sodium storage

Journal Pre-proof Encapsulating yolk-shell FeS2@carbon microboxes into interconnected graphene framework for ultrafast lithium/sodium storage Peng Jing, Qiong Wang, Boya Wang, Xu Gao, Yun Zhang, Hao Wu PII:

S0008-6223(19)31295-3

DOI:

https://doi.org/10.1016/j.carbon.2019.12.060

Reference:

CARBON 14910

To appear in:

Carbon

Received Date: 18 October 2019 Revised Date:

9 December 2019

Accepted Date: 23 December 2019

Please cite this article as: P. Jing, Q. Wang, B. Wang, X. Gao, Y. Zhang, H. Wu, Encapsulating yolkshell FeS2@carbon microboxes into interconnected graphene framework for ultrafast lithium/sodium storage, Carbon (2020), doi: https://doi.org/10.1016/j.carbon.2019.12.060. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Author contribution statement Peng Jing: Conceptualization, Methodology, Writing-Original draft preparation, Writing- Reviewing and Editing. Qiong Wang, Xu Gao, Boya Wang: Methodology, Investigation, Data Curation. Yun Zhang: Discussion and Supervision. Hao Wu: Supervision, Discussion, Writing- Reviewing and Editing.

Graphical abstract

An exquisite hierarchical yolk-shell microbox structure consisting of FeS2-enriched inner core and outer N, S co-doped carbon shell is fabricated and successfully encapsulated within three-dimensional N-doped graphene framework, which exhibits superior capability, long cycling life, and stable fast charging feature for both lithiumand sodium-ion batteries.

Encapsulating Interconnected

Yolk-shell

FeS2@Carbon

Graphene

Framework

Microboxes for

into

Ultrafast

Lithium/Sodium Storage Peng Jing,a Qiong Wang,a Boya Wang,a Xu Gao,b Yun Zhang,a and Hao Wu*,a a

Department of Advanced Energy Materials, College of Materials Science and Engineering,

Sichuan University, Chengdu, 610064, China b

College of Chemistry and Chemical Engineering, Central South University, Changsha

410083, China * Corresponding authors. E-mail: [email protected]

Abstract: Pyrite FeS2 has been accepted as one promising anode candidate for lithium(LIBs) and sodium-ion batteries (SIBs) due to its extremely high theoretical capacity of 894 mAh g-1. However, the practical capabilities including rate performance and cycling stability of FeS2-based materials are still restricted by exaggerated volume variation and sluggish ions kinetics. Herein, we upgrade an interesting dual-carbon decorated strategy to address these issues. The resultant material features a hierarchical architecture with yolk-shell FeS2@carbon microboxes as well as interconnected graphene framework (GF/FeS2@C). By virtue of the dual-carbon protection effect and binary channel for electrons/ions transfer, the GF/FeS2@C composites exhibit superior Li and Na storage performance. For LIBs, it shows superior rate capability of 455 mAh g-1 at 20 A g-1 and long-term cycle stability (755 mAh g-1 at 2 A g-1 after 400 cycles). As for SIBs, it delivers a reversible capacity of 341 mAh g-1 at high current density of 10 A g-1 and ultra-long lifespan at 0.5 A g-1 (314 mAh g-1 after 600

1

cycles) and 2 A g-1 (203 mAh g-1 after 600 cycles). Such excellent high-rate capabilities delivered by the GF/FeS2@C feature the predominant fast charging properties for potential application in portable electric technology.

Keywords: Carbon microbox; pyrite FeS2; graphene network; lithium/sodium-ion batteries; fast charging ability

1. Introduction Recently, rechargeable lithium ion batteries (LIBs), a renewable energy storage device, show extraordinary talents in the fields of electric vehicles (EVs), hybrid electric vehicles (HEVs) and electricity grids owing to their decent energy density, stability in use and environment friendly nature.[1-5] Additionally, considering the sufficient sodium source, low cost, and similar working mechanism comparing with LIBs, the renewable SIBs have also attracted extensive attention for next-generation electrical energy storage.[6-12] Although the LIBs and SIBs exhibit special advantages in various energy storage systems, their practical application still require improvement in terms of capacity, stability and charge time. To achieve these goals, one of effective strategies is to explore appropriate electrode materials which could play a dual role in both high Li-storage and Na-storage. It should be pointed out that the current commercial graphite, as the state-of-the-art anode material, not only cannot meets the requirement for the further development of LIBs due to it low theoretical capacity (372 mA h g-1) but also is unsatisfied for SIBs because the higher energy barrier caused by larger ions radius has obstructed the Na+ insert into the graphite layer, leading to a lower capacity of 34 mAh g-1.[13-15] Given this, exploring superior negative electrode materials beyond the current graphited anode for both LIBs and SIBs is imperative.

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For accessible anode candidates, transition-metal disulfides (TMSs) have captured significant concerns owing to their high capacity, robust structure, easy-synthesis, and natural abundance.[16-19] Triggered by these metrics, many TMSs (like MnS, CuS, CoS2, FeS2 and so on) with excellent electrochemical performances have been studied.[20-28] Among them, FeS2 as one member of typical TMSs with high theoretical capacity (894 mAh g-1), is recognized as the most attracted anode material.[29-31] Unfortunately, a large volume swelling has generated during the process of FeS2 reaction with Li+/Na+, which will lead to the structural pulverization and exfoliation of electrode materials, thus resulting in rapidly capacity decay.[32-34] In addition, like other conversion-type materials, FeS2 faces the challenge of sluggish electron/ions kinetics, which leading to a poor rate capability.[35-40] To ameliorate these issues and full use the host talent of FeS2, various efforts have been proposed. The most popular approach is to design anode materials with porous nanoarchitecture, such as yolk-shell, hollow sphere, core–sheath and microboxes, make it possess appropriate void space so as to tolerate the volume variation and guarantee the integrity of entire electrode during charging/discharging processes, thus enhancing the cyclic stability.[23, 24, 28, 41] For instance, Xu et al reported a carbon nanotube encapsulated nanoFeS2 hybrids for LIBs application.[42] By confining the electrochemical reaction of FeS2 in semi-enclosed zones, the final FeS2 anode exhibited a long lifespan with high specific capacity of 525 mAh g-1 after 1000 cycles at 2 A g-1. Even though, the energy storage performance especially the fast charging capability are still need to be enhanced for the portable electric vehicles. Tuning the surface of the afore-mentioned architecture by using conducting polymer with electronic activity (e.g. graphene, CNTs, and functionalized carbon) is expected to boost charging process and improve cycle stability.[43-45] However, this method has rarely been applied to FeS2-based anodes and the corresponding charge-storage mechanism still remains a topic of discussion.

