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A General Method to Fabricate Free-Standing Electrodes: Sulfonate Directed Synthesis and their Li+ Storage Properties Yue Ma,* Habtom Desta Asfaw, and Kristina Edström Ångström Advanced Battery Centre (ÅABC), Department of Chemistry − Ångström Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden S Supporting Information *

ABSTRACT: For materials based on a spatially varied conversion reaction, Li+ storage properties largely hinge on the rational design of the concurrent electronic and ionic pathways in the electrode. We herein present a scalable approach for integrating size-tunable Fe3O4 nanocrystals with hierarchical porous carbon foam by employing sulfonated high internal phase emulsion polymers (polyHIPE) as the carbon source and substrate. To verify the feasibility of our configuration design, the electrodes of such a type were directly evaluated in pouch cells without using an auxiliary binder or a metallic current collector: The best performing composite electrode, with optimized oxide size range, exhibits a good capacity retention of 89.7% of the first charge capacity after 100 cycles and high rate durability up to 4 A g−1. Furthermore, this synthetic approach was also applied to develop carbon/FeS free-standing anodes using the sulfonate groups as the sulfur source, demonstrating its generic applicability to the fabrication of other freestanding electrodes with enhanced Li+ storage properties.

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INTRODUCTION Environmental concerns such as harmful gases (CO2, SO2, NOx) and particulate emissions from combustion processes combined with a depletion of oil reserves have galvanized endeavors to innovate renewable energy conversion and storage systems. Due to a higher energy density and longer lifespan, lithium-ion batteries (LIB) stand out as the most viable energy accumulators, and they have successfully dominated the market of portable electronics during the past two decades. However, for the large scale applications of electric vehicles and smart grids, the widespread commercialization of LIB still requires the technological innovation of electrode materials with higher energy density, greater power, and durability at a lower cost.1,2 Iron-based electroactive species that operate via conversion reaction with Li+ (e.g., Fe3O4, FeS) have attracted much attention as anode candidates for replacing commercially used graphite due to their low cost, natural abundance, and relative high theoretical gravimetric capacities (927 mA h g−1 for Fe3O4 and 609 mA h g−1 for FeS compared to 372 mA h g−1 for graphite).3−5 However, their bulk counterparts suffer from rapid capacity fading due to sluggish electrode kinetics and intrinsically drastic volume changes during the conversion reaction. Research over the years has identified several strategies to circumvent the aforementioned problems: (1) downsizing to nanoscale tends to reduce the solid state electron/Li+ diffusion length and dissipate the mechanical stress induced by the volume variation during the lithiation/ delithiation processes.6−8 (2) Packaging discrete nanoparticles into micro-/nanostructures can improve usability of nanoma© XXXX American Chemical Society

terials through the ease of material handling and processing while retaining the nanoscale advantages,9−13 and (3) compositing with conductive and buffering media improves the electrode kinetics and the tolerance of volumetric fluctuations.14−17 Besides the design ingenuity of electroactive materials, the electrochemical performance also relies onfor conventional slurry-cast electrode preparationa uniform distribution of the carbon additive, the active material, and auxiliary binder so that reactant particles are electrically well connected to the metal foil (nickel/copper) current collector.18 At the same time, the porosity should be preserved to facilitate electrolyte percolation and thus allow for a good supply of Li+ throughout the electrode. Failure to achieve this ideal design would result in an electrode with poor current distribution, incomplete discharge of the battery, and poor rate performance. Fabrication of hierarchical nanostructures with controlled geometry, component arrangement, and internal microstructure open up many new possibilities for energy storage applications. One recently emerging concept is to directly integrate nanoscale electroactive building blocks with a conductive substrate so as to manufacture a complete electrode configuration.19 By virtue of its structural design, an integrated electrode offers many advantages such as intimate component coupling and their synergistic interaction, avoiding the use of auxiliary additives, and the excellent structural robustness that Received: March 5, 2015 Revised: May 5, 2015

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DOI: 10.1021/acs.chemmater.5b00853 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Scheme 1. Schematic Figure of the Preparation Procedures for the CF/Fe3O4−FS Electrode, CF/FeS-FS Electrode, and polyHIPE-Carbon Electrode by Using the polyHIPE as the Structural Template

thereby eliminating the necessary usage of binder and carbon additives. Moreover, the supportive 3D carbon foam functions as the current collector as well. This replacement of the commonly employed metallic foils (nickel or copper) dramatically improves the gravimetric energy density of the anode.31,32 When cycled galvanostatically, the free-standing electrode with an optimized size range of oxide nanocrystals (CF/Fe3O4−FS-2) was observed to deliver a high reversible capacity (∼479 mA h g−1 at a current density of 500 mA g−1), tolerably low capacity fading (∼10% for 100 cycles), and remarkable rate performance up to 4 A g−1. Finally, we further extended to develop various CF/FeS composites of similar electrode configuration by using the sulfonate groups as the anchoring sites as well as the sulfur source. The sulfonated polyHIPE is hydrophilic and is readily wetted by aqueous precursor solutions. This in turn allows for the fabrication of free-standing carbon foam electrodes coated uniformly in oxides, sulfides, and other active materials.

