NiS1.03 Hollow Spheres and Cages as Superhigh

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Jul 13, 2018 - KEYWORDS: hollow spheres and cages, sodium-ion batteries, anode material, ultrahigh performance, ... negative electrodes, especially for SIBs.16−18 Nickel sulfides are ...... Electrolytes: A Case Study of ZnS Nanospheres.

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NiS1.03 Hollow Spheres and Cages as Superhigh Rate Capacity and Stable Anode Materials for Half/Full Sodium-Ion Batteries Caifu Dong,† Jianwen Liang,† Yanyan He,† Chuanchuan Li,† Xiaoxia Chen,† Lijun Guo,† Fang Tian,† Yitai Qian,† and Liqiang Xu*,†,‡ Downloaded via SHANDONG UNIV on November 20, 2018 at 09:40:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.



Key Laboratory of Colloid and Interface Chemistry, Ministry of Education and School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China ‡ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Nickle sulfides as promising anode materials for sodium-ion batteries have attracted tremendous attention owing to their large specific capacity and good electrical conductivity. However, the relative large volume changes during the sodiation/desodiation process usually result in a fast capacity decay, poor cycling stability, and sluggish electrode kinetics which hinder their practical applications. Herein, NiS1.03 porous hollow spheres (NiS1.03 PHSs) and porous NiS1.03 hollow cages (NiS1.03 PHCs) with high yield are designed and selectively fabricated via a simple solvothermal and subsequent annealing approach. The obtained NiS1.03 PHSs display long-term cycling stability (127 mAh g−1 after 6000 cycles at 8 A g−1) and excellent rate performance (605 mAh g−1 at 1 A g−1 and 175 mAh g−1 at 15 A g−1). NiS1.03 PHCs also show high rate capability and outstanding cycling stability. In addition, the analyses results of in situ and ex situ XRD patterns and HRTEM images reveal the reversible Na-ion conversion mechanism of NiS1.03. It is also worth noting that the NiS1.03 PHSs//FeFe(CN)6 full cell is successfully assembled and exhibits an initial reversible capacity of 460 mAh g−1 at 0.5 A g−1, which further evidence that NiS1.03 is a kind of prospective anode material for SIBs. KEYWORDS: hollow spheres and cages, sodium-ion batteries, anode material, ultrahigh performance, full cell

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for half/full SIBs is urgently needed especially for the increasing requirement of electric vehicles and smart grids. Transition-metal sulfides as anodes usually show higher reversible capacities compared with those of the insertion-type negative electrodes, especially for SIBs.16−18 Nickel sulfides are considered to be suitable host materials for SIBs due to its low cost, environmental friendliness, and high theoretical specific capacity. However, the aggregation of nanograins and the collapse of electrode structures are the main obstacles of nickel sulfides during sodiation/desodiation processes, which easily result in fast decline of specific capacity and poor rate capability. To solve these problems, tremendous efforts have been made to develop high-performance anode materials for SIBs. An effective way to address these challenges is to design nanostructures with hollow interiors, which have highly accessible surface areas for sufficient electrode/electrolyte

odium-ion batteries (SIBs) are promising and attractive as next-generation battery candidates because sodium is abundant in the earth, its cost is low, and its intercalation chemistry is similar to that of Li.1−4 However, the radius of sodium ions is about 55% larger than that of lithium ions, and sodium is less electropositive than Li.5−7 The volume effect of electrode material aroused by the insertion/ extraction process of Na+ is significant, which easily causes the serious structure damage of electrode material and eventually results in the severe sluggish kinetics and poor electrochemical performances.8−10 Therefore, great challenges still remain for the performance improvement and commercialization of electrode materials, especially the anodes for SIBs.11 During the past years, various carbon related materials, transitionmetal oxides, and alloy related materials all have been utilized as anodes for SIBs, and obvious progresses have been made on the performance improvement of these materials. 12−15 Although obvious progresses have been made, the rational designation and development of electrodes with superhigh rate-capacity and stable cycling performance as anode materials © 2018 American Chemical Society

