We present the design and synthesis of three-dimensional (3D)-networked NiCo2S4 nanosheet arrays (NSAs) grown on carbon cloth along with their novel application as anodes in lithium-ion batteries. The relatively small (~60%) volumetric expansion of NiCo2S4 nanosheets during the lithiation process was confirmed by in situ transmission electron microscopy and is attributed to their mesoporous nature. The 3D network structure of NiCo2S4 nanosheets offers the additional advantages of large surface area, efficient electron and ion transport capability, easy access of electrolyte to the electrode surface, sufficient void space and mechanical robustness. The fabricated electrodes exhibited outstanding lithium-storage performance including high specific capacity, excellent cycling stability and high rate of performance. A reversible capacity of ~1275 mAh g−1 was obtained at a current density of 1000 mA g−1, and the devices retained ~1137 mAh g−1 after 100 cycles, which is the highest value reported to date for electrodes made of metal sulfide nanostructures or their composites. Our results suggest that 3D-networked NiCo2S4 NSA/carbon cloth composites are a promising material for electrodes in high-performance lithium-ion batteries.
Rechargeable lithium-ion batteries (LIBs) are the most widely used electrochemical energy storage devices because of their inherent advantages including high energy density, long life span, lack of memory effect and environmental nontoxicity.1, 2, 3 The increasing applications of LIBs in daily electronic devices—along with industry demands for further improvement in energy density, durability, rate capability and safety—have driven the development of new electrode materials and new electrode structures.4, 5, 6, 7 Among the great variety of anode materials studied, metal oxides (MOs) and metal sulfides (MSs) such as Co3O4,8 SnO2,9 Fe3O4,10 V2O5,11 FeS2,12 NiS2,13 MoS214 and WS2 15 comprise an important class of materials that can be charged and discharged through redox reactions. Compared with their MO counterparts, MSs generally feature higher electrical conductivity, better mechanical and thermal stability, and richer redox chemistry, making them attractive anode materials for high-performance LIBs.16, 17 Various MS nanostructures including nanoparticles,18 nanosheets,19 nanowires,20 and nanotubes21 have been synthesized and offer superior electrochemical energy storage capacity compared with bulk MSs. However, the large volumetric change of MS nanostructures during electrochemical reactions leads to reduced capacity and poor cycling stability. Moreover, the necessary addition of conductive additives and binders inevitably lessens overall energy storage capacity, and binders in particular hinder electron transport from MS nanostructures to the electron collector, limiting their practical applications.16, 17
Recently, direct growth of self-supported NiCo2O4 nanowires and ZnCo2O4 nanowire arrays on current-collecting substrates has been shown to overcome the drawbacks of mixing active electrode materials with conductive additives and binders and to increase endurance of fast charge and discharge processes.22, 23 Furthermore, it has been reported that 3D network structures (for example, graphene networks,7 MO/3D graphene network composites24 and diamond/carbon nanotubes25) can provide open channels for efficient electron/ion transport and large surface area for reactions, both of which are favorable properties for materials and devices used in electrochemical applications. In this work, we report the synthesis of 3D-networked NiCo2S4 nanosheet arrays (NSAs) directly on carbon cloth and their application as an anode material in LIBs; to our knowledge, this is the first such demonstration. NiCo2S4 possesses an electric conductivity ~104 times higher than conventional MO semiconductors.26 In previous studies, NiCo2S4 was shown to have outstanding electrochemical characteristics when used as an electrode material in supercapacitors.27, 28 Our work demonstrates that the direct growth of NiCo2S4 NSAs on carbon cloth enables their use as anodes in LIBs without requiring the addition of carbon additives and binders. In situ transmission electron microscopy (TEM) observations verify that the volumetric expansion of NiCo2S4 nanosheets during the lithiation process is relatively small (~60%) as a result of their mesoporous nature. NiCo2S4 NSA/carbon cloth electrodes exhibit remarkable lithium storage properties that include the highest specific capacity for MS electrodes reported thus far (~1275 mAh g−1 at a current density of 1000 mA g−1), excellent cycling stability (~89% retention after 100 cycles) and high rate capability, suggesting that it is a favorable anode material for high-performance LIBs.
