Improved Na Storage Performance with the Involvement of Nitrogen

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Improved Na Storage Performance with the Involvement of Nitrogen Doped Conductive Carbon into WS2 Nanosheets Xin Wang, Jianfeng Huang, Jiayin Li, Liyun Cao, Wei Hao, and Zhanwei Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06032 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 30, 2016

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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 41

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 Materials & Interfaces

Improved Na Storage Performance with the Involvement of Nitrogen Doped Conductive Carbon into WS2 Nanosheets Xin Wang a, Jianfeng Huang a,*, Jiayin Li a,*, Liyun Cao a, Wei Hao b, Zhanwei Xu a a

School of Material Science and Engineering, Shaanxi University of Science and

Technology, Xi’an, Shaanxi 710021, P. R. China b

School of Material Science and Engineering, Shanghai Jiao Tong University, 800

Dongchuan Road, Shanghai 200240, P. R. China

*Corresponding author: Jianfeng Huang E-mail: [email protected]. Tex./Fax: +86-029-86168802 *Corresponding author: Jiayin Li E-mail: [email protected] Tex./Fax: +86-029-86168688

1

ACS Paragon Plus Environment


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

ABSTRACT Tungsten disulfide (WS2) material is regarded as one of the most promising anode candidate for sodium ion batteries (SIBs). However, the exploration of this material still remains a great challenge to improve its cycling capacity. In this paper, nitrogen-doped conductive carbon/WS2 nanocomposites (WS2-NC) was fabricated based on the synthesis of the pure WS2 and conductive carbon/WS2 (WS2-C) nanocomposites. The reversible capacity of the as-prepared WS2-NC is stabilized at ~360 mA h g-1 at the density of 100 mA g-1, even ~200 mA h g-1 at 1 A g-1, presenting much better cycling performance than pure WS2 and conductive carbon/WS2 (WS2-C) samples. This excellent performance is further attributed to obviously promoted interfacial reaction in WS2 nanosheets at a low voltage platform (0.3~0.0 V), which is considered to closely relate to the incorporation of nitrogen doped conductive carbon into WS2 nanosheets. Generally, this work presents an obviously enhanced Na storage performance by the incorporation of N-doped carbon into WS2 nanosheets to promote their interfacial reaction at low voltage platform. It could provide guidelines to create other high-capacity anode sulfide materials for SIBs. KEYWORDS: Tungsten disulfide; Nitrogen-doped conductive carbon; Electronic conductivity; Electrochemical performance; Sodium ion batteries. 1. INTRODUCTION High-energy lithium-ion batteries (LIBs) have already played a significant role in the development of portable electronics, electric vehicles, and energy storage.1-3 Meanwhile, high capacity anode materials in LIBs hold the key to meeting future high 2


Page 2 of 41

Page 3 of 41

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


energy density, longer cycle life and extreme safety requirements.4, 5 Therefore, there is increasing demand for designing the next generation low-cost rechargeable batteries.6, 7 Sodium ion batteries (SIBs) have rapidly drawn an attractive attention because of abundant sodium resources and lower cost than that of LIBs.8 Additionally, SIBs exhibit the lower standard half-reaction potential of sodium (2.714 V for Na/Na+) compared with that of lithium (3.045 V for Li/Li+), further guaranteeing the safety of SIBs in application. 9, 10 According to previous reports, transition metal dichalcogenides (TMDs) (MS2, M = W, Mo, Zn, Mn, Ni, Fe, Ti; X = S, Se) have applied extensively as electronic materials.11-13 Particularly, tungsten disulfide (WS2) has been extensively investigated as anode materials for SIBs due to their pronouncedly layered structure (S-W-S).14 On the other hand, the low electronic conductive and poor cycle stability caused by the volume expansion of WS2 limit its application in batteries.15, 16 Therefore, some efforts have been made to address this issue for improving the electrochemical performance. In the previous literatures, WS2 was composited with three-dimensional reduced grapheme oxide,17 double carbon coating

18

and other carbon materials as

SIBs anode materials to buffer volume expansion, and obtain outstanding electrochemical performances. Besides, N-doped carbon nanofibers were embedded into single layers of WS2 nanoplates (WS2@NCNFs) as anode materials for LIBs.19 The as-obtained WS2@NCNFs nanocomposites prepared by N-doping method has increased electronic conductivity. It is beneficial for their electrochemical performances (an initial charge capacity of 590.4 mA h g-1 at a current density of 0.1 3



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

Page 4 of 41

A g-1). However, these carbon materials used in these previous literatures are high cost and difficult to obtain. Simultaneously, N-doping carbon with WS2 has not been reported as anode materials for SIBs. In this work, nitrogen-doped conductive carbon/WS2 nanosheets composites (WS2-NC) as anode material for SIBs was prepared by a sample method, the same procedures as conductive carbon/WS2 nanosheets composites (WS2-C) and bare WS2. 20

All of the samples are performed as anodes for electrochemical tests. Compared

with the WS2-C nanocomposites and bare WS2, the effect of nitrogen-doped carbon on electrochemical performances of the WS2-NC nanocomposites was mainly investigated and its mechanism was also discussed and revealed. We focused on the transport mechanism of Na ions and electrons in WS2-NC anode materials during the cycling

process.

