Semimetallic 1Tâ² WTe2 Nanorods as Anode Material for the Sodium

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Batteries and Energy Storage

Semimetallic 1T’-WTe2 nanorods as anode material for sodium ion battery Meiling Hong, Jie Li, Wenfeng Zhang, Shantang Liu, and Haixin Chang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00454 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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Semimetallic 1T’-WTe2 nanorods as anode material for sodium ion battery MeilingHong,†,‡ Jie Li, ‡ Wenfeng Zhang, ‡ Shantang Liu, †,* Haixin Chang‡,*

†

College of Chemistry and Environmental Engineering, Wuhan Institute of

Technology, Wuhan 430073, China. ‡

Center for Joining and Electronic Packaging, State Key Laboratory of Material

Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.

* Email: [email protected], [email protected].

Abstract: Highly crystalline semimetallic 1T’-WTe2nanorods (WTe2 NRs) and WTe2nanoflowers (WTe2 NFs) are applied as the anode materials for sodium ion battery (SIB) for the first time. WTe2 NRs and NFs are synthesized through a novel two-step process with hydrothermal derived WO3 transformed into WTe2 NRs and NFs after a chemical vapor deposition process. The performance of WTe2 SIB anode is highly influenced by WTe2 morphology. WTe2 NRs have shown high capacity in sodium ions storage with excellent rate and cycling stability. The initial discharge capacity for WTe2 NRs is 442 mA h g-1 at the current density of 0.1 A g-1, and remains 221 mA h g-1 after 100 cycles, while WTe2 NFs show 324 mA h g-1 initial capacity and remain 260 mA h g-1 after 40 cycles. The coulombic efficiency of both WTe2 NRs and NFs anodes are as high as 98.83% and 97.96% from the second cycle, respectively.

1. Introduction Compared to lithium ion battery (LIB), sodium ion battery SIB is receiving

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more and more attention nowadays owing to its outstanding electrochemical performance,1-8 However, it has not been applied in commercialized energy storage area due to the lack of suitable materials and skillful techniques for Na+ storage. Profited from adjacent positions in the periodic table, Na is extremely similar to Li in physical and chemical properties, as well as SIB, it resembles closely to LIB in structure, consists, and mechanism.9,10,30 Na is a kind of light metal with theoretical capacity up to 1165 mA h g-1, and has an abundant reserves in the earth in the meanwhile, which is nearly 1000 times greater than lithium. Therefore, SIB is supposed to be the most promising substitute for LIB which is at a period of stagnation owing to the lithium decay gradually. However, It is an enormous challenge to search suitable and stable materials for sodium storage due to larger radius,Na+ (1.13Å) is a little bigger than Li+ (0.76Å), leading to the poor performance in both cycling and stability.For the purpose of settling these kinds of problems, many materials have been applied in SIB.11-21 Carbon-based materials are used asthe most commonly anode materials,16,

22-28

the capacity such as graphite up to 360 mA h g-1, which is

nearly approaching tothe theoretical capacity. Carbon nano-materials for instance, carbonnanotube20 and nanowire,25 exhibit a better electrochemical performance. Wang has synthesized a variety of N-doped porous carbon material, it exhibits a high capacity up to 349 mA h g-1 at a current density of 50 mA g-1.28 Titanium-based materials also demonstrate a considerable sodium anode.29 TiO2 with three-dimensional structure produces a reversible capacity over 150 mA h g-1. Titanates such as NaTi3O731and NaTi6PO1332 have already been applied as electrode. Senguttuvan et. al discovered that the NaTi2(PO4)3 electrode exhibits a long and stable plateau located at 2.1V, and provides a capacity which is approaching to the theoretical value of 133 mA h g-1.33

Metal materials are prospective electrode for SIB,34-45,64 and could provide higher capacity by the way of forming alloy with a large amount of sodium.36-41 Numbers of metal elements have been studied so far. Many transition metals and other elements or