Materials and Methods
All chemicals used in this work are commercially available from Sigma-Aldrich (Shanghai, China) and were used as received without further purification. The 3D-networked NiCo2S4 NSAs were synthesized on carbon cloth using a hydrothermal reaction combined with a simple subsequent sulfurization process, as described in Supplementary Information I.
Sample morphology and microstructure were characterized by scanning electron microscopy (Philips XL 30FEG, Amsterdam, Netherlands) and TEM (JEM-2100F equipped with an energy-dispersive X-ray spectrometer, Tokyo, Japan). X-ray diffraction patterns were recorded using a Philips X’Pert MRD X-ray diffractometer with Cu Kα radiation (Amsterdam, Netherlands). In situ TEM observation of the lithiation process was performed using a scanning tunneling microscopy-TEM holder from Nanofactory Instruments AB (Gothenburg, Sweden) within the JEOL 2100F TEM, which was operated at 200 kV.
The 3D-networked NiCo2S4 NSA/carbon cloth composites were used as working electrodes without adding any ancillary materials. The loading density of the NiCo2S4 NSAs (that is, the active material) was calculated to be 1.02–1.36±4% mg. To allow electrochemical measurements of the control sample, the working electrode was prepared by mixing the NiCo2S4 flowers, conductive agent (carbon black) and binder (sodium alginate) in a weight ratio of 70:20:10. Coin-type cells (CR2032) were fabricated using lithium metal as the counter electrode, Celgard 2400 (Charlotte, NC, USA) as the separator and LiPF6 (1 M) in ethylene carbonate-dimethyl carbonate (1:1 vol%) as the electrolyte. Cyclic voltammetry (CV) measurements were conducted at 0.1 mV s−1 over the range of 0.01–3.0 V (vs Li/Li+) on a CHI 600D (Shanghai, China) electrochemical workstation. Electrochemical impedance spectroscopy was carried out with a ZAHNERelektrik IM 6 (Kronach, Germany) electrochemical system over a frequency range of 100 kHz to 0.01 Hz.
Results and Discussion
The 3D-networked NiCo2S4 NSAs were synthesized by a two-step process as schematically illustrated in Figure 1a. In the first step, strips of carbon cloth (Supplementary Figure S1) were immersed into an aqueous solution containing Ni2+, Co2+ and methenamine, and NiCo precursor nanosheets arrays were grown uniformly on the surface of the cloth, as shown in Supplementary Figure S2. In the second step, the NiCo precursor NSAs were converted to 3D-networked and mesoporous NiCo2S4 NSAs with well-retained morphology via a simple sulfurization treatment, and densely packed, highly ordered NiCo2S4 NSAs were formed on the cloth, as shown in Figures 1b and c. NiCo2S4 NSAs uniformly covered the entire surface of the carbon cloth, as depicted in the low-magnification scanning electron microcopy image shown in Figure 1b. Close observation (Figure 1d) revealed that the nanosheets were interconnected and intersected each other to form a 3D network structure with voids clearly observed (marked by red arrows) that was similar to the structure of sponge. The retention of the NiCo interconnected structure during sulfurization can be attributed to the robust support of the carbon cloth and the slow sulfurization rate at the relatively low temperature. This 3D network structure is concluded to be superior for use in LIB anodes because (1) it provides better mechanical strength and excellent transport properties for both Li+ ions and electrons24, 25 and (2) the abundant empty space between the nanosheets may promote electrolyte penetration, facilitating the lithiation and delithiation of electrodes.6, 29 Moreover, NiCo2S4 NSAs are ultrathin (~10-nm thick), which greatly increases the specific surface area and promotes electrochemical reactions between electrode and electrolyte. Figure 1e presents the X-ray diffraction pattern of NiCo2S4 NSAs scratched from carbon cloth. The diffraction peaks at 31.6°, 38.3°, 47.4°, 50.5° and 55.3° are attributed to the 311, 400, 422, 511 and 440 planes of the cubic phase NiCo2S4, respectively. No other diffraction peaks were observed, verifying the formation of phase-pure NiCo2S4. A possible mechanism is proposed for the conversion process of NiCo precursor NSAs to 3D-networked and mesoporous NiCo2S4 NSAs. In the aqueous solution, Na2S dissolves to form S2− ions that are simultaneously hydrolyzed to generate HS− and H2S species. These species serve as the sulfur sources for the ion-exchange reaction that converts NiCo precursor to NiCo2S4. Three-dimensional-networked NiCo precursor NSAs with abundant empty space allow these sulfur sources to diffuse easily into each nanosheet. Because of the presence of OH− and O2− in the NiCo precursor NSAs, a surface redox reaction takes place on their surfaces to form NiCo2S4 NSAs. The edges of these NiCo2S4 NSAs subsequently join to form 3D-networked NiCo2S4 NSAs.