Most

importantly,

it

also

casts

light

on

innovative

composition-modulated designation of electrode materials in advanced SIBs. 2. EXPERIMENTAL SECTION 2.1 Synthesis of WS2-NC Nanosheets. WO3 were prepared by a precipitation reaction. The detail procedure was referred in our previous work.20 0.10 g Super P (conductive carbon) dispersed in 30 mL H2O2 (30 wt% in purity), then 1.50 g W powders (A.R.) were slowly dissolved in it. After reaction, the suspension was formed and 10 mL isopropyl alcohol (IPA) was added to the above suspension to form the precursor solution. The precursor solution was dried at 60 °C to obtain ~2.20 g the conductive carbon/WO3 precursor. Afterwards, the mass ratio of the above conductive carbon/WO3 precursor and urea is mW : mN = 1 : 1. The mixture was loaded into a 4


Page 5 of 41

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


small alumina boat, which is nested in a large alumina boat. Thioacetamide (TAA) loaded into the large alumina boat with a cover. These boats were heated in vacuum atmosphere at 900 °C for 2 h. The WS2-NC nanocomposites were produced finally. It is notable that the WS2-C nanocomposites were produced without urea and the bare WS2 was produced without Super P and urea. 2.2 Materials Characterizations. The crystalline structures of the as-prepared powders were characterized by a powder X-ray diffraction (XRD, Rigaku D/max-2000) with Cu Ka radiation (k= 0.15406 nm). Thermogravimetry analysis (TG, Bruker, STA449 F3) was performed to examine the weight change of nanocomposites at the heating rate of 10 °C min-1 in air from room temperature to 600 °C. The morphologies of the samples were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) equipped with energy dispersive X-ray spectroscopy (EDS) and transmission electron microscope (TEM, FEI Tecnai G2 F20). The X-ray photoelectron spectroscopy (XPS, PHI-5400) spectrum was performed with a Surface Science Instruments Spectrometer focused monochromatic Al Kα radiation 1486.6 eV. Raman spectrum measurements of the as-received samples were conducted on the Renishaw-invia with a laser at 532 nm. 2.3 Electrode Preparation and Electrochemical Measurements. The capacities and cycling properties of the WS2-NC, WS2-C and WS2 samples were determined with 2032-type coin cells. The electrodes were prepared from a mixture containing 70 wt% active material, 20 wt% Super P and 10 wt% sodium carboxymethyl cellulose (Na-CMC) binder. Sodium metal and glass fiber were used as a counter electrode and 5



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

separator, respectively. The electrolyte was a solution of 1 M NaPF6 (Aldrich) in a 1:1 volume mixture of ethylene carbonate/diethyl carbonate (EC/DEC) (Aldrich). Moreover, the charge-discharge characteristics of the samples were determined by cycling in the potential range 0.0-3.0 V at fixed current densities with a multichannel battery testing system (Shenzhen, Neware, China). Cyclic voltammograms (CV), recorded at scan rates of 0.1 mV/s, and electrochemical impedance spectroscopy (EIS) over a frequency range of 100 kHz-0.01 Hz were performed by the CHI660E electrochemical station (Shanghai Chenhua, China). All electrochemical tests were conducted at room temperature. 3. RESULTS AND DISCUSSION

Figure 1. XRD patterns (a) and TG curves (b) of the WS2-NC, the WS2-C nanocomposites and the bare WS2 nanosheets. The phase composition of the as-prepared WS2 powders sulfidated at 900 °C for 2 h is shown in Figure 1a. The XRD patterns of the sulfidated powders displays hexagonal WS2 (PDF card No. 08-0237) structure without other impurity peaks. Figure 1b shows the thermogravimetry (TG) curves of the three samples. The 7.42% weight loss of the WS2 sample demonstrates the oxidation of WS2 into WO3.21 In terms of WS2-C and WS2-NC samples, WS2 was oxidized into WO3 corresponding to 6


Page 6 of 41

Page 7 of 41

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


the weight loss of about 3.91% and 5.92%, respectively. The weight loss of 5.90 % and 3.61% represents the decomposition of the conductive carbon and N-doped carbon,22 which is consistent with the less carbon content in the WS2-C and WS2-NC nanocomposites, respectively.

Figure 2. SEM images of WS2-NC (a), WS2-C (c) nanocomposites and the bare WS2 (d), EDS spectrum (b) and element mapping images of WS2-NC nanocomposites (e-i). The morphologies of the WS2-NC, the WS2-C nanocomposites and the bare WS2 were characterized by SEM, as shown in Figure 2. As depicted in Figure 2a, the WS2-NC product exhibits nanosheet-like structure with uniform smaller size than the WS2-C and bare WS2 products. The WS2-C product also presents wrinkled and curled nanosheet-like structure, which is formed a larger nanosheets than WS2-NC (Figure 2c) and the bare WS2 sample shows similar particle size with WS2-C (Figure 2d). The EDS spectrum of WS2-NC nanocomposites is displayed in Figure 2b, it can be seen that the composition of them contains C, N, W and S elements. From the EDS element mapping images of WS2-C nanocomposites (Figure 1S), the uniform distributions of C and S components shows the uniform combination of WS2 7



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

nanocrystals and conductive carbon. Furthermore, the EDS element mapping images shown in Figure 2e-2i demonstrate homogeneous distribution of C, N, W and S elements all over the WS2-NC nanocomposites. It can be found that N, C and S are overlapped well to distribution in WS2-NC sample, which can reveal that nitrogen has been involved in WS2-NC product.23

Figure 3. TEM and HRTEM images of the WS2-NC (a, b), WS2-C (c, d) nanocomposites and the bare WS2 (e, f). The detailed morphology and microstructure information on these WS2-NC, WS2-C nanocomposites and bare WS2 were further characterized by TEM and HRTEM, as presented in Figure 3. All the three samples exhibit homogenous 8


Page 8 of 41

Page 9 of 41

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


nanosheets structures, which agree well with the SEM observation in Figure 2. The size of WS2-NC nanosheets is smaller than the other samples obviously. From HRTEM observation, the WS2 nanosheets are cladding on the conductive carbon uniformly in WS2-NC and WS2-C samples. In addition, the WS2 nanosheets displays visible lattice stripes and the lattice d-spacing of 0.63 nm corresponds to the (002) plane of hexagonal WS2 phase, which is consistent with the XRD analysis. Other planes or d-spacing values because of the existence of the impurity phases are not distinctly detected.