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compounds have been applied as SIB anode materials14,43,60-62. Tin-based materials generate theoretical capacity up to 660 mA h g-1,9,13,35,37,40,52 while Sn combines 15/4 of Na to form Na15Sn4 alloy. Kamaba reported sodium storage property of Sn electrode at the earliest, it generated capacity of 500 mA h g-1, and cycled for 20 cycle.9 Nevertheless, metal electrodes are not doing well in cycling and stability due to the swinging electrode structure damage caused by mechanical stress while alloying/de-alloying process. For the sake of overcoming the disadvantage of tremendous expanding during reaction process, a lot of efforts have been donesuch as electrolyte addictive. The most common sodium-ion battery electrolyte formulations are NaPF6 or NaClO4 as salts and carbonates, particularly propylene carbonate (PC) is a solvent, however, the metal sodium anode is constantly corroded in the presence of the most commonly used organic electrolyte, and affecting the final electrochemical performance. Therefore, the additive of the electrolyte is particularly important. Recently, it has been observed that fluoroethylene carbonate (FEC) can form a passivation film on carbon, so that the metal sodium anode is more stable, and the electrode can be further react with solvent.56 Other additives such as vinylene carbonate (VC) have also been used. The electrolyte of the sodium-ion battery is a polar organic solvent system of carbonates. Therefore, the binder must be stable enough in a complex electrochemical environment. Water-based binder such as PVA and PTFE are insufficient to meet the requirements. Ethylene rubber and methyl cellulose sodium are currently the most widely used.57 New SIB electrode materials such as transition alloy materials have been intensely explored recently. Antimony-based materials, for instance, AlSb58 and Mo3Sb759 provide a capacity of around 400 mA h g-1. Tin-based materials, such as Sn-Cu,37-39 Sn-Sb40,41,50 draw a lot of attention. Xiao synthesized Sn-Sb via a simple high energy ball milling way. It exhibited higher capacity of 540 mA h g-1 and better cycling performance which remained 500 mA h g-1 after 100 cycle.40, 41 However, alloy SIB electrodes still exist big problem in cycling, stability and rate performance.

Herein, we report a kind of new transition alloy materials as SIB anode for the


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first time. Semimetallic, 1T’ phase WTe2 nanocrystals with two kinds of morphologies, nanorods and nanoflowers, have been synthesized via a novel two step process: a facile hydrothermal treatment to obtain two different precursors of WO3 followed by a chemical vapor deposition process to convert WO3 into WTe2. The sodium storage abilities including capacity, cycling, and rate performance of WTe2 with two kinds of different morphologies are discussed in this study. WTe2 NRs show high capacity, excellent rate and cycling performance. The initial discharge capacity for WTe2 NRs is 442 mA h g-1 at the current density of 0.1 A g-1, and remains 221 mA h g-1 after 100 cycles. While WTe2 NFs show 324 mA h g-1and remain 260 mA h g-1 after 40 cycles. The coulombic efficiencies of both WTe2 NRs and NFs electrodes are 98.83% and 97.96% from the second cycle, respectively.

2. Experimental section 2.1 Preparation of materials Preparation of WO3 NRs precursors The precursors of WO3 NRs were prepared through a facile hydrothermal treatment. Firstly, 1.5 g ammonium metatungstate hydrate (AMT) was dissolved in 60 mL deionized water uniformly, and then 1 g thiourea was added into the aqueous solution. After sonication for 10 minutes, the mixture solution was transferred into a 100 mL Taflon-lined stainless autoclave and heated to 180℃ for 12 h. Blue products w ere collected after w ashed w ith deionized w ater and anhydrous ethanol for several times after the autoclave cooled down to room temperature. Finally, the materials were dried at 60℃ overnight.

Preparation of WO3 NFs presursors The WO3 NFs were synthesized through a hydrothermal treatment as well. Firstly, 125 mg tungsten hexachloride was dissolved in 10 mL anhydrous ethanol. After sonication for 10 min, the mixture solution was transferred into a 50 mL Taflon-lined stainless autoclave and heated to 180℃ for 12 h. L ight blue


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powder were collected and washed with deionized water and anhydrous ethanol for several times after after autoclave was cooled down to room temperature. Finally, the materials were dried at 60℃ overnight.

Preparation of WTe2 The products of WTe2 NRs and NFs were prepared through a similar chemical vapor deposition process. Typically, 0.6 g tellurium powder and 0.1 g as-prepared WO3 powder were placed at the ends of a quartz boat separately. The quartz boat was put inside the center of the tube furnace and vacuumed the tube subsequently. After that, the furnace was heated to 650℃ at a heating rate of 5℃/min and kept for 1 h under a N 2/H2 flowing gas (N2=15 sccm, H2=20 sccm). Finally, collected the products after the furnace cooled down to room temperature naturally.

2.2 Electrochemical measurements The electrochemical performances of WTe2 with different morphologies were tested via the 2032-type coin cells assembled and sealed in an argon filled glove box (99% coulombic efficiency, indicating the electrodes possess excellent reversibility during discharge/charging. Compared with WTe2 NFs, the charge and discharge plateau of WTe2 NRs is much longer, and the capacity is larger, indicating that the Na+ storage capacity is stronger. This may result from their different morphology. The particle size of WTe2 NRs is much smaller than that of WTe2 NFs, and has more interface contacts with the electrolyte than WTe2 NFs. In addition, WTe2 NRs can provide more reactive active sites and is beneficial to increase the energy storage capacity of the electrode.

