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Hierarchical architectured NiS@SiO2 nanoparticles enveloped in graphene sheets as anode material for lithium ion batteries
Electrochimica Acta 155 (2015) 85–92
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hierarchical architectured NiS@SiO2 nanoparticles enveloped in graphene sheets as anode material for lithium ion batteries Zijia Zhang a , Hailei Zhao a,b, * , Zhipeng Zeng a , Chunhui Gao a , Jie Wang a , Qing Xia a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Materials and Technology, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 September 2014 Received in revised form 18 November 2014 Accepted 11 December 2014 Available online 2 January 2015
A well-designed hierarchical architecture NiS@SiO2/graphene is prepared through electrostatic selfassembly between (3-aminopropyl) triethoxysilane (APTES)-modified NiS and graphene in aqueous solutions at room temperature. The obtained composite possesses a unique structure with SiO2 ultrasmall nanoparticles (3–5 nm) derived from the pyrolysis of APTES homogeneously anchored on the surface of NiS nanoparticles (100 nm), forming NiS@SiO2 core-shell hybrid particles, which are well enveloped in graphene sheets. The SiO2 nanoparticles act as pillars to form open space between graphene sheets and NiS particles, which can buffer the volume change and afford easy electrolyte-wetting and fast lithium ion transport channels. The graphene sheets can not only significantly enhance the overall electrical conductivity of the NiS@SiO2/graphene electrode, but also serve as a blanket to wrap NiS particle and so as to avert its exfoliation from electrode due to large volume change during cycling. The prepared NiS@SiO2/graphene nanocomposite exhibits high reversible capacity (750 mAh g1 for 100 cycles), remarkable cycling stability and impressive rate capability. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: nickel sulfide graphene electrochemical properties anode lithium ion batteries
1. Introduction In recent years, great efforts have been devoted to develop electric vehicles (EVs) in an attempt to reduce dependence on the limited fossil fuel resources and to decrease the greenhouse gas emission. Rechargeable lithium ion batteries (LIBs) with high power density, long cycle life and low cost are crucial for EVs’ development [1–3]. However, the relatively low specific capacity of conventional graphite (372 mAh g1 in theory) limits the capacity and energy densities of LIBs and thereby fails to meet the increasing requirement for EVs’ applications [4]. To address this issue, a great deal of research interest has been shifted to seek new materials that can deliver higher capacities. Nickel sulfide has attracted great attention due to its high theoretical capacity (590 mAh g1). However, the practical application of nickel sulfide has been impeded by the dramatic volume change during cycling [5,6], which could cause pulverization of the NiS particles, cracking of the electrode, and subsequent electrical
* Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel.: +86 10 82376837; fax: +86 10 82376837. E-mail address: [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.electacta.2014.12.074 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
disconnection from current collectors, leading to quick capacity fading upon extended cycling. To circumvent this problem, one of the promising strategies is to design porous or hollow structures of NiS to accommodate the large volume change. Recently, Zhao et al. [7] prepared NiS hollow spheres consisting of nanoflakes by a dodecanethiol-assisted hydrothermal process. Yang et al. [8] fabricated a porous nanostructured NiS on Ni foam substrate, which showed not only high discharge-charge capacities, but also stable cycling performance. Ni and co-workers [9] synthesized hollow structured NiS/ Ni composite that can yield a stable cycling of 100 times at 0.15 C. Another effective attempt is integrating carbon materials with nanostructured NiS, where the carbon component can mitigate the volume expansion and thus improve the cycling stability. Dai et al. [10] have reported the synthesis of a composite that Ni3S2 nanoparticles grown on the backbone of conductive multiwalled carbon nanotubes (MWCNTs) using a glucose-assisted hydrothermal method. Kang and co-workers [11] fabricated NiS/C composite with NiS nanocrystals being uniformly distributed inside the spherical carbon matrix, which exhibited an excellent discharge capacity of 472 mAh g1 at a high current density of 1000 mA g1, and even after 5000 cycles a high capacity retention of 86% was achieved. Graphene, a honeycomb network for sp2 carbon atoms, has attracted enormous interest as an intriguing substrate to build
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nanohybrids for LIBs because of its extraordinary electrical conductivity, outstanding mechanical flexibility and large specific surface area [12–14]. Recently, graphene was introduced to NiS to improve the rate-capability and cycling performance of electrode. Several works concerning NiS/graphene composites with NiS particles being encapsulated by graphene sheets were reported [15–19]. In this work, we presented a novel NiS@SiO2/graphene composite with hierarchical architecture that the NiS particles were enveloped by graphene sheets and between them nano-SiO2 particles were anchored on NiS particle surface, forming a void space inside composite particles. The NiS@SiO2/graphene composite was prepared by electrostatic self-assembly route at room temperature. This assembly enables an efficient encapsulation of NiS nanoparticles by graphene sheets with nano-SiO2 as a connector. The graphene sheets are instrumental in providing a conducting network for fast electron transfer between the active materials and the collector, and acting as a flexible twodimensional carbon matrix to prevent the loss of active sulfur component from the electrode. Notably, the SiO2 nanoparticles anchored on NiS nanoparticles serve as pillars to support the graphene sheets forming open spaces between the sheets and NiS particles, which could accommodate the volume change of NiS particles, improve electrode/electrolyte wettability and promote lithium ion transport in bulk electrode. The prepared NiS@SiO2/ graphene composite displays superior electrochemical performance with excellent cycling stability and high rate capability. 2. Experimental Section 2.1. Preparation of NiS nanoparticles The NiS nanoparticles were synthesized by a facile hydrothermal route. Typically, 0.38 g of Ni(Ac)2 4H2O was dissolved in 70 ml of deionized water under stirring for 30 min at room temperature. Then 0.588 g of C6H5Na3O7 2H2O was added into the above solution and stirred for 30 min, followed by the addition of 0.242 g of L-cysteine and stirred until it thoroughly dissolved. The pH value of the solution was adjusted to 10 by ammonia. Subsequently, the mixture was kept stirring for 1 h and then transferred into a 100 ml Teflon-lined stainless steel autoclave and heated in an oven at 180 C for 24 h. Finally, the autoclave was allowed to cool down to room temperature naturally. The products were filtered, washed with deionized water/ethanol and dried in air at 60 C overnight. 2.2. Preparation of hierarchical NiS@SiO2/graphene composite Hierarchical NiS@SiO2/graphene composite was fabricated by mixing NiS and graphene aqueous suspensions through the electrostatic attraction between positively charged NiS that is modified with APTES and negatively charged graphene, followed