Figure 4. XPS spectra: broad scan spectra (a, b, c) and N 1s (d), C 1s (e), W 4f (f) and S 2p (g) of the WS2-NC, WS2-C and WS2 samples. XPS analysis was performed to gain insight into the chemical states in the three samples (Figure 4). The broad XPS spectrum of WS2-NC powders in Figure 4a confirms the presence of W, S, C, and N elements. Importantly, the N 1s peak could 9



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

only be found in Figure 4a rather than Figure 4b and 4c. It indicated that N atom is not existed in WS2-C and WS2 samples. In the high resolution XPS spectrum of N element (Figure 4d), nitrogen atoms present with three different forms as the N 1s peaks of WS2-NC samples can be assigned to pyridinic-N, pyrrolic-N, and graphitic-N. These are detected at 398.1, 401.2, and 403.1 eV, respectively.19, 24, 25 Simultaneously, the N 1s peak spectra of the WS2-C and WS2 samples are both invisible and disorderly, which means the N atom exists scarcely in the WS2-C and WS2 samples. As can be obtained from Figure 4e, the C 1s peaks of WS2-NC sample are divided into C-C, C-N, C-OR and C=O, which are located at 284.5, 285.5, 286.3 and 287.6 eV,26 and these peaks shift to higher binding energy about 0.4 eV than that of WS2-C. This means that chemical bonding have been formed between C and N in WS2-NC powders. The peaks of the three products (Figure 4f) found at 32.5, 34.7, 35.2 and 38.1 eV are assigned to W 4f7/2, W 4f5/2, W 5p5/2 and W 5p3/2, respectively,27, 28

indicating that all of the W elements in three products are W4+ of WS2. Doublet

peaks of S 2p1/2 and S 2p3/2 at 162.1 and 163.3 eV are characteristic of S2- in WS2-NC, WS2-C and WS2 samples.17, 18 Moreover, there is no apparent shift of W and S peak in the three samples. The above results reveal that the involvement of nitrogen can not change the chemical states of W and S elements in the three samples. Thus, the N-doped Super P is achieved by the involvement of nitrogen and the existence of N is able to create defects and vacancies in its structure,29 which can be also verified with Raman Spectrum by the intensity of D-band and G-band (Figure S2). Significantly, the N-doping Super P is beneficial for improving the electrical conductivity of the 10


Page 10 of 41

Page 11 of 41

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


Super P, leading to rapid pathways for electrons and facile charge transfer in the application of WS2-NC anode materials.30

Figure 5. Electrochemical performances of the three samples (WS2-NC, WS2-C, WS2): cycling performances at a current density of 100 mA g-1 (a), 1 A g-1 (c) and rate performances (b); selected galvanostatic discharge/charge profiles of WS2-NC at a current density of 100 mA g-1 (d); cyclic voltammetry curves of WS2-NC (e), WS2-C (f) scanned in the voltage range of 0.01-3.00 V (d). The Na-ion storage performances of the pure WS2 nanosheets, WS2-C and WS2-NC nanocomposites were evaluated by galvanostatic charge-discharge cycling using two-electrode coin cells. As shown in Figure 5a, the bare WS2 has low capacity with fast fading rate (32 mA h g-1 after 100 cycles) at a current density of 100 mA g-1. The cycling capacity of WS2-C anodes is increased to 93 mA h g-1 after 100 cycles when the Super P is added into the WS2 nanosheets. But the capacity of WS2 anodes is improved slightly by incorporating conductive carbon into them. Interestingly, the WS2-NC nanocomposites exhibit an excellent cycling performance with high initial 11



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

coulombic efficiency of 79.8 %. From 2nd to 50th cycle, the capacity possesses rising tendency and is grown up to 371 mA h g-1 at 50th cycling. A highly likely explanation for the rising tendency capacity is activation, the cycling-induced formation of a polymer gel on the surface of the nanostructured conversion electrode is achieved during electrochemical reaction.31,32 After 50th cycle, the reversible capacity is stabilized at ~360 mA h g-1. At the density of 1 A g-1, The cycling capacity of WS2-NC anodes can maintain to ~200 mA h g-1, as depicted in Fig. 5c. This phenomenon indicates that WS2-NC electrodes deliver higher capacity and better cycling stability than WS2-C and WS2 electrodes. The rate performance of these pure WS2 nanosheets, WS2-C and WS2-NC nanocomposites for Na-ion batteries are shown in Figure 5b. The addition of conductive carbon into WS2 nanosheets can act as the conductive network to improving electronic conductive in WS2-C electrode. This incorporated structure is beneficial for accelerating the discharge/charge processes and the Na+ transportation. Thus, the WS2-C electrode possesses faster Na+ diffusion and more reactive Na+ sites than WS2 electrode. Consequently, both the capacity and rate performances of WS2-C nanocomposites electrode were enhanced comparing with bare WS2 electrode. In addition, at various current densities of 100, 300, 500 and 1000 mA g-1, the WS2-NC electrode can still displays desirable average capacities of 349, 313, 282, and 258 mA h g-1, respectively. When the current density was changed to return back to 100 mA g-1, this electrode still delivers a capacity of 305 mA h g-1 at the 50th cycle. This also shows the good rate performance of the WS2-NC electrodes. 12


Page 12 of 41

Page 13 of 41

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


The selected galvanostatic discharge/charge curves of the WS2-NC electrodes (Figure 5d) can further verify the excellent reversible reaction. The curves of 50th and 100th cycle are overlapped each other perfectly. Significantly, the WS2-NC electrodes with good cycling stability has been achieved when N-doped conductive carbon is incorporated into WS2 nanosheets. To further clarify the excellent electrochemical performances of the WS2-NC electrodes, the CV measurements were carried out at room temperature. Figure 5e shows the CV curves of the first three cycles of WS2-NC electrodes at a scanning rate of 0.1 mV s-1. During the first cathodic scanning, the cell exhibits a broad peak potential at approximately 0.30 V, which can be ascribed to the conversion reaction, the formation of an SEI film and decomposition of electrolyte. It corresponds to the chemical reaction equation (1).27, 33,34 4Na + + WS2 + 4e-  W + 2Na 2S

(1)

