Hollow Carbon Nanospheres with Extremely Small Size as Anode

Jan 27, 2016 - *E-mail: [email protected]., *E-mail: [email protected]., ... The high reversible capacity, improved high-rate capability, and ...
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Hollow Carbon Nanospheres with Extremely Small Size as Anode Material in Lithium-ion Batteries with Outstanding Cycling Stability Qiulai Huang, Shiliang Wang, Yan Zhang, Bowen Yu, Lizhen Hou, Geng Su, Songshan Ma, Jin Zou, and Han Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10455 • Publication Date (Web): 27 Jan 2016 Downloaded from http://pubs.acs.org on February 2, 2016

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Hollow Carbon Nanospheres with Extremely Small Size as Anode Material in Lithium-ion Batteries with Outstanding Cycling Stability Qiulai Huang1, Shiliang Wang1,2,*, Yan Zhang3, Bowen Yu1,2, Lizhen Hou4, Geng Su5,*, Songshan Ma1, Jin Zou2, Han Huang2,* 1

School of Physics and Electronics, Central South University, Changsha 410083, China

2

School of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia

3

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

4

School of Physics and Information Science, Hunan Normal University, Changsha 410081, China

5

College of Material Science and Engineering, Central South University of Forestry and Technology, Changsha 410075, China

Abstract: Hollow carbon nanospheres (HCNSs) were fabricated by annealing the Cu-C core-shell nanoparticles at 1250 oC in vacuum. The as-obtained HCNSs have ultra-thin shell of 1 - 3 nm in thickness, small size of about 20 nm in diameter, high surface area of 300 m2 g-1, and ultra-small pores below 5 nm within the C shells. The HCNSs exhibit excellent electrochemical performance as anode materials for lithium-ion batteries. A reversible capacity of 400 mAh g-1 and capacity retention of nearly 100% are achieved at the current density of 186 mA g-1 after 100 charging-discharging cycles. The high reversible capacity, improved high-rate capability and outstanding cycling stability could be attributed to their unique structural characteristics including the extremely small diameter, the high surface area and the hollow structure with porous, ultrathin shell.

*

Corresponding author. E-mail: [email protected], [email protected], [email protected] 1

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1. Introduction Anode materials play a decisive role in the development and commercialization of the lithium-ion batteries.1-3 Carbon anodes, due to their low cost and excellent stability, have been widely used in lithium-ion batteries (LIB).4-6 Recently, apart from the traditional graphitic carbons and hard carbons,4 large number of new carbon materials with various structures, such as hollow carbon nanospheres/microspheres (HCNSs/HCMSs),7-12 hollow carbon nano-onions (HCNOs),13 carbon nanocages (CNCs),14 porous carbon particles (PCPs),15-17 carbon nanotubes (CNTs),18 carbon nano-onions,19 carbon nanobeads,20 graphenes21 and their composites,22-23 have been widely investigated. At the same time, many other new anode materials, such as the nanostructures of silicon,3 tin,24 and metallic oxides,25-26 also exhibit high potentials as the anode materials of LIB. Among these new materials, the hollow nanostructural materials have attracted intensive attention, because they can effective reduce the capacity loss,10 hinder the agglomeration of the active materials, and provide enough space to tolerate the volume expansion during the cycling process of the anode materials.27-30 Combining the advantages of carbon materials and the hollow nanostructures, hollow carbon nanomaterials are expected to be promising candidates for anode materials. As a result, the fabrication and electrochemical performance of hollow carbon nanomaterials have attracted increasing attention in recent years.6-18 So far, various hollow carbon nanomaterials, 2

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such as HCNSs/HCMSs, HCNOs, CNCs and PCPs, can be synthesized by many different methods,6-18

and

their

excellent

electrochemical

performance

have

been

widely

investigated.6-18 However, most of the reported hollow carbon nanomaterials have a relatively large particle size of over 100 nm, possibly due to the significant difficulties in the large-scale fabrication of the smaller hollow carbon nanomaterials. Consequently, their corresponding electrochemical performance remains unknown, which needs an attention. In our previous works, we developed a simple method based on one-step metal-organic chemical vapour deposition (MOCVD) for the controlled synthesis of Cu-C core-shell nanostructures with an average size of about 20 nm on a large scale. 31-32 Here, we report that HCNSs of ~ 20 nm in diameter can be easily obtained by simply annealing the Cu-C core-shell nanoparticles at 1250 oC in vacuum, and demonstrate that the as-synthesized HCNSs are a promising candidate as the anode material for LIB with substantially high electrochemical performance.