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Herein, we have rational designed and synthesized a unique dual carbon decorated FeS2based hybrid (denoted as GF/FeS2@C) by using Prussian blue microcubes as the selfsacrificial templates, in which yolk-shell FeS2@C microboxes were homogeneously encapsulated in a N, S co-doped graphene framework. In this design, the yolk-shell structure is excepted to endurance the repeating volume change and guarantee the mechanical integrity of the working electrode during Li+/Na+ insertion and extraction, while the continuously graphene framework with abundant defects can enrich the number of electrochemically active sites, and provide fast electron pathways. Benefitting from the synergistic effect of these elaborated characteristics, the as-prepared GF/FeS2@C composites display high reversible capacity, long lifespan, as well as fast charging ability when evaluated as anode material for both LIBs and SIBs. Furthermore, the kinetics analysis predicts that the electrochemical reaction is mainly boosted by pseudo-capacitance behavior, thus endow the optimized GF/FeS2@C anodes with an ultra-fast capability. 2. Experimental Section 2.1Materials Synthesis of Prussian blue (PB) microcubes: PB microcubes were synthesized according to a previous report.[46] In a typical procedure, 1.52 g polyvinylpyrr-olidone (PVP) and 0.22 g K4Fe(CN)6·3H2O were added in a 100 mL 0.1 M HCl solution under continuous stirring for 1 h. Then, the mixture was transferred into a sealed glass bottle and heated at 80 °C for 24 h. After cooling, the blue PB precipitate was collected by filtration and washed with deionized water for several times, and dried in a vacuum oven at 60 °C. Synthesis of core-shell PB@PDA microcubes: Typically, 80 mg of PB microcubes were dispersed in 60 mL of Tris-buffer aqueous solution (PH≈8.5) to form a homogeneous suspension. Subsequently, 40 mg of dopamine was slowly added to the mixture with

4

magnetic stirring for 10 h. The core-shell PB@PDA microcubes were collected after washed with deionized water and ethanol and dried at 60 °C. Synthesis of GO/PB@PDA composites: The as-prepared PB@PDA microcubes were firstly functionalized with positive charge by physical adsorption poly (diallyldimethylammoni-um chloride) (PDDA), a cationic polyelectrolyte. Briefly, 80 mg PB@PDA microcubes were dispersed into 100 mL of 5 wt % PDDA aqueous solution and ultrasonically treated for 1 h to form a uniform suspension. After that, the mixture was centrifugated and washed using deionized water for several times to remove the redundant PDDA. Then, the PDDA-modified PB@PDA microcubes were re-dispersed in 100 mL deionized water, followed by slowly dropped into 20 mL graphene oxide (GO) aqueous suspension (0.75 mg mL-1) under magnetic stirring for 0.5 h. Finally, the GO/PB@PDA composites were obtained through centrifugation and freeze-drying. Synthesis of GF/FeS2@C composites: Typically, the GO/PB@PDA was mixed with sulfide powder with a mass ratio of 1:16, and then loaded in a graphite crucible with cover. Subsequently, the samples were firstly heated to 155 °C for 1h and continually heated to 400 °C for 6 h with a heating rate of 3 °C min-1 under Ar atmosphere. Eventually, the GF/FeS2@C composites were yielded after cooled to room temperature. For comparison, The FeS2 and FeS2@C samples were also synthesized by directly sulfidation of PB and PB@PDA microcubes followed by the same synthetic procedure, respectively. 2.2 Characterization Field emission scanning electron microscopy (FE-SEM, Hitachi, S-4800, Japan) and field emission transmission electron microscopy (TEM, FEI, Titan themis 200, USA) were employed to observe the morphology and microstructure of the materials. Powder X-ray diffraction (XRD, Philips X’pert TROMPD, Cu Kα1 radiation, λ=1.54178 Å) was applied to

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identify the crystalline phases. The chemical composition and their valences were further investigated by X-ray photoelectron spectrometer (XPS, Escalab 250, Thermo Fischer Scientific, USA). Raman spectra were recorded on a Raman spectrophotometer (Horiba Jobin Yvon, HR800, France) with 532.17 nm laser radiation. The elemental contents were determined through elemental analysis technique. 2.3 Electrochemical Measurements The electrochemical properties of all the samples were tested with a half-cell LIB and SIB configuration. To prepared the working electrode, the 70 wt.% active materials, 20 wt.% acetylene black, and 10 wt.% carboxymethyl cellulose be pre-mixed in deionized water to from a homogenous slurry, and then the slurry was cast onto copper foil and dried at 80 °C in a vacuum for 4 h. After dried treatment, the sheet was punched into disc (Φ=12 mm) as the final working electrode, where the mass loading of active materials (e.g. GF/FeS2@C) is about 1.0-1.5 mg cm-1. For LIBs, the half cells (type CR2032) were assembled by using lithium metal foil and Celgard 2400 as counter electrode and separator, respectively. The electrolyte was consisted of 1 M LiPF6 in ethylene carbonate/diethyl carbonate (1:1 by volume) with the addition of volumetric 5 % fluoroethylene carbonate. For half cells of SIBs (type CR2016), sodium metal foil and Whatman glass fiber were used as counter electrode and separator, respectively. And 1.0 M NaCF3SO3 in diglyme (DGM) was used as the electrolyte. Electrochemical performances including rate ability, cycling capability and dischargecharge curves were tested using an automatic NEWARE battery cycler (Neware, China). For LIBs and SIBs, the testing voltage window was 0.01-3 V and 0.5-3 V, respectively. Cyclic voltammetry (CV) and Electrochemical impedance spectra (EIS) analysis were performed by a PARSTAT multichannel electrochemical workstation (Princeton Applied Research, PMC1000DC, USA). EIS measurements were conducted at a frequency range of 0.01–100

