Raspberry-Like Nanostructured Silicon Composite Anode for High

Keywords: silicon, carbon coated, hollow structure, Li-ion batteries, anode ... molten salt reduction is precisely designed to synthesise a raspberry-...
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Raspberry-like Nanostructured Silicon Composite Anode for HighPerformance Lithium-Ion Batteries Shan Fang,†,‡ Zhenkun Tong,† Ping Nie,† Gao Liu,‡ and Xiaogang Zhang*,† †

College of Materials Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China Energy Storage and Distributed Resources Division, Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States

‡

S Supporting Information *

ABSTRACT: Adjusting the particle size and nanostructure or applying carbon materials as the coating layers is a promising method to hold the volume expansion of Si for its practical application in lithium-ion batteries (LIBs). Herein, the mild carbon coating combined with a molten salt reduction is precisely designed to synthesize raspberry-like hollow silicon spheres coated with carbon shells (HSi@C) as the anode materials for LIBs. The HSi@C exhibits a remarkable electrochemical performance; a high reversible specific capacity of 886.2 mAh g−1 at a current density of 0.5 A g−1 after 200 cycles is achieved. Moreover, even after 500 cycles at a current density of 2.0 A g−1, a stable capacity of 516.7 mAh g−1 still can be obtained. KEYWORDS: silicon, carbon coated, hollow structure, Li-ion batteries, anode

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binders are the main strategies.16−21 It is well-known that nanometer ranged Si materials can alleviate the mechanical stress induced by lithiation and enhance the resistance to pulverization and crack, which in turn can highly improve the electrochemical performance of the whole electrode.22 All kinds of nanostructured Si materials, including nanoparticles (NPs),23−26 nanowires (NWs),27−30 nanotubes,31−34 and hollow nanospheres,35−37 have been designed and prepared, and encouraging results have been achieved through these efforts. Among them, void core−shell structures have attracted particular interests, where the void can efficiently accommodate the large volume changes, while the outer coated carbon shell serves as an electric pathway to improve the electron conductivity. Herein we precisely design and synthesize raspberry-like hollow nanospheres with highly conductive carbon shells (HSi@C) by the molten salt magnesiothermic reduction of mesoporous hollow silica nanospheres (HSiO2) and polymerization of dopamine on the surface of HSi, followed with carbonization and without any other template. This distin-

INTRODUCTION A high specific energy of a reliable and rechargeable battery is critically important for wide applications such as portable, transportable, and smart power grid. Lithium-ion batteries (LIBs) offer the higher energy density, power capability, and comparatively good life cycling, dominating the market for energy storage.1−5 However, LIBs based on traditional graphite anode and lithium metal oxides cathodes provide a specific energy of ∼150 Wh kg−1, which is too low to meet the need for an electric vehicle.6−9 To this end, tremendous efforts have been made to developing electrode materials with higher specific capacities and power capability. For the anode material, silicon, which possesses an unparalleled theoretical capacity of 3579 mAh g−1, low lithiation potential, and is environmental friendly, has been considered as one of the most encouraging anode candidates for the next-generation high energy density LIBs.10−13 Unfortunately, the rapid capacity fading due to various modes such as electrical isolation, the active material pulverization, and an unstable solid−electrolyte interphase (SEI) layer continuous formation caused by large volume expansion (>300%) during charge and discharge process largely limit the successful applications of silicon anode.14,15 To address these challenges, nanostructure design of silicon particles, coating with buffer layers, and adding conductive © 2017 American Chemical Society

Received: March 4, 2017 Accepted: May 15, 2017 Published: May 15, 2017 18766

DOI: 10.1021/acsami.7b03157 ACS Appl. Mater. Interfaces 2017, 9, 18766−18773

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the preparation of HSi@C nanocomposite.

