Rational Synthesis and Assembly of Ni3S4 Nanorods for Enhanced

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Rational Synthesis and Assembly of Ni3S4 Nanorods for Enhanced Electrochemical Sodium Ion Storage Jun Deng, Qiufang Gong, Hualin Ye, Kun Feng, Junhua Zhou, Chenyang Zha, Jinghua Wu, Junmei Chen, Jun Zhong, and Yanguang Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08625 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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Rational Synthesis and Assembly of Ni3S4 Nanorods for Enhanced Electrochemical Sodium Ion Storage Jun Deng,† Qiufang Gong,† Hualin Ye, Kun Feng, Junhua Zhou, Chenyang Zha, Jinghua Wu, Junmei Chen, Jun Zhong and Yanguang Li* Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China †

These two authors contribute equally;

Correspondence to: [email protected]

Abstract: Even though advocated as the potential low-cost alternatives to current lithium-ion technology, the practical viability of sodium-ion batteries remains illusive and depends on the development of high-performance electrode materials. Very few candidates available at present can simultaneously meet the requirements on capacity, rate capability and cycle life. Herein, we report a high-temperature solution method to prepare Ni3S4 nanorods with uniform sizes. These colloidal nanorods readily self-assemble side by side, and form microsized superstructure, which unfortunately negates the nanoscale feature of individual nanorods. To this end, we further introduce two-dimensional graphene nanosheets as the spacer to interrupt nanorod self-assembly. Resultant composite presents a marked advantage toward electrochemical storage of Na+ ions. We demonstrate that in half cells, it exhibits large reversible specific capacity in excessive of 600 mAh/g, high rate capability with >300 mAh/g retained at 4 A/g, and great cycle life at different current rates. This anode material can also be combined with NASICON-type Na3V2(PO4)3 cathode in full cells to enable large capacity and good cycleability. KEYWORD: sodium-ion battery, nickel sulfide nanorods, self-assembly, composite, full 1

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battery Sodium-ion batteries (SIBs) have attracted rapidly growing attention over recent years as one of the most promising post-lithium-ion technologies by virtue of the natural abundance and low cost of sodium.1-3 They also share a similar working principle to lithium-ion batteries (LIBs), and therefore the accumulated knowledge about LIBs can be readily transplanted to facilitate their research.4,

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In spite of the great potential, current SIBs still cannot rival

existing lithium-ion technology. A primary cause is the lack of suitable electrode materials that are able to accommodate the reversible electrochemical uptake and release of larger-sized Na+ ions with a significant capacity and cycle life.5-10 High-capacity materials such as P and Sb are usually short in cycle life,11, 12 whilst long-cycle-life materials such as hard carbon have rather limited capacity (600 mAh/g), rate capability (~300 mAh/g at 4 A/g) and cycling performance (>200 cycles) at the half cell level as well as satisfactory performance when coupled with the Na3V2(PO4)3 cathode in full cells.

Results and discussion Ni3S4 nanorods (NRs) were prepared by reacting NiCl2 and 1-dodecanethiol in oleylamine at 250°C (see Experimental Methods for details). Solid product at the end of reaction was readily dispersible in non-polar solvents such as cyclohexane to form a colloidal solution stable for months, presumably due to the surface passivation of Ni3S4 NRs with oleylamine. Figure 1a (insert) showed a ~100 mL cyclohexane solution of Ni3S4 NRs at a concentration of 1.6 mg/mL from one reaction batch. It could be further scaled up to yield product at the gram scale. When added with polar solvents such as ethanol, the dispersion became destabilized, and Ni3S4 NRs were immediately precipitated. X-ray diffraction (XRD) analysis (Figure 1a) of the product revealed that it was cubic Ni3S4 (JCPDS No.076-1813) free of other impurity phase. Typical scanning electron microscope (SEM) image illustrated that it consisted of one-dimensional nanorods with largely uniform sizes (Figure 1b). Many of these nanorods were aligned side by side, and 3