by a pyrolysis of APTES. In a typical experiment, the obtained NiS nanoparticles (0.117 g) was dispersed into 50 ml of deionized water and ethanol (10:1, v/v) via sonication for 1 h, followed by dropwise addition of 0.5 ml of APTES and stirred for 1 h to render the surface of NiS positively charged. Meanwhile, graphene (30 mg) was dispersed in a water-ethanol (1:1, v/v) mixture system with vigorous ultrasonic agitation for 1 h. Then, the APTES-modified NiS suspension was added into the graphene suspension under mild magnetic stirring. After mixing for 1 h, 0.14 ml of hydrazine (80 wt. %) was added into the above suspension. Then the NiS@SiO2/ graphene composite was collected by centrifugation, washed with deionized water and ethanol, and followed by a thermal treatment to pyrolyze APTES in a conventional tube furnace at 500 C for 1 h under nitrogen atmosphere with a ramping rate of 10 C min1. The NiS/graphene composite was synthesized by the same method as NiS@SiO2/graphene without the addition of APTES. 2.3. Structure characterization Zeta potential (z) of the samples was determined by Zetasizer Nano ZS (Malvern) at room temperature of 25 C for characterization of surface charge properties. The phase structure of the products was identified by X-ray diffraction (XRD, Rigaku, D/maxA, Cu Ka, l = 1.5406 Å) and Raman spectroscope (LabRAM HR Evolution, excited by 523 nm laser). The morphology and lattice structure of the samples were characterized using field emission scanning electron microscopy (FE-SEM, SUPRA55) and transmission electron microscopy (TEM, Tecnai F20, 200 kV) equipped with high resolution TEM (HR-TEM) and energy dispersive X-ray spectroscopy (EDX). The porosity and Brunauer-Emmett-Teller (BET) surface area of NiS@SiO2/graphene were determined by N2 adsorption/desorption technique (QUADRASORB SI-MP, Quantachrome). The results were calculated by applying the Barrett-JoynerHalenda (BJH) model and the linear part of the BET plot, respectively. 2.4. Electrochemical measurements The working electrode was prepared by mixing 70 wt. % active material (pure NiS, NiS/graphene or NiS@SiO2/graphene composite), 15 wt. % acetylene black (AB) and 15 wt. % carboxymethylcellulose (CMC) with water under magnetic stirring for 24 h. The formed homogeneous slurry was then coated onto Ni foam, dried at 70 C for 6 h under vacuum and punched into disks with a diameter of 8 mm. Before cell assembling, the electrode was further dried at 120 C for 24 h under vacuum. The loading of active material in each electrode is 1.6 mg cm2. The 2032 coin-type cells were assembled in an Ar-filled glove box with active material as the working electrode, lithium foil as the counter electrode and Celgard 2400 as the separator. The electrolyte was composed of 1 M LiPF6 dissolved in ethylene carbonate (EC), diethyl carbonate
Scheme 1. Schematic illustration of synthetic procedure of NiS@SiO2/graphene.
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(DEC) and dimethyl carbonate (DMC) (1:1:1, in vol %). Galvanostatic charge/discharge testing was performed in the potential window from 0.01 to 3.0 V (vs. Li/Li+) on a Land battery tester (LAND CT-2001A tester). Cyclic voltammetry (CV) measurements were carried out at a scanning rate of 0.1 mV s1. Electrochemical impedance spectra of pure NiS and NiS@SiO2/graphene composite were analyzed in the frequency range from 106 to 0.1 Hz, while the disturbance amplitude was 5 mV. 3. Results and discussion The strategy to synthesize hierarchical NiS@SiO2 nanocomposite enveloped in graphene sheets through electrostatic selfassembly method is depicted in Scheme 1. With the hydrolization and condensation of APTES, the surface of NiS was modified with amine functional groups, by which the NiS nanoparticles are positively charged. The negatively charged surface of graphene is ascribed to the ionization of the oxygen-containing groups [20]. The opposite charge features of APTES-modified NiS and graphene sheets make the NiS nanoparticles be well enveloped with graphene sheets through the electrostatic attraction. After reducing the functional groups of graphene and pyrolyzing the APTES derivative on the surface of NiS, NiS@SiO2/graphene composite can be attained. The final composite has a unique feature that SiO2 nanoparticles (3–5 nm) are anchored on the surface of NiS nanoparticles (100 nm), forming hierarchical NiS@SiO2 core-shell particles, while the graphene sheets are intimately bounded to the composite particles. The graphene sheets can not only improve the electrical conductivity of the NiS@SiO2/graphene electrode by forming an efficient electrically conductive network [21–23], but also wrap the active particle to prevent its exfoliation from electrode due to the pulverization during charge/discharge processes. In addition, the anchored SiO2 nanoparticles can act as pillars to form open spaces between NiS nanoparticles and graphene sheets, which can effectively relieve the volume expansion of NiS nanoparticles to improve the structure stability of the electrode and at the same time provide a fast lithium ion migration pathway via enhanced electrolyte permeation into the active particles. The electrostatic self-assembly process of NiS nanoparticles with graphene sheets can be elucidated with the zeta potentials (z) of different components. As shown in Fig. 1, the suspension of pure NiS and graphene reveal negatively charged surfaces with zeta potential values of -10.3 and -11.5 mV, respectively. However, the
Fig. 1. Zeta potentials of pure NiS, graphene, and APTES-modified NiS.
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APTES-modified NiS shows an opposite zeta potential value of +8.6 mV, indicating that the negatively charged surface of NiS has been successfully changed after modification with APTES. The positively charged APTES-modified NiS and the negatively charged graphene can spontaneously assemble to construct composite with intimate interfacial contact by the electrostatic attraction. Fig. 2 presents the XRD patterns of the NiS@SiO2/graphene composite and pure NiS. All the identified peaks can be indexed to the hexagonal NiS phase (JCPDS card No. 65-3419). Compared to pure NiS, the NiS@SiO2/graphene composite shows a diffraction hump in the range of 20–25 (inset of Fig. 2a), which originates from graphene sheets [24,25] and nanocrystalline SiO2 [26]. The Raman spectrum was recorded to further clarify the structure characteristics of NiS@SiO2/graphene composite. As shown in Fig. 3a, two prominent peaks centered at 1351 and 1596 cm1 are observed, corresponding to the well documented D and G band, respectively [27]. The D band is ascribed to the disordered aromatic structure of the sp3 bonded carbons [28], while the G band relates to the first-order scattering of the stretching vibration mode E2 g observed for sp2 carbon domains [27,29]. The peak at 2684 cm1 known as 2D band and the peak at 2931 cm1 associated with the (D + G) band indicate a certain extent of disorder of the graphene sheets [30]. The peak marked by an ellipse in Fig. 3a is enlarged and displayed as Fig. 3b. The broad asymmetric peak can be deconvoluted into two peaks located at 286 and 320 cm1, which are assignable to the A1 vibration mode of NiS [31,32] and Si-O-Si bending vibration of SiO2 [33–35], respectively. This outcome validates the existence of SiO2 in the composite. The morphology of the synthesized samples was observed by FE-SEM and TEM. Fig. 4a shows the typical FE-SEM image of pure NiS particles, which have spherical appearance but aggregate severely to form large micro-sized congregation. The high magnification FE-SEM image (inset in Fig. 4a) reveals that the NiS particles with spherical structure are composed of agglomerated nanoparticles with size of ca. 100 nm. The FE-SEM images of the NiS/graphene and NiS@SiO2/graphene composites are presented in Fig. 4b and c. It is clear that the NiS particles and graphene sheets aggregate separately for NiS/graphene composite, while the NiS particles are well enveloped with graphene sheets and the typically wrinkled graphene sheets form a network for NiS@SiO2/graphene composite. This result implies that with the
Fig. 2. XRD patterns of NiS@SiO2/graphene composite (a) and pure NiS (b).
