N-doped carbon composite as anodes for lithium-ion batteries

N-doped carbon composite as anodes for lithium-ion batteries

Electrochimica Acta 206 (2016) 328–336 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

3MB Sizes 0 Downloads 72 Views

Electrochimica Acta 206 (2016) 328–336

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Highly stable SiOx/multiwall carbon nanotube/N-doped carbon composite as anodes for lithium-ion batteries Yurong Rena , Ximin Wub , Mingqi Lib,* a b

School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637009, China

A R T I C L E I N F O

Article history: Received 11 January 2016 Received in revised form 26 April 2016 Accepted 26 April 2016 Available online 27 April 2016 Keywords: Lithium-ion battery Nonstoichiometric silicon oxide Multiwall carbon nanotube N-doped carbon Anode Electrochemical performance

A B S T R A C T

Fabricating high-capacity electrode materials with long cycle life is essential to developing high-power energy storage and conversion systems. SiOx is a very attractive anode material for lithium-ion batteries, but both low electronic conductivity and volume effect severely hamper its practical application. In this work, multiwall carbon nanotube (MWCNT) and N-doped carbon are combined to improve the electrochemical properties of SiOx. The synthesized composite (labeled as SiOx/MWCNT/N-doped C) has a network structure, in which MWCNT serves as a highly conductive and porous scaffold facilitating electron and ion transport, while N-doped C improves electric contact between SiOx/MWCNT particles and prevents the physical and electrochemical agglomeration of SiOx. The electrochemical measurement shows that the SiOx/MWCNT/N-doped C exhibits excellent cyclic stability and rate capability. At a current density of 100 mA g1, a stable discharge capacity of about 620 mAh g1 is achieved and the capacity can be preserved up to 450 cycles. The enhanced conductivity and stable electrode structure should be responsible for the excellent electrochemical performance. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction The rapid development of both electrical vehicles and largescale energy storage systems has created great demands for lithium-ion batteries with high energy density and long cycle life [1]. Since the electrode materials play predominant part in deciding the energy density of lithium-ion batteries, developing high-capacity electrode materials has been pursued [2,3]. Silicon is regarded as the most ideal anode material for lithium-ion batteries because of a high specific capacity of about 3600 mAh g1 at room temperature, satisfying lithium insertion/extraction potentials and abundant resources [4]. However, the exceptional lithium-storage capability is accompanied by a huge volume variation upon cycling, which leads to structural instability of the electrode and becomes the biggest challenge for the application of Si-based materials [5,6]. In recent two decades, although some important progress has been achieved, the large-scale application of siliconbased materials in lithium-ion batteries still faces many challenges [7–10]. Especially when a high mass loading of active materials is used per unit area, it is still difficult to achieve stable cycle performance in prolonged cycling. Compared with pure silicon,

* Corresponding author. Tel.:+ +86 1811393086. E-mail address: [email protected] (M. Li). http://dx.doi.org/10.1016/j.electacta.2016.04.161 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.

silicon oxides not only have a relatively small volume change, but also Li2O and silicate salts formed during the initial lithiation severe as the dispersion and buffer media of silicon in subsequent cycles [11–13]. Overall, silicon oxide electrodes have better cyclic stability than the elemental silicon electrodes. Other than commercial SiO, nonstoichiometric SiOx (x>1) is synthesized readily by simple liquid phase chemical method, which is costeffective and offers much more choices to improve its electrochemical performance. In the recent several years, SiOx has attracted the researchers’ attention and exhibits promising prospect [14–20]. For instance, Guo used cellulosic substances as template to prepare SiOx nanotubular materials [21]. The SiOx nanotube exhibited a reversible capacity of 940 mAh g1 and the capacity retention kept over 91.5% after 50 cycles. SiOx/C composite synthesized by Zhao’ group via a template assisted hydrothermal route and a carbon-coating process demonstrated a specific capacity of 780 mA h g1 with a 0.02% decay per cycle [20]. Lately, we fabricated graphene nanoplatelets-supported SiOx composite, which showed a stable reversible capacity of about 630 mAh g1 and the capacity retention could be kept up to 250 cycles [22]. However, the cyclic stability of these materials is still far from the requirement of practical application and thus more advanced material structural design is needed. In this work, networkstructural SiOx/MWCNT/N-doped C ternary composite is

Y. Ren et al. / Electrochimica Acta 206 (2016) 328–336

329

effectively improve the conductivity of the electrode and buffer the volume change of SiOx to a large extent. Considering the interface contact between SiOx/MWCNT particles is not good enough because of low conductive SiOx on the surface of MWCNT and the exposed SiOx nanoparticles tend to agglomeration during annealing and cycling, high conductive N-doped carbon is used to coat SiOx/MWCNT. The electrochemical measurement shows that such structural composite exhibits prominent cycle stability with a high reversible capacity.

