Hierarchically Interconnected Ni3S2 Nanofibers as Binder-Free

Subscriber access provided by Iowa State University | Library

Article

Hierarchically Interconnected Ni3S2 Nanofibers as Binder-free Electrodes for High-Performance Sodium-Ion Energy Storage Devices Mao Cheng Liu, Jun Li, Qing-Qing Yang, Yan Xu, Ling-Bin Kong, Rui-Juan Bai, Wenwu Liu, Wenjun Niu, and Yu-Lun Chueh ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02341 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Hierarchically Interconnected Ni3S2 Nanofibers as Binder-free Electrodes for HighPerformance Sodium-Ion Energy Storage Devices Mao-Cheng Liu *ab, Jun Li a, Qing-Qing Yang a, Yan Xu a, Ling-Bin Kong ab, Rui-Juan Bai d, Wen-Wu Liu ab, Wen-Jun Niu ab, Yu-Lun Chueh c, e * a

State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of

Technology, Lanzhou 730050, Gansu, China b

School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, Gansu,

China c

Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan.

d

Sanya Technology Institute for Quality and Technical Supervision of Hainan Province, Sanya 572000, P. R.

China e

Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University,

Hsinchu 30013, Taiwan, Republic of China *Corresponding author: [email protected] and [email protected] Abstract- Direct growth of hierarchically interconnected Ni3S2 nanofibers as a binder-free electrode for high-performance sodium ion battery was demonstrated by a facile one-step hydrothermal method on a nickel (Ni) foam. The hierarchically interconnected Ni3S2 nanofibers can effectively relieve volume expansion of Ni3S2 and shorten diffusion paths of sodium ions (Na+) that can enhance the high electronic conductivity during the electrochemical reaction, yielding high specific capacitance, excellent cycling stability and outstanding rate capability. As a result, a high discharge specific capacity of 584.2 mAh g-1 at a current density of 0.2 A g-1 in the first sodiation process with a capacity retention of 91.9 % after 100 cycles (retains 536.9 mAh g-1) can be achieved. The asymmetric Na+ ion capacitor based on the hierarchically interconnected Ni3S2 nanofibers as a binder-free anode material and the activated carbon as a cathode material exhibits high energy and power density. Interestingly, the conversion reaction mechanism of Na+ ion storage in the hierarchically interconnected Ni3S2 nanofibers was also investigated by the ex-situ XRD studies, suggesting a promising material as the binder-free electrode for advanced sodium ion energy storage.

1 ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords: Sodium ion energy storage, Ni3S2 nanofibers, Anode material, Impedance analysis, Conversion reaction mechanism, Sodium ion capacitors


Page 2 of 28

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Clean and renewable energy is a key issue to achieving sustainable development for the human society. Lithium (Li+) ion batteries (LIBs) have been regarded as a priority candidate with its advantages of high energy density, long cycle life and environmental friendliness, which can achieve efficient energy storage and conversion.1-5 However, the limit and non-uniform distribution of Li resources coupled with the extensive use of LIBs makes Li resources more scarce with a high price, which is not suitable for the applications in the field of large-scale energy storage. Sodium ion batteries (SIBs) have attracted enormous attention due to its low cost and the environmental abundance,6-8 and they have been considered as the best candidate for replacement of LIBs in the large-scale electrical energy storage.9 Na and Li belong to an identical atomic family group and have the similar physical and chemical properties. However, Na ion has a much larger radius (ca. 0.106 nm) than that of Li-ion (ca. 0.076 nm),10 which limits the reversible and rapid ion insertion/extraction from the host materials. Therefore, it is not suitable to use the LIBs electrode material as the SIBs electrode material directly. The high reactivity of metallic Na, which can result in seriously safe risks, limits the Na metal being used as an anode directly.11 Therefore, developing advanced alternative anode materials with an increasement in the energy density and the capability are the key motivation to the practical application of SIBs. Up to date, many advanced anode materials of SIBs have been synthesized, including alloy types metals,12-13 carbon materials,14-15 metal oxides/chalcogenides16-17 and non-metal materials.18-19 Among these anode materials, metal sulfide and their composites, which store Na+ ions in a similar reaction manner to the Li counterpart, have been considered as promising anode materials in SIBs because of their unique physical and chemical properties.20-22 Nickel sulfides (Ni3S2) have focused much attention as attractive anode materials,23 owing to its high theoretical capacity, rich resources, low cost and environmental benignity.24-28 However, the poor electrical conductivity (1.2×10-4 Ω cm-1) compared that with pure metals29 and severe volume expansion during Na+ insertion/extraction processes not only leading poor rate capability and cycle performance, but also impeding the practical applications of the Ni3S2 for SIBs. Though many efforts have been devoted to solving the volume expansion of



Ni3S2 during Na+ insertion/extraction processes, the design of different nanostructures is still an effective way to alleviate obstacles mentioned above and improve the Na+ storage performance. Selfstanding 1D nanostructure can provide enough buffer spaces to accommodate the large volume expansion through the rapid Na+ sodiation/desodiation processes.30-32 Compared to the traditional Ni3S2 electrodes using the coating method, the design of Ni3S2 NWs directly grown on the current collector exhibits a good adhesion behavior and better point contacts, leading to the excellent structural stability. Although nickel foam based electrode materials exhibiting excellent electrochemical performance, which is still face non-uniformity in the thickness and area density. Compared with traditional copper foil, its inherent conductivity is relatively low, leading to narrow utilization of the Ni foam electrode material in commercial application. Ni foam is an ideal substrate with low resistance, providing a unique porous structure and a benificial contacts between electrode materials and electrolytes.33 Binder-free electrode materials directly prepared by the deposited method will be able to avoid the influence of reducing the electrical conductivity by the binder, yielding the enhancement in the rate capability of the anode electrode. Recently, Shang et al. built Ni3S2 on the Ni foam substrate followed by the deposition of the PEDOT, namely Ni3S2-PEDOT electrodes, stably exhibited the reversible capacity of 280 mAh g-1 after 30 cycles.24 Xia et al. reported Ni3S2/CST arrays on a Ni foam substrate, which can deliver a first cycle reversible capacity of 887 mAh g-1 at 50 mA g-1 and 212 mAh g-1 capacity remains after 260 cycles.34 Therefore, designing and preparing binder-free Ni3S2 nanostructures on the Ni foam by a simple hydrothermal deposition method is imperative. In this regard, we have designed a facile one-step hydrothermal route to synthesize hierarchically interconnected Ni3S2 nanofibers on the surface of nickel foam as the binder-free electrode for SIBs. The hierarchical Ni3S2 nanofibers exhibit a unique three dimensional (3D) porous architecture where the nickel foam substrate acts as a solid skeleton to grow the Ni3S2 nanofibers. Interestingly, the conversion reaction mechanism of Na+ storage in Ni3S2 was investigated by the ex-situ XRD studies. As anticipated, the hierarchically interconnected Ni3S2 nanofibers deliver improved electrochemical performance and enhanced charge transfer efficiency during charge and discharge process, yielding a


