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rGO nanocomposite towards high-performance lithium storage
Journal Pre-proof A new magnesium hydride route to synthesize morphology-controlled Si/rGO nanocomposite towards high-performance lithium storage Feixiang Bian, Jiage Yu, Wenlong Song, Hui Huang, Chu Liang, Yongping Gan, Yang Xia, Jun Zhang, Xinping He, Wenkui Zhang PII:
S0013-4686(19)32119-X
DOI:
https://doi.org/10.1016/j.electacta.2019.135248
Reference:
EA 135248
To appear in:
Electrochimica Acta
Received Date: 20 August 2019 Revised Date:
12 October 2019
Accepted Date: 7 November 2019
Please cite this article as: F. Bian, J. Yu, W. Song, H. Huang, C. Liang, Y. Gan, Y. Xia, J. Zhang, X. He, W. Zhang, A new magnesium hydride route to synthesize morphology-controlled Si/rGO nanocomposite towards high-performance lithium storage, Electrochimica Acta (2019), doi: https://doi.org/10.1016/ j.electacta.2019.135248. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical Abstract Synthesized by a mild process of MgH2 reduction reaction, the morphology-controlled Si/rGO composite exhibits great electrochemical performance as LIBs anode.
A new magnesium hydride route to synthesize morphology-controlled Si/rGO nanocomposite towards high-performance lithium storage
Feixiang Bian ‡,a, Jiage Yu ‡,a, Wenlong Song b, Hui Huang a,*, Chu Liang a, Yongping Gan a, Yang Xia a, Jun Zhang a, Xinping He a, Wenkui Zhang a,* a
College of Materials Science and Engineering, Zhejiang University of Technology,
Hangzhou 310014, China b
Zhejiang Tianneng Energy Technology Co ., Ltd, Huzhou, 313100, China
Corresponding authors & E-mails: [email protected] ‡ These authors contributed equally to this work.
Abstract Si/graphene composites have attracted great attention for application as anode materials of Li-ion batteries owing to their superior capacity and cycle stability. Magnesiothermic reduction of silica is a quite scalable and cost-effective method to synthesize Si/graphene composite. However, this remains a considerable challenge because the intense heat accumulation during the violent exothermic reaction makes it difficult to remain the desired nanostructure of the silica precursor and results in severe aggregation of silicon grains. Herein, we offer a mild and scalable route to efficiently convert silica into morphology-controlled Si nanoparticles by magnesium hydride (MgH2) reduction at a low temperature. The Si nanoparticles that are produced from silica template structure are fine and narrowly distributed, showing the ability to preserve morphology characteristics. The as-synthesized Si/rGO composite exhibits outstanding electrochemical properties, delivering a superior rate capability (513 mA h g−1 at 5 A g−1) and a high reversible capacity of 894 mA h g−1 over 100 cycles at 0.2 A g−1. We believe MgH2 reduction of silica demonstrates its great potential in fabricating Si based anode materials.
Keywords:
Si/graphene
composites;
Morphology-controlled;
reduction; Magnesium hydride; Li-ion batteries
Magnesiothermic
1. Introduction Lithium-ion batteries (LIBs) have been widely investigated as an energy storage system for electric vehicles and portable electronic devices due to their great power density, environmental benignity and long cycle life [1-3]. The traditional graphite anode suffers a limited capacity (372 mA h g−1), which has encouraged a great interest in pursuing potential substitutes [4]. Silicon is considered as one of the most promising candidates for its unparalleled lithium storage of 3579 mA h g−1 (Li15Si4), low voltage plateau (~0.4 V vs. Li/Li+) and abundance [5, 6]. Unfortunately, one fatal flaw is that Si sustains dramatic volume variation during lithiation process (>300%), giving rise to the cracking and pulverization of active particles and repetitive growth of the solid-electrolyte interface (SEI) [7-9]. In addition, Si exhibits poor electrical conductivity and Li-ion diffusion capacity for its semiconductor nature [10]. These result in low initial Coulombic efficiencies, rapid capacity fading in few cycles especially under a high current density. To tackle the aforementioned drawbacks, the effect of integrating nanostructured Si with graphene matrix has been proved beyond contradiction [11, 12]. Nano Si possesses high tolerance to volume change and shortened Li+ diffusion length, while graphene can not only stabilize the growth of SEI film and accommodate the volume variation, but also improve the conductivity of electrons and lithium ions [13, 14]. The Si/graphene composite can be directly synthesized by mechanical mixing, layer-by-layer deposition and self-assembly [15-18]. However, the bond between graphene and Si via physical routes is not firm enough to maintain the tight connection and the involving of high cost commercial nano silicon hinders practical applications. Therefore, much interest has been paid to chemical synthesis methods. Direct growth of graphene on Si nanoparticles or depositing Si on graphene by
chemical vapor deposition (CVD) is also adapted to fabricate Si/graphene structures [19, 20]. But CVD requires toxic gas precursor and delicate equipment, and suffers from an extremely low yield [21]. In contrast, template assisted magnesiothermic reduction (MR) of SiO2/graphene becomes a simple and scalable route for the preparation of Si/graphene composite [22-27]. The reaction enthalpy is calculated as −291.6 kJ mol−1 at 25 oC for Mg (s) and −577.1 kJ mol−1 at 650 oC for Mg (g), respectively, indicating that MR is a violent exothermic reaction (Reaction 1). The vast heat releases and accumulates sharply in short time. Thus, MR is uncontrollable, the actual temperature is even higher than the melting point of silicon (1414 o
C),leading to the restack of graphene and the sintering of the as-obtained Si particles
[28]. Currently, the large challenge of MR is not easy to preserve the morphology of silica template in silicon analogue. Another challenge is controlling the reaction extent to avoid the undesirable process of HF etching silica. Thus, it is highly desirable to develop a mild, controllable reduction reaction to replace MR.
2Mg + SiO2 → 2MgO + Si
(1)
2MgH 2 + SiO 2 → 2 MgO + Si + 2H 2 (g)
(2)
Herein, we propose a promising alternative to MR for the synthesis of Si/rGO composite with superior lithium storage performance (Figure 1). The pre-designed SiO2/rGO composites can efficiently convert to Si/rGO via magnesium hydride reduction (MHR) at a moderate temperature (500 oC), much lower than the commonly reported temperature of MR (650 oC). The reaction enthalpy of MHR is about −139.3 kJ mol−1 at 25 oC and −134.6 kJ mol−1 at 500 oC (Reaction 2). This MHR method is mild, controllable and scalable, showing outstanding ability to permit template assisted design of silicon structures. We believe that this strategy provides a new opportunity for efficient and controllable nano silicon synthesis.