6

kHz with the voltage perturbation at 5 mV. The capacity of the electrode was calculated based on the total weight of the samples. 3. Results and discussion 3.1 Material synthesis and characterization

Figure 1 Schematic diagram of the preparation for GF/FeS2@C composites. A representative synthetic procedure of the target GF/FeS2@C material is illustrated in Figure 1a. Initially, the cubic Prussian blues (PB, Fe4Fe(CN)63, XRD result shown in Figure S1), a typical metal-organic framework (MOF), with a length of about 1.5 µm (Figure 1b and Figure S2) are synthesized by a facile hydrothermal method, which then transformed into core-shell PB@PDA microcubes (Figure 1c and Figure S3) by coated with polydopamine as reported in our previous work.[40, 46] Subsequently, to construct a stable architecture with graphene/microcubes/graphene configuration, the as-prepared PB@PDA are first decorated by physisorption PDDA to produce positively charged PB@PDA microparticles, so that it could tightly bond with the negatively charged graphene oxide (GO) sheets together under the electrostatic interaction. In this process, the exposed graphene sheets are also cross-linked with others owning to the electrostatic effect between residual PDDA and oxygen-containing groups exposed on the graphene surface, eventually forming the ideal three-dimensional (3D) hierarchical graphene matrix. Hence, core-shell PB@PDA

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microcubes wrapped within an interconnected graphene network are successfully obtained (denoted as GO/PB@PDA, Figure 1d and Figure S4). After freeze-drying treatment, the precursor of GO/PB@PDA undergoes a thermal reaction process with sulfur powder, resulting in the final target products (marked as GF/FeS2@C, Figure 1e). During this process, inner PB core reaction with sulfur to generate FeS2 nanoparticle aggregates, and the outer PDA layer decomposes into carbon shell on the surface of inner FeS2, meanwhile, the GO sheets are simultaneously reduced to reduced graphene framework (GF).

Figure 2 SEM images (a-c), TEM and HRTEM imagines (d-f), SEAD pattern (g), and EDS mapping (h) of the GF/FeS2@C. Benefiting from the dual-protection of carbon shell and GF, the target product of GF/FeS2@C adopts the elaborated 3D hierarchical foam-like morphology without structural

8

collapse or destroy as observed by SEM and TEM observation in Figure 2. In detail, as shown in the SEM imagines (Figure 2a-c), the FeS2@C microboxes are embedded homogeneously

in

a

graphene

framework,

which

themselves

stack

up

in

to

graphene/microboxes/graphene configuration. TEM imagine at low magnification (Figure 2d) also confirms that multilayer graphene sheets omnibearingly surround on the surface of FeS2@C, forming an interconnected graphene network. This wrapped network could link all FeS2@C microreactors together, which could boost the charge transfer, thereby making it possible to enhance the electrochemical kinetics.[44, 47, 48] Moreover, from the highmagnification TEM imagine (Figure 2e) and FESEM imagine (Figure 2h), a small gap between carbon shell (with a thickness of ~30 nm) and FeS2 core can be apparently found owing to the shrinkage of PB cubes, rusting in the formation of yolk-shell FeS2@C. This unique yolk-shell structure, according to the previous reports, not only effectively facilitate the penetration of electrolyte but offer a buffer zone for the volume variation.[23, 49, 50] Compare with the GF/FeS2@C, nevertheless, the SEM imagines (Figure S5 a-c) of FeS2@C sample reveal that its single carbon encapsulated morphology retain well except that the outer carbon shell shows slight shrink. In contrast, the micro-sized FeS2 samples (Figure S5 d-f), synthesized by directly sulfurizing pure PB microcubes, are composed of cobblestone-like FeS2 nanospheres, which cannot maintain their original cubic shape. That is to say, the original cubic structure of PB can be destroyed easily without the confine of carbon shell during sulfidation. The difference between all FeS2 samples manifests that encapsulating PB into PDA shell is a critical step for maintaining the integrity of inner FeS2 aggregates. Moreover, the high-resolution TEM (HRTEM) imagine (Figure 2f) of GF/FeS2@C shows a clearly parallel fringes with a d-spacing of 0.24 nm, corresponding to the (211) plane of pyrite FeS2 (JCPDS no. 42-1340). Furtherly, the selected area electron diffraction (SAED) pattern (Figure 2f) shows several diffraction rings, corresponding to the (111), (211), (200),

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and (221) planes of FeS2, respectively, suggesting the high crystallinity.[30, 33] In addition, the EDS mapping was performed to detect the elemental distribution in the final GF/FeS2@C products. As Figure 2g shows, the C signal can be detected in the whole particles and outer graphene sheets, implying the N-doped carbon shell and graphene layer, while the Fe and S elements are well distributed along with the shape of inner FeS2 yolk, confirming the yolkshell structure. Furthermore, the corresponding EDX result demonstrates that the atomic ratio of Fe/S is close to 1:2 (Figure S6), verifying again the successful chemical reaction process.