Page 4 of 28

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


high discharge specific capacity of 584.2 mAh g-1 at a current density of 0.2 A g-1 in the first sodiation process and an excellent capacity retention (91.9 % after 100 cycles). The results suggest that this hierarchically interconnected Ni3S2 nanofibers is a promising binder-free electrode for advanced sodium ion energy storage.

Experimental Section Synthesis of the Ni3S2: The hierarchically interconnected Ni3S2 nanofibers were synthesized through a facile one-step hydrothermal method using the nickel foam as both Ni source and substrate, simultaneously. In a typical process, Ni foam (5 cm × 3 cm) was cleaned by the ethanol and acetone many times. 30 mg thiourea was dissolved in 60 mL deionized water to obtain a mixture solution and then transferred into Teflon-lined autoclave with a piece of nickel foam in it. After maintained at 160 °C for 20 h, the sample was cleaned by the alcohol and distilled water for several time, and then dried in the oven over night. Finally the hierarchically interconnected Ni3S2 nanofibers as the binder-free electrode can be achieved. The weight of Ni3S2 on the Ni foam was obtained using the reaction 3Ni+2S  Ni3S2

, and the equation:

m Ni3S2 =Δm

M Ni3S2 2MS

, where Δm is the weight received by the Ni foam

before and after the hydrothermal reaction, which was measured by a precise balance.35 Structure

and

Electrochemical

characterizations:

Structure

and

electrochemical

characterizations are described in the supporting information.

Results and discussion The fabrication procedure of hierarchically interconnected Ni3S2 nanofibers is depicted in Fig 1a. In a typical synthesis process, thiourea was used as the sulfur source reacted with the porous Ni foam to produce Ni3S2 nanofibers, where the possible chemical reactions can be expressed (2S2- + O2 + 2H2O + 3Ni  Ni3S2 + 4OH-). After maintained at 160 °C for 20 h, the nickel foam was covered by Ni3S2 nanofibers. Fig. 1b illustrates the corresponding XRD results to confirm the Ni3S2 phase of nanofibers where three strong peaks contributed from the Ni substrate (JCPDS 65-2865) were found.



Furthermore, the remaining peaks are in well accordance with pure Ni3S2 phase (JCPDS 44-1418). The peak intensity of the Ni3S2 phase is obviously weaker than that of the Ni substrate because of the small amount of the Ni3S2 deposited on the Ni foam substrate. No other peaks can be observed, confirming that the hierarchically interconnected Ni3S2 nanofibers on the Ni foam have been successfully synthesized. In Figure S1, the peaks arising from Ni 2p and S 2p in the survey XPS spectrum can be assigned to Ni3S2. The chemical identities and valence states of the Ni3S2 phase as displayed in Fig. 1c and 1d. Two distinct peaks located at 873.2 and 855.4 eV related to Ni 2p1/2 and Ni 2p3/2. Other two peaks corresponding to the accompanied satellite peaks of Ni 2p1/2 and Ni 2p3/2, were confirmed as well.36 The XPS spectra of S 2p (Fig. 1d) exhibits two peaks located at 162.3 and 164.1 eV, which related to 2p3/2 and 2p1/2 of S2-, respectively. The peak located at ~168.3 eV can be expected to the residual S2O32- on the surface of the Ni3S2, with which the influences of S2O32- on electrochemical performance can be ignored.35 As a result, peaks in Ni 2p and S 2p spectra exhibit obvious characteristics of Ni3S2, which are consistent well with XRD results. The morphologies and structures of the hierarchically interconnected Ni3S2 nanofibers were further investigated by SEM and TEM (shown in Fig. 2). The insert of Fig. 2a reveals that the structure of the Ni foam still maintain structural stability after the hydrothermal reaction, only the hierarchically interconnected Ni3S2 nanofibers are formed on the surface. Figure S2 depicts the porous structure of the pristine Ni foam used as substrate with the high electronic conductivity for the synthesis of Ni3S2 nanofibers. High magnification SEM images revealed that uniform Ni3S2 nanofibers are grew on the surface of the Ni foam as a hierarchically interconnected structure, which can improve the ion diffusion during charged and discharged processes. Fig. 2c shows a low magnified TEM image where the porous configuration can be confirmed. In addition, a high resolution-TEM image of the Ni3S2 nanofibers was showed in Fig 2d, the measured lattice spacing of 0.28 nm related to the (110) interplanar distance of Ni3S2. The inset in Fig. 2d shows the corresponding selected area electron diffraction pattern (SAED). The ring patterns with different planes made up of discrete spots, and the d-spacings are match well with the (220) and (200) planes of Ni3S2 phase. The results are consistent with that of XRD results. Furthermore, the elemental mapping analysis of the Ni3S2 are shown in Fig.