In the anodic scans, the peak at 0.34 V corresponds to the reverse reaction [34]. The distinct two oxidation peaks at 1.92 V and 0.64 V are attributed to the sodium extraction from Na X WS2 , detailed reaction equation (2) is as follows:14, 27, 35

Na X WS2  xNa + + WS2 + xe-

(2)

The positions of three WS2-C electrode peaks at ~1.9 V possess obviously different values in initial three cycles. It reveals that the polarization is occured in WS2-C electrode. Notably, the locations of these peaks in WS2-NC curves are almost same, which indicates that the ability of ionic/electronic transportation is remarkably improved. Therefore, the polarization phenomenon has been avoided by involving 13



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

nitrogen into WS2-C product. Discharge/charge profile for the first cycle of the three samples has been divided into three stages, as shown in Figure 6a. The three areas (I: 2.2~0.5 V; II: 0.5~0.3 V and III: 0.3~0.0V) are reported as the process of insertion, conversion, and interfacial reaction of Na+ with WS2,19 respectively (Figure 6b). The specific capacity of the WS2-NC electrode was larger than the capacities of WS2-C and WS2 electrode. This can be attributed to additional sodium storage sites, which are derived from interfacial sites of WS2-NC nanocomposites. In the electrochemical reaction process, partially negatively charged W nanoparticles (Wσ-) will be produced after conversion reaction ( 4Na + + WS2 + 4e-  W + 2Na 2S ) on the surface of W particles and Na2S. Then, these Wσ- can absorb additional Naσ+ on the interface. Since W does not alloy with sodium, sodium can be adsorbed on W nanoparticles with a partial charge transfer.36-39 Furthermore, the capacity contribution of these reactions in WS2-NC, WS2-C and WS2 electrodes was calculated from actual voltage platforms, respectively, as shown in Table 1. The three electrodes exhibit similar capacities in insertion reaction and conversion reaction. Especially, the WS2-NC electrode displays high capacity in the interfacial reaction. The interfacial reaction (III: 0.3~0.0 V) not only provides the majority of the discharge capacity in each electrode, but also contributes to the main difference in the electrochemical performance of these three electrodes. These phenomena mean that WS2-NC electrode possesses a better interfacial adsorption of Na+ than other two electrodes in SIBs during cycling. This interfacial adsorption is related to the ionic/electronic transportation rate. We propose that the WS2-NC 14


Page 14 of 41

Page 15 of 41

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


electrode delivers enhanced ionic and electronic transportation during the charge/discharge process in SIBs. This transportation can further provide rapid pathways for electrons. It also plays a significant role in the interfacial reaction. Thus, WS2-NC electrode with fast charge transfer have ability to participate in the interfacial reaction, providing high capacity in interfacial process, which agrees well with the results discussed in Figure 4e and 4f.

Figure 6. Discharge/charge profile for the first cycle of the three electrodes at a current density of 100 mA g-1 (a), schematic diagram of the insertion, conversion, and interfacial reaction of Na+ with WS2 during discharge process (b). Table 1. The as-obtained three samples capacities of the different reaction stages during first discharge process. Capacity (mA h g-1) Sample Ⅰ(Insertion)

Ⅱ (Conversion)

Ⅲ (Interfacial)

Total

WS2-NC

10

108

324

442

WS2-C

26

97

265

388

WS2

1

69

199

269

15



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

The above result proves that N-doped conductive carbon with WS2 nanosheets can provide rapid routes for electrons diffusion, also allowing rapid charge transfer during interfacial reactions. It is further supported by the electrochemical impedance spectra of WS2-NC, WS2-C and WS2 electrodes, which are displayed in Figure 7. The impedance plots consist of a high frequency semicircle and a low frequency slope line. The semicircle at high frequency corresponds to the charge transfer reaction on the surface of particles. Besides, the slope line corresponds to the diffusion resistance of Na+ at low frequency region.

40, 41

According to Figure 7d, Rs, Rct, Zw and CPE

represent the solution resistance, charge transfer resistance, Warburg resistance and constant phase element. The charge transfer resistance of the three electrode after 0th are higher than after 1st cycle. This phenomenon is attributed to the incompletely activation effect in the electrode materials before cycling. Apparently, the charge transfer impedance of the WS2-NC electrode demonstrates a much larger Rct that increases drastically before 3 cycles and decreases evidently from 10000 Ω (3rd cycle) to 957 Ω (100th cycle). The high charge transfer impedance may be ascribed to the formation of inorganic SEI on the surface of particles and lack of electrolyte wetting completely. These factors can provide low conductivity and slow charge transfer. The WS2-C electrode is examined with the increase of transfer impedance from 1040 Ω (3rd cycle) to 2480 Ω (100th cycle) during cycling. At the same time, that of the WS2 electrode also largely increases from 3840 Ω (3rd cycle) to 8900 Ω (100th cycle). These impedance changes are mainly attributed to the instable structure of our electrodes. With the ceaseless pulverization of the WS2-C and bare WS2 electrodes 16


Page 16 of 41

Page 17 of 41

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


during cycling, the new SEI over the active materials will be formed over again and again. The continually formed SEI will increase the impedance between the interfaces and impede the transfer of electrons and ions, which can result in the increased impedance of the electrodes finally.42,

43

The decrease of WS2-NC electrode is

attributed to faster transportation rate of ion and electron during repeated Na+ insertion/extraction than WS2-C and WS2 electrodes. The low Rct of WS2-NC electrode is beneficial for the better cycling behavior and rate performance than those of WS2-C and WS2 electrodes.44

Figure 7. Electrochemical impedance spectra (EIS) of WS2-NC (a), WS2-C (b) and WS2 electrodes (c) after the 0th, 1st, 3rd and 100th cycle, Equivalent circuit for electrodes (d).

