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Accepted Manuscript
Core-shell SiO@F-doped C composites with interspaces and voids as anodes
for high-performance lithium-ion batteries
Lingzhi Guo, Hongyan He, Yurong Ren, Chao Wang, Mingqi Li
PII:
DOI:
Reference:
S1385-8947(17)31859-4
https://doi.org/10.1016/j.cej.2017.10.145
CEJ 17929
To appear in:
Chemical Engineering Journal
Received Date:
Revised Date:
Accepted Date:
25 July 2017
17 October 2017
23 October 2017
Please cite this article as: L. Guo, H. He, Y. Ren, C. Wang, M. Li, Core-shell SiO@F-doped C composites with
interspaces and voids as anodes for high-performance lithium-ion batteries, Chemical Engineering Journal (2017),
doi: https://doi.org/10.1016/j.cej.2017.10.145
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Core-shell SiO@F-doped C composites with interspaces and voids as anodes for
high-performance lithium-ion batteries
Lingzhi Guo a, Hongyan He a, Yurong Ren b, Chao Wang c, Mingqi Li a, *
a
College of Chemistry and Chemical Engineering, China West Normal University,
Nanchong 637009, China
b
School of Materials Science and Engineering, Changzhou University, Changzhou
213164, China
c
Clean Energy Materials and Engineering Center, School of Microelectronics and
Solid-state Electronics, University of Electric Science and Technology of China,
Chengdu 611731, China
Abstracts: Core-shell F-doped carbon coated SiO composites with interspaces and
some small voids (SiO@F-doped C) are successfully fabricated using bulk SiO as
starting material. With the assistance of polytetrafluoroethylene (PTFE), both etching
and carbon coating of SiO are firstly performed in a single step, which largely lowers
the production cost of materials. The SiO@F-doped C composite obtained by
annealing the mixture of SiO and PTFE with a mass ratio of 5:3 at 650 oC shows the
highest stable discharge capacity with excellent cycling stability. It delivers a stable
discharge capacity of about 975 mAh g -1 at a current density of 100 mA g-1. Even after
the current density increases to 1600 mA g -1, a stable discharge capacity of about 553
mAh g-1 can be achieved. At a current density of 400 mA g-1, the composite maintains
Corresponding Authors: *E-mail: lmingq888@aliyun.com. Tel.: +86-817 2321461.
1
a discharge capacity of 752 mAh g-1 after 350 cycles, the retention of which is 99.2%
versus that in the second cycle. Compared with the annealed SiO, in addition to higher
conductivity, the ions diffusion rate of the SiO@F-doped C composite increases 1.94
times while the volume expansion falls by more than half, which should be
responsible for its significantly enhanced electrochemical performance.
Keywords: Silicon monoxide; F-doped carbon; Anode; Lithium-ion battery;
Electrochemical performance
1. Introduction
Electrode materials play a critical role in the development of lithium ion batteries
with high energy and power density. Silicon has been thought as the most ideal
alternative to graphite because of the highest theoretic capacity among the known
materials, satisfying lithium insertion/extraction potentials and rich resources.
However, silicon particles experience a huge volume change of ~300% during
alloying/dealloying, which leads to serious internal cracking of electrodes and
delamination from the current collector, resulting in rapid capacity fading and
electrode failure [1-2]. In the recent ten years, although some significant progress in
improving the cycling performance of silicon electrodes has been achieved [3-5],
large scale application still faces many challenges, especially cycling life,
charge-discharge efficiency and costs. As an alternative to silicon, SiO with the same
particle sizes experiences a smaller volume change than silicon and thus shows
relatively better cycling performance. Morita firstly reported that SiO could be
transformed into nanosilicon cluster-SiOx composites at 1000 oC and the formed SiOx
2
had buffer to the volume change of nanosilicon cluster during lithiation/delithiation
[6]. After that, a number of efforts have been done to improve the electrochemical
performance of SiO [7-28]. The adopted main measures include reducing particle
sizes followed by carbon coating [21, 22], etching with HF/NaOH [29, 30], and
fabricating SiO alloy composite materials [31, 32]. Although SiO experiences a
relatively smaller volume change than silicon, the strain from this volume change can
still destroy the microstructure of electrode. Reserving a room for the volume
expansion is still the most effective means to largely enhance the cycling stability of
SiO. To do this, HF/NaOH has been used to etch SiO [29, 30]. However, the use of
toxic HF and corrosive NaOH does harm to operators and needs troubling subsequent
processing. Moreover, in the previous reports, etching and carbon coating are done
step by step, which pushes up production cost.