Figure 2. (a) XRD patterns of SiO2, HSi, and HSi@C composites. (b) Raman spectra of HSi and HSi@C composites. (c) TGA curve for HSi@C composites. (d) Cycling performance of HSi@C and HSi measured at 0.5 A g−1. The Brunauer−Emmett−Teller specific surface area and the Barrett−Joyner−Halenda pore volume calculated from the N2 sorption of HSi@C are 253.15 cm2 g−1 and 0.59 cm3 g−1, respectively, and the pore size distribution is centered around 12 nm (d, inset).

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guished material design brings about multiple fascinating merits: (a) the large stress and volume changes of Si during cycling can be effectively alleviated by the plentiful porous structure of the hollow nanospheres; (b) the transportation distance of the ionic and electron can be extremely reduced benefit from the hollow nanospheres with a thin shell; (c) thermal decomposition of carbon precursors as coatings provide high electrical conductivity of the electrode as well as a buffer layer to remission the stress of volume changes. Outstanding cycling stability and distinguished rate capability are obtained when such novel HSi@C are evaluated as anode materials in LIBs. Therefore, these specially designed hollow structures exhibit a stable and high capacity of 886.2 mAh g−1 for 200 cycles with a high Coulombic efficiency (CE) up to 99.4%; a high capacity of 516.7 mAh g−1 can be maintained at 2.0 A g−1 even after 500 cycles, which worked as encouraging candidates for practical high energy storage LIBs.

EXPERIMENTAL SECTION

Sample Synthesis. Synthesis of HSi@C composite: the hollow SiO2 spheres were synthesized via the spontaneous self-transformation approach according to the previous literature,38 0.5 g of HSiO2 and magnesium powder were mixed with a weight ratio of 1:1, and the amounts of NaCl and KCl have the same molar ratio of Mg (1:1:1); after that, the mixture was placed in a stainless steel autoclave reactor (Figure S1) and sintering at 700 °C for 6 h with an inert atmosphere. The HSi@PDA was obtained by selectively removing MgO and in situ polymerization of dopamine according to our previous literature.39 The HSi@PDA was sintering under an N2 for 2 h at 400 °C and 3 h at 800 °C to obtain the HSi@C. Material Characterization. A scanning electron microscope (SEM, Hitachi S-4800), a transmission electron microscope (TEM, JEOL JEM-2010), and a dual-beam focused ion beam (FIB) microscope were used to characterize the morphologies of the products. X-ray diffraction (XRD) (Bruker D8 Advance) with Cu Kα radiation was applied to characterize the crystal structures of the samples. The surface area of the materials was characterized by an ASAP-2010 surface area analyzer. The Raman spectrum was 18767

DOI: 10.1021/acsami.7b03157 ACS Appl. Mater. Interfaces 2017, 9, 18766−18773

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a, b) SEM and TEM images of SiO2 hollow spheres. (c) SEM images of HSi@C. (d, e, f) TEM images and HRTEM images of HSi@C. Insets: SAED pattern of silicon cubic structure. performed with 514.5 nm wavelength to check the compositions, and the carbon content of the product was check by EA (Vario EL-III). A thermogravimetric test was implemented on a TGA device (NETZSCH STA 409 PC) from 30 to 900 °C in air. Electrochemical Test. The electrodes were fabricated by a mixture of active materials, carbon black, and sodium alginate (Alg) binder (7:2:1); DI water was used as solvent. Then, the slurry was coated on a copper foil uniformly by a doctor blade and dried in a vacuum oven at 80 °C overnight. The average mass loading of the active material was about 0.7 mg cm−2. The thickness of the electrode is ∼25 μm, so that the electrode density is calculated as 0.7/0.7 × 10−3 mg/2.5 × 10−3 cm3 = 0.4 g cm−3. The electrolyte solution consisted of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) by volume ratio of 1:1, and 2% (V) fluoroethylene carbonate (FEC) was used as an electrolyte additive. A CT2001A cell test device (LAND Electronic Co.) was used to conduct electrochemical characterization by galvanostatic discharge/charge at a voltage of 0.01−1.5 V (vs Li/Li+). Electrochemical impedance spectroscopy (EIS) was studied in the frequency range of 100 kHz−0.01 Hz, with an amplitude of 5 mV.