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Fig. 3. Raman spectrum (a) and enlarged partial spectrum (b) of NiS@SiO2/graphene composite.
Fig. 4. FE-SEM images of pure NiS (a), NiS/graphene (b) and NiS@SiO2/graphene (c), and TEM image of NiS@SiO2/graphene (d). The inset of (a) is a high magnification FE-SEM image of pure NiS.
modification of APTES, the graphene sheets and NiS particles have been well integrated together with an intimate interfacial contact by the electrostatic self-assembly approach. TEM observation (Fig. 4d) reveals that the SiO2 particles with a size of 3–5 nm homogeneously cover the surface of NiS particles, forming hierarchical structured NiS@SiO2 nanocomposite. A small amount of SiO2 nanoparticles scatter on the sheets of graphene. The nano-
sized SiO2 is originated from the pyrolysis of APTES, which was employed to couple the NiS particles with graphene sheets. EDX analysis was carried out to check the elemental compositions of NiS@SiO2/graphene composite. The result is displayed in Fig. 5a and the corresponding STEM image is shown in the inset of Fig. 5a. The elements of Ni, S, Si, O are detected in region A, which further confirm the existence of SiO2 in addition to NiS. The peaks
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Fig. 5. EDX profile (inset: the corresponding STEM image) of NiS@SiO2/graphene (a) and HR-TEM image of a single SiO2 nanoparticle in the composite (b).
assignable to Cu come from the Cu foil substrate. The HR-TEM image of a single SiO2 nanoparticle, as is shown in Fig. 5b, reveals some lattice fringes with an interplanar spacing of 0.206 nm, corresponding to (2 2 2) plane of SiO2, which indicates its crystalline feature. The porosity of NiS@SiO2/graphene composite was characterized by nitrogen adsorption/desorption isotherms. As shown in Fig. 6, the BET surface area and pore volume of NiS@SiO2/graphene are 195 m2 g1 and 0.45 cm3 g1, respectively. The main size of the pores as indicated in the inset of Fig. 6 is 3.8 nm, which is comparable with the size of SiO2 (3–5 nm), demonstrating that the SiO2 nanoparticles anchored on the surface of NiS particles can act as pillars to create empty space between graphene sheets and NiS particles. This porous structure is expected to be capable of offering a large number of open channels as passages of the electrolyte and thus accelerating the diffusion rate of lithium ions. On the other hand, it can provide space to tolerate the volume change of NiS particles during charge/discharge cycles. The electrochemical properties of pure NiS, and NiS@SiO2/ graphene were evaluated by galvanostatic charge/discharge cycling at a current density of 100 mA g1 with a potential window of 0.01–3.0 V. The representative charge/discharge voltage profiles of the initial 10 cycles of NiS@SiO2/graphene and NiS are shown in Fig. 7a and b. The NiS@SiO2/graphene electrode exhibits a wide
Fig. 6. N2 adsorption/desorption isotherms of NiS@SiO2/graphene (inset: BJH pore size distribution of the corresponding sample).
voltage plateau at 1.5–1.2 V in the first discharge curve, while it divides into two plateaus at 1.8 and 1.4 V in subsequent discharge processes. The increase of the voltage plateau is indicative of the structural change of the material [36]. The charge curves show no distinct variation in the initial 10 cycles, exhibiting a voltage plateau at 1.9 V, which can be attributed to the regeneration of NiS [37]. In the case of NiS electrode, similar charge and discharge curves can be observed. The discharge plateau of the NiS electrode shrinks remarkably with increasing cycle number while the NiS@SiO2/graphene electrode displays overlapped charge/discharge curves except for the initial cycle, revealing the excellent electrochemical stability and reversibility of the NiS@SiO2/graphene composite. Typical cyclic voltammetry (CV) characteristics of pure NiS and NiS@SiO2/graphene composite are illustrated in Fig. 8. As for pure NiS electrode (Fig. 8a), the CV curves of the 2nd and 3rd cycles are similar, whereas an obvious difference between the first and subsequent two cycles is observed. In the 1st cathodic scan, a strong peak at 1.0 V and a weak peak at 0.6 V are attributed to the generation of Li2S accompanying the reduction of NiS to Ni and the formation of solid electrolyte interphase (SEI) film [9], respectively. In the subsequent cycles two peaks at 1.3 and 1.8 V are displayed, which could be assigned to the two step reaction of NiS with lithium during discharge process [6,38–40], as described in Eqs. (1) and (2). 3NiS + 2Li+ + 2e ! Ni3S2 + Li2S
(1)
Ni3S2 + 4Li+ + 4e ! 3Ni + 2Li2S
(2)
In the three anodic scans, there is a strong oxidation peak at 2.0 V and a weak peak at 2.2 V, which are resulted from the extraction of lithium ions from Li2S and the reproducing of NiS [41]. The CV curves of NiS@SiO2/graphene electrode (Fig. 8b) are akin to those of pure NiS except for the presence of a weak reduction peak at 0.4 V in the 1st scan and an oxidation peak at 1.3 V. The reduction peak becomes boarder and shifts to higher potential (0.75 V) in subsequent cycles. This pair of peaks are ascribed to the insertion/extraction of Li+ into/from graphene, suggesting that the graphene in the composite is also electroactive for lithium storage [42–44]. Fig. 9a illustrates the cycling performances of pure NiS, NiS/ graphene and NiS@SiO2/graphene electrodes with a cut off voltage of 0.01–3.0 V vs. Li/Li+ at a current density of 100 mA g1 at room temperature (25 C) as well as the NiS@SiO2/graphene electrode at elevated temperature (50 C). The NiS electrode shows extremely
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Fig. 7. Charge/discharge voltage profiles of the initial 10 cycles of NiS@SiO2/graphene (a) and pure NiS (b).
Fig. 8. Cyclic voltammetry (CV) curves of pure NiS (a) and NiS@SiO2/graphene composite (b).
Fig. 9. (a) Cycling performances of pure NiS and NiS/graphene at RT, and NiS@SiO2/graphene at RT and 50 C, (b) rate performances of pure NiS, NiS/graphene and NiS@SiO2/ graphene at RT.
fast capacity degradation with discharge capacities of 775, 450, and 83 mAh g1 in the 1st, 2nd, and 100th cycles, respectively. The higher initial discharge capacity of the pure NiS electrode than the theoretical capacity of NiS (590 mAh g1) is most likely owing to the irreversible formation of solid electrolyte interface (SEI) film [45,46]. In the absence of SiO2, the NiS/graphene electrode shows a relatively stable cycle performance in the first few cycles but then the capacity decays rapidly as the pure NiS electrode does.
Contrarily, the NiS@SiO2/graphene electrode exhibits much better cycling performances at both 25 and 50 C. The enhanced cycling stability of NiS@SiO2/graphene composite electrode is evidently attributed to the existence of graphene sheets and the unique structure of NiS@SiO2/graphene. The graphene sheets can not only provide good electronic conductivity for electrode reaction but also serve as a protective layer to prevent the loss of active sulfur component and thus maintain the integrity of the electrode.