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formed a continuous film over the substrate. Their diameter and length were analyzed by randomly counting >100 nanorods from multiple SEM images and were presented in the histograms of Figure 1c. We noted a narrow size distribution with a predominant length of 35-40 nm and a predominant diameter of 5-6 nm. Very interestingly, under slow solvent evaporation, these nanorods were observed to spontaneously self-assemble and form superstructures up to microsize. A representative image in Figure 1d depicted such a superstructure consisting of roughly hexagonal close-packed nanorods standing on the substrate and some horizontally lying nanorods scattered around the perimeter. This would be impossible if it was not for the strong interaction among nanorods and for their uniform sizes. It was widely accepted that the self-assembly of nanorods was driven by the reduction of interfacial energy.31 Given its structure anisotropy, the side-by-side assembly of nanorods to form a hexagonal close-packed lattice could maximize their mutual interaction and thereby was energetically more favorable than the head-to-head assembly.32,

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In addition, we

investigated the influence of several synthetic parameters on the product morphology, and results were summarized in Supporting Information, Figure S1. Under transmission electron microscopy (TEM), small bundles of aligned Ni3S4 NRs were clearly discernible (Figure 1e). They were survived from the vigorous sonication during the TEM sample preparation, and again reflected the strong interaction among nanorods. High-resolution TEM image revealed that each nanorod was single-crystalline and displayed obvious lattice fringes assignable to the (311) plane of cubic Ni3S4 (Figure 1f). Based on the corresponding fast Fourier transform pattern as shown in the insert, we determined that the nanorod longitudinal axis was along the [110] direction. Such structural anisotropy was suggested to be facilitated by the selective binding of Cl- ions on the surface.34 Despite the appealing superstructure ordering, the spontaneous self-assembly of Ni3S4 NRs during solvent drying would greatly diminish available surface areas, negate their 4


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nanosized feature and therefore was detrimental to electrochemical applications. To alleviate this problem, we introduced a small amount of graphene oxide nanosheets as the spacer and support to the cyclohexane solution of as-prepared Ni3S4 NRs. Graphene oxide nanosheets were selected for their flexible two-dimensional (2D) structure, large surface areas, rich surface functionalities, good dispersion in most solvents and facile conductivity restoration by thermal reduction.35 They could readily capture and immobilize Ni3S4 NRs from the solution presumably via their surface functionalities. After brief sonication, resultant composite solid was precipitated upon ethanol addition, and subsequently subjected to annealing at 400 o C in Ar. The final product was named Ni3S4 NRs/rGO. Raman spectrum of Ni3S4 NRs/rGO was measured as shown in Figure 2a. It exhibited several vibrational bands at the low-wavenumber region that were characteristic to Ni3S4,36 as well as the pronounced D and G bands between 1200-1700 cm-1 from the defective graphene support. Ni 2p and S 2p X-ray photoelectron spectroscopy (XPS) spectra as well as C K-edge and Ni L-edge X-ray absorption near edge structure (XANES) spectra of Ni3S4 NRs/rGO were consistent with those of NiSx and rGO support (Supporting Information, Figure S2).37, 38 A low-magnification TEM overview revealed that Ni3S4 NRs were attached side-on to graphene nanosheets with a uniform distribution and mostly free of aggregation (Figure 2b, c). There was no free nanorod identified outside the support, highlighting the effectiveness of two-dimensional graphene nanosheets for immobilizing Ni3S4 NRs and suppressing their aggregation. Close TEM examination showed that individual nanorod retained its sizes and single-crystallinity (Figure 2c). Elemental mapping by energy dispersive spectroscopy (EDS) was also carried out under scanning transmission electron microscopy (STEM) (Figure 2d-f). The distribution of both Ni and S species had high spatial correlation and agreed well with the physical location of Ni3S4 NRs on the graphene support. Furthermore, thermogravimetric analysis (TGA) in air evidenced that the graphene content in Ni3S4 NRs/rGO was ~11.8 wt% 5