(a) SiOx/MWCNT/N-doped C

2. Experimental

SiOx/MWCNT Intensity/A.U.

2.1. Material preparation

SiOx

MWCNT 10

20

30

40

50

60

70

80

90

2Theta/Degree

(b)

SiOx/MWCNT/N-doped C

SiOx/MWCNT

474 804

1641 Transmittance Rate/A.U.

2.2. Physical characterization

1091 1384

SiOx

1091

1629

3438

804 1114

474

MWCNT

1697

3382

A certain amount of MWCNT (US Research Nanomaterials, Inc.) was dispersed in a solution composed of 0.2 g of cetyltrimethylammonium bromide (CTAB), 25 mL of ethanol and 58 mL of distilled water via ultrasonication. Next, 2.5 mL of ammonium hydroxide was added and the solution was stirred at room temperature for 15 min. Subsequently, 1.8 mL of (C2H5O)3SiC2H5 was added dropwise to the above dispersion and stirring was continued until the solution completely became clear. The precipitate was collected by filtration, washed with distilled water and dried at 100  C in vacuum for 8 h. Then, 0.8 g of the above dried sample was added into a polyacrylonitrile (PAN) solution, which was composed of 0.15 g of PAN (Sigma) and 9 ml of Nmethylpyrrolidinone (NMP), and stirred for 0.5 h. Subsequently, distilled water was rapidly poured into the above dispersion under vigorous stirring to obtain PAN-coated product. Finally, the resultant product was thermally treated in a tube furnace at 1000  C with an argon flow of 100 mL min1 for 3 h, and then the furnace was naturally cooled to room temperature. The final product was labeled as SiOx/MWCNT/N-doped C. For comparison, SiOx and SiOx/MWCNT were also synthesized by similar method. PAN-C was prepared by directly pyrolyzing polyacrylonitrile polymer at 1000  C under the protection of argon.

1140

X-ray diffraction (XRD) patterns were recorded using D8 Discover (Bruker) equipped with Cu Ka (l = 0.15406 nm) radiation at a scan rate of 5.0 min1 from 10 to 90 . The carbon mass fraction in the samples was determined by elemental analysis (Elementar analyzer, Elementar Americas INC). Scanning electron microscopy (SEM, Leo-1530, Zeiss), transmission electron microscopy (TEM, Philips CM12) and energy-dispersive x-ray spectroscopy (EDS) were conducted to investigate the morphology, microstructure and elemental distribution of the as-prepared composites. During TEM measurement, copper mesh was used as the supporter for the sample. Fourier-transform infrared reflection (FTIR) spectra were recorded on a vertex 70 FTIR spectrometer (Bruker). The specific surface area was determined by the Brunauer–Emmet–Teller method (BET, ASAP 2020, Micromeritics). 2.3. Electrochemical measurement

1558 3500

3000

2500

2000

1500

Wave Number/cm

1000

500

-1

Fig. 1. XRD patterns and FTIR spectra of the MWCNT, SiOx, SiOx/MWCNT and SiOx/ MWCNT/N-doped C.

fabricated. Herein, MWCNT is used as the scaffold of anchoring SiOx. Because of high conductivity, excellent flexibility and large specific area [23], the introduction of MWCNT is expected to

2032 coin cells were assembled to evaluate electrochemical performance of the as-prepared materials. Working electrodes were composed of 75 wt% of the as-prepared materials as active material, 10 wt% of acetylene black (AB) as conductive agent and 15 wt% of sodium alginate as binder. Cu foil was used as current collector. The electrode was dried at 105  C in vacuum for 8 h. The mass loading of active material was 1.3-1.4 mg cm2. A lithium disc was used as counter electrode as well as reference electrode. Electrolyte was 1 M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in weight) with