2. Experimental 2.1 Materials synthesis 2.1.1 Preparation of SiO2/rGO precursor Graphene oxides (GO) were obtained from flake graphite (Aladdin, ~300 mesh) by a modified Hummers method [29]. SiO2/rGO composite was fabricated via a supercritical CO2 fluid-assisted method as described in our previous work [30-32]. Typically, 300 mg GO sheets, 0.1 mL ammonia solution and 0.1 mL DI water were dispersed in 50 mL absolute ethyl alcohol. Then 2.0 mL tetraethoxysilane (TEOS) was dropwise added to the alcohol dispersion to undergoing a three-hour stirring. The mixture was then placed in a stainless steel container. The container was vacuumized prior to filling with 80 bar CO2. The container was kept at 35 oC with rotating at 350 r.p.m. overnight. Thereafter, the sample was sealed in a Teflon-lined autoclave with hydrothermal treatment at 150 oC for 12 h. Finally, the sample was collected by filtration and freeze-dried. 2.1.2 Preparation of MgH2 powder 5 g magnesium powder (Aladdin, 99.5%) was sealed into a custom-made reactor, then the chamber was vacuumized prior to filling with 70 bar of hydrogen. Next the reactor was heated to 420 oC and refilled with hydrogen every 2 hours to keep hydrogen pressure higher than 70 bar. After 24 h treatment, the sample was transferred to a stainless steel container together with ZrO2 balls at ball-to-powder mass ratio of 40:1. Then it was milled at 500 r.p.m. for 24 h under 50 bar of hydrogen. After repeating the above several times, the MgH2 powder was successfully synthesized and characterized by XRD (Fig. S1). 2.1.3 Preparation of Si NPs/rGO composite 200 mg SiO2/rGO composite and 150 mg MgH2 powder were mixed and sealed
into a custom-made reactor in argon-filled glove box. The reactor was vacuumized prior to being heated to 500 oC (ramp rate 3 oC/min) for 300 min. Then the reacted powders were immersed in diluted hydrochloric acid to remove MgO with subsequent wash by DI water and ethanol for three times. Lastly, Si/rGO composite reduced by MgH2 (MHR-Si/rGO) was collected by freeze-drying. For comparison, Si/rGO reduced by Mg denoted as MR-Si/rGO was also prepared. Briefly, 200 mg of SiO2/rGO composite and 138 mg of Mg powder were used as raw materials with the same molar ratio as MHR. The sample was synthesized at 650 oC as reported in many previous reports.
2.2 Materials characterization X-ray diffraction (XRD) analysis was carried out on an X'Pert Pro diffractometer using copper Kα radiation (λ = 0.15418 nm). Thermogravimetric analysis (TGA) was acquired by TA Instruments SDT Q600 under air atmosphere. Raman spectrums were obtained by Renishaw InVia Reflex spectrometer (excitation wavelength = 532 nm). Scanning electron microscopy (SEM) images of composites were determined via FEI Nova NanoSEM 450, and transmission electron microscopy (TEM) images were carried out using FEI, Tecnai G2 F30.
2.3 Battery test Electrochemical characterization of samples was tested by CR 2032 coin cell. To fabricate the working electrode, Si/rGO composite was stirred with Super-P carbon and polyvinylidene fluoride (PVDF) binder in weight ratio of 7:1.5:1.5 in N-methyl-2-pyrrolidone (NMP). The obtained viscous slurry was spread on a Cu foil and dried at 85 oC under vacuum for 8 h. The active material mass loading was controlled at 1.0–1.2 mg cm−2. The testing cell was assembled in a MBRAUM glovebox under Ar atmosphere by using lithium foil as counter electrode, Cellgard
2500 membrane as separator film. And the electrolyte was 1.0 M LiPF6 in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) in equal volumes. Before test, the cells were placed at 30 oC for 24 h to ensure the full penetration of the electrolyte. Cyclic voltammetry (CV) profiles were obtained by a CHI 660E electrochemical workstation. Galvanostatic charging/discharging was conducted in the potential window of 0.01–2.0 V on CT-3008W Neware battery test systems. Electrochemical impedance spectroscopy (EIS) was measured by a Zahner Zennium electrochemical workstation over the frequency range of 1 MHz–0.1 Hz.
3. Results and Discussion The temperature and gaseous pressure during the reaction processes were recorded by online monitors for better understanding of MR and MHR. It has been revealed by Fig. S2 that MR doesn’t occur at 500 oC, so the holding temperature of MR in our experiment was set as 650 oC, the melting point of magnesium [27, 33]. As shown in Fig. 2a, b, the real-time temperature of MHR increases steadily, while a sharp temperature rising from 510 oC to 860 oC is observed in the MR system. Then after several minutes, the temperature drops rapidly to the normal temperature range, owing to the reactants are limited. This phenomenon indicates that the exothermic reaction of MR is much more ferocious than MHR. As for the gaseous pressure, a significant increase is observed at 380 oC in the MHR system, implying that MHR can be triggered at low temperature. To ensure the MHR can be fully carried out, some calculations have been made in Supplementary Material. These observations confirm the superiority of MHR reaction. Fig. S3 displays the XRD pattern of the residue collected from MHR at 500 oC, where two major phases of MgO and Si and a minor phase of Mg2Si can be observed. Besides, no peaks of MgH2 can be detected, confirming that MgH2 has been totally
consumed in the MHR process. The presence of Mg2Si may be a result of the side reaction between MgH2 and Si, which can be summarized as Reaction 3.