Received: May 10, 2018 Accepted: July 13, 2018 Published: July 13, 2018 8277

DOI: 10.1021/acsnano.8b03541 ACS Nano 2018, 12, 8277−8287

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indispensible role for the formation of NiS hollow sphere precursors (NiS HSs) in the first step. In the absence of it, NiS solid microspheres consisting of interconnected nanosheets were obtained. While in the presence of it, NiS hollow microspheres self-assembled by smaller interconnected nanosheets were produced. After a calcination process under argon atmosphere, NiS1.03 PHSs and NiS1.03 PHCs were obtained finally. Figure 1a shows the typical FESEM image of the as-prepared precursor with hollow spherical structure and diameters in the range of 1−2 μm. Figure S1a displays the SEM image of a cracked sphere, in which the hollow structure is clearly observed, and the microsphere is composed with tightly aggregated nanosheets with the size of 100 to 200 nm, and the large interior hollow space is further confirmed by TEM images (Figure 1b). FESEM images of NiS solid spheres at different magnifications (Figure S1b,c) and TEM images (Figure S1d) reveal that NiS solid spheres assembled by larger and looser nanosheets with sizes ranging from 200 to 300 nm were produced. It can be seen from Figure 1c,d that shape remained evolution from the NiS precursor to NiS1.03 PHSs was achieved. The porous feature of the obtained NiS1.03 PHSs (composed of small nanosheets) revealed by SEM and TEM images can not only remarkably shorten the transmission distances of ions and electrons but also can slow down the volume expansion during the charging and discharging processes. The average thickness of the shell of the hollow spheres is ∼30 nm (inset in Figure 1d). The lattice distance of 0.251 nm displayed in Figure 1e corresponds to the (102) planes of NiS1.03. After the precursor was calcined in argon atmosphere for 1 h, an interesting structural evolution from solid microspheres into porous hollow microspheres is observed, and the size of the nanosheets almost has no obvious changes (Figure 1f,g). The energy-dispersive X-ray spectroscopy (EDS) mappings of NiS1.03 PHSs and NiS1.03 PHCs presented in Figure 1h and Figure S1e confirm the uniform elemental distributions of Ni, S, C, and N elements in both samples. Figure S2a shows the XRD pattens of the hollow and solid spherical precursors, which can be indexed to be hexagonal nickel sulfide (JCPDS card no. 02-1280, space group P63/ mmc). Combining the analyses results of TGA curves (the precursors, Figure S2b) and the XRD patterns, it is found that NiS transformed into NiS1.03 after annealing at 400 or 500 °C for 1 h in Ar (Figure 2a and Figure S2c). However, NiS converted into multiphase (NiS, NiXS6, NiS1.03) after the precursor was annealed at 650 °C (Figure S2d). SEM images show that there are obvious morphological changes after thermal treatment of the NiS at 500 and 650 °C (Figure S2e,f). Thus, 400 °C for 1 h in Ar atmosphere is the optimal calcination temperature for the fabrication of NiS1.03 (NiS HSs and NiS SSs) from NiS precursors. Figure 2a shows XRD patterns of the products after the calcination, in which all the diffraction peaks with high diffraction intensity can be indexed as the hexagonal-phase NiS1.03 (JCPDS card no. 02-1273) with high crystallinity. No obvious diffraction peaks related to other materials were detected, indicating NiS1.03 of pure phase has been obtained. The schematic crystal structure of NiS1.03 is exhibited in Figure 2b. There are two Ni2+ ions and two S2− ions in one crystal cell. The Ni2+ ion is coordinated with six S2− ions displaying a distorted octahedral geometry. XPS analyses were applied to further verify the chemical composition and

contacts and can provide relative large internal space to accommodate the volumetric changes during cycling, thus enhancing the cycling stability of the electrode. For instance, NiS spheres fabricated by refluxing method show a charge capacity of 499.9 mAh g−1 at a current density of 100 mA g−1 after 50 cycles.19 Another effective strategy to combat these challenges is to combine nickel sulfides with carbon, which can improve the electronic conductivity as well as restrain the volume variation of the electrode. For example, nickel sulfide/ RGO composite prepared via a microwave-assisted method achieves a reversible specific capacity of ∼391.6 mAh g−1 at a current density of 100 mA g−1 after 50 cycles.20 Along with the above two approaches, the property improvement of the electrode materials could also be achieved by tuning the electrochemical window to largely reduce the voltage drop and electrochemical polarization degrees.21,22 For example, mesoporous NiS2 nanospheres synthesized via a polyvinylpyrrolidone-assisted method achieve a charge capacity of 253 mAh g−1 at 5 A g−1 (rate performance).23 Although the cycling performances especially the cycle stability have been obviously improved, long cycle life (>300 cycles) and high rate performance are still great challenges for nickel sulfides. Herein, both NiS1.03 hollow spheres (NiS1.03 PHSs) and NiS1.03 hollow cages (NiS1.03 PHCs) that are composed of nanosheets are obtained by a simple solvothermal and subsequent annealing method. Both of them display superior rate capability and outstanding cycling stability when used as anode materials for SIBs. Take the sample of NiS1.03 PHSs as an example, it exhibits excellent rate performance (605 mAh g−1 at 1 A g−1 and 175 mAh−1 at 15.0 A g−1) and demonstrates a high reversible capacity of 127 mAh g−1 after 6000 cycles at 8 A g−1. The sodium ion storage mechanism of NiS1.03 is investigated by in situ XRD patterns and HRTEM. Moreover, the morphology of the electrode in the cell was examined by SEM at different sodiation/desodiation states in the first cycle. The impressive electrochemical performance combining with simple synthesis strategy reveals that the NiS1.03 is a promising electrode for SIBs, and this strategy may be extended to design many other high-performance energy storage systems.

RESULTS AND DISCUSSION As is shown in Scheme 1, NiS1.03 porous hollow spheres (NiS1.03 PHSs) and porous NiS1.03 hollow cages (NiS1.03 PHCs) that are composed of closely interconnected ultrathin nanosheets were prepared via a simple solvothermal method followed by annealing at relative low temperature. It is worth mentioning that 2-methylimidazole (2-MIN) plays an Scheme 1. Brief Illustration of the Synthesis Process of NiS1.03 PHSs and NiS1.03 PHCs

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Figure 1. FESEM and TEM images of the precursor (a, b), FESEM, TEM and HRTEM images of the NiS1.03 PHSs (c−e) and NiS1.03 PHCs (f, g). (h) STEM image of NiS1.03 PHSs and the corresponding elemental mappings of Ni, S, N and C elements (as labeled).