enhances the electrochemical performance. So far, several types of nanostructured electroactive materials have been grown on conductive metallic substrates through a hydro/solvothermal approach or by template-assisted synthesis.19−23 Another category of free-standing electrodes is based on stacked films of nanostructured active materials and expensive carbon nanostructures, such as carbon nanotubes/nanofibers or graphene.24−29 These synthetic methods however involved the usage of costly metal and carbon substrates, hindering costeffective mass-production of the electrodes. Additionally, the incompatibility issue among the components of the composite would prevent the precise control of the electrode features, e.g. the distribution and microstructure of electroactive materials, porosity, and overall electrode dimension.21,29,30 In this paper, we report a simple synthetic strategy for the fabrication of free-standing composite electrodes with hierarchical porosities. Using sulfonated high internal phase emulsion polymers (polyHIPE) as the substrate and carbon source, Fe3O4 nanocrystals (NCs) were in situ nucleated on the pyrolyzed carbon foam (CF) upon the controlled annealing process. The as-developed configuration has several novel advantages: (1) The optimized size range of Fe3O4 NCs reduces the solid state Li+/electron diffusion length. (2) The interconnected macroporosity of the carbon foam affords ion reservoirs and thoroughfares to facilitate electrolyte percolation; in addition, there is an improved wettability and electrolyte accessibility through channelling of the mesopores. (3) The uniform distribution and encapsulation of Fe3O4 NCs within carbon guarantee the structural robustness allowing for volumetric changes during the conversion reactions of Fe3O4,

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RESULTS AND DISCUSSION The preparation of carbon foam encapsulated Fe-based electroactive species as free-standing electrodes is illustrated in Scheme 1. Our synthesis begins with the development of macroporous polymeric foam via a High Internal Phase Emulsion (HIPE) method (process I), in which the oil phase of the emulsion gradually polymerizes into a monolithic, porous polymer, while the aqueous phase, initially serving as the liquid droplet template, gradually evaporates, leaving the interconnected macropores behind.18 Then the sulfonation of the polymer is conducted to prevent depolymerization and stabilize B


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Chemistry of Materials the microstructure of the polymer during carbonization. As shown in the field-emission scanning electron microscopy (FESEM) images at different magnifications in Figure S1, the sulfonated HIPE polymer exhibits a “continuous” system with interconnected macropores with the diameter ∼1−5 μm. The development of the carbon foam-based free-standing electrodes is realized via a facile method which involves the immersion of the sulfonated polymers in the precursor solution (process II, Scheme 1). In brief, Fe3+ ions are anchored with the sulfonate groups of the polyHIPE polymer, forming a “Fe3+impregnated polymer.” During the ensuing calcination process at 260 °C, the Fe3+ ions partially convert into the amorphous oxide phase accompanied by the oxidative decomposition of the sulfonate groups. At the final stage, pyrolysis is done in a N2 atmosphere to completely carbonize the polyHIPE polymer to the macro−mesoporous carbon foam matrix, into which ultrafine Fe3O4 nanocrystals are incorporated with uniform distribution. Important features of the free-standing electrodes, such as shape, size, and thickness of the electrode, could be customized using different polyHIPE polymers as structural supports. In an effort to investigate the impact of temperature on the particle size of Fe3O4, a number of carbon foam encapsulated Fe3O4 nanocrystal free-standing electrodes (CF/ Fe3O4−FS) were produced for this study: they are CF/Fe3O4− FS-1, CF/Fe3O4−FS-2, CF/Fe3O4−FS-3, CF/Fe3O4−FS-4, and CF/Fe3O4−FS-5 with increasing annealing temperatures of 500, 550, 600, 650, and 700 °C, respectively. The asprepared CF/Fe3O4−FS-2 electrode, shown here as an example, has a rectangular shape of 10 mm × 7 mm (inset photograph in Figure 1a) with a rough thickness around 0.62 mm (cross-sectional FESEM image in Figure S2). The top view of the CF/Fe3O4−FS-2 in the FESEM images at low magnification exhibits a three-dimensional structure of the electrode with the interconnected macropores (Figure 1a), suggesting the well-preserved microstructure of the sulfonated HIPE polymer upon the carbonization process. A closer scrutiny by FESEM images, shown in Figure 1b and c, reveals a very uniform distribution of ultrafine nanocrystals (NCs) over the carbon matrix. In spite of the heavy loading, most of the NCs have been encapsulated within the carbon matrix without agglomeration. Another noteworthy feature is the firm binding of the Fe3O4 nanocystals onto the carbon foam substrate; no particle is observed to be detached from the carbon substrate even if ultrasonic pretreatment of the composite (20 kW, 120 Hz) was conducted for 10 min before the sample preparation for electron microscopy (EM) characterization. Crystallinity is confirmed by high resolution transmission electron microscopy (TEM; Figure 1c, inset) where well-defined lattice fringes spaced 0.25 nm apart corresponding well to (311) planes of cubic spinel Fe3O4 were observed. Energy-dispersive X-ray spectroscopy (EDS) elemental maps for the CF/Fe3O4−FS-2 composite (Figure 1f) shows the concordance of C, O, and Fe signals, suggesting the uniform distribution of Fe3O4 NCs throughout the carbon matrix. If the Fe3+-impregnated polyHIPE polymer was annealed at 500 °C while keeping all other experimental parameters unchanged, the CF/Fe3O4−FS1 composite demonstrates incomplete carbonization of the polyHIPE: the partially polymeric nature led to gradual material degradation upon the excessive heating during the FESEM characterization (Figure S3a). When the annealing temperature was increased to 600, 650, and 700 °C, the composite electrodes show the uniform distribution of well-crystallized nanocrystals with diameters of ∼20 nm for CF/Fe3O4−FS-3