Page 6 of 28

Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2(e-g), confirming that Ni and S elements are homogeneously distributed on the Ni foam. As shown in Fig. S3, the prepared sample was sonicated for 20 minutes to further confirm that whether the hierarchically interconnected Ni3S2 nanofibers are tightly adhered on the Ni foam. It is clearly that hierarchically interconnected Ni3S2 nanofibers remain on the Ni foam after the ultra-sonication process. To explore the electrochemical behaviors, the hierarchically interconnected Ni3S2 nanofibers were assembled by two-electrodes coin cell with the Na metal as the counter electrode. The cyclic voltammetry (CV) of the hierarchically interconnected Ni3S2 nanofibers in the first four cycles were recorded within the potential range of 0.01–2.8 V at a scan rate of 0.1 mV s-1 (show in Fig. 3a). During the initial discharge progress, the cathodic peak at ~0.56 V can be related to the formation of the solid electrolyte interphase (SEI) layer on the active material surface, In the subsequent cycles, the above peaks disappeared and a couple of redox peaks were observed at 0.89 and 1.73 V. The strong peak at 0.89 V is attributed to the insertion of Na+ into Ni3S2, while the peak at 1.73 V is related with Na+ extraction from Ni3S2 phase, Note that the following CV from 2nd to 4th cycles remains almost overlapped, demonstrating the excellent reversibility and cycle stability for the storage of Na+. To further verify the Na+ diffusion performance in the hierarchically interconnected Ni3S2 nanofibers, CV results at various scan rates were performed as shown in Figure S4. The anodic peaks moved to high voltages while the cathodic peaks shifted to lower ones with an increase in the scan rate. This phenomenon can be explained by the occurrence of the redox reaction on the entire surface of hierarchically interconnected Ni3S2 nanofibers at a low scan rate. However, diffusion of Na+ will be hampered with the increase in scan rates because of the time constraint. This means that the diffusion rate of Na+ is relatively slow compared with the faster electrode reaction rate.37-38 Therefore, only the outer active surface on hierarchically interconnected Ni3S2 nanofibers is involved in the redox reaction, which leads to a position shift of both reduction and oxidation peaks.6 There are obvious redox peaks in the CV curves with an increase in the scan rate, suggesting excellent reversibility because of the good conductivity in the Ni3S2 nanofibers electrode during the sodiation/desodiation processes. Fig. 3b presents the first three cycles galvanostatic discharge and charge curves of the hierarchically interconnected Ni3S2 nanofibers at 0.1 A g-1. The first discharge and charge voltage plateau agree well



with the CV scans. It can be seen that the first discharge plateau located at 0.56 V relates to the intercalation of Na+ and conversion reactions while it disappears after the first cycle. The potential plateau was found to be replaced by a long sloped curve from 1.1 to 0.7 V. During the first charge process, the voltage plateau located at 1.7 V is ascribed to the conversion reaction, suggesting the high reversibility of the Ni3S2 nanofibers electrode. The initial discharge and charge capacities can reach to 906.1 mAh g-1 and 708.5 mAh g-1, respectively, relevanting to a coulombic efficiency (CE) of 78.1 %. Note that the initial discharge specific capacity is much higher than that of the subsequent cycles attributed to the formation of SEI and decomposition of the electrolyte, which is commonly found for most anode material.39 However, there is no significant capacity degradation found during the whole cycling measurements. The high reversible capacity can be contributed to the hierarchically interconnected structure that can effectively relieve volume expansion of Ni3S2 nanofibers and shorten the diffusion path of Na+ during the electrochemical reaction. The Ni3S2 nanofibers electrode also exhibits an excellent rate capability. Fig. 3c shows the rate capability behaviours of interconnected Ni3S2 nanofibers as the binder-free electrode, it delivers average reversible specific discharge capacities of 608, 552, 476 and 355 mAh g-1 at the current rates of 0.1 A g-1, 0.2 A g-1, 0.5 A g-1 and 1 A g-1, respectively. The discharge specific capacity can set back to the initial value as the current density return from 1 A g-1 to 0.1 A g-1, meaning that the hierarchically interconnected Ni3S2 nanofibers has a very stable and durable rate capability. Moreover, in the subsequent cycles, the capacity tends to stay steady and the CE gradually picked up to 99 %. The impressive rate performance can be attributed to the following reasons. On the one hand, the binder-free electrode contributed an excellent electrical conductivity between the hierarchically interconnected Ni3S2 nanofibers and the Ni foam. On the other hand, the structural stability of hierarchically interconnected Ni3S2 nanofibers can effectively buffer the volume variations of Ni3S2 nanofibers and shorten the Na+ diffusion path during the electrochemical reaction. The typical discharge-charge profiles at various rates are shown in Fig. 3d. It is obvious to find the lower discharge plateau and higher charge potential at the higher current density. The shapes of discharge/charge curves are similar, indicating the excellent rate performance. Fig. 3e shows the cycling stability of the hierarchically interconnected


Page 8 of 28

Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ni3S2 nanofibers, a reversible specific capacity of 517.2 mAh g-1 is maintained after 100 cycles at 0.1 A g-1, the coulombic efficiency increases from 78.1 % to ~99.9 % during the first cycle and remains nearly ~99.9 % in the subsequent cycles. The cycle performance at the current density of 0.2 A g-1 was further evaluated as shown in Figure S5, a dramatic enhancement in cycle performance with the reversible discharge specific capacity of 584.2 mAh g-1 and 91.9 % of this specific capacity (536.9 mAh g-1) after 100 cycles is achieved. These results clearly confirm that the binder-free and interconnected Ni3S2 nanofibers electrode can provide the structural integrity against huge volume change over extend cycling and result in the excellent cycling stability. Notably, in comparison with other Ni3S2 based anodes in SIBs, the hierarchically interconnected Ni3S2 nanofibers exhibit more enhanced capacities and cycling stability, which attributed to the enough void space that can effectively buffer the stresses change during the charge/discharge process. Electrochemical impedance spectroscopy (EIS) was performed on the hierarchically interconnected Ni3S2 nanofibers as the binder-free electrode before the cycling test and in the 10 th and 60 th fully charged state as shown in Figure 4a. Distinctly, the inset in Fig. 4a shows the corresponding equivalent circuits used to fit the EIS results where Re denotes the resistance of electrolyte and cell components, Rsf and Rct represent the resistance of surface film and charge transfer, CPEi is a constant phase element and the Warburg diffusion impedance (Ws) reflects Na+ transfer process in the electrode materials, respectively.11,