In this work, we suggest a new and scalable method to fabricate core-shell
SiO@F-doped C anode composites with interspaces and some small voids
(SiO@F-doped C). With the assistance of polytetrafluoroethylene (PTFE), which acts
as etching agent and F-doped C source, both carbon coating and etching of SiO are
firstly performed in one single step. The interspaces/voids formed during preparation
provide an extra space for the volume expansion. Like N, O and S elements [33],
since F has higher electronegativity than C, F-doped C is expected to further enhance
the conductivity of the electrode and contribute to lithium insertion. To our
knowledge best, it is first time that F-doped C is used to coat SiO anode material.
Besides, we also investigate the influence of both annealing temperature and material
3
ratio on the electrochemical performance of the synthesized composites.
Materials Synthesis: Commercial SiO (200 meshes, Aladdin) was firstly milled
in an argon atmosphere at 500 rpm for 6 h. Subsequently, the milled SiO was mixed
with PTFE (Aladdin) at different mass ratios by milling at 500 rpm for another 1 h.
Ball milling was performed on a planetary ball mill (Nanjing University, China). Next,
the mixture was transferred to a tube furnace and annealed in an argon atmosphere
flow of 80 mL min-1 at a desired temperature for 3 h. The ramp rate was 5 oC min-1.
Then, the tube furnace was automatically cooled to the room temperature. For
comparison, the milled SiO without PTFE was also annealed at 650 oC for 3 h.
(labeled as annealed SiO).
Materials Characterization: X-ray diffraction (XRD) patterns of the samples
were recorded on a D8 Discover (Bruker) equipped with Cu Ka (λ = 0.15406 nm)
radiation at a scan rate of 3.0 o min-1 from 10 to 80 o. 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 composition of the as-prepared composites.
Thermogravimetric (TG) analysis was performed on a TA Instrument (TA2960, US).
Raman spectra were recorded on a Raman micro spectrometer (InVia, UK). Nitrogen
adsorption and desorption isotherms were determined by nitrogen physisorption at 77
K on a Micrometritics ASAP 2020 analyzer. X-ray photoelectron spectroscopy (XPS)
analysis was performed by a multi-technique ultra-high vacuum Imaging XPS
Microprobe system (ESCALab 250, Thermo VG Scientific) and the CasaXPS
4
software (Version 2.3.16) was used to analyze the recorded spectra.
Electrochemical Measurement:2032 coin-type cells were assembled to
evaluate the electrochemical performance of the synthesized samples at the room
temperature. Working electrodes were composed of 75 wt% the synthesized samples
as active material, 10 wt% acetylene black (AB) as conductive agent and 15 wt%
sodium alginate as binder. The mass loading of the active material was 1.5-1.7 mg
cm-2. Electrolyte was 1 mol L-1 LiPF6 in a mixture of ethylene carbonate, dimethyl
carbonate and ethyl methyl carbonate with a volume ratio of 1:1:1. To form stable SEI
film, 10 wt% fluorinated ethylene carbonate (FEC) and 2 wt% vinylene carbonate
(VC) were added as additives. Lithium foil was used as counter electrode. Separator
was Celgard 2400 membrane. The assembly of cells was performed in a Braun glove
box (LabMaster, Germany) filled with ultra-high purified argon. Constant current
charge-discharge measurement was carried out on Neware battery testers (China) with
a voltage window of 0.02-1.2 V. The coating thickness changes on current collector
before and after cycling were measured by a micrometer in the glove box above.
Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were
carried out on an EC-Lab (VMP3, France). The scan rate for CV measurement was
0.2 mV s-1. EIS measurement was performed at open circuit potential by applying a
sine wave with an amplitude of 10 mV over a frequency range from 100 kHz to 0.01
Hz. The ions diffusion coefficient was obtained by the fitting analysis of EIS.