2MgH2 +Si →Mg2Si +2H2 (g)
(3)
With excess MgH2, the formation of Mg2Si would reduce the yield of silicon, yet in the lack of MgH2, the undesirable HF etching should be introduced to eliminate the SiO2 residue. Thus a range of controlled experiments were performed to optimize the reactant mass ratio, which was finally restricted as 4:3 of SiO2/rGO and MgH2. As the SiO2 content in the SiO2/rGO composites in this case was determined to be 79.2 wt% (shown in Fig. S4), the mole ratio of SiO2/rGO and MgH2 in reactants was 1:2.16. The XRD results of rGO, SiO2/rGO, MR-Si/rGO and MHR-Si/rGO are shown in Fig. 2c. The XRD pattern of rGO delivers a large broad peak at 23.8o and a small peak at 43.0o, respectively. The broad peak corresponds to the (002) plane with a d-spacing of 0.372 nm. After the SiO2 deposition, a new broad peak appears between 20o and 23o, indicating the amorphous nature of SiO2 particles [34]. MR-Si/rGO and MHR-Si/rGO have the same (002) peak of rGO, but it’s worth noting that the (002) peak in MR-Si/rGO is relatively more intense, implying the rGO lamellae have restacked to graphite-like agglomerates. In addition, the patterns of MR-Si/rGO and MHR-Si/rGO both exhibit three peaks at 28.4o, 47.3o and 56.1o, corresponding to (111), (220) and (311) of crystal Si (JCPDS 27-1402), respectively. Although MR can achieve a successful synthesis of Si, the amorphous SiO2 phase still remains in the pattern of MR-Si/rGO sample, demonstrating the chemical reaction between Mg and SiO2 is incomplete. The XRD analysis also reveals the superiority of employing MgH2 as a reducing agent. Fig. 2d shows the Raman spectra of the four samples, all of which present characteristic peaks at about 1340 cm−1 for D band and 1590 cm−1 for G band of graphene [35]. The overall spectra of MR-Si/rGO and MHR-Si/rGO are
similar except for the Raman shift of Si peaks. The band at about 520 cm−1 of crystal-Si shifts towards negative with decreasing of the Si particle size owing to the nano silicon induced confinement effect [26, 36, 37]. So it’s confirmed the Si nanoparticles of MHR-Si/rGO with Raman shift at 505 cm−1 have a smaller size than that of MR-Si/rGO whose Raman shift at 512 cm−1. The rGO contents in two Si/rGO composites were tested by TGA as presented in Fig. S4. The convergent initial mass loss up to 200 oC of two samples is due to the evaporation of adsorbed water, and the obvious mass loss in the temperature from 500 oC to 650 oC is ascribed to the decomposition of rGO, while the subsequent mass gain corresponds to the Si oxidation [38]. By excluding the adsorbed water, it can be calculated that MR-Si/rGO and MHR-Si/rGO contain 34.3 wt% and 35.2 wt% rGO, respectively. The SEM images of SiO2/rGO, MR-Si/rGO and MHR-Si/rGO are revealed in Fig. 3. Fig. 3a, b exhibits the feature of SiO2/rGO prepared by supercritical fluid assisted strategy. The low-magnification SEM image shows the rGO sheets possess a thin thickness of several nanometers with smooth surface. The high-magnification SEM image indicates SiO2 particles are uniformly dispersed on rGO sheets with no agglomeration. The morphology of products after MR reaction has changed dramatically as shown in Fig. 3c, d. The platelet morphology of rGO has been severely destroyed, becoming rough and thick. The fine SiO2 particles disappear and the large-size Si particles have generated and aggregated at the edge of rGO, which may be caused by the abnormal growth of crystal grains at high temperature due to violent exothermic reaction of MR. But for MHR, the rGO sheet still keeps a smooth surface and no large-size Si particles exists, which is clearly displayed in Fig. 3e, f. The reduced Si particles are much smaller, and are firmly bonded to rGO sheet with a good distribution, contrasting starkly with Fig. 3d.
The microstructures of the three composites were further analyzed by TEM. Fig. 4a shows the SiO2 nanoparticles whose average size is 20 nm are uniformly dispersed on rGO sheet. The HRTEM image in Fig. 4b indicates the rod-like SiO2 particles are well-anchored and amorphous. Fig. 4c illustrates the morphology after MR reaction and HCl treating. The size distribution of the as-formed Si crystals is very wide from tens to more than a hundred nanometers. By contrast, the crystal grains of Si reduced by MgH2 (Fig. 4e) own similar morphology with the SiO2 precursor, verifying the outstanding ability of shape preservation derived from mild reaction characteristic of MHR. Fig. 4d and 4f display the HRTEM images of MR-Si/rGO and MHR-Si/rGO, respectively. The former shows overlapping nanoparticles with an interplanar spacing of 0.19 and 0.31 nm for the (220) and (111) lattice planes of cubic silicon crystal. The latter displays Si nanocrystals, presenting the (111) plane are well-anchored to the rGO sheet. In addition, the STEM elemental mappings of MHR-Si/rGO in Fig. S5 further verify a uniform distribution of silicon can be achieved by MHR reaction. To illustrate the superiority of MHR-Si/rGO to MR-Si/rGO composite, their electrochemical performance as LIBs anodes was evaluated by half-cells. Fig. 5a demonstrates three initial CV curves of MHR-Si/rGO at a scan rate of 0.1 mV s−1, which are in good agreement with the Si/graphene anode as previously reported [39]. In the first lithiation process, two minor cathodic peaks at 1.2 V and 0.45 V, which disappear in subsequent CV curves, are detected. The former one is owing to the irreversible reaction between Li+ with edges and functional groups on rGO [27], which is also observed in the first CV profiles of rGO and SiO2/rGO anodes in Fig. S6a–6b. The other broad one around 0.45 V mainly comes from the reaction of Silicon and electrolytes to form SEI films. Si-Li alloying transition from crystalline Si to Li15Si4 is uncovered by the steep peak at around 50 mV. During the anodic sweep,
two remarkable humps at 0.35 and 0.52 V result from dealloying reaction from Li15Si4 to amorphous Si (a-Si) [35]. In the following cycles, an additional cathodic peak centered at 0.17 V arises in accordance with the reaction of a-Si and Li to form a-LixSi [6]. Moreover, the peak intensity increases with cycling, which can be ascribed to the activation effect [27]. Fig. 5b exhibits the first three discharge/charge curves of MHR-Si/rGO anode at 100 mA g−1. The initial discharge profile shows a pseudo-plateau at 1.2 V, a slope plateau at 1.0–0.2 V and another long plateau at ~0.1 V, which is consistent with the observation from CV curve. Afterwards, the discharge and charge profiles of different cycles are almost overlapped, demonstrating good reversibility of MHR-Si/rGO for lithium storage [40]. Fig. 5c illustrates the cycling stability of rGO, SiO2/rGO, MR-Si/rGO and MHR-Si/rGO at 200 mA g−1 in 100 cycles. The rGO and SiO2/rGO anodes show good reversibility, but their reversible capacities are very low (less than 380 mA h g−1). MR-Si/rGO delivers the highest initial discharge capacity, yet decays rapidly from 1610 to 489 mA h g−1 after 100 cycles. According to Fig. 3c, the platelet morphology of rGO in MR-Si/rGO has been severely destroyed and the generated Si particles are large, so the deteriorated performance of MR-Si/rGO might originate from these bad rGO platelets and the remarkable volume change of Si particles during cycling. By comparison, the MHR-Si/rGO anode delivers an outstanding reversible capacity of 894 mA h g−1 over 100 cycles with great cycling stability, verifying the structural advantages of MHR-Si/rGO composite. It can be further confirmed by the cycling capability of two composites at a high current rate of 1 A g−1. MHR-Si/rGO in Fig. 5d exhibits much improved cycling stability than MR-Si/rGO. After 400 cycles, MHR-Si/rGO composite retains a reversible capacity of 530 mA h g−1, whereas