Figure 1. (a) FESEM image at low magnification and photograph (inset) of CF/Fe3O4−FS-2 electrode. (b) FESEM image of CF/ Fe3O4−FS-2 with the uniform distribution of Fe3O4 NCs on the carbon matrix. (c) High magnification FESEM image and HRTEM image (inset) of the CF/Fe3O4−FS-2 composite. (d) FESEM image of CF/Fe3O4−FS-2 and corresponding EDS element maps of C, O, and Fe. High magnification FESEM image and HRTEM image (inset) of (e) CF/Fe3O4−FS-3 composite and (f) CF/Fe3O4−FS-4 composite.

(Figure 1e and inset), 40−50 nm for CF/Fe3O4−FS-4 (Figure 1f and inset), and 100−150 nm for CF/Fe3O4−FS-5 (Figure S3b), respectively. Although the particle size of the Fe3O4 dramatically increases upon elevating the annealing temperature, all these free-standing electrodes exhibit uniform distributions of Fe3O4 NCs in the carbon foam support without forming separate dispersed nanoparticles or agglomerates. This structural feature could be ascribed to the sulfonate groups which play a crucial role in enhancing the wettability of the HIPE polymer in the aqueous precursor solution by providing anchoring sites for the metal ions. For control experiments where the sulfonated polyHIPE polymer was used without pH neutralization, densely packed Fe3O4 aggregates composed of nanocubes of ∼40−50 nm (CF/Fe3O4−W/O− N) were sparsely distributed on the carbon foam, as shown in the FESEM images at different magnifications (Figure S3c and inset). It is thus speculated that the pH adjustment of the sulfonated polyHIPE polymer has activated the sulfonate groups on the polymer surface by providing the negatively charged anchoring sites (sulfonate groups RSO3−) for Fe3+ adsorption and thus increasing the mass loading of the C


