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C yolk-shell anode by one-step magnesiothermic reduction
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Facile preparation of Hollow Si/SiC/C yolk-shell anode by one-step magnesiothermic reduction Yuefei Chena, Jinqiu Zhanga,∗, Xueqin Chenb,∗∗, Peixia Yanga, Maozhong Ana a MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, 150001, PR China b School of Astronautics, Harbin Institute of Technology, Harbin, 150001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: One-step magnesiothermic Silicon carbon anode Hollow yolk-shell Lithium ion battery
Silicon is a promising anodic material used in lithium-ion battery (LIB) to improve the specific energy. While the huge volume change during lithiation/delithiation and low intrinsic conductivity has been a bottle neck of silicon anode. For this sake, carbon shell was used to enhance the conductivity, and hollow structure to buffer the volume swelling/shrinking during cycles. In this work, we synthesized a hollow yolk-shell structure Si/SiC/C (SSC) anode by one-step magnesiothermic reductive reaction. At first, polydopamine (PODA) was used as carbon source to optimize the dosage of magnesium powder and reaction temperature. Then resorcinol formaldehyde (RF) resin was used as another carbon source to prepare several samples with different carbon shell thicknesses. A vital parameter, critical thickness, about 10nm of final shell was proposed to keep the hollow yolk-shell morphology. During this one-step exothermic reaction, Si and C spontaneously reacted and formed inter SiC layer with high modulus. The half-cell with SSC anode delivered a capacity of 505.8 mA h g-1 after 220 cycles at a current density of 100 mA g-1. This novel method is facile to realize and might be used in other fields like composite plating and material modification etc.
1. Introduction With the booming requirement of new green energy to replace traditional petroleum energy, higher specific capacity of lithium-ionbattery are needed [1,2]. Silicon is one of the highest theoretical mass capacity anode materials [3]. When the silicon negative electrode is above 100 °C, the final product of lithiation [4] is Li22Si5, which has the highest theoretical specific capacity (4200 mA h g-1), above 10 times more than graphite. Below 85 °C, the final product of lithiation is Li15Si4, and the corresponding theoretical specific capacity is 3579 mA h g-1. However, silicon anode has several insurmountable intrinsic defects: 1) The volume expansion ratio of silicon during charging and discharging exceeds 300% [(volume after lithiation - initial volume)/ initial volume] [5], and it is difficult to be limited by external force [6]; 2) As a semiconductor, silicon has a poor conductivity [7]. These two drawbacks of their own lead to a series of problems with silicon anodes. The first is pulverization of materials during expansion and contraction processes. During the lithiation process, Li+ is embedded into the crystal lattice of crystalline silicon, destroying the structure of the
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original crystal, leading to crystallized composite fracture and partial powder pulverization [6]. Secondly, the proportion of irreversible capacity loss in the first circle is high [8,9], generally around 20%–50%. In addition, the cycle performance is poor [10]. The capacity decays quickly, and the coulombic efficiency is not high enough. Due to the good electrical conductivity and electrochemical activity of carbon materials [11,12], the preparation of silicon-carbon composite materials has become the focus of researchers today [10,13–15]. In addition to increasing the electrical conductivity of materials, the recombination of the carbonaceous material can also limit the volume expansion of the silicon material to some extent. However, the limitation of external force alone does not offset the volume expansion of the silicon material. In order to prevent the destruction of structures, it is also necessary to provide a buffer space for material volume expansion inside the material [16]. Therefore, many researchers have proposed models of hollow or porous structural materials, including hollow nanotubes [17], hollow nanospheres [18], hollow nanocubes [19,20], and porous silicon carbon materials [21]. The advantage of hollow yolkshell structure [22] is that they can isolate the contact of silicon materials with electrolytes by outer coatings [8]. The energy level of the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (X. Chen).
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https://doi.org/10.1016/j.ceramint.2019.05.255 Received 15 April 2019; Received in revised form 14 May 2019; Accepted 23 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yuefei Chen, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.05.255
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hydrochloride was added as a monomer for polymerization reaction, stirred at room temperature for 12 h, centrifuged for 3 times, and dried at 80 °C for 12 h. Thus, SiO2/PODA was obtained. The conformal coating of RF on silica was prepared as followed. After dispersing 3 g SiO2 into 300 ml ultrapure water by sonicating 30min, added 9 g cetyl trimethyl ammonium bromide (CTAB) into former solution and remarked as solution A. 0.3 g resorcinol was dissolved into 150 ml absolute ethanol. 1 ml ammonia was added to aforesaid solution and remarked as solution B. Mixing solution B to solution A with agitate stirring. Added 0.4 ml formaldehyde solution to the mixed solution. After agitate stirring 6 h and aging for 12 h, collected the powder by centrifugation. The obtained SiO2/RF was washed three times with water and ethanol and dried at 70 °C for 12 h.
Fig. 1. Schematic diagram during lithiation/delithiation of hollow Si/SiC/C yolk-shell structure.
electrode material enables a more stable solid-liquid reaction interface. Traditionally, the preparation of silicon carbon materials by magnesium thermal method is often carried out by a stepwise reduction method [23,24]. First, the silica is reduced to silicon using a magnesiothermic method. Then compound using carbon sources and be carbonized to form a final silicon carbon material. Both of these reactions need to be carried out in a high temperature tube furnace, which is cumbersome, energy intensive, and difficult to conduct in practical production applications. In this work, a new hollow Si/SiC/C model (Fig. 1) is realized via one-step magnesiothermic reduction, which is facile to realize. Hollow Si/SiC/C structure can effectively alleviate volume swelling by the synergistic effect of SiC interlayer [25,26] and inner hollow space [8,15,16,27]. And outer carbon shell improves conductivity with effect. The difference to most before researches is the one-step magnesiothermic reductive method being adopted here and the hollow spherical yolk-shell structure kept. SiC with high modulus is effective to restrict volume expanding while difficult to prepare at conventional conditions. In the process of preparing silicon-carbon composite materials, the initiation temperature required for the magnesium thermal reduction is low [28]. A lot of heat is given off, the actual temperature of the system rises high. And some reactions with harsh reacting conditions, such as the formation of silicon carbide or the initiation of carbothermic reaction, can be performed in magnesiothermic reductive system at a relatively low temperature. Therefore, we adopted magnesiothermic reductive reaction to prepare SiC for confining Si swelling here. We applied different Mg ratios (Mg: SiO2) and temperatures to probe into the differences between diverse reduction conditions using polydopamine (PODA) as carbon source. By altering the dosage of resorcinol formaldehyde (RF) resin, the thickness of carbon source coatings is proved to be a vital parameter during one-step reductive reaction.
