Tap Density Anodes for Rechargeable Batteries

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Micro intertexture carbon-free iron sulfide as advanced high-tap density anodes for rechargeable batteries Ying Xiao, Jang-Yeon Hwang, and Yang-Kook Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13239 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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ACS Applied Materials & Interfaces

Micro Intertexture Carbon-Free Iron Sulfide as Advanced HighTap Density Anodes for Rechargeable Batteries Ying Xiao, † Jang-Yeon Hwang, † Yang-Kook Sun*,† †

Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea

KEYWORDS: micro iron sulfide, carbon-free, high tap density, electrochemical performance, rechargeable batteries

ABSTRACT: Numerous materials have been considered as promising electrode materials for rechargeable batteries; however, developing efficient materials to achieving good cycling performance and high volumetric energy capacity simultaneously remains a great challenge. Considering the appealing properties of iron sulfides, which include low cost, high theoretical capacity, and favorable electrochemical conversion mechanism, in this work, we demonstrate the feasibility of carbon-free microscale Fe1-xS as high-efficiency anode materials for rechargeable batteries by designing hierarchical intertexture architecture. The as-prepared intertexture Fe1-xS microspheres constructed from nanoscale units take advantage of both the long cycle life of nanoscale units and the high tap density (1.13 g cm-3) of micro intertexture Fe1-xS. As a result, high capacities of 1089.2 mAh g-1 (1230.8 mAh cm-3) and 624.7 mAh g-1 (705.9 mAh cm-3) were obtained after 100 cycles at 1 A g-1 in Li-ion batteries and Na-ion batteries, respectively, demonstrating one of the best performance for iron sulfides based electrodes. Even after deep cycling at 20 A g-1, satisfactory capacities could be retained. Related results promote the practical application of metal sulfides as high-capacity electrodes with high rate capability for next-generation rechargeable batteries.

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1. INTRODUCTION By virtue of their long life span and high energy density, lithium-ion batteries (LIBs) have been accepted as the dominant power source for portable devices, electric vehicles (EVs), and hybrid EVs.1,2 Considering the rapid technological developments taking place in society, researching low-cost batteries with high energy capacity and high power density has become increasingly essential and urgent. On one hand, considerable effort has been made to search for appropriate anode materials to substitute for commercialused graphite in current LIBs. On the other hand, in light of the limited reserves and high cost of lithium resource in the earth, along with the similarities between its properties and those of the more abundant sodium,3,4 Na-ion batteries (SIBs) have been intensively investigated as a low-cost alternative to LIBs.5 However, as with LIBs, one of the critical issues in the development of SIBs is deficiency of high-performance anode materials. As promising electrodes in both LIBs and SIBs, metal sulfides have received significant interest in recent years owing to their properties including natural abundance, intrinsic safety, and high theoretical capacity.6,7 Compared with their metal oxide counterparts, metal sulfides display smaller volume change and greater reversibility during electrochemical reaction process,8,9 contributing to improved mechanical stability and higher first-cycle efficiency. Among all metal sulfides, iron sulfides exhibited appealing properties such as low-cost, rich earth reserve, low toxicity and large theoretical capacity,10,11 and thus showing great promise for practical applications. However, till now, most of the reported iron sulfides electrodes refer to carbon-based nanocomposites.


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In the past, numerous carbon-containing nanomaterials with various architectures have been developed and applied as electrodes in LIBs and SIBs.12,13 When used in electrodes for rechargeable batteries, nanostructures can endow the materials with remarkably superior properties, which include an accommodation effect under volume expansion resulting from large interior spaces between nanoparticles,14,15 increased interface area and reaction sites between the electrolyte and active materials benefiting from the high surface area,16 and reduced diffusion and transport lengths for the electrolyte and ions due to nanosized components.17,18 Additionally, the presence of carbon components could improve the electrical conductivity of the electrode and further alleviate the effects of large volume changes.16,19 Many studies have thus focused on synthesizing and investigating

