SiO2@C hollow sphere anodes for lithium-ion batteries

Xuelian Liu a, Yuxi Chen a,*, Hongbo Liu a, Zhi-Quan Liu b

a College of Materials Science and Engineering/Hunan Province Key Laboratory for Spray Deposition Technology and Application, Hunan University,Changsha 410082, Chinab Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

A R T I C L E I N F O

Article history:Received 16 May 2016Received in revised form7 July 2016Accepted 12 July 2016Available online 23 December 2016

Keywords:SilicaHollow spheresCarbon coatingAnodeLithium-ion batteries

As anode materials for lithium-ion batteries, SiO2 is of great interest because of its high capacity, lowcost and environmental affinity. A facile approach has been developed to fabricate SiO2@C hollow spheresby hydrolysis of tetraethyl orthosilicate (TEOS) to form SiO2 shells on organic sphere templates fol-lowed by calcinations in air to remove the templates, and then the SiO2 shells are covered by carbon layers.Electron microscopy investigations confirm hollow structure of the SiO2@C. The SiO2@C hollow sphereswith different SiO2 contents display gradual increase in specific capacity with discharge/charge cycling,among which the SiO2@C with SiO2 content of 67 wt% exhibits discharge/charge capacities of 653.4/649.6 mA h g−1 over 160 cycles at current density of 0.11 mA cm−2. The impedance fitting of theelectrochemical impedance spectroscopy shows that the SiO2@C with SiO2 content of 67 wt% has the lowestcharge transfer resistance, which indicates that the SiO2@C hollow spheres is promising anode candi-date for lithium-ion batteries.

Copyright © 2017, The editorial office of Journal of Materials Science & Technology. Published byElsevier Limited.

1. Introduction

Rechargeable lithium-ion batteries (LIBs) have a variety of appli-cations, such as portable electronic devices, electrical vehicles andrenewable energy storage [1–3]. Development of advanced LIBs re-quires incorporation of high-performance materials to replace thecurrently commercial graphite anodes [4]. Si is the most promisinganode candidate due to its moderate operating voltage (0.1–0.5 V vsLi/Li+) and high theoretical capacity of 4200 mA h g−1, which is overten times higher than that of commercial graphite (372 mA h g−1) [5–7].However, the large volume expansion of Si particles during lithiation(over 300%) may lead to internal cracks and the solid electrolyte inter-phase (SEI) layers formed on the Si surfaces unstable, which resultin its poor cyclic performance [8]. To eliminate the volume variationeffects, Si nanoparticles [9], nanotubes [10,11] and nanofibers [12]have been explored. In addition, combination of Si with amorphouscarbon has been demonstrated to be a very effective way to improvethe cyclic stability, since the amorphous carbon can act as buffer layersto accommodate the large volume variation [13–15].

In recent years, SiO2 has attracted much attention as anode can-didate for LIBs. Previous investigations indicate that nanostructuredSiO2 can be an alternative way to improve the cyclic performanceof Si-based anode [16,17]. During the initial lithiation of SiO2, nano-

Si domains and inert Li4SiO4 or Li2O domains are uniformly formed,which help to keep the mechanical integrity of the electrodes andthus improve the cyclic performance [18,19]. However, the low elec-tronic conductivity and volume variation of SiO2 remain challengesfor its further applications. Nanostructured SiO2/carbon compos-ites anode is believed to be an effective way, such as SiO2-carboncomposite [20], SiO2/C nanofibers [21], three dimensional SiO2@graphene composite [19], etc. Herein, we report a facile wet chemicalsynthesis of SiO2@C hollow spheres. Further, their electrochemi-cal performance as anode for LIBs has also been evaluated.

2. Experimental

2.1. Materials preparation

The schematic of fabrication process of SiO2@C hollow spheresis illustrated in Fig. 1. In the first, 0.6 mL Polyacrylic acid (PAA)was added dropwise into 100 mL ethanol containing 4 mL ammo-nium hydroxide and the sphere template was formed withinseconds. Then, 4.2 mL TEOS were added dropwise into the solu-tion under vigorous stirring. The reaction was preceded for 10 h.Then the product was washed with ethanol and then dried invacuum overnight. The SiO2 hollow spheres could be obtainedafter the template was removed by calcinations at 600 °C in air.

0.5 g as-prepared SiO2 product, certain weight of sucrose and 3 mLconcentrated hydrochloric acid were added into 30 mL deionized water

* Corresponding author.E-mail address: yxchen@hnu.edu.cn (Y. Chen).

http://dx.doi.org/10.1016/j.jmst.2016.07.0211005-0302/Copyright © 2017, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.

