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C composites based on KI-assisted magnesiothermic reduction for high performance Li-ion batteries
Applied Surface Science 481 (2019) 933–939
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Morphology-controlled synthesis of hollow Si/C composites based on KIassisted magnesiothermic reduction for high performance Li-ion batteries
T
Pei Hana,b, Weicheng Sunb, Dongzhi Lib, Donghai Luob, Yanzi Wangb, Bo Yangb, Cuihua Lib, ⁎ ⁎ Yiping Zhaoa, , Li Chena, Jian Xuc, Caizhen Zhub, a
School of Materials Science and Engineering of Tianjin Polytechnic University, Tianjin 300387, China Institute of Low-dimensional Materials Genome Initiative, College of Chemistry and Environmental Engineering of Shenzhen University, Shenzhen 518060, China c Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b
ARTICLE INFO
ABSTRACT
Keywords: Magnesiothermic reduction Potassium iodide Si carbide Hollow Si composites Lithium-ion batteries
Magnesiothermic reduction of silicon oxide (SiO2) is thought to be an effective method for preparing nanostructured silicon (Si). However, the magnesiothermic reduction can hardly maintain the original shape of the SiO2 due to the harsh reaction condition. Carbon coating is an efficient strategy to prevent the original structure collapse and improve its conductivity, while the by-product silicon carbide (SiC) is prone to form due to the overheating phenomenon during the process, which is useless to the Li-ion storage. Herein, potassium iodide (KI) is used as heat scavenger for the first time to control the formation of SiC, in which the melt of KI can absorb excess heat during the harsh reduction process. Ideally, hollow Si/C nanospheres and nanorods with little content of SiC are successfully synthesized by the KI-assisted magnesiothermic reduction from carbon coated SiO2 precursors. These novel hollow Si/C nanospheres and nanorods endow it with an excellent electrochemical performance as anodes for Li-ion battery. In detail, Si/C nanospheres achieve a specific capacity of 1792 mAh g−1 after 100 cycles at 1 A g−1, and 1271 mAh g−1 even at a high current density of 3 A g−1 after 200 cycles. This novel strategy provides an effective and facile way to reduce SiC generation during magnesiothermic reduction reaction and can be applied to obtain Si/C composites with various morphology.
1. Introduction Si has been identified as one of the most hopeful alternatives to commercial graphite for LIBs for a long time owing to its high theoretical capacity of ~4200 mAh g−1, environment-friendliness, and abundance in nature [1,2]. However, the huge volume change during lithiation-delithiation process, especially for those grain size > 150 nm, causes not only the loss of mechanical/electrical contacts between Si and current collector, but also the continuous formation of unstable solid-electrolyte interphases (SEIs), which consumes too many lithiumions and leading to rapid capacity fading [3–5]. To address those issues above, various Si nanostructures, such as nanorods, [6] nanowires, [7–9] thin films, [10,11] nanocrystallines, [12–16] and porous nanostructures, [17–21] have been successfully synthesized with greatly improved electrochemical performance. However, expensive and toxic raw material as SiCl4 or SiH4 and complicated processing with low yield (e.g. electrochemical etching, electrospinning and chemical vapor deposition (CVD)) are generally used in the preparation of above mentioned Si nanostructure. Apart from
⁎
nanostructured Si, the introduction of void spaces in Si has been raised to be another valid strategy to buffer the volume change of Si during cycling [22–24]. Various core-shell hollow structures have thus been fabricated for further stabilizing the volume changing [25–31]. For instance, yolk-shell Si particles were synthesized by using SiO2 as sacrificial template, which achieved a specific capacity of 1500 mAh g−1 at 1 C after 1000 cycles [32]. Similarly, granadilla-like Si was achieved via removing sacrificial layer of calcium carbonate, which could retain 1100 mAh g−1 at 250 mA g−1 after 200 cycles [33]. The void spaces indeed can relieve the Si volume change during cycling, however the introduction and removal of sacrificial layer makes this process complicated. Therefore, it is necessary to exploit a facile, low-cost and environmentally friendly strategy for synthesizing hollow Si. Among varieties of approaches to fabricate nanostructured Si, magnesiothermic reduction [5,34,35] has been widely applied in the transformation of SiO2 into Si with relatively low power consumption (~650 °C). The reaction formula is shown as follows:
2Mg(s) + SiO2 (s)
Corresponding authors. E-mail addresses: [email protected] (Y. Zhao), [email protected] (C. Zhu).