MR-Si/rGO suffers a terrible capacity fading. As well as a satisfactory cycling stability, MHR-Si/rGO composite also delivers an impressive rate performance shown in Fig. 5e. When the anodes are cycled at different current densities from 0.2 to 0.3, 0.5, 1, 2, 3, and 5 A g−1, the MHR-Si/rGO anode delivers the highest stable capacities from 1158 to 1060, 958, 844, 754, 640, and 513 mA h g−1 respectively. After such a violent cycle travel, the reversible capacity remains 1045 mA h g−1 as the current rate returns to 0.2 A g−1, indicating superior reversibility of the MHR-Si/rGO anode. Fig. 5f reveals the Nyquist plots of rGO, SiO2/rGO, MR-Si/rGO and MHR-Si/rGO composites. The EIS profiles of these samples exhibit an oblique line in low frequency and a depressed semicircle in high frequency, which can be interpreted by the equivalent model (inset of Fig. 5f) [41]. In general, the electrolyte resistance (Re) corresponds to the first intercept of semicircle on the Z’ axis, the charge transfer resistance (Rct) is determined by the diameter of the semicircle, and the resistance of Li-ion diffusion (Zw) is related to the linear slope. These four anodes exhibit similar Re value of 3 Ω and almost the same Zw values, indicating the infiltration state of the electrolyte and the Li-ion diffusivity of these anodes are similar [42]. However, the Rct value of these electrodes varies. Although the Rct value of MHR-Si/rGO (43 Ω) increases slightly compared to rGO (31 Ω), it’s distinctly smaller than MR-Si/rGO (52 Ω). The low Rct value enables faster charge transfer, achieving better rate capability of MHR-Si/rGO electrode. For a deep insight into the outstanding performance, the MHR-Si/rGO composite after 90 cycles at 0.2 A g−1 are characterized by postmortem SEM and TEM. As shown in SEM (Fig. 6a and b), the Si NPs still adhere to rGO sheets after repeated cycles and the particle size has barely changed, confirming the bonding is firm enough to tolerate the volume variation. The HRTEM images in Fig. 6c and d reveal
the nano Si is partly crystalline, confirming the structure change after delithiation as discussed in CV analysis. The STEM elemental mapping in Fig. 6e further confirm the existence of Si and its uniform dispersion, verifying the active Si did not fall off upon cycling. Combined with the above analysis, the favorable electrochemical performance of MHR-Si/rGO can be explained as the following synergistic effect [18, 26]. Si NPs with high lithium storage capacity possess good damage tolerance and fast electron transportation; the rGO sheets serve as excellent conductivity matrix and prevent the aggregation of Si NPs, enabling the formation of stable SEI film. Owing to the superior reduction effect of MgH2, the desired structure of MHR-Si/rGO can be achieved. On the other hand, the structure of the composite obtained by magnesiothermic reduction is far from expected, leading to the unsatisfactory performance.
4. Conclusions In this paper, an efficient, scalable and mild alternative route to magnesiothermic reduction for the synthesis of Si/rGO composite has been successfully developed. Magnesium hydride (MgH2) reduction method is proved to synthesize silicon from silica at a low temperature, which can permit template assisted design of silicon structure. The as-formed MHR-Si/rGO shows small Si nanoparticles, high dispersion, replicated morphology from silica. As LIBs anode, MHR-Si/rGO composite delivers a superior reversible capacity of 894 mA h g−1 at 0.2 A g−1, remarkable rate performance (513 mA h g−1 at 5 A g−1), and much improved cycling stability than MR-Si/rGO composite particularly at high current rate. We believe this effective strategy can expand the field of synthesizing advanced silicon based anode materials.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant nos. 51677170, 51777194 and 51572240), Natural Science Foundation of Zhejiang Province (Grant nos. LY17E020010, LY16E070004 and LY18B030008).
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Figure captions
Fig. 1 Schematic illustration of the synthesis of the Si/rGO composite via MgH2 reduction. Fig. 2 Real-time dependence of temperature and gaseous pressure curves in (a) MR, (b) MHR; (c) XRD patterns and (d) Raman spectra of the synthesized samples. Fig. 3 SEM images of (a, b) SiO2/rGO; (c, d) MR-Si/rGO; (e, f) MHR-Si/rGO. Fig. 4. TEM images of (a) SiO2/rGO, (c) MR-Si/rGO, (e) MHR-Si/rGO; HRTEM images of (b) SiO2/rGO, (d) MR-Si/rGO, (f) MHR-Si/rGO. Fig. 5. Electrochemical performance of various materials: (a) Typical CV curves of MHR-Si/rGO at 0.1 mV s−1; (b) Galvanostatic discharge/charge profiles of MHR-Si/rGO at 0.2 A g−1; (c, d) Cycling performance at 0.2 A g−1 and 1 A g−1; (e) Rate capability at various current rates from 0.2 A g−1 to 1 A g−1; (f) Nyquist plots of various materials. Fig. 6 SEM images (a, b); TEM images (c, d); STEM elemental mappings (e) of MHR-Si/rGO composite after 90 cycles at 0.2 A g-1.
1
Fig. 1 Schematic illustration of the synthesis of the Si/rGO composite via MgH2 reduction.
2
Fig. 2 Real-time dependence of temperature and gaseous pressure curves in (a) MR, (b) MHR; (c) XRD patterns and (d) Raman spectra of the synthesized samples.
3
Fig. 3 SEM images of (a, b) SiO2/rGO; (c, d) MR-Si/rGO; (e, f) MHR-Si/rGO.
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Fig. 4. TEM images of (a) SiO2/rGO, (c) MR-Si/rGO, (e) MHR-Si/rGO; HRTEM images of (b) SiO2/rGO, (d) MR-Si/rGO, (f) MHR-Si/rGO.
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Fig. 5. Electrochemical performance of various materials: (a) Typical CV curves of MHR-Si/rGO at 0.1 mV s−1; (b) Galvanostatic discharge/charge profiles of MHR-Si/rGO at 0.2 A g−1; (c, d) Cycling performance at 0.2 A g−1 and 1 A g−1; (e) Rate capability at various current rates from 0.2 A g−1 to 1 A g−1; (f) Nyquist plots of various materials.
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Fig. 6 SEM images (a, b); TEM images (c, d); STEM elemental mappings (e) of MHR-Si/rGO composite after 90 cycles at 0.2 A g−1.
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Highlights •
We develop a new MgH2 reduction route to synthesize Si/rGO nanocomposite.