Figure 3 XRD patterns (a) for all three samples, XPS profiles (b), Fe 2p (c), S 2p (d), C1s (e) and N 1s (f) spectrum of the GF/FeS2@C composites. Raman spectra (g) of FeS2@C and GF/FeS2@C composites. Elemental analysis result (h) and electronic conductivity (i) for all samples.

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The crystalline structure and chemical composition of all as-prepared FeS2 were further investigated by XRD and XPS technologies. Figure 3a presents the XRD patterns of all FeS2 samples, it can be seen that all diffraction peaks are well matched with the standard pyrite FeS2 (JCPDS no. 42-1340) without any impurity, indicating that all the as-prepared samples possess high purity.[22, 34, 51] And there are no detectable peaks related with elemental sulfur, suggesting the completely chemical reaction. XPS measurement was carried out to fully analysis the chemical composition and valence bond of the GF/FeS2@C composites. The survey spectrum, as shown in Figure3b, clearly verify the presence of Fe, S, C, N, O elements. From the high resolution of Fe 2p spectrum (Figure 3c), two characteristic peaks located at 707.1 and 719.7 eV can be observed, which correspond to the Fe 2p3/2 and Fe 2p1/2 levels in FeS2, respectively.[28, 52] Another two peaks at 711.7 and 725.4 eV are fitted into Fe-O bond, which may be ascribed to the slight oxidation of surficial FeS2.[24, 41] Figure 3d exhibits the emission spectrum of S 2p, in which the two prominent peaks appeared at 162.5 and 164.1 eV belong to typical S 2p3/2 and 2p1/2 levels in FeS2 species. Specifically, the CS/C=S bond can be detected at 165.4 eV, further confirming that a part of S atoms has been doped into the carbon shell and GF component. Another peak related with -SOx- bond at 168.3 eV is also observed, which is related to the slight oxidation of superficial FeS2.[53, 54] Furthermore, as shown in Figure 3e, the C 1s spectrum is fitted into four five peaks appearing at 284.3, 284.8, 285.4, and 286.3 eV, which are associated with C-C/C=C, C-S/CS-Fe, C-N, and C-O/C-O-C species, correspondingly.[20, 55, 56] The N 1s spectrum, as seen in Figure 3f, is composed of three distinct peaks at 398.3, 399.3, and 400.4 eV, which can be ascribed to pyridinic-N, pyrrolic-N and graphitic-N, respectively.[44, 57] These results clearly identify the existence of C-N and C-S bonds, that is to say, N and S atoms have been successfully doped into the carbon shell and graphene framework during thermal sulfidation process, simultaneously. According to the previous reports, N, S co-doped carbon or

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graphene may possess a higher electronic conductivity because N and S heteroatoms can serve as electron donor to enrich the density of charge-carrier, thereby improving the charge kinetics and rate ability.[58-60] Raman spectroscopy was also conducted to light the detailed graphitic structure of GF/FeS2@C and control samples. As displayed in Figure 3g, there are two characteristic peaks located at 1341 and 1601 cm-1 corresponding to the disordered carbon structure (D band) and ordered graphitic structure (G band), respectively.[61] Notably, the peak intensity ratio (ID/IG) of GF/FeS2@C is lower than that of FeS2@C, suggesting more defects existed in GF/FeS2@C sample. Such an abundant defects may provide extra active sites for capturing Li+ and Na+.[47] The C and N contents of FeS2@C and GF/FeS2@C composites are further determined through elemental analysis. As shown in Figure 3h, the carbon content of GF/FeS2@C is 25.7%, only 5.2% ahead of the FeS2@C sample, indicating the low weight fraction of GF. Interestingly, the nitrogen content is determined to 7.3% for GF/FeS2@C and 6.7% for FeS2@C, suggesting that the N atoms are doped into both carbon shell and GF. Such a high N doping mainly derives from cyanide ligands in PB.[52, 62] According to the above analysis, it can conclude that the hierarchically monolithic GF/FeS2@C architecture, integrating

with

yolk-shell

structure,

sandwich-like

graphene/microboxes/graphene

configuration, appropriate chemical composition, as well as heteroatom-N, S co-doping, was successfully engineered, which may hold great potential for electrochemical applications. Besides, the four-probe method (Figure 3i) was applied to measure the electronic conductivity of GF/FeS2@C (8.06×10-2 S cm-1), which is much higher than that of FeS2@C (3.30×10-2 S cm-1), and FeS2 (1.67×10-2 S cm-1), indicating that the carbon shell and N, S co-doped graphene effectively enhance the electron transfer of FeS2-based materials. 2.1 Electrochemical properties for LIBs

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In light of the superior structure design and chemical composition, the electrochemical properties of GF/FeS2@C were exposed as anode material for LIBs and SIBs. Cyclic voltammetry (CV) measurement, voltage-time curve, and ex-situ XRD were performed to indepth explore the possible Li-storage mechanism. Figure 4a gives the CV curves of GF/FeS2@C anode at a scan rate of 0.1 mV s-1 in the voltage window range from 0.01 to 3V. During the first scan cycle, as seen, two pair of reduction/oxidation peaks (1.4/2.5V, 0.9/1.9V) can be observed, suggesting a complicatedly multistep electrochemical reaction between active GF/FeS2@C and Li ions.[17, 63] In the initial voltage-time plots (Figure 4b), there are two discharge platforms at around 1.4 and 1.1V and two charge plateaus at around 1.8 and 2.4V, greatly coinciding with the CV peaks, which also confirms that a rich redox reaction occurs during cycling. The detail of this multistep lithiation/de-lithiation process was further figured out by ex-situ XRD in Figure 4c. For the XRD pattern of pristine GF/FeS2@C electrode (stage A), six sharp peaks can be well index into pure FeS2 (JCPDS no. 42-1340). After discharging to 1.5 V (stage B), the peaks belong to FeS2 slightly diminish and no other peaks can be observed. After discharging to 0.9 V, diffraction peaks appear at 2θ=26.3° and 39.1° correspond to the generation of Li2FeS2 intermedia phase while the peak at 2θ=21.3° is related with Li2S, suggesting that Li+ ions insert into FeS2 lattice to form intermediate phase Li2FeS2 companying with its reduction to from Li2S phase. While full discharging to 0.01 V (stage D), all diffraction peaks are replaced by that of Li2S phase, indicating the complete conversion reaction from FeS2 to Li2S phase. In the following charge procedure from stage E to F, characteristic peaks belonging to Li2S markedly fade while diffraction peaks belonging to LiFeS2 gradually appear, which can be ascribed to the reverse process of stage D and C. It should be pointed out that the characteristic peaks of active FeS2 component have not fully regenerated when full charging to 3.0 V (stage E), which may demonstrate that the amorphous solid electrolyte interface (SEI) film has thoroughly covered on the surface of