https://doi.org/10.1016/j.apsusc.2019.03.051 Received 13 December 2018; Received in revised form 22 February 2019; Accepted 4 March 2019 Available online 05 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
2MgO (s) + Si (s)
(1)
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However, the magnesiothermic reduction process could hardly maintain the original shape of various SiO2 precursors, such as nanospheres, [36] nanofibers, [37] nanorods, [6] aerogels [38] and prone to crush into disordered microporous structure, caused by the harsh replacement reaction condition between SiO2 and Mg. A precoating protective carbon layer before reduction process is thought to be an effective strategy to prevent the structure collapse. Nevertheless, the formation of Si carbide (SiC) is hard to control during the magnesiothermic reduction process of SiO2/C composites. One probably reason is that, the massive heat from the exothermic magnesiothermic reaction (∆H = −593 kJ molSi−1) could not successfully released, [39] leading to a local overheating beyond the SiC formation temperature (~1100 °C). [40] Up to now, however, there is not an efficient way to control the thermodynamic byproduct of SiC. In this work, a facile strategy for the morphology-controlled preparation of hollow Si/C composites was reported by directly reducing carbon-coated silica using potassium iodide (KI) as the heat scavenger. SiO2 nanospheres and nanorods synthesized by soft-template method were selected as the precursors. It was found that the fusion of KI (m.p. 681 °C; ∆Hfusion (KI) = 15.05 kJ mol−1) could effectively absorb excessive heat released from the exothermic magnesiothermic reaction, which prevented a local overheating as well as the generation of SiC. The fusion heat of KI is calculated as the Eq. (2) (∆fHmθ(KI, l) = − 312.85 kJ mol−1; ∆fHmθ(KI, s) = − 327.90 kJ mol−1):
Hfusion (KI ) =
f
Hm (KI , l)
f Hm (KI , s )
comparison, silica spheres and rods without carbon coating were directly reduced by Mg using the same method mentioned above, which products were abbreviated as R-Si NSs and NRs. 2.3. Characterization The morphology was characterized by scanning electron microscope (SEM, JEOL 7500F) and transmission electron microscope (TEM, FEI Tecnai G2 F20), respectively. X-ray diffraction (XRD) patterns were taken by a Rigaku MiniFlex600 (with CuKa radiation) instrument, Raman spectra were collected on confocal HR800 spectrometer of HORIBA Jobin Yvon, X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALab220i-XL electron spectrometer from VG Scientific. Al-Kα radiation was used as the X-ray source and operated at 300 W. N2 (at 77.3 K) sorption measurements were performed using QUADRASORB SI/MP from Quantachrome Instruments. 2.4. Electrochemical measurements Electrodes were prepared by blade-coating a slurry of 60% of K-HSi samples, 20% super P and 20% of sodium alginate (SA) dispersed in deionized water on copper foil, and drying at 70 °C for 6 h and punched into circular discs for coin-cell fabrication, representing ~1 mg cm−2 of loading density. The cathode electrodes for full cell were blade-coating a slurry of 90% of LiCoO2, 5% super P and 5% of Poly (vinylidene fluoride) (PVDF) dispersed in N-Methyl pyrrolidone (NMP) on aluminum foil. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), fluorinated ethylene carbonate (FEC) (in a volume ration of 5:70:25). 2032 type coin cells were assembled in an argon-filled glove box using lithium metal as the counter electrode (LiCoO2 electrodes for full cell) and glass fiber as separator. The galvanostatic charge-discharge test was conducted by a Wu-Han land CT2001A testing system in the voltage range between 0.005 V and 2 V. Electrochemical impedance spectra (EIS) characterization were taken using a Solatron 1260 Impedance Analyzer. Cyclic voltammetry (CV) measurements were performed on a CHI660D electrochemical workstation with a scan rate of 0.1 mV s−1 in the voltage range of 0.005–1.5 V (vs. Li/Li+).
(2)
After KI-assisted magnesiothermic reduction, the reduced Si materials well inherit the initial morphology of silica precursors, owing to the confinement of precoated carbon layer. In addition, the subsequent aid treatment to remove MgO and unreacted SiO2 could create micropores and void spaces in the Si. The KI-assisted magnesiothermic reduced hollow Si/C nanospheres and nanorods abbreviated as K-HSi NSs and K-HSi NRs, respectively, were used as anode in Li-ion batteries, which exhibit a good cycling stability and a high specific capacity of 1792 mAh g−1 after 100 cycles at 1 A g−1, and 1271 mAh g−1 even at a high current density of 3 A g−1 after 200 cycles. 2. Experimental section
3. Results and discussion
2.1. Synthesis of SiO2 NSs and NRs
Strategy for fabricating hollow Si/C NSs and NRs is illustrated in Fig. 1. Firstly, the silica particles were encapsulated in polydopamine, which can maintain their morphology during the harsh magnesiothermic reduction process. Then the obtained particles were mixed with magnesium and KI powder, and calcined at 650 °C under Ar atmosphere. In this process, KI is used to absorb the excessive heat, thus the formation of SiC is inhibited. Finally, the hollow Si/C composites were obtained after the decomposition of MgO and unreacted SiO2 with acid washing. The SEM images for K-HSi NSs and NRs depicted that the resulted Si/C composites well inherit the sphere and rod-like morphology from silica used with a rough surface (Fig. 1). A hollow structure can be identified at some broken spheres for K-HSi NSs with a diameter of ~200 nm (Fig. 1b), caused by the remove of unreacted silica, [43] which were further confirmed by TEM images (Fig. 2a). K-HSi NRs demonstrate a uniformly rod-like nanostructure with an average width of ~100 nm and length of ~300 nm. Moreover, porous architecture can be observed in Fig. 1b. By comparison, the silica spheres and rods directly reduced by Mg without carbon coating (Fig. 1c and f) lose the primary shape, and agglomerate into a bulk, owing to the volume change and breakdown during the reduction process. The TEM images (Fig. 2a and b) demonstrate hollow structure for KHSi NSs and NRs, and a porous feature can also be identified, which may be caused by the reduction process and the removal of MgO. Energy dispersive spectroscopy (EDS) elemental mappings were employed
SiO2 NSs were prepared by previous reported Stöber method [36]. Typically, 500 mL ethanol, 20 mL ammonium hydroxide and 40 mL deionized water were added into a flask. Then, 15 mL tetraethyl orthosilicate (TEOS) was quickly added into the flask under stirring. The SiO2 nanoparticles were obtained after 12 h. SiO2 NRs were simply achieved by soft-template method at room temperature [37]. Typically, F127 (3.075 g), CTAB (4.56 g) and 37.5 mL NH3·H2O were dissolved in 735 mL deionized water under stirring to form a clear solution, to which TEOS (15 mL) was added for 10 min. The mixture was placed under static condition for 5 h, and filtrated to achieve silica nanorods. 2.2. Preparation of hollow Si/C NSs and NRs The silica precursors were first coated with polydopamine: silica spheres or rods (1 g) were mixed with dopamine (0.5 g) in a Tris-buffer solution (500 mL, 10 mM; pH 8.5) for 20 h [41,42]. The carbon coated silica was collected by filtration. In a typical magnesiothermic reduction process, carbon coated silica (0.1 g), Mg (0.8 g) and KI (0.4 g) were ground together in agate mortar, and then carried into a tube furnace at 650 °C for 3 h under Ar atmosphere with a heating rate of 1 °C min−1. The resulting powder was washed in 2 M HCl for 3 h and 10% HF for 30 min to remove MgO and unreacted SiO2, respectively. For 934
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Fig. 1. Schematic illustration of K-HSi NSs and NRs and SEM images for (a, d) carbon-coated SiO2 NSs and NRs before reduction; (b, e) K-HSi NSs and K-HSi NRs; (c, f) R-Si NSs and NRs (SiO2 NSs and NRs reduced without carbon coating).
to determine the element distribution. The results show a homogeneous distribution of Si throughout the particles. A typical high-resolution TEM image for K-HSi NSs (Fig. 2d) reveals a lattice space of 0.311 nm matching to the Si (111) spacing, and amorphous carbon layer can also
be observed. The hollow and porous feature can offer free void for Si volume change during lithiation process, and their homogeneous particle morphology can give a uniform and stable lithium ion storage value.