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Fig. 10. Cycling performances of the NiS@SiO2/graphene composite under different charge/discharge conditions.
Meanwhile, the open spaces formed by SiO2 nanoparticles between NiS particles and graphene sheets can help cushion the volume change of NiS and therefore improve the structural stability of the electrode. Both of them guarantee the excellent cycling performance. Without the addition of APTES, NiS/graphene composite is an inhomogeneous mixture as both of their surfaces are negatively charged. Hence, the NiS/graphene composite can improve the electrical conductivity and accommodate the volume change to a degree in the first few cycles. However, the capacity starts to decline rapidly in the subsequent cycles, which may originate from the pulverization of NiS particles and loss of electrical contact between graphene sheets and NiS particles caused by volume expansion/shrinkage during long-term cycling. The NiS@SiO2/graphene electrode at room temperature delivers the initial lithiation and delithiation capacities of 1275 and 867 mAh g1, respectively, corresponding to a Coulombic efficiency of 68%. This irreversible capacity could be associated with the formation of a solid electrolyte interface (SEI) film resulting from the decomposition of electrolyte on the surface of the electrode [38] and the reaction of oxygen-containing functional groups on graphene with lithium ions [47–50]. Fortunately, the Coulombic efficiency reaches nearly 100% and remains relatively stable in the subsequent cycles. After 100 cycles, the NiS@SiO2/graphene composite keeps a stable and reversible capacity of 750 mAh g1, which is much higher than the theoretical capacity of NiS (590 mAh g1). The extra capacity is most likely associated with the existence of graphene sheets and nano-SiO2. Except for the graphene sheets, the SiO2 nanoparticles with 3–5 nm in diameter can react with Li and contribute to the overall capacity [51,52]. In addition, a polymer gel-like film resulting from decomposition of the electrolyte at low voltage plays an indispensable role in this extra specific capacity [53]. The electrochemical performance of the prepared NiS@SiO2/ graphene composite at elevated temperature (50 C) is further evaluated by setting the coin cell in an electric oven. The sample shows a higher reversible capacity of 930 mAh g1 compared to that at room temperature (750 mAh g1), and a stable cycling performance, demonstrating an enhanced electrode kinetic reaction process and a good structure stability of the composite electrode at high temperature. The rate capabilities of the NiS, NiS/graphene and NiS@SiO2/ graphene electrodes were evaluated at different current densities. The cells were first cycled at 100 mA g1, and then switched to 200, 300, 500, 800, 1200, 1600, 2000 mA g1, successively. As anticipated, the NiS@SiO2/graphene composite presents excellent rate
Fig. 11. Nyquist plots of the NiS and NiS@SiO2/graphene electrodes at fully charged state after 15 cycles at 100 mA g1.
performance, as shown in Fig. 9b. At high current densities of 200, 300, 500, 800, 1200, 1600 mA g1, the composite can deliver reversible specific capacities of 680, 640, 610, 570, 540, 500 mAh g1, respectively. A relatively stable capacity of 470 mAh g1 can be still obtained at high current density of 2000 mA g1. Moreover, the discharge capacity can be recovered when the current density is restored to 100 mA g1, showing a great merit for an abuse tolerance of the NiS@SiO2/graphene electrode with varied current rates. In stark contrast, the capacity of NiS/graphene composite drops promptly from 800 to 185 mAh g1 when the rate increases from 100 to 2000 mA g1, and the capacity of NiS electrode decreases sharply with increasing C-rate and it cannot be recovered well, indicating the poor rate capabilities of NiS and NiS/graphene electrodes. The excellent rate performance of the NiS@SiO2/graphene composite is attributed to the mutual effects of the presence of graphene and the unique structure of NiS@SiO2/graphene composite with open spaces between graphene sheets and NiS particles formed by SiO2 nanoparticles. The former provides an electronically conductive network for fast electrode reaction and serves as a blanket to avoid the loss of active NiS from electrode and thus ensure the structural robustness of electrode during cycling. The latter offers an open space to allow the liquid electrolyte to infiltrate into NiS@SiO2/graphene particles and form a good contact with NiS particle surface, which are favorable for the lithium ion diffusion and the electrode reaction kinetics. To examine the difference in lithiation and delithiation kinetics, the NiS@SiO2/graphene electrode was subjected to cycle under different charge/discharge conditions. Two regimes were employed: I) charging at 800 mA g1 and discharging at 100 mA g1; II) charging at 100 mA g1 and discharging at 800 mA g1. Fig. 10 shows the results. The electrode delivers stable cycling performances under both charge/discharge conditions. Apparently, regime I leads to higher specific capacity than regime II, suggesting that the delithiation (charge) kinetics of NiS@SiO2/ graphene electrode is much faster than the lithiation (discharge) kinetics. EIS measurements were further conducted to understand the different electrochemical behaviors of pure NiS and NiS@SiO2/ graphene electrodes. Fig. 11 depicts the Nyquist plots of the electrodes at fully charged state after 15 cycles. The first semicircle at high frequency region reflects the resistance associated with Li+
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migration through the SEI film (Rf), while the second semicircle refers to the interfacial charge transfer resistance (Rct), which is a measure of the difficulty involved for charges crossing the boundary between the electrode and electrolyte. The straight line slope in the low frequency region corresponds to the diffusion of Li+ within the electrode (Warburg impedance). The Rct of the NiS@SiO2/graphene electrode tested at room temperature is much smaller than that of the NiS electrode, indicating the faster reaction kinetics of the composite electrode. It is ascribable to the graphene sheets and open spaces between the sheets and NiS particles, which ensure fast electronic and ionic transport within the NiS@SiO2/graphene electrode. The decrease of the Rct of the composite electrode at 50 C suggests the thermal activation feature of charge transfer process at the interface. 5. Conclusion In summary, we demonstrate a novel NiS@SiO2/graphene composite with unique hierarchical architecture that the NiS nanoparticles are well enveloped by graphene sheets, and on the surface of NiS particles many SiO2 minute nanoparticles are anchored, which support graphene sheets to form open spaces between the sheets and NiS particles. The space can effectively cushion the volume change of NiS and provide channels to increase electrode/electrolyte wettability and thereby promote lithium ion diffusion. The graphene sheets offer good electronic conductivity for electrode reaction and function as a blanket to prevent the exfoliation of NiS from electrode due to the volume change during charge/discharge processes. As a result, the NiS@SiO2/graphene electrode exhibits high specific capacity, excellent cyclic stability and good rate capability. The prepared NiS@SiO2/graphene composite displays a fast delithiation but a slow lithiation kinetics. Moreover, the synthesized NiS@SiO2/graphene composite demonstrates remarkable electrochemical performance at elevated temperature (50 C) in terms of specific capacity and cycling stability. Considering the superior electrochemical properties, the prepared NiS@SiO2/graphene composite is a promising candidate as anode material in LIBs. Acknowledgements This work was financially supported by National Basic Research Program of China (2013CB934003), “863”program (2013AA050902), National Nature Science Foundation of China (21273019) and Guangdong Industry-Academy-Research Alliance (2013C2FC0015). References [1] [2] [3] [4]