330

Y. Ren et al. / Electrochimica Acta 206 (2016) 328–336

2% vinylene carbonate (VC) and 5% fluoroethylene carbonate (FEC) as additives. Cells were cycled at different current densities over a voltage window of 0.0-3.0 V. Cyclic voltammetry (CV) was carried out at a scan rate of 0.2 mV s1 on an EC-Lab (Biologic Science Instruments). After cell charging ended and the cell was kept at an open circuit state for 1 h, EIS measurements were performed at the open circuit potential by applying a sine wave with an amplitude of 10 mV over a frequency range from 100 k to 0.01 Hz. 3. Results and discussion The SiOx/MWCNT/N-doped C is synthesized by self-assembly, in situ polymerization reactions and physical coating, followed by heat treatment. Firstly, CTA+ cations were adsorbed on MWCNT surface by intermolecular interaction. With the help of aqueous ammonia, negatively charged oligomeric silicate species produced by the hydrolysis of (C2H5O)3SiC2H5 were adsorbed on the surface of CTA+-coated MWCNT by electrostatic interaction [24,25]. The absorbed oligomeric alkyl silicate species polymerized each other to form alkyl silicate polymer/MWCNT composite. Since PAN is insoluble in water, it was precipitated from NMP solution to form PAN-covered alkyl silicate [email protected] composite with the addition of water. After annealing, organic components were decomposed or carbonized while silicate polymer was transformed into SiOx [26]. Fig. 1(a) shows the XRD patterns of the SiOx/MWCNT/N-doped C, SiOx/MWCNT, SiOx and raw MWCNT, respectively. Only one broad and weak diffraction peak appear in the XRD pattern of the SiOx, indicating an amorphous structure. The raw MWCNT shows two pronounced diffraction peaks at 2theta = 25.8 and 42.4 , respectively, indicating a crystalline structure. The XRD pattern of the SiOx/MWCNT is the overlapping of both SiOx and MWCNT, in which no new peaks are observed and the diffraction peak positions of MWCNT have no shift, suggesting SiOx is not reduced by MWCNT and does not also intercalate the crystal interlayers of MWCNT. After N-doped carbon coating, only a new small shoulder peak appears at 2theta = 21.6 , indicating the N-doped carbon is also amorphous. Fig. 1(b) is the FT-IR spectrum of the SiOx, MWCNT, SiOx/MWCNT and SiOx/MWCNT/N-doped C. As for the typical FTIR spectrum for the MWCNT, the characteristic peak at 1558 cm1 should be attributed to an in-plane mode E1u stretching vibration of carbon wall. The peak at 1140 cm1 corresponds to CO stretching vibration, while the peak at around 1697 cm1 is assigned to C¼O stretching vibration in both carboxyl group and carbonyl group. The broad and weak peak at 3348 cm1 is attributed to H  O H stretch vibration [27]. These results show that the raw MWCNT is oxidized partly. In the FTIR spectrum of the SiOx, 474 cm1 is for the Si–O rocking vibration, 804 cm1 for symmetric Si–O–Si stretching vibration, 1114 cm1 for asymmetric Si O Si stretching vibration [15], 1629 cm1 for H O bond bending vibration in absorbed H2O molecules and 3438 cm1 for O H stretch vibration in Si-O-H groups or absorbed H2O molecules [28]. After SiOx is anchored on the MWCNT, beside the asymmetry Si–O–Si bond stretching vibration peak shifts from 1114 to 1091 cm1, the peaks for MWCNT shift also toward low wave number direction, indicating there is a strong molecular interaction between MWCNT and SiOx. It is completely possible that the highly reactive functional groups in MWCNT such as carboxyl and alcohol can participate in polymerization reaction with the oligomeric silicate species under basic conditions. SEM and TEM are used to characterize the morphology and microstructure of the samples. From Fig. 2, the outside diameter of the raw MWCNT is about 10–20 nm and the length is on a micro scale. Other than the prepared SiOx which is some irregular bulks (Fig. 1s), the SiOx/MWCNT is network-structured aggregates, in which MWCNT is coated by pyrolyzed SiOx and some SiOx/MWCNT

Fig. 2. SEM images of the raw MWCNT (a), SiOx/MWCNT (b and c) and SiOx/ MWCNT/N-doped C (d and e) with different magnification; Elemental mappings of the SiOx/MWCNT/N-doped C (f–i); TEM images of the SiOx/MWCNT/N-doped C (j and k) with different magnification.

tubes are integrated by SiOx. The SiOx/MWCNT/N-doped C is also liable to form network-structured aggregates but the interface contact between neighbor tubes is relatively loose, which should be attributed to the presence of the PAN-C, preventing the physical agglomeration of SiOx to some degree during annealing. Elementary analysis shows that the weight fraction of C in the SiOx/ MWCNT is 34.8 wt%, while C and N in the SiOx/MWCNT/N-doped C account for 41.4 wt% and 1.7 wt%, respectively. N element comes from the PAN polymer, which has widely been used as the precursor of N-doped carbon. Considering the pyrolysis of both

Y. Ren et al. / Electrochimica Acta 206 (2016) 328–336

Fig. 2. (Continued)

CTAB and alkyl silicate polymer can yield little carbon, EA is also used to investigate the carbon content in the as-prepared SiOx and it is found that the value is about 4.8 wt%. Based on the above analysis results and the presumption that the yield ratio of SiOx and pyrolyzed carbon from the CTAB and the alkyl silicate polymer keeps constant, it can be calculated that the SiOx/MWCNT is composed of 31.5 wt% MWCNT, 3.3 wt% C and 65.2 wt% SiOx, while the SiOx/MWCNT/N-doped C is composed of 27.5 wt% MWCNT, 13.9 wt% C, 1.7 wt% N and 56.9 wt% SiOx. The EDS analysis shows that the atomic ratio of O/Si is about 1.54 in the SiOx/MWCNT/Ndoped C. Our previous study has demonstrated that the Si element in the SiOx is multi-valence, but no elemental silicon presents [26].