carbon-containing

nanomaterials

to

improve

the

electrochemical

performance of materials and devices, in spite of their low tap density and intrinsically large irreversible capacity at initial cycle. In contrast, microscale materials can introduce high tap density and generate high volumetric capacity when they are utilized as electrodes for rechargeable batteries. However, more serious disintegration and long ion/electron transport pathways occur within microscale anodes.20 Inspired by these findings, microscale materials constructed from nanoscale primary blocks are considered a feasible and promising approach to harness the advantages of each while avoiding the aforementioned shortcomings.21-25 As a consequence, good cycling stability, high energy density, and high tap density can be expected, which would be favorable properties for practical applications. For instance, our group reported a microscale CNT-Si composite anode used in highly efficient Li-ion batteries.26,27 The compact nature of the CNT–Si composite anode provided a high tap density of 1.10 g cm-


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and can delivered high capacity around 2300 mAh g-1 at 400 mA g-1, along with a

satisfactory cycle life and rate capability. Chen et al. investigated the Na-storage performance of micro-nanostructured CuO/C spheres. Their results indicated excellent cycling stability and rate capability, attributed to the high electrochemical reactivity of nanoscale CuO and the stable anode structure.22 In this work, carbon-free microscale Fe1-xS spheres with small secondary units were successfully synthesized and demonstrated promise for use as high tap density anodes in LIBs and SIBs. Specifically, it exhibits a high Coulombic efficiency at initial cycles for both systems and delivered high capacities of 1089.2 (1230.8 mAh cm-3) and 624.7 mAh g-1 (705.9 mAh cm-3) at 1 A g-1 after 100 cycles, respectively. Even at high current densities, the cells could still produce satisfactory capacities. For instance, approximately 358.8 mAh g-1 (405.4 mAh cm-3) for SIB was retained after deep cycled at 20 A g-1. The measured performance was comparable with that of nanoscale samples and superior to that of previous reports. This can be attributed to the interconnected secondary units that ensure the structural stability of the microspheres; the small size of the building blocks resulted in a large interface area and reduced the transport path length of ions and electrons. 2. EXPERIMENTAL SECTION Synthesis of Carbon-Free Fe1-xS Microspheres In a typical procedure, iron nitrate and thioacetamide (TAA) in a molar ratio of 1:3 were dissolved in ethanol solution, then 10 mmol of urea were added to the homogeneous solution. The as-prepared light yellow solution was transferred into a 100 mL Teflonlined stainless steel autoclave and held at 180 °C for 36 h. Finally, the Fe1-xS products were generated by annealing in a N2 atmosphere at 600 °C for 2 h (denoted hereafter as


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micro intertexture Fe1-xS). As a comparison, an Fe1-xS sample with nanosheets morphology (nanosheets Fe1-xS) was produced using a similar method that used a mixed solution of ethanol and water as a solvent. Fe1-xS micro bulk material was prepared using commercial Fe2O3 as an iron source (commercial Fe1-xS).

Material Characterizations The phase and composition were characterized using X-ray diffraction (XRD, Rint2000, Rigaku). The microstructure was observed by scanning electron microscopy (SEM) (HitachiS-4800) and transmission electron microscopy (TEM) (JEOL 2010). Surface chemical composition was investigated by X-ray photoelectron spectroscopy (XPS) (Perkin-Elmer, PHI 5600). Inductively-coupled plasma spectrometry (ICP) was tested on a Jarrel-ASH (ICAP-9000). The Brunauer-Emmett-Teller (BET) surface area was measured using a Quantachrom Autosorb-1.

Electrochemical Measurements The working electrode was prepared by mixing 80 wt% active material, 10 wt% carbon nanotubes, and 10 wt% sodium carboxymethyl cellulose (CMC) binder (as 1 wt% CMC solution) using a mortar. The resulting homogeneous slurry was cast onto Cu foil with an applicator and dried at 80 °C for 6 h in a glass vacuum oven. Coin cells (CR2032) with Li metal and Na metal as the cathodes were used to study the Li- and Na-storage performance of the samples, respectively. A 1 M solution of LiPF6 and a 1 M solution of NaClO4 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, vol%) with 5 wt% of 4-Fluoro-1,3-


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dioxolan-2-one (FEC) were used as electrolytes for assembling lithium ion batteries and sodium ion batteries, respectively. The mass loading for the active electrodes are 2.0 mg cm-2.