2. Experimental Section 2.1. Material synthesis and structure characterization HCNSs were prepared by annealing Cu-C core-shell nanoparticles at 1250 oC in vacuum. The Cu-C core-shell nanoparticles were synthesized by MOCVD, and the detailed process can be found in our previous studies.31-34 Briefly, metal-organic precursor Cu(acac)2 was evaporated at 150 oC, and then transported to the deposition region by the H2 carrier gas in a horizontal tube furnace. Cu-C core-shell nanoparticles were collected at the deposition region of the furnace, and their shell thickness and core size could be tuned by changing the deposition temperature and the flow of the carrier gas.31 The obtained Cu-C nanoparticles were then loaded in a corundum boat, and annealed in vacuum at 1250 oC for 10 h in the horizontal tube furnace. After cooled down to room temperature, the annealed produces did not show obvious difference from the previous Cu-C nanoparticle before annealing, but lots 3

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of reddish-brown spherical particles with diameter of 1 - 2 mm appeared in the corundum boat. The structural and defective characteristics of the synthesized and annealed products were examined by X-ray diffraction (XRD; Rigaku D max 2500 VB), Raman Spectroscopy (Lab RAMHR 800), transmission electron microscopy (TEM; FEI Tecnai F20 operated at 200 kV). The surface characteristics of the products were characterized by nitrogen adsorption/ desorption isotherms at 77 K on a Quadrasorb SI sorption analyzer. The specific surface areas were calculated according to the Brunauer–Emmett–Teller (BET) model,35 and the pore-size distribution was derived from the density functional theory model.36

2.2. Electrochemical Measurement: Electrochemical tests were carried out in 2016 coin-type cells (diameter: 20 mm; thickness: 1.6 mm). The working electrodes were prepared by coating the slurry of the HNCSs (80 wt%), carbon black (10 wt%) and polyvinylidene fluoride (10 wt%) dissolved in n–methyl pyrrolidinone (NMP) onto a Cu foil substrate, and then dried in a vacuum oven at 100 °C for 12 h. The loading mass of the active material of the electrodes was 0.7 mg cm-2. Li metal foil was utilized as the counter electrode, and Celgard 2400 was used as the separator. The electrolyte was composed of 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 by volume). Half-cells were assembled in a MBraun glovebox (H2O < 0.5 ppm, O2 < 0.5 ppm) filled with argon. Charge-discharge measurements and cycling tests were performed with an Arbin batteries cycler (BT2000) at room temperature. Cyclic voltammetry measurements were carried out on an electrochemistry workstation (PARSTAT 2273) in the potential range from 0.01 to 3 V versus Li/Li+ at the scan rates of 0.1, 0.2, 0.5 mV/s. The electrochemical impedance measurements were performed on a Solartron Analytical at an AC voltage of 5 mV amplitude in the range from 0.01 to 100 kHz.

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3. Results and Discussion Figure 1(a) shows the XRD patterns of the products before and after annealing. In addition to the strong ddiffraction peaks from Cu for the sample before annealing, a relatively weak broad shape in the region of C (0002) peak can also be distinguished (see the inset in Figure 1(a)). This indicates that the C shells have a relatively low crystallinity. On the other hand, only an obvious diffraction peak for C (0002) can be detected from the sample after annealing. This suggests that the Cu cores should have been evaporated during the annealing process and the crystallinity of C shells has been improved. To further clarify the effects of the annealing process on the crystallinity of the C shells, the two samples were further characterized by Raman spectroscopy. As can be seen from the Raman spectra in Figure 1(b), two characteristic peaks at around 1337 and 1579 cm−1 can be ascribed to the D band arising from the finite particle-size effect or defects in the hexagonal C network, and the G band from the in-plane bond-stretching motion of pairs of C sp2 atoms.37-40 Although the intensity ratios, ID/IG, for the carbon shells are close, 1.01 (before) and 1.06 (after), the full width at half maximum of G peak increased after annealing. This also suggests that the annealing process improved the crystallinity of C shells, and is consistent with the previous reports.30, 41-42

