Solid-Solution Sulfides Derived from Tunable Layered Double

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Solid-Solution Sulfides Derived from Tunable Layered Double Hydroxide Precursors/Graphene Aerogel for Pseudocapacitors and Sodium-Ion Batteries Yajie Song, Hui Li, Lan Yang, Daxun Bai, Fazhi Zhang, and Sailong Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13622 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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ACS Applied Materials & Interfaces

Solid-Solution Sulfides Derived from Tunable Layered Double Hydroxide Precursors/Graphene Aerogel for Pseudocapacitors and Sodium-Ion Batteries Yajie Song, Hui Li, Lan Yang, Daxun Bai, Fazhi Zhang, and Sailong Xu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China *Corresponding Author. E-mail: [email protected]

Abstract Transition metal sulfides (TMSs) are suggested as promising electrode materials for electrochemical pseudocapacitors and lithium-/sodium-ion batteries, however, typically involve mixed composites or conventionally stoichiometric TMSs (such as NiCo2S4 and Ni2CoS4). Herein we demonstrate a preparation of solid-solution sulfide (Ni0.7Co0.3)S2 supported on three-dimensional graphene aerogel (3DGA), via a sulfuration of NiColayered double hydroxide (NiCo-LDH) precursor/3DGA. The electrochemical tests show that the (Ni0.7Co0.3)S2/3DGA electrode exhibits a capacitance of 2165 F g–1 at 1 A g–1, 2055 F g–1 at 2 A g–1, and 1478 F g–1 at 10 A g–1, which still preserves 78.5% capacitance retention upon 1000 cycles for pseudocapacitors, and, in particular, possesses a relatively high charge capacity of 388.7 mA h g–1 after 50 cycles at 100 mA g–1 as anode nanomaterials for sodium-ion batteries. Furthermore, the electrochemical performances are readily tuned by varying cationic type of the tunable LDH precursors to prepare different solid-solution sulfides, such as (Ni0.7Fe0.3)S2/3DGA and (Co0.7Fe0.3)S2/3DGA. Our results

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show that engineering LDH precursors can offer an alternative to prepare diverse transition metal sulfides for energy storage. Keywords: Transition metal sulfide; Layered double hydroxide precursor; Graphene aerogel; Pseudocapacitors; Sodium-ion batteries

1. Introduction The development of both high-efficiency and tunable electrode nanomaterials for supercapacitors and lithium-/sodium-ion batteries has been of particular interest to match rapid growth in energy consumption.1-4 Electrochemical capacitors, especially pseudocapacitors, have been demonstrated attractive for energy storage devices, owing to possessing larger specific capacitances than the electrical double layer capacitors.5,6 Among various electrode nanomaterials, for instance, carbon (activated carbon, grapheme and carbon nanotube),7 transition metal oxides and hydroxides (RuO2, Co3O4, and Ni(OH)2),8 as well as conducting polymers (polyaniline, polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene)),9 transition metal sulfide (TMS) composites have been intensively identified as one type of promising electrode materials for supercapacitors, because of the higher electrical conductivity and the richer electroactive sites compared with the counterparts of single TMSs or transition metal oxides.10-14 Diverse mixed TMS nanostructures were created, such as Ni3S4@MoS2 nanospheres,15 nanotriangular pyramid Ni3S2@CoS core/shell arrays grown on Ni foam,16 amorphous nickelsulfide@CoS double-shelled polyhedral nanocages.17 The TMS composites were able to deliver greatly enhanced specific capacitances compared with the single TMSs, owing to the synergistic interfacial charge storage of the bi-component-active components.18,19 Furthermore, by virtue of the interface contact created down to the atomic scale, ternary TMS nanostructures have


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been demonstrated to exhibit the greatly boosted electrochemical performances, such as NiCo2S4@polypyrrole nanotube core/shell grown on Ni foam,20 caterpillar-like NiCo2S4,21 NiCo2S4 and Ni2CoS4 nanoboxes,22 as well as NiCo2S4 nano-onion.23 However, the ternary TMSs only involve conventionally stoichiometric NiCo2S4 and Ni2CoS4, most of which were derived from the specific Co3O4 precursor. Therefore, it is of genuine interest to overcome the difficulty in preparing controllable precursors to develop diverse and tunable TMSs for energy storage. Inspired by these previous studies, we herein describe a preparation of solid-solution (Ni0.7Co0.3)S2 supported on three-dimensional graphene aerogel (3DGA), through a sulfuration of cation type-tunable layered double hydroxide (LDH) precursors/3DGA (Scheme 1). LDHs, also