3. RESULTS AND DISCUSSION Figure S1 and Figure 1a shows the XRD patterns of the precursor and the annealed products, respectively. All reflection peaks belonging to the precursor could be assigned to the composite of Fe3S4 (JCPDF Card No.23-1122) and FeS2 (JCPDF Card No.24-76) with cubic phases. After

Figure 1. (a) XRD pattern of the annealed product (the black represents the XRD pattern of the standard product). (b,c) SEM images of the precursor. The inset in Figure 1b is the particle size distribution curve of the precursor. (d–g) SEM images of the annealed product. The inset in Figure 1d is the particle size distribution curve the annealed product. (h,i) Cross-sectional SEM images of the micro intertexture Fe1-xS product.


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annealing, all of the peaks could be well indexed to the planes of pure hexagonal Fe1-xS (JCPDF Card No. 29-726), suggesting the successful formation of pure sulfides. The atomic mass of Fe was calculated to be 42.7 % by ICP-MS measurement. From the SEM image of the precursor shown in Figure 1b, we could observe a uniform spherical morphology with a diameter of ~5.1 µm (calculated from the size distribution curve corresponding to the SEM image). The magnified images (Figure 1c) reveal that the as-prepared sphere displayed a rough surface and was assembled from thick connected sheets with a thickness of ~900 nm. After annealing, the sample retained an almost spherical morphology and had an average diameter of ~3.0 µm (Figure 1d), smaller than that of the precursor, showing that obvious shrinkage occurred during the annealing process. Compared with that of the precursor, the surface of the annealed sample was rougher, and the connections between the secondary units (thick sheets) were tighter (Figures 1d and e), which may be caused by the bond growing and the possible decomposition of organic components during calcining process.28,29 Interestingly, the edges of the secondary units became smoother and had a radial length of 100–200 nm (Figures 1f and g). The assembly of the secondary blocks contributed to the formation of pores within the spherical

structure,

which

were

beneficial

for

the

resulting

electrochemical

performance.7,30 The Brunauer-Emmett-Teller (BET) surface area and average pore size of the annealed sample were calculated to be ca. 42.1 m2 g-1 and 19.2 nm (Figure S2) respectively, facilitating the electrochemical uptake and release of Li+/Na+ with respect to the bulk materials. Additionally, in order to confirm the favorable structure of the asprepared microspheres, the cross-section of one sphere was investigated using SEM


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technique. The particles were prepared by embedding in an epoxy and grinding flat. As shown in Figures 1g and h, interconnected sheets with a basically radical distribution could be detected in the cross-section of the particle, indicating the formation of an intertexture structure using the present approach, which is beneficial for retaining structural stability during electrochemical performance testing.

Figure 2. (a–d) SEM and corresponding elemental mapping images of micro intertexture Fe1-xS. (e) EDS spectrum of micro intertexture Fe1-xS.

EDS mapping images shown in Figure 2a–d clearly indicate that Fe and S elements were distributed uniformly. The corresponding EDS spectrum suggests that Fe and S were present at an atomic ratio of 0.997:1. The tap density of the micro intertexture Fe1-xS was determined to be 1.13 g cm-3. In order to further investigate the microstructure of the as-prepared sample, TEM imaging was performed. Figure 3a displays a typical TEM image of the annealed sample, which confirms the formation of well-defined microspheres, consistent with the SEM


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results. Figures 3b and c show magnified TEM images of the edge of an individual sphere, in which it is clear that some loose particles made up the surface of the sphere, which increased the interface area and had a positive effect on the electrochemical performance.7,31 The interplane spacing shown in Figure 3d was calculated to be 0.299 nm, which agrees well with the (200) plane of hexagonal Fe1-xS. From this image, we cannot observe the interior structure in detail, which may be due to the high-density distribution of the building blocks, suggesting close connections between the thick sheets.