formation method by diffuse the silica spheres in water. Then, HSi was produced through the reduction of HSiO2 in a mixture of salt (NaCl and KCl) by magnesium in a small stainless steel reactor. The mechanism of this reaction is analogous to the Kirkendall effect, and the reaction with SiO2 would occur only at the interphase boundary. The mixed molten salt not only can scavenge amounts of the heat generated by the exothermic reaction to completely prohibit the structure from being deconstructive and the formation of Mg2Si but also remarkably improve the homogeneity of the magnesiothermic reduction due to the homogeneous distribution and transportation of Mg around the mesoporous hollow spheres, thus promoting a highyield conversion of silicon at a temperature of 700 °C in N2. Finally, in situ self-polymerizing dopamine on the surface of the HSi and carbonized at a high temperature in N2 to get the final products. The shell thickness of hollow SiO2 spheres can be tailored by the control of incubation temperature (Figure S2). Since Si has a large volume change (>300%) during lithiation and delithiation, there is a relationship between shell thickness and the radius (Figure S3), according to the calculation, when the radius of Si hollow sphere is 430 nm and the corresponding shell thickness is about 27.1 nm; it can meet the 3 times volume expansion of the Si hollow spheres. On the basis of this

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RESULTS AND DISCUSSION The major preparation procedure employed in the present work is illuminated in Figure 1. The uniform HSiO 2 nanospheres were fabricated by an autonomous self-trans18768

DOI: 10.1021/acsami.7b03157 ACS Appl. Mater. Interfaces 2017, 9, 18766−18773

Research Article

ACS Applied Materials & Interfaces

Figure 4. Discharge/charge profiles of (a) HSi@C and (b) bare HSi at a current density of 0.1 A g−1. (c) Rate capability tests for HSi@C and pure HSi at various C-rates. (d) Cycling performance of HSi@C and HSi measured at 0.5 A g−1. (e) Long-term cycling performance of HSi@C composite at 2 A g−1.

calculation, we control the thickness of the SiO2 shell to be ∼30 nm for further experiments. The X-ray diffraction (XRD) characterization is used to identify the crystal structure of the sample (Figure 2a). The XRD pattern of the HSiO2 displays only one broad peak centered between 20° and 30° and demonstrates an amorphous structure of the samples. After the molten salt magnesiothermic reduction reaction and acid treatment, all diffraction peaks of the HSi can be ascribed to a cubic phase of crystalline Si (JCPDS card No. 27-1402). The color changes from white to yellow (Figure S4), indicating that SiO2 is successfully transformed into Si. For the HSi@C sample, in addition to the characteristic peak of crystalline Si, a broad peak appearing at around 22° indicates the amorphous character of the carbon coating. Raman spectra were further applied to certify the success reduction of silica to silicon during this magnesiothermic reduction process. As shown in Figure 2b, the commonly observed peaks corresponding to amorphous surface oxides in the range of 300−450 cm−1, and the amorphous Si peak at 480 cm−1 was not detected. The Raman peak corresponding to crystalline Si was distinctively observed at 508 cm−1 (lower than that of bulk Si at 520 cm−1),40 further implying the phase structure of the samples. The raspberry-like HSi@C composite shows a disorder-induced D band at about 1360 cm−1 and a graphitic G band at about 1595 cm−1, indicating the successful carbon coating on the surface of HSi. The intensity ratio ID/IG is around 0.82, revealing the domination of disordered structure