40

The semicircle represents the resistance of the

surface film and the charge-transfer (Rsf+ct) at the interface between the electrode and the electrolyte. An approximately the straight line at 45° angle can be ascribed to the diffusion of Na+ near the electrode. As a result, Re and Rsf+ct were calculated by a Zview software as shown in Table 1. The asprepared cell delivers an impedance of 19.5 Ω, which is mainly ascribed to Rct. Note that the Rsf+ct gradually raises with the increase in cycle numbers (from 45.3 Ω to 79.9 Ω), indicating the growth of SEI film on the electrode surface or the formation of new SEI during the charge and discharge cycles, which is due to the morphology reconstruction and electrolyte decomposition on the reconstruction surface of Ni3S2 electrode. The Re of the hierarchically interconnected Ni3S2 nanofibers ﬂuctuate while staying in a stable range, indicating the excellent stability. EIS was also employed to calculate the



Page 10 of 28

transfer coefficient of Na+ ion in the electrode material.40-41 The Na+ diffusion coefficient (D) can be obtained from the Equation 1 where R, T, A, n, F, C and σ represent the gas constant, the absolute temperature, the surface area of the anode, the number of electrons transferred during electrochemical reaction, the Faraday constant, the concentration of ions and the Warburg coefficient, respectively. There is a correlation between σ and Warburg impedance and the value can be obtained by the equation 2 and the corresponding line is shown in Fig. 4b with data points from the low-frequency region taken from the EIS curve in Fig. 4a. The σ was calculated to be 214.3 and 55.3 after 10 and 60 cycles, respectively. Clearly, the diffusion coefficient (D) after 60 cycles is much larger than that of 10 cycles, assuming other parameters are the same. With the increase in the charge and discharge cycles, the diffusion rate of Na+ can be accelerated, indicating the better sodiation and desodiation reaction kinetics. D=(R 2 T 2 )/(2A 2 n 4 F4 C2 σ 2 )

(1)

Zw =σ -1/2

(2)

Figure S6 shows a high magnification SEM image of the hierarchically interconnected Ni3S2 nanofibers after 60 cycles where a large number of nanosized particles can be found. This morphology variation can be related to the electrochemical reconstruction,42 with which the reduced size of the particles results in an increase in specific surface area and better contact between the Ni3S2 and the Ni foam substrate. The results are consistent with the conclusion in Fig. 4b. The extraordinary electrochemical performance of the hierarchically interconnected Ni3S2 nanofibers as the anode can be attributed to the interconnected nanofibers with the binder-free on the Ni foam. Firstly, the hierarchically interconnected Ni3S2 nanofibers possess a larger surface area, provide more contact sites and shorten the transmission path of electrons and ions, resulting in the rapid ion diffusion with the excellent rate capability. Secondly, the self-supporting porous structure can effectively relieve the volume expansion caused by the sodiation and desodiation during the cycling processes, leading to the high cyclic stability. Thirdly, the Ni foamed substrate also has a unique porous structure acting as a solid skeleton to grow the interconnected Ni3S2 nanofibers with the improved mechanical adhesion strength and the electrical connection, yielding the enhanced rate performance. In comparison with the


Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


similar anodes based on Ni3S2 as shown on in Table 2, the hierarchically interconnected Ni3S2 nanofibers electrode can deliver much enhanced capacities.24, 34-35, 43-44 In order to clarify the Na+ storage mechanism of the hierarchically interconnected Ni3S2 nanofibers, ex-situ XRD results (taken from the in the 3rd cycle) were measured at different charged and discharged conditions from stages a to f as shown in Fig. 5a and 5b. The fresh anode shows clear patterns of Ni3S2 phase (stage a) while the strong peaks located at 44.3° and 51.9° are ascribed to the Ni foam substrate. Note that no obvious difference except the weakened intensity of diffraction peaks after the twice fully charged states (stage b). Interestingly, a new peak located at ~27° appears during the 3rd cycle and can be related to the formation of the NaS phase when the cell is discharged to 0.87 V (stage c), revealing a conversion reaction (Ni3S2 + 2Na+ + 2e-  3Ni + 2NaS) from Ni3S2 to Ni and NaS in the sodiation process.45 No Ni3S2 phase can be observed after the electrode is further discharged to 0.1 V (stage d), implying that the discharge products are in amorphous states while the NaS peak gets more prominence.39 When the reverse charging was applied to 1.96 V (stage e), the diffraction peak of the Ni3S2 reappears with the stronger intensity while the peak intensity referring to NaS gradually decreases, which is related to the recombination of Ni3S2 and Na during the discharge process. At the fully charged state (stage f), the Ni3S2 reappears but the peak intensity decreases compared with the stage b while the NaS phase almost disappears. In addition, the Ni3S2 phase peak located at 21.7° shows a slight shift, which means Na+ sodiation in inner structure during the discharge process. As the full charge state, the diffraction peak is almost as same as the initial stage, demonstrating an excellent cyclic reversibility. The peak intensity of the Ni3S2 electrode is broader and weaker compared with the initial state, suggesting that the lattice of Ni3S2 expands during the sodiation and the desodiation processes. Due to the excellent properties of the porous Ni3S2 electrode in the half-cell, the Na+ ion capacitor was further assembled using the hierarchically interconnected Ni3S2 nanofibers as the anode and the activated carbon (AC) as the cathode, respectively. Fig. 6a shows the corresponding CV curves at different scan rates. The shapes of the CV curves are well remained as the scan rate increases. It is