331

Elemental mapping images in Fig. 2(f–i) show that Si, O and N elementals have a relatively uniform distribution in the composite. The TEM images further confirm that the SiOx/MWCNT/N-doped C has a network structure and the outside surface of MWCNT is covered by amorphous substance. Moreover, the amorphous substance can also be observed in the inside hole of big MWCNT. Fig. 3(a) is the N2 absorption/desorption isotherms of the SiOx/ MWCNT/N-doped C at 77 K, which shows an obvious hysteresis loop, indicating the presence of mesopores [29]. After the relative pressure exceeds 0.8, the absorbed quantity increases sharply, suggesting the composite contains macropores. Calculated with its BET model, the specific surface area and total pore volume of the SiOx/MWCNT/N-doped C are about 44.6 m2 g1 and 0.24 cm3 g1, respectively. From Fig. 3(b), the pore size of the composite is concentrated at about 3.8 nm. Fig. 4(a) shows the CV curves of the SiOx/MWCNT/N-doped C for the first five consecutive cycles at a scan rate of 0.2 mV s1 in a voltage window of 3.0–0 V. The CV behavior is generally consistent with that reported previously in literature [17,30,31], suggesting the same electrochemical reaction pathway. Specifically, in the first cathodic scan, the peaks ranging from 1.75 to 1.0 V are associated with the irreversible decomposition of the electrolyte additive FEC and the formation process of the solid electrolyte interface (SEI) layer, while the peak between 0.9-0.5 V is ascribed to the reduction reactions of VC and EC [32]. The weak peak from 0.5 to 0.2 V mainly corresponds to the reduction of SiOx [26,33,34]. In the reduction process, beside elemental silicon, Li2O, Li2Si2O5 and Li4SiO4 are formed, among which silicon and Li2Si2O5 are active to lithium. The dominant cathodic peak nearly 0 V corresponds to the alloying processes of the reduced silicon and pyrolyzed carbon. The broad anodic peaks at 0.1-1.5 V correspond to the dealloying processes of C and Si alloys, while the anodic peak at 2.1 V is attributed to the lithium extraction process from Li2Si2O5. In subsequent cycles, with the activation of the electrode, the lithium insertion peak of silicon gradually shifts positively and a new cathodic peak appears at about 0.3 V. Other than elemental silicon electrodes, no sharp anodic peak appears in the CV curves. Such phenomenon is often observed in the glassy silicon oxide electrodes, which is ascribed to slow dynamics caused by the low conductive Li2O and silicate salts [31,33]. Fig. 4(b and c) show the charge-discharge profiles of the SiOx, SiOx/MWCNT and SiOx/MWCNT/N-doped C in the first two cycles. In the first cycle, the SiOx shows a discharge capacity of 998.7 mAh g1 with a coulombic efficiency of about 37.0%. The low coulombic efficiency should be attributed to the electrolyte decomposition and the irreversible reduction, low electronic conductivity and volume change of SiOx. As mentioned previously, SiOx experiences a reduction process during the first lithiation, in which some products are irreversible. Both drastic contraction and low electronic conductivity lead to the formation of died lithium because some particles are completely isolated. After SiOx is anchored on MWCNT, its electrochemical behavior is significantly improved. Calculated on the total weight of the SiOx/MWCNT, the composite delivers a discharge capacity of 1053 mAh g1 and a charge capacity of 638 mAh g1, with a coulombic efficiency of 60.6%. The electrochemical performance of the SiOx/MWCNT/Ndoped C is further enhanced. The composite presents a discharge capacity of 1093 mAh g1 and a charge capacity of 720 mAh g1, with a coulombic efficiency of about 66%. Based on the first reversible capacity of N-doped C (301 mAh g1) and MWCNT (179 mAh g1) in Fig. 2s and the aforementioned composition of the as-prepared composites, it can be calculated that the individual SiOx in the SiOx/MWCNT and SiOx/MWCNT/N-doped C contributes a first reversible capacity of 832 and 1096 mAh g1, respectively, which are significantly higher than that of the SiOx without MWCNT and N-dope carbon. Additionally, from the charge-