of carbon coatings.41 Then the TGA is used to evaluate the Si content of the HSi@C composite. The main weight loss at 400−600 °C should be ascribed to the decomposition of carbon in the air. According to the calculation, the fraction of Si is identified to be 70.1 wt % (Figure 2c). The specific surface area and pore structure of the sample were investigated by N2 adsorption−desorption isotherms. It was observed that the HSiO2 spheres showed type IV features unveiling features of mesoporous materials with narrow pore size distribution. Based on the calculation, the pore volume and surface area are calculated to be as high as about 0.82 cm3 g−1 and 846 m2 g−1, respectively (Figure S5). After magnesiothermic reduction and carbon coating, the surface area of HSi@C composite is 253 m2 g−1. The relatively high surface area is a benefit to Li-ion diffusion during the lithiation/delithiation procedure since it increases the contact area between active material and electrolyte, which in turn shortens the diffusion distance in the electrode. The average pore size distribution is broadly in the range of 12.7−300 nm (inset of Figure 2d); this perhaps originated from the hollow space between the core and the shell. And the average pore volume is 0.592 cm3 g−1 The microstructure of the samples was observed by SEM and TEM. As shown in the SEM image, the silica nanospheres are highly uniform with an average diameter of ∼430 nm (Figure 3a). The thickness of the SiO2 shell can be tailored by the control of the temperature of incubating in the water. TEM images in Figure S2 show that the wall thickness in the hollow 18769

DOI: 10.1021/acsami.7b03157 ACS Appl. Mater. Interfaces 2017, 9, 18766−18773

Research Article

ACS Applied Materials & Interfaces

Figure 5. SEM images of HSi@C before (a, b, c) and after 50 cycles (d, e, f) at a current density of 0.5 A g−1.

formation of amorphous Li−Si alloy (a-LixSi).28,39 The major discharge potential plateau which appears close to 0.1 V (vs Li/ Li+) is attributed to the phase transition from a-LixSi to crystalline Li15Si4.42,43 During the first cycle, the composite electrode achieves a specific discharge and charge capacities are 1970.6 and 1032.6 mAh g−1, respectively, corresponding to the initial Coulombic efficiency (CE) of 52.4%. The irreversible capacity is generally attributed to the inevitable formation of a SEI film as well as side reactions between the electrolyte and the active material. Noticeably, the second and third profiles overlapped very well, illustrating a good reversible reaction between the HSi@C electrode and lithium ion. There should be noted that when the same experimental procedure was carried on the bare HSi sample, distinctly different cycling behavior was observed (Figure 4b). The discharge/charge profiles of bare HSi show an obvious plateau between 0.1 and 0.01 V, demonstrating that the lithiation of crystalline Si. Then, during the following delithiation process of Si, a plateau at around 0.4 V could be detected. The initial lithiation and delithiation capacities of HSi were 2073.1 and 1111.9 mAh g−1, respectively; the higher initial discharge capacity is because of the more amount of Si contained in the bare HSi samples. Upon cycling, a slow capacity fade can be observed in the voltage curves, displaying the decreasing in the capacities. The relatively low reversibility of bare HSi can be attributed to the cracking and collapse of the structure of the electrode. The rate capabilities and electrode kinetics of these anode materials are appraised at various current densities, which are shown in Figure 4c. The HSi@C anode delivers the reversible charge capacities of 1124.5, 1070.0, 863.5, 697.0, 531.0, 360.5, and 259.5 mAh g−1 at 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 A g−1, respectively. Notably, after cycling at a high rate, a stable capacity of 1037.5 mAh g−1 could still be obtained as the current density is returned to 0.1 A g−1 after a total of 40 cycles. When the current density was increased from 0.1 to 6.4 A g−1 step by step, the HSi without coatings exhibited a serious decrease in normalized capacity ratio, indicating the beneficial effect of the carbon shell on the cycling stability. The cycling performances of these two samples are compared to a 0.5 A g−1 as shown in Figure 4d. The raspberry-like structure of HSi@C composite electrode demonstrated admirable capacity retention during the cycling. HSi@C nanospheres maintained a specific discharge capacity of 886.2 mAh g−1 after 200 cycles, which is

