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Hollow Structured Silicon Anode with Stabilized Solid Electrolyte Interphase Film for Lithium-ion Batteries Qiuliang Lv, Yuan Liu, Tianyi Ma, Wentao Zhu, and Xinping Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b05970 • Publication Date (Web): 24 Sep 2015 Downloaded from http://pubs.acs.org on September 27, 2015

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ACS Applied Materials & Interfaces

Hollow Structured Silicon Anode with Stabilized Solid Electrolyte Interphase Film for Lithium-ion Batteries Qiuliang Lv,Yuan Liu, Tianyi Ma, Wentao Zhu, Xinping Qiu* Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China.

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ABSTRACT: Silicon has been considered as a promising anode material for the next generation lithium-ion batteries due to its high specific capacity. Its huge volume expansion during alloying reaction with lithium spoils the stability of interface between electrode and electrolyte, resulting in capacity degradation. Herein, we synthesized a novel hollow structured silicon material with the interior space for accumulate the volume change during the lithiation. The as-prepared material shows excellent cycling stability, with reversible capacity of ~1650 m Ah g-1 after 100 cycles, corresponding to 92% retention. The electrochemical impedance spectroscopy and differential scanning calorimetry were carried out to monitor the growth of SEI film, and the results confirm the stable solid electrolyte interphase film on the surface of hollow structured silicon.

KEYWORDS: Anode, silicon, coulombic efficiency, interface stability, lithium-ion battery

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Introduction Silicon has been regarded as a promising anode material for the next generation of lithium ion batteries (LIBs)[1-4], due to its low discharge potential and high specific capacity. However, huge volume expansion during alloying reaction with lithium causes pulverization of particles, disintegration of electrode[5-7] and rupture of existed interface layer, which promotes the continual formation of SEI film. Excessive growth of SEI film, observed in previous work,[8, 9] results in the increase of impedance, irreversible capacity loss and low coulombic efficiency during cycle.[10-12] The anode with low coulombic efficiency continuously consumes lithium ions from the cathode and causes rapid capacity degradation of the balanced full battery. Nanostructured silicon materials, such as hollow nanospheres,[13-15] silicon core-hollow

carbon

shell

nanocomposites[16-19]

and

1D

hollow

silicon

nanotubes[20-23], with void inside, have been reported with improved cycle performance. Voids in the materials can efficiently accommodate the large strain from the lithiation/dethiathtion reaction. The nanostructure can also absorb the electrolyte and shorten the diffusion paths of lithium ions, which are benefit for rate capability. In spite of the enhanced electrochemical performances, direct and effective characterization of SEI film stability is still absence of evidence, also the vital process to engineer the voids in these materials is low efficient. In this work, we report a facile approach to prepare hollow structured silicon material through a chemical vapor deposition (CVD) method with calcium carbonate (CaCO3) as template. With the built-in space, this product presents remarkable capacity retention and fairly high coulombic efficiency. The stable of SEI film is demonstrated as well.

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Experimental Preparation of hollow structured silicon material. Commercial nano CaCO3 (ShanXi NanoMaterials Technology Co., Ltd.) was directly used as the template in CVD process. Silicon deposition was conducted in a horizontal tube furnace at 480 °C with mixed gas of 5 wt % SiH4 and Ar as silicon source. The wall thickness of silicon form was controlled by the time of deposition, the samples denoted as HS-10, HS-15 and HS-20 corresponding to the deposition time of 1.0 h, 1. 5 h and 2 h, respectively. The CaCO3 template was removed with hydrochloric acid (5 wt %). SiOx on the surface of hollow structured silicon was removed in hydrofluoric acid (10 wt %) solution. Structural characterization. Morphologies of samples were observed on a transmission electron microscopy (TEM, JEOL model JEM-2011) operating at 80 keV and a scanning electron microscopy (SEM, HITACHI S-5500) operating at 5 keV. X-ray diffraction (XRD) spectra were collected using an X-ray diffractometer (Bruker D8 Advance) with a Cu Kα radiation source, values of 2θ were scanned in the range of 10° and 90º at a rate of 8° min-1 with a step of 0.02°. X-ray photoelectron spectroscopy (XPS) measurements were conducted with a PHI Quantera SXM spectrometer using a focused and monochromatized Al Kα radiation, the binding energy scale was calibrated using the C 1s peak at 284.8 eV. N2 sorption measurements were carried out on a Quantachrome NOVA 1000e at 77.3 K, specific surface