•
This strategy avoids the intense heat release in reaction process.
•
The product show great ability to preserve morphology feature.
•
The composite exhibits superior cycle performance and excellent rate capability.
Conflict of Interest The authors declare no conflict of interest.
S0013-4686(19)32119-X
DOI:
https://doi.org/10.1016/j.electacta.2019.135248
Reference:
EA 135248
To appear in:
Electrochimica Acta
Received Date: 20 August 2019 Revised Date:
12 October 2019
Accepted Date: 7 November 2019
Please cite this article as: F. Bian, J. Yu, W. Song, H. Huang, C. Liang, Y. Gan, Y. Xia, J. Zhang, X. He, W. Zhang, A new magnesium hydride route to synthesize morphology-controlled Si/rGO nanocomposite towards high-performance lithium storage, Electrochimica Acta (2019), doi: https://doi.org/10.1016/ j.electacta.2019.135248. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Graphical Abstract Synthesized by a mild process of MgH2 reduction reaction, the morphology-controlled Si/rGO composite exhibits great electrochemical performance as LIBs anode.
A new magnesium hydride route to synthesize morphology-controlled Si/rGO nanocomposite towards high-performance lithium storage
Feixiang Bian ‡,a, Jiage Yu ‡,a, Wenlong Song b, Hui Huang a,*, Chu Liang a, Yongping Gan a, Yang Xia a, Jun Zhang a, Xinping He a, Wenkui Zhang a,* a
College of Materials Science and Engineering, Zhejiang University of Technology,
Hangzhou 310014, China b
Zhejiang Tianneng Energy Technology Co ., Ltd, Huzhou, 313100, China
Corresponding authors & E-mails: [email protected] ‡ These authors contributed equally to this work.
Abstract Si/graphene composites have attracted great attention for application as anode materials of Li-ion batteries owing to their superior capacity and cycle stability. Magnesiothermic reduction of silica is a quite scalable and cost-effective method to synthesize Si/graphene composite. However, this remains a considerable challenge because the intense heat accumulation during the violent exothermic reaction makes it difficult to remain the desired nanostructure of the silica precursor and results in severe aggregation of silicon grains. Herein, we offer a mild and scalable route to efficiently convert silica into morphology-controlled Si nanoparticles by magnesium hydride (MgH2) reduction at a low temperature. The Si nanoparticles that are produced from silica template structure are fine and narrowly distributed, showing the ability to preserve morphology characteristics. The as-synthesized Si/rGO composite exhibits outstanding electrochemical properties, delivering a superior rate capability (513 mA h g−1 at 5 A g−1) and a high reversible capacity of 894 mA h g−1 over 100 cycles at 0.2 A g−1. We believe MgH2 reduction of silica demonstrates its great potential in fabricating Si based anode materials.
Keywords:
Si/graphene
composites;
Morphology-controlled;
reduction; Magnesium hydride; Li-ion batteries
Magnesiothermic
1. Introduction Lithium-ion batteries (LIBs) have been widely investigated as an energy storage system for electric vehicles and portable electronic devices due to their great power density, environmental benignity and long cycle life [1-3]. The traditional graphite anode suffers a limited capacity (372 mA h g−1), which has encouraged a great interest in pursuing potential substitutes [4]. Silicon is considered as one of the most promising candidates for its unparalleled lithium storage of 3579 mA h g−1 (Li15Si4), low voltage plateau (~0.4 V vs. Li/Li+) and abundance [5, 6]. Unfortunately, one fatal flaw is that Si sustains dramatic volume variation during lithiation process (>300%), giving rise to the cracking and pulverization of active particles and repetitive growth of the solid-electrolyte interface (SEI) [7-9]. In addition, Si exhibits poor electrical conductivity and Li-ion diffusion capacity for its semiconductor nature [10]. These result in low initial Coulombic efficiencies, rapid capacity fading in few cycles especially under a high current density. To tackle the aforementioned drawbacks, the effect of integrating nanostructured Si with graphene matrix has been proved beyond contradiction [11, 12]. Nano Si possesses high tolerance to volume change and shortened Li+ diffusion length, while graphene can not only stabilize the growth of SEI film and accommodate the volume variation, but also improve the conductivity of electrons and lithium ions [13, 14]. The Si/graphene composite can be directly synthesized by mechanical mixing, layer-by-layer deposition and self-assembly [15-18]. However, the bond between graphene and Si via physical routes is not firm enough to maintain the tight connection and the involving of high cost commercial nano silicon hinders practical applications. Therefore, much interest has been paid to chemical synthesis methods. Direct growth of graphene on Si nanoparticles or depositing Si on graphene by
chemical vapor deposition (CVD) is also adapted to fabricate Si/graphene structures [19, 20]. But CVD requires toxic gas precursor and delicate equipment, and suffers from an extremely low yield [21]. In contrast, template assisted magnesiothermic reduction (MR) of SiO2/graphene becomes a simple and scalable route for the preparation of Si/graphene composite [22-27]. The reaction enthalpy is calculated as −291.6 kJ mol−1 at 25 oC for Mg (s) and −577.1 kJ mol−1 at 650 oC for Mg (g), respectively, indicating that MR is a violent exothermic reaction (Reaction 1). The vast heat releases and accumulates sharply in short time. Thus, MR is uncontrollable, the actual temperature is even higher than the melting point of silicon (1414 o
C),leading to the restack of graphene and the sintering of the as-obtained Si particles
[28]. Currently, the large challenge of MR is not easy to preserve the morphology of silica template in silicon analogue. Another challenge is controlling the reaction extent to avoid the undesirable process of HF etching silica. Thus, it is highly desirable to develop a mild, controllable reduction reaction to replace MR.
2Mg + SiO2 → 2MgO + Si
(1)
2MgH 2 + SiO 2 → 2 MgO + Si + 2H 2 (g)
(2)
Herein, we propose a promising alternative to MR for the synthesis of Si/rGO composite with superior lithium storage performance (Figure 1). The pre-designed SiO2/rGO composites can efficiently convert to Si/rGO via magnesium hydride reduction (MHR) at a moderate temperature (500 oC), much lower than the commonly reported temperature of MR (650 oC). The reaction enthalpy of MHR is about −139.3 kJ mol−1 at 25 oC and −134.6 kJ mol−1 at 500 oC (Reaction 2). This MHR method is mild, controllable and scalable, showing outstanding ability to permit template assisted design of silicon structures. We believe that this strategy provides a new opportunity for efficient and controllable nano silicon synthesis.