Fig. 2. TEM and EDS elemental mapping images of (a) K-HSi NSs; (b) K-HSi NRs; (c) The relationships between XRD patterns and the ratio of silica to KI for K-HSi NSs; and (d) Typical HRTEM images for K-HSi NSs. 935
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Fig. 3. (a) XRD patterns for K-HSi NSs using different heat scavenger with a mass ratio of 1:4; Characterization of K-HSi NSs and NRs. (b) Raman spectra; (c) Si 2p XPS survey; (d) N2 adsorption-desorption isotherm and BJH pore size distribution inset.
The phase evolution of products with relationship to the amount of KI used is analyzed by using X-Ray diffraction (XRD). As shown in Fig. 2c, the silica to KI mass ratio varied from 1:1 to 1:4. The XRD patterns clearly demonstrate that the resultant crystalline structure is varied for products obtained from reactant with different amount of KI. Solely SiC product was achieved from silica reduced without KI, which peaks can be indexed to the (111), (220) and (311) planes of SiC structure (PDF 29-1129). Otherwise, if reactants mixed with KI, Si crystalline peaks can be observed clearly at 28.4, 47.3, 56.1, 69.1, 76.3 and 88.0o corresponding to (111), (220), (311), (400), (331) and (442) planes for Si (PDF 27-1402), respectively. Moreover, the Si crystalline proportion increases linearly with increasing of KI contents in the reactants, and peaks for SiC almost invisible in the case of silica to KI mass ratio of 1:4. These results strongly support that KI indeed restrict the formation of SiC. Other similar heat scavenger like KCl [43] and NaCl [39] has also been attempted in Fig. 3a. It can be seen that single peaks of SiC is demonstrated for the experiment using NaCl as heat scavenger, indicating that NaCl have done little effect for inhibiting the formation of SiC. As for KCl, weak peaks of Si can be observed, however, the strong peaks of SiC is still exist, suggesting KCl cannot reduce the content of SiC in the products effectively as KI achieved. As we all know, NaCl and KCl have been commonly used to stabilizing the magnesiothermic reduction, the reason for their inferior performance in controlling the generation of SiC maybe ascribe to their different melting point. As the melting point of KI is 681 °C, which is lower than the melting point of NaCl (801 °C) and KCl (770 °C). Besides, the melting point of KI is very closed to the reaction temperature of magnesiothermic reduction (650 °C), making KI is more sensitive than NaCl and KCl to controlling the reaction temperature, and thus reducing the amount of the byproduct of SiC. In order to further realize their chemical and structural properties for the products achieved by KI-assisted magnesiothermic reduction, a series of characterization methods were carried out. The Raman
spectrum of K-HSi NSs and NRs are shown in Fig. 3b. A strong peak at around 503 cm−1 and two wide peaks at around 291 and 921 cm−1 ascribing to TA (X) and TO (L), [44] respectively, can be observed. These peaks can be indexed to Si [45,46]. Due to the Raman intensity and content of the carbon layer is relatively lower than Si, which causes their Raman peaks can hardly observed, we magnified the Raman spectrum range from 1100 to 1800 cm−1. As is shown in the inset in Fig. 3b, two additional wide peaks at around 1344 and 1590 cm−1 can be observed, corresponded to the D and G band of the carbon layer, respectively, which reveal the existence of the carbon layer. [47] The Si component is further analyzed by the X-ray photoelectron spectroscopy (XPS). The High resolution XPS of Si were deconvoluted into four peaks at 95.5 eV for Si0, 101.2 eV for Si2+, 102.2 eV for Si3+, and 103.5 eV for Si4+, respectively. Si2+ is attributed to the formation of SiO or SiC, whereas residual peaks of Si3+ and Si4+ correspond to byproducts such as MgxSiOy and SiO2. So the weak peaks at Si2+ for both K-HSi NSs and NRs indicate their low content of SiC in the products. The N2 adsorption-desorption measurement is conducted to investigate the pore structure of K-HSi samples (Fig. 3d). The obtained N2 adsorption-desorption isotherm is type IV. [48–53] K-HSi NSs shows a higher N2 uptake than K-HSi NRs with a specific surface area of 154 and 64 m2 g−1, which is calculated by Brunauer-Emmett-Teller (BET) method. Both samples show a mesoporous structure with a wide pore size distribution of 2–5 nm. In addition, K-HSi NSs shows an extra macropore distribution of 10–40 nm with most probable pore size of 17 nm, which may be attributed to the fracture of the Si spheres which can be observed in the SEM images. Their pore volume is 0.06 and 0.03 cm3 g−1, respectively. The electrochemical performance of K-HSi as anode for LIBs is investigated in a half-cell. Fig. 4a shows voltage profiles for K-HSi NSs, the first discharge and charge specific capacity were 3346 and 2299 mAh g−1, respectively, achieving an initial Coulombic efficiency of 68.7%. The sloping voltage plateau at around 1.12–1.40 V in the first discharge curve and disappeared in subsequent cycles could be attributed to the formation of SEIs, electrode polarization and electrolyte 936
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Fig. 4. (a) Voltage profiles of K-HSi NSs at 0.1 A g−1 between 0.005 and 2 V; (b) CV curves of the first 5 cycles for K-HSi NSs at a scan rate of 0.1 mV s−1 in the voltage range of 0.005–1.5 V; (c) Cycling performance at 0.1 A g−1 for initial 5 cycles and 1 A g−1 for remaining cycles (The current density for KCl and NaCl-Si NSs is 0.1 A g−1 for all cycles); (d) Rate performance; and (e) Nyquist plots after 10 cycles for K-HSi NSs and K-HSi NRs and equivalent circuit of the electrochemical impedance (Rs: the electrolyte resistance; Rct: the charge transfer resistance at the electrode-electrolyte interface; CPE: the resistor with constant phase elements; W0: the Warburg impedance term for the solid state diffusion of lithium ions).