Figure 2. (a) XRD patterns of the obtained NiS1.03 PHSs and NiS1.03 PHCs. (b) Crystal structure of NiS1.03. (c−f) XPS spectra of the obtained NiS1.03 PHSs.

electronic states of the NiS1.03 PHSs. Figure 2c shows the Ni 2p XPS spectrum, in which two main peaks located at 854.0 and 871.1 eV can be attributed to Ni2+ and another two main peaks at 856.1 and 873.5 eV corresponding to Ni3+, while the peaks centered at 861.2 and 879.8 eV correspond to two satellite peaks for Ni 2p.24−26 Figure 2d displays the S 2p XPS spectrum, in which two main peaks at 161.7 and 162.8 eV can be assigned to S 2P3/2 and S 2P1/2 of Ni−S bondings, another

two peaks located at 168.7 and 170.0 eV can be ascribed to the O−S and OS bonds, respectively.26,27 The N 1s spectrum of NiS1.03 PHSs is shown in Figure 2e, which can be deconvoluted into two peaks centered at 399.6 and 400.4 eV, corresponding to pyrrolic N and graphitic N, respectively.24,28 Nitrogen doping can change the electronic properties of the neighboring carbon matrix and enhance the electronic conductivity, providing more active sites for Na+ insertion.29−31 The C 1s 8279

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Figure 3. (a) CV curves of the NiS1.03 PHSs electrode obtained at a scan rate of 0.1 mV s−1. (b) The charge−discharge profiles of NiS1.03 PHSs at 1 A g−1. (c) Rate performance of NiS1.03 PHSs and NiS1.03 PHCs electrodes in the voltage range of 0.01−2.3 V vs Na+/Na. (d) Rate performances of electrodes based on NixSy. (e) CV curves of the NiS1.03 PHSs electrode at different scan rates. (f) Linear relationship between log (i) and log (v) on each redox peak.

spectrum in Figure 2f exhibits two peaks at 284.8 and 288.5 eV, which can be assigned to C−N bond and graphitic carbon, respectively.18 In order to further examine the carbon content, thermal analysis was performed with a heating rate of 10 °C min−1 from 25 to 800 °C in air (TGA curve in Figure S3a,b). The drastic weight loss (∼6 wt %) occurred at ∼550 °C could be attributed to the loss of carbon.32 The nitrogen content of NiS1.03 hollow spheres is 0.41 wt % based on the element analysis. Based on the Brunauer−Emmett−Teller (BET) method, the BET surface areas of NiS1.03 PHSs and NiS1.03 PHCs are 6.6 and 4.3 m2 g−1, respectively (Figure S4). Benefiting from the structural and compositional merits, NiS1.03 PHSs and NiS1.03 PHCs are expected to display excellent electrochemical performance as anode materials for SIBs. The time-dependent experiments for the formation of NiS hollow and NiS solid microspheres were investigated to understand the function of 2-MIN. Figure S5 shows the SEM images of the NiS hollow microspheres obtained within different solvothermal times. The results indicate that the reaction time is crucial for the formation of the NiS hollow microspheres. It can be seen that the solid spheres with smooth surfaces appeared after 1 h. The surfaces of the solid spheres become rougher and are decorated with very small nanoparticles when the reaction time was increased to 2 h. After 4 h, the nanoparticles self-assembled into nanosheets. An interesting structural evolution from solid spheres to yolk−shell structure spheres is observed when the reaction time was 8 h. Almost all of the spheres present a hollow structure character when the reaction time is further increased to 72 h. This is consistent with the observation results from TEM images (Figure S6). For NiS solid microspheres, initially, some nanoparticles appeared in the solid microspheres (Figure S7a,b) and then rapidly self-assembled into nanosheets after 4 h (Figure S7c). After that, the morphology of the solid microspheres has almost no changes after the time was

increased to 72 h. This is also consistent with the observation results from the TEM images (Figure S8). In summary, the formation mechanism of hollow microspheres composed of nanosheets can be explained by nanoparticles self-assembling into nanosheets on the outside of the solid spheres, and then Ostwald ripening process was processed, in which 2-MIN plays an indispensable role. The effects of different sulfur sources on the morphology of the precursor have been investigated, and the related SEM images are shown in Figure S9. When thiosemicarbazide was used as a sulfur source, hollow spheres transform into interconnected cubes (Figure S9 a) finally. Figure S9b,c exhibits the SEM images of the precursor at different magnifications when thiourea was used instead of thioacetamide (TAA). At low magnification, the hollow spheres are observed. At high magnification, one can see that these hollow spheres consist of nanoparticles. When using Na2S as sulfur source, nanoparticles were formed instead of hollow spheres (Figure S9d). To demonstrate the versatility of the presented synthesis approach, the precursors with different morphologies were prepared by changing the component ratios of TAA, 2-MIN, Ni(NO3)2·6H2O, and sulfur sources in the synthesis process. When changing the component ratios of TAA (240 mg), Ni(NO3 ) 2·6H2O (58.2 mg), and 2-MIN (131.2 mg), pomegranate-like microspheres were obtained through the same method (Figure S10a,b). After the above precursors were calcined at 400 °C and maintained for 1 h in argon atmosphere, the NiS/NiS2 porous microspheres could be obtained (Figure S10c,d). The XRD diffraction peaks of precursor after calcined at 400 °C can be indexed to NiS (JCPDS card no. 02-1280) and NiS2 (JCPDS card no. 110099) (Figure S11). By virtue of the appealing structures of hierarchical NiS1.03 PHSs and NiS1.03 PHCs, coin-type cells were assembled to evaluate their electrochemical performances as anodes for SIBs. Figure S12 displays the cycle performance, and electrochemical 8280