obvious that the CV is asymmetric, suggesting both of the faradaic and non-faradaic reactions for the Na+ storage. The hybrid device combines not only the advantages of both the batteries and supercapacitors but also works at a higher operating voltage, pushing the higher energy and power density. Fig. 6b shows the galvanostatic discharge and charge curves of Na+ ion capacitor, the specific capacitance of Ni3S2//AC was calculated according to discharge curves (shown in Fig. 6c). Distinctly, the specific capacitance of 29.5 F g-1 can be achieved at 0.1 A g-1 while 40 % of this value remains as the current density increases to 2 A g-1. The energy and power densities of the Ni3S2//AC capacitor can be calculated using Equations S2 and S3 as described in the experimental section based on galvanostatic discharge and charge measurements. A high energy density of 65.6 Wh kg-1 can be obtained at a power density of 204.6 W kg-1. On the same time, the Na+ ion capacitor can also deliver a high power output of 3704.3 W kg-1 with the energy density of 25.6 Wh kg-1. the Ragone plots of the Ni3S2//AC capacitor compared to the other reported sodium ion capacitors are shown in Fig. 6d. It is obvious that other reported capacitors have the lower energy density than the Ni3S2//AC capacitor, such as NiCo2O4//AC,46 V2O5/CNT//AC47 and TiO2 mesocagegraphene nanocomposite (denoted as MWTOG)//AC.48 The long-term cycling stability was also tested at 0.1 A g-1 as shown in Fig 6e. It is found that 98 % of the initial capacity is remained after 2000 cycles with a high coulombic efficiency of nearly 100 %, demonstrating the excellent electrochemical reversibility and long-term electrochemical stability of the Ni3S2//AC Na+ ion capacitor.

Conclusion In summary, the hierarchically interconnected Ni3S2 nanofibers with excellent electrochemical performance were synthesized by the hydrothermal route and delivered a high initial discharge specific capacity of 906.1 mAh g-1 at 0.1 A g-1 as well as a high initial coulombic efficiency of 78.1 %. It also show excellent rate capability and superior cycle performance. It is believed that the excellent electrochemical performance of the Ni3S2 electrode is attributed to the interconnected structure and its good adhesion to the 3D porous Ni foam. Furthermore, the reaction mechanism of the Ni3S2 electrode was investigated in ex-suit XRD, suggesting reversible conversion processes in the sodiation and


Page 12 of 28

Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


desodiation reactions. Finally, a Ni3S2//AC sodium ion capacitor was assemabled and exhibited a maximum energy density of 65.6 Wh kg-1 at a power density of 204 W kg-1, presenting a very promising application in the high-rate energy storage fields.

Supporting Information Stucture and electrochemical characterization of hierarchically hierarchically interconnected Ni3S2 nanofibers Ni3S2 nanofibers, SEM of Ni foam and hierarchically interconnected Ni3S2 nanofibers after 60 cycles are contained in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements This work was supported by the Natural Science Funds for Distinguished Young Scholars of Gansu Province (No. 1606RJDA320), the Foundation for Innovation Groups of Basic Research in Gansu Province (No.1606RJIA322) and the Natural Science Foundation of Hainan Province (No. 517301). In addition, the research is also supported by Ministry of Science and Technology through Grant through grants no, 107-2923-E-007-002 -MY3, 107-2112-M-007-030-MY3, 106-2923-E-007006-MY2, 105-2119-M-009-009 and 107-3017-F-007-002. Y.L. Chueh greatly appreciates the use of the facility at CNMM.

References (1) Tan, J.; Liu, L.; Guo, S.; Hu, H.; Yan, Z.; Zhou, Q.; Huang, Z.; Shu, H.; Yang, X.; Wang, X., The Electrochemical Performance and Mechanism of Cobalt (II) Fluoride as Anode Material for Lithium and Sodium Ion Batteries. Electrochimica Acta 2015, 168, 225-233. (2) Wang, H.; Jia, G.; Guo, Y.; Zhang, Y.; Geng, H.; Xu, J.; Mai, W.; Yan, Q.; Fan, H. J., Atomic Layer Deposition of Amorphous TiO2 on Carbon Nanotube Networks and Their Superior Li and Na Ion Storage Properties. Advanced Materials Interfaces 2016, 3, 1600375-1600383.



(3) Sun, X.; Yan, C.; Chen, Y.; Si, W.; Deng, J.; Oswald, S.; Liu, L.; Schmidt, O. G., ThreeDimensionally “Curved” NiO Nanomembranes as Ultrahigh Rate Capability Anodes for Li-Ion Batteries with Long Cycle Lifetimes. Advanced Energy Materials 2014, 4, 1300912-1300917. (4) Wang, C.; Niu, Y.; Jiang, J.; Chen, Y.; Tian, H.; Zhang, R.; Zhou, T.; Xia, J.; Pan, Y.; Wang, S., Hybrid thermoelectric battery electrode FeS2 study. Nano Energy 2018, 45, 432-438. (5) Wang, Y.; Yan, D.; Hankari, S. E.; Zou, Y.; Wang, S., Recent Progress on Layered Double Hydroxides and Their Derivatives for Electrocatalytic Water Splitting. Advanced Science 2018, 5, 1800064-1800096. (6) Zhang, F.; Xia, C.; Zhu, J.; Ahmed, B.; Liang, H.; Velusamy, D. B.; Schwingenschlögl, U.; Alshareef, H. N., SnSe2 2D Anodes for Advanced Sodium Ion Batteries. Advanced Energy Materials 2016, 6, 1601188-1601197. (7) Sun, R.; Wei, Q.; Sheng, J.; Shi, C.; An, Q.; Liu, S.; Mai, L., Novel Layer-by-Layer Stacked VS2 Nanosheets with Intercalation Pseudocapacitance for High-Rate Sodium Ion Charge Storage. Nano Energy 2017, 35, 396-404. (8) Niu, Y.; Xu, M.; Cheng, C.; Bao, S.; Hou, J.; Liu, S.; Yi, F.; He, H.; Li, C. M., Na3.12Fe2.44(P2O7)2/Multi-Walled Carbon Nanotube Composite as a Cathode Material for Sodium-Ion Batteries. Journal of Materials Chemistry A 2015, 3, 17224-17229. (9) Kundu, D.; Talaie, E.; Duffort, V.; Nazar, L. F., The Emerging Chemistry of Sodium Ion Batteries for Electrochemical Energy Storage. Angewandte Chemie 2015, 54, 3431-3448. (10) Zhang, S.; Yu, X.; Yu, H.; Chen, Y.; Gao, P.; Li, C.; Zhu, C., Growth of ultrathin MoS2 Nanosheets with Expanded Spacing of (002) Plane on Carbon Nanotubes for High-Performance Sodium-Ion Battery Anodes. ACS applied materials & interfaces 2014, 6, 21880-21885. (11) Guo, X.; Xie, X.; Choi, S.; Zhao, Y.; Liu, H.; Wang, C.; Chang, S.; Wang, G., Sb2O3/MXene(Ti3C2Tx) Hybrid Anode Materials with Enhanced Performance for Sodium-Ion Batteries. J. Mater. Chem. A 2017, 5, 12445-12452. (12) Tang, J.; Ni, S.; Chao, D.; Liu, J.; Yang, X.; Zhao, J., High-Rate and Ultra-Stable Na-Ion Storage for Ni3S2 Nanoarrays via Self-Adaptive Pseudocapacitance. Electrochimica Acta 2018, 265, 709-716.