Scheme 1. Schematic illustration of solid-solution MS2/3DGA composites derived from cationtunable LDH precursors. well-known as a large family of anionic clay compounds,24-26 possess the general chemical formula of [C2+1-xC3+x(OH)2]x+(An–)x/n•yH2O, which contains layered structure similar to the Mg(OH)2-like layers, readily tunable cations (C2+ and C3+), and anions (An–), as well as molar ratio of C2+/C3+ ranging from 2 to 5. In distinct contrast to other synthesis routes,27 LDH-based approaches has a very convenient flexibility of tailoring the LDH precursors toward multifunctional nanomaterials, especially in energy storage. In this case, the NiCo-LDH precursor is chosen to prepare (Ni0.7Co0.3)S2 supported on 3DGA. The (Ni0.7Co0.3)S2 is endowed with the following advantages: (i) the good distribution of CoS2 and NiS2 at the atomic scale, and (ii) the improved conductivity



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of the underlying of 3DGA. Significantly, the as-prepared (Ni0.7Co0.3)S2/3DGA composite exhibits high electrochemical performances for pseudocapacitors and sodium storage. In particular, this LDH precursor-derived synthesis route is readily extended to prepare different ternary TMS (such as (Ni0.7Fe0.3)S2/3DGA and (Co0.7Fe0.3)S2/3DGA) with the comparable electrochemical performances, by easily altering tunable LDH cations. 2. Experimental Section 2.1 Materials All chemicals were purchased without any further purification. Ni(NO3)2•6H2O, Fe(NO3)2•6H2O, Co(NO3)2•6H2O, NH3•H2O, 30% H2O2, citric acid, sulfur powder, graphite powder, NaHSO3, Na2S•9H2O, NaNO3, KMnO4, 85% N2H4•H2O and 98% H2SO4, 10% HCl from J&K scientific Co. Ltd. (China). Graphene oxide (GO) was prepared on the basis of a modified Hummer’s method.28 2.2 Preparation of tunable LDH precursors/3DGA The NiCo-LDH precursor was in situ grown on 3DGA support via a traditional co-precipitation method. In brief, a GO-containing suspension A was obtained through dispersion of the asprepared GO (1 mg) into a 50 mL distilled water for 2 h, and a solution B was available by dissolving bivalent salts (3 mmol Ni(NO3)2•6H2O and 1 mmol Co(NO3)2•6H2O), 200 mg citric acid, and H2O2 (1 mL) into a 100 mL deionized water. Then the as-prepared solution B was added dropwisely into the above-obtained solution A with strong stirring until pH = 6.8 adjusted by using 1% NH3•H2O. The hydrothermal treatment was conducted to the slurry at 150 °C for 24 h. The NiCo-LDH/3DGA precursor was obtained after the centrifugation, washing with enough deionized water, ethanol several times, and eventually freeze-drying overnight for use.


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The NiFe- and CoFe-LDH precursors were also prepared under the same experimental conditions only by varying type of metal salts. 2.3 Preparation of solid-solution MS2/3DGA The collected LDH precursor/3DGA was calcined with sulfur powder at a precursor/sulfur powder weight ratio of 1 : 3. The precursor and sulfur powder were loaded into two different porcelain boats, the latter of which was put separately at the upstream side of the furnace. The samples were then heated at a temperature of 400 °C with a heating speed of 2 °C /min in a N2 atmosphere, and kept for 4 h. For comparison, the pure sulfides and graphene aerogel were also prepared under the same experimental conditions except for no addition of GO or metal salts, respectively. 2.4 Characterization X-ray diffraction (XRD) measurement was conducted to samples on a diffractometer (Rigaku XRD-6000) equipped with a graphite-filtered Cu Kα source (Kα = 1.54178 Å). Scanning electron microscope (SEM, ZEISS Supra 55), and high-resolution transmission electron microscope (TEM, JEOL JEM-2100) were utilized to image the morphology, dimensional sizes, and crystalline phase of the as-obtained samples. X-ray photoelectron spectroscopy (XPS) of samples was used on a Xray photoelectron spectrometer possessing an Al Kα radiation of 1486.6 eV (Kratos Axis ULTRA). Raman spectroscopy of samples was acquired on a confocal Raman spectrometer (Renishaw RM2000). Specific surface area and pore size distribution of samples were determined by using N2 adsorption-desorption isotherms on Quanta chrome apparatus (Nova 1200). The thermogravimetric analysis (TGA) was conducted in air atmosphere from 25°C to 1000 °C at a rate of 10 °C /min. Inductively coupled plasma-emission spectrometer (Shimadzu, ICP-ES) was utilized for elemental analysis.