3. Results and Discussion
The as-prepared SiO@F-doped C composites are black in appearance, indicating
5
that brown SiO particles have been coated by carbon layer. Figure 1(a, b) show the
SEM images of the SiO@F-doped C composite obtained by annealing the mixture of
SiO and PTFE with a mass ratio of 5:3 at 650 oC. The composite consists of irregular
aggregates from roughly dozens to hundreds of nanometers in diameters. Compared
with the annealed SiO (Fig. S1 (a, b)), a layer of carbon shell can be observed in the
SiO particle surfaces of the SiO@F-doped C composite. The TEM images in Fig. 1 (c)
confirm that the composite shows a core-shell structure and there are interspaces
between core and shell. From the HRTEM image shown in Fig. 1 (c), some small
nano-voids appear on the surface of material. The EDS obtained by plane scanning
reveals that the composite is composed of Si, O, C and F elements, in which F:C is
about 1:5 at weight ratio, and the four elements have relatively homogenous
distribution (Fig. 1 (e)). From the TG curve in Fig. 1 (f), the composite contains about
7.6% F-doped C. Combining the TG curve of the SiO and PTFE mixture in argon (Fig.
S2 (b)), it can be calculated that when annealing the mixture of SiO and PTFE with a
mass ratio of 5:3 at 650 oC, the pyrolitic carbon yield of PTFE is about 12.7 wt%
while about 9.3 wt% SiO is chemically etched. Meanwhile, considering the
decomposition of SiO cannot happen at 650 oC, it can be concluded that the observed
interspaces and voids in the TEM and HRTEM images may come from the etching of
HF. Herein, it should be noted that individually annealing PTFE can barely produce
F-doped C solid at 650 oC in argon atmosphere (Fig. S2 (a)). However, although
PTFE in argon atmosphere could not directly generate HF gas at high temperature, the
difluorocarbon radicals (RCF2) formed at the initial decomposition would react with
6
the hydrogen containing, leading to hydrofluorocarbon formation which on
subsequent decomposition gave rise to HF gas and form carbon solid [34-36].
Therefore, in this work,it can be concluded that the hydroxyl groups on the surface of
SiO particles have strong interaction with PTFE during annealing, in which SiO is
coated by the generated F-doped C as well as etched by the released HF gas.
Brunauer–Emmett–Teller (BET) measurements confirm the porous nature of this
composite. Figure 1 (g) and (h) show the N2 adsorption/desorption isotherms and
pore-size distribution of the SiO@F-doped C composite, respectively. An obvious
hysteresis loop means the presence of mesopores. The absorbed N2 amount sharply
increases with increasing pressure after p/p0 > 0.8, indicating the presence of
macropores. From the pore-diameter distribution curve, the composite contains
micro-meso-macro pores. The BET specific surface area and pore volume of the
SiO@F-doped C are 36.3 m2 g-1 and 0.164 cm3 g-1, respectively. In contrast with the
SiO@F-doped C, only a not obvious hysteresis loop appear at the
absorption-desorption isotherms of the SiO sample annealed at 650 oC for 3 h and no
other peaks are observed except the peak at 3.9 nm in its pore-diameter distribution
curve (Fig. S1 (c, d)). Moreover, the annealed SiO shows only a BET specific surface
area of 14.62 m2 g-1 and a pore volume of 0.043 cm3 g-1, respectively. Therefore, it
can be concluded that the increased BET specific surface area and pore volume of the
SiO@F-doped C mainly come from both the pyrolitic F-doped C and the interspaces
and voids generated from SiO etching.