2.2. Synthesis of hollow Si/SiC/C particles Molten salt method was carried out in this process. NaCl was used as a heat sink for heat dissipation. 0.5 g SiO2/RF (or SiO2/PODA) prefabricated was dispersed into 20 ml ultrapure water containing 4 g NaCl and stirred under 80 °C until the solvent volatilizing completely. Collect SiO2/RF coated with NaCl and grind 30min with 0.8 g Mg powder in an agate mortar. The blended powder was transferred to a stainless steel crucible and calcined in tube furnace at 700 °C for 5 h with a heating rate of 10 °C min-1, as Fig. 2 shown. The product was washed in turn by ultrapure water, 6 M HCl, 5% HF for 2 times, 3 times and 1 time, respectively. After HF treating, the sample was washed with ultrapure water and absolute ethanol for 3 times, and then collected by configuration and desiccated in a blast air oven for 12 h at 60 °C. (The carbon sourced from polydopamine is abbreviated as PC for distinguish, and products sourced from SiO2/PODA noted as 100xSi/PC-y after magnesiothermic reduction, herein x denoting Mg proportion, y representing reactive temperature). 2.3. Materials characterizations The morphology and structure features were investigated by scanning electron microscope (SEM, Supra 55, Zeiss) and transmission electron microscope (TEM, JEOL 2100, JEOL). The phase information was measured by X-ray Diffraction (XRD, PANalytical X'Pert PRO) with Cu Kα radiation in the 2-θ range from 10° to 90°. The surface area was acquired based on the nitrogen adsorption and desorption isotherms by the Brunauer-Emmett-Teller (BET) method, and the Barrett-JoynerHalenda (BJH) mode was used to assure the pore size distributions by using porosity analyzer at 77.3 K. 2.4. Electrochemical characterization The as-synthesized active materials, acetylene black, and sodium carboxymethyl cellulose (CMC) binder with a weight ratio of 6:2:2 were dispersed in ultrapure water and stirred for 12 h to form a uniform slurry. Before stirring, active materials, conductive agent acetylene black and CMC were ground for 10min to mix them homogenously. The
2. Experimental 2.1. Preparation of silica particles and carbon sources coatings Silica was synthesized by modified StÖber method [29]. Tetraethoxysilane (TEOS) was dissolved in absolute ethanol and dispersed by ultrasonic for 5 min. 25% NH3·H2O catalyst was dissolved in a certain amount of ethanol and mixed uniformly. The NH3·H2O ethanol solution was quickly poured into the TEOS ethanol solution, and the ultrasonic reaction was carried out for 2 h. Mixed solution with 1:1 vol% of absolute ethanol and water were used to repeatedly ultrasonicate and centrifuge for three times collecting SiO2, and dried in a blast drying oven at 80 °C for 12 h to obtain monodispersed SiO2 micro balls for subsequent use. The average diameter of silica obtained was about 350 nm. The uniform coating of polydopamine (PODA) on silica was prepared as followed. 3 g of SiO2 was ultrasonically dispersed in 200 ml of Tris-HCl buffer solution (pH = 8.5), then 0.4 g of dopamine
Fig. 2. Sketch map of reactor during one-step magnesiothermic reduction. 2
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resulting mixture slurry was pasted onto the copper current collector using the doctor-blade method and then dried in a vacuum oven at 60 °C for 12 h. Finally punched into circular discs, the mass loading of each copper foil electrode was ∼0.6 mg cm-2. The CR2025 button cells were assembled in an Ar-filled glove box (water and oxygen concentration were both kept less than 0.1 ppm) with Celgard 2400 as the separator membrane. The electrolyte consisted of a 1 M LiPF6 solution in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) (1:1, v/v) containing 10 wt% fluoroethylene carbonate (FEC), and metal Li foil was used as the counter electrode and reference electrode. Galvanostatic discharge/charge measurements were performed in a potential range of 0.01–3 V vs. Li/ Li+ using a multichannel battery testing system (Neware CT-3008). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested by electrochemical workstation (CHI660E). Fig. 4. XRD patterns of a) SiO2, Si/PC reacted in different temperatures: b) 600 °C; c) 700 °C; d) 800 °C and PDF cards of e) Si; f) SiC.
3. Results and discussion 3.1. Effect of Mg dosage and temperature on one-step reduction
ratio of 0.8 was chosen for next discussion. When altering the temperatures, silica and SiC proportion two indicators were taken into consider. SiC contributes structure stability while no electrochemical activity. A large amount of SiC was formed according to the usual magnesium thermal temperature of 700 °C (Fig. 4c). A large amount of silica remained at 600 °C (Fig. 4b). Nevertheless, when the temperature rose to 800 °C, we found the amount of formative silicon carbide was reduced (Fig. 4d). Generally, the formation of silicon carbide was due to the release of excessive heat in reductive reaction, and the local temperature could be risen to above 2000 °C, and sometimes the temperature could even melt silicon. However, at this time, the amount of silicon carbide was reduced on the contrary. There are two reasons according to the conjecture: on the one hand, the melting point of coolant sodium chloride is 801 °C, very close to 800 °C. When the magnesium thermal reaction began to occur, systematic temperature is easily higher than 801 °C. Therefore, when the temperature was controlled at 800 °C, the reactants and the heat-dissipating agent were mixed in the liquid phase instead of solid phase blending, and the heat dissipation effect was more valid than that of the solid contact [32]. On the other hand, at a higher temperature, the reductive reaction might proceed more fully, and some carbon or silicon carbide underwent a carbothermal reaction [33] when reacted with excess SiO2, and was oxidized to CO or CO2, so that the amount of silicon carbide by-product remaining was small. By adjusting the Mg dosage and temperatures during magnesiothermic reduction using PODA as carbon sources, a suitable Mg ratio (Mg: SiO2) of 0.8 and a proper temperature of 800 °C were identified.
The catechol functional group of polydopamine gives a property of being easily adsorbed on almost any surface, and has an autophobic effect after growing to a certain thickness [30]. A nearly uniform coating layer can be easily obtained while maintaining a large concentration [31]. Therefore, polydopamine was selected here as a carbon source to explore the one-step reductive conditions such as the temperature and the proportion of magnesium powder. SiO2/PODA was used as precursor at first, preparing several samples reacted with different Mg proportions and in different temperatures to ascertain the most appropriate Mg proportion and temperature. From the results as shown in Fig. 3, magnesium reduction with different Mg ratio (Mg: SiO2) would result in different morphologies. Dopamine became taupe after oxidation (Fig. 3f and g). After the magnesia reductive reaction with different proportions of magnesium powder, when the mass ratio of SiO2 to Mg powder was 1:0.6, few materials maintained a spherical structure, and most of the material structures collapsed and the yield of products remained low. In the ratio of 1:0.8 (Fig. 3c), A few hollow carbon shells could be seen (as arrows pointing), which was formed by carbonization of PODA. When the proportions of magnesium powder were higher than 1: 1, the products were agglomerated (Fig. 3d and e). It was speculated that might be due to excessive magnesium dosage, which would cause excessive reaction during the reductive process. Excessive heat, which was difficult to dissipate caused the silicon melt and during the cooling process, they would condense into larger particles or even blocks as the temperature of system decreased. Considering the yields and products’ structure, a Mg
Fig. 3. SEM image of a) SiO2/PODA; products reduced by Mg:SiO2 mass ratio of b) 0.6; c) 0.8; d) 1.0; e) 1.2; and physical diagrams of f), g) SiO2/PODA. 3
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Fig. 5. SEM images of products by one-step magnesiothermic reduction of SiO2/RF (a-e gradually thinning) of different thickness resins; and pure silicon f) obtained under the same conditions; g) SEM image of SiO2/RF corresponding to image c); and TEM images of h) yolk-shell structure of SSC; i) yolk part of SSC.
about 200 mA h g-1 (Fig. S4). Besides, a hollow yolk-shell structure Si/SiC/C product (Fig. 5c) was noticed, noted as SSC and confirmed by TEM (Fig. 5h and i). The yolkshell structure was identified from Fig. 5i. Fig. 5g corresponded to the SiO2/RF precursor of SSC, the thickness of corresponding RF shell was measured to be ∼50 nm. As precursor shell growing thinner (Fig. 5d and e), both RF shell thickness are less than 50 nm, products collapsed and agglomerated. And Fig. 5f was pure Si (shell thickness = 0) prepared by identical process, the structure of which was also broken. To understand these results, a schematic diagram for reaction process of one-step magnesiothermic reduction was exhibited (Fig. 6). During the one-step magnesiothermic reduction process, the magnesium powder was melted and brought into contact with SiO2. At a temperature sufficient to initiate the reaction, the following reaction occurred as Eq. (1) shown, releasing a large amount of heat.