17



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 8. SEM, TEM and HRTEM images of the WS2-NC electrode after 50 cycles (a, e, and i), after 100 cycles (b, f, and j); WS2-C electrode (c, g, and k) and WS2 electrode (d, h, and l) after 100 cycles, respectively. To further reveal the enhanced electrochemical performance of WS2-NC nanocomposites, Figure 8 presents the changed morphologies of the WS2-NC nanocomposites after 50 and 100 cycles, WS2-C nanocomposites and the bare WS2 electrodes after 100 cycles. From SEM observations, the particles in WS2-C and WS2 electrode present a much rougher surface than that in WS2-NC electrode, which can suggest that a smooth SEI layer is formed on stable structure of WS2-NC, rough SEI layers can be constantly produced with the ceaselessly pulverization of WS2-NC and WS2-C electrodes.45 Compared with WS2-C and WS2, the electrochemically active WS2-NC can be coated on the surface of particles more uniformly to form a better ionic conductive film. With this ionic conductive film produced completely, a stable and homogeneous SEI layer can be formed in initial cycling. 18


Page 18 of 41

Page 19 of 41

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


From the HRTEM analyses after 50 and 100 cycles, the lattice fringes are clearly enough to be observed, and the interlayer distance of (002) plane is estimated to be 0.64 nm. On the contrary, the lattice fringes of WS2-C and WS2 electrodes are not clear to be identified because the structures exhibit volume expansion to some extent. The above results demonstrate that WS2-C and WS2 electrodes present obvious collapse and pulverization while the particles in WS2-NC electrode are still closely attached with each other without obvious structural change. Meanwhile, the stable maintenance of this favorable structure is beneficial for the electronic conductive during cycling.

Figure 9. The comparison of charge transfer resistance after 1st, 100th cycle and the particle size after 100th cycle of the three samples. To further explore the differences on the electrochemical and structural stability after 100th cycle of the three samples, Figure 9 shows the charge transfer resistance after 1st, 100th cycle and the particle size after 100th cycle of the three samples. It can be seen that WS2-NC sample possesses larger particle size (100nm) after 100th charge/discharge process, but exhibiting the smaller charge transfer resistance (957Ω) 19



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

than other two samples, which show significant small particle size and high charge transfer resistance. These results reveal that pulverization phenomenon can mainly result in the charge transfer resistance of WS2-C and WS2 samples increased during charge/discharge process. Therefore, the structure of WS2 nanosheets in the WS2-NC sample is stabilized with nitrogen doped conductive carbon significantly, presenting the low charge transfer resistance. This may be related to different SEI formation. 46, 47 The WS2-NC samples with N-doped conductive carbon has stable structure. Besides, a stable and homogeneous SEI layer can be formed on the surface of WS2-NC particles, causing fast charge transfer and the charge transfer resistance decreased effectively. However, the WS2-C and WS2 samples possess instable structure with volume expansion and ceaselessly pulverization. Meanwhile, the instable SEI layer is also formed in both WS2-C and WS2 samples. Finally, all of them can lead to WS2-C and WS2 samples with large charge transfer resistances.

Figure 10. Schematic illustration for the mechanism of involving nitrogen into carbon with WS2 in the WS2-NC anode for application in SIBs. In order to make explanation the reasons for the WS2-NC anode with enhanced 20


Page 20 of 41

Page 21 of 41

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


electrochemical performances, Figure 10 illustrates the mechanism of the interfacial reaction and charge transfer process in the WS2-NC anode for application in SIBs. The above structural and electrochemical results reveal that the discharge process of WS2 is divided into three reaction steps (I: insertion, II: conversion, III: interfacial reaction), in which the electrochemical performance of WS2 nanosheets is mainly determined by their interfacial reaction. During the conversion reaction, Na+ and WS2 react together with e- to generate negatively charged Wσ- eventually.36,37 Then, these Wσ- can adsorb additional Naσ+ to provide high capacity contribution to the interfacial reaction,38 which offers main capacity to WS2 materials. Simultaneously, the electronic transportation rate can affect the yield of Wσ- in conversion reaction. In conclusion, the faster electronic transportation rate is, the more Wσ- are generated in the conversion reaction and the more Naσ+ adsorbed in interfacial reaction. During the electrochemical reaction process, the diffusion of Na+ is mainly through the electrolyte, and the electronic transfer is occurred between the electrode materials. Therefore, the charge transfer is generated through the electron transportation from WS2 nanosheet to conductive carbon then to another WS2 nanosheet in WS2-NC electrode, building a rapid pathway for electronic transportation. When the rapid pathway is formed in WS2-NC electrode, the mechanism discussed above will be significantly promoted. The capacity attributed to the interfacial reaction is also improved.

Consequently,

the

WS2-NC

electrode

exhibits

superior

Na+

charge/discharge performance. On the contrary, the electronic transportation capability of bare WS2 and WS2-C electrodes can not be notably enhanced during 21



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

charge/discharge process, this result has been verified by EIS analysis. The interfacial reaction in bare WS2 and WS2-C electrodes is not promoted effectively and they exhibit poor cycling performance finally. 4. CONCLUSION In summary, the WS2-NC nanocomposites have been synthesized as anode for SIBs by doping N element into conductive carbon with WS2 nanosheets. Compared with pure WS2 and WS2-C electrodes, the as-obtained WS2-NC electrode exhibits better capacity retention (~360 mA h g-1 at the density of 100 mA g-1, ~200 mA h g-1 at the density of 1 A g-1 after 100 cycles) and rate performance. The excellent electrochemical performances can be ascribed to abundant adsorbed Na+ participating in interfacial reaction, which is able to facilitate ionic/electronic transportation in conversion reaction. These can be considered as the main reasons for possessing high and stable capacity at high current density. Furthermore, doping N element into conductive carbon with WS2 nanosheets can also maintain the original structure of WS2-NC electrode after cycling. This result suggests that involving N-doped carbon in other anode materials is an effective route to pursue excellent performance in SIBs. ACKNOWLEDGEMENTS This work has been supported by the 973 Special Preliminary Study Plan (No. 2014CB260411), Scientific Research Innovation Team Foundation of Shaanxi Province (No. 2013KCT-06), Innovation Team Assistance Foundation of Shaanxi University of Science & Technology (No. TD12-05), Doctoral initial foundation (No. BJ14-16), Scientific special fund of Shaanxi Province Office of Education (No. 22