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2.5 Electrochemical measurement To evaluate the electrochemical performances for pseudocapacitors, all the samples were examined on a commercial electrochemical workstation (CHI 760E, Shanghai, China), which was conducted by using a three-electroded cell, 6 M KOH was used as the aqueous electrolyte. Platinum foil was used as the counter electrode, and the Hg/HgO electrode was used as reference electrode. The working electrode was fabricated via manually blending the electroactive material, polymer binder (polyvinylidenediuoride, PVDF), and carbon black (super-P) with a weight ratio of 80 : 10 : 10 with the solvent (N-methyl-2-pyrrolidone, NMP). The working electrode was obtained after pasted with the current collector of Ni foam, with a quantitatively determined mass loading of ca. 3.0 mg cm–2 for the active material. Galvanostatic charge/discharge measurement was performed to detect supercapacitive performance between 0 and 0.41 V (vs. Hg/HgO) at current densities. Capacitances were calculated, based on the loadings of the active material in the electrode by using the following equation: Cm = (IDt)/(MDV), Cm represents the specific capacitance (F g−1), ID stands for the discharge current (A), t stands for the discharge time (s), MD means the mass loading for the active material (g), and V stands for the potential window during discharge (V). Electrochemical impedance spectroscopy (EIS) was performed with a frequency, which is between 0.01 Hz and 100 kHz at the open circuit potential. An asymmetric pseudocapacitor was assembled, with the (Ni0.7Co0.3)S2/3DGA electrode as the positive electrodes, and activated carbon (AC) (1 × 1 cm2) as the negative electrodes. Electrochemical evaluation was conducted in a two-electrode cell, 6 M KOH was used as the aqueous electrolyte. According to the following equations: E =0.5 Csp ΔV2, P = E/t, energy density E (Wh kg−1) and power density P (W kg−1) were quantitatively determined, where Csp is specific capacitance (F g−1), ΔV means window potential (V), and t is discharge time (s).


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To evaluate the electrochemical performances for sodium-ion batteries (SIBs), the electrochemical measurement was implemented with 2032 coin cells, which were assembled in an Ar-filled glove box. Electroactive material, carbon black (super-P), and PVDF, which were mixed at 70 : 20 : 10 (wt %), and Cu foil was used as the current collector to prepare the working electrodes, and metallic sodium as counter electrode, glass fiber (Whatman) was used as the separator, a propylene carbonate solution which contain 1 M NaClO4 was used as the electrolyte for SIBs. Galvanostatic discharge/charge cycling (0.01–2.7 V) was evaluated on LAND CT 2001A battery tester. 3. Results and Discussion The NiCo-LDH precursor/3DGA exhibits the characteristic diffraction peaks with 2θ centered at 11.1°, 22.1°, 34.4°, and 38.6° (Figure S1a), which can be indexed to the typical reflection peaks of (003), (006), (009), and (015) of the hydrotalcite-like compound, respectively. SEM image confirms that the NiCo-LDH nanoplatelets are well dispersed on the underlying 3DGA architecture (Figure S1b). After the completion of subsequent calcination of the NiCo-LDH precursor/3DGA with sulfur powder, (Ni0.7Co0.3)S2/3DGA composite was obtained. The resulting (Ni0.7Co0.3)S2/3DGA shows the characteristic diffraction peaks (Figure 1a), which can be indexed to the lattice planes of NiS2 phase (JCPDS No. 11-0099). Note that a slight shift of each reflection peak is clearly visible with regard to those of the standard PDF cards of NiS2 (JCPDS No. 11-0099) and CoS2 (JCPDS No. 27-0341). The apparent shift can be attributed to the doping of Co cation into the NiS2 phase. The atomic radius of Co (0.083 nm) is much larger than that (0.0745 nm) of the Ni, the Co-doping into NiS2 thus leads to the peak shift to the right. Furthermore, the ICP-ES result yields a Ni/Co molar ratio of 2.7, thus giving rise to the (Ni0.7Co0.3)S2/3DGA formula of