The XRD patterns of the raw SiO, annealed-SiO and SiO@F-doped C obtained
7
by annealing the mixture of SiO and PTFE with a mass ratio of 5:3 at 650 oC are
presented in Fig. 2 (a). The raw SiO sample shows strong SiO 2 characteristic peaks
(JCPDS 46-1045) which accords well with the observed SiO2 crystalline plane in the
HRTEM image, weak Si peaks (JCPDS 27-1402), and a weak and broad peak of
amorphous silicon oxides between 20 and 30o, indicating that the commercial SiO
also contains little crystalline silicon. As for the number and position of peaks, no
obvious difference can be observed in the XRD patterns of the annealed SiO and
SiO@F-doped C when compared with that of the raw SiO, indicating that both ball
milling and heat treatment at 650 oC in the material preparation process neither
change the crystalline structure of SiO nor lead to the decomposition of SiO. Morita
has demonstrated that the disproportionation reaction temperature of SiO is close to
1000 oC [6]. Three characteristic peaks at 471, 1347 and 1585 cm-1 in the Raman
spectroscopy of the SiO@F-doped C correspond to Si-Si stretching vibration of
amorphous silicon [37], sp3-bonded disordered carbon (D band) and sp2-bonded
graphitic carbon (G band), respectively, further confirming the presence of amorphous
elemental silicon and the successful coating of carbon (Fig. 2 (b)). The peak area ratio
of D band and G band is about 1.2, indicating that the main component of the
pyrolytic carbon is amorphous, and the formed carbon shell contains a large number
of defects [4]. The absence of the peak for crystalline Si may be due to its too low
content in the sample. The chemical compositions of the SiO@F-doped C,
particularly with regard to C and F, are further confirmed by X-ray photoelectron
spectroscopy. The C1s, O1s, F1s and Si2p peaks are an accurate representation of the
8
elemental composition of the composite (Fig. 2 (c-f)). In the C1s spectra, the four
prominent fitting peaks around 284.6 (285.3), 286.2 and 287.5 eV correspond to the
C−C, C−O and C−F chemical bands, respectively [38]. The fitting peaks of O1s at
532.8 and 531.2 eV indicate that O atoms combine with C and Si atoms. The fitting
peaks of F1s at 686.8 and 684.9 eV indicate that F atoms not only combine with C
atoms but are also doped in the SiO particles. In brief, these results confirm that SiO
particles are indeed coated by a layer of F-doped carbon.
The electrochemical performance of the SiO@F-doped C obtained by annealing
the mixture of SiO and PTFE with a mass ratio of 5:3 at 650 oC is evaluated using
CR2032 coin cells in a voltage range of 0.02-1.2 V vs Li+/Li. For comparison, the
annealed SiO without carbon coating is also investigated. Figure 3 (a) and (b) show
their charge-discharge profiles in the first three cycles at a current density of 100 mA
g-1. Unlike the SiO annealed at 1000 oC [6], a long slope instead of a flat plateau
appears between 0.3-0.0 V in their first discharge profiles, which is typical
characteristics of the undisproportionated SiO [13]. The annealed SiO exhibits a first
discharge capacity of 1718 mAh g-1 with a first coulombic efficiency of 54.7%. The
initial irreversible capacities denote the utilization of these capacities for the
formation of Li2O and Li4SiO4 which act as buffer components for improving the
cycle behavior [39]. The SiO@F-doped C shows a lower first discharge capacity of
1518 mAh g-1 but with a slightly higher first coulombic efficiency of 56.2%. To
observe their electrochemical behaviors in more detail, the CV curves of the two
materials are shown in Fig. 3 (c) and (d). The two electrodes show weak reduction
9
peaks near 1.20 and 0.75 V in the first cycle, which mainly corresponds to the
decomposition of electrolyte additives FEC and VC, respectively [40]. Except of the
mentioned point above, they present some obviously different behaviors in the CV
curves. With the increase of cycle number, in the cathodic branch, the reduction peak
of the annealed SiO appears near 0 V all the time, while that of the SiO@F-doped C
gradually shifts positively to 0.15 V. In the anodic branch, with the increase of cycle
number, the oxidation peak of the annealed SiO gradually shifts toward positive while
that of the SiO@F-doped C electrode has kept stable at about 0.54 V. These results
show that F-doped carbon coating effectively improves the electrode process kinetics
of SiO and the structural stability of the electrode.