Page 22 of 41

Page 23 of 41

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


15jk1074) and Graduate Innovation Foundation of Shaanxi University of Science and Technology. ASSOCIATED CONTENT Supporting Information Distribution of W, S and C elements in the as-prepared WS2-C nanocomposites are examined by EDS element mapping analysis. Characterization of WS2-NC, WS2-C nanocomposites and bare Super P (conductive carbon) by Raman spectra analyses. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

23



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

Page 24 of 41

REFERENCES (1) Li, W.; Sasaki, A.; Oozu, H.; Aoki, K.; Kakushima, K.; Kataoka, Y.; Nishiyama, A.; Sugii, N.; Wakabayashi, H.; Tsutsui, K.; Natori, K.; Iwai, H. Improvement of Charge/Discharge Performance for Lithium Ion Batteries with Tungsten Trioxide Electrodes. Microelectron. Reliab. 2015, 55, 402-406. (2) Zhang, L.; Huang, Y.; Zhang, Y.; Fan, W.; Liu, T. Three-Dimensional Nanoporous Graphene-Carbon Nanotube Hybrid Frameworks for Confinement of SnS2 Nanosheets: Flexible and Binder-Free Papers with Highly Reversible Lithium Storage. ACS Appl. Mater. Interfaces 2015, 7, 27823-27830. (3) Kim, D. M.; Kim, S. J.; Lee, Y. W.; Kwak, D. H.; Park, H. C.; Kim, M. C.; Hwang; B. M.; Lee, S.; Choi, J. H.; Hong, S.; Park K. W. Two-Dimensional Nanocomposites Based on Tungsten Oxide Nanoplates and Graphene Nanosheets for High-Performance Lithium Ion Batteries. Electrochim. Acta 2015, 163, 132-139. (4) Wang, H.; Ma, D.; Huang, X. L.; Huang, Y.; Zhang, X. B. General and Controllable

Synthesis

Strategy

of

Metal

Oxide/TiO2

Hierarchical

Heterostructures with Improved Lithium-Ion Battery Performance. Sci. Rep. 2011, 2, 136-136. (5) Zhao, K.; Liu, F.; Niu, C.; Xu, W.; Dong, Y.; Zhang, L.; Xie, S.; Yan, M.; Wei, Q.; Zhao, D.; Mai, L. Graphene Oxide Wrapped Amorphous Copper Vanadium Oxide with Enhanced Capacitive Behavior for High-Rate and Long-Life 24


Page 25 of 41

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


Lithium-Ion Battery Anodes. Chem. Rev. 2015, 100, 4443-4464. (6) Song, L. J.; Liu, S. S.; Yu, B. J.; Wang, C. Y.; Li, M. W. Anode Performance of Mesocarbon Microbeads for Sodium-Ion Batteries. Carbon 2015, 95, 972-977. (7) Chen, W.; Deng, D. Deflated Carbon Nanospheres Encapsulating Tin Cores Decorated on Layered 3-D Carbon Structures for Low-Cost Sodium Ion Batteries. ACS Sustainable Chem. Eng. 2015, 3, 63-70. (8) Hayashi, A.; Noi, K.; Sakuda, A.; Tatsumisago, M. Superionic Glass-Ceramic Electrolytes

for

Room-Temperature

Rechargeable

Sodium

Batteries.

Nat.Commun. 2012, 3, 199-202. (9) Ellis, B. L.; Nazar, L. F. Sodium and Sodium-Ion Energy Storage Batteries. Curr. Opin. Solid State Mater. Sci. 2012, 16, 168-177. (10) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-González, J.; Rojo, T. Na-Ion Batteries, Recent Advances and Present Challenges to Become Low Cost Energy Storage Systems. Energy Environ. Sci. 2012, 5, 5884-5901. (11) Xu, X.; Rout C.S.; Yang J.; Cao R.; Oh P.; Shin H.S.; Cho J. Freeze-Dried WS2 Composites with Low Content of Graphene as High-Rate Lithium Storage Materials. J. Mater. Chem. A 2013,1,14548-14554. (12) Rui, X.; Tan, H.; Yan. Q. Nanostructured Metal Sulfides for Energy Storage. Nanoscale 2014, 6, 9889-9924. (13) Pan, H. Room-temperature Stationary Sodium-Ion Batteries for Large-Scale Electric Energy Storage. Energy Environ. Sci. 2013, 6, 2338-2360. (14) Shiva, K.; Matte, H. S. S. R.; Rajendra, H. B.; Bhattacharyya, A. J.; Rao, C. N. R. Employing Synergistic Interactions Between Few-Layer WS2 and Reduced Graphene Oxide to Improve Lithium Storage, Cyclability and Rate Capability of Li-Ion Batteries. Nano Energy 2013, 2, 787-793. 25