Page 14 of 28

Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Jackson, E. D.; Green, S.; Prieto, A. L., Electrochemical Performance of Electrodeposited Zn4Sb3 Films for Sodium-Ion secondary Battery Anodes. ACS applied materials & interfaces 2015, 7, 74477450. (14) Cao, Y.; Xiao, L.; Sushko, M. L.; Wang, W.; Schwenzer, B.; Xiao, J.; Nie, Z.; Saraf, L. V.; Yang, Z.; Liu, J., Sodium Ion Insertion in Hollow Carbon Nanowires for Battery Applications. Nano letters 2012, 12, 3783-3787. (15) Jache, B.; Adelhelm, P., Use of Graphite as a Highly Reversible Electrode with Superior Cycle Life for Sodium-Ion Batteries by Making Use of Co-Intercalation Phenomena. Angewandte Chemie 2014, 53, 10169-10173. (16) Li, D.; Yan, D.; Ma, J.; Qin, W.; Zhang, X.; Lu, T.; Pan, L., One-step Microwave-Assisted Synthesis of Sb2O3 /Reduced Graphene Oxide Composites as Advanced Anode Materials for SodiumIon Batteries. Ceramics International 2016, 42, 15634-15642. (17) Yan, L.; Chen, G.; Sarker, S.; Richins, S.; Wang, H.; Xu, W.; Rui, X.; Luo, H., Ultrafine Nb2O5 Nanocrystal Coating on Reduced Graphene Oxide as Anode Material for High Performance Sodium Ion Battery. ACS applied materials & interfaces 2016, 8, 22213-22218. (18) Zhao, L.; Zhao, J.; Hu, Y.-S.; Li, H.; Zhou, Z.; Armand, M.; Chen, L., Disodium Terephthalate (Na2C8H4O4) as High Performance Anode Material for Low-Cost Room-Temperature Sodium-Ion Battery. Advanced Energy Materials 2012, 2, 962-965. (19) Park, Y.; Shin, D. S.; Woo, S. H.; Choi, N. S.; Shin, K. H.; Oh, S. M.; Lee, K. T.; Hong, S. Y., Sodium Terephthalate as an Organic Anode Material for Sodium Ion Batteries. Advanced materials 2012, 24, 3562-3567. (20) Sun, W.; Rui, X.; Zhang, D.; Jiang, Y.; Sun, Z.; Liu, H.; Dou, S., Bismuth sulfide: A HighCapacity Anode for Sodium-Ion Batteries. Journal of Power Sources 2016, 309, 135-140. (21) Liu, Z. M.; Lu, T. C.; Song, T.; Yu, X. Y.; Lou, X. W.; Paik, U., Structure-Designed Synthesis of FeS2@C Yolk-Shell Nanoboxes as a High-Performance Anode for Sodium-Ion Batteries. Energy & Environmental Science 2017, 10, 1576-1580.



(22) Zhang, Y.; Zhu, P.; Huang, L.; Xie, J.; Zhang, S.; Cao, G.; Zhao, X., Few-Layered SnS2 on FewLayered Reduced Graphene Oxide as Na-Ion Battery Anode with Ultralong Cycle Life and Superior Rate Capability. Advanced Functional Materials 2015, 25, 481-489. (24) Shang, C.; Dong, S.; Zhang, S.; Hu, P.; Zhang, C.; Cui, G., A Ni3S2-PEDOT Monolithic Electrode for Sodium Batteries. Electrochemistry Communications 2015, 50, 24-27. (25) Duan, W.; Yan, W.; Yan, X.; Munakata, H.; Jin, Y.; Kanamura, K., Synthesis of Nanostructured Ni3S2 with Different Morphologies as Negative Electrode Materials for Lithium Ion Batteries. Journal of Power Sources 2015, 293, 706-711. (26) Wang, Y.; Niu, Y.; Li, C. M. The Effect of the Morphologies of Ni3S2 Anodes on the Performance of Lithium-Ion Batteries. ChemistrySelect 2017, 2, 4445-4451. (27) Dai, C. S.; Chien, P. Y.; Lin, J. Y.; Chou, S. W.; Wu, W. K.; Li, P. H.; Wu, K. Y.; Lin, T. W., Hierarchically Structured Ni3S2/Carbon Nanotube Composites as High Performance Cathode Materials for Asymmetric Supercapacitors. Acs Applied Materials & Interfaces 2013, 5, 12168-12174. (28) Lin, T.-W.; Dai, C.-S.; Hung, K.-C., High Energy Density Asymmetric Supercapacitor Based on NiOOH/Ni3S2/3D Graphene and Fe3O4/Graphene Composite Electrodes. Scientific Reports 2014, 4, 7274-7284. (29) P.A.MetcalfP.FanwickZ.Ka̧kolJ.M.Honig, Synthesis and Characterization of Ni3S2 Single Crystals. Journal of Solid State Chemistry 1993, 104, 81-87. (30) Li, D.; Li, X.; Hou, X.; Sun, X.; Liu, B.; He, D., Building a Ni3S2 Nanotube Array and Investigating its Application as an Electrode for Lithium Ion Batteries. Chemical communications 2014, 50, 9361-9364. (31) Li, Y.; Kong, L.-B.; Liu, M.-C.; Zhang, W.-B.; Kang, L., Facile Synthesis of Co3V2O8 Nanoparticle Arrays on Ni Foam as Binder-free Electrode with Improved Lithium Storage Properties. Ceramics International 2017, 43, 1166-1173. (32) Lai, C.-H.; Huang, K.-W.; Cheng, J.-H.; Lee, C.-Y.; Lee, W.-F.; Huang, C.-T.; Hwang, B.-J.; Chen, L.-J., Oriented Growth of Large-Scale Nickel Sulfide Nanowire Arrays via a General Solution


