C Hybrids during Molten

Feb 21, 2019 - When evaluated as an anode for lithium-ion batteries, the obtained Si@C@Si ... electrolytic silicon; lithium-ion battery; molten salt e...
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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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In Situ Pyrolysis Concerted Formation of Si/C Hybrids during Molten Salt Electrolysis of SiO2@Polydopamine Wei Weng, Chen Zeng, and Wei Xiao* School of Resource and Environmental Sciences, Hubei International Scientific and Technological Cooperation Base of Sustainable Resource and Energy, Wuhan University, Wuhan 430072, P. R. China

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by WEBSTER UNIV on 02/23/19. For personal use only.

S Supporting Information *

ABSTRACT: Aiming to enhanced productivity and improved functionality of electrolytic silicon from electroreduction of solid silica in molten salts, we herein report a one-pot electrochemical preparation of Si/C hybrids via pyrolysis-cumelectrolysis (PCE) of SiO2@polydopamine (SiO2@PDA) in molten NaCl−CaCl2 at 800 °C. The obtained hybrids, denoted Si@C@Si, are composed of outmost silicon thin layers due to electrodeposition, sandwiched N-doped carbon hollow spheres derived from pyrolysis of PDA, and encapsulated silicon nanoparticles stemming from direct electrodeoxidation of SiO2. The PCE protocol shows intriguing merits on accelerated electroreduction of SiO2 and retarded generation of inconvenient SiC. The preparation conditions of Si@C@Si are optimized by varying electrolysis time and applied voltage, with the optimal conditions being identified as PCE at 2.6 V for 2 h. When evaluated as an anode for lithium-ion batteries, the obtained Si@C@Si exhibits a reversible specific capacity of 904 mAh g−1 after 100 galvanostatic charge/discharge cycles at 500 mA g−1. The proposed PCE method is highlighted as an intensified Si extraction method for advanced lithium-ion batteries, promising practical applications. KEYWORDS: electrolytic silicon, molten salt electrolysis, Si/C, pyrolysis-cum-electrolysis, lithium-ion battery

1. INTRODUCTION Direct electroreduction of solid silica in molten salts is highly regarded as a low-temperature and short-process silicon extraction, in which the solid SiO2 cathode is electrochemically deoxidized to solid Si in molten chlorides at voltages lower than that of decomposition of molten salts and at temperatures lower than that of the melting point of silicon.1−3 Many unusual interfacial phenomena in the process render an elaborate tune of components and microstructures of electrolytic Si, as highlighted in the successful preparation of Si-based nanowires,4−7 nanotubes,8 encapsulated nanoparticles,9 thin films,10−13 and p-n junctions.14 In lithium-ion batteries (LIBs), Si anodes exhibit a theoretical specific capacity 10 times higher than that of state-of-the-art graphite.15−18 Si-based materials are therefore promising for next-generation high-performance LIBs. In 2016, a tonne-grade scale-up pilot of the molten salt Si extraction has initialized in China (http://www.glabat.com/), aiming to production of silicon-based anode materials for LIBs. The overall solid-to-solid reaction pathways as well as intrinsically low conductivity of Si are the main barriers for satisfactory preparation yields.19 As anodes for LIBs, vast volume changes upon electrochemical polarizations occur in Si, causing inferior cycling stability. Moreover, the intrinsically low conductivity of Si tends to deteriorate the high-rate performance of the Si anode for LIBs. To be commercially implemented, the preparation efficiency and lithium storage capability of the electrolytic Si such as the cycle life and high© XXXX American Chemical Society