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hierarchical architectured NiS@SiO2 nanoparticles enveloped in graphene sheets as anode material for lithium ion batteries Zijia Zhang a , Hailei Zhao a,b, * , Zhipeng Zeng a , Chunhui Gao a , Jie Wang a , Qing Xia a a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Lab of New Energy Materials and Technology, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 September 2014 Received in revised form 18 November 2014 Accepted 11 December 2014 Available online 2 January 2015
A well-designed hierarchical architecture NiS@SiO2/graphene is prepared through electrostatic selfassembly between (3-aminopropyl) triethoxysilane (APTES)-modified NiS and graphene in aqueous solutions at room temperature. The obtained composite possesses a unique structure with SiO2 ultrasmall nanoparticles (3–5 nm) derived from the pyrolysis of APTES homogeneously anchored on the surface of NiS nanoparticles (100 nm), forming NiS@SiO2 core-shell hybrid particles, which are well enveloped in graphene sheets. The SiO2 nanoparticles act as pillars to form open space between graphene sheets and NiS particles, which can buffer the volume change and afford easy electrolyte-wetting and fast lithium ion transport channels. The graphene sheets can not only significantly enhance the overall electrical conductivity of the NiS@SiO2/graphene electrode, but also serve as a blanket to wrap NiS particle and so as to avert its exfoliation from electrode due to large volume change during cycling. The prepared NiS@SiO2/graphene nanocomposite exhibits high reversible capacity (750 mAh g1 for 100 cycles), remarkable cycling stability and impressive rate capability. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: nickel sulfide graphene electrochemical properties anode lithium ion batteries
1. Introduction In recent years, great efforts have been devoted to develop electric vehicles (EVs) in an attempt to reduce dependence on the limited fossil fuel resources and to decrease the greenhouse gas emission. Rechargeable lithium ion batteries (LIBs) with high power density, long cycle life and low cost are crucial for EVs’ development [1–3]. However, the relatively low specific capacity of conventional graphite (372 mAh g1 in theory) limits the capacity and energy densities of LIBs and thereby fails to meet the increasing requirement for EVs’ applications [4]. To address this issue, a great deal of research interest has been shifted to seek new materials that can deliver higher capacities. Nickel sulfide has attracted great attention due to its high theoretical capacity (590 mAh g1). However, the practical application of nickel sulfide has been impeded by the dramatic volume change during cycling [5,6], which could cause pulverization of the NiS particles, cracking of the electrode, and subsequent electrical
* Corresponding author at: School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Tel.: +86 10 82376837; fax: +86 10 82376837. E-mail address: [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.electacta.2014.12.074 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
disconnection from current collectors, leading to quick capacity fading upon extended cycling. To circumvent this problem, one of the promising strategies is to design porous or hollow structures of NiS to accommodate the large volume change. Recently, Zhao et al. [7] prepared NiS hollow spheres consisting of nanoflakes by a dodecanethiol-assisted hydrothermal process. Yang et al. [8] fabricated a porous nanostructured NiS on Ni foam substrate, which showed not only high discharge-charge capacities, but also stable cycling performance. Ni and co-workers [9] synthesized hollow structured NiS/ Ni composite that can yield a stable cycling of 100 times at 0.15 C. Another effective attempt is integrating carbon materials with nanostructured NiS, where the carbon component can mitigate the volume expansion and thus improve the cycling stability. Dai et al. [10] have reported the synthesis of a composite that Ni3S2 nanoparticles grown on the backbone of conductive multiwalled carbon nanotubes (MWCNTs) using a glucose-assisted hydrothermal method. Kang and co-workers [11] fabricated NiS/C composite with NiS nanocrystals being uniformly distributed inside the spherical carbon matrix, which exhibited an excellent discharge capacity of 472 mAh g1 at a high current density of 1000 mA g1, and even after 5000 cycles a high capacity retention of 86% was achieved. Graphene, a honeycomb network for sp2 carbon atoms, has attracted enormous interest as an intriguing substrate to build
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nanohybrids for LIBs because of its extraordinary electrical conductivity, outstanding mechanical flexibility and large specific surface area [12–14]. Recently, graphene was introduced to NiS to improve the rate-capability and cycling performance of electrode. Several works concerning NiS/graphene composites with NiS particles being encapsulated by graphene sheets were reported [15–19]. In this work, we presented a novel NiS@SiO2/graphene composite with hierarchical architecture that the NiS particles were enveloped by graphene sheets and between them nano-SiO2 particles were anchored on NiS particle surface, forming a void space inside composite particles. The NiS@SiO2/graphene composite was prepared by electrostatic self-assembly route at room temperature. This assembly enables an efficient encapsulation of NiS nanoparticles by graphene sheets with nano-SiO2 as a connector. The graphene sheets are instrumental in providing a conducting network for fast electron transfer between the active materials and the collector, and acting as a flexible twodimensional carbon matrix to prevent the loss of active sulfur component from the electrode. Notably, the SiO2 nanoparticles anchored on NiS nanoparticles serve as pillars to support the graphene sheets forming open spaces between the sheets and NiS particles, which could accommodate the volume change of NiS particles, improve electrode/electrolyte wettability and promote lithium ion transport in bulk electrode. The prepared NiS@SiO2/ graphene composite displays superior electrochemical performance with excellent cycling stability and high rate capability. 2. Experimental Section 2.1. Preparation of NiS nanoparticles The NiS nanoparticles were synthesized by a facile hydrothermal route. Typically, 0.38 g of Ni(Ac)2 4H2O was dissolved in 70 ml of deionized water under stirring for 30 min at room temperature. Then 0.588 g of C6H5Na3O7 2H2O was added into the above solution and stirred for 30 min, followed by the addition of 0.242 g of L-cysteine and stirred until it thoroughly dissolved. The pH value of the solution was adjusted to 10 by ammonia. Subsequently, the mixture was kept stirring for 1 h and then transferred into a 100 ml Teflon-lined stainless steel autoclave and heated in an oven at 180 C for 24 h. Finally, the autoclave was allowed to cool down to room temperature naturally. The products were filtered, washed with deionized water/ethanol and dried in air at 60 C overnight. 2.2. Preparation of hierarchical NiS@SiO2/graphene composite Hierarchical NiS@SiO2/graphene composite was fabricated by mixing NiS and graphene aqueous suspensions through the electrostatic attraction between positively charged NiS that is modified with APTES and negatively charged graphene, followed