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up to 477.7 mAh g−1, and NiS1.03 PHCs can deliver a reversible capacity of 482.6 mAh g−1. It is worth noting that the capacities of NiS1.03 PHSs and NiS1.03 PHCs remained from the second to the 100th cycle and have stable plateaus during the discharge and charge processes. As is shown in Figure S17, after 100 cycles, the hollow structure of NiS1.03 PHSs and NiS1.03 PHCs was retained when the electrodes were used as anode for SIBs, displaying the excellent structural stability of the present electrode during the Na-storage process. The rate performances of NiS1.03 PHSs and NiS1.03 PHCs electrodes at different current densities are investigated (Figure 3c). As expected, both NiS1.03 PHSs and NiS1.03 PHCs electrodes display superior rate capability. For NiS1.03 PHSs, when cycled at current densities of 1, 2, 3, 5, 8, 10, and 12 A g−1, the reversible capacities of NiS1.03 PHSs are ∼604.8, 505.5, 470.6, 404.0, 322.4, 272.3, and 226.6 mAh g−1, respectively. Notably, even at ultrahigh rates of 15 A g−1, the reversible capacity still can remain at 175 mAh g−1. If the current density is switched abruptly back to 1 A g−1, the reversible capacity can recover to 541.4 mAh g−1 and remain stable during next 20 cycles, displaying its fast reaction kinetics. Similarly, the NiS1.03 PHCs show reversible capacities of 577.2, 482.4, 443.4, 356.9, 235.6, 208.2, 176.7, and 100 mAh g−1 at current densities of 1, 2, 3, 5, 8, 10, 12, and 15 A g−1, respectively. When the current density was reset to 1 A g−1, the electrode still can deliver a reversible capacity of 547.8 mAh g−1. Figures S18a,b and 19a,b show the morphologies of the materials observed by SEM and TEM after the rate performances. It is found that both of the samples still well maintain hollow structure and no obvious pulverization phenomena after rate performance detections, indicating that the hollow structure can largely accommodate the volume expansion. The elemental mapping results (Figures S18c and S19c) reveal the homogeneous distribution of Ni, S, C, N elements in the sample, further indicating that the robust mechanical strength of hollow structure can act as a mechanical buffer against the huge volume changes during the sodiation/desodiation process. Compared with previously reported NixSy-based anodes for SIBs, NiS1.03 PHSs and NiS1.03 PHCs electrodes exhibit superior rate performance8,16,19,20,23,34,35,40−42 (see Figure 3d). The excellent rate capability exhibited by NiS1.03 PHSs motivates us to further investigate the electrochemical mechanism from aspects of electrochemical kinetics. The CV curves of the NiS1.03 PHSs electrode obtained at different scan rates of 0.2, 0.3, 0.5, 0.8, and 1.0 mV s−1 are presented in Figure 3e. It is observed that these curves show similar shape and display a slight peak shift, and the cathodic and anodic peaks are also strengthened along with the increased scan rate from 0.2 to 1.0 mV s−1, suggesting low polarization degrees of NiS1.03 PHSs in the electrolyte of tetraethylene glycol dimethyl ether and the co-existence of capacitance- and diffusioncontrolled charge-storage behaviors. 23 The relationship between measured current (i) and the sweep rate (v) (eq 3) is used to qualitatively evaluate the contribution of capacitance in the battery system.6,23

impedance spectroscopy (EIS) results of NiS1.03 PHSs in different electrolytes. It can be seen that the cells with NaSO3CF3/TGM as electrolytes possess a stable capacity and smaller charge-transfer resistances (Rct). Figure S13 displays the morphology of NiS1.03 PHSs in different electrolytes after 100 cycles at 1 A g−1. It can be seen that the morphology of NiS1.03 PHSs electrode maintains optimal when using NaSO3CF3/TGM as the electrolyte. Hence, NaSO3CF3/ TGM was selected as the electrolyte to obtain superior performance. Figure S14a displays the cycle performance of NiS1.03 PHSs in different cutting-off voltages at 1 A g−1, and the cycling performance has been significantly enhanced by tuning the cutting-off voltage to 2.3 V. The morphology of the electrodes almost has no changes (Figure S14b−d). Therefore, the subsequent electrochemical performances were carried out in the voltage range of 0.01−2.3 V using NaSO3CF3/TGM as electrolyte. The CV curves of the two samples for the initial five cycles at a scanning rate of 0.1 mV s−1 in the potential range of 0.01−2.3 V are shown in Figure 3a and Figure S15a. It can be observed from the CV curves of these two samples that they display similar shapes. In the first cycle of NiS1.03 PHSs, two main reduction peaks located at 1.10 and 0.81 V can be observed, which are consistent with the discharge profile (Figure S16a). The peaks at 1.10 and 0.81 V can be attributed to the transformation from NiS1.03 to Ni3S2, the conversion from Ni3S2 to metallic Ni and Na2S, and the formation of SEI films on the surface of electrode during this processes, respectively. For the oxidation process, the main peak at 1.35 V, 1.69 V, and a peak at 2.04 V could be attributed to the oxidation of Ni to Ni3 S 2 and the decomposition of SEI.16,19,33−36 A pair of redox peaks near 0 V can be ascribed to the insertion/extraction of Na+ from the carbon.37−39 According to the above analysis, the electrochemical reaction mechanism can be described by the following equations: ΝiS1.03 + 2Na + + 2e− → Νi3S2 + Na 2S