2. Experimental 2.1 Materials synthesis 2.1.1 Preparation of SiO2/rGO precursor Graphene oxides (GO) were obtained from flake graphite (Aladdin, ~300 mesh) by a modified Hummers method [29]. SiO2/rGO composite was fabricated via a supercritical CO2 fluid-assisted method as described in our previous work [30-32]. Typically, 300 mg GO sheets, 0.1 mL ammonia solution and 0.1 mL DI water were dispersed in 50 mL absolute ethyl alcohol. Then 2.0 mL tetraethoxysilane (TEOS) was dropwise added to the alcohol dispersion to undergoing a three-hour stirring. The mixture was then placed in a stainless steel container. The container was vacuumized prior to filling with 80 bar CO2. The container was kept at 35 oC with rotating at 350 r.p.m. overnight. Thereafter, the sample was sealed in a Teflon-lined autoclave with hydrothermal treatment at 150 oC for 12 h. Finally, the sample was collected by filtration and freeze-dried. 2.1.2 Preparation of MgH2 powder 5 g magnesium powder (Aladdin, 99.5%) was sealed into a custom-made reactor, then the chamber was vacuumized prior to filling with 70 bar of hydrogen. Next the reactor was heated to 420 oC and refilled with hydrogen every 2 hours to keep hydrogen pressure higher than 70 bar. After 24 h treatment, the sample was transferred to a stainless steel container together with ZrO2 balls at ball-to-powder mass ratio of 40:1. Then it was milled at 500 r.p.m. for 24 h under 50 bar of hydrogen. After repeating the above several times, the MgH2 powder was successfully synthesized and characterized by XRD (Fig. S1). 2.1.3 Preparation of Si NPs/rGO composite 200 mg SiO2/rGO composite and 150 mg MgH2 powder were mixed and sealed
into a custom-made reactor in argon-filled glove box. The reactor was vacuumized prior to being heated to 500 oC (ramp rate 3 oC/min) for 300 min. Then the reacted powders were immersed in diluted hydrochloric acid to remove MgO with subsequent wash by DI water and ethanol for three times. Lastly, Si/rGO composite reduced by MgH2 (MHR-Si/rGO) was collected by freeze-drying. For comparison, Si/rGO reduced by Mg denoted as MR-Si/rGO was also prepared. Briefly, 200 mg of SiO2/rGO composite and 138 mg of Mg powder were used as raw materials with the same molar ratio as MHR. The sample was synthesized at 650 oC as reported in many previous reports.
2.2 Materials characterization X-ray diffraction (XRD) analysis was carried out on an X'Pert Pro diffractometer using copper Kα radiation (λ = 0.15418 nm). Thermogravimetric analysis (TGA) was acquired by TA Instruments SDT Q600 under air atmosphere. Raman spectrums were obtained by Renishaw InVia Reflex spectrometer (excitation wavelength = 532 nm). Scanning electron microscopy (SEM) images of composites were determined via FEI Nova NanoSEM 450, and transmission electron microscopy (TEM) images were carried out using FEI, Tecnai G2 F30.
2.3 Battery test Electrochemical characterization of samples was tested by CR 2032 coin cell. To fabricate the working electrode, Si/rGO composite was stirred with Super-P carbon and polyvinylidene fluoride (PVDF) binder in weight ratio of 7:1.5:1.5 in N-methyl-2-pyrrolidone (NMP). The obtained viscous slurry was spread on a Cu foil and dried at 85 oC under vacuum for 8 h. The active material mass loading was controlled at 1.0–1.2 mg cm−2. The testing cell was assembled in a MBRAUM glovebox under Ar atmosphere by using lithium foil as counter electrode, Cellgard
2500 membrane as separator film. And the electrolyte was 1.0 M LiPF6 in the mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) in equal volumes. Before test, the cells were placed at 30 oC for 24 h to ensure the full penetration of the electrolyte. Cyclic voltammetry (CV) profiles were obtained by a CHI 660E electrochemical workstation. Galvanostatic charging/discharging was conducted in the potential window of 0.01–2.0 V on CT-3008W Neware battery test systems. Electrochemical impedance spectroscopy (EIS) was measured by a Zahner Zennium electrochemical workstation over the frequency range of 1 MHz–0.1 Hz.
3. Results and Discussion The temperature and gaseous pressure during the reaction processes were recorded by online monitors for better understanding of MR and MHR. It has been revealed by Fig. S2 that MR doesn’t occur at 500 oC, so the holding temperature of MR in our experiment was set as 650 oC, the melting point of magnesium [27, 33]. As shown in Fig. 2a, b, the real-time temperature of MHR increases steadily, while a sharp temperature rising from 510 oC to 860 oC is observed in the MR system. Then after several minutes, the temperature drops rapidly to the normal temperature range, owing to the reactants are limited. This phenomenon indicates that the exothermic reaction of MR is much more ferocious than MHR. As for the gaseous pressure, a significant increase is observed at 380 oC in the MHR system, implying that MHR can be triggered at low temperature. To ensure the MHR can be fully carried out, some calculations have been made in Supplementary Material. These observations confirm the superiority of MHR reaction. Fig. S3 displays the XRD pattern of the residue collected from MHR at 500 oC, where two major phases of MgO and Si and a minor phase of Mg2Si can be observed. Besides, no peaks of MgH2 can be detected, confirming that MgH2 has been totally
consumed in the MHR process. The presence of Mg2Si may be a result of the side reaction between MgH2 and Si, which can be summarized as Reaction 3.