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volume of 0.20 cm3 g−1 (cumulative pore volume for the mesopores of 2−50 nm) for CF/Fe3O4−FS-2. The increased interfacial contact between electrolyte and electrode material is crucial for the sustenance of a high Li+ flux across the solid/ liquid interface. From the Barrett−Joyner−Halenda (BJH) pore size distribution plot in the inset image of Figure S4b, the broad pore size distribution mainly locates at two ranges: (1) less than 5 nm (the micro/mesopores for the polyHIPEderived carbon) and (2) larger than 80 nm (macropores derived from the polyHIPE polymer precursor; Figure 2e, inset). This incorpration of interconnected macropores, theoretically, is effective in buffering the volume variation in the conversion reaction, and for reducing the diffusion limitations in purely micro-/mesoporous materials.34 The galvanostatic discharge/charge tests of polyHIPE pyrolyzed carbon were conducted at the current density of 100 mA g−1 (Figure S5a); the reversible specific capacities were evaluated as 102.3 mA h g−1, 132.5 mA h g−1, 164.7 mA h g−1, 176.2 mA h g−1, and 190.6 mA h g−1, respectively (obtained from discharge capacity of the fifth cycle for the prelithiated electrodes). On the basis of the gravimetric loading of Fe3O4 in these composites obtained from thermal gravimetric analysis (TGA) results (Figure S5b), the theoretical capacities of the electrodes CF/Fe3O4−FS-1, CF/Fe3O4−FS-2, CF/Fe3O4−FS3, CF/Fe3O4−FS-4, and CF/Fe3O4−FS-5 are estimated to be 319.3 mA h g−1, 431.7 mA h g−1, 463.0 mA h g−1, 521.1 mA h g−1, and 551.2 mA h g−1, respectively. The capacities are approximated according to the following equation: theoretical capacity = mass fraction of Fe3O4 in the composite × theoretical gravimetric capacity of Fe3O4 (926 mA h g−1) + mass fraction of carbon in the composite × reversible capacity of carbon at corresponding annealing temperature). The macroporous carbon foam not only affords a facile pathway for electrical/ionic conductivity but also encapsulates the electroactive iron oxides within the carbon matrices to accommodate the volume fluctuations of Fe3O4 during cycling. Additionally, the tight integration of the carbon foam and the Fe3O4 NCs in this configuration ensures reliable electrical contact and structural integrity of the electrode, eliminating the use of a heavy metallic current collector and polymeric binders. To further highlight the merits of the electrode design for Li+ storage, the CF/Fe3O4−FS composites were directly used as anode electrodes in a Li/composite half-cell for galvanostatic measurements without processing. It is clear from Figure 3a that the CF/Fe3O4−FS-1 electrode has the lowest specific capacity among the electrodes at a current density of 500 mA g−1. Although a reasonable stable cyclability is observed (charge capacity decays from 370 mA h g−1 to 342 mA h g−1 after 70 cycles), the Coulombic efficiency (CE) for this composite (49.9% for the first cycle and less than 95% for first 10 cycles) is however discouraging. The CF/Fe3O4−FS-2 electrode demonstrates a dramatic improvement in cycling performance, with a much higher initial CE of 73.0% and enhanced charge capacity of 479 mA h g−1 viable after 100 cycles (89.7% of the charge capacity during the first cycle). The increase in reversibility and accessible specific capacity of the CF/Fe3O4−FS-2 electrode could be attributed to the better carbonization degree of polyHIPE polymer and increased weight ratio of crystallized Fe3O4 in the composite. With the continued increase of the size range of Fe3O4 NC, the capacity retention gradually became poorer due to incomplete encapsulation of the larger Fe3O4 nanocrystals within the carbon matrix and increased solid state Li+/e− diffusion length: The electrode based on CF/Fe3O4−

electroactive Fe3O4. If no PVA surfactant was added in the precursor solution while keeping all other experimental parameters identical as CF/Fe3O4−FS-2, the composite (CF/ Fe3O4−W/O-PVA) exhibits obvious agglomeration of irregular Fe3O4 particles with a much larger size range compared with the CF/Fe3O4−FS-2 (Figure S3d and inset image). It can be concluded that the linear chain of surfactant PVA could thus provide a constrained oxidative environment of Fe3+ which retards the oxidation of Fe3+ and crystal growth during the annealing process, leading to the ultrafine Fe3O4 NCs encapsulated in the carbon matrix. The X-ray diffraction (XRD) patterns of the CF/Fe3O4−FS composites are shown in Figure 2 where all the diffraction

Figure 2. XRD patterns of various CF/Fe3O4−FS free-standing electrodes.

peaks can be indexed to those of the cubic magnetite structure (ICCD 00-003-0863) without other crystalline phases or impurities. It is noted that the diffraction peaks for the composite material obtained at lower temperatures are obviously broadened, which indicates a nanoscale crystallite nature. According to the calculation from the Debye−Scherrer equation (using the Fe3O4 (311) diffraction peak), the average crystallite sizes are estimated as 7.7 nm for CF/Fe3O4−FS-1, 10.3 nm for CF/Fe3O4−FS-2, 21.7 nm for CF/Fe3O4−FS-3, 38.2 nm for CF/Fe3O4−FS-4, and 108.2 nm for CF/Fe3O4− FS-5. These results are generally consistent with the estimation from the aforementioned electron micrographs. Raman spectroscopy of various CF/Fe3O4−FS composites are recorded in Figure S4a to reveal the nature of polyHIPE derived carbon obtained at different annealing temperatures. It is noted that two distinguishable peaks at about 1348 cm−1 (D band) and 1590 cm−1(G band) gradually become pronounced as the annealling temperature increases from 500 to 700 °C, implying that the carbonization degree has improved. Figure S4b depicts the nitrogen adsorption and desorption isotherms of CF/Fe3O4−FS-2 displaying a classical type IV isotherm with type H4 hysteresis with no adsorption limit at high p/p0. This isotherm type and hysteresis loop confirms the assemblage of slit-shaped mesopores in the macroporous structure.33 The presence of mesoporosity resulted in a large Brunauer− Emmett−Teller (BET) surface of 294.6 m2 g−1 and a pore D


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Figure 3. (a) Cycling performance of various CF/Fe3O4−FS composites measured at a current density of 500 mA g−1. (b) The first, second, 10th, 50th, and 100th charge−discharge curves of CF/Fe3O4−FS-2 electrode. (c) Cycling stability of the CF/Fe3O4−FS-2 electrode at different current densities. (d) Nyquist plots of various CF/Fe3O4−FS composites after 100 cycles of discharge and recharge at 500 mA g−1. The frequency range used for the measurements was 100 kHz−0.1 Hz.