decomposition [54–56]. In the cyclic voltammogram (CV) scan curves in Fig. 4b, a broad peak at 1.12–1.40 V in first scan is in accordance with the results in voltage profiles (Fig. 4a). The reduction peaks at 0.21 and 0.27 V can be ascribed to the typical lithiation of Si and lithium-ion insertion in amorphous carbon layer, while the oxidization peaks at 0.32 and 0.5 V can be assigned to the phase transition between the LixSi alloy and amorphous Si formed by the dealloying reaction [57–59]. The cyclic performance of K-HSi NSs and K-HSi NRs at a current density of 1 A g−1 and compared with the electrode of Si reduced by the assistance of KCl (KCl-Si NSs) and NaCl (NaCl-Si NSs) with a mass ratio of 1:4 and Si reduced directly from silica sphere (R-Si NSs) are demonstrated in Fig. 4c. It can be seen that the specific capacity of R-Si NSs dramatically decreased from 3831 mAh g−1 to 527 mAh g−1 after 100 cycles and 194 mAh g−1 350 cycles. Because a lot of SiC was mixed into KCl-Si NSs, it demonstrated a lower initial capacity of 2049 mAh g−1, and decreased to 430 mAh g−1 after 100 cycles. As for NaCl-Si NSs with a much high content of SiC, its initial capacity was 1435 mAh g−1 and dramatically reduced to 183 mAh g−1 just after 5 cycles. In contrast, The K-HSi NSs and NRs exhibited more stable cyclic performance up to 350 cycles than R-Si NSs. In particular, the specific capacity of KHSi NSs anode is 2670 mAh g−1 at 0.1 A g−1 after first 5 cycles, which retained 1792 mAh g−1 after 100 cycles and 1365 mAh g−1 even after 350 cycles at 1 A g−1, reaching a capacity of 87.5% and 66.6%, respectively. K-HSi NRs shown a relatively low capacity of 1250 mAh g−1 after 100 cycles and 966 mAh g−1 after 350 cycles, but achieved a higher capacity retention of 90.6% and 70.1%, respectively. The stable cyclic performance is related to the ordered hollow porous structure which can buffer the volume expansion effectively. Besides, different
electrochemical performance of K-HSi NSs and NRs may cause by their different particle size and morphology as well as their inner characteristic porous Si feature. Fig. 4d depicts their rate performance. In detail, K-HSi NSs delivers an excellent specific capacity of 2275, 1997, 1810, 1548, 1310 and 1128 mAh g−1 at 0.1, 0.5, 1, 2, 3 and 4 A g−1, respectively. Remarkably, it can retain a capacity of 1551 mAh g−1 at 2 A g−1 after 128 cycles, and 1271 mAh g−1 even at a high current density of 3 A g−1 after 200 cycles. By comparison, K-HSi NRs delivers a lower specific capacity of 1303, 1099, 1017, 927, 894 and 831 mAh g−1 at 0.1, 0.5, 1, 2, 3 and 4 A g−1, respectively. It may ascribe to its specific area is smaller and K-HSi NSs possess a better porous structure. Besides, their electrochemical impedance spectroscopy spectra further confirmed that K-HSi NSs possessed lower charge transfer resistance and ionic diffusion resistance, as verified by its smaller diameter of the high frequency semicycle and higher slope of the low frequency line. These results were verified by the fitted data based on equivalent circuit. The Rct of K-HSi NSs and NRs was 9.671 and 16.7 Ω (Table S1), respectively, indicating the electrode of KHSi NSs processed faster charge transfer kinetics. The outstanding cyclic and rate performance results can be attributed to various comprehensive factors: (1) The uniform controlled nano-morphology gives a homogeneous electrode slurry and provide an excellent conductivity and a stable structure; (2) The hollow structure and macro-, meso-pores can buffer the volume change of Si during lithiation; (3) The carbon layer can serve as skeleton to maintain the structure stability during cycling. To further confirm the structure stability of composites, SEM images of K-HSi NSs and NRs electrodes before and after 200 cycles are shown in Fig. 5 (a, b) and (d, e), respectively. The spherical and rodlike 937
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Fig. 5. SEM images for electrodes before and after 200 cycles of (a, b) K-HSi NSs and (d, e) K-HSi NRs, respectively; (c) Voltage profiles for full cell based on K-HSi NSs as anode material and LiCoO2 as cathode material; (f) A photo of the assembled full cell lighting a LED screen.
morphology of K-HSi samples almost remains after cycling, indicating their durable structure, but the particle size decreases a little caused by the repeated volume change. Additionally, In order to evaluate their potential for application, a full cell based on K-HSi NSs electrode as anode and LiCoO2 electrode as cathode was established in CR2032 cointype cell with the anode capacity/cathode capacity of about 1.2. The as assembled full cell was cathode limited and tested at a current density of 1C (0.273 A g−1) based on the weight of cathode active materials between 2.5 and 4.2 V. As is shown in Fig. 5c, it delivered an initial charge and discharge specific capacity of 129 and 109 mAh g−1, respectively, yielding a Coulombic efficiency of 84.5%. The specific capacity slight decays to 71 mAh g−1 after 100 cycles with a capacity retention of 65.1%. In addition, the as assembled full cell was used to light a LED screen in Fig. 5f, which operating voltage is beyond 3.5 V. The full cell can light the screen with ~52 LED element for about 8 s.
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4. Conclusion In summary, an effective method was proposed to control the formation of SiC during magnesiothermic reduction of carbon coated SiO2 composites, through which Si/C composites with a shape of nanosphere and rod were achieved. It has been proved that the rigid protective carbon layer can perfectly retain the morphology from silica used, and restrict the Si particle agglomeration during Mg reduction. Meanwhile, the removal of MgO and unreacted SiO2 can further create void space, which can buffering the volume change during lithiation and delithiation process. In addition, KI was first used as the heat scavenger to control the formation of SiC. The obtained hollow Si/C composites were used in LIBs, which demonstrate an excellent specific capacity of 1365 mAh g−1 even after 350 cycles at 1 A g−1. We think this method can be extended to explore a wide range of hollow Si/C composites with various structures. Acknowledgements This work was supported by National Natural Science Foundation of China (51673117, 51574166 and 51602199), the Shenzhen Science and Technology Innovation Commission (JCYJ20150529164656097, JSGG20160226201833790, JCYJ20160520163535684, JCYJ20160422144936457, JCYJ20170818093832350, JCYJ20170818112409808, JSGG20170824112840518).