rate performance need further improvement. Protocols toward the intensified process of the novel silicon extraction are therefore of prime importance and urgency. Composites between silicon and carbon are ideal anode materials for LIBs. The introduced highly conductive carbon is able to alleviate polarizations over Si particles and increase conductivity of the Si/C, promising enhanced cycling performance and high-rate capability of Si/C anodes. Such merits of Si/C anodes provoke direct preparation of Si/C via molten salt Si extraction. The presence of conductive carbon in the solid SiO2 cathode might enhance the conductivity of the solid cathode and might be helpful for enhanced production yields. However, generation of silicon carbide (SiC) via the combination reaction between generated Si and present C is thermodynamically spontaneous (Reaction 1), resulting in the formation of SiC instead of Si/C via molten salt electrolysis of SiO2/C cathodes.20−22 Direct preparation of Si/C composites based on the molten salt Si extraction approach has been a cherished-but-failed task, yet. Si(s) + C(s) = SiC(s), ΔG = − 63.4 kJ mol−1at800 °C (1) Received: January 5, 2019 Accepted: February 13, 2019

A

DOI: 10.1021/acsami.9b00265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. TGA curve and MS curves for main (a) and minor (b) components during the pyrolysis of pure PDA in Ar. Cathodic H2 (c) and CO (d) concentrations detected by GC during the PCE of SiO2@PDA at 2.6 V for 0.5 h. nitrogen source, was coated onto the surface of SiO2 by the following steps: 1.5 g of SiO2 spheres was dispersed in 300 mL of deionized (DI) water under continuously ultrasonication for 15 min, followed by adding 0.3634 g of Tris and 0.75 g of dopamine hydrochloride to the above solution. The suspension was then stirred for 10 h at room temperature. The final sample was obtained by centrifugation and washing with DI water several times.24 The resulting SiO2@PDA has a rough surface (Figures S1b and S2b). According to transmission electron microscope (TEM) images (Figure S2a,b), the diameter of the SiO2 spheres is 300 nm and the thickness of the assembled PDA shell is 30 nm. The presence of PDA is confirmed by Fourier transform infrared (FT-IR) spectra shown in Figure S2d, as evidenced by the peak at 1517 cm−1 assigned to the N−H band of PDA.25 Besides, according to the EDS element distribution results (Figure S3), N is distributed only on the outer surface of the SiO2@PDA sphere, which further verifies the successful coating of PDA. Gas composition during PDA pyrolysis in an Ar atmosphere was analyzed by a thermogravimetric analyzer (TGA) coupled with a mass spectrometer (MS). The in situ released gases from the cathode during PCE were also analyzed by a gas chromotograph (GC). To avoid the interference of anodic gases, the cathodic gases were solely led out by an alumina tube for analysis. The experimental details are presented in the SI.

We herein propose an unprecedented protocol of one-pot preparation of Si/C hybrids through simultaneous pyrolysis and electrolysis of mixture between SiO2 and organic mass in molten salts. The proposed method is verified for successful preparation of Si@C@Si composites via pyrolysis-cumelectrolysis (PCE) of polydopamine (PDA, [C8H11O2N]n)coated SiO2 spheres in molten chlorides. The interfacial mechanism of PCE in molten salts is investigated and rationalized. In the PCE of SiO2@PDA, PDA is pyrolyzed into N-doped carbon and SiO2 is simultaneously electrodeoxidized into Si. Pyrolysis of PDA results in gas evolution (viz., CO and H2) in the interface between Si and C species, functioning as a physical buffer between the generated Si and C to restrain the formation of inconvenient SiC. The ionized oxygen tends to react with the neighboring SiO2, with the formation of SiO32−. The generated SiO32− then dissolves in the melts and gets electrodeposited over the outer surface of the generated carbon via the previously documented dissolution−electrodeposition mechanism,19,23 leading to the formation of the Si@C@Si microstructure. The merits of PCE in molten salts are highlighted as accelerated reduction of SiO2, constrained SiC formation, and formation of Si@C@Si with enhanced lithium storage capability. The present study establishes solid scientific grounds toward high-efficiency and short-process preparation of Si/C hybrids. The proposed intensified process for electrolytic silicon extraction in terms of enhanced production efficiency and lithium storage capability also provides hints on promoting its commercialization.