332

Y. Ren et al. / Electrochimica Acta 206 (2016) 328–336

Fig. 2. (Continued)

discharge profiles in Fig. 4(b and c), the SiOx electrode shows a maximum polarization degree, while the polarization degree of the SiOx/MWCNT/N-doped C electrode is minimized. These results indicate that the combination of MWCNT and N-doped carbon effectively improves the electrochemical performance of SiOx and enhances its utilizations. However, it should be noticed that although the initial coulombic efficiency of the SiOx/MWCNT/Ndoped C shows a significant increase compared with those of the SiOx and SiOx/MWCNT, it is still low, which should mainly be attributed to the large specific surface area of nano-material and the irreversible reduction of SiOx. Fig. 5(a and b) shows the cycle performance profiles of the SiOx, SiOx/MWCNT and SiOx/MWCNT/N-doped C at a current density of 100 mA g1. Although the SiOx delivers a first discharge capacity of 998.7 mAh g1, the electrode shows rapid capacity degradation in

subsequent cycles. After 100 cycles, the discharge capacity degrades to 243 mAh g1. Although the discharge capacity of both SiOx/MWCNT and SiOx/MWCNT/N-doped C also shows degradation tendency in the initial 10 cycles, they deliver much higher stable discharge capacity and better cyclic stability than the SiOx in subsequent cycles. The stable discharge capacity of the SiOx/ MWCNT is about 550 mAh g1 and can be kept up to the 272th cycle. The enhanced electrochemical performance of the SiOx/ MWCNT should be attributed to the function of MWCNT, which provides not only efficient electron wiring and a porous scaffold, facilitating electron and Li+ ion transportation, but also good buffer to volume change of active components [35]. Compared with the SiOx/MWCNT, the SiOx/MWCNT/N-doped C exhibits more prominent electrochemical performance. After 450 cycles, the discharge capacity is still as high as 621 mAh g1. Except for the first cycle, the

Y. Ren et al. / Electrochimica Acta 206 (2016) 328–336

333

80

(a)

Quantity Absobed/mmol g

-1

70 60 50

Absorption Desorption

40 30 20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure/P/P*

3 -1

BJH Desorption dV/dW/cm g nm

-1

0.012

(b) 0.010

0.008

0.006

0.004

0.002

0.000 0

10

20

30

40

50

60

Pore Width/nm Fig. 3. N2 absorption/desorption isotherms (a) and BJH analysis of the desorption isotherm (b) of the SiOx/MWCNT/N-doped C.

average coulombic efficiency is about 99.2% for the SiOx/MWCNT and 99.4% for SiOx/MWCNT/N-doped C. From Fig. 5(c and d), the rate capability of the SiOx/MWCNT/N-doped C is obviously superior to that of the SiOx/MWCNT. Although the two composite shows stable cyclic performance after about 10 cycles, the discharge capacity of the former is higher than that of the latter at different densities. At big current densities, the difference becomes more obvious. For instance, at 800 mA g1, the stable discharge capacity is about 388 mAh g1 for the SiOx/MWCNT/Ndoped C and 313 mAh g1 for the SiOx/MWCNT, respectively. These results indicate that N-doped carbon plays an important role in further improving the comprehensive electrochemical performance of the SiOx/MWCNT. In order to understand the reason for the improvement of the electrochemical performance of the as-prepared composites, EIS was carried out to analyze the resistance evolution of the electrodes during cycling. The high-frequency semicircle in EIS

stands for interface (SEI and charge transfer) impedance, while the low frequency line represents the ion diffusion resistance [36]. Fig. 6 shows the Nyquist plots of the SiOx, SiOx/MWCNT and SiOx/ [email protected] C electrodes before cycling and after different cycles. The equivalent circuit in Fig. 3s is used to fit these spectra and the corresponding results are listed in Table 1s. Before cycling, the high-frequency semicircle diameter is about 248.3 V cm2 for the SiOx, 170.8 V cm2 for the SiOx/MWCNT and 102.6 V cm2 for the SiOx/MWCNT/N-doped C. Moreover, the intercept of the semicircle of the SiOx/MWCNT/N-doped C electrode in high frequency is also the smallest among the three. These results suggest that the combination of MWCNT and N-doped C effectively improve the conductivity of SiOx. Although MWCNT has good conductivity, it can be foreseen that the interface contact between SiOx/MWCNT tubes/particles is still poor after low conductive SiOx is deposited on the surface of MWCNT, especially when the deposited SiOx layer is thick or the big SiOx aggregates appear. After high conductive

334

Y. Ren et al. / Electrochimica Acta 206 (2016) 328–336

0.5

1200

(a)

SiOx

1000

I/mA

anode branch 1st 2nd 3rd 4th 5th

0.3 V

-1.0

1st 2nd 3rd 4th 5th

-1.5

I/mA

0.3

SiOx/[email protected] C

Discharge Capacity/mAh g

0.4

-0.5

0.2

800 -1

Current : 100 mA g 600

400

200

0.1

0 0

0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

50

100

150

0.5

1.0

1.5

200

250

300

350

400

450

Cycle Number

+

E/V vs Li /Li

0.0

(a)

SiOx/MWCNT

-1

0.0

2.0

2.5

3.0

+

E/V vs Li /Li

3.0

Couclombic Efficiency/%

100

(b)