Journal of Materials Science & Technology 33 (2017) 239–245

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and 40 mL ethanol under stirring, as described elsewhere [14]. Afterbeing ultrasonicated, the solution was stirred for 3 h at 60 °C, andthen dried at 120 °C overnight. The resulting 0.5 g SiO2 mixed withsucrose was transferred into a corundum boat installed in the middleregion of a tube furnace. The furnace was purged with pure argongas flow and then heated to 800 °C for 2 h. The SiO2@C hollow sphereswere finally obtained and denoted as SiO2@C-1.5, SiO2@C-2 and SiO2@C-2.5, respectively, when the added sucrose was 0.75 g, 1 g and 1.25 g,respectively.

Certain weight of the three samples was transferred into threecorundum boats, respectively, and then put into a muffle furnace.The SiO2 contents were measured by calculating weight loss of thesamples after heating samples to 800 °C for 5 h in air. The resultsindicate that SiO2 contents of SiO2@C-1.5, SiO2@C-2 and SiO2@C-2.5 are 75 wt%, 67 wt% and 58 wt%, respectively.

2.2. Characterization

The morphology and structure of the SiO2@C hollow spheres wereinvestigated using a S-4800 field emission scanning electron mi-croscope (SEM), a Titan G2 60-300 transmission electron microscope

(TEM), a ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS) andBruker D8 Advance X-ray diffraction (XRD) using Cu Kα radiation.

2.3. Electrochemical measurements

The electrochemical performance of the SiO2@C hollow sphereswere measured using 2025-type coin cells assembled with Li metalsas counter electrodes in an argon-filled glove box. The electrode ma-terials were prepared by mixing active material (70 wt%), carbonblack (10 wt%) and PVDF (20 wt%) dissolved in N-methylpyrrolidone(NMP) with constant stirring for 6 h to form a homogeneous slurry.Then, the slurry was pasted onto a Cu foil, and dried at 80 °C for6 h. The loading weight of the mixture on the electrode was ca. 5 mg,and the area of electrode is 1.13 cm−2. The electrolyte solution wascomposed of 1 M LiPF6 dissolved in dimethylcarbonate (DMC) andethylenecarbonate (EC) mixed solvent of 1:1 by volume. Cyclicvoltammetry (CV) data were recorded on an Arbin BT 2000 systemat a scanning rate of 0.1 mV s−1. The electrode capacities were mea-sured using a galvanostatic discharge-charge method at a currentdensity of 0.11 mA cm−2 in a potential range of 0.001–3 V on a LandCT2001A system. EIS measurements were performed on an elec-trochemical workstation (CHI660D) by applying a sine wave withamplitude of 5.0 mV over the frequency range from 100 kHz to0.01 Hz. The current density (mA cm−2) of the discharge/charge testwas calculated based on the area of the electrodes. The specific ca-pacities were calculated according to the weight of the SiO2@C hollowspheres.

3. Results and Discussion

Fig. 2 illustrates XRD profile of SiO2@C-2. The broadening peakbetween 2θ = 20° and 25° is probably due to the presence of amor-phous SiO2 and carbon. Previous studies have proposed thatcompared with crystalline silicon oxide, amorphous silicon oxidehas lower activation energy barrier and structure strain during phasetransition of electrochemical lithiation-delithiation [20,22].

Fig. 3(a–c) shows SEM images of SiO2@C-1.5, SiO2@C-2 and SiO2@C-2.5, respectively. All the nanocomposites display spherical particle

Fig. 1. Schematic of fabrication process of the SiO2@C hollow spheres. SiO2 shell is formed on the PAA sphere template, and then carbon coating layer covers the shell aftercalcination of the template.

Fig. 2. XRD profile of SiO2@C-2.

Fig. 3. SEM images of (a) SiO2@C-1.5, (b) SiO2@C-2 and (c) SiO2@C-2.5.

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shape. The diameter of the particles varies from 50 to 350 nm andthe surface is relatively rough. TEM images (Fig. 4) confirm the hollowsphere structure of the three nanocomposites. It can be seen thatthe hollow spheres display almost uniform shell thickness, whichis around 35 nm. From Fig. 4(a), it is difficult to distinguish the carbonlayers in SiO2@C-1.5 which has the lowest carbon content. However,

the connection of the hollow spheres by carbon is clearly visiblein Fig. 4(c) of SiO2@C-2.5.

In order to investigate microstructural feature of carbon coatinglayer, SiO2@C-2 was selected to perform high angle annular darkfield (HAADF) characterization, as shown in Fig. 5(a). The corre-sponding O, Si and C element mapping of SiO2@C-2 has also been

Fig. 4. TEM images of (a) SiO2@C-1.5, (b) SiO2@C-2 and (c) SiO2@C-2.5.