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Full length article
Morphology-controlled synthesis of hollow Si/C composites based on KIassisted magnesiothermic reduction for high performance Li-ion batteries
T
Pei Hana,b, Weicheng Sunb, Dongzhi Lib, Donghai Luob, Yanzi Wangb, Bo Yangb, Cuihua Lib, ⁎ ⁎ Yiping Zhaoa, , Li Chena, Jian Xuc, Caizhen Zhub, a
School of Materials Science and Engineering of Tianjin Polytechnic University, Tianjin 300387, China Institute of Low-dimensional Materials Genome Initiative, College of Chemistry and Environmental Engineering of Shenzhen University, Shenzhen 518060, China c Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b
ARTICLE INFO
ABSTRACT
Keywords: Magnesiothermic reduction Potassium iodide Si carbide Hollow Si composites Lithium-ion batteries
Magnesiothermic reduction of silicon oxide (SiO2) is thought to be an effective method for preparing nanostructured silicon (Si). However, the magnesiothermic reduction can hardly maintain the original shape of the SiO2 due to the harsh reaction condition. Carbon coating is an efficient strategy to prevent the original structure collapse and improve its conductivity, while the by-product silicon carbide (SiC) is prone to form due to the overheating phenomenon during the process, which is useless to the Li-ion storage. Herein, potassium iodide (KI) is used as heat scavenger for the first time to control the formation of SiC, in which the melt of KI can absorb excess heat during the harsh reduction process. Ideally, hollow Si/C nanospheres and nanorods with little content of SiC are successfully synthesized by the KI-assisted magnesiothermic reduction from carbon coated SiO2 precursors. These novel hollow Si/C nanospheres and nanorods endow it with an excellent electrochemical performance as anodes for Li-ion battery. In detail, Si/C nanospheres achieve a specific capacity of 1792 mAh g−1 after 100 cycles at 1 A g−1, and 1271 mAh g−1 even at a high current density of 3 A g−1 after 200 cycles. This novel strategy provides an effective and facile way to reduce SiC generation during magnesiothermic reduction reaction and can be applied to obtain Si/C composites with various morphology.
1. Introduction Si has been identified as one of the most hopeful alternatives to commercial graphite for LIBs for a long time owing to its high theoretical capacity of ~4200 mAh g−1, environment-friendliness, and abundance in nature [1,2]. However, the huge volume change during lithiation-delithiation process, especially for those grain size > 150 nm, causes not only the loss of mechanical/electrical contacts between Si and current collector, but also the continuous formation of unstable solid-electrolyte interphases (SEIs), which consumes too many lithiumions and leading to rapid capacity fading [3–5]. To address those issues above, various Si nanostructures, such as nanorods, [6] nanowires, [7–9] thin films, [10,11] nanocrystallines, [12–16] and porous nanostructures, [17–21] have been successfully synthesized with greatly improved electrochemical performance. However, expensive and toxic raw material as SiCl4 or SiH4 and complicated processing with low yield (e.g. electrochemical etching, electrospinning and chemical vapor deposition (CVD)) are generally used in the preparation of above mentioned Si nanostructure. Apart from
⁎
nanostructured Si, the introduction of void spaces in Si has been raised to be another valid strategy to buffer the volume change of Si during cycling [22–24]. Various core-shell hollow structures have thus been fabricated for further stabilizing the volume changing [25–31]. For instance, yolk-shell Si particles were synthesized by using SiO2 as sacrificial template, which achieved a specific capacity of 1500 mAh g−1 at 1 C after 1000 cycles [32]. Similarly, granadilla-like Si was achieved via removing sacrificial layer of calcium carbonate, which could retain 1100 mAh g−1 at 250 mA g−1 after 200 cycles [33]. The void spaces indeed can relieve the Si volume change during cycling, however the introduction and removal of sacrificial layer makes this process complicated. Therefore, it is necessary to exploit a facile, low-cost and environmentally friendly strategy for synthesizing hollow Si. Among varieties of approaches to fabricate nanostructured Si, magnesiothermic reduction [5,34,35] has been widely applied in the transformation of SiO2 into Si with relatively low power consumption (~650 °C). The reaction formula is shown as follows:
2Mg(s) + SiO2 (s)
Corresponding authors. E-mail addresses: [email protected] (Y. Zhao), [email protected] (C. Zhu).
https://doi.org/10.1016/j.apsusc.2019.03.051 Received 13 December 2018; Received in revised form 22 February 2019; Accepted 4 March 2019 Available online 05 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
2MgO (s) + Si (s)
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However, the magnesiothermic reduction process could hardly maintain the original shape of various SiO2 precursors, such as nanospheres, [36] nanofibers, [37] nanorods, [6] aerogels [38] and prone to crush into disordered microporous structure, caused by the harsh replacement reaction condition between SiO2 and Mg. A precoating protective carbon layer before reduction process is thought to be an effective strategy to prevent the structure collapse. Nevertheless, the formation of Si carbide (SiC) is hard to control during the magnesiothermic reduction process of SiO2/C composites. One probably reason is that, the massive heat from the exothermic magnesiothermic reaction (∆H = −593 kJ molSi−1) could not successfully released, [39] leading to a local overheating beyond the SiC formation temperature (~1100 °C). [40] Up to now, however, there is not an efficient way to control the thermodynamic byproduct of SiC. In this work, a facile strategy for the morphology-controlled preparation of hollow Si/C composites was reported by directly reducing carbon-coated silica using potassium iodide (KI) as the heat scavenger. SiO2 nanospheres and nanorods synthesized by soft-template method were selected as the precursors. It was found that the fusion of KI (m.p. 681 °C; ∆Hfusion (KI) = 15.05 kJ mol−1) could effectively absorb excessive heat released from the exothermic magnesiothermic reaction, which prevented a local overheating as well as the generation of SiC. The fusion heat of KI is calculated as the Eq. (2) (∆fHmθ(KI, l) = − 312.85 kJ mol−1; ∆fHmθ(KI, s) = − 327.90 kJ mol−1):
Hfusion (KI ) =
f
Hm (KI , l)
f Hm (KI , s )