2MgH2 +Si →Mg2Si +2H2 (g)
(3)
With excess MgH2, the formation of Mg2Si would reduce the yield of silicon, yet in the lack of MgH2, the undesirable HF etching should be introduced to eliminate the SiO2 residue. Thus a range of controlled experiments were performed to optimize the reactant mass ratio, which was finally restricted as 4:3 of SiO2/rGO and MgH2. As the SiO2 content in the SiO2/rGO composites in this case was determined to be 79.2 wt% (shown in Fig. S4), the mole ratio of SiO2/rGO and MgH2 in reactants was 1:2.16. The XRD results of rGO, SiO2/rGO, MR-Si/rGO and MHR-Si/rGO are shown in Fig. 2c. The XRD pattern of rGO delivers a large broad peak at 23.8o and a small peak at 43.0o, respectively. The broad peak corresponds to the (002) plane with a d-spacing of 0.372 nm. After the SiO2 deposition, a new broad peak appears between 20o and 23o, indicating the amorphous nature of SiO2 particles [34]. MR-Si/rGO and MHR-Si/rGO have the same (002) peak of rGO, but it’s worth noting that the (002) peak in MR-Si/rGO is relatively more intense, implying the rGO lamellae have restacked to graphite-like agglomerates. In addition, the patterns of MR-Si/rGO and MHR-Si/rGO both exhibit three peaks at 28.4o, 47.3o and 56.1o, corresponding to (111), (220) and (311) of crystal Si (JCPDS 27-1402), respectively. Although MR can achieve a successful synthesis of Si, the amorphous SiO2 phase still remains in the pattern of MR-Si/rGO sample, demonstrating the chemical reaction between Mg and SiO2 is incomplete. The XRD analysis also reveals the superiority of employing MgH2 as a reducing agent. Fig. 2d shows the Raman spectra of the four samples, all of which present characteristic peaks at about 1340 cm−1 for D band and 1590 cm−1 for G band of graphene [35]. The overall spectra of MR-Si/rGO and MHR-Si/rGO are
similar except for the Raman shift of Si peaks. The band at about 520 cm−1 of crystal-Si shifts towards negative with decreasing of the Si particle size owing to the nano silicon induced confinement effect [26, 36, 37]. So it’s confirmed the Si nanoparticles of MHR-Si/rGO with Raman shift at 505 cm−1 have a smaller size than that of MR-Si/rGO whose Raman shift at 512 cm−1. The rGO contents in two Si/rGO composites were tested by TGA as presented in Fig. S4. The convergent initial mass loss up to 200 oC of two samples is due to the evaporation of adsorbed water, and the obvious mass loss in the temperature from 500 oC to 650 oC is ascribed to the decomposition of rGO, while the subsequent mass gain corresponds to the Si oxidation [38]. By excluding the adsorbed water, it can be calculated that MR-Si/rGO and MHR-Si/rGO contain 34.3 wt% and 35.2 wt% rGO, respectively. The SEM images of SiO2/rGO, MR-Si/rGO and MHR-Si/rGO are revealed in Fig. 3. Fig. 3a, b exhibits the feature of SiO2/rGO prepared by supercritical fluid assisted strategy. The low-magnification SEM image shows the rGO sheets possess a thin thickness of several nanometers with smooth surface. The high-magnification SEM image indicates SiO2 particles are uniformly dispersed on rGO sheets with no agglomeration. The morphology of products after MR reaction has changed dramatically as shown in Fig. 3c, d. The platelet morphology of rGO has been severely destroyed, becoming rough and thick. The fine SiO2 particles disappear and the large-size Si particles have generated and aggregated at the edge of rGO, which may be caused by the abnormal growth of crystal grains at high temperature due to violent exothermic reaction of MR. But for MHR, the rGO sheet still keeps a smooth surface and no large-size Si particles exists, which is clearly displayed in Fig. 3e, f. The reduced Si particles are much smaller, and are firmly bonded to rGO sheet with a good distribution, contrasting starkly with Fig. 3d.
The microstructures of the three composites were further analyzed by TEM. Fig. 4a shows the SiO2 nanoparticles whose average size is 20 nm are uniformly dispersed on rGO sheet. The HRTEM image in Fig. 4b indicates the rod-like SiO2 particles are well-anchored and amorphous. Fig. 4c illustrates the morphology after MR reaction and HCl treating. The size distribution of the as-formed Si crystals is very wide from tens to more than a hundred nanometers. By contrast, the crystal grains of Si reduced by MgH2 (Fig. 4e) own similar morphology with the SiO2 precursor, verifying the outstanding ability of shape preservation derived from mild reaction characteristic of MHR. Fig. 4d and 4f display the HRTEM images of MR-Si/rGO and MHR-Si/rGO, respectively. The former shows overlapping nanoparticles with an interplanar spacing of 0.19 and 0.31 nm for the (220) and (111) lattice planes of cubic silicon crystal. The latter displays Si nanocrystals, presenting the (111) plane are well-anchored to the rGO sheet. In addition, the STEM elemental mappings of MHR-Si/rGO in Fig. S5 further verify a uniform distribution of silicon can be achieved by MHR reaction. To illustrate the superiority of MHR-Si/rGO to MR-Si/rGO composite, their electrochemical performance as LIBs anodes was evaluated by half-cells. Fig. 5a demonstrates three initial CV curves of MHR-Si/rGO at a scan rate of 0.1 mV s−1, which are in good agreement with the Si/graphene anode as previously reported [39]. In the first lithiation process, two minor cathodic peaks at 1.2 V and 0.45 V, which disappear in subsequent CV curves, are detected. The former one is owing to the irreversible reaction between Li+ with edges and functional groups on rGO [27], which is also observed in the first CV profiles of rGO and SiO2/rGO anodes in Fig. S6a–6b. The other broad one around 0.45 V mainly comes from the reaction of Silicon and electrolytes to form SEI films. Si-Li alloying transition from crystalline Si to Li15Si4 is uncovered by the steep peak at around 50 mV. During the anodic sweep,
two remarkable humps at 0.35 and 0.52 V result from dealloying reaction from Li15Si4 to amorphous Si (a-Si) [35]. In the following cycles, an additional cathodic peak centered at 0.17 V arises in accordance with the reaction of a-Si and Li to form a-LixSi [6]. Moreover, the peak intensity increases with cycling, which can be ascribed to the activation effect [27]. Fig. 5b exhibits the first three discharge/charge curves of MHR-Si/rGO anode at 100 mA g−1. The initial discharge profile shows a pseudo-plateau at 1.2 V, a slope plateau at 1.0–0.2 V and another long plateau at ~0.1 V, which is consistent with the observation from CV curve. Afterwards, the discharge and charge profiles of different cycles are almost overlapped, demonstrating good reversibility of MHR-Si/rGO for lithium storage [40]. Fig. 5c illustrates the cycling stability of rGO, SiO2/rGO, MR-Si/rGO and MHR-Si/rGO at 200 mA g−1 in 100 cycles. The rGO and SiO2/rGO anodes show good reversibility, but their reversible capacities are very low (less than 380 mA h g−1). MR-Si/rGO delivers the highest initial discharge capacity, yet decays rapidly from 1610 to 489 mA h g−1 after 100 cycles. According to Fig. 3c, the platelet morphology of rGO in MR-Si/rGO has been severely destroyed and the generated Si particles are large, so the deteriorated performance of MR-Si/rGO might originate from these bad rGO platelets and the remarkable volume change of Si particles during cycling. By comparison, the MHR-Si/rGO anode delivers an outstanding reversible capacity of 894 mA h g−1 over 100 cycles with great cycling stability, verifying the structural advantages of MHR-Si/rGO composite. It can be further confirmed by the cycling capability of two composites at a high current rate of 1 A g−1. MHR-Si/rGO in Fig. 5d exhibits much improved cycling stability than MR-Si/rGO. After 400 cycles, MHR-Si/rGO composite retains a reversible capacity of 530 mA h g−1, whereas