3. RESULTS AND DISCUSSION 3.1. Interfacial Mechanism of PCE. PDA pyrolysis and SiO2 electrodeoxidation occur simultaneously during the PCE process, in which the in situ released gases from PDA pyrolysis may trigger a distinctive microinterfacial phenomenon, affecting the reaction kinetics and final product composition in PCE. Therefore, the gaseous composition during PDA pyrolysis is essential for better understanding the physicochemical reactions in PCE. The main gaseous products and the TGA curve for heating PDA in Ar are shown in Figure 1a. The weight loss of PDA monitored by TGA begins at ∼250 °C, whereas the gas evolution detected by MS starts at ∼390 °C. The temperature difference is caused by the time gap needed to lead the gaseous products from TGA to MS through a heated capillary. As shown in Figure 1a, the main gaseous products released from PDA pyrolysis in Ar are CO (or N2), CO2, and H2O with the mass-to-charge (m/z) ratios being 28, 44, and 18, respectively.26 According to Figure 1b, C2H6, C2H4, and

2. EXPERIMENTAL SECTION The experimental details are provided in the Supporting Information (SI). Molten NaCl−CaCl2 at 800 °C was employed as the reaction medium. The samples subjected to pyrolysis-cum-electrolysis (simultaneous pyrolysis and electrolysis) are notated as PCE. The electrolysis-after-pyrolysis treatment was also investigated, with the corresponding samples being notated as EAP. In the EAP process, the pyrolysis duration is 2 h and the following electrolysis was performed at 2.6 V for 2 h. The samples subjected to individual pyrolysis and electrolysis are represented as P and E, respectively. SiO2 microspheres, as the Si source, were prepared by the Stöber method (SI), which show a regular spherical structure with a smooth surface (Figures S1a and S2a in the SI). PDA, as the carbon and B

DOI: 10.1021/acsami.9b00265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces CH3OH (with the m/z ratios being 29, 27, and 31 respectively, as shown in Figure 1b) with marginal concentrations were also detected,27 whose ion currents are about 100 times lower than those in Figure 1a. To further understand the released components during the PCE process in the molten salt, the cathodic gases were further analyzed by a GC analyzer. It was found that the released gases from the cathode mainly consists of H2 and CO (Figure 1c,d). CO2 and H2O were not detected in the cathodic gaseous products. The depletion of CO2 during the practical PCE process is likely due to the absorption of CO2 by O2− in molten salt (reaction 2).19,23 CO2 is also possibly consumed by C to generate CO (reaction 3), in which C is derived from the pyrolysis of PDA. H2O is generated from PDA pyrolysis in the cathode (Figure 1a). Therefore, generation of H2 during the PCE process should be originated from the pyrolysis of H2O with the aid of PDA-derived carbon (reaction 4). CO2 (g) + O2 −(dissolved) = CO32 −(dissolved)

(2)

CO2 (g) + C(s) = 2CO(g), ΔG = −17.5 kJ mol−1 at 800 °C

Figure 2. X-ray powder diffraction (XRD) patterns of the samples (a) and current−time curves during electrolysis (b) under different conditions.

(3)