Figure 4 (a) shows the cycling performance profiles of the annealed SiO and
SiO@F-doped C obtained by annealing the mixture of SiO and PTFE with a mass
ratio of 5:3 at 650 oC at 100 mA g-1. Although the annealed SiO exhibits much better
cycling stability when compared with silicon nanoparticles [41], its capacity still
degrades slowly with the increase of cycle number. In contrast, the SiO@F-doped C
exhibits rather stable cycling performance. It shows a stable discharge capacity of
about 975 mAh g-1 and obvious capacity degradation is not observed from the second
cycle. Moreover, from about 30 cycles, the SiO@F-doped C has been higher than the
annealed SiO in the charge-discharge efficiency, indicating F-doped C coating
enhances the reversibility of SiO. From Fig. 4 (b), after 350 cycles at 400 mA g -1, the
SiO@F-doped C shows a discharge capacity of 752 mAh g-1, while the discharge
capacity of the annealed SiO fades to 402 mAh g-1. Relative to the discharge capacity
10
in the second cycle, the capacity retention for the former is as high as 99.2% while the
value for the latter decreases to 49.2%, indicating the SiO@F-doped C can
accommodate better the strains from the volume change of active components during
cycling. As shown in Fig. 4 (c), the SiO@F-doped C has also much more excellent
rate capability than the annealed SiO. The SiO@F-doped C delivers stable discharge
capacities of about 985, 946, 880, 742 and 553 mAh g−1 at 100, 200, 400, 800 and
1600 mA g−1, respectively, and recovers to 990 mAh g−1 at 100 mA g−1. The bigger
the used current density is, the more obvious the electrochemical performance
difference between the two electrodes is. For instance, at 1600 mA g -1, the stable
discharge capacity of the SiO@F-doped C roughly doubles that of the annealed SiO
(271 mAh g-1). To understand the improved reason of the electrochemical
performance for the SiO@F-doped C, the coating thickness changes on the collectors
are measured before and after the specified cycling, and the obtained results are listed
in Tab. 1 and 2. After the cells are discharged to 0.02 V in the tenth cycle, the
annealed SiO electrodes expand an average volume of about 72%, while the
SiO@F-doped C electrodes show only an average volume expansion of about 28%,
indicating that the extra voids/caves in the SiO@F-doped C composite is really
helpful to accommodate the volume expansion of SiO. After the cells are charged to
1.2 V in the tenth cycle, the average volume of the annealed SiO electrodes is about
21% bigger than that before cycling, while the SiO@F-doped C electrodes show only
an average expanded volume of about 4.0%, indicating the latter have better
restoration ability during cycling, which should be mainly attributed to its special
11
structure and composition. Compared with the annealed SiO, in addition to the gaps
formed in the secondary particles during milling, the interspaces and voids formed
from HF etching not only provides extra space for the volume expansion, but also
further reduces SiO particle sizes. Meanwhile, F-doped C improves the conductivity
of SiO. The resistance measurement further confirms the enhanced conductivity and
structural stability of the SiO@F-doped C electrode. From Fig. 4 (d), before cycling,
the resistance diameter of the semicircle for the SiO@F-doped C electrode is only
about half of that for the annealed SiO electrode, indicating that F-doped C
significantly improves the conductivity of SiO. Figure 4 (e) shows that the diameter of
semicircle for the SiO@F-doped C electrode at high-medium frequency region which
mainly reflects the resistance of solid-electrolyte film and charge transfer [41], has
basically kept unchanged from the 10th to 100th cycle. The SEM images of the
SiO@F-doped C electrode after 200 cycles with different magnifications are shown in
Fig. S3. Except a layer of SEI film, the delamination and pulverization of the
electrode material are not observed. These further confirm that the SiO@F-doped C
electrode can well accommodate the volume change during cycling. The influence of
the etching and F-doped C on the kinetics of the SiO electrode process is also
evaluated. The fitting analysis of EIS shows that the diffusion coefficient of lithium
ions in the SiO@F-doped C electrode is about 2.94 times of that in the annealed SiO
electrode (Fig. S4). Therefore, the excellent electrochemical performance of the
synthesized SiO@F-doped C electrode should be attributed to the significantly
improved structural stability, electronic conductivity and lithium ion transferring
12
ability.