Fig. 5. (a) HAADF image of SiO2@C-2 and the corresponding element mapping images of (b) O, (c) Si and (d) C.

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conducted, as shown in Fig. 5(b–d), respectively, which imply thatthe amorphous carbon layers are uniformly covered on the SiO2

shells. The structure and morphology of SiO2@C-1.5 and SiO2@C-2.5 are very close to SiO2@C-2 (not shown), because the onlydifference among them is carbon content.

CV performances of SiO2@C-1.5, SiO2@C-2 and SiO2@C-2.5 wereevaluated within 0.001–3 V voltage window at a scanning rate of0.1 mV s−1, as shown in Fig. 6. The three samples exhibit similar CVprofiles. Two cathodic peaks appear at ca. 0.75 V and ca. 1.25 V in thefirst CV scanning, respectively, which are probably associated withirreversible reactions between the electrode and electrolyte and theformation of the solid electrolyte interface (SEI) layer [23–25]. Theformer disappears while the latter moves to low potential (ca. 1 V)in the following two cycles. The big cathodic peak at ca. 0. 1 V is dueto the Li-Si alloy process [25]. Two anodic at ca. 0.2 V and ca. 1.2 Vcan be observed in the three cycles, corresponding to delithiation ofthe active materials.

The voltage profiles of SiO2@C-1.5, SiO2@C-2 and SiO2@C-2.5 withcurrent density of 0.11 mA cm−2 and cutoff voltages of 0.001−3 V areshown in Fig. 7(a–c), respectively. It can be seen that the lithiationpotential of SiO2@C-1.5 starts at ca. 1 V, followed by a voltage plateauat ca. 0.75 V, which is replaced by voltage slopes in the following cycles.The delithiation potential starts at ca. 0.1 V and increases gradually

until 3 V. SiO2@C-2 and SiO2@C-2.5 exhibit similar voltage profileswith SiO2@C-1.5.

Fig. 8 shows cyclic behavior of the three samples. As a compari-son, cyclic performance of the SiO2 hollow spheres without carboncoating has also been displayed. SiO2 hollow spheres exhibit very lowspecific capacities, which are around 90 mA h g−1. For the three sampleswith carbon coating layers, the reversible capacities gradually in-crease with cycling. SiO2@C-1.5 which has the least carbon contentshows a slight rise in reversible capacity from the initial 86 mA h g−1

to 200.3 mA h g−1 over 160 cycles. In contrast, SiO2@C-2 exhibits a dra-matic increase from the initial 153.6 mA h g−1 to 649.6 mA h g−1 over160 cycles. SiO2@C-2.5 with the highest carbon content exhibits mod-erate increase tendency, from the initial 147.2 mA h g−1 to328.1 mA h g−1. It can be seen that the initial coulombic efficiencies(CEs) of the three samples are low, which are 40.7%, 48.2% and 46.3%for SiO2@C-1.5, SiO2@C-2 and SiO2@C-2.5, respectively. However, theCEs of the three samples are generally over 97% after 10 cycles.

Up to date, the electrochemical lithiation mechanism of SiO2 isnot fully clear. Previous reports indicate that the lithiation process

Fig. 6. CV profiles of (a) SiO2@C-1.5, (b) SiO2@C-2 and (c) SiO2@C-2.5 in the 0.001–3.0 V voltage window.

Fig. 7. Voltage profiles of (a) SiO2@C-1.5, (b) SiO2@C-2 and (c) SiO2@C-2.5 with currentdensity of 0.11 mA cm−2 and cutoff voltages of 0.001–3.0 V.

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includes complex irreversible and reversible reactions of SiO2

[16,23,25]. The phenomenon of gradual increase in specific capac-ity has been observed in previous studies and it was explained bynon-full utilization of SiO2 in the initial cycling and gradual growthof Si phase during later lithiation-delithiation cycling [20,26].

To better understand electrochemical performance of SiO2@C, XPSwas carried out to confirm the chemical state of the nanocompositeafter cycling test. The XPS spectrum of SiO2@C-2 after 50 cycles isshown in Fig. 9. As can be seen from Fig. 9(b, c), the Si2p peak at103.5 eV and O1 s peak at 532.25 eV correspond to SiO2, and severalSi2p peaks at around 98.8 eV are attributed to Si phase. Therefore,SiO2 and Si both exist in the SiO2@C after cycling.

Fig. 10 displays morphology and structure of the electrode ofSiO2@C-2 after 160 lithiation-delithiation cycles. Compared with Fig.3(b), it can be seen that most of the active SiO2@C-2 materials main-tain deformed sphere morphology. However, some of them havebeen broken, which may be resulted from repeated volume varia-tion of SiO2@C during lithiation-delithiation cycling.