Figure 3. (a) Low-magnification TEM image of the micro intertexture Fe1-xS spheres. (b) Highmagnification TEM image of one micro intertexture Fe1-xS sphere. (c,d) HR-TEM images of the micro intertexture Fe1-xS spheres. High-resolution (e) Fe 2p XPS spectrum and (f) S 2p XPS spectrum.

Such a good connection could effectively prevent the structural deformation of the microspheres during the discharge/charge process, thus ensuring the stable performance of the product. In order to further confirm the surface chemical composition of the


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product, XPS testing was carried out. In the XPS spectrum (Figure S3), characteristic peaks belonging to Fe and S could be detected. The presence of C 1s and O 1s in the spectrum was possibly a result of carbonaceous materials adsorbed on the surface from the air.30 A high-resolution Fe 2p XPS spectrum is shown in Figure 3e, which could be deconvoluted into six peaks. The peaks observed at approximately 724.0 and 710.3 eV correspond to Fe3+, whereas those centered at approximately 720.0 and 707.1 eV correspond to Fe2+, consistent with the composition of Fe1-xS.32,33 In the high-resolution S 2p XPS spectrum, five peaks at about 161.1, 162.3, 163.8, 165.2, and 168.0 eV were detected (Figure 3f). The peaks at about 161.1, 162.3, 163.8, 165.2 eV correspond to Fe1-xS, whereas the peak at 168.0 eV corresponds to oxidized groups (SO32-).32-34 These characterizations verify the formation of hierarchical Fe1-xS microspheres. In order to explore the formation mechanism of the novel Fe1-xS microspheres, timedependent SEM and TEM images were analyzed. As shown in Figures 4a and b, the sample collected after 3 h of reaction time displayed small particles with a diameter of ~5 nm. When the time was extended to 6 h (Figures 4c and d), the nanoparticles assembled into porous solid spheres with diameters ranging from 40 nm to 150 nm. Ostwald ripening occurred when reaction time increased to 12 h, as shown in Figures 4e and f, and a hollow structure was formed. Compared with the former solid one, this hollow sphere exhibited a slightly larger diameter, and the shell constructed from secondary units was loosely. An interesting phenomenon appeared when the reaction time was increased to 18 h. As seen in Figures 4g and h, novel hierarchical microspheres with a diameter of ~5 µm formed at this stage instead of nanospheres, suggesting a reassembly process. Such microspheres


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were constructed from thick sheets, forming an intertexture structure. Upon further increase in time to 24 h, some particles appeared on the surface of the spheres (Figures 4i and j), which may be attributed to the trace amount of FeS2 shown in the

(a)

(c)

(e)

(g)

(i)

(b)

(d)

(f)

(h)

(j)

&

# Fe2O3 & Fe3S4 Φ FeS2

&

(k)

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Φ

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36 h 24 h

&

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18 h

#

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12 h 6h 3h

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2θ (degree)

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(l)

Figure 4. Formation mechanism investigation based on time-dependent experiments. SEM images of the sample prepared with different solvothermal reactions: (a,b) 3 h, (c,d) 6 h, (e,f) 12 h, (g,h) 18 h, (i,j) 24 h, and (k) the corresponding XRD patterns of the samples prepared with different times. (l) Schematic illustration of the formation process of micro intertexture Fe1-xS spheres.

corresponding XRD results (Figure 4k). After 36 h of reaction, the microspheres showed no obvious changes except an increasing number of particles appearing, consistent with


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the XRD pattern. In addition to the morphology changing, we can see that the composition of the sample was changed from iron oxide (before 18 h) to iron sulfide (after 12 h) with the reaction time increasing (Figure 4k), indicating the occurring of chemical transformation during hydrothermal process, which may be caused by the Ksp difference between iron oxide and iron sulfide. These results suggest that particle assembly, Ostwald ripening, and an interesting reconstruction as well as chemical transformation process led to the formation of the micro intertexture iron sulfides. Then, the annealing post-treatment results in the formation of pure Fe1-xS microspheres. Based on the aforementioned results and analysis, a possible formation process was proposed in Figure 4l.