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GF/FeS2@C hybrids after the first cycle. In the subsequent CV cycles, the reduction curves show a visible variation, in which the two reduction peaks shifted to higher potential of 2.1 and 1.4 V correspond to the intercalation and conversion reactions, respectively. Noticeably, a new reduction peak at around 0.7 V appeared since the second cycle, which is associated with the reformation of SEI layers and in good agreement with the above ex-situ XRD result. Moreover, the CV curves are overlapped well in the following three cycles, indicating an excellent reversible nature of the electrochemical reactions in the GF/FeS2@C anode.[34, 41] The discharge/charge voltage profiles of the GF/FeS2@C anode at a current density of 100 mAh g-1 are shown in Figure 4d. It is clear that the GF/FeS2@C delivers the highest discharge/charge capacity of 1239/1099 mAh g-1, with an initial Coulombic efficiency (CE) of 88.7%. The voltage curves tend to overlap since the second cycle, demonstrating the good reversibility of the lithiation/de-lithiation process.[36, 41, 54] For comparison, the voltage profiles of FeS2@C and FeS2, as displayed in Figure S7, exhibit the relatively low initial discharge capacity of 1199 and 1120 mAh g-1, only corresponding to the low CE of 76.7% and 82.9%, respectively. The initial capacity loss is mainly ascribed to the irreversible side reaction between the defect of carbon shell-GF layer and Li+ as well as the formation of SEI films.[39, 44] Figure 4e compares the cycling performance of GF/FeS2@C, FeS2@C and FeS2 at a current density of 1 A g-1. In the first three cycles, as seen, the capacity of three electrode display a visible fading owning to the afore-mentioned side reaction and active process. However, the GF/FeS2@C electrode delivers remarkable cycling stability with negligible capacity loss since from 4th cycle. It delivers the highest discharge capacity of 881 mAh g-1 in the 4th cycle and eventually stabilizes at 934 mAh g-1 after 200 cycles, delivering a capacity retention of 100%. In contrast, the continuous capacity damping can be found from 100 to 200 cycles in the FeS2@C and FeS2 electrode, and eventually give discharge capacity as low as 635 and 423 mAh g-1, yielding unsatisfied retention of 83.3% and 73.3% compare

14

to their capacity at 4th cycle, respectively. The stable Li-storage ability of GF/FeS2@C should be mainly ascribed to the rational design of encapsulating yolk-shell structure within elastic GF matrix, which may induces three roles: i) yolk-shell structure offers limited interior cavity to contain the inward volume expansion; ii) robust carbon shell confines the electrochemical reaction of active materials to a closed room, avoiding its contact with electrolyte and thereby ensuring its weight without loss; iii) the elastic GF layer acted as a buffer cushion to migrate the excessive stress caused by serious volume swelling, maintaining the structural integrity. The correlation between cycling stability and structure features of the as-prepared materials was also certified by postmortem SEM imagines of cycling-after electrodes. Benefiting from the dual-carbon decorated structure, as shown in Figure S8 a and b, the GF/FeS2@C electrode cycling at 1 A g-1 after 100 cycles mainly keeps its original integrity, where the GF/FeS2@C particles still maintain the graphene coated cubic structure without visible structural collapse. In contrast, the individual FeS2@C particle, as shown in Figure S8 c and d, undergoes uncomplete structural fracture, and even the FeS2 (Figure S8 e and f) suffers from complete destruction. Moreover, it is worth noting that a robust SEI film formed on the outer surface of the elastic GF layer owing to the omnibearing isolation of the GF while the SEI layer cannot be distinguished with the FeS2@C or FeS2 particles, indicating that the SEI film formed on the GF/electrolyte interface is more stable than that on the FeS2@C/electrolyte and/or FeS2/electrolyte interface. Those results indicate the synergistical effect of carbon shell and elastic GF matrix in protecting the structure integrity of the entire GF/FeS2@C electrode. The rate capabilities were also evaluated at various current densities from 0.1 to 20 A g-1 and shown in Figure 4f. It is clear that GF/FeS2@C anode exhibits the best rate capability and excellent restorability compare to its counterparts. It delivers a reversible capacity of 1050, 924, 850, 752, 683, 603, 530 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 A g-1,

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respectively. Even at ultra-high rate of 20 A g-1, it shows the reversible capacity as high as 428 mAh g-1. When the current rate is returned back to 0.1 A g-1, its capacity can be recovered to 1068 mAh g-1. As a comparation, the FeS2@C and FeS2 anode gives an inferior rate capability (as seen from Table S1) and the difference in capacity become more and more obvious with the rising of current rate.