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(Figure 2g). Above structural characterizations corroborated the successful synthesis of Ni3S4 NRs with uniform sizes, and upon the incorporation of graphene nanosheets as the spacer, the formation of intimate Ni3S4 NRs/rGO composite. Such a composite preserved the nanosized feature of individual Ni3S4 NRs. The introduction of graphene support could also provide high electric conductivity needed for electrochemical applications. In what follows, we examined the performance of Ni3S4 NRs/rGO for the electrochemical storage of Na+ ions. Its half-cell measurements were carried out by pairing this composite material with Na metal disks in standard CR 2032 type coin cells (see Experimental Method for details). The electrolyte in use was 1 M NaClO4 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 by volume) with the addition of 8 vol% fluoroethylene carbonate (FEC). The sodiation/desodiation process of Ni3S4 NRs/rGO was suggested to proceed via a conversion mechanism to/from Ni and Na2S as schematically illustrated in Figure 3a. All conversion materials were subjected to an irreversible structural change during the first cycle, and Ni3S4 was no exception. Figure 3b presented the typical cyclic voltammetry (CV) curves of Ni3S4 NRs/rGO between 0.05 V and 2.8 V (versus Na+/Na, the same hereafter) at a scan rate of 0.1 mV/s for the first three cycles. During the initial negative sweep, multiple cathodic peaks centered at 0.66 V, 1.0 V and 1.2 V were discernible, which attested to the stepwise sodiation of Ni3S4, likely accompanied with the formation of solid electrolyte interface (SEI). All ensuing cycles featured two cathodic peaks at 0.79 V and 1.27 V, and two anodic peaks at 1.71 V and 1.38 V — indicative of permanent structure change at the first discharge and, subsequently, reversible structure conversion. It was also noteworthy that the stabilized CV curves of Ni3S4 NRs/rGO resembled those of NiS-based materials previously reported in literature.23, 39 It inferred that despite different starting compositions, both of them transformed to a similar active structure for the reversible uptake and release of Na+ ions. 6


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Galvanostatic charge/discharge voltage profiles of Ni3S4 NRs/rGO were collected at 50 mA/g and depicted in Figure 3c. Its discharge curve exhibited a plateau approximately around 1.0 V, followed by a long sloping tail from 0.9 V down to the cutoff voltage. The corresponding charging plateau was at ~1.6 V. Ni3S4 NRs/rGO delivered a specific capacity of ~830 mAh/g at the first discharge, and recovered ~700 mAh/g upon the subsequent recharge. This recovered capacity was close to the theoretical value of 705 mAh/g assuming the complete conversion to Ni and Na2S, and the irreversible capacity loss during the first cycle (~130 mAh/g) was likely associated with the SEI formation. Ex-situ XRD and TEM analysis showed that at discharge Ni3S4 converted to Ni and Na2S, and upon recharge, rhombohedral Ni3S2 phase emerged rather than original cubic Ni3S4 (Supporting Information, Figure S3). This was probably because Ni3S2 was more thermodynamically stable than Ni3S4 within the corresponding potential region. Worth noting was that an earlier work similarly reported Ni3S2 as the recharge product using NiS as the starting electrode material.23 As further cycling proceeded, the specific capacity of Ni3S4 NRs/rGO reached a stable value of ~630 mAh/g and retained this value for at least 100 cycles as shown in Figure 3d. Its Coulombic efficiency after the third cycle maintained 97~100%. The contribution of rGO support to the specific capacity was found to be small (Supporting Information, Figure S4). Control experiments with commercial Ni3S4 nanopowders and Ni3S4 NRs alone were also carried out. When assessed under the same conditions, the former demonstrated a smaller initial discharge capacity of ~500 mAh/g that quickly declined to ~300 mAh/g after 50 cycles, whereas the latter, despite its similar initial capacity to Ni3S4 NRs/rGO, was also suffered from notably inferior cycling stability and ended up with ~490 mAh/g after 90 cycles. In addition, we also noted that most NiSx-based electrode materials previously reported for SIBs had insufficient specific capacity (200~500 mAh/g) and/or limited cycling stability (sometimes < 50 cycles) (Supporting Information, Table S1).37-40 The greatly enhanced 7