The cycling performances of two kinds of WTe2nanostrucures measured at a current density of 0.1 A g-1 is shown in Fig. 5e. As the number of cycles increased, the capacity of WTe2 NRs slowly decreased, and the reversible specific capacity remained as high as 221 mA h g-1 after 100 cycles, which is approaching to its theoretical capacity (theoretical capacity: 244mA h g-1). However, the cycling performance of the WTe2 NFs electrode is much worse. The capacity remains 260 and 55 mA h g-1 after 40 and 100 cycles, respectively. Significant capacity decrease in WTe2 NFs was observed after the 40th cycle, while the same phenomenon did not appear on the WTe2 NRs electrode. This may be due to the fact that structure of the WTe2 NFs electrode material is irreversibly damaged during the cycling tests. This destructive force comes from many sources, including Na+ intercalation/de-intercalation and alloying/dealloying reactions. Benefiting from finite-size effect, WTe2 NRs are subjected to mechanical stress-induced volume expansion far smaller than the WTe2 NFs electrodes. Therefore, the WTe2 NRs can still maintain a capacity close to the theoretical specific capacity after 100 cycles, which is far superior to WTe2 NFs in


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cycle performance.51-53 The performance comparison of alloy anode elements and compounds

14,43,60-62

applied in SIB have been provided in Table S1 (see supporting

information). The performance is comparable even better than recently reported MoTe2 SIB anode. Materials based on sodium alloying/de-alloying mechanism mostly need to enhance their electrochemical properties by forming composites with carbon materials like graphene as a buffer to relieve volume expansion. However, WTe2 do not need carbon modifications for its semimetallic properties.

As shown in Fig. 5f, the rate performances of WTe2 NRs and WTe2 NFs are measured with the current density increasing from 0.1, 0.2, 0.4, 0.6, 0.8, to 1 A g-1 and returned to 0.1 A g-1. The corresponding reversible capacity were 313, 274, 250, 227, 196, 143, 217 mA h g-1 for WTe2 NRs and 258, 221, 186, 144, 120, 78, 191 mA h g-1 for WTe2 NFs. It shows a great rate performance even at a high current density of 1 A g-1, which is approximately corresponding to 4C, considering the theoretical capacity of WTe2 were 244 mA h g-1 according to the Faraday’s Law. The higher capacity than theoretical capacity due to the interfacial and defective storage of sodium ions as shown in many layered transition metal dichalcogenides (TMDs) nanostructures. Although capacity of WTe2 NRs and NFs decrease with the increasing of current density, the capacity increase from 143, 78 mA h g-1 to 217, 191 mA h g-1, respectively, when the current density decrease to 0.1 A g-1. The WTe2 NRs also have a better performance in rate ability compared to WTe2 NFs. It demonstrates the superiority of WTe2 NRs in SIB.

The EIS spectra of WTe2 NRs were measured to investigate electrochemical process further. As shown in Fig. 6a-c, the Nyquist plots of cells before cycling, after 1 cycle and 70 cycles are obtained through deconvolution with a Randle-type equivalent-circuit model, which includes three electrochemical reaction steps: Na ion pass through the SEI layer, electron transfer reaction, and Na ion diffusion through the active material. It is observed that the solution


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resistance (RS) is 3.68 (before cycling), 3.46 (1 cycle) and 3.29 (70 cycles), and the charge-transfer resistance (Rct) derived from semicircle in middle frequency change slightly from 32.15，41.06, to 85.1 Ω before cycling, after 1 cycle and 70 cycles, respectively. The slight change in Rct further confirms the better performance of WTe2 NRs in SIB electrode.

4. Conclusion In this study, a new kind of anode alloy material for sodium ion battery have been synthesized through a novel two-step method. The performance of WTe2 SIB anode are highly influenced by WTe2 morphology. The WTe2 NRs have shown better electrochemical performances in sodium ions storage than NFs. The initial discharge capacity is 442 mA h g-1 at the current density of 0.1 A g-1, and remains 221 mA h g-1 after 100 cycles, while WTe2 NFs show 324 mA h g-1 initial capacity and remain 260 mA h g-1 after 40 cycles. The coulombic efficiency of both WTe2 NRs and NFs anodes are as high as 98.83% and 97.96% from the second cycle, respectively.