H 2O(g) + C(s) = H 2(g) + CO(g), ΔG = −18.1 kJ mol−1 at 800 °C

1:0.20. Therefore, direct addition of carbon in the precursor tends to place the electroreduction of SiO2 for Si extraction into a dilemma, viz., the enhanced reduction but deteriorated purity. The above dilemma, however, can be tackled by the PCE method. As shown in Figure 2a, for the XRD pattern of the PCE sample ([email protected] V + 2 h), the intensities of undesired SiC peaks significantly decrease, with the Si/SiC peak ratio being 1:0.05. In comparison to that in the E-SiO2 process, the electrolysis current in the PCE increases (Figure 2b), indicating enhanced electroreduction of SiO2 in PCE. In the PCE of SiO2@PDA, pyrolysis of PDA into carbon and electroreduction of SiO2 into Si occur simultaneously. Besides, PDA-derived carbon can also increase the conductivity of cathode precursors. Significantly, gas evolution takes place during the pyrolysis process of PDA, which mitigates the direct contact between Si and C and resultantly retards SiC formation. Therefore, PCE is an effective method combining enhanced Si yields and restrained SiC formation. To further confirm the inhibition effect of gas evolution on the formation of SiC, the electrolysis of SiO2 after pyrolysis of PDA was conducted (denoted [email protected] V + 2 h). In this method, no gas evolution occurs during the SiO2 electrolysis process. As shown in Figure 2a, the intensities of characteristic peaks of SiC in the cathode product are obvious and the Si/SiC peak ratio is 1:0.39. Due to the presence of carbon, the electroreduction of SiO2 is also enhanced in the EAP, compared with the electrolysis of only SiO2 (Figure 2b). The above results clearly reveal that in situ gas evolution in PCE is essential to inhibit SiC formation. Another factor affecting the formation of SiC is the reaction duration. Less reaction time is helpful to restrain SiC formation but is disadvantageous to the electrodeoxidation of SiO2. A plausible solution is to increase applied voltages in PCE. At higher voltages, electroreduction is accelerated and less time is needed for thorough deoxidation. Upon PCE at 2.6 V, 2 h electrolysis is adequate for thorough deoxidation. With decreasing the PCE voltage to 2.2 V, minor residual SiO2 remains in the

(4)

The above results reveal that CO and H2 are released during the PCE process of SiO2@PDA. Therefore, gas evolution, carbon generation by PDA pyrolysis, and Si formation by electrolysis coexist in the SiO2@PDA cathode during the PCE process. Predictably, the released gases can function as a physical buffer between the generated Si and C, restraining the formation of inconvenient SiC. It should be mentioned that the formation of SiC by reaction between SiO2 and CO or C is also ruled out due to positive Gibbs free energy changes, as shown in Figure S4. Furthermore, the PDA-pyrolysis-derived carbon with high conductivity can possibly promote the electrodeoxidation of SiO2. To verify the above speculations, SiO2, SiO2@PDA, and the SiO2−graphite mixture were treated in molten salts. As shown in Figure 2a, when SiO2 spheres were electrolyzed at 2.6 V for 2 h (denoted as E-SiO2-2.6 V + 2 h), a large proportion of unreacted SiO2 is detected, as evidenced by the shoulder at around 22° ascribed to amorphous silica. The appearance of major residual SiO2 and minor Si in the sample means that the electrochemical deoxidization of pure SiO2 is sluggish. After addition of 5 wt % graphite into the SiO2 precursor, the characteristic peak of SiO2 disappears in the sample derived from the same electrolysis condition (E-(SiO2 + 5 wt % graphite)-2.6 V + 2 h). Instead, the intensities of the three peaks at 28.4, 47.3, and 56.1° all belonging to cubic Si (JCPDS no. 27-1402) get intensified, verifying that Si becomes the main electrolysis product. The accelerated electroreduction is also confirmed by the increased electrolysis current after the addition of graphite in the precursor (Figure 2b). Such an enhancement is due to the enhanced conductivity of the cathode precursor. However, undesired SiC is also generated in the product due to the combination reaction between Si and C (Reaction 1). The ratio of the strongest diffraction peak ascribed to Si and SiC is used as an internal index, representing the proportion of SiC in the sample. The deduced Si/SiC peak ratio in the E-(SiO2 + 5 wt % graphite)-2.6 V + 2 h sample is C

DOI: 10.1021/acsami.9b00265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces sample based on the XRD patterns (Figure 3) even after 12 h of PCE treatment. The Si/SiC peak ratio is 1:0.22, indicating

Figure 3. XRD patterns of PCE products conducted at different voltages and times.