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electrochemical performance of our Ni3S4 NRs/rGO over control samples and previous NiSx-based materials was believed to benefit from the nanoscale feature of Ni3S4 NRs and their effective hybridization with the rGO support. Here, 2D rGO nanosheets not only provided the high electrical conductivity desirable to battery applications, but also suppressed the nanorod self-assembly and preserved their individual nanoscale feature. We found that if rGO was replaced with the same amount of Ketjenblack carbon at the mixing step, Ni3S4 nanorod agglomeration was not effectively suppressed, and the electrochemical performance of thus-formed composite (Ni3S4 NRs/C) was considerably inferior to Ni3S4 NRs/rGO (Supporting Information, Figure S5). Moreover, we examined the morphology of the recharge product under TEM (Supporting Information, Figure S6). The rod-like morphology was preserved after the 2nd cycle, but it gradually transformed to small (~3 nm) nanoparticles attached to rGO nanosheets afterwards. Such a morphology transformation was understandable given the large volume change during charge and discharge. But it was remarkable that all these nanoparticles stayed attached to the support, which might explain the observed excellent cycle life after the initial capacity loss. Next, we evaluated and compared the rate capability of Ni3S4 NRs/rGO and Ni3S4 NRs. As shown in Figure 3e, their specific current was programmed to ramp stepwise from 50 mA/g up to 4 A/g, and then symmetrically reverting back to 50 mA/g. Ni3S4 NRs/rGO exhibited remarkable high-rate performance, and was able to deliver a significant reversible capacity of ~310 mAh/g even at 4 A/g. By stark contrast, Ni3S4 NRs alone showed poor rate capability, and its specific capacity dropped close to zero at 4 A/g. Such drastically different high-rate performances underscored the markedly enhanced charge transfer kinetics in Ni3S4 NRs/rGO over Ni3S4 NRs due to the intimate contact between individual nanorods and the conductive graphene support. Electrochemical impedance spectroscopy (EIS) analysis of these two electrode materials at the end of the 3rd charge showed that the interfacial charge 8


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transfer resistance of Ni3S4 NRs/rGO estimated from the semicircle diameter was only a small fraction of that of Ni3S4 NRs (Figure 3f). Furthermore, we found that the cycling stability of Ni3S4 NRs/rGO was not noticeably compromised under high current rates. At 1 A/g, it delivered a reversible specific capacity of ~460 mAh/g with 99~100% Coulombic efficiency for 200 cycles, whereas Ni3S4 NRs alone suffered from a quick capacity loss and retained only ~40 mAh/g after 200 cycles (Figure 3g). As far as we were aware, the capability of our composite material to sustain large and stable capacity even at high current rates exceeded most existing NiSx-based electrode materials (Supporting Information, Table S1)37-40 as well as other popular SIB candidate materials such as hard carbon, MoS2, SnS2 and so on,41-43 and therefore rendered it attractive for potential power applications. Encouraged by the impressive performance of Ni3S4 NRs/rGO as the SIB anode material, we constructed sodium-ion full cells by pairing this anode material with nanostructured Na3V2(PO4)3 (NVP) cathode (Figure 4a). NVP had a three-dimensional Na+ ion superionic conductor (NASICON) framework with large interstitial channels for fast Na+ ion diffusion, and was well known for its great cycling stability and high-rate performance.44 In our study, nanostructured NVP was prepared via a facile solution precipitation method followed by high-temperature annealing.45, 46 The final product was analyzed to be pure NVP by XRD and revealed to consist of nanoflake-assembled hierarchical microflowers under SEM (Supporting Information, Figure S7). When evaluated in the half-cell configuration in 1 M NaClO4/EC/DEC, NVP exhibited the characteristic flat plateau at ~3.4 V (Figure 4b). Its specific capacity reached ~120 mAh/g at 30 mA/g, and retained 93% of the initial capacity at the end of 100 cycles, typical to NVP reported in literature.46, 47 NVP//Ni3S4 full cells were assembled by pairing 10 mg/cm2 NVP and 1 mg/cm2 Ni3S4 NRs/rGO in standard coin cells. The absolute capacity of the cathode was ~40% larger than that of the anode in order to offset the irreversible capacity loss of Ni3S4 NRs/rGO during the first cycle and to ensure that Ni3S4 9


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NRs/rGO was the capacity-limiting electrode. Figure 4c showed the galvanostatic charge and discharge voltage profile of the NVP//Ni3S4 full cell between 0.1−3.0 V at 50 mA/g (normalized to the weight of anode material). It displayed a discharge voltage plateau between 1.7−2.0 V, and delivered a capacity of 530-580 mAh/g (also normalized to the weight of anode material) for the first three cycles. At last, the cycling stability of NVP//Ni3S4 full cell was also assessed. It was charged and discharged at 50 mA/g for the first five cycles, and then at 100 mA/g for up to 100 cycles. The capacity retained >400 mAh/g with corresponding Coulombic efficiency over 98% (Figure 4d).