2.0

+

Potential/V vs Li /Li

2.5

90

(b) SiOx SiOx/MWCNT

80

SiOx/[email protected] C 70

SiOx

1.5

SiOx/MWCNT

60 0

SiOx/MWCNT/N-doped C

1.0

50

100

150

200

250

300

350

400

450

Cycle Number

0.5 1100

(c)

1000

200

400

600

800

Specific Capacity/mAh g

1000

1200

-1

3.0

(c)

2.5

Discharge Capacity/mAh g

0

-1

0.0

[email protected]@N-doped C

900

[email protected]

800 700

100 mA g

100 mA g

-1

200 mA g

600

-1

-1

400 mA g

-1

500

800 mA g

-1

400

200

+

0

10

20

30

40

50

60

70

80

90

100

Cycle Number

1.5

1.0

100

SiOx SiOx/MWCNT

0.5

0.0 0

200

400

Specific Capacity/mAh g

(d)

95

SiOx/MWCNT/N-doped C

600

800

-1

Fig. 4. CV curves of the SiOx/MWCNT/N-doped C for the first five consecutive cycles at a scan rate of 0.2 mV s1 (a) (The inset is the anode branch of CV curves); Chargedischarge profiles of the SiOx, SiOx/MWCNT and SiOx/MWCNT/N-doped C in the first (b) and second (c) cycles.

Coulombic Efficiency/100%

Potential/V vs Li /Li

300

2.0

[email protected]@N-doped C

90

[email protected]

85 80 75 70 65 60 0

20

40

60

80

100

Cycle Number

N-doped carbon is introduced, the poor electric contact is overcome. Additionally, N-doped C coating alleviates the physical aggregation of SiOx during annealing, which further facilitates the conductive improvement of SiOx. After the first cycle, the highfrequency semicircle diameter decreases to 130.3 V cm2 for the

Fig. 5. Cycle performance profiles of the SiOx, SiOx/MWCNT and SiOx/MWCNT/Ndoped C at a current density of 100 mA g1 (a and b); Rate capability of the SiOx/ MWCNT and SiOx/MWCNT/N-doped C (c and d).

Y. Ren et al. / Electrochimica Acta 206 (2016) 328–336

335

(a)

(b)

(c)

(d)

Fig. 6. EIS of the SiOx, SiOx/MWCNT and SiOx/MWCNT/N-doped C: before cycling (a) and after the first cycle (b), 100th cycle and 350th cycle.

SiOx, 91.3 V cm2 for the SiOx/MWCNT and 28.2 V cm2 for the SiOx/ MWCNT/N-doped C, respectively, due to activation. After 100 cycles, the high-frequency semicircle diameter of the SiOx increases to 168.4 V cm2, while the high-frequency semicircle diameters of the SiOx/MWCNT and SiOx/[email protected] C further decrease to 62.4 and 23.1 V cm2, respectively, indicating the structure of the SiOx has been destroyed. After 350 cycles, the resistance of the SiOx/MWCNT increases to 89.7 V cm2, while that of the SiOx/MWCNT/N-doped C electrode is still kept at about 32.3 V cm2, indicating the SiOx/MWCNT/N-doped C electrode has better structural stability than the SiOx/MWCNT electrode. The SEM images of the SiOx/MWCNT and SiOx/MWCNT/N-doped C electrodes after 350 cycles confirm the above speculation (Fig. 4s). The two electrodes show significant difference in morphology and microstructure. Some consecutive big and long gaps presence at the SiOx/MWCNT electrode due to the electrochemical agglomeration of materials, which leads to a low utilization of active materials and thus should be responsible for the capacity degradation. For the SiOx/MWCNT/N-doped C electrode, although a thick SEI layer is formed on the electrode surface, no consecutive crack is observed in the electrode, indicating N-doped carbon coating prevents the electrochemical aggregation of SiOx during cycling. Therefore, the significantly improved electrochemical performance of SiOx/MWCNT/N-doped C should be attributed to the following facts. Firstly, after anchored on the surface of MWCNT, SiOx is obviously improved in conductivity and high flexible MWCNT provides a good buffer for its volume change. Secondly, N-doped C coating not only improves the interface contact between SiOx/MWCNT tubes/particles, but also alleviates the physical and electrochemical agglomeration of SiOx during

annealing and cycling. Thirdly, nanostructured composite with pores largely shortens the transport pathway of ions and electronics. 4. Conclusions In summary, we successfully fabricate a network-structured SiOx/MWCNT/N-doped C anode composite for lithium-ion batteries. The SiOx/MWCNT/N-doped C composite exhibits excellent cyclic stability with a high capacity. After 450 deep charge– discharge cycles, the discharge capacity is still as high as 621 mAh g1. The composite also demonstrates good rate capability. The excellent electrochemical performance of the SiOx/ MWCNT/N-doped C should be attributed to the combination of MWCNT and N-doped carbon, which improves the conductivity of the electrode, provides a good buffer for the volume change of SiOx, shortens the transport pathway of ions and electronics and prevents the agglomeration of SiOx. Scalable synthesis, high reversible capacity and excellent cyclic stability suggest that the composite is promising to be a candidate anode material for highenergy lithium-ion batteries. The work not only offers a new routine to largely improve the electrochemical performance of SiOx, but also the proposed material designing concept is easily extended to the studies of other electrode materials. Acknowledgment This research was financially supported by Natural Science Foundation of China (51374175 and 21576030), Lithium-ion Battery Innovative Team Project of China West Normal University