area

and

pore

Brunauer-Emmett-Teller

size

(BET)

distribution and

were

calculated

Barrett-Joyner-Halenda

using

(BJH)

the

method,

respectively. The differential scanning calorimetry (DSC) was performed on a Mettler-Toleda 821 calorimeter. The LixSi material was scratched from the electrode in the argon glove box and sealed in a hermetic high-pressure stainless steel crucible 4


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(gold plated, METTLER TOLEDO). DSC measurements were performed from 30 to 300°C at a temperature ramp of 2 °C min−1. Electrochemical measurements. The electrodes for electrochemical tests were prepared by coating slurries, containing 80 wt % active material, 10 wt % carbon black (Super P Li, TIMCAL) and 10 wt % polyacrylic acid (PAA), onto copper foil. The loading of active materials in the electrode is 0.4-0.6 mg cm-2. 2025 coin cells were assembled with lithium foil as the counter electrode. The electrolyte solution were composed of 1 M LiPF6, dissolved in a non-aqueous mixture of ethylene carbonate/dimethyl carbonate/ethylmethyl carbonate (EC/DMC/EMC, 1:1:1 in volume ratio) with 2.5 wt % vinylene carbonate (VC) as additive (Zhangjiagang Guotai-Huarong New Chemical Materials Co., Ltd). Galvanostatic tests were performed between 0.05 V and 1.5 V vs Li/Li+ with a relaxation period of 1 min at the end of each charge/discharge process. Three electrode cells (ECC-REF model, EL-CELL) were assembled for electrochemical impedance spectra (EIS) tests, using Li foil as reference and counter electrodes. EIS measurements were conducted on PARSTAT2273 electrochemical workstation (AMETEK). AC potential was controlled with the amplitude of 5.0 mV in the frequency range of 105-0.02 Hz. Before tests, a constant current density of 100 mA g-1 was applied in the discharge process until the voltage reached 1 mV vs. Li/Li+, then the cells were remained at open-circuit for 2 h to stabilize their potential. All cells were assembled in an Ar-filled glove box with moisture and oxygen contents below 1 ppm.

Results and discussion We fabricated hollow strucutred silicon with a simple template method as illustrated in Figure 1. The silicon was deposited onto nano-sized CaCO3 template by 5



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a CVD process. CaCO3 template was washed away in hydrochloric acid solution. The resulted silicon was then treated in hydrofluoric acid to remove the SiOx. The morhpology of nano CaCO3 particles is nearly spherical shape, as shown in Figure 2 a and b, with a narrow distributed particle size of 50 nm. After deposition, a layer of silicon can be observed on the surface of nano CaCO3, Figure 2c, d. An interconnected hollow structured silicon material were left after the remove of template in hydrochloric acid, Figure 2e, f. The wall thickness of porous silicon can be tailored by the control of time of CVD in a fixed flow rate of precursor gas, TEM images in Figure 3 show that the wall thickness in the hollow structured silicon increases with the deposition time. Broad diffuse rings in the selected area electron diffraction (SAED) pattern of HS-10 sample (inset of Figure 3a) reveal that silicon is amorphous. In the XRD patterns of all samples, as shown in figure 3d, characteristic peaks of crystalline silicon (PDF#65-1060) around 28°, 47° and 56° are absent, indicating the amorphous structure of silicon, in agreement with the results of SAED. XPS spectra of Si 2p in Figure 3e are fitted well using a nonlinear Shirley-type background[24] and a combination of 80% Gaussian, 20% Lorentzian line shapes. The first main 3/2-1/2 doublet (the spin-orbit splitting is 0.6 eV and the intensity ratio is 3:1), located at 99.1-99.7 eV corresponds to Si0 (75 % content). The component located at higher binding energy (100.0 eV) is associated with SiOx formed on the surface with a proportion of 25%.[25] The nitrogen adsorption/desorption isotherms in Figure S1a (Supporting Information) show a sharp capillary condensation step at high relative pressures (P/P0 = 0.85-0.99), indicating the existence of large pores. Corresponding pore size distributes mainly in the range of 20 nm and 100 nm (Figure S1b), which is attributed to the removal of site-occupying CaCO3 template. With the increase of 6