Figure 5 (a) shows the cycling performance of the SiO@F-doped C composites
obtained by annealing the mixture of SiO and PTFE with different mass ratios at 650
o
C at a current density of 400 mA g -1. It can be seen that the mass ratios of SiO and
PTFE affect both the specific capacity and cycling stability of the synthesized
composites. The SiO@F-doped C obtained with a mass ratio of 5:1 shows the highest
discharge capacity, but slow capacity degradation happens, which is attributed to both
low carbon content and low SiO etching degree (see below), not enough for buffering
the volume effect during cycling. With the increase of PTFE dosage, the specific
capacities of the synthesized composites decrease gradually. Despite of their capacity
difference, the other composites exhibit excellent cycling stability. In comparison, the
SiO@F-doped C obtained with a mass ratio of 5:3 exhibits the highest stable
discharge capacity, accompanied with good cycling stability. The TG and BET
analysis results of these composites are shown in Tab. S1. With increasing PTFE
dosage, the carbon content increases to 35.5% from 3.4%, while the specific surface
increases to 69.13% from 19.82%. The etching function of PTFE is further confirmed
by the XRD patterns in Fig. S5 (a). With the increase of PTFE dosage, the peaks of
crystalline SiO2 weaken gradually, indicating more and more SiO is etched away
during annealing. Figure S6 shows the SEM and TEM images of the SiO@F-doped C
composite obtained by annealing the mixture of SiO and PTFE with a mass ratio of
1:5 at 650 oC. Although it still keeps core-shell structure, a thicker coating layer can
be observed. Therefore, the decreased specific capacity of the composites should be
13
ascribed to the decrease of SiO component. Figure 5 (b) shows the cycling
performance of the SiO@F-doped C composites obtained by annealing the mixture of
SiO and PTFE with a mass ratio of 5:3 at different temperatures at a current density of
400 mA g-1. The composite synthesized at 550 oC shows relatively poor cycling
stability. After 300 cycles, the discharge capacity fades to 568 mAh g-1 from 758 mAh
g-1 in the 2nd cycle. The specific capacities of the two composites synthesized at 750
and 850 oC are slightly lower than that of the composite synthesized at 650 oC, but the
three composites show excellent cycling performance in the tested cycles. Although
the composite synthesized at 950 oC also shows good cycling performance, its stable
discharge capacity is only about 550 mAh g-1 and a longer activation time is needed.
From their XRD patterns (Fig. S5 (b)), the composites synthesized at less than 950 oC
do not show obvious difference, which keep the same characteristics as the annealed
SiO. However, more pronounced peaks of crystalline Si can be observed in the XRD
pattern of the composite synthesized at 950 oC, indicating the part disproportionation
reaction of SiO has happened. From their composition and BET analysis results (Tab.
S2), the composite synthesized at 550 oC has the highest F-doped C content and the
lowest total pore volume, suggesting that the pyrolysis of PTFE is not complete, thus
responsible for the relatively poor cycling stability. With the increase of annealing
temperature, the carbon content slightly increases, indicating that the temperature has
little influence on the etching of SiO after over than 650 oC, which may be due to low
pyrolysis temperature of PTFE. It should be noted that compared with the other
composites, although the composite synthesized at 950 oC shows only little difference
14
in the carbon content, its specific capacity is a lot smaller and a longer activation time
is needed. Figure S7 shows its SEM and TEM images. It is found that the composite
has been sintered to form big bulks, which accords with its low specific area and
small pore volume in BET analysis, resulting in a low utilization of active
components.
4. Conclusion
A simple, scalable and cost-effective routine is developed to synthesize new
core-shell SiO@F-doped C composites, using commercial SiO and PTFE as starting
materials, in which both etching and carbon coating of SiO are performed in a single
step. The influences of material ratios and annealing temperatures on the
electrochemical performance of the synthesized composites are investigated. The
composite synthesized at the optimized conditions exhibits high capacity with
excellent cycling performance and rate capability. The enhanced electrochemical
performance is attributed to the stable electrode structure and the significantly
improved conductivity and lithium ion diffusion ability. Considering the simplicity of
preparation procedure and the excellent electrochemical performance, the optimized
composite is promising to become an anode candidate for high-performance
lithium-ion batteries in the future.