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

(15) Li, H.; Yu, K.; Fu, H.; Guo, B.; Lei, X.; Zhu, Z. Multi-Slice Nanostructured WS2@RGO with Enhanced Li-Ion Battery Performance and a Comprehensive Mechanistic Investigation. Phys. Chem. Chem. Phys. 2015, 17, 29824-29833. (16) Chen, R.; Zhao, T.; Wu, W.; Wu, F.; Li, L.; Qian, J.; Xu, R.; Wu, H.; Albishri H. M.; Al-Bogami, A. S.; El-Hady, D. A.; Lu, J.; Amine K. Free-Standing Hierarchically Sandwich-Type Tungsten Disulfide Nanotubes/Graphene Anode for Lithium-Ion Batteries. Nano Lett. 2014, 14, 5899-5904. (17) Choi, S.H.; Kang, Y.C. Sodium Ion Storage Properties of WS2-Decorated Three-Dimensional Reduced Graphene Oxide Microspheres. Nanoscale 2015, 7, 3965-3970. (18) Du, Y.; Zhu, X.; Si, L.; Li, Y.; Zhou, X.; Bao, J. Improving the Anode Performance of WS2 Through a Self-Assembled Double Carbon Coating. J. Phys. Chem. C 2015, 119, 15874-15881. (19) Yu, S.; Jung, J. W.; Kim, I. D. Single Layers of WS2 Nanoplates Embedded in Nitrogen-Doped Carbon Nanofibers as Anode Materials for Lithium-Ion Batteries. Nanoscale 2015, 7, 11945-11950. (20) Huang, J.; Wang, X.; Li, J.; Cao, L.; Xu, Z.; Hao, W. WS2-Super P Nanocomposites Anode Material with Enhanced Cycling Stability for Lithium Ion batteries. J. Alloys Compd. 2016, 673, 60-66. (21) Choi, S. H.; Boo, S. J.; Lee, J. H.; Kang, Y. C. Electrochemical Properties of Tungsten Sulfide-Carbon Composite Microspheres Prepared by Spray Pyrolysis. Sci. Rep. 2014, 4, 5755-5755. 26


Page 26 of 41

Page 27 of 41

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


(22) Zhang, G.; Zhu, J.; Zeng, W.; Hou, S.; Gong, F.; Li, F.; Li, C. C.; Duan, H. Tin Quantum Dots Embedded in Nitrogen-Doped Carbon Nanofibers as Excellent Anode for Lithium-Ion Batteries. Nano Energy 2014, 9, 61-70. (23) Yap, P. S.; Cheah, Y. L.; Srinivasan, M.; Lim, T. T. Bimodal N-Doped P25-TiO2/AC Composite: Preparation, Characterization, Physical Stability, and Synergistic Adsorptive-Solar Photocatalytic Removal of Sulfamethazine. Appl. Catal., A 2012, 427-428, 125-136. (24) Zhao, Q. N.; Li, C. L.; He, X.; Zhao, X. J. XPS Study of N-Doped TiOx Thin Films Prepared by DC Reactive Magnetron Sputtering. Key Eng. Mater. 2003, 249, 457-462. (25) Liu, Y.; Tang, N.; Wan, X.; Feng, Q.; Li, M.; Xu, Q.; Liu, F.; Du, Y. Realization of Ferromagnetic Graphene Oxide with High Magnetization by Doping Graphene Oxide with Nitrogen. Sci. Rep. 2013, 3, 2566-2566. (26) Jiang, Z. Z.; Wang, Z. B.; Chu, Y. Y.; Gu, D. M.; Yin, G. P. Ultrahigh Stability Carbon Riveted Pt/TiO2 Catalyst Prepared by in Situ Carbonized Glucose for Proton Exchange Membrane Fuel Cell. Energy Environ. Sci. 2011, 4, 2558-2566. (27) Yongchang, L.; Ning, Z.; Hongyan, K.; Minghui, S.; Lifang, J.; Jun, Chen. WS2 Nanowires as a High-Performance Anode for Sodium-Ion Batteries. Chem.-Eur. J. 2015, 21, 11878-11884. (28) Alphazan, T.; Bonduelle-Skrzypczak, A.; Legens, C.; Gay, A. S.; Boudene, Z.; Girleanu, M.; Ersen, O.; Coperet, C.; Raybaud, P. Highly Active Nonpromoted Hydrotreating Catalysts Through the Controlled Growth of a Supported 27



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

Hexagonal WS2 Phase. ACS Catal. 2014, 4, 4320-4331. (29) Zhang, B.; Yu, Y.; Xu, Z. L.; Abouali, S.; Akbari, M.; He, Y. B.; Kang, F.; Kim, J. K. Correlation Between Atomic Structure and Electrochemical Performance of Anodes Made from Electrospun Carbon Nanofiber Films. Adv. Energy Mater. 2014, 4, 1634-1642. (30) Nan, D.; Huang, Z. H.; Lv, R.; Yang, L.; Wang, J.; Shen, W.; Lin, Y.; Yu, X.; Ye, L.; Sun, H.; Kang, F. Nitrogen-Enriched Electrospun Porous Carbon Nanofiber Networks as High-Performance Free-Standing Electrode Materials. J. Mater. Chem. A 2014, 2, 19678-19684. (31) Xu, Z.; Wang, H.; Li, Z.; Kohandehghan, A.; Ding, J.; Chen, J.; Cui, K.; Mitlin, D. Sulfur Refines MoO2 Distribution Enabling Improved Lithium Ion Battery Performance. J. Phys. Chem. C 2014, 118, 18387-18396. (32) Gachot, G.; Grugeon, S.; Armand, M.; Pilard, S.; Guenot, P.; Tarascon, J. M.; Laruelle, S. Deciphering The Multi-Step Degradation Mechanisms of Carbonate-Based Electrolyte in Li Batteries. J. Power Sources 2008, 178, 409-421. (33) Zhu, C.; Kopold, P.; Li, W.; Aken, P. A.; Maier, J.; Yu, Y. Engineering Nanostructured Electrode Materials for High Performance Sodium Ion Batteries: A Case Study of a 3D Porous Interconnected WS2/C Nanocomposite. J. Mater. Chem. A 2015, 3, 20487-20493. (34) Ko, Y. N.; Choi, S. H.; Park, S. B.; Kang, Y. C. Hierarchical MoSe2 Yolk-Shell Microspheres with Superior Na-ion Storage Properties. Nanoscale 2014, 6, 28