To give further insight into the morphology and structure of the as-prepared 3D-networked NiCo2S4 NSA/carbon cloth composites, TEM and high-resolution TEM images are shown in Figure 2. Consistent with the scanning electron microcopy observations, the low-magnification TEM image (Figure 2a) reveals the network structure of NiCo2S4 nanosheets. Interestingly, the as-synthesized NiCo2S4 nanosheets have a mesoporous structure with a pore size of ~5 nm (Figure 2b). The high-resolution TEM image in Figure 2c demonstrates the crystalline nature of NiCo2S4 nanosheets, and the existence of lattice discontinuity patches further verifies their mesoporous structure. As denoted in the figure, the interplanar distances were measured to be 0.33 nm and 0.28 nm, which correspond to the d-spacing of (220) and (311) planes, respectively, in cubic NiCo2S4. Figure 2d presents the energy-dispersive X-ray spectrum (EDX) of the NiCo2S4 nanosheets, which clearly indicates the presence of Ni, Co and S elements. (The Cu signals originate from the TEM grid.)
The N2 adsorption/desorption isotherm curve and the corresponding pore size distribution plot in Supplementary Figure S3 demonstrate the porous nature of the as-synthesized NiCo2S4 nanosheets. The Brunauer–Emmett–Teller surface area of NiCo2S4 nanosheets was calculated to be 109 m2 g−1. The porous NiCo2S4 nanosheets exhibited a pore size distribution ranging from 4 to 8 nm as measured by the Barrett–Joyner–Halenda method, agreeing with the TEM image in Figure 2b. The formation of the mesoporous structure could be due to the replacement of O2− by S2− during sulfurization. In previous work, it was determined that more interfacial bonding exists in mesoporous nanostructures, which not only offer more active sites for Li+ storage but also facilitate Li+ transfer, resulting in improved specific capacity.2, 30
Generally, the large volumetric strain of conventional MS nanostructures during charge/discharge processes can lead to cracking, fracture and electrical disconnect from the current collector. This mechanism has been recognized as one of the major causes for rapid capacity fading in LIBs with MS electrodes.2, 3 In this work, we carried out in situ TEM experiments to investigate mechanical strain in NiCo2S4 nanosheets during their reaction with Li ions using a dual-probe biasing TEM holder (Nanofactory Instrument).31, 32 The configuration for in situ TEM measurements is shown schematically in Figure 3a. NiCo2S4 nanosheets were decorated on the Cu wire probe, and a small piece of Li/Li2O was attached to the tip of the opposite W wire probe. By manipulating a piezoelectric motor on the TEM holder, a NiCo2S4 nanosheet was placed in contact with the lithium oxide and a bias voltage of −3 V was applied to drive Li ions through the solid-state electrolyte (Li2O) toward the NiCo2S4 nanosheet.