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silicon wall thickness, the specific surface area and pore volume decrease obviously (Table S1, Supporting Information) owing to the integrated silicon layer and increased density of material. Galvanostatic tests were performed at a constant current densities of 100 mA g-1 for the first three cycles and 500 mA g-1 for later cycles. The specific capacity was calculated based on the mass of silicon. Figure 4a shows the first and second charge/discharge profiles of HS-10 at a rate of 100mA/g. A long platform is observed in first discharge process and disappeare in subsequent cycle, corresponding to the SEI film formation.[26] The sloping voltage profile suggests a typical lithiation reaction of amorphous silicon.[6, 27]

For the first discharge and charge cycle, the

capacities reach 2973 mAh g-1 and 2367 mAh g-1, respectively, corresponding to coulombic efficiency of 73.4%.

Plots of differential capacity of HS-10 in different

cycles are presented in Figure 3b. The peak at 0.2 V in the first discharge suggests the lithiation process in amorphous silicon. During the first charge, two peaks at 0.32 and 0.48 V indicate the formation of amorphous silicon during the dealloying reaction , being consistent with the previously report in the literatures for amorphous silicon electrodes.[14, 28-30] Peak value from 50 to 100 cycles are almost unchanged, indicating little capacity losses during cycling.[15] Figure 4c illustrates the cycle performance and coulombic efficiency. Among these samples, HS-10 shows the highest capacity retention (92%) after 100 cycles, corresponding reversible capacity of 1654 mAh g-1. Benefit from the short lithium ion diffusion distance in this nanostructure, excellent rate capability is also demonstrated (Figure 4d). Increase current density from 0.1 to 2 A g-1, the specific capacity of HS-10 is more than 1000 m Ah g-1, when the current density changes back to 0.1 A g-1, 98.5% capacity of first cycle is recoverable. The increase of the wall thickness of 7



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hollowed silicon decreases capacity retention obviously (76% for HF-15 and 73% for HS-20 after 100 cycles), this is possible due to the limited interior space for volume change. In order to investigate the evolution process of SEI during cycles, the electrode of HS-10, after 100 cycles, was taken out from the coin cell and wash off the electrolyte residue in dimethyl carbonate. Primary hollow structured silicon covered with SEI film can be observed in Figure 5a, the surface of hollow structured silicon and the carbon black additives can be clearly distinguished, demonstrating no excessive growth of SEI film. 1 mM acetic acid solution was used to remove the SEI film.[31] The shape (Figure 5b) and the wall (Figure 5c) were maintained without fractures and cracks. This indicates that hollow structured silicon can withstand the volume change. Electrochemical impedance spectroscopy (EIS) measurements were further conducted. Typical Nyquist plots of hollow structured silicon electrode are illustrated in Figure 6a. The semicircle in mid-frequency regime is generally associated with interfacial phenomena, either the charge transfer or the occurrence of SEI film between the active particles and the liquid electrolyte;[11] the impedance tail in low-frequency region can be attributed to the bulk diffusional effects. RSur here is used to evaluate the whole surface resistance (equivalent circuit is shown in inset of Figure 6a). In Figure 6a and 6b, RSur maintains around 20 Ohm for HS-10, which indicates the stable SEI during cycling. After 5 cycles, the constancy of the characteristic frequency (2.63 kHz) suggests that the kinetics of the charge transfer reaction and the SEI film conductivity do not change. By contrast, continuous increase of RSur is observed for the electrode with Si nano-particles in our previous research,[32] indicating the excessive SEI growth. To evaluate the amount of SEI film directly, a method based on differential 8