(1)

Νi3S2 + 4Na + + 4e− F 3Ni + 2Na 2S

(2)

After the first cycle, three reduction peaks and three oxidation peaks are observed, which is consistent with the following charge−discharge profiles (Figure S16b), and the electrochemical reaction mechanism can be described by the equations 2. In addition, after the first cycle, the reduction peaks of both samples shift to higher voltages due to improved kinetics of the electrodes. The oxidation and reduction peaks are almost overlapped in the flowing cycles, indicating the good cycling stability of the obtained material. Figure 3b and Figure S15b show the charge and discharge curves of NiS1.03 PHSs and NiS1.03 PHCs at 1 A g−1 within a cutoff voltage window of 0.01−2.3 V, respectively. It is found that these two materials show similar charge and discharge profiles, both of them show a short platform at ∼1.1−1.0 V and a long plateau at ∼0.8−0.6 V in their first discharge curves. Additionally, an distinct platform at ca. 1.6−1.8 V and the sloping segments in the range of 1.15−1.45 V are observed during the charging process. The initial discharge capacities of NiS1.03 PHSs and NiS1.03 PHCs are 813.2 and 782.5 mA h g−1, and their initial charge capacities are 631.7 and 630.5 mA h g−1, corresponding to the Coulombic efficiency of 77.7% and 80.6%, respectively. The irreversible capacity loss can be attributed to the decomposition of the electrolyte and some other irreversible side reactions.16,23,35 After 100 cycles, the NiS1.03 PHSs electrode can deliver a high reversible capacity of

i = a × (v)b

(3)

where a and b are adjustable constants. Figure 3f shows the relationship of the log (i) versus log (v), where the value of b is obtained by fitting the slope. If the value of b is more close to 1, then the greater the contribution of the capacitive process. The calculated b values in this study are 0.859 for O1 and 0.707 for R1, where the kinetics are largely a pseudocapacitive 8281

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Figure 4. Cycle performances of NiS1.03 PHSs and NiS1.03 PHCs at 1 A g−1 (a) and 2 A g−1 (b). (c, d) Long-term cycle performances of NiS1.03 PHSs and NiS1.03 PHCs at 8 A g−1 and 10 A g−1, respectively.

behavior in the NiS1.03 PHSs electrodes under high rates. Figure S20 lists pseudocapacitive contributions at various scan rates of the obtained electrode. The contribution percentages are 85.1%, 87.0%, 88.9%, 91.2%, and 92.9% at the scan rates of 0.2, 0.3, 0.5, 0.8, and 1.0 mV s−1, respectively, revealing that the pseudocapacitive behaviors occupy a high percentage of the total capacity, which can be considered as a main reason responsible for the reversible capacity exceeding the theoretical capacity. The reversible capacities of the metal sulfides exceeding their theoretical capacities when applied as electrodes for rechargeable batteries are a frequently encountered phenomenon during cycling up to data.19,23,43 Such a high capacitive contribution for NiS1.03 PHSs electrodes is important for achieving excellent rate performances.44 The excellent cycle performance at high current densities is of great significance, especially for large-scale energy storage systems and electric vehicles. Therefore, the cycling stability of NiS1.03 PHSs and NiS1.03 PHCs was tested at a high current density of 1 A g−1 (Figure 4a). The initial charge-specific capacities of NiS1.03 PHSs and NiS1.03 PHCs are as high as 575 and 542 mAh g−1, respectively. After 200 cycles, their reversible capacities remained as high as 364 and 355 mAh g−1, respectively. The corresponding coulomb efficiencies of both materials are close to 100%. The NiS1.03 PHSs and NiS1.03 PHCs electrodes also show very stable cyclability at 2 A g−1

(Figure 4b), and they still deliver reversible capacities of 314.5 and 330.2 mAh g−1(with Coulombic efficiencies of approaching 100%) after 300 cycles, respectively. The ultralong cycling performance of NiS1.03 PHSs and NiS1.03 PHCs was further examined at ultrahigh current densities of 5, 8, and 10 A g−1. As can be seen from Figure S15c, after 1500 charge−discharge cycles at a current density of 5 A g−1, the reversible capacities of NiS1.03 PHSs and NiS1.03 PHCs can remain at 167 and 156 mA h g−1, respectively. The capacity decay with cycling may be attributed to the following factors: (1) the electrode partial pulverization associated with volume change of the conversiontype electrode material, which is usually the major cause for the fast capacity loss in SIBs.35,45 The morphology of the electrode materials after 50 and 100 cycles at 1 A g−1 are observed by ex situ SEM. It can be seen from Figure S21b,d that the hollow structure of NiS1.03 PHSs remained, but the nanosheets are not well maintained after 100 cycles. Figure S21f,h displays the SEM images of the NiS1.03 PHSs electrodes after 200 and 500 cycles at 5 A g−1, respectively. A similar phenomenon can be observed. (2) The reductive reaction induced decomposition of the electrolyte, the formation and stabilization of the SEI film, and some irreversible reactions during the discharge− charge processes might be other reasons for the capacity decay.34,46,47 After 6000 charge−discharge cycles under a current density of 8 A g−1, the NiS1.03 PHSs and NiS1.03 PHCs 8282