Figure 5. Li-storage performance of the as-prepared samples. (a) CV curves of micro intertexture Fe1-xS. (b) Charge/discharge curves of micro intertexture Fe1-xS. (c) Cycle performance of micro intertexture Fe1-xS, nanosheet Fe1-xS, and commercial Fe1-xS at 1000 mA g-1. (d) Rate capability of micro intertexture Fe1-xS, nanosheet Fe1-xS, and commercial Fe1-xS with current density ranging from 0.2 to 20 A g-1.


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To explore the benefits granted by the intertexture microspheres, the electrochemical performance of Fe1-xS as LIB and SIB anodes was investigated using CR2032-type coin cells. Figure 5a displays the cyclic voltammetry (CV) curves of the intertexture microsphere anodes in LIBs between 0.01 and 3 V. During the first negative scan, two peaks at about 1.09 and 0.77 V corresponded to the reduction of Fe1-xS into Fe and the side reaction at the interface of electrode that formed the solid electrolyte interface (SEI) film, respectively.35 After the first cycle, the reduction peaks at about 1.39 and 2.03 V corresponded to the step-by-step formation of Li2Fe1−xS2. During the cathode sweep process, the peak detected at about 1.95 V was ascribed to the oxidation of Fe to Li2Fe1−xS2.36 Based on the analysis and the literature,16,37,38 corresponding equation could be described as follows: Fe1 - xS + 2Li + + 2e - → Li2S + Fe

(1)

Li2S + Fe ↔ Li2Fe1 − xS2 + 2Li + + 2e -

(2)

The peaks overlapped well in the following cycles, implying its good electrochemical reversibility. Figure 5b shows the charge/discharge profiles of micro intertexture Fe1−xS at 0.2 A g-1. In the first discharge curve, voltage plateaus were observed between 1.37 and 1.24 V and between 1.24 and 1.01 V, in addition to a small slope below 1.01 V. In the following cycles, long voltage plateaus between 1.45 and 1.10 V for the reduction process and between 1.77 and 2.0 V for the oxidation reaction were observed, in agreement with the CV curves. Additionally, the micro intertexture Fe1-xS anode had a first discharge capacity of about 1056.2 and subsequent charge capacity of ~866.1 mAh g-1, corresponding to an initial Coulombic efficiency of about 82%. The irreversible capacity loss was probably caused by the formation of solid state interface (SEI) films and the irreversible decomposition of the


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electrolyte.16,37 Figure 5c presents the comparison between the cycling performance of the micro intertexture sphere, the nanosheet Fe1-xS (Figures S4 and S5), and the commercial micro Fe1-xS (Figure S6). For the micro intertexture Fe1-xS, a high capacity of about 1089.2 mAh g-1 was retained after 100 cycles at 1000 mA g-1, displaying cycling stability and Li performance competitive with that of nanosheet Fe1-xS (1007.1 mAh g-1) and superior to that of commercial Fe1-xS (455.1 mAh g-1). These results demonstrate the merits of the intertexture structure and its promise as an anode material for LIBs. Moreover, a capacity increasing can be observed for the cycling curve of micro intertexture Fe1-xS and nanosheets Fe1-xS samples. This phenomenon is common for transition metal-based materials, which could be ascribed to the gradual activation of the porous materials, the progressive formation of electro-chemistry active polymeric gel-like films, the interfacial Li-storage, or the generation of LiOH and its subsequent reversible reaction with Li to form Li2O and LiH.16,38,39 Figure 5d and Figure S7 show the rate capability of the samples tested at different current densities. Micro intertexture Fe1-xS displayed excellent stability, similar to that of nanosheet Fe1-xS and better than that of commercial Fe1-xS. After deep cycling at a high current density of 20 A g-1, a capacity of about 958.7 mAh g-1 was delivered when returned to 0.2 A g-1. However, the nanosheet Fe1-xS only recovered to a discharge capacity of about 312.9 mAh g-1 at 0.2 A g-1, and the commercial Fe1-xS delivered only about 87.5 mAh g1

at 10 A g-1. These results suggest excellent reversibility of the intertexture Fe1-xS. Owing to the deficiency of appreciating electrode materials for SIBs, we also investigated the

electrochemical performance of the as-prepared Fe1-xS in SIBs. The CV curves shown in Figure 6a indicated that a sharp reduction peak appeared in the first negative scan, resulting from the decomposition of electrolyte and the formation of SEI.40 In the following cycles, a clear reduction peak at about 0.31 V and an oxidation peak at about 1.42 V corresponded to the