structured silica decreases with the increase of the temperature. In order to meet the volume expansion, HSiO2 spheres with a shell thickness of ∼30 nm which was incubated in distilled water at 120 °C for 48 h was used as precursor materials. After magnesiothermic reduction, an interconnected hollow structured silicon sphere was obtained (Figure 3c), and the EDX mapping of element images of Si, C, and N further certified the Si is homogeneously coated with carbon (Figure S6). The TEM images of Figure 3d,e demonstrate the shell of the HSi@C nanosphere is composed of silicon nano hollow spheres of ∼40 nm in diameter with a shell thickness of about 5 nm, which structure looks like a raspberry. The shell of this hollow sphere had a polycrystalline structure with several ordered continuous lattice fringes in the HRTEM image (Figure 3f). The inset of Figure 3f gives the d-spacings as 3.13, 1.92, and 1.11 Å corresponding to (111), (220), and (422) planes of cubic Fd3m (227) silicon phase. The lithium storages properties of the prepared Si-based composite are evaluated by a series of electrochemical tests in half-cell configuration. Cyclic voltammetry (CV) was tested in a potential range 0.01−2.5 V versus Li/Li+ with a scanning rate of 0.1 mV s−1 (Figure S7). As shown in Figure S7, two broad cathodic peaks centered at around 1.4 V (vs Li/Li+) and 0.8 V (vs Li/Li+) have been observed in the first cycle, which could be ascribed to the SEI formation on the surface of the electrode due to the reaction of the electrode and the electrolyte, and the electrolyte decomposes, respectively. Both peaks disappear after the first cycle shows their irreversible procedure. This is consistent with the first discharge curve of the HSi@C; a sloping plateau at 1.2 V has been observed (Figure 4a). The lithiation of Si starts from 0.3 V (vs Li/Li+). The anodic peaks at 0.33 and 0.52 V are characteristic of lithium extraction process of LixSi to Si. Galvanostatic tests were performed at constant current densities of 0.1 A g−1 for the first three cycles and 0.5 A g−1 for the later cycles at a voltage range of 0.01−1.5 V (vs Li/Li+). The specific capacity was calculated based on the mass of HSi@C composite. Figure 4a shows the first three lithiation/delithiation curves of HSi@C at a current density of 0.1 A g−1. During the first lithiation process, sloping voltage plateaus which disappeared in the second cycle between 1.5 and 0.25 V are assigned to the SEI formation on the surface of the electrode, electrolyte decomposes, and an electrochemicaldriven solid-state amorphization of silicon results in the 18770

DOI: 10.1021/acsami.7b03157 ACS Appl. Mater. Interfaces 2017, 9, 18766−18773

ACS Applied Materials & Interfaces

Research Article

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CONCLUSIONS For a conclusion, a raspberry-like HSi@C nanocomposite was successfully fabricated by a facile method including the magnesiothermic reduction of HSiO2 in molten salt and polymerization of PDA on the surface of HSi without any template. Benefits from this outstanding stable nanostructure, a high reversible capacity, good rate capability, and excellent cycling ability have been achieved. This attractive electrochemical performance was by virtue of the synergetic effect of the porous hollow structure and the carbon coatings. The plentiful void space in the hollow spheres can not only accommodate the large volume expansion but also advantage to the Li+ and electrolyte diffusion into the electrode. The electronic conductivity and the stability of the whole electrode of the HSi nanospheres can be extremely improved by the surface carbon coatings. Furthermore, a scalable and facile synthesis procedure exploited the HSi@C nanocomposite as a promising candidate for energy storage application for LIBs.