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Figure 5. (a−c) In situ XRD patterns of the NiS1.03 PHSs electrode during the first discharge process at different voltages. The corresponding voltage curve is plotted at the left. (d) Ex situ XRD patterns of the NiS1.03 PHSs composite electrode at various charge−discharge states during the first charge−discharge process. The corresponding voltage curve is plotted at the right. (e−g) HRTEM images of the electrode after first discharged to 0.01 V and charged to 2.3 V (f, g).

still can deliver a reversible capacities of 126.7 and 78 mA h g−1, respectively (Figure 4c). The cycling performance of NiS1.03 PHSs and NiS1.03 PHCs at 10 A g−1 is displayed in Figure 4d. As a result, the reversible specific capacities are maintained at 80.0 and 106.5 mAh g−1 after 10,000 cycles. The current hollow spheres exhibit competitive electrochemical performance not only in rate capability (Figure 3d) but also in terms of the capacity, cycle number, and capacity decay rate (Table S1) when compared with previously reported transition-metal dichalcogenides anodes and their related carbon composites in SIBs. These results strongly verify that the porous hollow structure of NiS1.03 PHSs and NiS1.03 PHCs can benefit the electrons and ions transportation and shorten

their diffusion distances, enhancing the rate capability and cycle performance of the electrode at ultrahigh current densities. To further explore the good performance of NiS1.03 PHSs and NiS1.03 PHCs, electrochemical impedance spectrum (EIS) measurements of the NiS1.03 PHSs and NiS1.03 PHCs for SIBs have also been performed (Figure S15d). It can be seen that both samples exhibit a small charge-transfer resistance (Rct) and Warburg coefficient. It is believed that the following advantages are largely responsible for their excellent performances: (1) The hollow structure and porous shell of the material can effectively accommodate the volume changes during charging and discharging processes and maintain the structural integrity. (2) The nanosheets self-assembled hollow 8283

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investigated by ex situ SEM (different sodiation/desodiation states in the first cycle). As is presented in Figure S23, no obvious morphology changes could be observed before the electrode was discharged to 1.1 V, which is consistent with the results of the above analysis. In the subsequent discharge process, a clear volume expansion phenomenon that might be aroused by the sodium ion intercalations is observed, and it seems much more obvious when further discharged to 0.01 V. During the charging process, the morphology of the material was gradually restored to original shape, indicating the porous hollow NiS1.03 microspheres can largely accommodate the problem of volume expansion during charging and discharging processes. To further reveal the electrochemical mechanisms, ex situ XRD patterns (Figure S24) and HRTEM images (Figure S25) are carried out to investigate the crystalline structure of the NiS1.03 PHSs at different charge and discharge states in the second cycle, and the results are consistent with the obtained conclusions from the first cycle. The outstanding rate capacity and ultralong cyclability of the NiS1.03 PHSs motivated us to further assemble full cells to demonstrate its practical application potential as an anode material for SIBs in the future. Recently, Prussian blue analogues have been extensively explored as low-cost and high-capacity cathode materials for Na-ion batteries.48,49 Among them, FeFe(CN)6 displays outstanding performance when applied as cathode material. The XRD diffraction peaks and morphology of FeFe(CN)6 (Figure S26a,b) are consistent with a previous report.50 As there is no Na element in its structure, the FeFe(CN)6 and NiS1.03 PHSs half cells were cycled in the voltage range of 0.01−2.3 V and 2−4 V (vs Na/ Na+) at 500 mA g−1, respectively. The anode material has been activated for the initial three cycles to avoid the influence of the high initial irreversible capacity of NiS1.03 PHSs anode. To ensure the FeFe(CN)6 cathode material has enough Na in its structure, we first discharged the half-cell to 2 V, then took the cathode electrode to match the activated anode electrode, and the NiS1.03 PHSs//FeFe(CN)6 full cells were assembled successfully. The electrolytes and separators of the full cells are same with those of the half cells. The initial three discharge−charge curves of the NiS1.03 PHSs//FeFe(CN)6 full cell at a current density of 0.5 A g−1 are shown in Figure S27a. It delivers a reversible capacity of ∼580 mAh g−1. After 30 cycles, the charge capacity maintains at 215 mAh g−1 at 500 mAg−1 (Figure S27b). The excellent performances of both NiS1.03 PHCs PHSs and FeFe(CN)6//NiS1.03 PHSs further evidence that the NiS1.03 PHSs is a promising anode material for SIBs in the future.