Figure 4 CV curves (a), voltage-time plots (b), the corresponding ex-situ XRD patterns (c) at different potential states and discharge-charge curves (d) of GF/FeS2@C electrode. Cyclic capacities (e) at 1 A g-1 and rate performances (f) of all three FeS2 electrodes. The long-term

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cycling stability (g) and the corresponding voltage-time curves (h and i) at large current density of 2 and 20 A g-1 for GF/FeS2@C electrode. The reversible ability (j) of GF/FeS2@C. Long-term cycling capability at high-rate current and charge time are the most important electrochemical properties of lithium ion batteries for high-power applications (EVs and HEVs). As shown in Figure 4g, the GF/FeS2@C presents a stable cycling capacity of 678 mAh g-1 with an average Coulombic efficiency close to 100% at 2 A g-1after 400 cycles, the capacity retention vs. 2nd (723 mAh g-1) is calculated to be 93.8%. The corresponding voltage-time curves (Figure 4h) show that one galvanostatic discharge/charge process is take in 24 mins and its stable platforms can be clearly observed, indicating the excellent fastdischarge-charge ability and repeatability. Impressively, even cycling at 20 A g-1 (Figure 4i), in which one lithiation/delithiation reaction process is completed only in 2.5 mins, the reversible capacity of GF/FeS2@C can still achieves 502 mAh g-1 at the second cycle and eventually sustains at 313 mAh g−1 after 400 cycles, indicating again its fast charging potential. Also, the GF/FeS2@C composites show impressively reversible abilities. When repeatedly tested at fluctuating current densities (2 A g-1, 10 A g-1) in Figure 4j, its capacity could be kept stability with negligible capacity fading. To gain insight into the in-depth reason of the outstanding Li-storage capacity for GF/FeS2@C, especially its ultra-fast charging nature, the detailed kinetics were investigated by CV and EIS measurements. The CV curves of all electrodes at stepwise scan rates from 0.2 to 1.0 mV s-1 were recorded in Figure 5a-c. As shown, all curves show similar shapes with two redox couples, greatly coinciding with the discharge-charge platforms. Ions diffusion coefficient is deemed as a key factor to judge the low or fast property of electrochemical reaction, according to the previous reports, which could be calculated by a classical equation as below: Ip=2.69×105n3/2AD1/2v1/2CLi+ (1)

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where, Ip, n, A, D, v, and CLi+ are the peak current, the transform electrons in the reaction, the electrode area, the ions diffusion coefficient, the scan rates, and the concentration of lithium ions, correspondingly.[43, 45, 64] Figure 5d and e give the linear fitting results between Ip and v1/2 at all peak positions, the fitting slope (peak1, 2, 3, 4) of Ip/v1/2 and the corresponding D are listed in Table S2. By comparison, it could be found that the target GF/FeS2@C shows larger D values than those of FeS2@C and FeS2 at all peaks, so that it delivers the largest average of 3.8×10-6 cm2 s-1 with no doubt, thereby revealing the most fasted ion shutting. Furthermore, benefiting from the Dunn’s work, the electrochemical reactions occurring on the energy storage systems may involve a diffusion-controlled process and/or pseudocapacitive behavior, which can be quantified by the relationship between i (current response) and v (sweep rate) according to the equations: i=avb (2) i=k1v+k2v1/2 (3) i/v1/2=k1v1/2+k2 (4) In Equation 2, the electrochemical response is completely dominated by diffusion-controlled behavior when b equals 0.5, while the b equals 1, indicating an absolute capacitive behavior.[65, 66] According to this theory, the b values for two pairs of redox peaks can be calculated by linear fitting the plots of log(i) vs. log(v). Clearly, the b-values of GF/FeS2@C (Figure 5f) are 0.88 for peak1, 0.77 for peak2, 0.79 for peak3, and 0.75 for peak4, revealing the capacitive dominated electrochemical behaviors. Specifically, all b-values of target GF/FeS2@C electrode are larger than those of others (Figure S9a and b), demonstrating the enhanced pseudocapacitive response, which perhaps derived from the significant structural merits. To further quantify the capacitive contribution in total Li-storage capacity, the whole current is deconvoluted two parts, capacitive (k1v) and diffusion-controlled (k2v1/2), as described in Equation 3, where the k1 and k2 values could be calculated by fitting the plots of

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i/v1/2 vs. k1v1/2 (as illustrated in Figure S9c) based on Equation 4 transformed by Equation 3. The diagrammatic illustrations of the capacitive response in the total current at 1 mV s−1 are depicted in Figure S9d-f. On the basis of this methodology, the capacitive contributions at different sweep rates can be determined. As listed in Figure 5g, the capacitance contributions in the GF/FeS2@C electrode can reach up to 75.6%, 80.1%, 84.2%, 86.1%, and 89.2% at scan rates of 0.2, 0.4, 0.6, 0.8, and 1.0 mV s−1, respectively, which demonstrates that the pseudocapacitive charge-storage amount does occupy a high portion of the whole capacity. Note that the capacitive contributions of GF/FeS2@C are well beyond its counterparts, the boosted capacitive behavior is conductive to enhance rate capability, which may due to the GF introduction. Moreover, the EIS technology was also adopted to further investigate the reaction kinetics. Figure S10 presents the Nyquist plots of the electrodes before cycling. As we all known, typical Nyquist plot is composed of a semicircle at high-medium frequency and a straight line at low frequency, which are related the charge transfer impedance (Rct) and the Warburg impedance, respectively.[19, 39] In Figure S10a, the value of Rct is approximate to 153.8 Ω for GF/FeS2@C, 223.5 Ω for FeS2@C, and 474.5 Ω for FeS2, indicating the high conductivity after GF introduction. In addition to Rct, the apparent Li diffusion coefficient (D) is also estimated by fitting the date of Warburg impedance, the detailed calculated process is summarized in Figure S10b. As calculated, the GF/FeS2@C delivers the highest D values of 7.7×10−17 cm2 s-1, this value is at least 2 orders of magnitude larger than that of FeS2@C (3.7×10−17 cm2 s-1) and FeS2 (8.1×10−18 cm2 s-1). This order of D values in EIS, not coincidentally, are in accordance with that of the CV test, verify again that the carbon shell and the three-dimensional graphene framework synergistically enhance the ionic transport kinetics, thereby remarkable improving the rate capability and cycling stability at high-rate.