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formation of Na-rich phases.16,31,37 Additionally, the following cycles displayed great repletion, indicating good cycling stability. The corresponding equation could be described as follows: Fe1 - xS + 2Na + + 2e - → Na2S + Fe

(3)

Na2S + Fe ↔ NaaFe1 − xSb + 2Na + + 2e - (a,b=2 or 3)

(4)

NaaFe1 - xSb ↔ NayFe1 - xSb + (a − y)Na + + (a - y) e -

(5)

The charge/discharge profiles for the intertexture Fe1-xS microspheres shown in Figure 6b further confirm this electrochemical process. In contrast with the long plateau that occurred between 0.76 and 0.94 V in the first discharge process, an obvious short plateau

Figure 6. Na-storage performance of the as-prepared samples. (a) CV curves of micro intertexture Fe1-xS. (b) Charge/discharge curves of micro intertexture Fe1-xS. (c) Cycle performance of micro intertexture Fe1-xS and nanosheets Fe1-xS at 1000 mA g-1. (d) Rate capability of micro intertexture Fe1-xS, nanosheets Fe1-xS,and commercial Fe1-xS with current density ranging from 0.2 to 20 A g-1.


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between 0.31 and 0.36 V dominated the reduction reaction. For the charge process, a very similar trend in voltage was observed. These results agree well with the CV results. Furthermore, initial discharge and charge capacities of about 905.8 and 711.4 mAh g-1 were delivered, respectively, representing a Coulombic efficiency of about 78.5%. The cycling performance of micro intertexture Fe1-xS, nanosheet Fe1-xS, and commercial Fe1xS

are plotted in Figure 6c. After 100 cycles, the micro intertexture Fe1-xS displayed a

high capacity of 624.7 mAh g-1at 1 A g-1, comparable capacity and cycling stability to the nanosheet Fe1-xS (644.7 mAh g-1), suggesting promise for this high-tap density anode in SIBs. Meanwhile, the commercial Fe1-xS shows inferior cycling stability and lower capacity (416.2 mAh g-1), further confirming the advantages of the intertexture structure. More importantly, the as-prepared micro intertexture Fe1-xS also displayed a rate capability that outperformed both nanosheets Fe1-xS and commercial Fe1-xS. As shown in Figure 6d and Figure S8, the rate cycling was tested at current densities ranging from 0.2 to 20 A g-1 every 10 cycles. Satisfactory stability and capacities of about 699.6, 660.6, 647.4, 642.3, 606.2, 595.2, 567.9, 524.5, 500.3, 422.0, and 358.8 mAh g-1 were retained at 0.2, 0.5, 0.8, 1, 2, 3, 5, 10, 15, and 20 A g-1, respectively. When returned to 0.2 A g-1, a discharge capacity of about 719.8 mAh g-1 was delivered, slightly higher than the initial capacity, confirming the high activity and good reversibility of the micro intertexture Fe1xS

spheres. In order to investigate the dynamic behavior of the micro intertexture Fe1-xS during

electrochemical processes, CV curves of the as-prepared micro intertexture Fe1-xS in LIBs and SIBs were analyzed at different scan rates. As shown in Figure 7a, at 0.1 mV s −1, one reduction peak at about 0.29 V and one oxidation peak at about 1.42 V were observed when the micro


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intertexture Fe1-xS was used as an anode in LIBs. The latter was used to investigate the dynamic behavior of the battery. Figure 7b presents a linear relationship between log (i) and log (v) plots in oxidation states. This relationship is shown in the following equation: Log(i) = b × Log(v) + Log(a)

Where i is the peak current, v is the scan rate, and a and b are adjustable parameters. The b value determines the type of Li+ insertion/extraction. When b = 0.5, the electrochemical

(a)

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0.6

(d)

0.4 0.2 0.0 -0.2 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

Log (v, scan rate)