MR-Si/rGO suffers a terrible capacity fading. As well as a satisfactory cycling stability, MHR-Si/rGO composite also delivers an impressive rate performance shown in Fig. 5e. When the anodes are cycled at different current densities from 0.2 to 0.3, 0.5, 1, 2, 3, and 5 A g−1, the MHR-Si/rGO anode delivers the highest stable capacities from 1158 to 1060, 958, 844, 754, 640, and 513 mA h g−1 respectively. After such a violent cycle travel, the reversible capacity remains 1045 mA h g−1 as the current rate returns to 0.2 A g−1, indicating superior reversibility of the MHR-Si/rGO anode. Fig. 5f reveals the Nyquist plots of rGO, SiO2/rGO, MR-Si/rGO and MHR-Si/rGO composites. The EIS profiles of these samples exhibit an oblique line in low frequency and a depressed semicircle in high frequency, which can be interpreted by the equivalent model (inset of Fig. 5f) [41]. In general, the electrolyte resistance (Re) corresponds to the first intercept of semicircle on the Z’ axis, the charge transfer resistance (Rct) is determined by the diameter of the semicircle, and the resistance of Li-ion diffusion (Zw) is related to the linear slope. These four anodes exhibit similar Re value of 3 Ω and almost the same Zw values, indicating the infiltration state of the electrolyte and the Li-ion diffusivity of these anodes are similar [42]. However, the Rct value of these electrodes varies. Although the Rct value of MHR-Si/rGO (43 Ω) increases slightly compared to rGO (31 Ω), it’s distinctly smaller than MR-Si/rGO (52 Ω). The low Rct value enables faster charge transfer, achieving better rate capability of MHR-Si/rGO electrode. For a deep insight into the outstanding performance, the MHR-Si/rGO composite after 90 cycles at 0.2 A g−1 are characterized by postmortem SEM and TEM. As shown in SEM (Fig. 6a and b), the Si NPs still adhere to rGO sheets after repeated cycles and the particle size has barely changed, confirming the bonding is firm enough to tolerate the volume variation. The HRTEM images in Fig. 6c and d reveal
the nano Si is partly crystalline, confirming the structure change after delithiation as discussed in CV analysis. The STEM elemental mapping in Fig. 6e further confirm the existence of Si and its uniform dispersion, verifying the active Si did not fall off upon cycling. Combined with the above analysis, the favorable electrochemical performance of MHR-Si/rGO can be explained as the following synergistic effect [18, 26]. Si NPs with high lithium storage capacity possess good damage tolerance and fast electron transportation; the rGO sheets serve as excellent conductivity matrix and prevent the aggregation of Si NPs, enabling the formation of stable SEI film. Owing to the superior reduction effect of MgH2, the desired structure of MHR-Si/rGO can be achieved. On the other hand, the structure of the composite obtained by magnesiothermic reduction is far from expected, leading to the unsatisfactory performance.
4. Conclusions In this paper, an efficient, scalable and mild alternative route to magnesiothermic reduction for the synthesis of Si/rGO composite has been successfully developed. Magnesium hydride (MgH2) reduction method is proved to synthesize silicon from silica at a low temperature, which can permit template assisted design of silicon structure. The as-formed MHR-Si/rGO shows small Si nanoparticles, high dispersion, replicated morphology from silica. As LIBs anode, MHR-Si/rGO composite delivers a superior reversible capacity of 894 mA h g−1 at 0.2 A g−1, remarkable rate performance (513 mA h g−1 at 5 A g−1), and much improved cycling stability than MR-Si/rGO composite particularly at high current rate. We believe this effective strategy can expand the field of synthesizing advanced silicon based anode materials.
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant nos. 51677170, 51777194 and 51572240), Natural Science Foundation of Zhejiang Province (Grant nos. LY17E020010, LY16E070004 and LY18B030008).
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Figure captions
Fig. 1 Schematic illustration of the synthesis of the Si/rGO composite via MgH2 reduction. Fig. 2 Real-time dependence of temperature and gaseous pressure curves in (a) MR, (b) MHR; (c) XRD patterns and (d) Raman spectra of the synthesized samples. Fig. 3 SEM images of (a, b) SiO2/rGO; (c, d) MR-Si/rGO; (e, f) MHR-Si/rGO. Fig. 4. TEM images of (a) SiO2/rGO, (c) MR-Si/rGO, (e) MHR-Si/rGO; HRTEM images of (b) SiO2/rGO, (d) MR-Si/rGO, (f) MHR-Si/rGO. Fig. 5. Electrochemical performance of various materials: (a) Typical CV curves of MHR-Si/rGO at 0.1 mV s−1; (b) Galvanostatic discharge/charge profiles of MHR-Si/rGO at 0.2 A g−1; (c, d) Cycling performance at 0.2 A g−1 and 1 A g−1; (e) Rate capability at various current rates from 0.2 A g−1 to 1 A g−1; (f) Nyquist plots of various materials. Fig. 6 SEM images (a, b); TEM images (c, d); STEM elemental mappings (e) of MHR-Si/rGO composite after 90 cycles at 0.2 A g-1.
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Fig. 1 Schematic illustration of the synthesis of the Si/rGO composite via MgH2 reduction.
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Fig. 2 Real-time dependence of temperature and gaseous pressure curves in (a) MR, (b) MHR; (c) XRD patterns and (d) Raman spectra of the synthesized samples.
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Fig. 3 SEM images of (a, b) SiO2/rGO; (c, d) MR-Si/rGO; (e, f) MHR-Si/rGO.
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Fig. 4. TEM images of (a) SiO2/rGO, (c) MR-Si/rGO, (e) MHR-Si/rGO; HRTEM images of (b) SiO2/rGO, (d) MR-Si/rGO, (f) MHR-Si/rGO.
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Fig. 5. Electrochemical performance of various materials: (a) Typical CV curves of MHR-Si/rGO at 0.1 mV s−1; (b) Galvanostatic discharge/charge profiles of MHR-Si/rGO at 0.2 A g−1; (c, d) Cycling performance at 0.2 A g−1 and 1 A g−1; (e) Rate capability at various current rates from 0.2 A g−1 to 1 A g−1; (f) Nyquist plots of various materials.
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Fig. 6 SEM images (a, b); TEM images (c, d); STEM elemental mappings (e) of MHR-Si/rGO composite after 90 cycles at 0.2 A g−1.
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Highlights •
We develop a new MgH2 reduction route to synthesize Si/rGO nanocomposite.
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This strategy avoids the intense heat release in reaction process.
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The product show great ability to preserve morphology feature.
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The composite exhibits superior cycle performance and excellent rate capability.
Conflict of Interest The authors declare no conflict of interest.