Conclusion In summary, we developed a high-temperature solution method for the rational synthesis of Ni3S4 NRs. They had uniform sizes, and could self-assemble side by side to form microsized superstructures upon slow solvent evaporation. In order to fully capitalize the nanoscale feature of individual nanorods for electrochemical applications, we further introduced two-dimensional graphene nanosheets as the spacer to interrupt nanorod self-assembly. Ni3S4 NRs became effectively immobilized side-on to graphene nanosheets with a uniform distribution and mostly free of aggregation in the resultant composite. When assessed as the SIB anode material at the half-cell level, Ni3S4 NRs/rGO composite exhibited impressive specific capacity over 600 mAh/g, excellent rate capability with >300 mAh/g retained at 4 A/g, and great cycling stability at different current rates. When coupled with the Na3V2(PO4)3 cathode in full cells, it enabled an average working voltage of ∼2 V, large reversible capacity of >400 mAh/g and decent cycling stability. Our study demonstrated how the rational synthesis and engineering of nanomaterials could lead to much enhanced electrochemical performance for traditionally problematic SIB electrode materials.

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Experimental Methods Synthesis of Ni3S4 NRs/rGO. We started with the preparation of Ni3S4 NRs. In a typical synthesis, 2 mmol NiCl2•6H2O was first dissolved in 40 mL of oleylamine inside a 100 mL three-necked flask. Moisture and oxygen was repelled from the solution by heating it up to 140°C under magnetic stirring and Ar protection. After cooled back to room temperature, the solution was added with 12 mL of 1-dodecanethiol via injection. Its temperature was raised to 250°C, and maintained at this temperature for 30 min. At the end of the reaction, solid product was collected by centrifugation at 8000 rpm, repetitively washed with cyclohexane and then redispersed in 100 mL cyclohexane. Subsequently, 26 mg of graphene oxide (GO) powder (prepared from the modified Hummers’ method48) was dispersed in the above cyclohexane solution of Ni3S4 NRs. After vigorous bath-sonication for 15 min, the solid composite was precipitated following the addition of ethanol, repetitively washed with the mixed solvent of cyclohexane and ethanol (~1:3 v/v), and lyophilized. The product was finally annealed under Ar at 400 °C for 1 h for the thermal reduction of GO to rGO. Structural characterizations. Powder X-ray diffraction (XRD) was performed on PANalytical X-ray diffractometer at a scan rate of 0.05o/s. Scanning electron microscopy (SEM) images were taken on Supera 55 Zeiss scanning electron microscope. Transmission electron microscopy (TEM) images, scanning transmission electron microscopy (STEM) images and energy dispersive spectroscopy (EDS) mapping were acquired using Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kV. Raman was conducted on LabRAM HR 800 Raman microscope using 514 nm laser excitation. Thermal gravimetric analysis (TGA) was carried out on Mettler Toledo TGA/DSC1 Thermal Analyzer in air from room temperature to 1000 °C at a heating rate of 10 °C/min. X-ray photoelectron