Figure 1. (a) XRD patterns and (b) Raman spectra of the Cu-C core-shell nanoparticles and the HCNSs. The inset in (a) shows the 8-fold magnification spectrum ranged from 10 to 40 5

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degree To understand the change of the structural characteristics during the annealing process, TEM investigations were performed. Figures 2(a) and 2(b) show the TEM images of the Cu-C core-shell nanoparticles before and after annealing, respectively. The insets in Figures 2(a) and 2(b) are the typical high resolution TEM images of the samples, confirming that the annealing process has led to the formation of HCNSs. It should be noted that, in both cases, the C shells have a thickness of ~ 2 nm with the lattice spacing of 0.36 nm. As a direct comparison, Figure 2(c) is the size distributions of the Cu-C core-shell nanoparticles and the HCNSs based on the statistical results summarised from TEM images. It is clear that the HCNSs exhibit a quite similar average size of ~ 20 nm to the Cu-C core-shell nanoparticles, but the size distribution shows some differences. The slight increase in the quantity of the C shells with diameters below 20 nm may be related to the reconstruction and crystallization of the small hollow C shells at the high annealing temperature. The appearance of large shell may be resulted from the break and distortion of the relatively large C shells with diameters above 20 nm.

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Figure 2. (a,b) TEM images of the Cu-C core-shell nanoparticles and HCNSs, respectively. The insets show the high resolution TEM images of the core-shell particles and HCNSs. (c) Particle-size distribution of the core-shell nanoparticles and HCNSs. (d) Nitrogen adsorption/desorption isotherms and the corresponding pore-size distribution (inset) of the HCNSs. Based on the above-mentioned XRD and TEM analyses, it can be concluded that the annealing process is a simple and effective strategy to remove the Cu cores from Cu-C core-shell nanostructures to ultimately obtain HCNSs. To understand the mechanism of Cu disappearance, i.e., the pathway and driving force, we notice the defective structure of the C shells and the annealing conditions. In our C shells, there were many discontinuities in our C shells, as shown by the arrows in the insets of Figure 2(b). These defects can significantly lower the barrier for the penetration of Cu atoms, and thus provide the pathway for the Cu atoms and atomic clusters to penetrate through the shells. In our experiment, the annealing temperature of 1250 oC is far above the melting point of 1083 oC for bulk Cu, and the actual melting point of the Cu nanocores of ~ 20 nm in diameter is expected to be far below the bulk value due to the considerable melting point depression from the size effects.43-45 The high annealing temperature is therefore able to generate a high Cu vapour pressure in the inner hole of the C shells, establishing a high pressure gradient between the inside and the outside of the shells. This gradient serves as the driving force for the penetration of Cu atoms and atomic 7

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clusters through the shell. To understanding the surface characteristics of the Cu-C core-shell nanoparticles before and after annealing, the samples were characterized by the nitrogen absorption/desorption experiment. Using the Brunauer - Emmett - Teller model,35 the specific surface areas were calculated to be 30 and 300 m2 g-1 for the core-shell nanoparticles before and after annealing, respectively. The significant increase of the specific surface area indicates that the evaporation of the Cu cores not only reduced the total mass of the sample, but also led to the hollow structure to increase the surface area, as previously observed by TEM. This also suggests that our HCNSs should be a potential electrode material.46 Figure 2(d) exhibits the nitrogen absorption/desorption isotherm of the obtained HCNSs. The pore distribution curve shown as the inset in Figure 2(d) was derived by the density functional theory (DFT) method. From the pore size distribution curve, a series of pore sizes of 1.4, 1.7, 2.9, 4.3, 7.5, 10.5 and 14.6 nm, can be observed. Compared with the particle-size distribution obtained by direct TEM measurement (Figure 2(c)), it is presumed that the relatively large pores with size above 5 nm should be the inner holes of the C shells. The small pores with size below 5 nm, which is the majority in the pore-size distribution cure but invisible in TEM images, should come from the flaws and defects in the C shells.