spherical structure feature of NiS1.03 PHSs provide sufficient electrochemical active sites which are beneficial for the penetration of electrolyte into the porous hollow spheres and for shortening the ion and electron transmission distances, thus the electrochemical reactions can easily take place in the outside and inside of spheres and endow their excellent cycling performances. (3) The use of either electrolyte can alleviate the dissolution of intermediate products and reduce side reactions during cycling processes. (4) Nitrogen-doped carbon improved the conductivity and stability of the electrode material. These advantages enable NiS1.03 PHSs and NiS1.03 PHCs to be excellent electrodes with excellent discharge capacity retention ability and high rate performances. To reveal the electrochemical mechanisms, in situ XRD patterns, ex situ XRD patterns, and HRTEM images are carried out to investigate the crystalline structure of the NiS1.03 PHSs at different charge and discharge states. Figure 5a shows the in situ XRD patterns, in which the diffraction peaks at the open circuit voltage state can be assigned to the NiS1.03 phase, while the peaks located at 45.8°, 51.0°, 52.8°, and 70.9° are derived from the diffractions of the Be window. During the initial discharge processes, it is found that obvious phase transition occurred at 0.1 A g−1, while in the subsequent discharge process, the characteristic peaks of NiS1.03 phase that centered at degrees of 30.0, 34.5, and 53.3 gradually disappear. Along with the above-mentioned peaks, the peaks located at degrees of 21.7, 31.1, 37.8, 44.3, 50.1, and 55.1, corresponding to the (101), (110), (003), (202), (211), and (122) planes of the Ni3S2 (JCPDS card no. 44-1418), appeared, respectively. At first, the characteristic peaks of Ni3S2 become more obvious along with the reduction of NiS1.03 (eq 1). Then the diffraction intensity of characteristic peaks of Ni3S2 significantly weakened owing to the gradual reduction of Ni3S2 into nickel element (eq 2). Meanwhile, another three peaks around 23.2°, 38.9°, and 56.1° appeared in the discharge process, corresponding to the Na2S (JCPDS card no. 23-0441). Figure 5b,c shows the partial curves of the in situ XRD patterns with high magnifications, clearly reflecting the phase transition during discharge process. Figure 5d presents the ex situ XRD patterns of the NiS1.03 PHSs electrode at different states as marked in the discharge−charge curves. The ex situ and in situ XRD reflects consistent phase transition, further confirming that the changes in eqs 1 and 2 occurred during the discharge process. Based on ex situ XRD results, only one peak at 38.8° was clearly observed during the charge process, which can be ascribed to the (220) planes of the Na2S. This peak can be frequently detected when the electrode was fully charged to 2.3 V. There are no obvious characteristic peak signals that related to Ni observed in the XRD pattern when the electrode was discharged to 0.01 V. But the lattice spacing of 2.460 Å in the HRTEM image (Figure 5e) can be assigned to (011) plane of Ni and the lattice fringes with interplanar distances of 2.536 and 3.420 Å corresponding to (311) and (420) crystal planes of Na2S, respectively (Figure 5f, Figure S22a). As is shown in Figure 5g, when the cell is fully charged to 2.3 V, the lattice fringes with interplanar distances of 2.13 Å correspond to the (021) crystal planes of Ni3S2, indicating that metallic nickel is oxidized to Ni3S2, which reveals the reversible Na+ insertion and extraction reactions. The average lattice spacing of 2.53 Å can be indexed to the (311) crystal planes of Na2S (Figure S22b), evidencing the presence of Na2S. This result is consistent with that of an XRD pattern. Moreover, the morphology changes of NiS1.03 PHSs during initial cycling are

CONCLUSION In summary, NiS1.03 PHSs and NiS1.03 PHCs of high yields have been rationally designed and conveniently fabricated by solvothermal and subsequent annealing methods. Their porous hollow structures could not only provide efficient electron- and ion-transfer pathways but also effectively suppress volume changes during charge and discharge processes. NiS1.03 PHSs and NiS1.03 PHCs spheres exhibit superior rate performance and outstanding stable cycling ability (127 mAh g−1 after 6000 cycles at 8 A g−1). In situ XRD, ex situ XRD, and HRTEM analyses confirmed the conversion energy storage mechanism for NiS1.03. In addition, a NiS1.03 PHSs//FeFe(CN)6 full cell has been assembled, which delivers a reversible capacity of 215 mAh g−1 at 500 mA g−1 after 30 cycles. Such long-life and stable cyclability combined with high rate properties 8284

DOI: 10.1021/acsnano.8b03541 ACS Nano 2018, 12, 8277−8287

Article

ACS Nano

ASSOCIATED CONTENT

demonstrate that NiS1.03 is an excellent and promising anode material for SIBs.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b03541. Morphology and phase characterization of NiS1.03 PHCs and FeFe(CN)6; TGA curves of NiS1.03 PHSs and NiS1.03 PHCs; nitrogen adsorption/desorption isotherms; TEM and SEM images of NiS hollow spherical precursors prepared with different reaction time; the effect of different sulfur sources on the morphology; electrolyte and the cutoff discharge voltage optimization of NiS1.03 PHSs batteries; SEM and TEM images of NiS1.03 PHSs after rate performances and different cycles; ex situ XRD patterns and ex situ HRTEM images of the NiS1.03 electrode recorded at various stages at 0.1 mA g−1 in the second cycle; performances of FeFe(CN)6//NiS1.03PHSs full cell (PDF)