And the sample (1008 PC-800) prepared in accordance with these two conditions was labeled as SPC. 3.2. The effect of thickness of carbon source coatings on structure Conformal carbon shell coatings of different thicknesses can be easily prepared from phenolic resin [34], thus we prepared hollow Si/ SiC/C used RF resin as carbon source. Nearly monodispersed SiO2 micro spheres (∼350 nm) were synthesized by modified StÖber method [29]. By controlling the concentration of resorcinol and formaldehyde, a few groups of SiO2/RF were prepared with different shell thickness (Supplementary Fig. S1, corresponding parameters are summarized in Table S1). After one-step magnesiothermic reduction under the same temperature and Mg ratio with SPC, different products were collected and characterized by scanning electron microscope (SEM). As-achieved images were shown in Fig. 5. From the electron microscopy images, several carbon shells after RF pyrolysis could maintain the spherical structure of the materials, and the hollow structure could be observed from b), c) in Fig. 5, but it was noted that the above mentioned materials were not all the hollow yolkshell structure, such as the white area in Fig. 5a) and b), represented the by-products in the core had not been removed. We noticed when the thickness of RF layer reached 100 nm (Fig. 5a), it was impossible to remove the magnesium oxide, which was identified by XRD (Fig. 7a), inside the solid core-shell structure even after repeated pickling with 6 M hydrochloric acid, because the excessively thick shell layer blocked the immersion of the hydrochloric acid solution. And there was no formation of magnesium fluoride after treatment with HF also confirmed this inference. To further study the solid byproducts in the core, SEM-mapping was used. Elemental analysis of the material using SEM-mapping, taking the three circles marked in Fig. S2 as an example, the distribution density of magnesium and silicon was significantly higher than surroundings. Combined with the SEM images, it could be seen that the distribution of magnesium and silicon was basically consistent, and all distributed inside the yolk-shell structure. It was concluded the solid core-shell structure was formed due to the residual MgO wrapped by thick carbon shell from RF resin. The presence of magnesium oxide greatly increased the amount of impurities, and the specific capacity dropped sharply to
2 Mg + SiO2 → 2MgO + Si
(1)
This heat could raise the temperature of the system above 2000 °C, enough to melt the reduced silicon particles [28], or promote the reaction of silicon with pyrolytic carbon to form a thin inter-layer of silicon carbide. When there was no resin shell layer or it's too thin (Fig. 6a), the structure of particles completely collapsed after reduction. While it's not perfect neither when the thickness of RF was too much. Once the too-thick resin shell was carbonized (Fig. 6c) and blocked up the access roads for liquid magnesium, the reductive reaction would be stopped, and MgO would be difficult to eliminate even using 6 M HCl (Fig. 6c). In this magnesium reductive process, liquid magnesium entered the yolk-shell structure, stayed in the core structure after the reaction, and the resin was gradually carbonized. The heat of carbonization mainly came from the heat released by the magnesium heat reaction, so it was much higher than the system set by the tube furnace. At that temperature, after MgO and Si formed by magnesiothermic reaction and sealed together, the pyrolysis carbon shell and Si underwent a secondary reaction to generate SiC due to excessive heat, and the formation of silicon carbide was mainly concentrated between the core and shell structure (Fig. 6b). When the final carbon layer is too thin, the structure is destroyed by
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Fig. 6. Diagrammatic drawing of magnesiothermic processes of precursors with: a) thinner shell coatings; b) coating of suitable thickness; c) thicker shell coatings.
of hollow space was estimated to be 1.6162 ml g-1, and the average pore diameter was about 3.82 nm (Fig. 7d).
high temperature during the thermal reduction of magnesium, resulting in collapse, as showed in Fig. 5d), e), f). While it is too thick, MgO will be resided inside solid core-shell structure, leading to a low capacity (Fig. 6c). Therefore, the corresponding thickness of SSC is suitable. And the result thickness of SSC was measured to be ∼10 nm by TEM. The phase change after the preparation process was detected by the method of non-in-situ powder X-ray diffraction. The diffraction peaks of SSC obtained after the reduction corresponded well to the PDF card of Si and SiC (Fig. 7a), indicating the Si was obtained after the reduction and MgO was removed. Taking Raman spectroscopy to characterize assynthesized SSC powder (Fig. 7b), the ID/IG was calculated be 1.452 after fitting. That means the pyrolytic carbon sourced from RF is hard carbon. And the final carbon thickness of SSC was measured to be 10 nm by TEM. The particles inside hollow shells are nano crystals, speculated to be silicon and inter SiC which are conformed by XRD (Fig. 7a). The specific surface area of SSC was determined to be 507 m2 g-1 by N2 adsorption/desorption isotherms (Fig. 7c). The volume
3.3. Electrochemical performance comparison among Si, SPC and SSC For clarifying the impact of different active materials’ structures on half-cell electrochemical performances, we compared the CV (Fig. 8a and b) and EIS (Fig. 8c and d) profiles of pure silicon and SSC as shown in Fig. 8. CV curves of pure Si and SSC are similar. The first ring of pure Si discharge (lithiation) at 0.66 V and 1.48 V appeared oblique line (Fig. 8a), which represented the decomposition of FEC [35] and the formation of solid electrolyte interface (SEI) film according to literature [9,36]. A voltage peak appeared at ∼0.1 V during the subsequent discharge process (Fig. 8a and b), indicating crystalline Si and lithium ions formed amorphous LixSi [37]. All of these same characteristic peaks of SSC are moved positively (Fig. 8b), indicating a smaller polarization caused by higher electrochemical activity. There were two oxidation
Fig. 7. a) XRD pattern of pure Si, solid core-shell structure and SSC; b) Raman spectroscopy of SSC; c) nitrogen adsorption/desorption isotherms and d) pore diameter scatter diagrams of SSC. 5
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Fig. 8. a) CV curves of pure silicon anode; b) CV curves of SSC; EIS spectroscopy and fitting results of c) silicon anode, d) SSC. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