by a pyrolysis of APTES. In a typical experiment, the obtained NiS nanoparticles (0.117 g) was dispersed into 50 ml of deionized water and ethanol (10:1, v/v) via sonication for 1 h, followed by dropwise addition of 0.5 ml of APTES and stirred for 1 h to render the surface of NiS positively charged. Meanwhile, graphene (30 mg) was dispersed in a water-ethanol (1:1, v/v) mixture system with vigorous ultrasonic agitation for 1 h. Then, the APTES-modified NiS suspension was added into the graphene suspension under mild magnetic stirring. After mixing for 1 h, 0.14 ml of hydrazine (80 wt. %) was added into the above suspension. Then the NiS@SiO2/ graphene composite was collected by centrifugation, washed with deionized water and ethanol, and followed by a thermal treatment to pyrolyze APTES in a conventional tube furnace at 500 C for 1 h under nitrogen atmosphere with a ramping rate of 10 C min1. The NiS/graphene composite was synthesized by the same method as NiS@SiO2/graphene without the addition of APTES. 2.3. Structure characterization Zeta potential (z) of the samples was determined by Zetasizer Nano ZS (Malvern) at room temperature of 25 C for characterization of surface charge properties. The phase structure of the products was identified by X-ray diffraction (XRD, Rigaku, D/maxA, Cu Ka, l = 1.5406 Å) and Raman spectroscope (LabRAM HR Evolution, excited by 523 nm laser). The morphology and lattice structure of the samples were characterized using field emission scanning electron microscopy (FE-SEM, SUPRA55) and transmission electron microscopy (TEM, Tecnai F20, 200 kV) equipped with high resolution TEM (HR-TEM) and energy dispersive X-ray spectroscopy (EDX). The porosity and Brunauer-Emmett-Teller (BET) surface area of NiS@SiO2/graphene were determined by N2 adsorption/desorption technique (QUADRASORB SI-MP, Quantachrome). The results were calculated by applying the Barrett-JoynerHalenda (BJH) model and the linear part of the BET plot, respectively. 2.4. Electrochemical measurements The working electrode was prepared by mixing 70 wt. % active material (pure NiS, NiS/graphene or NiS@SiO2/graphene composite), 15 wt. % acetylene black (AB) and 15 wt. % carboxymethylcellulose (CMC) with water under magnetic stirring for 24 h. The formed homogeneous slurry was then coated onto Ni foam, dried at 70 C for 6 h under vacuum and punched into disks with a diameter of 8 mm. Before cell assembling, the electrode was further dried at 120 C for 24 h under vacuum. The loading of active material in each electrode is 1.6 mg cm2. The 2032 coin-type cells were assembled in an Ar-filled glove box with active material as the working electrode, lithium foil as the counter electrode and Celgard 2400 as the separator. The electrolyte was composed of 1 M LiPF6 dissolved in ethylene carbonate (EC), diethyl carbonate
Scheme 1. Schematic illustration of synthetic procedure of NiS@SiO2/graphene.
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(DEC) and dimethyl carbonate (DMC) (1:1:1, in vol %). Galvanostatic charge/discharge testing was performed in the potential window from 0.01 to 3.0 V (vs. Li/Li+) on a Land battery tester (LAND CT-2001A tester). Cyclic voltammetry (CV) measurements were carried out at a scanning rate of 0.1 mV s1. Electrochemical impedance spectra of pure NiS and NiS@SiO2/graphene composite were analyzed in the frequency range from 106 to 0.1 Hz, while the disturbance amplitude was 5 mV. 3. Results and discussion The strategy to synthesize hierarchical NiS@SiO2 nanocomposite enveloped in graphene sheets through electrostatic selfassembly method is depicted in Scheme 1. With the hydrolization and condensation of APTES, the surface of NiS was modified with amine functional groups, by which the NiS nanoparticles are positively charged. The negatively charged surface of graphene is ascribed to the ionization of the oxygen-containing groups [20]. The opposite charge features of APTES-modified NiS and graphene sheets make the NiS nanoparticles be well enveloped with graphene sheets through the electrostatic attraction. After reducing the functional groups of graphene and pyrolyzing the APTES derivative on the surface of NiS, NiS@SiO2/graphene composite can be attained. The final composite has a unique feature that SiO2 nanoparticles (3–5 nm) are anchored on the surface of NiS nanoparticles (100 nm), forming hierarchical NiS@SiO2 core-shell particles, while the graphene sheets are intimately bounded to the composite particles. The graphene sheets can not only improve the electrical conductivity of the NiS@SiO2/graphene electrode by forming an efficient electrically conductive network [21–23], but also wrap the active particle to prevent its exfoliation from electrode due to the pulverization during charge/discharge processes. In addition, the anchored SiO2 nanoparticles can act as pillars to form open spaces between NiS nanoparticles and graphene sheets, which can effectively relieve the volume expansion of NiS nanoparticles to improve the structure stability of the electrode and at the same time provide a fast lithium ion migration pathway via enhanced electrolyte permeation into the active particles. The electrostatic self-assembly process of NiS nanoparticles with graphene sheets can be elucidated with the zeta potentials (z) of different components. As shown in Fig. 1, the suspension of pure NiS and graphene reveal negatively charged surfaces with zeta potential values of -10.3 and -11.5 mV, respectively. However, the
Fig. 1. Zeta potentials of pure NiS, graphene, and APTES-modified NiS.
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APTES-modified NiS shows an opposite zeta potential value of +8.6 mV, indicating that the negatively charged surface of NiS has been successfully changed after modification with APTES. The positively charged APTES-modified NiS and the negatively charged graphene can spontaneously assemble to construct composite with intimate interfacial contact by the electrostatic attraction. Fig. 2 presents the XRD patterns of the NiS@SiO2/graphene composite and pure NiS. All the identified peaks can be indexed to the hexagonal NiS phase (JCPDS card No. 65-3419). Compared to pure NiS, the NiS@SiO2/graphene composite shows a diffraction hump in the range of 20–25 (inset of Fig. 2a), which originates from graphene sheets [24,25] and nanocrystalline SiO2 [26]. The Raman spectrum was recorded to further clarify the structure characteristics of NiS@SiO2/graphene composite. As shown in Fig. 3a, two prominent peaks centered at 1351 and 1596 cm1 are observed, corresponding to the well documented D and G band, respectively [27]. The D band is ascribed to the disordered aromatic structure of the sp3 bonded carbons [28], while the G band relates to the first-order scattering of the stretching vibration mode E2 g observed for sp2 carbon domains [27,29]. The peak at 2684 cm1 known as 2D band and the peak at 2931 cm1 associated with the (D + G) band indicate a certain extent of disorder of the graphene sheets [30]. The peak marked by an ellipse in Fig. 3a is enlarged and displayed as Fig. 3b. The broad asymmetric peak can be deconvoluted into two peaks located at 286 and 320 cm1, which are assignable to the A1 vibration mode of NiS [31,32] and Si-O-Si bending vibration of SiO2 [33–35], respectively. This outcome validates the existence of SiO2 in the composite. The morphology of the synthesized samples was observed by FE-SEM and TEM. Fig. 4a shows the typical FE-SEM image of pure NiS particles, which have spherical appearance but aggregate severely to form large micro-sized congregation. The high magnification FE-SEM image (inset in Fig. 4a) reveals that the NiS particles with spherical structure are composed of agglomerated nanoparticles with size of ca. 100 nm. The FE-SEM images of the NiS/graphene and NiS@SiO2/graphene composites are presented in Fig. 4b and c. It is clear that the NiS particles and graphene sheets aggregate separately for NiS/graphene composite, while the NiS particles are well enveloped with graphene sheets and the typically wrinkled graphene sheets form a network for NiS@SiO2/graphene composite. This result implies that with the
Fig. 2. XRD patterns of NiS@SiO2/graphene composite (a) and pure NiS (b).