Page 28 of 41

Page 29 of 41

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


10511-10515. (35) Ko, Y. N.; Kang, Y. C.; Park, S. B. Superior Electrochemical Properties of MoS2 Powders with a MoS2@void@MoS2 Configuration. Nanoscale 2014, 6, 4508-4512. (36) Maier, J. Size Effects on Mass Transport and Storage in Lithium Batteries. J. Power Sources 2007, 174, 569-574. (37) Jamnik, J.; Maier, J. Nanocrystallinity Effects in Lithium Battery Materials. Phys. Chem. Chem. Phys. 2003, 5, 5215-5220. (38) Zhu, C.; Mu, X.; Aken, P. A. V.; Yu, Y.; Maier, J. Single-Layered Ultrasmall Nanoplates of MoS2 Embedded in Carbon Nanofibers with Excellent Electrochemical Performance for Lithium and Sodium Storage. Angew. Chem., Int. Ed. 2014, 53, 2152-2156. (39) Dai, H. Q.; Zhou, Y. N.; Qian, S.; Fang, L.; Fu, Z. W. Enhanced Electrochemical Properties of NiO-NiS Nanocomposite Thin Film. Electrochim. Acta 2012, 76, 145-151. (40) Xu, X.D.; Liu, W.; Kim, Y.; Cho, J. Nanostructured Transition Metal Sulfides for Lithium Ion Batteries: Progress and Challenges. Nano Today 2014, 9, 604-630. (41) Chen, J.; Yang, L.; Fang, S.; Hirano, S. I.; Tachibana, K. Synthesis of Hierarchical Mesoporous Nest-Like Li4Ti5O12 for High-Rate Lithium Ion Batteries. J. Power Sources 2012, 200, 59-66. (42) Zhang, G.; Zhu, J.; Zeng, W.; Hou, S.; Gong, F.; Li, F.; Li, C. C.; Duan, H. Tin Quantum Dots Embedded in Nitrogen-Doped Carbon Nanofibers as Excellent 29



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

Page 30 of 41

Anode for Lithium-Ion Batteries. Nano Energy 2014, 9, 61-70. (43) Hou, X.; Jiang, H.; Hu, Y.; Li, Y.; Huo, J.; Li, C. In Situ Deposition of Hierarchical Architecture Assembly From Sn-Filled CNTs for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2013, 14, 6672-6677. (44) Wan, N.; Lu, X.; Wang, Y.; Zhang, W.; Bai, Y.; Hu, Y. S.; Dai, S. Improved Li Storage Performance in SnO2 Nanocrystals by a Synergetic Doping. Sci. Rep. 2016, 6, 1-11. (45) Fan, Q.;

Zhang, W.;

Duan, J.;

Hong, K.;

Xue, L.;

Huang. Y. Effects of

Binders on Electrochemical Performance of Nitrogen-Doped Carbon Nanotube Anode in Sodium-Ion Battery. Electrochim. Acta 2015, 174, 970-977. (46) Hirasawa, K.A.; Nishioka, K.; Sato, T.; Yamaguchi, S.; Mori, S. Investigation of Graphite Composite Anode Surfaces by Atomic Force Microscopy and Related Techniques. J. Power Sources 1997, 69, 97-102. (47) Li, J.; Le, D. B.; Ferguson, P. P.; Dahn, J.R. Lithium Polyacrylate as a Binder for Tin-Cobalt-Carbon Negative Electrodes in Lithium-Ion Batteries. Electrochim. Acta 2010, 55, 2991-2995.

30


Page 31 of 41

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 1. XRD patterns (a) and TG curves (b) of the WS2-NC, the WS2-C nanocomposites and the bare WS2 nanosheets. 80x30mm (300 x 300 DPI)



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. SEM images of WS2-NC (a), WS2-C (c) nanocomposites and the bare WS2 (d), EDS spectrum (b) and EDS element mapping images of WS2-NC nanocomposites (e-i). 160x57mm (300 x 300 DPI)


Page 32 of 41

Page 33 of 41

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 3. TEM and HRTEM images of the WS2-NC (a, b), WS2-C (c, d) nanocomposites and the bare WS2 (e, f). 80x95mm (300 x 300 DPI)



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. XPS spectra: broad scan spectra (a, b, c) and N 1s (d), C 1s (e), W 4f (f) and S 2p (g) of the WS2NC, WS2-C and WS2 samples. 119x98mm (300 x 300 DPI)


Page 34 of 41

Page 35 of 41

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 5. Electrochemical performances of the three samples (WS2-NC, WS2-C, WS2): cycling performances at a current density of 100 mA g-1 (a), 1 A g-1 (c) and rate performances (b); selected galvanostatic discharge/charge profiles of WS2-NC at a current density of 100 mA g-1 (d); cyclic voltammetry curves of WS2-NC (e), WS2-C (f) scanned in the voltage range of 0.01-3.00 V (d). 119x60mm (300 x 300 DPI)



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. Discharge/charge profile for the first cycle of the three electrodes at a current density of 100 mA g-1 (a), schematic diagram of the insertion, conversion, and interfacial reaction of Na+ with WS2 during discharge process (b). 33x14mm (300 x 300 DPI)


Page 36 of 41

Page 37 of 41

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 7. Electrochemical impedance spectra (EIS) of WS2-NC (a), WS2-C (b) and WS2 electrodes (c) after the 0th, 1st, 3rd and 100th cycles, Equivalent circuit for electrodes (d). 80x62mm (300 x 300 DPI)



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 8. SEM, TEM and HRTEM images of the WS2-NC electrode after 50 cycles (a, e, and i), after 100 cycles (b, f, and j); WS2-C electrode (c, g, and k) and WS2 electrode (d, h, and l) after 100 cycles, respectively. 160x101mm (300 x 300 DPI)


Page 38 of 41

Page 39 of 41

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 9. The comparison of charge transfer resistance after 1st, 100th cycle and the particle size after 100th cycle of the three samples. 60x46mm (300 x 300 DPI)



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 10. Schematic illustration for the mechanism of involving nitrogen into carbon with WS2 in the WS2NC anode for application in SIBs. 99x74mm (300 x 300 DPI)


Page 40 of 41

Page 41 of 41

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


Table 1. The as-obtained three samples capacities of the different reaction stages during first discharge process. 150x59mm (300 x 300 DPI)


Improved Na Storage Performance with the Involvement of Nitrogen

Recommend Documents