alloyed to form amorphous LixSi during the first cycle. Silicon was a diamond-type structure, the bond energy was high and difficult to break. The lithium-silicon compound after lithiation was amorphous, and the diffusion of solid phase ions was difficult. Therefore, some lithium will enter the silicon lattice and be trapped in the subsequent cycles. That was why the coulomb efficiency of the second and third cycles were still low. The SSC was tested for charge and discharge. The first ring discharge capacity was 1194.2 mA h g-1, the charge specific capacity was 455.05 mA h g-1, and the initial coulomb efficiency (ICE) was 38% (Fig. 9c), only slightly higher than pure silicon material, which was due to silicon carbide. The presence of solid phase diffusion of the particles was more difficult, resulting in a further increase in irreversible capacity. However, in subsequent cycles, the irreversible capacity loss decreased rapidly, and the coulombic efficiencies of the second and third cycles were 83% and 87%, respectively. The charge and discharge curves of the second and third circles were close to each other, indicating that subsequent reactions tended to be stable, and the same conclusion could be drawn from the latter cycles. The charge and discharge test and the rate cycle test were also performed on the SPC. The first cycle discharge specific capacity was 1267.9 mA h g-1, the charge specific capacity was 539.21 mA h g-1, and the ICE was 42.59% (Fig. 9b). The initial coulombic efficiency was slightly enhanced, and the discharge curve of the first cycle is different from subsequent cycles, while the charging curve of the first cycle and subsequent cycles are relatively similar. The consistency of the second and the third cycles’ discharge curves is also high. The coulomb efficiency of the second and third cycle lapped and returned to 83.7% and 88.37%, respectively. The rate properties of Si, SPC and SSC were valued by different current densities, Fig. 9d showed the charge-discharge specific capacity and coulombic efficiency of pure silicon material at current densities of
peaks appeared near 0.3 V and 0.46 V during pure Si charging processes, indicating de-alloying reaction [17], which were verified by differential capacitance curves (Fig. S4). What is different between, is that the current density of SSC materials is greater than that of pure silicon materials. That verified the greater electrochemical activity of SSC. The both fitting EIS lines matched raw data well. We built a same model inserted in Fig. 8c) and d) to analyze the electrochemical process of reaction. Where Rs represents the electrolyte resistance, CPE represented the electric double layer capacitance, and Zw was the Warburg impedance caused by mass transfer. Rct represented the electrochemical reaction resistance of the electrode material. From the EIS files, it's clearly showed the resistance of as-assembled half cells was slashed. The resistance of pure silicon electrode is about 80Ω, while the hollow Si/SiC/C electrode's is below 60Ω. This can be ascribed to high conductivity of the outer carbon shell. Comparing SPC (Fig. S4) with them, we found the resistance of SPC was between the resistances of Si and SSC, that's because the partially broken structure of SPC didn't promote the conductivity of materials as much as hollow yolk-shell structure. Fig. 9 has shown the charge-discharge files of Si, SPC and SSC. Fig. 9a was the curves of first three turns of pure silicon material charging and discharging. The first ring discharge capacity was 2209 mA h g-1, the first cycle charge specific capacity was 738 mA h g-1, and the initial coulomb efficiency (delithiation capacity/lithiation capacity) was 33.4% (Fig. 9a). The high irreversible capacity of the first ring was caused by several reasons. First, FEC was easily decomposed under the pure silicon electrode system to form an SEI film, and the formation of SEI film generally caused an irreversible capacity of 20% or more; Secondly, silicon was semiconductor, and the electron conductivity was poor. Therefore, the electrochemical polarization of the silicon negative electrode system was large, and the crystalline silicon 6
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Fig. 9. Charge/discharge profiles of a) pure silicon, b) SPC and c) SSC in the first three cycles; d) rate performance of pure silicon and SSC; e) cycle performance of SPC; f) cycle performance of SSC. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
0.1, 0.2, 0.5, 1, 2, 5 A g-1, which delivered low capacity of 312, 260, 210, 150, 120, 80, 40 mA h g-1 respectively. Under the same conditions, SPC delivered higher capacity of 486.87, 366.79, 284.12, 187.81, 158.32 and 119.44 mA h g-1 respectively. After 110 cycles (Fig. 9e), SPC remained a discharge capacity of 362 mA h g-1, better than pure Si anode. SSC delivered the highest capacity of 468.10, 422.21, 364.27, 340.59, 290.16, 246.42 and 209.49 mA h g-1 accordingly. After 220 cycles at 100 mA g-1, a high capacity of 505.8 mA h g-1 had been kept, which was far higher than former two materials and the theoretical specific capacity of graphite anode. SPC showed a higher capacity at low-rate tests during initial cycles, while fading soon and performing not good as SSC at high-rate tests. The same was galvanostatic chargedischarge process. SPC performed just between Si and SSC, which is in agreement with the thickness of their carbon shell, the structural integrity and the resistances. The superior electrochemical performance of SSC could be ascribed to its hollow yolk-shell structure, which provided a stable interface and inner volume buffering Si expansion and contraction.
4. Conclusions In summery. Hollow Si/SiC/C structure (SSC) was successfully synthesized via one-step magnesiothermic reductive reaction. An appropriate temperature 800 °C and a suitable Mg proportion (Mg: SiO2 = 0.8) were selected by using SiO2/PODA precursor. A critical thickness about ∼10 nm of the pyrolytic carbon shell was noticed by adjusting the carbon thickness from RF, which is a crucial parameter of one-step magnesiothermic reduction, and the suitable RF coating thickness was determined to be ∼50 nm. Thicker shell (> 100 nm) caused unremovable MgO residues in yolk-shell structure and thinner shell (< 50 nm) couldn't keep the shape of hollow spheres. A proper size of SiC/carbon shell was prepared by adjusting the dosage of resorcinol and formaldehyde solution. This hollow SSC material delivered a high capacity of 505.8 mA h g-1 after 220 cycles at a current of 100 mA g-1, better than SPC and Si anodes. The obtained specific capacity was higher than traditional graphite electrode while ICE was low. That is because SiC is non-active and inner core (yolk structure) 7
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might lose contact with outer shell. The next improvement could be applied to improve the adsorption between yolk and shell part. The one-step magnesiothermic method is facile to realize, and could be also used in other domains like preparing ceramic materials at low temperatures or composite materials modifying.