FS-3 could only deliver a charge capacity of 415 mA h g−1 at the end of 100 cycles; CF/Fe3O4−FS-4 showed an even severe capacity fading from 497 mA h g−1 to 332 mA h g−1 upon 50 discharge−charge cycles. Above observations suggest some complementary factors should have collectively contributed to the cycle performance: (1) the complete carbonization degree of the polyHIPE polymer with well-preserved macropores, (2) the small Fe3O4 nanocrystals which shorten the Li+/e− solid state diffusion length, and (3) the intimate encapsulation of Fe3O4 NCs within the pyrolyzed carbon matrix for structural robustness. These factors are apparently most balanced in CF/ Fe3O4−FS-2, resulting in the best performance among the asfabricated electrodes. Figure 3b records the discharge/charge curves of the CF/ Fe3O4−FS-2 electrode during the 1st, 2nd, 10th, 50th and 100th cycle. The discharge curve for the first cycle exhibits prominent features characteristic of Fe3O4: lithium insertion into the spinel framework observed at the initial stage of voltage drop from open-circuit potential to 0.8 V, SEI formation evidenced by the irreversible consumption of lithium (plateau around 0.8 V), and finally the conversion of the oxides to metallic Fe. The sloping curve also associates with (1) the Li+ intercalation into the carbon foam and (2) the interfacial storage mechanism of Li+ on the surface of oxides which is well documented in the literature.35−37 For the first charge curve, the sloping plateau at the voltage between 1.55 and 2.0 V is generally an indication of the reoxidation of Fe0 to Fe3O4. The

unsatisfactory irreversible capacity loss during the initial cycle (379 mA h g−1) could be attributed to Li+ consumption during the formation of the SEI layer and irreversible trapping of Li+ by the functional groups of the carbon foam. The discharge plateau is upshifted to around 1.0 V from the second cycle onward, implying a more facile electrochemical reduction to zerovalent Fe after the first cycle. The CE for CF/Fe3O4−FS-2 approaches 92.7% during the second cycle and higher than 98% after 10 cycles, showing that only a negligible amount of Li ions has been further consumed in parasitic SEI-forming reactions due to the electrolyte degradation. The superimposability of the second, 10th, 50th, and 100th discharge−charge curves demonstrates the good structural robustness of CF/Fe3O4− FS-2 upon the cycling. Since the rate capability determines the output power, it is crucial to investigate the cycling stability of the CF/Fe3O4−FS2 electrode at various current densities (Figure 3c). Initially, the capacity of the prelithiated electrode was stabilized to 550 mA h g−1 by cycling it at 200 mA g−1. Then, the current density was increased to 500 mA g−1, 1 A g−1, 2 A g−1, and 4 A g−1 in a stepwise manner for every subsequent 10 discharge−charge cycles, where stable capacities of 426 mA h g−1, 484 mA h g−1, 435 mA h g−1, and 376 mA h g−1, respectively, were retrievable. Considering the electrode dimension, the calculated areal density of 5.1 mA h cm−2 was obtained at a high current density of 54.3 mA cm −2 . This remarkable rate performance demonstrated the best results among the Fe-based free-standing E


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Figure 4. (a) FESEM micrographs of the CF/FeS NS composite at low and high (inset) magnifications. (b) TEM image of the CF/FeS NS composite at low magnification. (c) Enlarged view of TEM image and HRTEM image (inset) of CF/FeS NS composite. (d) Low magnification TEM image of the CF/FeS NR composite. (e) HRTEM image of CF/FeS NR composite. (f) FESEM image of the CF/FeS NF composite at low and high (inset) magnifications.