Page 16 of 28

Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Route for Lithium-Ion Battery Cathode Applications. Journal of Materials Chemistry 2009, 19, 72777782. (33) Zhang, Z.; Zhao, H.; Xia, Q.; Allen, J.; Zeng, Z.; Gao, C.; Li, Z.; Du, X.; Świerczek, K., High Performance Ni3S2/Ni Film with Three Dimensional Porous Architecture as Binder-free Anode for Lithium Ion Batteries. Electrochimica Acta 2016, 211, 761-767. (34) Xia, X.; Xie, J.; Zhang, S.; Pan, B.; Cao, G.; Zhao, X., Ni3S2 Nanosheet-anchored Carbon Submicron Tube Arrays as High-Performance Binder-Free Anodes for Na-Ion Batteries. Inorganic Chemistry Frontiers 2017, 4, 131-138. (35) Song, X.; Li, X.; Bai, Z.; Yan, B.; Li, D.; Sun, X., Morphology-Dependent Performance of Nanostructured Ni3S2/Ni Anode Electrodes for High Performance Sodium Ion Batteries. Nano Energy 2016, 26, 533-540. (36). Wang, J.; Liu, J.; Yang, H.; Chao, D.; Yan, J.; Savilov, S. V.; Lin, J.; Shen, Z. X., MoS2 Nanosheets Decorated Ni3S2@MoS2 Coaxial Nanofibers: Constructing an Ideal Heterostructure for Enhanced Na-Ion Storage. Nano Energy 2016, 20, 1-10. (37) Tong, Z.; Lv, H.; Zhang, X.; Yang, H.; Tian, Y.; Li, N.; Zhao, J.; Li, Y., Novel Morphology Changes from 3D Ordered Macroporous Structure to V2O5 Nanofiber Grassland and its Application in Electrochromism. Scientific reports 2015, 5, 16864-16864. (38) Tong, Z. Q.; Yang, H. W.; Na, L.; Qu, H. Y.; Zhang, X.; Zhao, J. P.; Li, Y., Versatile Displays Based on a 3-Dimensionally Ordered Macroporous Vanadium Oxide Film for Advanced Electrochromic Devices. Journal of Materials Chemistry C 2015, 3, 3159-3166. (39) Zhao, X.; Wang, H.-E.; Massé, R. C.; Cao, J.; Sui, J.; Li, J.; Cai, W.; Cao, G., Design of Coherent Anode Materials with 0D Ni3S2 Nanoparticles Self-Assembled on 3D Interconnected Carbon Networks for Fast and Reversible Sodium Storage. Journal of Materials Chemistry A 2017, 5, 73947402. (40) Gan, Q.; Zhao, K.; Liu, S.; He, Z., Solvent-Free Synthesis of N-Doped Carbon Coated ZnO Nanorods Composite Anode via a ZnO Aupport-Induced ZIF-8 in-Situ Growth Strategy. Electrochimica Acta 2017, 250, 292-301.



(41) Xiang, J. Y.; Tu, J. P.; Qiao, Y. Q.; Wang, X. L.; Zhong, J.; Zhang, D.; Gu, C. D., Electrochemical Impedance Analysis of a Hierarchical CuO Electrode Composed of Self-Assembled Nanoplates. The Journal of Physical Chemistry C 2011, 115, 2505-2513. (42) Sun, Y.; Hu, X.; Luo, W.; Xia, F.; Huang, Y., Reconstruction of Conformal Nanoscale MnO on Graphene as a High-Capacity and Long-Life Anode Material for Lithium Ion Batteries. Advanced Functional Materials 2013, 23, 2436-2444. (43) Han, Y.; Liu, S.-y.; Cui, L.; Xu, L.; Xie, J.; Xia, X.-K.; Hao, W.-K.; Wang, B.; Li, H.; Gao, J., Graphene-immobilized Flower-Like Ni3S2 Nanoflakes as a Stable Binder-Free Anode Material for Sodium-Ion Batteries. International Journal of Minerals, Metallurgy, and Materials 2018, 25, 88-93. (44) Xia, X.; Xie, J.; Zhang, S.; Pan, B.; Cao, G.; Zhao, X., Wrinkled Graphene-Reinforced Nickel Sulfide Thin Film as High-Performance Binder-Free Anode for Sodium-Ion Battery. Journal of Materials Science & Technology 2017, 33, 775-780. (45) Kim, J. S.; Ahn, H. J.; Ryu, H. S.; Kim, D. J.; Cho, G. B.; Kim, K. W.; Nam, T. H.; Ahn, J. H., The Discharge Properties of Na/Ni3S2 Cell at Ambient Temperature. Journal of Power Sources 2008, 178, 852-856. (46) Ding, R.; Qi, L.; Wang, H., An Investigation of Spinel NiCo2O4 as Anode for Na-Ion Capacitors. Electrochimica Acta 2013, 114, 726-735. (47) Chen, Z.; Augustyn, V.; Jia, X.; Xiao, Q.; Dunn, B.; Lu, Y., High-Performance Sodium-Ion Pseudocapacitors Based on Hierarchically Porous Nanowire Composites. ACS nano 2012, 6, 43194327. (48) Le, Z.; Liu, F.; Nie, P.; Li, X.; Liu, X.; Bian, Z.; Chen, G.; Wu, H. B.; Lu, Y., Pseudocapacitive Sodium Storage in Mesoporous Single-Crystal-Like TiO2-Graphene Nanocomposite Enables HighPerformance Sodium-Ion Capacitors. ACS nano 2017, 11, 2952-2960. (49) Zhao, F.; Gong, Q.; Traynor, B.; Zhang, D.; Li, J.; Ye, H.; Chen, F.; Han, N.; Wang, Y.; Sun, X.; Li, Y., Stabilizing Nickel Sulfide Nanoparticles with An Altrathin Carbon Layer For Improved Cycling Performance in Sodium Ion Batteries. Nano Research 2016, 9, 3162-3170.