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Acknowledgments Thanks for the funding support by Power System Technology Project (30508020207). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.05.255. References [1] X.B. Cheng, R. Zhang, C.Z. Zhao, Q. Zhang, Toward safe lithium metal anode in rechargeable batteries: a review, Chem. Rev. 117 (2017) 10403–10473. [2] J. Yang, Y. Wang, W. Li, L. Wang, Y. Fan, W. Jiang, W. Luo, Y. Wang, B. Kong, C. Selomulya, H.K. Liu, S.X. Dou, D. Zhao, Amorphous TiO2 shells: a vital elastic buffering layer on silicon nanoparticles for high-performance and safe lithium storage, Adv. Mater. 29 (2017) 1700523. [3] J. Lang, B. Ding, T. Zhu, H. Su, H. Luo, L. Qi, K. Liu, K. Wang, N. Hussain, C. Zhao, X. Li, H. Gao, H. Wu, Cycling of a lithium-ion battery with a silicon anode drives large mechanical actuation, Adv. Mater. 28 (2016) 10236–10243. [4] V.L. Chevrier, J.W. Zwanziger, J.R. Dahn, First principles study of Li–Si crystalline phases: charge transfer, electronic structure, and lattice vibrations, J. Alloy. Comp. 496 (2010) 25–36. [5] L.Y. Beaulieu, K.W. Eberman, R.L. Turner, L.J. Krause, J.R. Dahn, Colossal reversible volume changes in lithium alloys, Electrochem. Solid State Lett. 4 (2001) A137–A140. [6] H. Wu, G. Chan, J.W. Choi, I. Ryu, Y. Yao, M.T. McDowell, S.W. Lee, A. Jackson, Y. Yang, L. Hu, Y. Cui, Stable cycling of double-walled silicon nanotube battery anodes through solid-electrolyte interphase control, Nat. Nanotechnol. 7 (2012) 310–315. [7] Y. Li, Z. Long, P. Xu, Y. Sun, K. Song, X. Zhang, S. Ma, A 3D pore-nest structured silicon–carbon composite as an anode material for high performance lithium-ion batteries, Inorgan. Chem. Front. 4 (2017) 1996–2004. [8] X. Li, P. Meduri, X. Chen, W. Qi, M.H. Engelhard, W. Xu, F. Ding, J. Xiao, W. Wang, C. Wang, J.G. Zhang, J. Liu, Hollow core–shell structured porous Si–C nanocomposites for Li-ion battery anodes, J. Mater. Chem. 22 (2012) 11014–11017. [9] S. Chen, Z. Chen, X. Xu, C. Cao, M. Xia, Y. Luo, Scalable 2D mesoporous silicon nanosheets for high-performance lithium-ion battery anode, Small 14 (2018) e1703361. [10] H. Akbulut, D. Nalci, A. Guler, S. Duman, M.O. Guler, Carbon-silicon composite anode electrodes modified with MWCNT for high energy battery applications, Appl. Surf. Sci. 446 (2018) 222–229. [11] J.M. Kim, V. Guccini, D. Kim, J. Oh, S. Park, Y. Jeon, T. Hwang, G. Salazar-Alvarez, Y. Piao, A novel textile-like carbon wrapping for high-performance silicon anodes in lithium-ion batteries, J. Mater. Chem. 6 (2018) 12475–12483. [12] Y.M. Sun, N.A. Liu, Y. Cui, Promises and challenges of nanomaterials for lithiumbased rechargeable batteries, Nat. Energy 1 (2016) 16071. [13] S. Chen, L. Shen, P.A. van Aken, J. Maier, Y. Yu, Dual-functionalized double carbon shells coated silicon nanoparticles for high performance lithium-ion batteries, Adv. Mater. 29 (2017) 1605650. [14] S. Choi, M.-C. Kim, S.-H. Moon, J.E. Lee, Y.K. Shin, E.S. Kim, K.W. Park, 3D yolk–shell Si@void@CNF nanostructured electrodes with improved electrochemical performance for lithium-ion batteries, J. Ind. Eng. Chem. 64 (2018) 344–351. [15] S. Guo, X. Hu, Y. Hou, Z. Wen, Tunable synthesis of yolk-shell porous silicon@ carbon for optimizing Si/C-based anode of lithium-ion batteries, ACS Appl. Mater.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Facile preparation of Hollow Si/SiC/C yolk-shell anode by one-step magnesiothermic reduction Yuefei Chena, Jinqiu Zhanga,∗, Xueqin Chenb,∗∗, Peixia Yanga, Maozhong Ana a MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, 150001, PR China b School of Astronautics, Harbin Institute of Technology, Harbin, 150001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: One-step magnesiothermic Silicon carbon anode Hollow yolk-shell Lithium ion battery
Silicon is a promising anodic material used in lithium-ion battery (LIB) to improve the specific energy. While the huge volume change during lithiation/delithiation and low intrinsic conductivity has been a bottle neck of silicon anode. For this sake, carbon shell was used to enhance the conductivity, and hollow structure to buffer the volume swelling/shrinking during cycles. In this work, we synthesized a hollow yolk-shell structure Si/SiC/C (SSC) anode by one-step magnesiothermic reductive reaction. At first, polydopamine (PODA) was used as carbon source to optimize the dosage of magnesium powder and reaction temperature. Then resorcinol formaldehyde (RF) resin was used as another carbon source to prepare several samples with different carbon shell thicknesses. A vital parameter, critical thickness, about 10nm of final shell was proposed to keep the hollow yolk-shell morphology. During this one-step exothermic reaction, Si and C spontaneously reacted and formed inter SiC layer with high modulus. The half-cell with SSC anode delivered a capacity of 505.8 mA h g-1 after 220 cycles at a current density of 100 mA g-1. This novel method is facile to realize and might be used in other fields like composite plating and material modification etc.
1. Introduction With the booming requirement of new green energy to replace traditional petroleum energy, higher specific capacity of lithium-ionbattery are needed [1,2]. Silicon is one of the highest theoretical mass capacity anode materials [3]. When the silicon negative electrode is above 100 °C, the final product of lithiation [4] is Li22Si5, which has the highest theoretical specific capacity (4200 mA h g-1), above 10 times more than graphite. Below 85 °C, the final product of lithiation is Li15Si4, and the corresponding theoretical specific capacity is 3579 mA h g-1. However, silicon anode has several insurmountable intrinsic defects: 1) The volume expansion ratio of silicon during charging and discharging exceeds 300% [(volume after lithiation - initial volume)/ initial volume] [5], and it is difficult to be limited by external force [6]; 2) As a semiconductor, silicon has a poor conductivity [7]. These two drawbacks of their own lead to a series of problems with silicon anodes. The first is pulverization of materials during expansion and contraction processes. During the lithiation process, Li+ is embedded into the crystal lattice of crystalline silicon, destroying the structure of the
∗
original crystal, leading to crystallized composite fracture and partial powder pulverization [6]. Secondly, the proportion of irreversible capacity loss in the first circle is high [8,9], generally around 20%–50%. In addition, the cycle performance is poor [10]. The capacity decays quickly, and the coulombic efficiency is not high enough. Due to the good electrical conductivity and electrochemical activity of carbon materials [11,12], the preparation of silicon-carbon composite materials has become the focus of researchers today [10,13–15]. In addition to increasing the electrical conductivity of materials, the recombination of the carbonaceous material can also limit the volume expansion of the silicon material to some extent. However, the limitation of external force alone does not offset the volume expansion of the silicon material. In order to prevent the destruction of structures, it is also necessary to provide a buffer space for material volume expansion inside the material [16]. Therefore, many researchers have proposed models of hollow or porous structural materials, including hollow nanotubes [17], hollow nanospheres [18], hollow nanocubes [19,20], and porous silicon carbon materials [21]. The advantage of hollow yolkshell structure [22] is that they can isolate the contact of silicon materials with electrolytes by outer coatings [8]. The energy level of the
Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (X. Chen).
∗∗
https://doi.org/10.1016/j.ceramint.2019.05.255 Received 15 April 2019; Received in revised form 14 May 2019; Accepted 23 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Yuefei Chen, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.05.255
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hydrochloride was added as a monomer for polymerization reaction, stirred at room temperature for 12 h, centrifuged for 3 times, and dried at 80 °C for 12 h. Thus, SiO2/PODA was obtained. The conformal coating of RF on silica was prepared as followed. After dispersing 3 g SiO2 into 300 ml ultrapure water by sonicating 30min, added 9 g cetyl trimethyl ammonium bromide (CTAB) into former solution and remarked as solution A. 0.3 g resorcinol was dissolved into 150 ml absolute ethanol. 1 ml ammonia was added to aforesaid solution and remarked as solution B. Mixing solution B to solution A with agitate stirring. Added 0.4 ml formaldehyde solution to the mixed solution. After agitate stirring 6 h and aging for 12 h, collected the powder by centrifugation. The obtained SiO2/RF was washed three times with water and ethanol and dried at 70 °C for 12 h.