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spectroscopy (XPS) was performed on Ultra DLD XPS Spectrometer. X-ray absorption spectra were collected at the XMCD endstation of the National Synchrotron Radiation Laboratory (NSRL). Electrochemical measurements. To prepare the working electrodes, Ni3S4 NRs powder was blended with Ketjenblack and carboxymethyl cellulose (CMC), and homogeneously dispersed in water. The slurry was then evenly blade-coated onto Cu foil, and vacuum dried at 60°C for 12 h. The areal loading of Ni3S4 NRs/rGO was 1~2 mg/cm2. Half-cell measurements were conducted in standard CR2032 coin cells by pairing the working electrode with a metallic Na disk, separated by electrolyte-saturated glass fiber membrane in an Ar-filled glovebox. The electrolyte was 1 M 1 M NaClO4 in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 v/v) with the addition of 8% fluoroethylene carbonate (FEC). Cyclic voltammetry (CV) was performed on CHI 660E potentiostat between 0.05−2.8 V at a scan rate of 0.1 mV/s. Galvanostatic charge and discharge measurements were carried out on NEWARE battery tester between 0.05−2.8 V at various current rates. All specific currents and specific capacities calculated based on the total weight of the composite. Electrochemical impedance spectroscopy (EIS) was collected on Gamry Interface 1000E electrochemical workstation. For NVP//Ni3S4 full cell measurements, 10 mg/cm2 NVP and 1 mg/cm2 Ni3S4 NRs/rGO were paired up in standard coin cells with the same electrolyte. Galvanostatic charge/discharge experiments were carried out between 0.10 and 3.0 V. Acknowledgements We acknowledge supports from the National Natural Science Foundation of China (51472173 and 51522208), the Natural Science Foundation of Jiangsu Province (SBK2015010320), the 12


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Priority Academic Program Development of Jiangsu Higher Education Institutions and Collaborative Innovation Center of Suzhou Nano Science and Technology. Supporting Information available: Influence of different synthetic parameters on the product morphology, XPS and XANES characterizations, XRD and TEM analysis of discharge and recharge products, electrochemical performance of rGO alone, TEM image and electrochemical performance of Ni3S4 NRs/C, TEM images of cycled products, SEM and XRD of NVP, and performance comparison with previously reported nickel sulfide materials for SIBs. The Supporting Information is available free of charge on the ACS Publications website.

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Figure 1. Structural characterizations of Ni3S4 NRs. (a) Powder XRD pattern, with the digital image (insert) showing ~100 mL of Ni3S4 NRs dispersed in cyclohexane. (b) SEM image and (c) corresponding size distribution histograms. (d) SEM image showing a representative superstructure self-assembled from nanorods. (e) Low-magnification TEM image. (f) High resolution TEM image, with the fast Fourier transform (FFT) pattern shown in the insert.

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Figure 2. Structural characterizations of Ni3S4 NRs/rGO composite. (a) Raman spectra of Ni3S4 NRs/rGO, Ni3S4 NRs and rGO. (b,c) TEM images at different magnifications. (d) STEM image of Ni3S4 NRs/rGO and (e,f) corresponding Ni and S EDS mapping of the enclosed area in (d). (g) TGA curve of Ni3S4 NRs/rGO in air.

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Figure 3. Electrochemical measurements of Ni3S4 NRs/rGO. (a) Schematic illustration showing the possible charge and discharge mechanism of Ni3S4 NRs/rGO. (b) CV curves for the first three cycles at a scan rate of 0.1 mV/s. (c) Galvanostatic charge and discharge curves for the first three cycles at 50 mA/g. (d) Cycling performance and Coulombic efficiency of Ni3S4 NRs/rGO in comparison with Ni3S4 NRs and commercial Ni3S4 at 50 mA/g. (e) Rate performances of Ni3S4 NRs/rGO and Ni3S4 NRs. (f) Nyquist plots of Ni3S4 NRs/rGO and Ni3S4 NRs at the end of the 3rd recharge. (g) Cycling performance and Coulombic efficiency of Ni3S4 NRs/rGO and Ni3S4 NRs at 1 A/g.

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Figure 4. Electrochemical measurements of NVP//Ni3S4 full cells. (a) Schematic illustration showing the full cell configuration. (b) Galvanostatic charge and discharge curves of NVP half cells at 30 mA/g and (insert) cycling performance. (c) Galvanostatic charge and discharge curves of NVP//Ni3S4 full cells at 50 mA/g. (d) Cycling performance and corresponding Coulombic efficiency of full cells at 50 and 100 mA/g.

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Rational Synthesis and Assembly of Ni3S4 Nanorods for Enhanced

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