Table 1 compares electrochemical performance of various SiO2

and SiO2-C composite anode materials in previous reports. TheSiO2@C hollow spheres have relatively lower specific capacity in theinitial cycle. However, its reversible capacity experiences a great in-crease to 649 mA h g−1 over 160 cycles, which is just lower than pureSiO2 nanocubes.

To understand electrochemical kinetics of the SiO2@C hollowspheres, EIS measurements coupled with impedance fitting havebeen performed. The Nyquist plots of SiO2, SiO2@C-1.5, SiO2@C-2 andSiO2@C-2.5 at different cycle numbers are exhibited in Fig. 11(a–d),

respectively. All measurements were carried out after full relax-ation of the cells. The plots can be considered to consist of onedepressed semicircle in the high frequency region, two depressedsemicircles in the high to middle frequency region, followed by aslopped line in the low frequency region. According to previousstudies [27–30], an equivalent circuit (Fig. 11(e)) is proposed to fitthe Nyquist plots. It consists of an Rs representing solution resis-tance, three units in series, which is composed of a resistor (Rsei,

Fig. 8. Cyclic behavior and coulombic efficiencies of SiO2, SiO2@C-1.5, SiO2@C-2 andSiO2@C-2.5.

Table 1Electrochemical performance of SiO2 and SiO2-C composite anodes for lithium-ion batteries.

Materials Content ofSiO2 (wt%)

Currentdensity

Initial discharge/charge specificcapacity (mA h g−1)

Reversible capacity(mA h g−1)/cycle number (n)

Ref.

SiO2 thin film 100 28 μA cm−2 539/388 510/100 [23]SiO2 nanocubes 100 100 mA g−1 3084/1457 919/30 [25]Carbon-coated SiO2 nanopaticles 50.1 50 mA g−1 536/Ca.900 Above 500/50 [17]Silicon oxide-carbon 95.8 70 mA g−1 1055/458 601/100 [20]Carbon-coated silica macroparticles — 0.2 mA cm−2 281/95 222/100 [26]SiO2@graphene composite — 500 mA g−1 — Ca. 300/110 [19]SiO2@C hollow spheres 67 0.11 mA cm−2 318.7/153.6 649.6/160 This work

Fig. 9. (a) XPS survey spectrum of the SiO2@C-2 after 50 discharge/charge cycles;Enlarged (b) Si2p and (c) O1 s spectra of the SiO2@C-2 after 50 discharge/charge cycles.

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Rint or Rct) and a constant phase element (CPEsei, CPEint or CPEct) inparallel and a Warburg diffusion element (Zw). The subscripts sei,int and ct represent the SEI film, interphase electronic contact andcharge transfer, respectively. The fitted impedances data denotedby solid lines agree well with the experimental ones.

Rct of the four nanocomposites at different cycles are obtainedand plotted as a function of cycle number in Fig. 12. It can be seenthat all the three composites experience the highest Rct value at the

3rd cycle, which are 426.7 Ω, 442.7 Ω, 173.9 Ω and 352.6 Ω for SiO2,SiO2@C-1.5, SiO2@C-2 and SiO2@C-2.5, respectively, and then de-crease to 104.5 Ω, 117.9 Ω, 26.01 Ω and 101.2 Ω at the 200th cycle,respectively. SiO2@C-2 exhibits the lowest Rcts among the threesamples, which is consistent with its highest cyclic capacities.

Fig. 10. SEM image of the electrode of SiO2@C-2 after 160 lithiation-delithiation cycles.

Fig. 11. The Nyquist plots of (a) SiO2, (b) SiO2@C-1.5, (c) SiO2@C-2 and (d) SiO2@C-2.5. The solid curves represent quantitative fitting data. (e) The equivalent circuit forquantitative simulation of the Nyquist plots.

Fig. 12. Plots of Rct as function of cycle number of SiO2, SiO2@C-1.5, SiO2@C-2 andSiO2@C-2.5.

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4. Conclusion

SiO2@C hollow spheres were fabricated by a template methodfollowed by carbon coating, which exhibit gradual increase of cycliccapacity with cycling. The SiO2@C with SiO2 content of 67 wt% dis-plays the best electrochemical performance. Its reversible capacitycan reach 649.6 mA h g−1 over 160 cycles. Consistently, its chargetransfer resistances are the lowest among three samples with dif-ferent SiO2 content. The SiO2@C hollow spheres are promising anodecandidate for high energy-density lithium-ion batteries.

Acknowledgement

This work was supported by the National Natural Science Foun-dation of China (Grant No. 51472083).

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