electrodes to the best of our knowledge.19,38−41 Note that the commonly used Ti foam, nickel, or copper foil current collector in the conventional battery configuration is ∼5−10 times heavier than the electroactive materials loaded. In this regard, this rate performance is really encouraging since the gravimetric capacity value for the anode part would not be compromised by the carbon additive, auxiliary binder or by a heavy metal current collector. The dependence of the reversible capacities on the current densities for the composite electrodes is also summarized in Figure S6. The capacity differences between the CF/Fe3O4− FS-2 and other composite electrodes become noticeably larger with the increase of current density, indicating the reduction in Li+ storage at high rates is limited by either the poor carbonization of the polyHIPE polymer (CF/Fe3O4−FS-1) or increased solid-state Li+ diffusion length (CF/Fe3O4−FS-3 and CF/Fe3O4−FS-4). The good rate performance of CF/Fe3O4− FS-2 could be again attributed to the optimal synergism between ultrafine Fe3O4 nanocrystals and its complete coupling with the macroporous carbon support to provide a mixedconducting property for facile electrode kinetics. Postcycling characterizations were conducted via electrochemical impedance spectroscopy (EIS) measurements to reveal the electrode kinetics. The Nyquist plots of all the electrodes after 100 discharge-recharge cycles (Figure 3d) shared the similar feature of a purely resistive response at the high frequency end, a semicircle in the high-to-middle

frequency region, and an inclined straight line in the low frequency region.42 The high-frequency ends of the plots intercepted the z′ axis at the same position due to the similar electrolyte resistance (Rs) used for all electrodes. The diameters of the semicircles decreased in the following order: CF/Fe3O4− FS-2 < CF/Fe3O4−FS-3 < CF/Fe3O4−FS-1 < CF/Fe3O4−FS4, which suggests the smallest charge transfer resistance in CF/ Fe3O4−FS-2. The straight line in the low frequency region may be categorically attributed to the Warburg impedance (Zw) associated with Li+ diffusion in the bulk of the material. Moreover, the Warburg-like response of CF/Fe3O4−FS-2 exhibits the largest slope, suggesting the highest solid-state lithium ion mobility as compared with other electrodes.43 These measurements corroborate the good electrochemical performance of CF/Fe3O4−FS-2 due to the facile electrode kinetics. FESEM images of the postcycling CF/Fe3O4−FS-2 composite taken after 50 and 100 cycles (Figures S7 and S8) further exhibit crucial morphological details: the interconnected macrostructure of the electrode has been preserved well, and no cracks are observed after 50 cycles (Figures S7a and S7b); EDX elemental maps of the cycled electrode show a superimposable Fe and O (Figures S7c and S8), implying that the homogeneous distribution of the oxide particles on the carbon foam has been well maintained. On the other hand, the F and P signals were derived from the species of unwashed SEI layer (e.g., LiF, poly(ethylene oxide) (PEO)-type polymer, and LiPF6 salt).44 These observations validate the strong structural F


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100 cycles at a current density of 500 mA g−1 and remarkable rate capability up to 4 A g−1. This design concept was also applied to develop various CF/FeS free-standing electrodes with the demonstration of good cyclability in Li+ storage. Hopefully, this scalable electrode fabrication process, using polyHIPE-derived carbon as the structural support, a conductive medium, and a current collector, could also be adapted to prepare other carbon foam/metal oxide (sulfide) composites for applications in high-performance LIBs, capacitors, catalysts, and sensors.

integrity of the CF/Fe3O4−FS-2 electrode upon the repeated discharge−charge cycles. The negatively charged sulfonate groups contained in the polyHIPE polymer could also serve as the sulfur source for the in situ formation of FeS on the supportive carbon foam. On the basis of the electron micrographs shown in Figure 4a and b, the nanosheets with a lateral length of ∼100−300 nm and a thickness of ∼5−10 nm (based on the observation of the vertically oriented nanosheets) were uniformly dispersed on the carbon foam. The HRTEM image (inset image in Figure 4c) of a typical single nanosheet reveals a lattice spacing of 5.1 Å corresponding to (100) planes of troilite FeS in the XRD pattern (Figure S9). A thin carbon coating layer on the surface of the nanosheet (marked by the blue arrow mark) was also observed. Increasing the annealing temperature from 500 to 600 °C transforms the FeS nanosheets into nanorods with widths of ∼20 nm and lengths of ∼50−80 nm (Figure 4d). A closer examination of a representative nanorod (Figure 4e) reveals similar well-defined lattice fringes of 5.1 Å and a very uniform amorphous carbon coating at the surface (thickness 3− 4 nm indicated by the blue mark). Theoretically, this carbon encapsulation could enable the conversion reaction of the FeS within the carbon and thus prevent the dissolution problem of polysulfides into the organic electrolyte.45 For comparison, a sulfonated polyHIPE polymer without adjusting the pH value was also employed in the preparation. The obtained CF/FeS composite exhibits a flower-like morphology of densely aggregated nanosheets which sparsely distributed on the carbon foam as shown in the FESEM images at different magnifications in Figure 4f. This observation further proves our conclusion that the activation of the negatively charged binding site (RSO3−) for the Fe3+ absorption on the polyHIPE polymer surface occurs via a neutralization process. The XRD patterns of all these composites (Figure S9, Supporting Information) correspond to the phase-pure troilite FeS (ICDD 01-0896926). The electrochemical performance of these CF/FeS freestanding composites is analyzed in detail in the Supporting Information (Figure S10). To the best of our knowledge, there is no pioneer investigation regarding the FeS-based freestanding electrode so far. Thus, this sulfonate anchored synthetic approach provides new possibilities to develop metal sulfide anodes with a free-standing electrode configuration. In summary, using sulfonated polyHIPE polymer as the structural support, we successfully fabricated various composites through the intimate integration of percolating carbon foam and Fe3O4 nanocrystals with tunable size ranges. The sulfonate functional groups grafted on the polymer surface ensure the increased wettability by the precursor solution, and PVA surfactant restricts the oxide growth and prevents the agglomeration of nanocrystals. This free-standing composite configuration demonstrates interesting electrochemical properties for Li+ storage: (1) An interconnected macroporous carbon network facilitates electrolyte percolation and tolerates the volume variations upon the conversion reaction of oxides. (2) The mesoporosity affords a numerous electrode/electrolyte contact to support a high Li+ flux across the interface. 3) The size range of oxide was also optimized to provide the best balance of uniform distribution, short solid state Li+ diffusion length, and intimate electrical wiring with carbon support. When directly used as a free-standing anode, the CF/Fe3O4− FS-2 electrode demonstrates the best overall electrochemical performance: a large reversible capacity of 479 mA h g−1 after

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EXPERIMENTAL SECTION

Materials. All chemicals were used as received. Iron(III) chloride hexahydrate (FeCl3· 6H2O, 98%) and poly(vinyl alcohol) (PVA, MW = 115 000) were obtained from Lancaster and BDH Chemicals, respectively. Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 98%), styrene (C6H5CHCH2, 99%), divinylbenzene (DVB, 80%), Span80, calcium chloride hexahydrate (CaCl2.6H2O, 99%), 1,2-dichloroethane (ClCH2CH2Cl, 99%), acetic anhydride ((CH3CO)2O, 99%), urea (H2NCONH2, 98%), and metallic Li foil (99.9%) were purchased from Sigma-Aldrich. Potassium persulfate (K2S2O8, 99%) was obtained from Merck KGaA. 4-Vinylbenzyl chloride (VBC, 90%) was purchased from Fluka. Ultrapure water (Millipore) with a resistivity higher than 18.2 MΩ cm was used as a solvent. Synthesis of PolyHIPE Polymer. The procedure for polyHIPE synthesis was described by Wang et al. and in our previous work.18,46 In brief, the the oil phase (0.6 mL styrene, 0.3 mL DVB, 0.1 mL VBC and 0.3 g surfactant Span 80) was mixed with an aqueous phase (6 mL distilled water, 0.165 g of stabilizing salt CaCl2·6H2O, and 0.0150 g of initiator K2S2O8) to form an emulsion under constant stirring. This emulsion phase was then transferred to a polytetrafluoroethylene (PTFE) mold and stored in an oven at 65 °C for 48 h. During this process, the monomers in the continuous oil phase started to polymerize while the aqueous phase evaporated, giving rise to interconnected macropores within the polymer. The polymer was cut into pieces of desired dimensions and washed with distilled water at 100 °C for 24 h to leach out salt residues and with ethanol at 60 °C for 48 h to remove the surfactant and unreacted monomers. Afterward, the dried polymer pieces were sulfonated by concentrated sulfuric acid at 90 °C for 24 h. The sulfur content in the sulfonated HIPE polymers can be controlled by adjusting the concentration of the sulfuric acid, temperature, or the duration of the sulfonation process. Development of CF/Fe3O4 Free-Standing Electrodes. A total of 200 mg of PVA was dissolved in 20 mL of deionized water under constant magnetic stirring. A calculated amount of FeCl3·6H2O (2.1 g) was then added to complete the preparation of the precursor solution. A total of 2 g of the polyHIPE polymer was first rinsed with a 0.1 M NaOH solution until the pH value of the decantate was neutral, then immersed in the precursor solutions at 45 °C for 4 h. The precursorimpregnated polymer was then removed, rinsed with deionized water, and dried in a vacuum at 100 °C for 30 min. The precursorimpregnated polymer was heated in a muffle furnace from ambient temperature to 260 °C with a ramping rate of 3 °C min−1 and then maintained for 30 min to remove the residue sulfonate functional groups. Afterward, the polymer was then carbonized in an alumina crucible under a N2 atmosphere in a tube furnace for 30 min (Heraeus Tube Furnace). The composites annealed at 500, 550, 600, 650, and 700 °C were designated as CF/Fe3O4−FS-1, CF/Fe3O4−FS-2, CF/ Fe3O4−FS-3, CF/Fe3O4−FS-4, and CF/Fe3O4−FS-5, in order of increasing calcination temperature. The as-fabricated composite was designated as CF/Fe3O4 W/O PVA if PVA surfactant was not employed in the precursor solution. When the sulfonated polyHIPE polymer was used without rinsing (pH value has not been adjusted to neutral), while keeping all other preparation conditions identical to those of CF/Fe3O4−FS-2, the fabricated composite was designated as CF/Fe3O4 W/O N. The sulfonated polyHIPE polymer was also directly calcinated into the carbon material under a N2 atmosphere for 30 min at different temperatures: the pyrolyzed carbon materials G



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obtained at 500, 550, 600, 650, and 700 °C were designated as polyHIPE-carbon-1, polyHIPE-carbon-2, polyHIPE-carbon-3, polyHIPE-carbon-4, and polyHIPE-carbon-5, respectively. Development of CF/FeS Composites with Tunable Morphologies. Macroporous carbon foam encapsulated iron sulfide nanosheet (CF/FeS-NS) composite was obtained when the precursorimpregnated polymer, as prepared in the preceding section, was directly calcinated in an alumina crucible under argon atmosphere at 500 °C for 30 min without pretreatment in air at 260 °C. When the annealing temperature increased to 600 °C, the as-fabricated composite was designed as CF/FeS-NR. A carbon foam encapsulated FeS nanoflower (CF/FeS NW) composite was prepared when the unrinsed sulfonated polyHIPE polymer (pH value has not been adjusted to neutral) was used in the preparation, while keeping all other preparation conditions identical such as CF/FeS-NS. Materials Characterization. A scanning electron microscope (SEM) characterization was performed on a Zeiss 1550 instrument operating at 5−10 kV. In situ energy-dispersive X-ray (EDX) analysis was carried out during the FESEM session using a Horiba EMAX attachment analyzer. Transmission electron microscope (TEM) and high resolution-TEM (HR-TEM) images were taken on a JEOL JEM2010F operating at 200 kV. X-ray powder diffraction patterns (XRD) of the composites were recorded on a Bruker D8 ADVANCE Diffractometer using Cu Kα radiation. Raman analysis was performed using a Renishaw Ramascope equipped with a Lieca LM optical microscope, a CCD camera, and an argon ion laser (λ = 514.5 nm) source. Thermal gravimetric analysis (TGA) was performed using a TA Instruments TGA Q500 where the temperature was ramped from 40 to 700 °C at 10 °C/min in the air. BET measurements were carried out on a Micromeritics ASAP 2020 analyzer at 77 K equipped with the V3.04 E software as a porosity analyzer. A specimen for post-mortem analysis was treated as follows: the composite electrode was cycled at 500 mA g−1 for 100 cycles and then left to rest at open circuit voltage for 24 h. The anode electrode was disassembled from the pouch cell in a glovebox followed by washing with NMP and vacuum drying at 100 °C for 12 h. The sample was mounted in the SEM holder under an argon atmosphere and then quickly transferred into the SEM, reducing the air exposure to less than 20 s. Electrochemical Measurements. The electrochemical properties of the composites for reversible Li+ storage were evaluated by galvanostatic discharging and charging. The as-developed CF/Fe3O4 composites were directly used as the working electrode without using a metal foil current collector. A lithium metal foil was used as the counter electrode, and the electrolyte was a 1 M LiPF6 solution in a 3:7 (v/v) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Battery cell assembly was carried out in a recirculating Ar glovebox (MBRAUN labmaster 130) where the moisture and oxygen contents were below 1 ppm each. The test cells were discharged (Li+ insertion) and charged (Li+ extraction) galvanostatically at room temperature in the 0.1 to 2.5 V voltage window at different current densities on a Digatron BTS-600 battery tester. A μAutolab Type III potententiostat/galvanostat with a FRA2 frequency analyzer and Nova 1.5 software was used for electrochemical impedance spectroscopy (EIS). Impedance was measured for electrodes in the fully delithiated state in the frequency range of 100 kHz to 0.1 Hz with a small perturbation of ±10 mV.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been funded by the Swedish Foundation for Strategic Research within the project Road to Load, Swedish Research Council and The Swedish Energy Agency. The Knut and Alice Wallenberg Foundation are acknowledged for an equipment grant for the electron microscopy facilities at Stockholm University.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

FESEM images, Raman spectra, nitrogen adsorption−desorption isotherms, TGA curves, XRD patterns, and electrochemical cell data. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.chemmater.5b00853. H


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