the occurrence of SiC. By increasing the duration of PCE at 2.6 V from 2 to 4 h, the SiC peak intensity increases by 4.4 folds (Figure 3). It is therefore concluded that a higher voltage and less duration in PCE are essential for thorough deoxidation and less SiC formation. The optimal PCE parameters are identified as 2.6 V and 2 h in the present study. By adopting a PCE strategy, both the acceleration of SiO2 electroreduction and the restraining of SiC formation can be realized. Acceleration of SiO2 electroreduction is attributed to the enhanced conductivity due to the formation of carbon from PDA pyrolysis. SiC formation is restrained due to the physical buffer effect of the released gas from PDA pyrolysis. In addition to the PCE strategy, proper electrolysis conditions are also important for promoting SiO2 electroreduction and avoiding SiC formation. A high voltage can promote SiO2 reduction, whereas a short electrolysis duration can avoid SiC formation, meaning the optimal PCE parameters being 2.6 V and 2 h. 3.2. Rationalization of the Microstructure. The composition and the microstructure are two critical aspects determining the functional properties of electrolysis products. The sample prepared by PCE of SiO2@PDA at 2.6 V for 2 h was analyzed by a field-emission scanning electron microscope (FESEM) and a transmission electron microscope (TEM). The product maintains the original spherical shape well, although accompanied by slight breakage (Figure 4a,b). In contrast, the SiO2 microspheres and the mixture between SiO2 and graphite are electroreduced into nanoparticles under the same conditions (Figure S1d,e). By PCE of SiO2@PDA at 2.6 V for 4 h or 2.2 V for 12 h, the structure of initial spheres is destroyed and the spheres are converted to substantial nanoparticles (Figure S1g,h). As for the product obtained from SiO2@PDA by electrolysis at 2.6 V for 2 h after pyrolysis, the spherical structure of the product does not change significantly, which explains that the carbon derived from PDA can play an important role to prevent structural destruction (Figure S1f). As shown in Figure 4c, the sample derived from PCE at 2.6 V for 2 h has a hollow core−shell structure with internal voids. Numerous nanoparticles are encapsulated in the sphere shell (zone 2). It is surprisingly noted that some nanoparticles also appear in the outer surface of the shells (zone 1). The dspacing value of such nanoparticles is observed to be 0.31 nm (Figure 4d,e), corresponding to the (111) plane of cubic Si. Figure 4f−h shows the typical electron energy loss spectrometer (EELS) elemental mapping of an individual sphere. As can be seen, the shell is N-doped carbon and the

Figure 4. FESEM images (a, b) and TEM images of SiO2@PDA after PCE at 2.6 V for 2 h (c−e, i). EELS mapping images for C, N, and Si (f−h, j−l). Panels (d) and (e) are based on zone 1 and zone 2 as indicated by rectangles in panel (c).

encapsulated nanoparticle is Si. The EELS regarding the interphase zone is shown in Figure 4i−l. The Si/N-doped carbon/Si tandem structure is confirmed. To understand the formation mechanism of the outer silicon thin layer, the SiO2@PDA pellet was immersed in the melt for 2 h (pyrolysis in the melt only) to investigate its change during pyrolysis. It is observed that the spherical core structure of SiO2 gets damaged due to the dissolution of SiO2 (Figures S1c and S2c). This phenomenon indicates that O2− is released during PDA pyrolysis and reacts with the neighboring solid SiO2 to generate soluble SiO32−,19,23 leading to the dissolution of SiO2. Therefore, the formation mechanism of the Si@C@Si microstructure is speculated as follows. During the PCE process, PDA is pyrolyzed into the N-doped carbon shell, whereas SiO2 is simultaneously electrodeoxidized into Si particles (reaction 5), which are encapsulated inside the carbon shell. Pyrolysis of PDA results in gas evolution and generation of O2− at the interface between SiO2 and PDA. The former functions as a physical buffer between the generated Si and C, restraining the formation of inconvenient SiC, as discussed in the above section. The latter tends to react with the neighboring SiO2 to generate soluble SiO32− (reaction 6). The SiO32− is electrodeposited over the outer surface of the generated carbon shell via the dissolution−electrodeposition mechanism (reaction 7),19,23 leading to the formation of the Si@C@Si microstructure. The dissolution of SiO2 upon pyrolysis is also beneficial to retard the direct contact between Si and C species, therefore further constraining the generation of SiC. D