comparison, silica spheres and rods without carbon coating were directly reduced by Mg using the same method mentioned above, which products were abbreviated as R-Si NSs and NRs. 2.3. Characterization The morphology was characterized by scanning electron microscope (SEM, JEOL 7500F) and transmission electron microscope (TEM, FEI Tecnai G2 F20), respectively. X-ray diffraction (XRD) patterns were taken by a Rigaku MiniFlex600 (with CuKa radiation) instrument, Raman spectra were collected on confocal HR800 spectrometer of HORIBA Jobin Yvon, X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALab220i-XL electron spectrometer from VG Scientific. Al-Kα radiation was used as the X-ray source and operated at 300 W. N2 (at 77.3 K) sorption measurements were performed using QUADRASORB SI/MP from Quantachrome Instruments. 2.4. Electrochemical measurements Electrodes were prepared by blade-coating a slurry of 60% of K-HSi samples, 20% super P and 20% of sodium alginate (SA) dispersed in deionized water on copper foil, and drying at 70 °C for 6 h and punched into circular discs for coin-cell fabrication, representing ~1 mg cm−2 of loading density. The cathode electrodes for full cell were blade-coating a slurry of 90% of LiCoO2, 5% super P and 5% of Poly (vinylidene fluoride) (PVDF) dispersed in N-Methyl pyrrolidone (NMP) on aluminum foil. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), fluorinated ethylene carbonate (FEC) (in a volume ration of 5:70:25). 2032 type coin cells were assembled in an argon-filled glove box using lithium metal as the counter electrode (LiCoO2 electrodes for full cell) and glass fiber as separator. The galvanostatic charge-discharge test was conducted by a Wu-Han land CT2001A testing system in the voltage range between 0.005 V and 2 V. Electrochemical impedance spectra (EIS) characterization were taken using a Solatron 1260 Impedance Analyzer. Cyclic voltammetry (CV) measurements were performed on a CHI660D electrochemical workstation with a scan rate of 0.1 mV s−1 in the voltage range of 0.005–1.5 V (vs. Li/Li+).
(2)
After KI-assisted magnesiothermic reduction, the reduced Si materials well inherit the initial morphology of silica precursors, owing to the confinement of precoated carbon layer. In addition, the subsequent aid treatment to remove MgO and unreacted SiO2 could create micropores and void spaces in the Si. The KI-assisted magnesiothermic reduced hollow Si/C nanospheres and nanorods abbreviated as K-HSi NSs and K-HSi NRs, respectively, were used as anode in Li-ion batteries, which exhibit a good cycling stability and a high specific capacity of 1792 mAh g−1 after 100 cycles at 1 A g−1, and 1271 mAh g−1 even at a high current density of 3 A g−1 after 200 cycles. 2. Experimental section
3. Results and discussion
2.1. Synthesis of SiO2 NSs and NRs
Strategy for fabricating hollow Si/C NSs and NRs is illustrated in Fig. 1. Firstly, the silica particles were encapsulated in polydopamine, which can maintain their morphology during the harsh magnesiothermic reduction process. Then the obtained particles were mixed with magnesium and KI powder, and calcined at 650 °C under Ar atmosphere. In this process, KI is used to absorb the excessive heat, thus the formation of SiC is inhibited. Finally, the hollow Si/C composites were obtained after the decomposition of MgO and unreacted SiO2 with acid washing. The SEM images for K-HSi NSs and NRs depicted that the resulted Si/C composites well inherit the sphere and rod-like morphology from silica used with a rough surface (Fig. 1). A hollow structure can be identified at some broken spheres for K-HSi NSs with a diameter of ~200 nm (Fig. 1b), caused by the remove of unreacted silica, [43] which were further confirmed by TEM images (Fig. 2a). K-HSi NRs demonstrate a uniformly rod-like nanostructure with an average width of ~100 nm and length of ~300 nm. Moreover, porous architecture can be observed in Fig. 1b. By comparison, the silica spheres and rods directly reduced by Mg without carbon coating (Fig. 1c and f) lose the primary shape, and agglomerate into a bulk, owing to the volume change and breakdown during the reduction process. The TEM images (Fig. 2a and b) demonstrate hollow structure for KHSi NSs and NRs, and a porous feature can also be identified, which may be caused by the reduction process and the removal of MgO. Energy dispersive spectroscopy (EDS) elemental mappings were employed
SiO2 NSs were prepared by previous reported Stöber method [36]. Typically, 500 mL ethanol, 20 mL ammonium hydroxide and 40 mL deionized water were added into a flask. Then, 15 mL tetraethyl orthosilicate (TEOS) was quickly added into the flask under stirring. The SiO2 nanoparticles were obtained after 12 h. SiO2 NRs were simply achieved by soft-template method at room temperature [37]. Typically, F127 (3.075 g), CTAB (4.56 g) and 37.5 mL NH3·H2O were dissolved in 735 mL deionized water under stirring to form a clear solution, to which TEOS (15 mL) was added for 10 min. The mixture was placed under static condition for 5 h, and filtrated to achieve silica nanorods. 2.2. Preparation of hollow Si/C NSs and NRs The silica precursors were first coated with polydopamine: silica spheres or rods (1 g) were mixed with dopamine (0.5 g) in a Tris-buffer solution (500 mL, 10 mM; pH 8.5) for 20 h [41,42]. The carbon coated silica was collected by filtration. In a typical magnesiothermic reduction process, carbon coated silica (0.1 g), Mg (0.8 g) and KI (0.4 g) were ground together in agate mortar, and then carried into a tube furnace at 650 °C for 3 h under Ar atmosphere with a heating rate of 1 °C min−1. The resulting powder was washed in 2 M HCl for 3 h and 10% HF for 30 min to remove MgO and unreacted SiO2, respectively. For 934
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Fig. 1. Schematic illustration of K-HSi NSs and NRs and SEM images for (a, d) carbon-coated SiO2 NSs and NRs before reduction; (b, e) K-HSi NSs and K-HSi NRs; (c, f) R-Si NSs and NRs (SiO2 NSs and NRs reduced without carbon coating).
to determine the element distribution. The results show a homogeneous distribution of Si throughout the particles. A typical high-resolution TEM image for K-HSi NSs (Fig. 2d) reveals a lattice space of 0.311 nm matching to the Si (111) spacing, and amorphous carbon layer can also
be observed. The hollow and porous feature can offer free void for Si volume change during lithiation process, and their homogeneous particle morphology can give a uniform and stable lithium ion storage value.