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Fig. 3. Raman spectrum (a) and enlarged partial spectrum (b) of NiS@SiO2/graphene composite.
Fig. 4. FE-SEM images of pure NiS (a), NiS/graphene (b) and NiS@SiO2/graphene (c), and TEM image of NiS@SiO2/graphene (d). The inset of (a) is a high magnification FE-SEM image of pure NiS.
modification of APTES, the graphene sheets and NiS particles have been well integrated together with an intimate interfacial contact by the electrostatic self-assembly approach. TEM observation (Fig. 4d) reveals that the SiO2 particles with a size of 3–5 nm homogeneously cover the surface of NiS particles, forming hierarchical structured NiS@SiO2 nanocomposite. A small amount of SiO2 nanoparticles scatter on the sheets of graphene. The nano-
sized SiO2 is originated from the pyrolysis of APTES, which was employed to couple the NiS particles with graphene sheets. EDX analysis was carried out to check the elemental compositions of NiS@SiO2/graphene composite. The result is displayed in Fig. 5a and the corresponding STEM image is shown in the inset of Fig. 5a. The elements of Ni, S, Si, O are detected in region A, which further confirm the existence of SiO2 in addition to NiS. The peaks
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Fig. 5. EDX profile (inset: the corresponding STEM image) of NiS@SiO2/graphene (a) and HR-TEM image of a single SiO2 nanoparticle in the composite (b).
assignable to Cu come from the Cu foil substrate. The HR-TEM image of a single SiO2 nanoparticle, as is shown in Fig. 5b, reveals some lattice fringes with an interplanar spacing of 0.206 nm, corresponding to (2 2 2) plane of SiO2, which indicates its crystalline feature. The porosity of NiS@SiO2/graphene composite was characterized by nitrogen adsorption/desorption isotherms. As shown in Fig. 6, the BET surface area and pore volume of NiS@SiO2/graphene are 195 m2 g1 and 0.45 cm3 g1, respectively. The main size of the pores as indicated in the inset of Fig. 6 is 3.8 nm, which is comparable with the size of SiO2 (3–5 nm), demonstrating that the SiO2 nanoparticles anchored on the surface of NiS particles can act as pillars to create empty space between graphene sheets and NiS particles. This porous structure is expected to be capable of offering a large number of open channels as passages of the electrolyte and thus accelerating the diffusion rate of lithium ions. On the other hand, it can provide space to tolerate the volume change of NiS particles during charge/discharge cycles. The electrochemical properties of pure NiS, and NiS@SiO2/ graphene were evaluated by galvanostatic charge/discharge cycling at a current density of 100 mA g1 with a potential window of 0.01–3.0 V. The representative charge/discharge voltage profiles of the initial 10 cycles of NiS@SiO2/graphene and NiS are shown in Fig. 7a and b. The NiS@SiO2/graphene electrode exhibits a wide
Fig. 6. N2 adsorption/desorption isotherms of NiS@SiO2/graphene (inset: BJH pore size distribution of the corresponding sample).
voltage plateau at 1.5–1.2 V in the first discharge curve, while it divides into two plateaus at 1.8 and 1.4 V in subsequent discharge processes. The increase of the voltage plateau is indicative of the structural change of the material [36]. The charge curves show no distinct variation in the initial 10 cycles, exhibiting a voltage plateau at 1.9 V, which can be attributed to the regeneration of NiS [37]. In the case of NiS electrode, similar charge and discharge curves can be observed. The discharge plateau of the NiS electrode shrinks remarkably with increasing cycle number while the NiS@SiO2/graphene electrode displays overlapped charge/discharge curves except for the initial cycle, revealing the excellent electrochemical stability and reversibility of the NiS@SiO2/graphene composite. Typical cyclic voltammetry (CV) characteristics of pure NiS and NiS@SiO2/graphene composite are illustrated in Fig. 8. As for pure NiS electrode (Fig. 8a), the CV curves of the 2nd and 3rd cycles are similar, whereas an obvious difference between the first and subsequent two cycles is observed. In the 1st cathodic scan, a strong peak at 1.0 V and a weak peak at 0.6 V are attributed to the generation of Li2S accompanying the reduction of NiS to Ni and the formation of solid electrolyte interphase (SEI) film [9], respectively. In the subsequent cycles two peaks at 1.3 and 1.8 V are displayed, which could be assigned to the two step reaction of NiS with lithium during discharge process [6,38–40], as described in Eqs. (1) and (2). 3NiS + 2Li+ + 2e ! Ni3S2 + Li2S
(1)
Ni3S2 + 4Li+ + 4e ! 3Ni + 2Li2S
(2)
In the three anodic scans, there is a strong oxidation peak at 2.0 V and a weak peak at 2.2 V, which are resulted from the extraction of lithium ions from Li2S and the reproducing of NiS [41]. The CV curves of NiS@SiO2/graphene electrode (Fig. 8b) are akin to those of pure NiS except for the presence of a weak reduction peak at 0.4 V in the 1st scan and an oxidation peak at 1.3 V. The reduction peak becomes boarder and shifts to higher potential (0.75 V) in subsequent cycles. This pair of peaks are ascribed to the insertion/extraction of Li+ into/from graphene, suggesting that the graphene in the composite is also electroactive for lithium storage [42–44]. Fig. 9a illustrates the cycling performances of pure NiS, NiS/ graphene and NiS@SiO2/graphene electrodes with a cut off voltage of 0.01–3.0 V vs. Li/Li+ at a current density of 100 mA g1 at room temperature (25 C) as well as the NiS@SiO2/graphene electrode at elevated temperature (50 C). The NiS electrode shows extremely
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Fig. 7. Charge/discharge voltage profiles of the initial 10 cycles of NiS@SiO2/graphene (a) and pure NiS (b).
Fig. 8. Cyclic voltammetry (CV) curves of pure NiS (a) and NiS@SiO2/graphene composite (b).
Fig. 9. (a) Cycling performances of pure NiS and NiS/graphene at RT, and NiS@SiO2/graphene at RT and 50 C, (b) rate performances of pure NiS, NiS/graphene and NiS@SiO2/ graphene at RT.
fast capacity degradation with discharge capacities of 775, 450, and 83 mAh g1 in the 1st, 2nd, and 100th cycles, respectively. The higher initial discharge capacity of the pure NiS electrode than the theoretical capacity of NiS (590 mAh g1) is most likely owing to the irreversible formation of solid electrolyte interface (SEI) film [45,46]. In the absence of SiO2, the NiS/graphene electrode shows a relatively stable cycle performance in the first few cycles but then the capacity decays rapidly as the pure NiS electrode does.
Contrarily, the NiS@SiO2/graphene electrode exhibits much better cycling performances at both 25 and 50 C. The enhanced cycling stability of NiS@SiO2/graphene composite electrode is evidently attributed to the existence of graphene sheets and the unique structure of NiS@SiO2/graphene. The graphene sheets can not only provide good electronic conductivity for electrode reaction but also serve as a protective layer to prevent the loss of active sulfur component and thus maintain the integrity of the electrode.