Figure 7. (a) CV curves and (b) log(i)/log(v) plots of micro intertexture Fe1-xS with different scan speeds in LIBs. (c) CV curves and (d) log(i)/log(v) plots of micro intertexture Fe1-xS with different scan speeds in SIBs.

reaction is controlled by ionic diffusion, and when b = 1, the process mainly relies on pseudocapacitive control. In this study, the b value was calculated to be 0.78. Accordingly, based on the CV characteristics (Figure 7c) of the micro intertexture Fe1-xS spheres at different scan


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rates in SIBs, the b values in Figure 7d were calculated to be 0.75 and 0.70, suggesting a similar mechanism during electrochemical processes. These results imply that the electrochemical reactions consist of partial pseudocapacitive behavior, leading to fast Li+/Na+ intercalation and extraction and excellent cycling stability.7,41 Additionally, to investigate the integrity of the micro intertexture electrode, the cells were disassembled after 100 cycles at 1 A g-1. As shown in Figure 8a and b, a similar anode morphology was observed before and after the cycling process in LIBs, indicating that the investigated anode remained free from electrode pulverization, which further suggests that the volume change of the anodes can be effectively overcome by the novel intertexture structure.42 When used as an anode in SIBs, the solid sphere morphology was retained, but the size was reduced to 150 nm~2 µm (Figure 8c and d) compared with the

(a)

(b)

(c)

(d)

Figure 8. SEM and TEM images of the micro intertexture Fe1-xS electrodes in (a,b) LIBs and (c,d) SIBs after cycling at 1 A g-1.


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electrode before cycling, suggesting possible re-assembly during the charge/discharge process. Compared with previous reports, the investigated carbon-free material displayed superior performance even compared to carbon-containing iron sulfides (Table S1).8,3537,43-49

According to the aforementioned results, the remarkable cycling performance and

rate capability of the micro intertexture Fe1-xS spheres with high tap density can be attributed to its unique hierarchical architecture. First, the nanoscale secondary blocks provided more interface between the electrode and electrolyte50 and enabled shorter transport pathways for electrons and ions, contributing to the high rate capability.25 Second, the intertexture structure not only enhanced the stability of the microspheres and subsequently led to satisfactory cycling performance during the charge/discharge process, but also placed every sheet in contact with the electrolyte, ensuring its participation in the electrochemical reaction.50 Finally, the hierarchical architecture with spherical morphology possessed low surface energy, which resulted in less self-aggregation during the electrochemical process.19,51 4. CONCLUSIONS In conclusion, an advanced high tap density anode has been successfully developed using a facile solvothermal method. The prepared anode is composed of carbon-free Fe1-xS microspheres with a unique intertexture structure. When applied in LIBs and SIBs, the assembled batteries display remarkable Li/Na storage performances with high efficiency, which are competitive with that of nanoscale counterparts and superior to that of commercial Fe1-xS bulk materials. This excellent performance is believed to be contributed by the unique architecture induced by the small size of the building blocks and the intertexture assembly of the nanosheets. This method


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provides a promising way to improve the performance of micro-electrodes without using carbon components, which is important and beneficial for practical applications. ASSOCIATED CONTENT Supporting Information. Supplementary XRD pattern of the precursor, N2 adsorption-desorption isotherms of Fe1-xS microsphere, XPS data of Fe1-xS microsphere, SEM and TEM image and XRD pattern of nanosheet Fe1-xS, XRD pattern and XEM iamge of commercial Fe1-xS, charge-discharge profiles of rate capability of micro, nanosheet and commercial Fe1-xS for LIBs. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Programme (2013M3A6B1078875) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning

and

supported

by

a

Human

Resources

Development

programme

(No.

20154010200840) of a Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Trade, Industry and Energy of the Korean government.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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Table of contents graphic

Micro Fe1-xS

Radical distribution

Micro intertexture carbon-free iron sulfide as advanced high-tap density anodes for rechargeable lithium- and sodium-ion batteries. The proposed material with high tap density and novel intertexture structure is developed to achieve good cycling stability, excellent rate performance and high volumetric energy capacity simultaneously.


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Tap Density Anodes for Rechargeable Batteries

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