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Figure 3. Electrochemical characteristics of the HCNS electrodes. (a) Galvanostatic charge/discharge curves at 186 mA g-1, (b) CVs of the first four cycles between 3 and 0 V at a scan rate of 0.1 mV s-1, (c) cycling performance at a rate of 186 mA g-1, (d) rate performance at different current rates. The inset in (a) displays the plots of capacity versus cell voltage during the charging process. The inset in (b) shows the CVs at scan rates of 0.1, 0.2 and 0.5 mV/s, respectively. Figure 3 shows the electrochemical characteristics of the HCNS electrodes. Figure 3(a) is the Galvanostatic charge/discharge curves measured at a current rate of 186 mA g-1, from which a large irreversible capacity of 1153 mAh g-1 (0.5C, one Li per six formula units (LiC6) in 2 h) occurs. This is a typical phenomenon for carbonaceous electrodes, due to the formation of solid electrolyte interface (SEI) film.30 The formation of SEI film on the surface of HCNSs is also supported by the plateau at 0.8 - 0.6 V that only occurs in the first charge process.47 The Li storage in the HCNSs electrodes can be further understood by the plots of capacity versus voltage for the first four cycling processes, shown as the inset of Figure 3(a). The two peaks at ~ 0.2 and ~ 1.1 V and a slope from 2.7 to 3.0 V indicate the multiple Li storage positions in the as-obtained HCNSs. The peaks at ~ 0.2 and ~ 1.1 V can be attributed to the Li extraction from C layers and the defects in the HCNSs, respectively.8 The slope from 2.7 to 3.0 V is generally attributed to the surface functional groups (C-H, C-OOH, or C-OH)8, 10, 18 or the surface charge accumulation.48 Since our HCNSs have been annealing at 1250 oC, the 9

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density of surface functional groups should be very low. It is therefore presumed that the slope may be related to the charge accumulation from the Li absorbed on the surface of the HCNSs.48 Figure 3(b) shows the cyclic voltammograms (CVs) for the first four cycles at a scan rate of 0.1 mV s-1. In the CVs, the cathodic peak at ~ 0.6 V appeared only in the first cycle should be related to the formation of the SEI. The sloping plateau at around 1.0 V in the CV curves has been frequently observed in HCNS-based electrodes,7,

11, 16-17

but the corresponding

mechanism remains unclear. The inset of Figure 3(b) shows the corresponding CVs obtained at different scan speeds of 0.1, 0.2 and 0.5 mV/s. The shapes of these CVs are quite similar, with the same peak positions. This suggests that the HCNSs electrodes should have an excellent reversibility. Figure 3(c) displays the cycling performance of the HCNS electrodes examined under long-term cycling up to 100 cycles. After the low initial values for the first several cycles due to the formation of SEI,8, 16, 30 the coulombic efficiency of our HCNS electrodes exhibits a value of 99%, and holds the value for the subsequent cycles. The first reversible specific capacity of ~ 400 mAh g−1 is achieved at a current density of 186 mA g-1, which is higher than the theoretical value of 372 mAh g-1 for graphite.30 The charge capacity exhibited a slight degradation during the first 52 cycles probably due to the structural instability of the HCNSs in the charging-discharging cycles. After the 52nd cycle, the charge capacity exhibited a slight increase, and then became stable at the 85th cycle. The slight increase in charge capacity after the 52nd cycle can be attributed to the activating process of the pores and cavities in the HCNSs.9, 49 After 100 cycles, the electrodes still maintain a specific reversible capacity of ~ 400 mAh g-1. The excellent cycling stability of the HCNS electrodes can be further verified by its capacities obtained at various charge/discharge current rates, as shown in Figure 3(d). The cell was first cycled at 186 mA g-1 for 5 cycles, followed by cycling with a stepwise 10