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Figure 5 CV curves with scan rates from 0.2 to 1.0 mV s-1 for (a) FeS2, (b) FeS2@C, and (c) GF/FeS2@C. Plots of I vs. v1/2 (d and e) used for calculating the diffusion coefficient. Relationship between the peak currents and scan rates in the logarithmic format liner (f) for GF/FeS2@C electrode. The capacitive contributions (g) of all electrodes at different scan rate. 2.2 Electrochemical properties for SIBs Inspired by the significant lithium storage performance, the GF/FeS2@C and FeS2@C samples were also employed as anode materials for SIBs. Figure 6a gives the typical CV curves of GF/FeS2@C at a scan rate of 0.1 mV s-1 in the voltage window of 0.5-3 V. As shown, two reduction peaks appear at around 1.2 and 0.9 V in the first cathodic scan, the former is ascribed to the intercalation of Na+ into FeS2 crystal to from NaxFeS2 (0
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film.[16, 17, 31] During the reverse anodic scan, three broad peaks at approximate 1.1, 2.2 and 2.5 V can be observed, corresponding to the stepwise extraction process of Na+ from host materials.[22, 63] In the second cathodic sweep, the reduction peaks show obvious variation and evolve into one hump at 2.0 V, indicate that an irreversible electrochemical reaction occurs in the initial cycle.[23, 35] This possible electrochemical reaction between the active GF/FeS2@C and sodium ion was substantiated further by ex situ XRD and displayed in Fig. S 11 (see details in Supporting Information). Note that all electrochemical peaks stay at the same positions in the following testing, indicating high reversibility of GF/FeS2@C.

Figure 6 CV curves (a) at a scan rate of 0.1 mV s-1 of GF/FeS2@C. Discharge-charge curves at a current density of 100 mA g-1 for (b) GF/FeS2@C and (c) FeS2@C electrode. Rate performance (d) and EIS plots (e) for all composites. CV curves (f) at different sweep rates, the linear relation of (g) I vs. v1/2 and (h) log(i) vs. log(v), as well as the capacitive ratios (i) at all stepwise scan rates for GF/FeS2@C electrode.

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Figure 6b shows the galvanostatic discharge/charge curves of GF/FeS2@C. Clearly, in the first cycle, it delivers the initial discharge/charge capacity of 431/396 mAh g-1, with an initial CE as high as 91.9%. For comparison, the FeS2@C (Figure 6c) delivers a high discharge capacity of 449 mAh g-1 but poor initial CE of 82.8%. The initial capacity decay for two electrodes is mainly ascribed to the inevitable composition of the electrolyte and the formation of SEI film on the surface in fresh electrode.[67, 68] Moreover, the discharge/charge curves of GF/FeS2@C tend to overlap but the capacities of FeS2 decreased to a large extent. These results indicate that the encapsulation of FeS2@C into GF matrix is indeed in favor of enhancing specific capacity and electrochemical reaction repeatability. Figure 6d compares the rate capability of two three electrodes at current densities from 0.1 to 10 A g-1. As shown, the GF/FeS2@C delivers the high reversible discharge capacity of 378, 354, 326, 309, 282, 237, and 187 mAh g-1 at 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, and 10.0 A g-1, respectively, and then recover to 371 mAh g-1 when the current set back to 0.1 A g-1. In the same rate, however, the both FeS2@C and FeS2 electrode offers an unsatisfactory discharge capacity in comparison to GF/FeS2@C of 378, 320, 271, 220, 169, 104, and 53 mAh g-1. The good rate performance of GF/FeS2@C is mainly attributed to the construction of 3D interconnected GF skeleton, which induces the fast electron response and rapid ion transport, simultaneously. That is to say, the electrochemical reaction kinetics of the GF/FeS2@C electrodes have been enhanced after the GF decorating, which is further demonstrated by EIS and CV measurements. As seen from Figure 6e, the GF/FeS2@C exhibits the low Rct (47.6 Ω) in comparison to that of FeS2@C (88.9 Ω) anode and FeS2 (225.7 Ω), which suggest that the N, S co-doped graphene networks can construct an efficiently bi-continuous pathway for both electron and Na+ transfer, hence, improving reaction kinetics.[19, 53] The in-depth kinetics analysis of GF/FeS2@C through CV curves were also investigated as displayed in

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Figure 6f-i. The CV plots in Figure 6f show similar shapes and hardly change at different scan rate,

Figure 7 The comparation of cycling performance (a) at 0.5 A g-1 for GF/FeS2@C and FeS2@C electrode. Ultralong-term cyclic stability (b) of GF/FeS2@C at 0.2 A g-1 and 2 A g-. Charge-discharge curves (c) from 400st-410th loops for GF/FeS2@C when cycling at 2 A g-1. indicating the stable electrochemical reaction. The relationship between peak current and scan rate (Figure 6g) are linear fitted, the matching slopes at peak1, 2, and 3 are -0.94, 0.72