336

Y. Ren et al. / Electrochimica Acta 206 (2016) 328–336

(CXTD2015-1) and Foundation of enterprises, universities and research institutes of Jiangsu Province (BY2014037-31). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2016.04.161. References [1] S. Habib, M. Kamran, U. Rashid, Impact analysis of vehicle-to-grid technology and charging strategies of electric vehicles on distribution networks—A review, J. Power Sources 277 (2015) 205–214. [2] P. Roy, S.K. Srivastava, Nanostructured anode materials for lithium ion batteries, J. Mater. Chem. A 3 (2015) 2454–2484. [3] S. Myung, K. Amine, Y. Sun, Nanostructured cathode materials for rechargeable lithium batteries, J. Power Sources 283 (2015) 219–236. [4] M.N. Obrovac, L.J. Krause, Reversible cycling of crystalline silicon powder, J. Electrochem. Soc. 154 (2007) A103–A108. [5] B. Liang, Y. Liu, Y. Xu, Silicon-based materials as high capacity anodes for next generation lithium ion batteries, J. Power Sources 267 (2014) 469–490. [6] U. Kasavajjula, C. Wang, A.J. Appleby, Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells, J. Power Sources 163 (2007) 1003– 1039. [7] X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B.W. Sheldon, J. Wu, Silicon-based nanomaterials for lithium-ion batteries: a review, Adv. Energy Mater. 4 (2014), doi:http://dx.doi.org/10.1002/aenm.201300882. [8] L. Shi, W. Wang, A. Wang, K. Yuan, Z. Jin, Y. Yang, Si nanoparticles adhering to a nitrogen-rich porous carbon framework and its application as a lithium-ion battery anode material, J. Mater. Chem. A 3 (2015) 18190–18197. [9] J. Lee, E. Lee, J. Park, S. Park, S. Lee, Ultrahigh-energy-density lithium-ion batteries based on a high-capacity anode and a high-voltage cathode with an electroconductive nanoparticle shell, Adv. Energy Mater. 4 (2014), doi:http:// dx.doi.org/10.1002/aenm.201301542. [10] Z. Zhang, Y. Wang, W. Ren, Q. Tan, Y. Chen, H. Li, Z. Zhong, F. Su, Scalable synthesis of interconnected porous silicon/carbon composites by the rochow reaction as high-performance anodes of lithium ion batteries, Angew. Chem. Int. Edit. 53 (2014) 5165–5169. [11] B. Yu, Y. Hwa, C. Park, H. Sohn, Reaction mechanism and enhancement of cyclability of SiO anodes by surface etching with NaOH for Li-ion batteries, J. Mater. Chem. A 1 (2013) 4820–4825. [12] C. Park, W. Choi, Y. Hwa, J. Kim, G. Jeong, H. Sohn, Characterizations and electrochemical behaviors of disproportionated SiO and its composite for rechargeable Li-ion batteries, J. Mater. Chem. 20 (2010) 4854–4860. [13] J. Lee, N. Choi, S. Park, Highly stable Si-based multicomponent anodes for practical use in lithium-ion batteries, Energ. Environ. Sci. 5 (2012) 7878–7882. [14] B. Guo, J. Shu, Z. Wang, H. Yang, L. Shi, Y. Liu, L. Chen, Electrochemical reduction of nano-SiO2 in hard carbon as anode material for lithium ion batteries, Electrochem. Commun. 10 (2008) 1876–1878. [15] P. Lv, H. Zhao, C. Gao, T. Zhang, X. Liu, Highly efficient and scalable synthesis of SiOx/C composite with core-shell nanostructure as high-performance anode material for lithium ion batteries, Electrochim. Acta 152 (2015) 345–351. [16] J. Wang, H. Zhao, J. He, C. Wang, J. Wang, Nano-sized SiOx/C composite anode for lithium ion batteries, J. Power Sources 196 (2011) 4811–4815. [17] Y. Yao, J. Zhang, L. Xue, T. Huang, A. Yu, Carbon-coated SiO2 nanoparticles as anode material for lithium ion batteries, J. Power Sources 196 (2011) 10240– 10243.