Acknowledgment
This research was financially supported by Natural Science Foundation of China
(No. 51374175 and No. 21576030), Lithium-ion Battery Innovative Team Project of
China West Normal University (No. CXTD2015-1), Scientific Research Found of
15
Sichuan Provincial Education Department (No.17TD0036) and Science and
Technology Foundation of Sichuan Province (No. 2017JY0015).
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21
Figure captions
Fig. 1. SEM (a) and magnified SEM (b) images, TEM (c) and HRTEM (d) images,
EDS spectra with elemental mapping images (e), TG curve (f), nitrogen
adsorption-desorption isotherms (g) and BJH desorption pore-size distribution (h) of
the SiO@F-doped C composite obtained by annealing the mixture of SiO and PTFE
with a mass ratio of 5:3 at 650 oC.
Fig. 2. XRD patters (a), Raman spectrum (b) and HRXPS spectra (c-f) of the
SiO@F-doped C composite obtained by annealing the mixture of SiO and PTFE with
a mass ratio of 5:3 at 650 oC.
Fig. 3. Charge-discharge profiles of the annealed SiO (a) and SiO@F-doped C
obtained by annealing the mixture of SiO and PTFE with a mass ratio of 5:3 at 650 oC
(b) in the first three cycles at 100 mA g-1 ; Cyclic voltammetry curves of the annealed
SiO (c) and SiO@F-doped C (d) at a scan rate of 0.2 mV s -1.
Fig. 4. Cycling performance profiles of the annealed SiO and SiO@F-doped C
obtained by annealing the mixture of SiO and PTFE with a mass ratio of 5:3 at 650 oC
at 100 mA g-1 (a) and 400 mA g-1 (b); Rate capability of the annealed SiO and
SiO@F-doped C (c); Resistance comparison of the annealed SiO and SiO@F-doped
C electrodes before cycling (d), and resistance change of the SiO@F-doped C
electrode with cycle number (e).
Fig. 5. Cycling performance profiles of SiO@F-doped C obtained by annealing the
mixture of SiO and PTFE with different mass ratios at 650 oC (a); Cycling
22
performance profiles of SiO@F-doped C obtained by annealing the mixture of SiO
and PTFE with a mass ratio of 5:3 at different temperatures (b).
23
Fig.1
24
105
70
(g)
(f)
Desorption
Adsorption
-1
60
95
3
Quantity Adsorbed / cm g
Weight Fraction / %
100
92.45%
90
85
80
75
50
40
30
20
10
70
0
100
200
300
400
500
600
700
0
800
0.0
0.2
0.4
0.6
0.8
o
Temperature / C
Relative Pressure / p/p
0.016
0.012
3
-1
BJH Desorption dV/dw / cm g nm
-1
(h)
0.008
0.004
0.000
0
10
20
30
40
50
60
70
Pore Width / nm
25
o
1.0
Fig. 2
(a)
Intensity / A.U.
SiO@F-doped C
(b)
G Band
Annealed SiO
D Band
Intensity / A.U.
amorphous Si
SiO2
Si
Raw SiO
10
20
30
40
50
60
70
80
200
400
600
2Theta / Degree
800
1000
1200
F-C
Intensity / A.U.
Intensity / A.U.
2000
(d)
C-C
290
1800
C-C
F1s
292
1600
-1
(c)
C1s
C-F
1400
Raman Shift / cm
F-Si
C-O
288
286
284
282
280
278
695
690
Binding Energy / eV
685
680
675
Binding Energy / eV
(e)
(f)
Si2p
O1s
O-C
Intensity / A.U.
Intensity / A.U.