Fig. 1. Schematic diagram during lithiation/delithiation of hollow Si/SiC/C yolk-shell structure.
electrode material enables a more stable solid-liquid reaction interface. Traditionally, the preparation of silicon carbon materials by magnesium thermal method is often carried out by a stepwise reduction method [23,24]. First, the silica is reduced to silicon using a magnesiothermic method. Then compound using carbon sources and be carbonized to form a final silicon carbon material. Both of these reactions need to be carried out in a high temperature tube furnace, which is cumbersome, energy intensive, and difficult to conduct in practical production applications. In this work, a new hollow Si/SiC/C model (Fig. 1) is realized via one-step magnesiothermic reduction, which is facile to realize. Hollow Si/SiC/C structure can effectively alleviate volume swelling by the synergistic effect of SiC interlayer [25,26] and inner hollow space [8,15,16,27]. And outer carbon shell improves conductivity with effect. The difference to most before researches is the one-step magnesiothermic reductive method being adopted here and the hollow spherical yolk-shell structure kept. SiC with high modulus is effective to restrict volume expanding while difficult to prepare at conventional conditions. In the process of preparing silicon-carbon composite materials, the initiation temperature required for the magnesium thermal reduction is low [28]. A lot of heat is given off, the actual temperature of the system rises high. And some reactions with harsh reacting conditions, such as the formation of silicon carbide or the initiation of carbothermic reaction, can be performed in magnesiothermic reductive system at a relatively low temperature. Therefore, we adopted magnesiothermic reductive reaction to prepare SiC for confining Si swelling here. We applied different Mg ratios (Mg: SiO2) and temperatures to probe into the differences between diverse reduction conditions using polydopamine (PODA) as carbon source. By altering the dosage of resorcinol formaldehyde (RF) resin, the thickness of carbon source coatings is proved to be a vital parameter during one-step reductive reaction.
2.2. Synthesis of hollow Si/SiC/C particles Molten salt method was carried out in this process. NaCl was used as a heat sink for heat dissipation. 0.5 g SiO2/RF (or SiO2/PODA) prefabricated was dispersed into 20 ml ultrapure water containing 4 g NaCl and stirred under 80 °C until the solvent volatilizing completely. Collect SiO2/RF coated with NaCl and grind 30min with 0.8 g Mg powder in an agate mortar. The blended powder was transferred to a stainless steel crucible and calcined in tube furnace at 700 °C for 5 h with a heating rate of 10 °C min-1, as Fig. 2 shown. The product was washed in turn by ultrapure water, 6 M HCl, 5% HF for 2 times, 3 times and 1 time, respectively. After HF treating, the sample was washed with ultrapure water and absolute ethanol for 3 times, and then collected by configuration and desiccated in a blast air oven for 12 h at 60 °C. (The carbon sourced from polydopamine is abbreviated as PC for distinguish, and products sourced from SiO2/PODA noted as 100xSi/PC-y after magnesiothermic reduction, herein x denoting Mg proportion, y representing reactive temperature). 2.3. Materials characterizations The morphology and structure features were investigated by scanning electron microscope (SEM, Supra 55, Zeiss) and transmission electron microscope (TEM, JEOL 2100, JEOL). The phase information was measured by X-ray Diffraction (XRD, PANalytical X'Pert PRO) with Cu Kα radiation in the 2-θ range from 10° to 90°. The surface area was acquired based on the nitrogen adsorption and desorption isotherms by the Brunauer-Emmett-Teller (BET) method, and the Barrett-JoynerHalenda (BJH) mode was used to assure the pore size distributions by using porosity analyzer at 77.3 K. 2.4. Electrochemical characterization The as-synthesized active materials, acetylene black, and sodium carboxymethyl cellulose (CMC) binder with a weight ratio of 6:2:2 were dispersed in ultrapure water and stirred for 12 h to form a uniform slurry. Before stirring, active materials, conductive agent acetylene black and CMC were ground for 10min to mix them homogenously. The
2. Experimental 2.1. Preparation of silica particles and carbon sources coatings Silica was synthesized by modified StÖber method [29]. Tetraethoxysilane (TEOS) was dissolved in absolute ethanol and dispersed by ultrasonic for 5 min. 25% NH3·H2O catalyst was dissolved in a certain amount of ethanol and mixed uniformly. The NH3·H2O ethanol solution was quickly poured into the TEOS ethanol solution, and the ultrasonic reaction was carried out for 2 h. Mixed solution with 1:1 vol% of absolute ethanol and water were used to repeatedly ultrasonicate and centrifuge for three times collecting SiO2, and dried in a blast drying oven at 80 °C for 12 h to obtain monodispersed SiO2 micro balls for subsequent use. The average diameter of silica obtained was about 350 nm. The uniform coating of polydopamine (PODA) on silica was prepared as followed. 3 g of SiO2 was ultrasonically dispersed in 200 ml of Tris-HCl buffer solution (pH = 8.5), then 0.4 g of dopamine
Fig. 2. Sketch map of reactor during one-step magnesiothermic reduction. 2
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resulting mixture slurry was pasted onto the copper current collector using the doctor-blade method and then dried in a vacuum oven at 60 °C for 12 h. Finally punched into circular discs, the mass loading of each copper foil electrode was ∼0.6 mg cm-2. The CR2025 button cells were assembled in an Ar-filled glove box (water and oxygen concentration were both kept less than 0.1 ppm) with Celgard 2400 as the separator membrane. The electrolyte consisted of a 1 M LiPF6 solution in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) (1:1, v/v) containing 10 wt% fluoroethylene carbonate (FEC), and metal Li foil was used as the counter electrode and reference electrode. Galvanostatic discharge/charge measurements were performed in a potential range of 0.01–3 V vs. Li/ Li+ using a multichannel battery testing system (Neware CT-3008). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were tested by electrochemical workstation (CHI660E). Fig. 4. XRD patterns of a) SiO2, Si/PC reacted in different temperatures: b) 600 °C; c) 700 °C; d) 800 °C and PDF cards of e) Si; f) SiC.
3. Results and discussion 3.1. Effect of Mg dosage and temperature on one-step reduction
ratio of 0.8 was chosen for next discussion. When altering the temperatures, silica and SiC proportion two indicators were taken into consider. SiC contributes structure stability while no electrochemical activity. A large amount of SiC was formed according to the usual magnesium thermal temperature of 700 °C (Fig. 4c). A large amount of silica remained at 600 °C (Fig. 4b). Nevertheless, when the temperature rose to 800 °C, we found the amount of formative silicon carbide was reduced (Fig. 4d). Generally, the formation of silicon carbide was due to the release of excessive heat in reductive reaction, and the local temperature could be risen to above 2000 °C, and sometimes the temperature could even melt silicon. However, at this time, the amount of silicon carbide was reduced on the contrary. There are two reasons according to the conjecture: on the one hand, the melting point of coolant sodium chloride is 801 °C, very close to 800 °C. When the magnesium thermal reaction began to occur, systematic temperature is easily higher than 801 °C. Therefore, when the temperature was controlled at 800 °C, the reactants and the heat-dissipating agent were mixed in the liquid phase instead of solid phase blending, and the heat dissipation effect was more valid than that of the solid contact [32]. On the other hand, at a higher temperature, the reductive reaction might proceed more fully, and some carbon or silicon carbide underwent a carbothermal reaction [33] when reacted with excess SiO2, and was oxidized to CO or CO2, so that the amount of silicon carbide by-product remaining was small. By adjusting the Mg dosage and temperatures during magnesiothermic reduction using PODA as carbon sources, a suitable Mg ratio (Mg: SiO2) of 0.8 and a proper temperature of 800 °C were identified.