Fig. 2. TEM and EDS elemental mapping images of (a) K-HSi NSs; (b) K-HSi NRs; (c) The relationships between XRD patterns and the ratio of silica to KI for K-HSi NSs; and (d) Typical HRTEM images for K-HSi NSs. 935
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Fig. 3. (a) XRD patterns for K-HSi NSs using different heat scavenger with a mass ratio of 1:4; Characterization of K-HSi NSs and NRs. (b) Raman spectra; (c) Si 2p XPS survey; (d) N2 adsorption-desorption isotherm and BJH pore size distribution inset.
The phase evolution of products with relationship to the amount of KI used is analyzed by using X-Ray diffraction (XRD). As shown in Fig. 2c, the silica to KI mass ratio varied from 1:1 to 1:4. The XRD patterns clearly demonstrate that the resultant crystalline structure is varied for products obtained from reactant with different amount of KI. Solely SiC product was achieved from silica reduced without KI, which peaks can be indexed to the (111), (220) and (311) planes of SiC structure (PDF 29-1129). Otherwise, if reactants mixed with KI, Si crystalline peaks can be observed clearly at 28.4, 47.3, 56.1, 69.1, 76.3 and 88.0o corresponding to (111), (220), (311), (400), (331) and (442) planes for Si (PDF 27-1402), respectively. Moreover, the Si crystalline proportion increases linearly with increasing of KI contents in the reactants, and peaks for SiC almost invisible in the case of silica to KI mass ratio of 1:4. These results strongly support that KI indeed restrict the formation of SiC. Other similar heat scavenger like KCl [43] and NaCl [39] has also been attempted in Fig. 3a. It can be seen that single peaks of SiC is demonstrated for the experiment using NaCl as heat scavenger, indicating that NaCl have done little effect for inhibiting the formation of SiC. As for KCl, weak peaks of Si can be observed, however, the strong peaks of SiC is still exist, suggesting KCl cannot reduce the content of SiC in the products effectively as KI achieved. As we all know, NaCl and KCl have been commonly used to stabilizing the magnesiothermic reduction, the reason for their inferior performance in controlling the generation of SiC maybe ascribe to their different melting point. As the melting point of KI is 681 °C, which is lower than the melting point of NaCl (801 °C) and KCl (770 °C). Besides, the melting point of KI is very closed to the reaction temperature of magnesiothermic reduction (650 °C), making KI is more sensitive than NaCl and KCl to controlling the reaction temperature, and thus reducing the amount of the byproduct of SiC. In order to further realize their chemical and structural properties for the products achieved by KI-assisted magnesiothermic reduction, a series of characterization methods were carried out. The Raman
spectrum of K-HSi NSs and NRs are shown in Fig. 3b. A strong peak at around 503 cm−1 and two wide peaks at around 291 and 921 cm−1 ascribing to TA (X) and TO (L), [44] respectively, can be observed. These peaks can be indexed to Si [45,46]. Due to the Raman intensity and content of the carbon layer is relatively lower than Si, which causes their Raman peaks can hardly observed, we magnified the Raman spectrum range from 1100 to 1800 cm−1. As is shown in the inset in Fig. 3b, two additional wide peaks at around 1344 and 1590 cm−1 can be observed, corresponded to the D and G band of the carbon layer, respectively, which reveal the existence of the carbon layer. [47] The Si component is further analyzed by the X-ray photoelectron spectroscopy (XPS). The High resolution XPS of Si were deconvoluted into four peaks at 95.5 eV for Si0, 101.2 eV for Si2+, 102.2 eV for Si3+, and 103.5 eV for Si4+, respectively. Si2+ is attributed to the formation of SiO or SiC, whereas residual peaks of Si3+ and Si4+ correspond to byproducts such as MgxSiOy and SiO2. So the weak peaks at Si2+ for both K-HSi NSs and NRs indicate their low content of SiC in the products. The N2 adsorption-desorption measurement is conducted to investigate the pore structure of K-HSi samples (Fig. 3d). The obtained N2 adsorption-desorption isotherm is type IV. [48–53] K-HSi NSs shows a higher N2 uptake than K-HSi NRs with a specific surface area of 154 and 64 m2 g−1, which is calculated by Brunauer-Emmett-Teller (BET) method. Both samples show a mesoporous structure with a wide pore size distribution of 2–5 nm. In addition, K-HSi NSs shows an extra macropore distribution of 10–40 nm with most probable pore size of 17 nm, which may be attributed to the fracture of the Si spheres which can be observed in the SEM images. Their pore volume is 0.06 and 0.03 cm3 g−1, respectively. The electrochemical performance of K-HSi as anode for LIBs is investigated in a half-cell. Fig. 4a shows voltage profiles for K-HSi NSs, the first discharge and charge specific capacity were 3346 and 2299 mAh g−1, respectively, achieving an initial Coulombic efficiency of 68.7%. The sloping voltage plateau at around 1.12–1.40 V in the first discharge curve and disappeared in subsequent cycles could be attributed to the formation of SEIs, electrode polarization and electrolyte 936
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Fig. 4. (a) Voltage profiles of K-HSi NSs at 0.1 A g−1 between 0.005 and 2 V; (b) CV curves of the first 5 cycles for K-HSi NSs at a scan rate of 0.1 mV s−1 in the voltage range of 0.005–1.5 V; (c) Cycling performance at 0.1 A g−1 for initial 5 cycles and 1 A g−1 for remaining cycles (The current density for KCl and NaCl-Si NSs is 0.1 A g−1 for all cycles); (d) Rate performance; and (e) Nyquist plots after 10 cycles for K-HSi NSs and K-HSi NRs and equivalent circuit of the electrochemical impedance (Rs: the electrolyte resistance; Rct: the charge transfer resistance at the electrode-electrolyte interface; CPE: the resistor with constant phase elements; W0: the Warburg impedance term for the solid state diffusion of lithium ions).
