pitaya-structured Fe3O4@C precursor. Scanning electron microscopy (SEM) images of these Fe3O4@C particles (Figure 1b,c) clearly display that they have a uniform spherical shape and size (about 200 nm in average diameter). The structure feature of pitaya-shaped Fe3O4@C, i.e., Fe3O4 tiny nanoparticles fully embedded in carbon sphere matrix, can be directly revealed in transmission electron microscopy (TEM) photograph (Figure 1e). The pure cubic magnetite phase (JCPDS file no. 19–0629) of the inner capsulated oxide nanoparticles was also directly confirmed by X-Ray diffractometry (XRD) analysis (Figure 1d). Hereafter, these uniform oxide-based nanospheres were used as suitable template to form pitaya-structured particles by hydrothermal sulfuration. The X-ray photoelectron spectroscopy (XPS) measurement was also performed to reveal the major component of Fe3O4@C precursor (Figure S1, in Supporting Information). As displayed in Figure S1a, the characteristic peaks of binding energies located at 284.7 eV, 531.1 eV, and 710.5 eV are associated with C 1s, O 1s and Fe 2p, respectively. In particular, the peaks located at 710.5 eV and 724.5 eV shown in Figure S1b are 5


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attributed to the Fe 2p3/2 and Fe 2p1/2, respectively,25,26 and the ones at 529.8 eV, 531.1 eV, 531.8 eV, 532.9 eV shown in Figure S1c are related to Fe-O-Fe, Fe-O, Fe-O-C and C-O, respectively.27,28 Figure S1d shows the C 1s XPS spectra of Fe3O4@C, in which the peak at 284.7 eV is related to C-C and C=C, and the ones at 286.0 eV and 288.9 eV are associated to C-O and O-C=O, respectively.25,26 As clearly displayed in Figure 2a, the XRD pattern of the final FeS2@C spheres can be assigned to pyrite FeS2 (JCPDS no. 42-1340), without any peaks from marcasite, greigite FeS2, sulfur or other impurities. The broadening intensity of the diffraction peaks in Figure 2a indicates the small crystallite size of these embedded FeS2 nanoparticles，which is normally benefits fast Li+ diffusion, local strain release, and high utilization of electrode materials. The Figure S2b (in Supporting Information) shows the feature peak of 707.2 eV, associated with the FeS2. The peak of 710.5 eV is attributed to Fe 2p3/2, and the peaks of 719.9 eV and 724.5 eV are associated with Fe 2p1/2.25,29 In Figure S2c (in Supporting Information), the S 2p peak at 162.6 eV is related to FeS2,30 the peaks of 164 eV and 165.1 eV are attributed to Sx2-,31,32 while the one located at 169.2 eV is related to S in the porous vesica-like carbon frameworks.31 The XPS results could support the Fe3+ was reduced to Fe2+ and forming FeS2 in the hydrothermal process. The mass content of electroactive FeS2 in pitaya-structured FeS2@C composites was measured with thermogravimetric analysis (TGA) in O2 atmosphere, which indicates a high content of about 86.1 wt% (Figure 2b), assuming the final product was Fe2O3. The microstructure and crystalline features of these FeS2@C particles were investigated by SEM and TEM measurements, as 6


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displayed in Figure 2c-f. The FeS2@C sample obtained via solution sulfuration appears to still be individual nanospheres with much similar as that of Fe3O4@C precursor (Figure 2c). A high-magnification image (Figure 2d) displays that these spherical particles seems smooth and encapsulated a great quantity of granules. The corresponding typical TEM images of these nanocomposites are included in Figure 2e. It could be seen that a clear pitaya-like structure with dense small seeds (FeS2 particles with size in the range of 10-15 nm) fully embedded in porous vesica-like carbon frameworks (Figure S3, in Supporting Information) was actually formed (inset shows the photographic cross-section of pitaya). Such kind of pitaya-like mesoporous structure (Figure S4, in Supporting Information) possesses a well dispersion of FeS2 tiny particles in carbon buffer framework, which can alleviate the big volume variation of FeS2 during cycle. Besides, the spherical shape could endow the FeS2@C electrode with a relatively high volumetric density. Actually, the tapped density of these obtained pitaya-structured FeS2@C particles about 1.3 g cm-3, comparable to commercial LiFePO4 cathode. The highly crystallized nature of these embedded nanoparticles is also supported by high-resolution TEM (HRTEM) measurement. As displayed in Figure 2f, the clear space of the lattice fringes with 0.270 nm and 0.242 nm can be assigned to the (200) and (210) planes of the cubic FeS2, respectively. The detailed Li-ion storage performance of these pitaya-structured FeS2@C spheres was characterized as the cathode of lithium batteries. It's should be noted that the component of carbon in FeS2@C will not provide capacity in the work voltage region of 1.0-3.0 V. Figure 3a illustrates the initial cyclic voltammetry (CV) profiles of the 7