Figure 3b depicts pristine NiCo2S4 nanosheets before lithiation. A slight area expansion of the NiCo2S4 nanosheet was observed after it underwent lithiation for 0.5 min (Figure 3c), and the nanosheet continued to expand with increasing lithiation time, as shown in Supplementary Figure S4. After 20 min (Figure 3d), no further expansion of the nanosheet could be detected, indicating reaction completion. Because the NiCo2S4 nanosheets have a thickness of ~10 nm, it is challenging to visualize the lithium migration process by in situ TEM.32, 33, 34 However, as shown by the selected area electron diffraction patterns (inserts in Figures 3b and d), the conversion of the NiCo2S4 nanosheet from its original crystalline phase to an amorphous phase after 20 min can be considered indirect evidence of the lithiation process. Moreover, the detectable expansion of the NiCo2S4 nanosheet itself indicates that lithiation occurred. In this work, the volumetric expansion was estimated by measuring the area increase of the nanosheet in the TEM images depicted in Figures 3b and d. An area increase of ~37% was obtained after lithiation for 20 min. If we assume that expansion is isotropic, the volumetric expansion of NiCo2S4 nanosheet can be estimated to be ~60%. Interestingly, the NiCo2S4 nanosheet displayed no cracking or fracture as a result of the lithiation process. The existence of the large quantity of mesopores in NiCo2S4 nanosheets compensated for the volumetric expansion during the lithiation process; consequently, changes in the dimensions and morphology of nanosheets were negligible, and the nanosheets retained their mechanical integrity.
The electrochemical properties of the 3D-networked NiCo2S4 NSA/carbon cloth composite as an anode material for LIB were evaluated. Figure 4a displays representative cyclic voltammetry (CV) for the first, second and fifth cycles at a scan rate of 1 mV s−1 in the voltage window of 0.01–3 V vs Li/Li+. To distinguish the oxidation and reduction reactions induced by the NiCo2S4 NSAs, the electrochemical characteristics of pure carbon cloth were studied under the same testing conditions, as shown in Supplementary Figure S5. By comparing the CV curves of pure carbon cloth and 3D-networked NiCo2S4 NSA/carbon cloth, the dominant irreversible cathodic peak of NiCo2S4 NSAs, which was located at ~1.1 V in the first cathodic sweep, could be assigned to the reduction of Ni2+ and Co3+ into metallic Ni and Co, respectively.35 Furthermore, the peaks at ~1.4 and 2.0 V in the subsequent anodic scan could be ascribed to oxidation of metallic Ni and Co to NiSx and CoSx, respectively. In subsequent cycles, the main cathodic peaks shifted toward a higher potential and the anodic peaks downshifted slightly. The upshift and intensity decrease of the main cathodic peak are due to the occurrence of certain irreversible reactions associated with the formation of a solid electrolyte interphase (SEI) film on the electrode surface.36 The overlap in CV curves after the second cycle indicates good reversibility of the electrochemical reactions. Figure 4b shows the galvanostatic discharge/charge profiles of the 3D-networked NiCo2S4 NSA/carbon cloth electrodes at a current density of 100 mA g−1 for the first three cycles. The first cycle reveals discharge and charge capacities of 2298 mAh g−1 and 1868 mAh g−1, respectively, indicating an initial Coulombic efficiency of 81.2%. The irreversible capacity loss for the first cycle can be attributed to the formation of an SEI layer and possible incomplete restoration of metallic Ni and Co into the original sulfide, an effect that has been commonly observed in NiCo2O4 electrode materials.22, 35 In the second and third cycles, the discharge capacities of the electrode decreased to 1912 mAh g−1 and 1891 mAh g−1, respectively, and the corresponding Coulombic efficiency increased to 98.1% and 98.8%, respectively. The sharp increase in Coulombic efficiency indicates that a stable SEI layer had already formed during the initial cycles.
The cycling stability of the NiCo2S4 NSA/carbon cloth electrodes was tested by performing discharge-charge measurements up to 100 cycles, as shown in Figure 4c. Note that the test was performed at the low rate of 100 mA g−1 for the first three cycles to enable the slow activation of NiCo2S4 NSAs and the development of a stable SEI layer; the current density was subsequently increased to 1000 mA g−1. The discharge capacity was 1275 mAh g−1 for the fourth cycle and 1137 mAh g−1 after 100 cycles, corresponding to a capacity retention of 89.2% (red and black represent the discharge and charge curves, respectively), which is higher than that of previously reported MS nanostructures and MS composite electrodes.12, 13, 14, 15, 18, 19, 20, 21, 37, 38, 39 In addition, the Columbic efficiency increased sharply from 81.2% for the first cycle to >99.8% after several initial cycles, and then stayed at this level until the 100 cycles were completed. The extremely high columbic efficiency indicates that a stable SEI layer formed during the initial cycles. The stability test was also performed at an elevated current density of 2000 mA g−1 (Supplementary Figure S6), at which the electrode was capable of delivering a high discharge capacity of 792 mAh g−1 after 100 cycles, retaining 68.4% of the fourth-cycle discharge capacity of 1158 mAh g−1. Significantly, a high Coulombic efficiency (>99.6%, except for the first few cycles) was also achieved. Notably, the corresponding capacity and stability evaluations of the carbon-only cloth electrode (Supplementary Figure S7) verified the negligible contribution of carbon cloth to the overall capacity of NiCo2S4 NSA/carbon cloth electrodes (~17 out of 1275 mAh g−1).