decomposition [54–56]. In the cyclic voltammogram (CV) scan curves in Fig. 4b, a broad peak at 1.12–1.40 V in first scan is in accordance with the results in voltage profiles (Fig. 4a). The reduction peaks at 0.21 and 0.27 V can be ascribed to the typical lithiation of Si and lithium-ion insertion in amorphous carbon layer, while the oxidization peaks at 0.32 and 0.5 V can be assigned to the phase transition between the LixSi alloy and amorphous Si formed by the dealloying reaction [57–59]. The cyclic performance of K-HSi NSs and K-HSi NRs at a current density of 1 A g−1 and compared with the electrode of Si reduced by the assistance of KCl (KCl-Si NSs) and NaCl (NaCl-Si NSs) with a mass ratio of 1:4 and Si reduced directly from silica sphere (R-Si NSs) are demonstrated in Fig. 4c. It can be seen that the specific capacity of R-Si NSs dramatically decreased from 3831 mAh g−1 to 527 mAh g−1 after 100 cycles and 194 mAh g−1 350 cycles. Because a lot of SiC was mixed into KCl-Si NSs, it demonstrated a lower initial capacity of 2049 mAh g−1, and decreased to 430 mAh g−1 after 100 cycles. As for NaCl-Si NSs with a much high content of SiC, its initial capacity was 1435 mAh g−1 and dramatically reduced to 183 mAh g−1 just after 5 cycles. In contrast, The K-HSi NSs and NRs exhibited more stable cyclic performance up to 350 cycles than R-Si NSs. In particular, the specific capacity of KHSi NSs anode is 2670 mAh g−1 at 0.1 A g−1 after first 5 cycles, which retained 1792 mAh g−1 after 100 cycles and 1365 mAh g−1 even after 350 cycles at 1 A g−1, reaching a capacity of 87.5% and 66.6%, respectively. K-HSi NRs shown a relatively low capacity of 1250 mAh g−1 after 100 cycles and 966 mAh g−1 after 350 cycles, but achieved a higher capacity retention of 90.6% and 70.1%, respectively. The stable cyclic performance is related to the ordered hollow porous structure which can buffer the volume expansion effectively. Besides, different
electrochemical performance of K-HSi NSs and NRs may cause by their different particle size and morphology as well as their inner characteristic porous Si feature. Fig. 4d depicts their rate performance. In detail, K-HSi NSs delivers an excellent specific capacity of 2275, 1997, 1810, 1548, 1310 and 1128 mAh g−1 at 0.1, 0.5, 1, 2, 3 and 4 A g−1, respectively. Remarkably, it can retain a capacity of 1551 mAh g−1 at 2 A g−1 after 128 cycles, and 1271 mAh g−1 even at a high current density of 3 A g−1 after 200 cycles. By comparison, K-HSi NRs delivers a lower specific capacity of 1303, 1099, 1017, 927, 894 and 831 mAh g−1 at 0.1, 0.5, 1, 2, 3 and 4 A g−1, respectively. It may ascribe to its specific area is smaller and K-HSi NSs possess a better porous structure. Besides, their electrochemical impedance spectroscopy spectra further confirmed that K-HSi NSs possessed lower charge transfer resistance and ionic diffusion resistance, as verified by its smaller diameter of the high frequency semicycle and higher slope of the low frequency line. These results were verified by the fitted data based on equivalent circuit. The Rct of K-HSi NSs and NRs was 9.671 and 16.7 Ω (Table S1), respectively, indicating the electrode of KHSi NSs processed faster charge transfer kinetics. The outstanding cyclic and rate performance results can be attributed to various comprehensive factors: (1) The uniform controlled nano-morphology gives a homogeneous electrode slurry and provide an excellent conductivity and a stable structure; (2) The hollow structure and macro-, meso-pores can buffer the volume change of Si during lithiation; (3) The carbon layer can serve as skeleton to maintain the structure stability during cycling. To further confirm the structure stability of composites, SEM images of K-HSi NSs and NRs electrodes before and after 200 cycles are shown in Fig. 5 (a, b) and (d, e), respectively. The spherical and rodlike 937
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Fig. 5. SEM images for electrodes before and after 200 cycles of (a, b) K-HSi NSs and (d, e) K-HSi NRs, respectively; (c) Voltage profiles for full cell based on K-HSi NSs as anode material and LiCoO2 as cathode material; (f) A photo of the assembled full cell lighting a LED screen.
morphology of K-HSi samples almost remains after cycling, indicating their durable structure, but the particle size decreases a little caused by the repeated volume change. Additionally, In order to evaluate their potential for application, a full cell based on K-HSi NSs electrode as anode and LiCoO2 electrode as cathode was established in CR2032 cointype cell with the anode capacity/cathode capacity of about 1.2. The as assembled full cell was cathode limited and tested at a current density of 1C (0.273 A g−1) based on the weight of cathode active materials between 2.5 and 4.2 V. As is shown in Fig. 5c, it delivered an initial charge and discharge specific capacity of 129 and 109 mAh g−1, respectively, yielding a Coulombic efficiency of 84.5%. The specific capacity slight decays to 71 mAh g−1 after 100 cycles with a capacity retention of 65.1%. In addition, the as assembled full cell was used to light a LED screen in Fig. 5f, which operating voltage is beyond 3.5 V. The full cell can light the screen with ~52 LED element for about 8 s.
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4. Conclusion In summary, an effective method was proposed to control the formation of SiC during magnesiothermic reduction of carbon coated SiO2 composites, through which Si/C composites with a shape of nanosphere and rod were achieved. It has been proved that the rigid protective carbon layer can perfectly retain the morphology from silica used, and restrict the Si particle agglomeration during Mg reduction. Meanwhile, the removal of MgO and unreacted SiO2 can further create void space, which can buffering the volume change during lithiation and delithiation process. In addition, KI was first used as the heat scavenger to control the formation of SiC. The obtained hollow Si/C composites were used in LIBs, which demonstrate an excellent specific capacity of 1365 mAh g−1 even after 350 cycles at 1 A g−1. We think this method can be extended to explore a wide range of hollow Si/C composites with various structures. Acknowledgements This work was supported by National Natural Science Foundation of China (51673117, 51574166 and 51602199), the Shenzhen Science and Technology Innovation Commission (JCYJ20150529164656097, JSGG20160226201833790, JCYJ20160520163535684, JCYJ20160422144936457, JCYJ20170818093832350, JCYJ20170818112409808, JSGG20170824112840518).
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