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scanning calorimetry (DSC) was developed. In detail, the cells were discharged at a constant current density of 100 mA g-1 to the voltage of 1 mV and remained at open-circuit for 2 h to stabilize their potential. Then they were carefully opened in the glove box, retrieved electrodes were rinsed in dimethyl carbonate solvent to remove the electrolyte and dried in vacuum overnight. LixSi material was scratched from the electrode and was sealed into a hermetic high-pressure stainless steel crucible for DSC tests. Figure 6c and 6d present the DSC curves for lithiated silicon after different cycles. An exothermic signal at 86-100 °C is visible for all the curves of HS-10 and nano Si material. By analogy with lithiated graphite and nano silicon electrode,[33-36] it is reasonable to suggest that these exothermic peaks are attributed to the transformation of metastable SEI layer components. At elevated temperatures, the metastable components in SEI film (ROCO2Li compounds, ROLi, polycarbonate species and lithium oxalate)[33],[34] decompose and react with lithium in LixSi. For nano Si, much larger exothermic heat at 86-100 °C is observed, indicating that the amount of SEI film increases continuously. Meanwhile, there is no obvious change of exothermic peak for HS-10, which suggests that SEI formation in hollow structured silicon is confined and the interface layer between electrode and electrolyte is more stable. Another remarkable phenomenon is that the peak position at higher temperature (167-190 °C) show shift to low temperature for nano Si material. According to previous literatures,[36-38] this peak can be associated with conversion of metastable α-LixSi phases to thermodynamically stable ones, where lithium content (x value) is account for the shape and position of the peak. Continuous formation of SEI film on nano Si consume more lithium in the reaction, resulting in decreased x value in α-LixSi and further shift of the peak position.[37, 38] In HS-10, exothermic peaks of α-LixSi conversion maintain the similar shape and position, which confirm 9



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that interface stability in hollow structured silicon material is much better. In summary, hollow structured silicon material is synthesized using a facile chemical vapor deposition method in combination with nano calcium carbonate as template. Influence of silicon shell thickness on electrochemical performance have been discussed. This material exhibits excellent cycling stability and rate performance without crush of the hollow structure. The coulombic efficiency during cycling maintains nearly 100% which is benefit for long cycle life. Furthermore, EIS and DSC observation on SEI film transformation lead us to a deeper understanding of the interface stability. In consideration of nano calcium carbonate is commercially available,[39] this hollow structured silicon material is a promising anode candidate for the next generation lithium-ion batteries.

ASSOCIATED CONTENT Supporting Information Details on experimental, additional characterization including nitrogen sorption measurement and particle size analysis.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (X.P. Qiu) Tel: +86-10-62794234 Fax: +86-10-62794234.

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ACKNOWLEDGMENTS The authors appreciate the supports from National Key Project on Basic Research (2013CB934000, 2015CB251104), National International Science and Technology Cooperation Project (2012DFG61480), China-Germany Electric Vehicle Project (2011AA11A290), China-US Electric Vehicle Project (2010DFA72760), Beijing Natural Science Foundation (2120001) and Tsinghua University independent research program (20111081039).

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Captions

Fig. 1 Schematic diagram of the fabrication process of porous silicon. Fig. 2 Transmission electron microscopy(TEM) images of (a, b) nano CaCO3, (c, d) CaCO3 deposited with silicon layer, (e, f) hollow structured silicon. Fig. 3 TEM images of hollow structured silicon with different thickness (a) HS-10 (inset is the corresponding selected area electron diffraction (SAED) pattern), (b) HS-15, (c) HS-20, (d) X-ray powder diffraction (XRD) patterns of HS samples, (e) X-ray photoelectron spectroscopy (XPS) analysis of Si 2p of HS-10. Fig. 4 Electrochemical performance of hollow structured silicon samples. (a) Charge/discharge voltage profiles of HS-10 at 0.1A/g rate in the first two cycles. (b) Differential capacity of HS-10 in different cycles. (c) Capacity retention during 100 cycles for HS-10, HS-15 and HS-20. (d) Capacity of HS-10 at various charge/discharge rate. Fig. 5 (a) SEM image of HS-10 electrode after 100 cycles, (b, c) TEM images of HS-10 after 100 cycles, where SEI film was wahsed away. Fig. 6 (a) Nyquist plot of HS-10 at the end of discharge after different cycles (inset is the equivalent circuit), (b) the evolution of the resistance of the semi-circle. (c) and (d) DSC curves for HS-10 and nano Si after different cycles.