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scattering of 2 TO-phonon mode of silicon, respectively.28,29 The D band at 1355 cm−1 represents the disordered structure of carbon, and the G band at 1593 cm−1 explains graphitic carbon.30 However, the two peaks have partial overlapping, reflecting a low graphitization degree of the carbon. Peaks at 795 and 973 cm−1 belonging to SiC are not observed, implying a low content of SiC (Figure 6a).31 Thermogravimetric analysis (TGA) was applied to determine the content of Si in the compound, and the variation of sample mass with the increasing temperature is correlated to the oxidation of Si and combustion of carbon (Figure 6b). A commercial nano-Si with a diameter of 50 nm (Figure S5a,b) is also tested, which is used as an internal reference for calibrating weight increase due to the formation of Si−O. The content of silicon in Si@C@Si is calculated to be 55.1 wt % according to the literature.32,33 As shown in Figure 6c, the nitrogen adsorption−desorption isotherms of Si@C@Si and the commercial nano-Si show obvious hysteresis loops, assigned to type-IV isotherms.34−36 Nano-Si has a specific surface area of 89 m2 g−1 with the pore volume of 0.16 cm3 g−1. In comparison, the Si@C@Si hybrids show much larger surface area (323 m2 g−1) and a much higher pore volume (0.61 cm3 g−1), which reflects that the addition of carbon contributes to the improvement of surface area. The pore size distribution of Si@C@Si is shown in Figure 6d, revealing its mesoporous features. Figure 7 shows the X-ray photoelectron spectra (XPS) of Si@C@Si. The C 1s spectrum consists of four chemical bonds

Electrodeoxidation route SiO2 + 4e− = Si + 2O2 −

(5)

Dissolution process SiO2 + O2 − = SiO32 −

(6)

Electrodeposition route SiO32 − + 4e− = Si + 3O2 −

(7)

As illustrated in Figure 5, the synergetic operation of electrodeoxidation of silica, dissolution−electrodeposition of

Figure 5. Schematic illustration of one-pot preparation of Si/C composites via pyrolysis-cum-electrolysis (PCE) of SiO2@PDA in molten salts.

dissolved silica, and pyrolysis of PDA lead to the formation of the Si/C hybrids with the unique Si@C@Si architecture, in which N-doped carbon located in the middle layer is sandwiched by inner encapsulated Si nanoparticles and outer Si thin films. The inner N-doped carbon is due to the pyrolysis of PDA, and the encapsulated Si nanoparticles are generated from direct electrodeoxidation of solid SiO2.7 The outer Si thin film is attributed to the electroreduction of soluble SiO32− by the dissolution−electrodeposition mechanism.19,23 Figure 6a exhibits the Raman spectra of the samples. The Raman spectrum of the Si@C@Si hybrids prepared by PCE of SiO2@PDA at 2.6 V for 2 h shows the obvious peaks at around 521 and 960 cm−1, which are attributed to the first-order scattering of Si−Si stretching vibration and second-order

Figure 7. C 1s (a), Si 2p (b), O 1s (c), and N 1s (d) XPS spectra of Si@C@Si.

(Figure 7a), which are C−Si (283.4 eV), CC (284.0 eV), C−C (285.0 eV), and C−O/C−N (285.5 eV).35,37,38 The existence of C−Si and C−N bonds further verifies the existence of SiC and N-doped carbon. The Si 2p spectrum (Figure 7b) is split into three components. The peaks at 99.2 and 101.9 eV are attributed to Si0, and another two peaks belong to SiC (103.2 eV) and SiOx (103.9 eV). SiOx is due to inevitable surface oxidation of Si in the air.39,40 As summarized in Table S1, the content of SiC is only 5.53 wt %, indicating effective restraining of SiC in the optimal PCE process. The O spectrum shows two peaks at 532.8 and 533.2 eV with approximately equal areas, attributed to Si−O and C−O bonds, respectively (Figure 7c). In Figure 7d, the N element originates from PDA pyrolysis and is composed of pyridinic N