As a control sample, pure NiCo2S4 flower nanostructures were synthesized (Supplementary Figure S8), and their cyclic stability as electrodes was also tested at 1000 mA g−1, as shown in Figure 4c (green and blue curves represent discharge and charge curves, respectively). The electrode was fabricated in a conventional way by mixing the NiCo2S4 flowers with binder and conductive carbon. In sharp contrast to the NiCo2S4 NSA/carbon cloth electrodes, the capacity of the NiCo2S4 flower electrode declined considerably from ~1050 mAh g−1 during the fourth cycle to 159 mAh g−1 after 50 cycles. Moreover, the Coulombic efficiency of the cell was only 96–98% (except for the first three cycles), indicating its low electrochemical reversibility (Supplementary Figure S9). The low Coulombic efficiency could be an indication of SEI layer instability, which is closely related to lithiation-induced capacity degradation.8 The results summarized above clearly demonstrate the superiority of 3D-networked NiCo2S4 NSA/carbon cloth for use as LIB anodes.
The rate performance of the 3D-networked NiCo2S4 NSA/carbon cloth electrodes was further studied by cycling at different current densities. As shown in Figure 4d, capacity decreased from 1950 to 1560, 1220, 1030 and 340 mAh g−1 when the current density was increased from 100 to 500 mA g−1, 1000 mA g−1, 2000 mA g−1, and 5000 mA g−1, respectively. Notably, a capacity of 340 mAh g−1 was retained at the relatively high current density of 5000 mA g−1, a value comparable to the theoretical capacity of carbon/graphite-based electrodes (372 mAh g−1),40 illustrating the high rate capability of 3D-networked NiCo2S4 NSA/carbon cloth. After cycling at the highest current density of 5000 mA g−1, the capacity recovered was ~1900 mAh g−1 as the current density was switched back to 100 mA g−1, corresponding to a capacity retention of ~97%, which indicates high stability as well as excellent reversibility of the electrode.
To characterize NiCo2S4 NSAs grown directly on carbon cloth, we collected electrochemical impedance spectra of the electrodes made of 3D-networked NiCo2S4 NSA/carbon cloth and NiCo2S4 flowers, as shown in Figure 5a. The Nyquist plots of each sample depict a semicircle in the high-to-medium frequency region and an inclined line in the low frequency region. The semicircle is attributed to the charge transfer process at the electrode/electrolyte interface, and the inclined line corresponds to lithium diffusion, or Warburg diffusion, into the bulk of the electrode. The modified equivalent circuit model of the system is provided in Figure 5b.41, 42, 43 The parameter Rs is the electrolyte resistance; CPE1 and Rf are the capacitance and resistance, respectively, of the passivation SEI film formed on the electrode surface; CPE2 and Rct are the double-layer capacitance and charge transfer resistance, respectively; and Zw is the Warburg impedance related to the diffusion of lithium ions into the bulk of the electrode. For the NiCo2S4 NSA/carbon cloth electrode, the fitted impedance parameters were Rf=17.42 Ω, and Rct=81.75 Ω; for the NiCo2S4 flower electrode, Rf=54.11 Ω and Rct=96.79 Ω. It is obvious that Rf and Rct for the NiCo2S4 NSA/carbon cloth electrode are significantly lower than those of the NiCo2S4 flower electrode. Therefore, the 3D-networked NiCo2S4 NSAs enable rapid electron transport and fast faradic reaction during electrochemical lithium insertion/extraction processes.