METHODS Synthesis of NiS Hollow Spherical Precursors and NiS Solid Spherical Precursors. In a typical process, TAA (360 mg, 4.80 mmol) and Ni(NO3)2·6H2O (348.9 mg, 1.52 mmol) were added into the mixed solvent of DMF (1 mL) and CH3OH (7 mL). The solution was then transferred into a Teflon-lined autoclave with capacity of 23 mL. After the solution was stirred for 12 h at room temperature, 2MIN (65.6 mg) was added into the solution and stirred for another 2 h. After that, the above mixed solution was transferred into a Teflonlined stainless steel autoclave and maintained at 85 °C for 72 h. The autoclave was cooled to room temperature naturally. 125 mg (yield = 90.4%, based on Ni) of NiS hollow spherical precursors was obtained by filtration and washing with methanol for several times. For comparison, 115 mg (yield = 83.1%, based on Ni) of NiS solid microspherical precursors were synthesized through the similar procedure but in the absence of 2-MIN. Synthesis of NiS1.03 Porous Hollow Spheres (NiS1.03 PHSs) and NiS1.03 Porous Hollow Cages (NiS1.03 PHCs). The above precursors were calcined at 400 °C and maintained for 1 h in argon atmosphere to obtain the final products of NiS1.03 PHSs and PHCs. Synthesis of FeFe(CN)6. The synthesis procedure was according to a previous report.50 In a typical procedure, 100 mL FeCl3 aqueous solution (0.1 mol L−1) was added into a three-neck flask. Then 50 mL K3Fe(CN)6 aqueous solution (0.1 mol L−1) was slowly added under continuous stirring, and the resulting solution was heated at 60 °C for 6 h. After the autoclave was cooled down naturally to room temperature, the inner dark green precipitate was collected and washed with deionized water and acetone. Finally, the product was dried in the vacuum oven at 60 °C for 6 h. Materials Characterization. X-ray powder diffraction pattern (XRD) was carried out on a Bruker D8 apparatus equipped with Cu Kα radiation. Field emission scanning electron microscope (FESEM, ZEISS Geminisem 300), a transmission electron microscope (TEM, JEM-1011), and the high-resolution TEM (HRTEM JEOL-2011) were used to test the morphology of the products. Elemental mappings were collected with QUANTAX energy dispersive spectroscopy (EDS). Elemental analysis was measured on a Vario EL CUBE. X-ray photoelectron spectroscopic (XPS) analysis was obtained on a Kratos AXIS Ultra DLD spectrometer using an Al Ka X-ray source to characterize the chemical bonds of products. Thermal gravimetric analysis (TGA) carried out by a MettlerToledoTGA/ SDTA851 was employed to evaluate thermal stability of samples, with a heating rate of 5 °C min−1 from 20 to 800 °C. Nitrogen adsorption/ desorption isotherms were performed using a QuadraSorb SI surface area analyzer (version 5.06). Electrochemical Measurements. The electrochemical tests were examined by CR2032 coin cells. The electrodes consisted of 70 wt % active material, 20 wt % acetylene black, and 10 wt % polyvinylidene difluoride (PVDF). The slurry was spread onto a copper foil and kept at 60 °C under vacuum for 6 h. The loading of the active material was about 1.05−1.33 mg cm−2 on each of discs. The sodium foils with the diameter of ∼14 mm were prepared from sodium bulk (Aladdin, 99.7%) in a glovebox. The glass fiber film (Whatman) was used as the separator. One M CF3NaO3S dissolved in the tetraethylene glycol dimethyl ether was employed as electrolyte. Galvanostatic cycle tests at different current densities were conducted on battery test station (LAND CT-2001A, Wuhan, China) from 0.01 to 2.3 V. Electrochemical impedance spectra (EIS) were obtained on an AutoLab PGSTAT302 electrochemical workstation in the frequency range from 100 kHz to 0.01 Hz at room temperature. Cyclic voltammetry (CV) profiles were carried out by a CHI 760E electrochemical workstation in the potential window of 0.01−2.3 V.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] ORCID

Liqiang Xu: 0000-0002-0453-120X Author Contributions

C.D., Y.H., C.L., X.C., and L.G. conducted the experiments, and all authors contributed to the writing of the manuscript and approved the final version. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by National Science Foundation of China (21471091), 111 Project (no. B12015), Chinese Academy of Sciences large apparatus United Fund, Guangdong Province Science (no. 11179043), Guangdong Province Science and Technology Plan Project for Public Welfare Fund and Ability Construction Project (2017A010104003), Shenzhen Science and Technology Research and Development Funds (JCYJ20170818104441521), and the Taishan Scholar Project of Shandong Province (ts201511004). REFERENCES (1) Yan, Y.; Yin, Y. X.; Guo, Y. G.; Wan, L. J. A Sandwich-Like Hierarchically Porous Carbon/Graphene Composite as a HighPerformance Anode Material for Sodium-Ion Batteries. Adv. Energy Mater. 2014, 4, 1301584. (2) Chen, C. J.; Wen, Y. W.; Hu, X. L.; Ji, X. L.; Yan, M. Y.; Mai, L. Q.; Hu, P.; Shan, B.; Huang, Y. H. Na+ Intercalation Pseudocapacitance in Graphene-Coupled Titanium Oxide Enabling Ultra-Fast Sodium Storage and Long-Term Cyclin. Nat. Commun. 2015, 6, 6929. (3) Luo, W.; Shen, F.; Bommier, C.; Zhu, H. L.; Ji, X. L.; Hu, L. B. Na-Ion Battery Anodes: Materials and Electrochemistry. Acc. Chem. Res. 2016, 49, 231−240. (4) Dong, C. F.; Xu, L. Q. Cobalt- and Cadmium-Based MetalOrganic Frameworks as High Performance Anodes for Sodium Ion Batteries and Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2017, 9, 7160−7168. (5) Hu, Z.; Liu, Q. N.; Chou, S. L.; Dou, S. X. Advances and Challenges in Metal Sulfides/Selenides for Next-Generation Rechargeable Sodium-Ion Batteries. Adv. Mater. 2017, 29, 1700606. (6) Zhang, L.; Hu, X. L.; Chen, C. J.; Guo, H. P.; Liu, X. X.; Xu, G. Z.; Zhong, H. J.; Cheng, S.; Wu, P.; Meng, J. S.; Huang, Y. H.; Dou, S. 8285

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