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pitaya-structured FeS2@C composites at a 0.1 mV s-1 scan rate. As shown in this figure, the initial CV curve displays a much different shape as compared with that of the following three cycles. In detailed, the first Li+ insertion CV curve shows only one strong cathodic peak located at about 1.24 V. This cathodic peak is associated with the conversion of FeS2 into Li2S and Fe, corresponding to the following reaction.21,36,38,39 FeS2 + 4Li+ + 4e- → Fe + Li2S

(1)

According previous reports, 36,39 two cathodic peaks should be emerged during the first discharge process theoretically, belonging to lithiation (eq. 2) and conversion (eq. 3) reactions. FeS2 + 2Li+ + 2e- → Li2FeS2

(2)

Li2FeS2 + 2Li+ + 2e- → Fe + Li2S

(3)

Actually, these two reactions (lithiation and conversion) are almost simultaneously happened as their potentials are very close and normally overlap with each other, thus forming only one big cathodic peak. The ex-situ XRD patterns of FeS2@C electrode during discharge/charge processes are illustrated in Figure S5 (in Supporting Information). It can be seen that when discharged to 1.5 V, FeS2 was transformed into Li2FeS2. When further discharged to 1.0 V, only one fresh peak of Li2S emerged, supporting that the Li2FeS2 was fully converted into Li2S. In the subsequent charge to 3.0 V, it can be observed the peaks of FeS2 and Fe7S8, proving that the electrode during the charge process turn back to FeS2 with a part of oxidized to Fe7S8 and S. The corresponding ex-situ Fe 2p and S 2p XPS spectra are also provided in Figure S6 and S7 (in Supporting Information), and the detailed variations of S 2p XPS spectra 8


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also support the current proposed lithiation/delithiation mechanism for FeS2. In the 1st cycle, the oxidation peaks are located at 1.82, 2.35 and 2.50 V, corresponding to these following reactions:36,39 Fe + 2Li2S → Li2FeS2 + 2Li+ 2eLi2FeS2 → Li2-xFeS2 + xLi+ + xe- (0.5 < x < 0.8) Li2-xFeS2 → 0.8ortho-FeS2 +0.2FeS8/7 + 0.175S + (2-x)Li+ + (2-x)e-

(4) (5) (6)

As the ortho-FeS2 and S are charged products in FeS2 electrode from the second cycle onwards, the discharge processes could be ascribed to a slightly different mechanism with a little different discharge profile (Figure 3b). The outstanding lithium storage properties of pitaya-structured FeS2@C nanospheres are certified by their superior cycling behavior, as depicted in Figure 3c-f. The FeS2@C cathode displays a large first discharge/charge capacity of about 800/720 mA h g-1 at 0.1 A g-1, with an initial efficiency of approximate 90%. These robust pitaya-structured FeS2@C particles are demonstrated with outstanding high-rate capacities ranging from 720 mA h g-1 at 0.1 A g-1 to 400 mA h g-1 at 10.0 A g-1 (Figure 3c,d). When the rate changes back into 0.1 A g-1, the specific capacity can be recuperated to 690 mA h g-1. Besides, well cycling stability is also obtained with this pitaya-structured FeS2@C electrode. After 100 cycles, a capacity of 614 mA h g-1 at 0.3 A g-1 (Figure 3e) and 455 mA h g-1 (Figure 3f) at 1.5 A g-1 is achieved, respectively. Such high rate capability of pitaya-structured FeS2@C is superior to recently reported researches on pyrite-based electrode for LIBs (Table S1, in Supporting Information). The discharge energy density of FeS2@C electrode initially reaches 1280 Wh kg-1 (calculated with the mass of FeS2) with a 9