Binder and conductive carbon are used in the NiCo2S4 flower electrode; the electroactive materials thus present considerable resistance to electron transport to and from the current collector substrate, as illustrated schematically in Figure 5c. Such particulate nanostructures have also been demonstrated to have a tendency during cycling to aggregate and form disconnected clusters that may further deteriorate electron conduction between the active materials and the current collector.44 Moreover, it is less likely that an electrode consisting of binder and conductive carbon can provide a large and effective contact area between the active material and the electrolyte, maintain a short diffusion distance for Li+ or suitably accommodate mechanical strain during cycling.45 In comparison with the NiCo2S4 flower electrode, the 3D-networked NiCo2S4 NSA/carbon cloth electrode possesses the following merits: (1) NiCo2S4 has inherently high electronic conductivity (~4 orders of magnitude higher than that of conventional MO semiconductors); and the nanosheets are anchored tightly to the (carbon fiber) current collector, forming good electrical contact. Moreover, the hybrid structure can be used directly as an anode without the addition of binders and conductive carbon, as illustrated in Figure 5d. Thus, it can facilitate fast Li+ ion and electron kinetics during the redox reaction, leading to high capacity and outstanding cycling stability. (2) Because of their mesoporous nature, NiCo2S4 nanosheets exhibit relatively small volumetric expansion during the lithiation process, as the existence of voids in the NSAs (Figure 1d) provides sufficient free space to ameliorate the tendency for expansion. Furthermore, the robust 3D interpenetrating network structure can withstand the strain induced by volumetric changes during electrochemical reactions and can thus maintain the integrity of the hybrid, leading to increased cycling stability during lithiation/delithiation processes. (3) The enlarged surface area of the ultrathin and mesoporous NiCo2S4 nanosheets and their 3D network assembly greatly enhances the electrolyte/active material contact area, which may facilitate electrolyte penetration and the transport of lithium ions. Thus, in a synergistic manner resulting from their 3D assembly, the nanosheets not only act as reservoirs for storage of Li+ ions but also reduce the transport length of Li+ ions, which improves specific capacity. The unique material composition, morphology and structure of the 3D-networked NiCo2S4 NSA/carbon cloth electrodes lead to their outstanding specific capacity, excellent cycling stability and high rate performance.
In summary, 3D-networked NiCo2S4 NSAs were successfully grown on carbon cloth substrates through a surfactant-assisted hydrothermal method combined with a simple sulfurization treatment. NiCo2S4 has inherently high electronic conductivity, and the NiCo2S4 NSA/carbon cloth hybrid can be directly employed as a high-performance electrode for LIBs without the addition of binders or conducting materials. Significantly, the NiCo2S4 nanosheets were found to exhibit only slight volumetric expansion (~60%) during the lithiation process as a result of their mesoporous nature. In addition, the 3D network structure and void space of NiCo2S4 nanosheets are expected to provide structural robustness, efficient electron transfer, fast ion transport, large surface area and easy access of electrolyte to electrode. As an anode material in LIBs, the NiCo2S4 NSA/carbon cloth hybrid delivered high specific capacity, excellent cycling stability and outstanding rate performance. At a current density of 1000 mA g−1, the relatively large capacity of 1275 mAh g−1 was demonstrated; this value decreased only minimally to 1137 mAh g−1 after 100 cycles. Even at the high current density of 5000 mA g−1, the composite material exhibited a capacity of 340 mAh g−1, which is comparable to the theoretical capacity of carbon/graphite-based electrodes. The outstanding overall properties of the 3D-networked NiCo2S4 NSA/carbon cloth hybrid make it a promising anode material candidate for high-performance LIBs.
This work was financially supported by the Applied Research Grant of the City University of Hong Kong (ARG 9667111), the National Natural Science Foundation of China (Grant Nos. 21171035, 51302035 and 51372213), the PhD Programs Foundation of the Ministry of Education of China (No. 20130075120001 and 20110075110008), the China Postdoctoral Science Foundation (2014M550203), the Science and Technology Commission of Shanghai Municipality (Grant No. 13ZR1451200) and the Fundamental Research Funds for the Central Universities (Grant No. 2232013D3-09).
Supplementary Information accompanies the paper on the NPG Asia Materials website (http://www.nature.com/am)
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