Page 18 of 28

Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(50) Wang, T.; Hu, P.; Zhang, C.; Du, H.; Zhang, Z.; Wang, X.; Chen, S.; Xiong, J.; Cui, G., Nickel Disulfide-Graphene Nanosheets Composites with Improved Electrochemical Performance for Sodium Ion Battery. Acs Appl Mater Interfaces 2016, 8, 7811-7817. (51) Zhang, D.; Sun, W.; Zhang, Y.; Dou, Y.; Jiang, Y.; Dou, S. X., Engineering Hierarchical Hollow Nickel Sulfide Spheres for High-Performance Sodium Storage. Advanced Functional Materials 2016, 26, 7479-7485. (52) Ryu, H. S.; Kim, J. S.; Park, J.; Park, J. Y.; Cho, G. B.; Liu, X.; Ahn, I. S.; Kim, K. W.; Ahn, J. H.; Ahn, J. P., Degradation Mechanism of Room Temperature Na/Ni3S2 Cells Using Ni3S2 Electrodes Prepared by Mechanical Alloying. Journal of Power Sources 2013, 244, 764-770. (53) Qin, W.; Chen, T.; Lu, T.; Chua, D. H. C.; Pan, L., Layered Nickel Sulfide-Reduced Graphene Oxide Composites Synthesized via Microwave-Assisted Method as High Performance Anode Materials of Sodium-Ion Batteries. Journal of Power Sources 2016, 302, 202-209.



Page 20 of 28

Table Captions

Table 1 Simulation results of the EIS spectra using the equivalent circuit as shown in Figure 4(a) sample Ni3S2

Re (Ω) fresh 10.1

10 th 4.7

Rct+sf (Ω) fresh 19.5

60th 3.9

10 th 45.3

60 th 79.9

Table 2 Comparison of specific capacity and capacity retention for Ni3S2 composite electrode with other Ni3S2 based anodes in SIBs. Initial Capacity Current Material cycles Retention (mAh g-1) density (A/g) self-standing hierarchically 0.2 100 92% interconnected Ni3S2 nanofibers on a 584 nickel foam (This work) Ni3S2-PEDOT 21 0.6 50 50% Ni3S2 nanosheet-anchored carbon 887 0.05 260 24% submicron tube arrays 29 Different morphologies of Ni3S2 on Ni 373 0.05 100 84% foam 31 Graphene-immobilized flower-like Ni3S2 682 0.05 100 37% nanoflakes 38 Graphene-reinforced Ni3S2 thin film 39 791 0.05 110 71% 12 Ultrathin Ni3S2 nanoarrays 445.3 0.15 200 77% NiSx/CNT49 410.9 0.1 130 73% NiS2-graphene nanosheets50 510 0.1 200 60% 51 NiS 590 0.1 50 85% Ni3S252 440 0.045 220 50% 53 Ni3S2-reduced graphene 510 0.1 50 78%


Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure 1. (a) Schematic illustration of the hierarchically interconnected Ni3S2 nanofibers on a Ni foam. (b) XRD results of Ni3S2 phase. XPS spectra of the Ni3S2: (c) Ni 2p, (d) S 2p. Figure 2. (a-b) SEM and (c-d) TEM images of the hierarchically interconnected Ni3S2 nanofibers at different magnification. (e-g) EDS elemental mapping images of Ni and S. Figure 3. The electrochemical cyclic performance of the hierarchically interconnected Ni3S2 nanofibers. (a) CV curves. (b) Charge-discharge curves at 0.1 A g-1. (c) Rate capability, (d) Charge-discharge curves at different current rates. (e) Cycling stability at 0.1 A g-1. Figure 4. (a) Nyquist plots for the hierarchically interconnected Ni3S2 nanofibers and the pure Ni for as electrodes with different cycles at 0.1 A g-1. (b) The corresponding Nyquist plots of the real parts of the complex impedance versus ω-1/2. Figure 5. (a) and (b) Ex-situ XRD results of the hierarchically interconnected Ni3S2 nanofibers taken from the third cycle under different discharge and charge states. (c) The corresponding charge-discharge curves at 0.1 A g-1. Figure 6. Electrochemical performance of the sodium-ion capacitor. (a) CV curves at different scan rates. (b) Charge-discharge curves at different current densities. (c) Specific capacitance calculated from the charge-discharge curves as a function of current density. (d) The Ragone plots of the Ni3S2//AC and other reported sodium ion capacitors from the literature. (e) Cycling stability at the current density of 0.1 A g-1.



Figure 1


Page 22 of 28

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2



Figure 3


Page 24 of 28

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4



Figure 5


Page 26 of 28

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6



TOC


Page 28 of 28

Hierarchically Interconnected Ni3S2 Nanofibers as Binder-Free

Recommend Documents