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Fig. 1 Schematic diagram of the fabrication process of porous silicon.

14


Page 14 of 26

Page 15 of 26

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


(a)

(b)

(c)

(d)

(e)

(f)

Fig.2 Transmission electron microscopy(TEM) images of (a, b) nano CaCO3, (c, d) CaCO3 deposited with silicon layer, (e, f) hollow structured silicon.

15



20

30

40

50

60

70

80

108 106 104 102 100

90

e

PSi3/2 PSi1/2 PSiOx

Intensity (a.u.)

HS-10 HS-15 HS-20 Std. Si

d

10

c

b

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

Page 16 of 26

98

96

Binding Energy / eV

2 θ (degree)

Fig. 3 TEM images of hollow structured silicon with different thickness (a) HS-10 (inset is the corresponding selected area electron diffraction (SAED) pattern), (b) HS-15, (c) HS-20, (d) X-ray powder diffraction (XRD) patterns of HS samples, (e) X-ray photoelectron spectroscopy (XPS) analysis of Si 2p of HS-10.

16


2.0 1.5 1.0 0.5 0.0 0

1000

2000

-1 -1

(a)

2.5

4.0k 2.0k

dQ/dV / m Ah g V

+

3.0

(b)

0.0 -2.0k

1st cycle 2nd cycle 50th cycle 100th cycle

-4.0k -6.0k -8.0k

-10.0k 0.0

3000

0.4

2000 1500 1000 500 0 0

95 90 85

(c) 20

80 40

60

80

-1

Capacity / m Ah g

HS-20 HS-15 HS-10

CE %

-1

100

2500

1.2

1.6

Potential / V (vs Li/Li )

3500 3000

0.8

+

Capacity (mAh/g)

Capacity / mAh g

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


Potential (V vs Li/Li )

Page 17 of 26

3500

2500 2000

Cycle number

0.1 A/g 0.5 A/g

0.1 A/g

1500 1.0 A/g

1000

2.0 A/g

500 0

100

(d)

3000

0

5

10

15

20

25

30

Cycle number

Fig. 4 Electrochemical performance of hollow structured silicon samples. (a) Charge/discharge voltage profiles of HS-10 at 0.1A/g rate in the first two cycles. (b) Differential capacity of HS-10 in different cycles. (c) Capacity retention during 100 cycles for HS-10, HS-15 and HS-20. (d) Capacity of HS-10 at various charge/discharge rate.

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

a

b

Fig. 5 (a) SEM image of HS-10 electrode after 100 cycles, (b, c) TEM images of HS-10 after 100 cycles, where SEI film was wahsed away.

18


Page 18 of 26

c

Page 19 of 26

-60

140

(a)

-40 -30 -20

100

2.63 kHz

-10

80 60 40 20

0 0

10

20

30

40

50

0

60

0

10

Z' / Ohm

30

40

50

1.0

10 20 30 50 100

60

10 20 30 50 100

(d)

-1

0.8

HF / W g

-1

(c)

0.6 0.4

20

Cycle number

1.0 0.8

(b)

Nano Si HS-10

120

1st 3rd 10th 50th

RSur / Ohm

Z" / Ohm

-50

HF / W g

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


exo

0.2 0.0

0.6 0.4

exo

0.2 0.0

50

100

150

200

250

o

300

50

Temperature / C

100

150

200

o

250

300

Temperature / C

Fig. 6 (a) Nyquist plot of HS-10 at the end of discharge after different cycles (inset is the equivalent circuit), (b) the evolution of the resistance of the semi-circle. (c) and (d) DSC curves for HS-10 and nano Si after different cycles.