[18] H. Takezawa, K. Iwamoto, S. Ito, H. Yoshizawa, Electrochemical behaviors of nonstoichiometric silicon suboxides (SiOx) film prepared by reactive evaporation for lithium rechargeable batteries, J. Power Sources 244 (2013) 149–157. [19] M.K. Kim, B.Y. Jang, J.S. Lee, J.S. Kim, S. Nahm, Microstructures and electrochemical performances of nano-sized SiOx (1.18  x  1.83) as an anode material for a lithium(Li)-ion battery, J. Power Sources 244 (2013) 115–121. [20] C. Gao, H. Zhao, P. Lv, C. Wang, J. Wang, T. Zhang, Q. Xia, Superior cycling performance of SiOx/C composite with arrayed mesoporous architecture as anode material for lithium-ion batteries, J. Electrochem. Soc. 161 (2014) A2216–A2221. [21] H. Guo, R. Mao, X. Yang, J. Chen, Hollow nanotubular SiOx templated by cellulose fibers for lithium ion batteries, Electrochim. Acta 74 (2012) 271–274. [22] M. Li, Y. Yu, J. Li, B. Chen, A. Konarov, P. Chen, Fabrication of graphene nanoplatelets-supported SiOx-disordered carbon composite and its application in lithium-ion batteries, J. Power Sources 293 (2015) 976–982. [23] J. Liu, J. Ni, Y. Zhao, H. Wang, L. Gao, Grapecluster-like [email protected]/CNT nanostructures with stable Li-storage capability, J. Mater. Chem. A 1 (2013) 12879–12884. [24] K. Zhang, L. Xu, J. Jiang, N. Calin, K. Lam, S. Zhang, H. Wu, G. Wu, B. Albela, L. Bonneviot, P. Wu, Facile Large-scale synthesis of monodisperse mesoporous silica nanospheres with tunable pore structure, J. Am. Chem. Soc. 135 (2013) 2427–2430. [25] Z. Teng, G. Zheng, Y. Dou, W. Li, C. Mou, X. Zhang, A.M. Asiri, D. Zhao, Highly Ordered mesoporous silica films with perpendicular mesochannels by a simple Stöber-solution growth approach, Angew. Chem. Int. Edit. 51 (2012) 2173–2177. [26] M. Li, Y. Zeng, Y. Ren, C. Zeng, J. Gu, X. Feng, H. He, Fabrication and lithium storage performance of sugar apple-shaped [email protected] nanocomposite spheres, J. Power Sources 288 (2015) 53–61. [27] J. Zhang, H. Zou, Q. Qing, Y. Yang, Q. Li, Z. Liu, X. Guo, Z. Du, Effect of chemical oxidation on the structure of single-walled carbon nanotubes, J. Phys. Chem. B 107 (2003) 3712–3718. [28] N. Yan, F. Wang, H. Zhong, Y. Li, Y. Wang, L. Hu, Q. Chen, Hollow porous SiO2 nanocubes towards high-performance anodes for lithium-ion batteries, Sci. Rep. 3 (2013), doi:http://dx.doi.org/10.1038/srep01568. [29] J.C. Groen, L.A.A. Peffer, J. Pérez-Ramı’rez, Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis, Micropor. Mesopor. Mat. 60 (2003) 1–17. [30] P. Lv, H. Zhao, C. Gao, Z. Du, J. Wang, X. Liu, SiOx-C dual-phase glass for lithium ion battery anode with high capacity and stable cycling performance, J. Power Sources 274 (2015) 542–550. [31] J. Meng, Y. Cao, Y. Suo, Y. Liu, J. Zhang, X. Zheng, Facile fabrication of 3D [email protected] aerogel composites as anode material for lithium ion batteries, Electrochim. Acta 176 (2015) 1001–1009. [32] M. Li, Y. Yu, J. Li, B. Chen, X. Wu, Y. Tian, P. Chen, Nanosilica/carbon composite spheres as anodes in Li-ion batteries with excellent cycle stability, J. Mater. Chem. A 3 (2015) 1476–1482. [33] W. Chang, C. Park, J. Kim, Y. Kim, G. Jeong, H. Sohn, Quartz (SiO2): a new energy storage anode material for Li-ion batteries, Energ. Environ. Sci. 5 (2012) 6895– 6899. [34] H. Wang, P. Wu, H. Shi, W. Tang, Y. Tang, Y. Zhou, P. She, T. Lu, Hollow porous silicon oxide nanobelts for high-performance lithium storage, J. Power Sources 274 (2015) 951–956. [35] B. Jin, E.M. Jin, K. Park, H. Gu, Electrochemical properties of LiFePO4multiwalled carbon nanotubes composite cathode materials for lithium polymer battery, Electrochem. Commun. 10 (2008) 1537–1540. [36] E. Pollak, G. Salitra, V. Baranchugov, D. Aurbach, In situ conductivity impedance spectroscopy, and ex situ raman spectra of amorphous silicon during the insertion/extraction of lithium, J. Phys. Chem. C 111 (2007) 11437– 11444.