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and 0.70, respectively, corresponding to the D value of 5.6×10-7, 3.3×10-7, and 3.1×10-7 cm2 s-1. In addition, the b values at various of redox peaks were also calculated by linear fitting the relation of log(i) and log(v). According to the above analysis, b-value is a key factor to estimate whether the electrochemical responses are dominated by capacitive behavior (b>0.5) or diffusion-controlled process (b=0.5).[38, 43] Obviously, the b-values for all three peaks are far beyond 0.5 in Figure 6h, indicate that GF/FeS2@C exhibits a portion of pseudocapacitive Na-storage.[32] Furthermore, the contribution ration of capacitive is determined by the above-mentioned methodology. As listed in Figure 6i, the ratio could reach 61.1-91.3% with increasing the scan rate, confirms that the capacitive behavior is prominent. Except the rate performance, electrode materials with stable cyclic capacity and long lifespan are significant for large-scale practical application of SIBs. Therefore, cycling performance of GF/FeS2@C and FeS2@C are also evaluated. As shown in Figure 7a, the GF/FeS2@C exhibits only a slight capacity decay in the first three cycles, and then stabilizes at approximately 257 mAh g-1 after 1000 cycles at the current density of 0.5 A g-1. In contrast, the capacity of FeS2@C undergoes a rapid decrease from 360 (1st) to 181 (500th) mAh g-1 in the same testing current, demonstrating the inferior capacity retention at the absence of GF protection.[52, 69] Moreover, the target GF/FeS2@C could delivers an initial capacity as high as 448 mAh g−1 at small current of 0.2 A g-1, and remains a capacity of 370 mAh g−1 after 1000 cycles in Figure 7b. Noticeable, from the postmortem SEM images (Figure S12), it can be found that the original morphology of GF/FeS2@C have not destroyed completely, suggesting its good structural stability. More inspiringly, even cycling at an ultra-high rate of 2 A g-1 for 600 cycles, it also holds a high capacity of 203 mAh g−1, with a capacity retention of 74.4% compare to the capacity at the second cycle. Also, Figure 7c displays the sodiation/desodiation curves of 400th to 410th, and the similar shape of

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profiles are detected, corresponding to charged-discharged platforms mentioned above. It is worth noting that one complete charged process only occurs within 6 min, which demonstrated that the GF/FeS2@C also deliver a fast charging feature when applied in SIBs. Such an outstanding cycling stability of GF/FeS2@C should be reasonably attributed to the protection of external GF layer in the graphene/yolk-shell/graphene configuration, which is beneficial for mitigating volume expansion, maintaining yolk-shell architecture, and protecting the SEI layer.[20, 44, 70] The GF/FeS2@C greatly outperforms the FeS2@C no matter in cyclic capacity and life-span. It confirms that the GF/FeS2@C exhibits an advantage of Na-storage ability comparing with FeS2@C, demonstrating the architectural superiority of dual-carbon decorated GF/FeS2@C for high-capacity and high rate sodium storage. Overall, the electrochemical performances of GF/FeS2@C displayed in this work in both LIBs and SIBs show a comparability even superiority in comparison to that of previously reported TMSs-based anode materials (Table S3 and S4), suggesting the effectiveness of our strategy in enhancing the energy storage performance. 3. Conclusions In summary, we have successfully fabricated a novel FeS2-based material by encapsulating yolk-shell FeS2@C microcube into an interconnected graphene framework. In our rational design, yolk-shell structure with an interior void space and protective carbon layer enable the material to tolerate repeating volume variation, and the resilient graphene layer could serve as a crash cushion and a highway for electron transfer. Benefiting from the structural merits decorated by dual carbon components and the feature of enhanced conductivity, this anode material exhibits superior rate capability, excellent ultra-long cycling stability and super-fast discharge-charge feature when applied in both LIBs and SIBs. Remarkably, it shows highrate capacity of 455 mAh g-1 at 20 A g-1 and decent long life-span (i.e., 755 mAh g-1 at 2 Ag-1 and 322 mAh g-1 at 20 A g-1 after 400 cycles) in LIBs. When used as electrode material for

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SIBs, it also delivers excellent rate performance (200 mAh g-1 at 10 A g-1), ultra-long cycling life (314 mAh g-1 at 0.5 A g-1 after 600 cycles) and reversible capacity of 203 mAh g−1 at an ultra-high current rate of 2 A g−1 after 600 cycles. Investigation into electrochemical kinetics analysis discloses that the pseudocapacitance behavior plays a key role in reinforce the Li/Na-storage performance of FeS2 anode especially its fast discharge-charge ability. Overall, the structural design insight developed in this study will open up a new way to produce other potential electrode materials suffered from the obstacles of large volume change and low electronic conductivity. Acknowledgments The authors acknowledge the financial support from the National Natural Science Foundation of China (51502180), the Fundamental Research Funds for the Central Universities (2016SCU04A18) and the Sichuan Province Science and Technology Support Program (No. 2017GZ0132). The authors would like to thank Ceshigo (www.ceshigo.com) for SEM and TEM characterization. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at xxx.xxx.xxx References [1] M. Armand, J.-M. Tarascon, Building better batteries, Nature 451 (2008) 652-657. [2] Y. Sun, N. Liu, Y. Cui, Promises and challenges of nanomaterials for lithium-based rechargeable batteries, Nat. energy 1 (2016) 16071. [3] J. Liu, P. Kopold, P. A. van Aken, J. Maier, Y. Yu, Energy Storage Materials from Nature through Nanotechnology: A Sustainable Route from Reed Plants to a Silicon Anode for Lithium-Ion Batteries, Angew. Chem., Int. Ed. Engl. 54 (2015) 9632-6. [4] Y. Zhong, X. Xia, W. Mai, J. Tu, H. J. Fan, Integration of Energy Harvesting and Electrochemical Storage Devices, Adv. Mater. Technol-US 2 (2017) 1700182.

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Declaration of Interest Statement Dear Editor, Here we submit this paper for consideration to be published on “Carbon”. The further information about the conflict of interest is in the following. All authors declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Encapsulating Yolk-shell FeS2@Carbon Microboxes into Interconnected Graphene Framework for Ultrafast Lithium/Sodium Storage” All authors have seen and approved the final version of the manuscript being submitted. The authors claim that the article is the authors' original work, hasn't received prior publication and isn't under consideration for publication elsewhere. Authors: Peng Jing,a Qiong Wang,a Boya Wang,a Xu Gao,b Yun Zhang,a and Hao Wu*,a Address:

a

Department of Advanced Energy Materials, College of Materials Science

and Engineering, Sichuan University, Chengdu, 610064, P. R. China b

College of Chemistry and Chemical Engineering, Central South

University, Changsha 410083, China