Acknowledgements. The authors would like to acknowledge support from the National Basic Research Program of China (No. 2015CB258400), National Key Research and Development Program of China (No. 2016YFB070070-2), National Natural Science Foundation of China (grant nos.51402118, 51502101, 61674063), and the Fundamental Research Funds for the Central Universities in Huazhong University of Science and Technology (grant no. 2015QN006). AFM, XRD, Raman, SEM, TEM and XPS tests from Analytical Center of Huazhong University are acknowledged.

Supporting Information: supporting Figures S1-S2 for additional SEM and XRD.


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Fig. 1 SEM of (a) WTe2 NRs, (b) WTe2 NFs. Elemental mapping of (c) WTe2 NRs, (d) WTe2 NFs.


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Fig. 2 TEM and HR-TEM of (a,b) WTe2 NRs, (c,d) WTe2 NFs. Inset in (b,d): SAED patterns for WTe2 NRs and NFs.

10nm

1 um


b

021 103 113 024 122 105 124 131 008 200

Intensity (a.u.)

002

WTe2 nanorods

WTe2 nanoflowers

004 021 103 113 024 122 105 124 131 008 200

a Intensity (a.u.)

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 & Fuels

002

Page 17 of 21

PDF Card: 24-1352

10

20

30

40 50 60 2θ/degree

70

80

PDF Card: 24-1352

10

20

30

40 50 60 2θ/degree

Fig. 3 XRD patterns of (a) WTe2 NRs, (b) WTe2 NFs.


70

80

Energy & Fuels

a

W 4d3/2

b

W 4d5/2

Te 3d5/2

Intensity (a.u.)

Te 3d3/2

Intensity (a.u.)

c

588

270 265 260 255 250 245 240 235 Binding Energy (eV)

584 580 576 Binding Energy (eV)

572

A7 1

Intensity (a.u.)

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 18 of 21

A3 1A4 1

100

A9 1

200 300 Raman shift (cm-1)

400

Fig. 4 (a,b) XPS spectra of Te 3d and W 4d in WTe2 NRs. (c) Raman spectrum of WTe2 NRs with 1T’ phase.


Page 19 of 21

a 0.6

0.5 1.0 1.5 2.0 2.5 Potential (V vs Na+/Na)

1.8

2st

e 400 300 200

Current (mA)

5st

100 200 300 400 Capacity (mA h g-1) WTe2 nanorods

WTe2 nanoflowers

120 100 80 60 40

100 0

2st 3st 4st 5st

-0.4

d 3.0

4st

0.6

1st

-0.2

-0.8 0.0

3st

1.2

0.0

-0.6

3.0

1st

0.2

Potential (V vs Na+/Na)

Potential (V vs Na+/Na)

1st 2st 3st 4st 5st

WTe2 nanorods

0

WTe2 nanoflowers

0.4

2.4

0.0

b 0.6

WTe2 nanorods

20 0 0 10 20 30 40 50 60 70 80 90 100 Cycle Number

f

Coulombic efficiency (%) Specific Capacity(mAh g-1)

Current (mA)

0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 0.0

c 3.0

Capacity (mA h g-1)

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 & Fuels

0.6

1.2

1.8 2.4 + Potential (V vs Na /Na)

3.0

WTe2 nanoflowers

2.4

1st

1.8

2st 3st

1.2

4st

0.6 0.0

5st

0

100 200 300 Capacity (mA h g-1)

500

WTe2 nanoflowers WTe2 nanorods

400 300 200

0.1

0.2 0.4

100 0

0.6 Unit: A g-1

0

0.1 0.8 1

10 20 30 40 50 60 70 Cycle Number

Fig. 5 CV curves, Initial discharge-charge profiles, cycle and rate performances for WTe2 NRs (a,c,e,f) and WTe2 NFs (b,d,e,f). Scan rate in (a,b): 0.01 mV s-1. Current density in (c,d,f): 0.1 A g-1.


Energy & Fuels

WTe2 nanorods

b500

0 0

-Z''(ohm)

100

c150

WTe2 nanorods

400

300 200

WTe2 nanorods

50 100 150 200 250 300 Z'(ohm)

300

-Z''(ohm)

a 400 -Z''(ohm)

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

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200 100 0 0

100 200 Z'(ohm)

300

100 50 0 0

100 200 Z'(ohm)

300

400

d

Fig. 6 Nyquist impedance plots of WTe2 NRs: before cycling (a), after 1 cycle (b) , and after 70 cycles (c), and the equivalent circuit of WTe2 NRs (d).


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Energy & Fuels

TOC Figure


Semimetallic 1Tâ² WTe2 Nanorods as Anode Material for the Sodium

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