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Fig. 10. Cycling performances of the NiS@SiO2/graphene composite under different charge/discharge conditions.
Meanwhile, the open spaces formed by SiO2 nanoparticles between NiS particles and graphene sheets can help cushion the volume change of NiS and therefore improve the structural stability of the electrode. Both of them guarantee the excellent cycling performance. Without the addition of APTES, NiS/graphene composite is an inhomogeneous mixture as both of their surfaces are negatively charged. Hence, the NiS/graphene composite can improve the electrical conductivity and accommodate the volume change to a degree in the first few cycles. However, the capacity starts to decline rapidly in the subsequent cycles, which may originate from the pulverization of NiS particles and loss of electrical contact between graphene sheets and NiS particles caused by volume expansion/shrinkage during long-term cycling. The NiS@SiO2/graphene electrode at room temperature delivers the initial lithiation and delithiation capacities of 1275 and 867 mAh g1, respectively, corresponding to a Coulombic efficiency of 68%. This irreversible capacity could be associated with the formation of a solid electrolyte interface (SEI) film resulting from the decomposition of electrolyte on the surface of the electrode [38] and the reaction of oxygen-containing functional groups on graphene with lithium ions [47–50]. Fortunately, the Coulombic efficiency reaches nearly 100% and remains relatively stable in the subsequent cycles. After 100 cycles, the NiS@SiO2/graphene composite keeps a stable and reversible capacity of 750 mAh g1, which is much higher than the theoretical capacity of NiS (590 mAh g1). The extra capacity is most likely associated with the existence of graphene sheets and nano-SiO2. Except for the graphene sheets, the SiO2 nanoparticles with 3–5 nm in diameter can react with Li and contribute to the overall capacity [51,52]. In addition, a polymer gel-like film resulting from decomposition of the electrolyte at low voltage plays an indispensable role in this extra specific capacity [53]. The electrochemical performance of the prepared NiS@SiO2/ graphene composite at elevated temperature (50 C) is further evaluated by setting the coin cell in an electric oven. The sample shows a higher reversible capacity of 930 mAh g1 compared to that at room temperature (750 mAh g1), and a stable cycling performance, demonstrating an enhanced electrode kinetic reaction process and a good structure stability of the composite electrode at high temperature. The rate capabilities of the NiS, NiS/graphene and NiS@SiO2/ graphene electrodes were evaluated at different current densities. The cells were first cycled at 100 mA g1, and then switched to 200, 300, 500, 800, 1200, 1600, 2000 mA g1, successively. As anticipated, the NiS@SiO2/graphene composite presents excellent rate
Fig. 11. Nyquist plots of the NiS and NiS@SiO2/graphene electrodes at fully charged state after 15 cycles at 100 mA g1.
performance, as shown in Fig. 9b. At high current densities of 200, 300, 500, 800, 1200, 1600 mA g1, the composite can deliver reversible specific capacities of 680, 640, 610, 570, 540, 500 mAh g1, respectively. A relatively stable capacity of 470 mAh g1 can be still obtained at high current density of 2000 mA g1. Moreover, the discharge capacity can be recovered when the current density is restored to 100 mA g1, showing a great merit for an abuse tolerance of the NiS@SiO2/graphene electrode with varied current rates. In stark contrast, the capacity of NiS/graphene composite drops promptly from 800 to 185 mAh g1 when the rate increases from 100 to 2000 mA g1, and the capacity of NiS electrode decreases sharply with increasing C-rate and it cannot be recovered well, indicating the poor rate capabilities of NiS and NiS/graphene electrodes. The excellent rate performance of the NiS@SiO2/graphene composite is attributed to the mutual effects of the presence of graphene and the unique structure of NiS@SiO2/graphene composite with open spaces between graphene sheets and NiS particles formed by SiO2 nanoparticles. The former provides an electronically conductive network for fast electrode reaction and serves as a blanket to avoid the loss of active NiS from electrode and thus ensure the structural robustness of electrode during cycling. The latter offers an open space to allow the liquid electrolyte to infiltrate into NiS@SiO2/graphene particles and form a good contact with NiS particle surface, which are favorable for the lithium ion diffusion and the electrode reaction kinetics. To examine the difference in lithiation and delithiation kinetics, the NiS@SiO2/graphene electrode was subjected to cycle under different charge/discharge conditions. Two regimes were employed: I) charging at 800 mA g1 and discharging at 100 mA g1; II) charging at 100 mA g1 and discharging at 800 mA g1. Fig. 10 shows the results. The electrode delivers stable cycling performances under both charge/discharge conditions. Apparently, regime I leads to higher specific capacity than regime II, suggesting that the delithiation (charge) kinetics of NiS@SiO2/ graphene electrode is much faster than the lithiation (discharge) kinetics. EIS measurements were further conducted to understand the different electrochemical behaviors of pure NiS and NiS@SiO2/ graphene electrodes. Fig. 11 depicts the Nyquist plots of the electrodes at fully charged state after 15 cycles. The first semicircle at high frequency region reflects the resistance associated with Li+
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migration through the SEI film (Rf), while the second semicircle refers to the interfacial charge transfer resistance (Rct), which is a measure of the difficulty involved for charges crossing the boundary between the electrode and electrolyte. The straight line slope in the low frequency region corresponds to the diffusion of Li+ within the electrode (Warburg impedance). The Rct of the NiS@SiO2/graphene electrode tested at room temperature is much smaller than that of the NiS electrode, indicating the faster reaction kinetics of the composite electrode. It is ascribable to the graphene sheets and open spaces between the sheets and NiS particles, which ensure fast electronic and ionic transport within the NiS@SiO2/graphene electrode. The decrease of the Rct of the composite electrode at 50 C suggests the thermal activation feature of charge transfer process at the interface. 5. Conclusion In summary, we demonstrate a novel NiS@SiO2/graphene composite with unique hierarchical architecture that the NiS nanoparticles are well enveloped by graphene sheets, and on the surface of NiS particles many SiO2 minute nanoparticles are anchored, which support graphene sheets to form open spaces between the sheets and NiS particles. The space can effectively cushion the volume change of NiS and provide channels to increase electrode/electrolyte wettability and thereby promote lithium ion diffusion. The graphene sheets offer good electronic conductivity for electrode reaction and function as a blanket to prevent the exfoliation of NiS from electrode due to the volume change during charge/discharge processes. As a result, the NiS@SiO2/graphene electrode exhibits high specific capacity, excellent cyclic stability and good rate capability. The prepared NiS@SiO2/graphene composite displays a fast delithiation but a slow lithiation kinetics. Moreover, the synthesized NiS@SiO2/graphene composite demonstrates remarkable electrochemical performance at elevated temperature (50 C) in terms of specific capacity and cycling stability. Considering the superior electrochemical properties, the prepared NiS@SiO2/graphene composite is a promising candidate as anode material in LIBs. Acknowledgements This work was financially supported by National Basic Research Program of China (2013CB934003), “863”program (2013AA050902), National Nature Science Foundation of China (21273019) and Guangdong Industry-Academy-Research Alliance (2013C2FC0015). References [1] [2] [3] [4]
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