much higher than that of the traditional graphite anode (372 mAh g−1), and the corresponding CE of HSi@C was nearly 100% upon cycling. It can be indicated that higher conductivity of the carbon coating promises better reaction kinetics of the active material. However, without the carbon coating, bare HSi shows worse cycling performance in the first few cycles, and then a stable cycling performance can be observed; it is evident that the raspberry-like nanostructure also has a good effect on the performance of the electrode. The HSi electrodes can deliver a discharge capacity of 454.9 mAh g−1, which is still higher than the traditional graphite anode. In addition, a dense electrode and higher active material loading electrode have been tested to further evaluate the electrochemical performance of HSi@C composite material. More details can be seen in Figures S8 and S9. The prolonged cycle life of the HSi@C electrode is tested at a current density of 2.0 A g−1 for 500 cycles. In the first few cycles, the electrode experiences gradual capacity decay and then stabilized. After 500 cycles, HSi@C can still deliver a high capacity of 516.7 mAh g−1; the discharge and charge curves of 100, 200, and 500 cycles are also provided in Figure S10. The discharge/charge curves of the electrode have similar profiles, demonstrating the good stability of the hollow sphere silicon structures. The fitted EIS of HSi@C and HSi after 200 cycles at a current density of 0.5 A g−1 with an equivalent circuit is shown in Figure S11. It is clear to see that the Rct of HSi@C is 66.5 Ω, which is much lower than that of HSi of 279.8 Ω; the different charge transfer resistances could be attributed to the conductivity of the carbon coatings. On the contrary, due to the relatively poor conductivity of Si, the HSi electrode shows the higher charge transfer resistance. In addition, the HSi@C electrodes at various state of charge (SOC) was also measured and discussed in Figure S12. Figure 5 shows the morphology of the electrode surface and inside of HSi@C before and after 50 cycles at a current density of 0.5 A g−1. Compared with the fresh electrode, the HSi@C electrode still maintains its integrity, and no crack or pulverization can be observed after 50 cycles. FIB cut was used to observe cross sections of the Si electrodes before and after 50 cycles. According to the FIB image, in terms of well-distributed pore size inside the electrode, the initial porosity appears quite uniform. However, after 50 cycles, some changes are observed. The porosity still remains but becomes bigger than that before cycling which could be ascribed to the aggregation of the nanospheres. This observation is nicely consistent with the SEM measurements. There should be noted that even inside of the electrode no cracks can be observed. The intact structure of the electrode after cycling demonstrates that the raspberry-like HSi@C nanostructure can completely accommodate the stress of volume expansion and prohibit the detachment of pulverized Si during discharge/charge; as a consequence, the rate capability and cycling stability have been highly improved. The improved electrochemical performance of the HSi@C could be ascribed to the internal hollow structure, which can hold the serious volume changes of silicon and decrease the lithium ion diffusion path during the discharge/charge cycling. Additionally, more active sites for the contacting between the electrolyte and active materials have been provided by the uniformly dispersed nanoparticle structures. All the aspects mentioned above contribute to improving the electrochemical performance of the Si for a high lithium storage capacity and long cycling stability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03157. Additional SEM and TEM images, BET analysis, EIS of HSi@C and HSi after 200 cycles at 0.5A g−1 from 100 kHz to 0.01 Hz, digital photos of the synthesized samples, theoretical calculation of the shell thickness, CV test, cycling performance of dense and higher loading electrode, EDX mapping of HSi@C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-025-5211291 8; Fax +86-025-52112626; e-mail [email protected] (X.Z.). ORCID

Shan Fang: 0000-0001-9497-994X Xiaogang Zhang: 0000-0003-4464-672X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangsu Province (No. BK2011030 and BK20150739), the National Natural Science Foundation of China (No. 51372116 and 51504139), the National Program on Key Basic Research Project of China (973 Program, No. 2014CB239701), and Prospective Joint Research Project of Cooperative Innovation Fund of Jiangsu Province, China (BY-2015003-7). S.F. acknowledges the Priority Academic Program Development of Jiangsu Higher Education Institutions and the China Scholarship Council (CSC) for support.

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DOI: 10.1021/acsami.7b03157 ACS Appl. Mater. Interfaces 2017, 9, 18766−18773

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b03157 ACS Appl. Mater. Interfaces 2017, 9, 18766−18773

Research Article

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DOI: 10.1021/acsami.7b03157 ACS Appl. Mater. Interfaces 2017, 9, 18766−18773