The catechol functional group of polydopamine gives a property of being easily adsorbed on almost any surface, and has an autophobic effect after growing to a certain thickness [30]. A nearly uniform coating layer can be easily obtained while maintaining a large concentration [31]. Therefore, polydopamine was selected here as a carbon source to explore the one-step reductive conditions such as the temperature and the proportion of magnesium powder. SiO2/PODA was used as precursor at first, preparing several samples reacted with different Mg proportions and in different temperatures to ascertain the most appropriate Mg proportion and temperature. From the results as shown in Fig. 3, magnesium reduction with different Mg ratio (Mg: SiO2) would result in different morphologies. Dopamine became taupe after oxidation (Fig. 3f and g). After the magnesia reductive reaction with different proportions of magnesium powder, when the mass ratio of SiO2 to Mg powder was 1:0.6, few materials maintained a spherical structure, and most of the material structures collapsed and the yield of products remained low. In the ratio of 1:0.8 (Fig. 3c), A few hollow carbon shells could be seen (as arrows pointing), which was formed by carbonization of PODA. When the proportions of magnesium powder were higher than 1: 1, the products were agglomerated (Fig. 3d and e). It was speculated that might be due to excessive magnesium dosage, which would cause excessive reaction during the reductive process. Excessive heat, which was difficult to dissipate caused the silicon melt and during the cooling process, they would condense into larger particles or even blocks as the temperature of system decreased. Considering the yields and products’ structure, a Mg
Fig. 3. SEM image of a) SiO2/PODA; products reduced by Mg:SiO2 mass ratio of b) 0.6; c) 0.8; d) 1.0; e) 1.2; and physical diagrams of f), g) SiO2/PODA. 3
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Fig. 5. SEM images of products by one-step magnesiothermic reduction of SiO2/RF (a-e gradually thinning) of different thickness resins; and pure silicon f) obtained under the same conditions; g) SEM image of SiO2/RF corresponding to image c); and TEM images of h) yolk-shell structure of SSC; i) yolk part of SSC.
about 200 mA h g-1 (Fig. S4). Besides, a hollow yolk-shell structure Si/SiC/C product (Fig. 5c) was noticed, noted as SSC and confirmed by TEM (Fig. 5h and i). The yolkshell structure was identified from Fig. 5i. Fig. 5g corresponded to the SiO2/RF precursor of SSC, the thickness of corresponding RF shell was measured to be ∼50 nm. As precursor shell growing thinner (Fig. 5d and e), both RF shell thickness are less than 50 nm, products collapsed and agglomerated. And Fig. 5f was pure Si (shell thickness = 0) prepared by identical process, the structure of which was also broken. To understand these results, a schematic diagram for reaction process of one-step magnesiothermic reduction was exhibited (Fig. 6). During the one-step magnesiothermic reduction process, the magnesium powder was melted and brought into contact with SiO2. At a temperature sufficient to initiate the reaction, the following reaction occurred as Eq. (1) shown, releasing a large amount of heat.
And the sample (1008 PC-800) prepared in accordance with these two conditions was labeled as SPC. 3.2. The effect of thickness of carbon source coatings on structure Conformal carbon shell coatings of different thicknesses can be easily prepared from phenolic resin [34], thus we prepared hollow Si/ SiC/C used RF resin as carbon source. Nearly monodispersed SiO2 micro spheres (∼350 nm) were synthesized by modified StÖber method [29]. By controlling the concentration of resorcinol and formaldehyde, a few groups of SiO2/RF were prepared with different shell thickness (Supplementary Fig. S1, corresponding parameters are summarized in Table S1). After one-step magnesiothermic reduction under the same temperature and Mg ratio with SPC, different products were collected and characterized by scanning electron microscope (SEM). As-achieved images were shown in Fig. 5. From the electron microscopy images, several carbon shells after RF pyrolysis could maintain the spherical structure of the materials, and the hollow structure could be observed from b), c) in Fig. 5, but it was noted that the above mentioned materials were not all the hollow yolkshell structure, such as the white area in Fig. 5a) and b), represented the by-products in the core had not been removed. We noticed when the thickness of RF layer reached 100 nm (Fig. 5a), it was impossible to remove the magnesium oxide, which was identified by XRD (Fig. 7a), inside the solid core-shell structure even after repeated pickling with 6 M hydrochloric acid, because the excessively thick shell layer blocked the immersion of the hydrochloric acid solution. And there was no formation of magnesium fluoride after treatment with HF also confirmed this inference. To further study the solid byproducts in the core, SEM-mapping was used. Elemental analysis of the material using SEM-mapping, taking the three circles marked in Fig. S2 as an example, the distribution density of magnesium and silicon was significantly higher than surroundings. Combined with the SEM images, it could be seen that the distribution of magnesium and silicon was basically consistent, and all distributed inside the yolk-shell structure. It was concluded the solid core-shell structure was formed due to the residual MgO wrapped by thick carbon shell from RF resin. The presence of magnesium oxide greatly increased the amount of impurities, and the specific capacity dropped sharply to
2 Mg + SiO2 → 2MgO + Si
(1)
This heat could raise the temperature of the system above 2000 °C, enough to melt the reduced silicon particles [28], or promote the reaction of silicon with pyrolytic carbon to form a thin inter-layer of silicon carbide. When there was no resin shell layer or it's too thin (Fig. 6a), the structure of particles completely collapsed after reduction. While it's not perfect neither when the thickness of RF was too much. Once the too-thick resin shell was carbonized (Fig. 6c) and blocked up the access roads for liquid magnesium, the reductive reaction would be stopped, and MgO would be difficult to eliminate even using 6 M HCl (Fig. 6c). In this magnesium reductive process, liquid magnesium entered the yolk-shell structure, stayed in the core structure after the reaction, and the resin was gradually carbonized. The heat of carbonization mainly came from the heat released by the magnesium heat reaction, so it was much higher than the system set by the tube furnace. At that temperature, after MgO and Si formed by magnesiothermic reaction and sealed together, the pyrolysis carbon shell and Si underwent a secondary reaction to generate SiC due to excessive heat, and the formation of silicon carbide was mainly concentrated between the core and shell structure (Fig. 6b). When the final carbon layer is too thin, the structure is destroyed by
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Fig. 6. Diagrammatic drawing of magnesiothermic processes of precursors with: a) thinner shell coatings; b) coating of suitable thickness; c) thicker shell coatings.
of hollow space was estimated to be 1.6162 ml g-1, and the average pore diameter was about 3.82 nm (Fig. 7d).