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retention of 1074 Wh kg-1 after 100 cycles (Figure 4a), much as higher than LiCoO2 cathode (about 550 Wh kg-1).21 Figure 4b shows the Ragone plots for the current pitaya-structured FeS2@C and some typical cathodes. The as-obtained FeS2@C owns a ultrahigh energy density of about 1200 W h kg-1 (the Y-axis, the calculation method is shown in Supporting Information), which is much higher than that of the modified LiNi0.5Mn1.5O4, LiMn2O4, and LiFePO4 cathodes.40-43 Referring to the power density, FeS2@C still displays a comparable specific power density of nearly 10000 W kg-1 (the X-axis), which endow this FeS2-based cathode with possibility of fast charging and discharging for practical applications. To further fully investigate the capacitive behavior of the pitaya-structured FeS2@C composites, their kinetics were also analyzed with CV measurements. As clearly shown in Figure 5a, the CV profiles at different sweep rates increased from 0.2 to 2.0 mV s-1 display a much similar shape. Normally, the scan rate (v) and the measured current (i) obey the following relationship44,45 i = avb

(7)

Where a, b stands for empirical parameters. According to previous researches,36,37 the b-value of 0.5 represents a diffusion controlled behavior, whereas the value of one indicates ideal capacitive one.44 In the current work, the b-values of the pitaya-structured FeS2@C for two cathodic peaks and one anodic peak can be determined to be 0.88, 0.69 and 0.81 (by plotting log i vs. log v as displayed in Figure 5b), respectively. It can be seen that all these values of b indicate fast kinetics resulted from the pseudocapacitive effect. Furthermore, the total capacitive contribution at a 10


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given scan rate could be distinguished into two parts (the diffusion-controlled fraction k1v1/2 and capacitor-like one k2v) at a fixed potential (V) according to the following equation 47,48 i (V) = k1v1/2 + k2v

(8)

i (V)/v1/2 = k1 + k2v1/2

(9)

By simple transforming eq. 8 to

k1 and k2 can be facilely achieved by plotting i(V)/v1/2 vs. v1/2, and thus the capacitive current ic(V) = k2v could be extracted from the total one with the value of k2.48 For example, at a give 0.6 mV s-1 sweep rate, the CV profile for the capacitive current compared with that of the total measured current is shown in Figure 5c, in which 80.5% is quantified as capacitive. Based on the same method, all contribution ratios of the capacitive capacity at other different sweep rates (increased from 0.2 to 2.0 mV s-1) were also determined. Figure 5d summarizes contributions of the pseudocapacitive behaviors at various scan rates. The contributions are 77.9%, 79.2%, 80.5%, 83.4%, 84.1%, 87.0%, and 90.5% at the scan rates of 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, and 2.0 mV s-1, respectively. These results clearly show that the pseudocapacitive charge-storage amount does occupy a high portion of the whole capacity. Based on the aforementioned microstructure characterization and the above analysis, the superior Li+ storage performance of the current FeS2@C could be closely related to the well-defined 3D architecture, as illustrated in Figure 5e. The 3D porous vesica-like carbon framework can effectively alleviate the large volume change, inhibit the coarsening of Fe nanoparticles, and reduce the dissolution of polysulfide. To confirm 11


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this, further SEM measurement after 100 cycles at 0.3 A g-1 was carried out. As exhibited in Figure S8 (Supporting Information), the 3D fully encapsulated spherical framework can also be basically maintained, indicating that the well stable electrode structure. To strongly support the claim of enhancing electrochemical performance through such kind of structure engineering, the Na-ion storage performance of the current FeS2@C electrode were also investigated. Both CV (Figure S10a, Supporting Information) and galvanostatic charge/discharge (Figure 6a) measurements show that a little larger irreversible specific capacity in the 1st cycle (122 vs. 80 mA h g-1). As clearly exhibited in Figure 6a, the pitaya-structured FeS2@C displays an initial discharge/charge capacity of about 701/579 mA h g-1 at 0.1 A g-1. After the first cycle, both CV and voltage-capacity profiles nearly overlap each other, indicating a good cycling performance. Figure 6b displays a long cycling performance of pitaya-structured FeS2@C at 0.6 A g-1 after activation with low rates of 0.1 and 0.3 A g-1. It is clearly that a relatively high specific capacity of around 415 mA h g-1can be well maintained during the whole 100 cycles, while the rate capability testing also shows that these structure-stable electrode has a well performance. As clearly shown in Figure 6c and d, a high and stable reversible capability of 580, 502, 463, 422, 374, and 307 mA h g−1 at 0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 A g-1, respectively, was achieved for Na-ion storage. The Na-storage performance of this pitaya-structured FeS2@C is also superior other FeS2-based electrodes.49-51 The electrochemical impedance spectroscopy (EIS) spectra of pitaya-structured FeS2@C for Li-ion and Na-ion half 12


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cell was provided in Figure S11 (in Supporting Information), directly revealing a higher diffusion resistance for Na-ion half cell. As compared with that of Li-ion storage, the electrochemical performance of these pitaya-structured FeS2@C for Na-ion storage is a little inferior, which may related to the larger radius of Na+ (0.9 vs. 0.65 Å) influenced the diffusion rate and the higher diffusion resistance .52-54

CONCLUSION In summary, we have fabricated pitaya-structured nanocomposites with conversion-type FeS2 nanoparticles embedded in porous vesica-like carbon frameworks via a facile in-situ encapsulation route, which consists of a first solvothermal treatment and a subsequent sulfuration process. The resultant uniform pitaya-structured nanohybrid cathode achieves a stable cycling stability and high rate capability for lithium batteries due to its bionic structure and desirable composition. Remarkably, they demonstrate an ultrahigh and stable specific energy density of about 1100 W h kg-1 at 300 mA g-1, which is much larger than that of the typical LiNi0.5Mn1.5O4, LiMn2O4, and LiFePO4 cathodes. The current simple synthesis approach opens up a way to synthesize other sulfides-based composites for energy storage systems.

METHODS Preparation of pitaya-structured FeS2@C The pitaya-structured FeS2@C composite was prepared via a typical solvothermal 13


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method. First, 0.75 g ferrocene was dissolved in the 75 mL acetone with continuous stirring for 30 min. Then, the mixture was poured into Teflon-lined autoclave and moved to an oven at 210 ºC for 24 h. The obtained precipitate was centrifuged with ultrapure water and ethanol three times; the collected power was dried at 80 ºC overnight. To get the Fe3O4@C, 0.3 g collected precursor powder was annealed at 500 ºC for 4 h. To prepare the FeS2@C, 0.2 g Fe3O4@C and 0.4 g thioactamide (TAA ) were added into Teflon-lined autoclave and dissolved with 80 mL ultrapure water, then maintained at 200 ºC for 15 h.55-58 Finally, the obtained products was centrifuged with ultrapure water and ethanol three times and dried at 80 ºC overnight under vacuum condition to get the FeS2@C composite. Materials characterization The X-ray diffraction patterns of these obtained Fe3O4@C and FeS2@C were recorded by a Rigaku-DMax 2400 diffractometer equipped with graphite monochromatized Cu-Kα radiation flux at a scanning rate of 0.02 °s–1. The morphology and microstructure information was characterized by Carl Zeiss Supra 40 scanning electron microscope, while TEM and HRTEM were characterized with JEM-2100 microscope. The content of FeS2 in the FeS2@C composites was determined by a thermogravimetric analysis in air atmosphere from 40 °C to 700 °C at a rate of 10 °C min-1 by using a TGA system (TG 209 F3 Tarsus). The nitrogen adsorption/desorption isotherms of pitaya-structured FeS2@C spheres were measured at 77 K with a Quadrachrome adsorption instrument. XPS analysis was recorded on an XPS/ESCA (electron spectroscopy for chemical analysis) instrument (Axis Ultra 14


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DLD from Kratos Inc.) using a monochromatized Al Kα X-ray source. Electrochemical measurements The homogenous slurry was prepared by mixing FeS2@C, polyvinylidene fluoride (PVDF) and Super P with a weight ratio of 80 : 10 : 10 in a mortar with appropriate N-methy1-2-pyrrolidone (NMP). Then the slurry was coated on Cu foil and transferred to a vacuum oven 80 ºC for 12 h. The Li//FeS2@C batteries were assembled in the glove box (with the O2 and H2O < 0.1 ppm) with Celgard2400 as separator and 1 M lithium bis-trifluoromethanesulfonylimide (LiTFSI) dissolved in 1,3-dioxolane (DOL)/diglyme (DME) (1:1 in volume) as electrolyte. For the Na//FeS2@C batteries, 1 M NaCF3SO3 in DME was adopted as electrolyte and Whatman GF/D was used as separator. The galvanostatic charge-discharge tests were performed on CT2001A, LAND test system, with cutoff potentials from 1.0 V to 3.0 V for Li-ion batteries and 0.5 V to 3.0 V for Na-ion batteries. Cyclic voltammetry (CV) profiles were carried out on Gamry Interface 1000 workstation with the voltage region from 1.0–3.0 V and the scan rate from 0.1 mV s-1 to 2 mV s-1.

ACKNOWLEDGMENTS This work was supported by the National key research and development program (no. 2016YFA0202603), Innovative Research Groups of the National Natural Science Foundation of China (no. NSFC51621001), the “1000 plan” from Chinese Government, and the Project of Public Interest Research and Capacity Building of Guangdong Province (no. 2017A010104004). 15


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Supporting Information available Supplementary TEM images of porous vesica-like carbon framework, BET data of uniform pitaya-structured FeS2@C nanospheres, SEM images of pitaya-structured FeS2@C cathode after 100 cycles, ex-situ XPS and XRD results of FeS2@C cathode during

charge/discharge

processes,

electrochemical

performance

of

the

pitaya-structured FeS2@C electrode for SIBs. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. (a) Schematic illustration of the formation process of pitaya-structured FeS2@C nanospheres via a facile in-situ encapsulation and sulfuration route; (b,c) low- and high-magnification images of uniform pitaya-structured Fe3O4@C precursor with a average diameter of 200 nm; (d) XRD pattern of pitaya-structured precursor showing the pure phase of well-crystallized Fe3O4; (e) TEM image of a typical Fe3O4@C nanosphere clearly displaying small Fe3O4 nanparticles are fully embedded in carbon framework.

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Figure 2. (a) XRD pattern of the pure phase of pitaya-structured FeS2@C products; (b) TGA curve of pitaya-structured FeS2@C nanospheres under O2 atmosphere from room temperature to 700 °C, indicating the carbon content is about 13.9%; (c,d) lowand high-magnification SEM images of FeS2@C products showing that they well retain the same spherical morphology of Fe3O4@C precursor; (e,f) TEM and HRTEM images of pitaya-structured FeS2@C nanospheres showing that FeS2 nanoparticles fully embedded in the amorphous carbon framework, the inset of Figure 2e shows a photo image of the cross section of a pitaya. 26


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Figure 3. Superior Li-ion storage performance of pitaya-structured FeS2@C nanospheres: (a) CV curves at a scanning rate of 0.1 mV s−1 in the voltage range of 1.0–3.0 V; (b) the first three voltage-capacity curves at 0.1 A g-1; (c) voltage-capacity curves of FeS2@C electrode at different rate (from 0.1 A g-1 to 10 A g-1); (d) rate performance at different rates (from 0.1 A g-1 to 10 A g-1); (e,f) long cycling performance of FeS2@C cathode at 0.3 A g-1 (e) and 1.5 A g-1 (f).

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Figure 4. (a) Discharge energy density vs cycling number at the material level (∼Wh/kg-FeS2); (b) ragone plots of typical cathode materials in LIBs and FeS2@C cathode in this work.

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Figure 5. (a-d) Kinetic analysis of the anode of pitaya-structured FeS2@C nanospheres: (a) CV curves with scan rates from 0.2 to 5 mV s-1; (b) CV peak current (Ip) logarithmically potted as a function of the sweep rates (v) to give the slops (b); (c) the separation of the capacitive and diffusion currents at a scan rate of 0.6 mV s-1, the capacitive contribution to the total current is shown by the shaded region; (d) the contribution ratio of the capacitive and diffusion controlled charge vs. scan rate; (e) schematic

illustration

of

the

pitaya-structured

nanospheres

during

lithiation/delithiation processes. 29


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Figure 6. Na-ion storage performance of pitaya-structured FeS2@C nanospheres: (a) the first three voltage-capacity curves at 0.1 A g-1; (b) long cycling performance of pitaya-structured FeS2@C at 0.6 A g-1 after activation with low current densities of 0.1 and 0.3 A g-1; (c) voltage-capacity curves of pitaya-structured FeS2@C nanospheres at different rate (from 0.1 A g-1 to 3.2 A g-1); (d) rate performance at different rates (from 0.1 A g-1 to 3.2 A g-1).

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Robust Pitaya-Structured Pyrite as High Energy Density Cathode for

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