Si
O-Si
544
540
536
532
528
524
520
Si
Si
4+
2+
Si
108
Binding Energ / eV
3+
106
104
102
+
Si
100
Binding Energy / eV
26
0
98
96
Fig. 3
1.5
1.5
+
+
0.9
0.6
0.3
(b)
1st
2nd
3rd
1.2
Potential / V vs Li /Li
1.2
Potential / V vs Li /Li
(a)
1st
2nd
3rd
0.9
0.6
0.3
0.0
0.0
0
200
400
600
800
1000
1200
Specific Capacity / mAh g
1400
1800
0
200
400
-1
600
800
1000
Specific Capacity / mAh g
0.65 V
0.5
1600
(c)
1200
0.54 V
1.0
1400
1600
-1
(d)
0.54 V
0.56 V
0.5
0.0
0.0
I / mA
I / mA
1.19V
0.78 V
1st
2nd
3rd
4th
5th
-0.5
-1.0
0.72 V
-0.5
1st
2nd
3rd
4th
5th
-1.0
-1.5
0.15 V
-2.0
0.0
0.3
0.6
0.9
1.2
0.0
1.5
0.3
0.6
0.9
+
Potential / V vs Li /Li
+
Potential / V vs Li /Li
27
1.2
1.5
Fig. 4
1800
2000
100
100
60
-1
800
40
400
-1
20
0
0
10
20
30
40
50
60
70
80
80
1200
60
400 mA g
900
-1
40
600
20
300
0
0
100
90
Annealed SiO
SiO@F-doped C
0
50
100
150
200
250
300
0
350
Cycle Number
Cycle Number
2000
100
80
1200
100
100
200
60
400
800
800
40
1600
400
20
0
0
10
20
30
40
50
60
70
80
Coulombic Efficiency / %
Discharge Capacity / mAh g
-1
(c)
Annealed SiO
SiO@F-doped C
-1
Current unit: mA g
1600
0
90
Cycle Number
200
20
(d)
Annealed SiO
SiO@F-doped C
(e)
After 10st cycle
After 100th cycle
15
Zim / Ω
Zim / Ω
150
100
50
10
5
0
0
50
100
150
0
200
5
Zre / Ω
10
15
20
25
Zre / Ω
28
30
35
40
Coulombic Efficiency / %
100 mA g
Discharge Capacity / mAh g
80
1200
(b)
1500
Annealed SiO
SiO/F-doped C
1600
Coulombic Efficiency /%
Discharge Capacity / mAh g
-1
(a)
Fig. 5
1400
(a)
SiO:PTFE=5:1
SiO:PTFE=5:5
SiO:PTFE=3:5
SiO:PTFE=1:5
-1
1200
400 mA g
(b)
o
550 C
o
750 C
o
850 C
o
950 C
1200
Discharge Capacity / mAh g
Discharge Capacity / mAh g
-1
1600
-1
800
400
0
1000
Current Density: 400 mA g
-1
800
600
400
200
0
0
50
100
150
200
250
300
0
Cycling Number
50
100
150
Cycle Number
29
200
250
300
Tab. 1. Coating thickness changes of the SiO@F-doped C electrodes obtained by
annealing the mixture of SiO and PTFE with a mass ratio of 5:3 at 650 oC after 10
cycles.
Electrode number
1
2
3
4
5
6
7
8
Before cycling/um
40.7
40.5
40.0
40.3
39.3
40.3
39.0
41.0
51.8
51.0
52.0
51.7
/
/
/
/
Charged to 1.2 V/um
/
/
/
/
40.3
41.6
41.7
42.0
Thickness ratios/times
1.27
1.26
1.30
1.28
1.02
1.03
1.07
1.02
Mean ± S*
1.28 ± 0.02
Discharged to
0.02V/um
1.04 ± 0.02
*S represents standard deviation.
30
Tab. 2.
Coating thickness changes of the annealed SiO electrodes after 10 cycles.
Electrode number
1
2
3
4
5
6
7
8
Before cycling/um
32.3
36.0
35.7
35.3
35.0
37.0
38.5
37.5
Discharged to 0.02 V/um
57.0
63.3
59.3
60.3
/
/
/
/
Charged to 1.2 V/um
/
/
/
/
43.0
44.3
45.7
46.0
Thickness ratios/times
1.76
1.76
1.66
1.71
1.23
1.20
1.19
1.23
Mean ± S*
1.72 ± 0.05
1.21 ± 0.02
*S represents standard deviation.
31
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