high temperature during the thermal reduction of magnesium, resulting in collapse, as showed in Fig. 5d), e), f). While it is too thick, MgO will be resided inside solid core-shell structure, leading to a low capacity (Fig. 6c). Therefore, the corresponding thickness of SSC is suitable. And the result thickness of SSC was measured to be ∼10 nm by TEM. The phase change after the preparation process was detected by the method of non-in-situ powder X-ray diffraction. The diffraction peaks of SSC obtained after the reduction corresponded well to the PDF card of Si and SiC (Fig. 7a), indicating the Si was obtained after the reduction and MgO was removed. Taking Raman spectroscopy to characterize assynthesized SSC powder (Fig. 7b), the ID/IG was calculated be 1.452 after fitting. That means the pyrolytic carbon sourced from RF is hard carbon. And the final carbon thickness of SSC was measured to be 10 nm by TEM. The particles inside hollow shells are nano crystals, speculated to be silicon and inter SiC which are conformed by XRD (Fig. 7a). The specific surface area of SSC was determined to be 507 m2 g-1 by N2 adsorption/desorption isotherms (Fig. 7c). The volume
3.3. Electrochemical performance comparison among Si, SPC and SSC For clarifying the impact of different active materials’ structures on half-cell electrochemical performances, we compared the CV (Fig. 8a and b) and EIS (Fig. 8c and d) profiles of pure silicon and SSC as shown in Fig. 8. CV curves of pure Si and SSC are similar. The first ring of pure Si discharge (lithiation) at 0.66 V and 1.48 V appeared oblique line (Fig. 8a), which represented the decomposition of FEC [35] and the formation of solid electrolyte interface (SEI) film according to literature [9,36]. A voltage peak appeared at ∼0.1 V during the subsequent discharge process (Fig. 8a and b), indicating crystalline Si and lithium ions formed amorphous LixSi [37]. All of these same characteristic peaks of SSC are moved positively (Fig. 8b), indicating a smaller polarization caused by higher electrochemical activity. There were two oxidation
Fig. 7. a) XRD pattern of pure Si, solid core-shell structure and SSC; b) Raman spectroscopy of SSC; c) nitrogen adsorption/desorption isotherms and d) pore diameter scatter diagrams of SSC. 5
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Fig. 8. a) CV curves of pure silicon anode; b) CV curves of SSC; EIS spectroscopy and fitting results of c) silicon anode, d) SSC. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
alloyed to form amorphous LixSi during the first cycle. Silicon was a diamond-type structure, the bond energy was high and difficult to break. The lithium-silicon compound after lithiation was amorphous, and the diffusion of solid phase ions was difficult. Therefore, some lithium will enter the silicon lattice and be trapped in the subsequent cycles. That was why the coulomb efficiency of the second and third cycles were still low. The SSC was tested for charge and discharge. The first ring discharge capacity was 1194.2 mA h g-1, the charge specific capacity was 455.05 mA h g-1, and the initial coulomb efficiency (ICE) was 38% (Fig. 9c), only slightly higher than pure silicon material, which was due to silicon carbide. The presence of solid phase diffusion of the particles was more difficult, resulting in a further increase in irreversible capacity. However, in subsequent cycles, the irreversible capacity loss decreased rapidly, and the coulombic efficiencies of the second and third cycles were 83% and 87%, respectively. The charge and discharge curves of the second and third circles were close to each other, indicating that subsequent reactions tended to be stable, and the same conclusion could be drawn from the latter cycles. The charge and discharge test and the rate cycle test were also performed on the SPC. The first cycle discharge specific capacity was 1267.9 mA h g-1, the charge specific capacity was 539.21 mA h g-1, and the ICE was 42.59% (Fig. 9b). The initial coulombic efficiency was slightly enhanced, and the discharge curve of the first cycle is different from subsequent cycles, while the charging curve of the first cycle and subsequent cycles are relatively similar. The consistency of the second and the third cycles’ discharge curves is also high. The coulomb efficiency of the second and third cycle lapped and returned to 83.7% and 88.37%, respectively. The rate properties of Si, SPC and SSC were valued by different current densities, Fig. 9d showed the charge-discharge specific capacity and coulombic efficiency of pure silicon material at current densities of
peaks appeared near 0.3 V and 0.46 V during pure Si charging processes, indicating de-alloying reaction [17], which were verified by differential capacitance curves (Fig. S4). What is different between, is that the current density of SSC materials is greater than that of pure silicon materials. That verified the greater electrochemical activity of SSC. The both fitting EIS lines matched raw data well. We built a same model inserted in Fig. 8c) and d) to analyze the electrochemical process of reaction. Where Rs represents the electrolyte resistance, CPE represented the electric double layer capacitance, and Zw was the Warburg impedance caused by mass transfer. Rct represented the electrochemical reaction resistance of the electrode material. From the EIS files, it's clearly showed the resistance of as-assembled half cells was slashed. The resistance of pure silicon electrode is about 80Ω, while the hollow Si/SiC/C electrode's is below 60Ω. This can be ascribed to high conductivity of the outer carbon shell. Comparing SPC (Fig. S4) with them, we found the resistance of SPC was between the resistances of Si and SSC, that's because the partially broken structure of SPC didn't promote the conductivity of materials as much as hollow yolk-shell structure. Fig. 9 has shown the charge-discharge files of Si, SPC and SSC. Fig. 9a was the curves of first three turns of pure silicon material charging and discharging. The first ring discharge capacity was 2209 mA h g-1, the first cycle charge specific capacity was 738 mA h g-1, and the initial coulomb efficiency (delithiation capacity/lithiation capacity) was 33.4% (Fig. 9a). The high irreversible capacity of the first ring was caused by several reasons. First, FEC was easily decomposed under the pure silicon electrode system to form an SEI film, and the formation of SEI film generally caused an irreversible capacity of 20% or more; Secondly, silicon was semiconductor, and the electron conductivity was poor. Therefore, the electrochemical polarization of the silicon negative electrode system was large, and the crystalline silicon 6
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Fig. 9. Charge/discharge profiles of a) pure silicon, b) SPC and c) SSC in the first three cycles; d) rate performance of pure silicon and SSC; e) cycle performance of SPC; f) cycle performance of SSC. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
0.1, 0.2, 0.5, 1, 2, 5 A g-1, which delivered low capacity of 312, 260, 210, 150, 120, 80, 40 mA h g-1 respectively. Under the same conditions, SPC delivered higher capacity of 486.87, 366.79, 284.12, 187.81, 158.32 and 119.44 mA h g-1 respectively. After 110 cycles (Fig. 9e), SPC remained a discharge capacity of 362 mA h g-1, better than pure Si anode. SSC delivered the highest capacity of 468.10, 422.21, 364.27, 340.59, 290.16, 246.42 and 209.49 mA h g-1 accordingly. After 220 cycles at 100 mA g-1, a high capacity of 505.8 mA h g-1 had been kept, which was far higher than former two materials and the theoretical specific capacity of graphite anode. SPC showed a higher capacity at low-rate tests during initial cycles, while fading soon and performing not good as SSC at high-rate tests. The same was galvanostatic chargedischarge process. SPC performed just between Si and SSC, which is in agreement with the thickness of their carbon shell, the structural integrity and the resistances. The superior electrochemical performance of SSC could be ascribed to its hollow yolk-shell structure, which provided a stable interface and inner volume buffering Si expansion and contraction.
4. Conclusions In summery. Hollow Si/SiC/C structure (SSC) was successfully synthesized via one-step magnesiothermic reductive reaction. An appropriate temperature 800 °C and a suitable Mg proportion (Mg: SiO2 = 0.8) were selected by using SiO2/PODA precursor. A critical thickness about ∼10 nm of the pyrolytic carbon shell was noticed by adjusting the carbon thickness from RF, which is a crucial parameter of one-step magnesiothermic reduction, and the suitable RF coating thickness was determined to be ∼50 nm. Thicker shell (> 100 nm) caused unremovable MgO residues in yolk-shell structure and thinner shell (< 50 nm) couldn't keep the shape of hollow spheres. A proper size of SiC/carbon shell was prepared by adjusting the dosage of resorcinol and formaldehyde solution. This hollow SSC material delivered a high capacity of 505.8 mA h g-1 after 220 cycles at a current of 100 mA g-1, better than SPC and Si anodes. The obtained specific capacity was higher than traditional graphite electrode while ICE was low. That is because SiC is non-active and inner core (yolk structure) 7
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might lose contact with outer shell. The next improvement could be applied to improve the adsorption between yolk and shell part. The one-step magnesiothermic method is facile to realize, and could be also used in other domains like preparing ceramic materials at low temperatures or composite materials modifying.
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