Figure 6. (a) Raman spectrum of the Si@C@Si composite, (b) TGA curves (from room temperature to 1000 °C at a rate of 10 °C min−1 in the air), (c) nitrogen adsorption−desorption isotherm, and (d) pore size distribution of commercial nano-Si and the Si@C@Si composite. E

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initial Coulombic efficiency herein. The Coulombic efficiency increases to 93% in the second cycle and further increases to approximately 100% in the following cycles (Figure 8b). The cycle performance at 500 mA g−1 is also shown in Figure 8b. No carbon coating exists for the Si layer on the outer surface of Si@C@Si, which is the reason for the capacity loss in the initial stage. The Si@C@Si electrode delivers a reversible specific capacity of 904 mAh g−1 after 100 galvanostatic charge/discharge cycles, much higher than that of the state-of-the-art graphite anode (less than 400 mAh g−1). The Si@C@Si electrode also outperforms the commercial nano-Si (Figure S5d). Even at a low current density of 200 mA g−1, the specific capacity of the commercial nano-Si electrode becomes lower than 400 mAh g−1 after 35 cycles. The rate performance of the Si@C@Si electrode is also tested, as shown in Figure 8c. The fluctuation in Figure 8c is caused by the following reason: in the initial stages of the charge/discharge process, the outmost Si layer in Si@C@Si is very unstable, leading to the disturbance of the electrode−electrolyte interface in initial cycles. The capacities of the Si@C@Si anode are about 1500, 1300, and 1000 mAh g−1 at 200, 500, and 1000 mA g−1, respectively. When the current density is reversed from 1000 to 200 mA g−1, the capacity returns to 1500 mAh g−1 again. Especially, even at a high current density of 1000 mAh g−1, the capacity still keeps a stable high value of ∼887 mAh g−1, with neglectful capacity decay after 100 cycles. Obviously, the Si@C@Si electrode also shows an outstanding rate performance. The enhanced capacity retention and high rate performance of Si@C@Si are due to its compositional and structural features. The internal voids can function as a cushion to curb strains arisen from volume change upon cycling. The introduced N-doped carbon can increase the electric conductivity of the electrode.44−46 After the galvanostatic charge/discharge cycling at different currents, the coin cells were disassembled. The active-materialloaded Cu films were then thoroughly rinsed in ethylenecarbonate (EC) and deionized water to eliminate the electrolyte and binder.47 The resulting films or powder was then subjected to TEM. The morphology of Si@C@Si after charge/discharge for five cycles at 200 mA g−1 still keeps intact, as shown in Figure S6, which explains the high current efficiency and good cycling performance of the Si@C@Si anode. It is noted that the nanoparticles around the spheres in Figure S6a are originated from the acetylene black introduced during the assembling process of the battery. No carbon coating exists for the Si layer on the outer surface of Si@C@Si, which is the reason for the capacity loss in the early cycles during the charge/discharge process of Si@C@Si. The morphology of the Si@C@Si particles after the completion of the rate performance is further analyzed. As shown in Figure S7a, most spheres still keep intact. However, some cracked spheres are also observed (marked by the arrows in Figure S7a). As further revealed by EELS mapping images (Figure S7b−d), no silicon particles are detected inside the cracked spheres, which is the main reason for capacity loss during the prolonged discharge/charge process. Guo et al. demonstrated that constructing Si/C microspheres with hierarchical buffering structures or 3D conducting networks can greatly enhance the stability and energy density, which can be a significant guidance for improving the performance of Si/C anodes in Li-ion batteries.48,49 Besides, the importance of the porous structure for enhancing the performance of the Si/C anode is

(398.2 eV), pyrrolic N (399.3 eV), and quaternary N (401.0 eV).33 Such N element peaks are characteristic bands of Ndoped carbon. 3.3. Performance as an Anode in LIBs. The Si@C@Si composites prepared herein may show an outstanding lithium storage ability as an anode material for LIBs because the Si/C hybrid with such a unique sandwichlike microstructure can buffer the strain caused by the vast volume changes during the lithiation/delithiation process. Moreover, the N-doped carbon layer can improve the conductivity of the Si/C hybrid, improving the high-rate performance of the Si anode for LIBs. The lithium storage capability of the PCE-derived Si@ C@Si is shown in Figure 8. Figure 8a shows the discharge−