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Figure 1. (a) XRD pattern, (b) SEM image, as well as (c) TEM and (d) HRTEM images of (Ni0.7Co0.3)S2/3DGA derived from NiCo-LDH precursor/3DGA. Insert of Figure 1d shows a histogram of dimensional size of the supported (Ni0.7Co0.3)S2 nanoparticles. the solid solution obtained. From a previous study,29 a similar cation doping was also reported for graphene/Mn0.25Co0.75O solid solution, which was calcined from a CoMn-LDH/GO precursor at 600 °C in Ar for 2 h. In addition, as reported previously,30 a bi-active NiCo2S4/Ni0.96S composite was derived from a NiCo-LDH precursor/3DGA, via a sulfuration of thioacetamide under the hydrothermal synthesis condition at 120 °C for 24 h. The difference in forming the (Ni0.7Co0.3)S2/3DGA solid solution and the NiCo2S4/Ni0.96S composite could be ascribed to the different sulfuration approaches. The morphology of (Ni0.7Co0.3)S2/3DGA was visualized by using SEM and TEM. The SEM image shows that the 3D architecture is well preserved after the calcination, and also that various nanoparticles are distributed on the curly graphenes with the interconnected pores ranging from submicrometer to several micrometers (Figure 1b). The TEM observation corroborates the good


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dispersion of (Ni0.7Co0.3)S2/3DGA nanoparticles on the underlying 3DGA support (Figure 1c). The HRTEM image reveals that a highly crystalline (Ni0.7Co0.3)S2 nanodomain is clearly visible, with a well-determined lattice spacing of 0.231 nm referring to the (211) crystal planes of (Ni0.7Co0.3)S2 (Figure 1d). Note that the value is slightly smaller than that of the standard card NiS2 phase (0.232 nm), indeed consistent with the shift of the XRD reflection peaks observed above in Figure 1a. The particle size distribution of the supported (Ni0.7Co0.3)S2 nanoparticles was measured from more than 50 nanoparticles. The histogram shows that the mean dimensional size of (Ni0.7Co0.3)S2/3DGA nanoparticles is determined to be 32.9 ± 0.79 nm (insert of Figure 1d), demonstrating a good dispersion of (Ni0.7Co0.3)S2 nanoparticles on the 3DGA support. The textural structure of the (Ni0.7Co0.3)S2/3DGA composite was examined using N2 adsorption/desorption isotherm measurement. Figure 2a depicts that the isotherms show a H3-type hysteresis loop that can be ascribed to type IV. The specific surface area is calculated to be 20.9 m2 g−1, and the pore size distribution ranges mainly between 2 and 10 nm (Figure 2b). The pore size distribution clearly manifests the typical mesoporous characteristic, which, together with

Figure 2. (a) N2 adsorption/desorption isotherm and (b) pore size distribution of the (Ni0.7Co0.3)S2/3DGA composite.



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the appropriate surface area, endows the 3D architecture with both the high interfacial contact to electrolyte and the rich open channels for electrolyte and charge transports, both of which are expected to highly improve the electrochemical performance.31 TGA technique was used to quantitatively determine the content of 3DGA in the composite. Figure S2 shows that the weight loss prior to 200 °C can be assigned to the surface adsorbed water adsorbed. Another weight loss occurring between 200 °C and 580 °C can be ascribed to the oxygen-containing functional groups of (Ni0.7Co0.3)S2 and the phase transformation during the airannealing process. The last weight loss which is between 580 °C and 800 °C can be attributed to the removal of 3DGA by oxidation.32 The TGA result shows that the composite contains 26% 3DGA support and 74% (Ni0.7Co0.3)S2. Raman spectroscopy was utilized to examine the graphitic characteristic of the composite. Figure S3 shows that the sample exhibits two typical peaks, i.e., the disorder (D) bands and graphite (G) bands, which are centered at 1350 and 1580 cm–1, respectively. The intensity ratio (ID/IG) is thus determined to be 1.11, much higher than that (0.97) of the pure GO.33 The high value apparently reflects the decrease in the average size of the sp2 domains and also the effective removal of the oxygen functional groups upon the completion of the reduction of the GO. XPS technique was used to characterize the elemental compositions and chemical states of (Ni0.7Co0.3)S2/3DGA. Figure 3a displays a survey spectrum showing the co-existence of all the peaks of S, Ni, O, Co, N, and C elements. In the case of Co 2p (Figure 3b), the peaks visible at 784 and 798.4 eV represent the existence of Co2+, and the ones at 776 and 803.6 eV represent the existence of Co3+. Concerning Ni 2p (Figure 3c), the signals at 857.4 and 875.5 eV can be


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Figure 3. XPS spectra of (Ni0.7Co0.3)S2/3DGA: (a) full scan survey, (b) Co 2p, (c) Ni 2p, (d) C 1s, (e) S 2p, and (f) N 1s. assigned to Ni 2p, respectively, thus conforming the Ni2+ oxidation state.34-36 The oxidation states of Co2+ and Co3+ support the Co cation-doping into the NiS2 phase, which is well consistent with the above-mentioned shifts of XRD reflection peaks. Concerning C 1s spectrum (Figure 3d), a strong peak is clearly observed at 284.8 eV, corresponding to the graphitic carbon, while the weak peaks centered at 288, 285.3, and 284.4 eV are ascribed to the co-existing oxygenated carbon species involving C=O, C–O–C, and C–OH,36 respectively, all of which strongly suggest the deoxygenation of GO during the carbon thermal reduction. For S 2p spectrum (Figure 3e), a major peak is centered at 168.9 eV, which corresponds to the metal–sulfur bonds.34 For N 1s spectrum (Figure 3f), three peaks, centered at 400.9, 398.2, and 402.6 eV, can be well related to the pyrrolic N, the pyridinic N, and the quaternary N, respectively. The N species are mainly derived from ammonia during the hydrothermal process.37,38 By combination of the above-obtained results of XRD, TEM, N2 adsorption/desorption isotherm, and Raman spectroscopy, as well as XPS spectroscopy, we conclude that the (Ni0.7Co0.3)S2/3DGA



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composite is successfully obtained, which is endowed with ternary (Ni0.7Co0.3)S2 solid solution, N-doped 3DGA support, appropriate specific surface area and broad mesopore size distributions. From our previous studies,29,30 it is well recognized that these advantages are expected to facilitate improving the electrochemical performances of the (Ni0.7Co0.3)S2/3DGA composite when used as electrode nanomaterials for supercapacitor. For comparison, the pure (Ni0.7Co0.3)S2 and 3DGA architecture were also prepared under the same experimental conditions except for no addition of GO or metal salts, respectively. Figure S4a show a strong XRD reflection peak pattern centered at 2θ = 11.3°, indicative of the formation of 3DGA, and SEM image further confirms 3D, wrinkle, and porous architecture (Figure S4b). The obtained pure (Ni0.7Co0.3)S2 nanoparticles exhibit the typical XRD characteristic of NiS2 (JCPDS No. 11-0099) and CoS2 (JCPDS No. 27-0341) (as shown in Figure S5a), and SEM observation clearly show the agglomerated nanoparticles of pure (Ni0.7Co0.3)S2 resulting from no 3DGA support (Figure S5b). We initially carried out CV measurements of the (Ni0.7Co0.3)S2/3DGA electrode at scan rates ranging between 5 and 50 mV s–1. Figure 4a shows that for the CV profiles, the anodic peaks and the cathodic peaks shift to a positive potential and the negative potential, as scan rates increase. The result strongly suggests that the (Ni0.7Co0.3)S2/3DGA electrode obviously possesses a high charge transfer stability during the sweeping process. Note that only a remarkable couple of redox peaks is clearly visible, which apparently strongly reflects the pseudocapacitor characteristic of the electrode, i.e., the reversible Faradic reactions. In addition, no significant difference in CV shape is found at the different scan rates, which suggests a fast charge/discharge capability of the electrode. Compared with the NiS2/GO electrode reported early,38 the (Ni0.7Co0.3)S2/3DGA


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Figure 4. Electrochemical properties of the (Ni0.7Co0.3)S2/3DGA electrode for pseudocapacitors. (a) CV curves at varying scan rates, (b) galvanostatic charge and profiles at current densities, (c) comparison of capacitance as a function of current density between the electrodes of (Ni0.7Co0.3)S2/3DGA, pure (Ni0.7Co0.3)S2, and 3DGA. (d) Specific capacitance as a function of cycle number. (e,f) Electrochemical performances of the (Ni0.7Co0.3)S2/3DGA//AC asymmetric pseudocapacitor: (e) CV curves at various scan rates, and (f) galvanostatic charge/discharge profiles at a constant current density of 10 A g–1.



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electrode shows remarkable shifts to the lower potential values, which can be attributed to the Co cation-doping. The faradic reaction occurring on the surface of our (Ni0.7Co0.3)S2/3DGA electrode in alkaline media can be described with the following equation:39 (Ni,Co)S2 + OH– ↔ (Ni,Co)S2OH + e– We then carried out galvanostatic charge/discharge measurement to evaluate the supercapacitive performance from 0 to 0.41 V (vs. Hg/HgO) at a vary current densities. Figure 4b displays that each pair of active charge curves and discharge curves exhibit a basically symmetric profile at a current scan rate which possibly features a greatly boosted electrochemical capacitive and electrochemical reversibility. The specific capacitances of

(Ni0.7Co0.3)S2/3DGA were

calculated to be 2264, 2154, 2003, 1895, 1793, 1690, 1400, 1000, 750 F g–1 at a current density of from 1, though 2, 5, 10, 15, 20, 30, and 40, to 50 A g–1, respectively. Apparently, the specific capacitances are much larger than those of pure (Ni0.7Co0.3)S2 and pure 3DGA (Figure 4c). The remarkable differences strongly suggest that 3DGA plays a crucial role in improving the specific capacitances of (Ni0.7Co0.3)S2 electrode, via increasing the specific surface area to support and confine the highly dispersed (Ni0.7Co0.3)S2 nanoparticles, and also via improving the conductivity. We also examined the long-term cycling performance of the electrode at 10 A g–1. Figure 4d manifests that albeit a gradual decrease of specific capacitance upon increasing cycles, the specific capacitance is still as high as 1478 F g–1 after 1000 cycles, leading to a high capacitive retention of 78%. To explore the origin of the slight decrease of cycling stability, the post-cycled (Ni0.7Co0.3)S2/3DGA was examined. The SEM image shows that the 3D architecture of (Ni0.7Co0.3)S2/3DGA after post-cycling is well preserved (Figure S6a), and a closer examination of SEM reveals both the wrinkle graphene aerogel of 3DGA and (Ni0.7Co0.3)S2 nanoparticles dispersed well on the underlying 3DGA support (Figure S6b). By contrast, the (Ni0.7Co0.3)S2


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nanoparticles are agglomerated upon cycling. This could inspire us to improve the cycling stability of our (Ni0.7Co0.3)S2/3DGA electrode by carbonaceous encapsulation.40 Comparison of specific capacitance was conducted between our (Ni0.7Co0.3)S2/3DGA and other electrodes reported early. We can see from Figure S7a that the electrochemical performances of our electrode material are higher than or comparable to those of the TMS electrodes.21-23,36,39-43 We can assign the improvement mainly to the atomic dispersion of the ternary (Ni0.7Co0.3)S2 solid solution. The dispersion of the solid solution can be inherited from the well-ordered arrangement of cations within each LDH crystalline layer, the latter of which has been reported by a previous study of NMR spectroscopy.44 As reported by previous studies,17-22 such a dispersion can promote enhancing the electrochemical behavior of the multiple-component composites with regard to the individual. On the other hand, the highly conductive 3D support has an appropriate specific surface area, as well as a wide pore size distribution of mesopores and macrospores. The advantages are able to greatly boost the electrochemical performances of the (Ni0.7Co0.3)S2/3DGA composite owing to the possibilities to provide mass-buffering reservoirs for the rapid ion diffusion at the interface of electrode and electrolyte, and the interconnected and stable framework for electron transfer.29,30 To support this point, we conducted EIS examination to the electrode of (Ni0.7Co0.3)S2/3DGA, as well as pure (Ni0.7Co0.3)S2 and 3DGA for comparison. Generally, in the low-frequency range, the straight lines with a large slopes illustrate the good capacitive characteristics with a low ion diffusion resistance. Obviously, the (Ni0.7Co0.3)S2/3DGA electrode exhibits a relatively lower slope than that of the pure 3DGA architecture, but a greater slope than that of the pure (Ni0.7Co0.3)S2 (Figure S7b). The contrasts strongly suggest that the (Ni0.7Co0.3)S2/3DGA possesses a high capacitive characteristic and appropriate electron transfer, thus leading to the good electrochemical performance.



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We further assembled a (Ni0.7Co0.3)S2/3DGA//AC asymmetric pseudocapacitor, with the (Ni0.7Co0.3)S2/3DGA electrode as the positive electrodes and the activated carbon (AC) (1 × 1 cm2) as the negative electrodes. The obtained CV curve of the asymmetric pseudocapacitor shows a narrow rectangular-like profile in the range between 0 and 1.4 V (Figure 4e), revealing the unique characteristic with a double layer/faradaic capacitance. In addition, the galvanostatic charge– discharge curves display that the asymmetric supercapacitor possesses a good symmetry after the completion of 200 cycles at a current density of 10 A g–1 (Figure 4f), yielding a maximum energy density of 56 Wh kg−1 (at a power density of 7 kW kg−1) at 10 A g−1. The energy density is comparable to other asymmetric pseudocapacitors,15,22,43 such as MoS2–NiO//MoS2–Fe2O3 (39.6 W h kg−1),15 NiCo2S4@Polypyrrole-50//AC (34.62 Wh kg−1),22 and Zn0.76Co0.24S/carbon nanotubes on N-doped graphene (NG/CNTs)//NG/CNTs (50.2 W h kg−1)43 under an approximate operating voltage ranging from 1.45 to 1.60 V. The result could deserve further investigation to enhance energy density by boosting the cycling stability. Furthermore, nanostructured TMSs are also promising anode materials for sodium storage.45-47 The (Ni0.7Co0.3)S2/3DGA composite was subjected to the electrochemical tests for sodium storage. The electrochemical tests demonstrate that the (Ni0.7Co0.3)S2/3DGA electrode delivers a relatively high initial discharge capacities and charge capacities of 660.5 and 701 mA h g–1 at 100 mA g–1 (Figure 5a), thus yielding a irreversible capacity loss owing to the contribution from the solidelectrolyte-interface (SEI) film. After the completion of the 50th cycling, the reversible capacity still maintains at 388.7 mA h g–1 (Figure 5b), and the Columbic efficiencies retain at ca. 98% since the second cycle. Table S1 shows a comparison of reversible capacity between our (Ni0.7Co0.3)S2/3DGA electrode and the nickel/cobalt sulfides reported previously.48-53 We find


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Figure 5. (a) Galvanostatic charge/discharge curves and (b) cycling performance at 100 mA g−1 of the (Ni0.7Co0.3)S2/3DGA anode nanomaterials for SIBs.

that our composite electrode is able to deliver the reversible charge capacity close to those of the electrodes. On the basis of the above-obtained good electrochemical performances, we can believe that the (Ni0.7Co0.3)S2/3DGA composite could be attractive for energy storage in supercapacitor and SIBs. From the previous studies,54-56 it can be known that LDHs have a unique feature of readily tuning types and molar ratios of metal cations, which has enabled LDHs to sever as one type of very convenient precursors toward multifunctional nanomaterials, especially in energy storage. As a proof of concept, we have explored the LDH precursor-based synthesis route to synthesize different TMSs as electrode materials for supercapacitor and SIBs. Two 3DGA-supported LDH precursors—NiFe- and CoFe-LDHs—were successfully prepared, as evidenced by XRD (Figure S1c and S1d) and SEM (Figure S1e and S1f), respectively. Further sulfuration was carried out to those above two precursors, giving rise to the formation of 3DGA supported TMS solid solutions, i.e., (Co0.7Fe0.3)S2 (JCPDS No. 11-0099) and (Ni0.7Fe0.3)S2 (JCPDS No. 22-0595), as shown in Figures S8a and S8b, respectively. Note that with respect to those of the standard PDF cards, an obvious shift of each XRD reflection peak can also be resolved for both solid solutions, strongly



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reflecting the cation doping. TEM images further reveal the good dispersion of the nanoparticles for (Ni0.7Fe0.3)S2 (Figure S8c) and (Co0.7Fe0.3)S2 (Figure S8d). The HRTEM images shows that the lattice spacings were 0.270 nm referring to the (200) crystal planes of (Ni0.7Fe0.3)S2, and 0.312 nm referring to the (111) crystal planes of (Co0.7Fe0.3)S2, as shown by Figure S8e and S8f, respectively. When used as the electrode for supercapacitor, the electrochemical test shows that the specific capacitances of the (Ni0.7Fe0.3)S2/3DGA were determined to be 2165, 2055, 1600, and 1245 F g–1 at 1, 2, 5, and 10 A g–1, respectively. In addition, the specific capacitance is still up to 1250 F g–1 after 1000 cycles, thus yielding a capacitive retention rate of 78.5% at 5 A g–1 (Figure S9). In the case of (Co0.7Fe0.3)S2/3DGA derived from the CoFe-LDH precursor/3DGA, the specific capacitances were measured to be 1518, 1330.5, 1022.5, and 965 F g–1 at 1, 2, 5, and 10 A g–1. In addition, the specific capacity is up to 875 F g–1 after 1000 cycles at 5 A g–1 (Figure S10), thus giving rise to a capacitive retention rate of 85.6%. Figure S11 displays the Nyquist plots of the (Ni0.7Fe0.3)S2/3DGA and (Co0.7Fe0.3)S2/3DGA electrodes. We can see that (Ni0.7Co0.3)S2/3DGA possesses a lower ion diffusion resistance than (Ni0.7Co0.3)S2/3DGA, which may be suspected to the influence of different cation elements. Furthermore, when used as anode nanomaterials for SIBs (Figure S12), the (Ni0.7Fe0.3)S2/3DGA is capable of possessing a reversible capacity of 287.2 mA h g–1 after 50 cycles at a current density of 100 mA g–1, while the (Co0.7Fe0.3)S2/3DGA electrode is capable of exhibiting a reversible capacity of 370.5 mA h g–1 after 100 cycles at 100 mA g–1. The electrochemical performances are close or comparable to those of the above (Ni0.7Co0.3)S2/3DGA electrode, indeed corroborating the tunability and reproducibility of the LDH precursor-derived TMS solid solution.


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4. Conclusions We have explored a general strategy to prepare solid-solution sulfides supported on 3D graphene aerogel

through

the sulfuration of tunable LDH precursors/3DGA. The

(Ni0.7Co0.3)S2/3DGA composite was endowed with the features capable of improving the electrochemical performances: ternary (Ni0.7Co0.3)S2 with atomic-level interfacial dispersion, highly conductive 3DGA support, as well as the appropriate surface area and wide pore size distributions of co-existing mesopores and macropores. Indeed, the (Ni0.7Co0.3)S2/3DGA electrode displayed a high capacitance of 2165 F g–1 at 1 A g–1, 2055 F g–1 at 2 A g–1 when used an electrode for pseudocapacitors; and showed a reversible capacity of 388.7 mA h g–1 after 50 cycles at 100 mA g–1 for sodium storage. Furthermore, the electrochemical performances were modulated for energy storage in terms of pseudocapacitor and sodium-ion battery, by virtue of the flexibility of steering cation type of LDH precursor to prepare different solid-solution sulfides, such as (Ni0.7Fe0.3)S2/3DGA and (Co0.7Fe0.3)S2/3DGA). The results demonstrate that the precursors-based synthesis route can offer a convenient alternative to design and synthesize diverse transition metal sulfides for energy storage. Supporting Information The Supporting Information, including XRD patterns, SEM and TEM images, the first three galvanostatic charge/discharge curves, is available free of charge on the ACS Publications website at http://pubs.acs.org Notes The authors declare no competing financial interest. Acknowledgments



This

work

was

financially

supported

by

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the

National

Basic

Research Program of China (973 Program, 2014CB932102), and the National Natural Science Foundation of China. References: 1. Kyeremateng, N. A.; Brousse, T. Pech D.; Microsupercapacitors as Miniaturized EnergyStorage Components for on-Chip Electronics. Nat. Nanotechnol. 2017, 12, 7–15. 2. Zhou, T.; Zheng, Y.; Gao, H.; Min, S.; Li, S.; Liu, H.; Guo, Z. Surface Engineering and Design Strategy for Surface-Amorphized TiO2@Graphene Hybrids for High Power Li-Ion Battery Electrodes. Adv. Sci. 2015, 2, 1500027. 3. Zheng, Y.; Zhou, T.; Zhao, X.; Pang, W.; Gao, H.; Li, S.; Zhou, Z.; Liu, H.; Guo, Z. Atomic Interface Engineering and Electric-Field Effect in Ultrathin Bi2MoO6 Nanosheets for Superior Lithium Ion Storage. Adv. Mater. 2017, 29, 1700396. 4. Liu, Y.; Zhou, T.; Zheng, Y.; He, Z.; Xiao, C.; Pang, W.; Tong, W.; Zou, Y.; Pan, B.; Guo, Z.; Xie. Y. Local Electric Field Facilitates High-Performance Li-Ion Batteries. ACS Nano 2017, 11, 8519−8526. 5. Li, P.; Li, J.; Zhao, Z.; Fang, Z.; Yang, M.; Yuan, Z.; Zhang, Q.; Hong, W.; Chen, X.; Yu, D. A General Electrode Design Strategy for Flexible Fiber Micro-Pseudocapacitors Combining Ultrahigh Energy and Power Delivery. Adv. Sci. 2017, 4, 1700003. 6. Lesel, Benjamin K., Ko, Jesse S.; Dunn, B.; Tolbert. Sarah H.; Mesoporous LixMn2O4 Thin Film Cathodes for Lithium-Ion Pseudocapacitors. ACS Nano 2016, 10, 7572–7581. 7. Chen, B.; Jiang, Y.; Tang, X.; Pan, Y.; Hu, S.; Fully-Packaged Carbon Nanotube Supercapacitors by Direct Ink Writing on Flexible Substrates. ACS Appl. Mater. Interfaces 2017, 9, 28433–28440.


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Solid-Solution Sulfides Derived from Tunable Layered Double

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