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increase of discharge/charge current density to as high as 3720 mA g-1. It is shown that the capacity of 390 mAh g–1 is reached again when the current density is lowered back to 186 mA g-1. For comparison, in Table 1 we list the capacity, coulombic efficiency and capacity retention of different electrodes based our HCNS and some other hollow carbon nanostructures. Basically, all these electrodes exhibit relatively high coulombic efficiencies of more than 95% and relatively high capacities. However, the corresponding capacity retentions of these electrodes may vary from ~ 56% to 100%, depending on their structures and the testing conditions. As a remarkable feature, the capacity retention of our HCNS electrode is nearly 100% after 100 cycles, which is significant high than most of other hollow carbon-based electrodes (also see Table 1). It is well-known that the electrochemical performance of an anode material significantly depends on its morphology and structure. So the excellent electrochemical performance of our HCNS electrodes can be understood as follows. Firstly, the high surface area and porous structure can provide a large quantity of accessible active sites for Li-ion insertion and thus may lead to a high reversible specific capacity than the theoretical value for graphite.30 Secondly, the ultrathin shells of 1 ~ 3 nm in thickness and the hollow structure can greatly shorten the diffusion length for Li ions and buffer the volumetric change during the Li-ion insertion and extraction especially at high rates, and consequently lead to the high cycling stability and improved rate capability. Thirdly, our HCNSs have a quite small average diameter of ~ 20 nm, and thus have a higher structural stability than those with relatively large diameters or sheet-like structures, when being subject to the internal stress induced by the Li insertion/extraction and the external stresses originated from van der Waals forces between the neighbouring shells or from the mechanical compression during the fabrication process. As a result, our HCNS electrodes have an excellent cycling stability as shown in Table 1.

Table 1. Electrochemical characteristics of various hollow carbon nanomaterials 11

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Diameter Coulombic Capacity Loading mass Capacity Cycle Scan rate Materials /thickness efficiency retention Ref. -1 /thickness (m Ah g ) number (mA g-1) (nm) (%) (%) This HCNSs 20 / 2 0.7 mg cm-2 400 99 100 100 186 work HCNSs

100 / 12

1-1.2 mg cm-2

310

100

~ 65

200

372

7

HCNSs

80 /10

-

370

98

~ 80

50

372

8

HCNSs

250 / 5

0.45 mg

760

100

~ 97

100

100

9

HCNSs

380 / 70

400 - 600

98

85 - 90

30

74

10

HCMSs

2000 /

475

100

~ 70

100

100

11

1000

200

100

~ 95

500

200

HCMSs

150/8

743

100

69

50

100

12

HCNOs

30 / 12

391

95

~ 78

60

37

13

PCPs

-

850

93

77

20

100

15

PCPs

-

500

98

~ 56

40

74

16

PCPs

-

250

99

95

100

372

17

365

99

91

100

37

200 - 600

95

60 - 77

50

25

CNTs

-

1.3-2.5 mg cm-2

0.3 mm

18

4. Conclusions In summary, HCNSs with ultra-thin, porous shells of 20 nm in diameter have been prepared on a large scale by simple annealing the Cu-C core-shell nanoparticles at high temperature. The as-obtained HCNSs are demonstrated to be a promising candidate as the Li-ion electrode materials with high reversible capacity, improved high-rate capability and outstanding capacity retention. The superior electrochemical performance of the HCNSs is attributed to their unique structural characteristics that allow improved solid-state Li-ion diffusion and excellent structural stability during the Li inserted-extraction process.

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Acknowledgements This study is financially supported by the National Natural Science Foundation of China (51074188, 11502080), Research Funding of Central South University (2014JSJJ024) and the Australia Research Council (ARC) under the Discovery Project program (DP130101828).

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and

Excellent

Stability

of

Lithium

Ion

Battery

Anode

Using

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