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

Fig. 1 Schematic diagram of the fabrication process of porous silicon.

1


Page 20 of 26

Page 21 of 26

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


(a)

(b)

(c)

(d)

(e)

(f)

Fig.2 Transmission electron microscopy(TEM) images of (a, b) nano CaCO3, (c, d) CaCO3 deposited with silicon layer, (e, f) hollow structured silicon.

1



20

30

40

50

60

70

80

90

108 106 104 102 100

2  (degree)

e

PSi3/2 PSi1/2 PSiOx

Intensity (a.u.)

HS-10 HS-15 HS-20 Std. Si

d

10

c

b

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

Page 22 of 26

98

96

Binding Energy / eV

Fig. 3 TEM images of hollow structured silicon with different thickness (a) HS-10 (inset is the corresponding selected area electron diffraction (SAED) pattern), (b) HS-15, (c) HS-20, (d) X-ray powder diffraction (XRD) patterns of HS samples, (e) X-ray photoelectron spectroscopy (XPS) analysis of Si 2p of HS-10.

1


-1

(a)

-1

2.5

dQ/dV / m Ah g V

+

3.0

2.0 1.5 1.0 0.5 0.0 0

1000

2000

4.0k

(b)

2.0k 0.0 -2.0k

1st cycle 2nd cycle 50th cycle 100th cycle

-4.0k -6.0k -8.0k

-10.0k 0.0

3000

2000 1500 1000

90 85

500 0

95

(c) 0

20

80 40

60

80

-1

Capacity / m Ah g

HS-20 HS-15 HS-10

CE %

-1

100

2500

0.8

1.2

1.6

Potential / V (vs Li/Li )

3500 3000

0.4

+

Capacity (mAh/g)

Capacity / mAh g

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


Potential (V vs Li/Li )

Page 23 of 26

3500

2500 2000

Cycle number

0.1 A/g 0.5 A/g

0.1 A/g

1500 1.0 A/g

1000

2.0 A/g

500 0

100

(d)

3000

0

5

10

15

20

25

30

Cycle number

Fig. 4 Electrochemical performance of hollow structured silicon samples. (a) Charge/discharge voltage profiles of HS-10 at 0.1A/g rate in the first two cycles. (b) Differential capacity of HS-10 in different cycles. (c) Capacity retention during 100 cycles for HS-10, HS-15 and HS-20. (d) Capacity of HS-10 at various charge/discharge rate.

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

a

b

Fig. 5 (a) SEM image of HS-10 electrode after 100 cycles, (b, c) TEM images of HS-10 after 100 cycles, where SEI film was wahsed away.

1


Page 24 of 26

c

Page 25 of 26

-50

140

(a)

-40 -30 -20

100

2.63 kHz

-10 0 0

10

20

30

40

50

80 60 40 20 0

60

0

10

Z' / Ohm

30

40

50

1.0

10 20 30 50 100

0.8

HF / W g-1

(c)

0.6 0.4

20

60

Cycle number

1.0 0.8

(b)

Nano Si HS-10

120

1st 3rd 10th 50th

RSur / Ohm

Z" / Ohm

-60

HF / W 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


exo

0.2 0.0

10 20 30 50 100

(d)

0.6 0.4

exo

0.2 0.0

50

100

150

200

250

Temperature / oC

300

50

100

150

200

250

Temperature / oC

300

Fig. 6 (a) Nyquist plot of HS-10 at the end of discharge after different cycles (inset is the equivalent circuit), (b) the evolution of the resistance of the semi-circle. (c) and (d) DSC curves for HS-10 and nano Si after different cycles.

6



Table of Content Graphic

3500

100

3000 HS-20 HS-15 HS-10

2500 2000

90 85

1500 1000 0

95

CE %

-1

Capacity / mAh g

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 26 of 26

80 20

40

60

Cycle number


80

100

Hollow Structured Silicon Anodes with Stabilized ... - ACS Publications

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