Figure 8. Charge−discharge curves (a), cycling stability at 500 mA g−1 between 0.01 and 1.5 V (b), and the rate performance (c) of the Si@C@Si electrode.

charge curves at 500 mA g−1, which shows obvious plateaus for the alloying/dealloying process. The plateau appearing at about 0.65 V during the first alloying process could be attributed to the formation of the solid electrolyte interface (SEI) film, which becomes undistinguishable in the subsequent cycles.41 Although the carbon layer in Si@C@Si is porous, the contact of the encapsulated Si particles with the electrolyte is time-consuming. Besides, the outmost Si layer in Si@C@Si can slow down the penetration of the electrolyte. After the initiation of discharge/charge process, the encapsulated Si particles begin to contact with the electrolyte, accompanied by the continuous formation of the SEI film. Therefore, the plateau at about 0.65 V vs Li/Li+, which is attributed to the formation of the SEI film, still remains in the subsequent cycles. It is also noted that the plateau in the second cycle is much smaller than that in the first cycle and almost vanishes in the 50th cycle. It means that Si particles and the electrolyte are fully contacted with the prolonging of testing. The plateau at ∼0.2 V and the slope below ∼0.15 V are attributed to the formation of amorphous LixSi and crystalline Li15Si4, respectively.30,36,42,43 The first alloying and dealloying capacities are 2400 and 2050 mAh g−1, respectively, meaning an initial Coulombic efficiency of 85.4%. The initial irreversible capacity loss is mainly caused by the decomposition of the electrolyte and the formation of the SEI film, which is common for nanostructured anodes.41 The outer thin Si film in the Si@ C@Si structure is beneficial to the swift formation of the SEI film, which may be the reason contributing to such a high F

DOI: 10.1021/acsami.9b00265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces well demonstrated by Su et al.50,51 Therefore, the following strategies can be adopted to further improve the stability of Si@C@Si: (1) increasing the thickness of coated PDA to increase the thickness of the carbon layer in Si@C@Si, (2) further coating a carbon layer on the outer surface of Si@C@Si to fabricate a Si@C@Si@C structure, and (3) increasing the porosity of the Si@C@Si structure.

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4. CONCLUSIONS In summary, Si@N-doped carbon@Si was successfully prepared via one-pot pyrolysis-cum-electrolysis (PCE) treatment of SiO2@PDA in molten NaCl−CaCl2 at 800 °C, with significantly enhanced electrodeoxidation of silica and effectively retarded formation of silicon carbide. Such an unusual phenomenon results from a synergetic combination between electrodeoxidation of silica in molten salts and carbonization of PDA in molten salts. PDA pyrolysis results in the formation of N-doped carbon, gas evolution, and dissolution of adjacent SiO2. The present N-doped carbon acts as a depolarizer toward the electrodeoxidation of silica. The latter two effectively retards SiC generation. The outer Si thin layer is generated through a dissolution−electrodeposition pathway. The optimal voltage and duration for the PCE method are specified as 2.6 V and 2 h, respectively. When evaluated as an anode for lithium-ion battery, the obtained Si@ C@Si exhibits a reversible specific capacity of 904 mAh g−1 after 100 cycles upon galvanostatic charge/discharge cycling at 500 mA g−1, promising its practical applications.

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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.9b00265. Experimental details, powder XRD patterns, EDX spectra, SEM images, HR-TEM images, FT-IR spectra, XPS spectra, and electrochemical tests of the samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei Xiao: 0000-0001-5244-797X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the funding support from the National Natural Science Foundation of China (51722404, 51674177, 51804221, and 91845113) and the Science & Technology Project of the General Administration of Quality Supervision, Inspection and Quarantine of China (2017IK055).

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REFERENCES

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DOI: 10.1021/acsami.9b00265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.9b00265 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX