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Novel design and synthesis of carbon-coated porous silicon particles as high-performance lithium-ion battery anodes
Journal of Power Sources 439 (2019) 227027
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Novel design and synthesis of carbon-coated porous silicon particles as high-performance lithium-ion battery anodes Tianting Zhao a, b, Delun Zhu a, b, Wenrong Li b, Aijun Li a, *, Jiujun Zhang b, ** a b
School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China Institute for Sustainable Energy/College of Sciences, Shanghai University, Shanghai, 200444, China
H I G H L I G H T S
� A effectively strategy to optimize the micro-sized Si-base anodes. � Porous Si/C composite prepared by Ag-assisted chemical etching and self-assembly. � We prepared a composite with 3D porous structure and Ag/C conductive network. � This composite used as LIBs anodes exhibited remarkable rate performance. A R T I C L E I N F O
A B S T R A C T
Keywords: Porous silicon Silver nanoparticles Lithium-ion batteries Carbon coating
Porous silicon-based materials are considered promising next generation lithium-ion battery anode materials. Here, we report the rational design of multiscale recombined porous Si/C composites via a controllable and simple Ag-assisted chemical etching process with subsequent heat treatment of porous Si and chitosan com posites (porous Si/CTS) which prepared by self-assembly process. Chitosan is a green, nontoxic and biode gradable carbon source. The three-dimensional porous (3D porous) structure accommodates the huge volume changes during lithiation and delithiation, which helps form more stable solid electrolyte interface (SEI) film and significantly improves the electrochemical stability. The multiplicity conductive network is constructed by Ag nanoparticles and carbon layer, which keep conductive contact and maintain structure stability. The Li-ion battery anode based on porous Si/C exhibits a reversible high capacity of 782.1 mAh/g at 0.5 C after 200 cy cles with excellent Coulombic efficiency. This work opens a promising approach for production of highperformance micro-sized silicon-based anode materials in Li-ion battery.
1. Introduction
highest theoretical capacity at room temperature (3579 mAh/g), low discharge voltage (<0.5 V vs Li/Liþ), abundant crustal reserve and environment friendly [8–10]. However, the widely practical imple mentation of Si-based anode is hampered by several severe problems such as its considerable volume change (~300%) during lithiation and delithiation processes and poor electronic conductivity. The huge vol ume change causes electrode pulverization leading to loss of electrical contact between electrode materials and detachment the electrode ma terials from current collector, and producing the unstable SEI layer leading to constant consumption of electrolyte [11–13]. These disad vantages are the main reasons for serious capacity fading and low cycling performance. To overcome these aforementioned defects, several strategies have
Li-ion batteries (LIBs) are widely used in portable electric devices, electro vehicles and renewable energy resources because of high energy density, lightweight, friendly environment and long cycle performance [1–3]. So far, the current LIBs need to improve the performance of higher energy density and higher power density to apply in upgraded products and many other applications [4–6]. It is known that the elec trode materials are considered to be one of the most important factors determining the performance of LIBs. Recently, silicon-based materials as anodes have been studied as the most promising candidates to alternative commercial graphite electrode materials which have limited theoretical capacity (372 mAh/g) [7]. Silicon-based anodes are the * Corresponding author. ** Corresponding author. E-mail address: [email protected] (A. Li).
https://doi.org/10.1016/j.jpowsour.2019.227027 Received 12 May 2019; Received in revised form 28 July 2019; Accepted 16 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 439 (2019) 227027
Fig. 1. The schematic illustration of the preparation of porous Si/C composite.
been applied to optimize the Si-base anodes. One of the common methods is designing specific nanostructure, like Si nanoparticle, Si nanofiber and Si nanotube can accommodate large stress root in large volume expansion [14–20]. Compared with nanostructured silicon ma terial, micro-sized silicon particles do not need high price and rigorous preparation conditions, but they suffer from unavoidable huge volume expansion during electrochemical cycling. Recently, the studies of micro-sized Si-based materials have been attracted attention by the re searchers. It is an effective strategy to construct the 3D porous structure or the hollow structure in micro-sized Si materials, since the porous structure or the hollow structure provided the void space to accommo date the large volume expansion during electrochemical cycling. Various methods have been used for fabrication of such electrode ma terials, mainly including magnesiothermic reaction, electrochemical etching and acid etching metal-Si alloy [21–25]. For example, He et al. prepared scalable micro-sized bulk porous Si by ball milling and etching using HCl/HF as the etchant of pristine Fe–Si powder [22]. T.lkonen et al. prepared mesoporous Si by electrochemical etching in an aqueous hydrofluoric acid/ethanol electrolyte [23]. Huang et al. prepared morphological controllable hollow Si such as hollow cubes, hollow spheres and flowers shapes using templates of carbonates, which were removed by hydrochloric acid (HCl) [10]. Although micro-sized Si-based anodes with the porous structure or the hollow structure can partially accommodate the large volume expansion, the problem of poor conductivity of Si materials remains unsolved. It is a universal and effective strategy to coat conductive carbon layer on silicon-based ma terial, since the carbon layer can alleviate the mechanical stress of electrode and increase the electronic conductivity [26–30]. However, the improvement of electrical conductivity caused by carbon materials is limited. Moreover, micro-sized Si is much larger than nano-sized Si, the carbon elements only concentrated on the surface by the carbon coating, and the conductivity inside the particles needs to be further improved. In this study, we use micro-sized Si as the raw materials instead of nano-sized Si. Then, we report a controllable and simple operable method to synthesize interconnected porous Si materials by Ag-assisted chemical etching process. The pore size, depth and morphology of porous Si can be turned by the amount of catalyst, the relative compo sition of etching bath and reaction time, and Ag particles were distrib uted within the porous Si [31–34]. The tiny void space can partially buffer the huge volume change and provide a valid path of Li-ion. The Ag particles can form a conductive structure inside the silicon. In order to further restrain the large volume change and improve the conduc tivity, we produced modifying porous Si by carbon coating using the self-assembly reaction. Electrostatic self-assembly is an effective strat egy to create well-mixed composites. First, the chitosan has plenty of amino groups which will protonated in an acidic liquid. Porous Si has negative charges as a result of the existence of silicon oxide on its sur face. Then, Porous Si materials were distributed in chitosan solution, by adjusting the pH of the dispersion, porous Si adsorbs chitosan through electrostatic interaction to change the charge from negative to positive [30]. Finally, the porous Si/C was produced by heat treatment, which with 3D porous structure and Ag/C conductive network. As a result, the synthesized porous Si/C materials were used as LIBs
anodes which exhibited high specific capacity, including a specific ca pacity of 2055 mAh/g at the initial cycle, 1263.4 mAh/g after 50 cycles, 1002.6 mAh/g after 100 cycles, 782.1 mAh/g after 200 cycles at 0.5C and remarkable rate performance, including an average specific capac ity of 1833, 1533, 1303, 1118, 910 and 730 mAh/g in the current densities of 0.2, 0.5, 1, 2, 5 and 8 C, which has no significant gap compared to other nano-sized Si-based materials. For example, Zhang et al. prepared a composite of nano-si with carbon coating layer, which performed a storage capacity of 2031 mAh/g at the initial cycle, 960 mAh/g over 100 cycles at 1 A/g, and an average specific capacity of 989, 911 and 780 mAh/g at the current densities of 1.0, 2.0, 3.0 A/g [14]. Li et al. fabricated a ball-milling-silicon@carbon/reduced-graphene-oxide (bmSi@C/rGO) composite, which performed a specific capacity of 2126.8 mAh/g at the initial cycle, 935.77 mAh/g after 100 cycles, and an average specific capacity of 908.2, 706.6 and 464.3 mAh/g at the current densities of 2.0, 5.0, 10.0 A/g [30]. Zhou et al. prepared a nanocomposite of silicon nanoparticles encapsulated in graphene (Si-NP@G), which performed a storage capacity of 2920 mAh/g at the initial cycle, about 1300 mAh/g after 100 cycles under a current density of 100 mA/g, and an average specific capacity of 1452, 1320 and 990 mAh/g at the current densities of 400, 800, 1600 mA/g [1]. This easy-handling and controllable process of micro-sized porous Si/C ma terials should contribute to practical application. 2. Experimental 2.1. Materials Commercially available Si powder was purchased from Shanghai Chaowei nano technology Co.Ltd. (5 μm, 99.9%). Ethanol (AR), hydro chloric acid (AR, 38 wt%), silver nitrate (AR), hydrofluoric acid (AR, 40 wt%), hydrogen peroxide (AR, 30 wt%), chitosan were purchased from Sinopharm chemical reagent Co.Ltd. Acetylene black (AR), carboxyl methyl cellulose (CMC), electrolyte including LiPF6, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), fluoroethylene carbonate (FEC) were purchased from Hongjing new energy Co.Ltd. All the chemical reagents were directly used without to any special handing. 2.2. Preparation of porous Si particles Commercially available Si powder was dispersed into absolute ethyl alcohol by ultrasonication for 30 min and dried under vacuum condi tion. This step is to remove grease and impurities from the silicon sur face. The porous Si was prepared by two steps, as shown in Fig. 1. Firstly, 1 g Si powder (Fig. 1a) was added into a solution of 2 M hydrofluoric acid (HF) and 15 mM silver nitrate (AgNO3) with stirring for 2 min. After washing with deionized water (DI) for 3 times, we obtained about 1.19 g Ag nanoparticles decorated Si powders (Fig. 1b). The ratio of Si to Ag is 21:4. Then, 1 g Ag-decorated Si particles were evenly dispersed into a solution of 0.05 M hydrogen peroxide (H2O2) and 0.03 M hydrofluoric acid (HF) at 30 � C for 30 min. We obtained about 0.62 g porous Si par ticles (Fig. 1c) by washing and drying. The weight loss is mainly due to 2
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Journal of Power Sources 439 (2019) 227027
Fig. 2. (a) XRD patterns of the Si, porous Si, porous Si/C, (b) Raman spectra of the Si, porous Si, porous Si/C, (c) TGA measure results of the porous Si, porous Si/C.
the consumption of Si, and the silver is almost not lost as a catalyst. Therefore the Si and Ag contents in porous Si are 74 wt% and 26 wt%.
2.4. Characterization X-ray diffraction patterns (XRD) of as-prepared Si, porous Si, porous Si/C samples were obtained on a Rigaku D/Max2500 Vþ/PC using the Kα radiation of Cu. The all the three samples were also characterized by Raman spectroscopy (inVia, RENISHAW (HONG KONG, England). Zeta potential of porous Si particles and porous Si/CTS particles were ob tained on Potential analyzer (ZETASIZER 3000HS). Besides, the mor phologies as well as particle size of samples were observed by the fieldemission scanning electron microscopy (SEM) (Hitachi S-8100) and transmission electron microscopy (TEM) (JEM 2100F, JEOL, Japan) with energy dispersive X-ray spectroscopy (EDS). Porosity and specific surface areas were carried out on ASAP-2020 (Micromeritics, America) were according to Brunauer-Emmett-Teller method (BET). Thermogra vimetric (TG) analysis was carried out on an STA449 F3 (NETZSCH Instruments, Germany) using a heating rate of 10 � C min 1 in air.
2.3. Preparation of porous Si/C composite Firstly, 1 g chitosan powder was putted into 50 ml of 4% (v/v) acetic acid aqueous solution by stirring for 10 min to prepare chitosan solution. Then, the porous Si particles were distributed in deionized water by ultrasonication. The 30 ml chitosan solution was injected into the porous Si aqueous solution and the pH value of the suspension was kept to 3 by adding a few 1 M hydrochloric acid aqueous solution (HCl). After washing and centrifuging with deionized water for 3 times to remove the untightly absorbed chitosan, we obtained the porous Si/CTS. Chitosan was chosen as the carbon source because it has several advantages that it is green, non-toxic, biodegradable and the second most abundant polysaccharide in nature [35,36]. Finally, the porous Si/CTS was annealed in a tube furnace at 750 � C for 2 h under an argon atmosphere to form an evenly carbon-coating on the porous Si powder, we prepared the porous Si/C composite materials (Fig. 1d).
2.5. Electrochemical test The working electrode was obtained by mixing the porous Si/C as active materials with acetylene black (AR) as the conductive additive and carboxyl methyl cellulose (CMC) as the binder with mass ratio of 3
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Journal of Power Sources 439 (2019) 227027
Fig. 3. Scanning electron microscopy (SEM) images of Si (a and b), Ag-deposited Si (c and d), porous Si (e and f), porous Si/C (g and h).
70%:20%:10%, using deionized water as a solvent. The above mixed paste was spread with a thickness of 50 μm and pressed onto a copper foil as the current collector. The geometric area of all the three elec trodes are 1.44 cm2. The electrode was dried overnight in vacuum at 70 � C. The electrochemical test was measured in CR2032-type cells with above prepared working electrode and pure Li metal as reference elec trode. Celgard2500 polyethylene was used as the separator and the electrolyte was a commercial solution of 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC) (1:1:1by weight) added another 5% fluoroethylene carbonate (FEC) as additives. Galvanostatic charge/discharge cycling, as well as rate performance, was carried out by the LANHE-CT2001A instrument in a fixed voltage
range between 0.01 and 1.5 V, and the rest time between charging/ discharging steps is 1 min. Cyclic voltammetry (CV) measurements were performed on a CHI760E electrochemical workstation with a scanning rate of 0.1 mV s 1. Electrochemical impedance spectroscopy (EIS) tests were carried out on an electrochemical workstation (CHI760E) with the frequency range from 100 kHz to 10 mHz and an ac-oscillation of 10 mV. All the electrochemical measurements were obtained at room temperature. 3. Results and discussion Fig. 1 schematically illustrates the preparation process of porous Si/ C composite. Note that the silver nanoparticles were deposited on the 4
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Journal of Power Sources 439 (2019) 227027
Fig. 4. Transmission electron microscopy (TEM) images of porous Si (a and b), porous Si/C (c and d), High resolution transmission electron microscopy (HRTEM) images of porous Si/C (e and f), (g) EDS elemental mapping results of porous Si/C: (g1) Ag, (g2) Si and (g3) C elements.
surface of silicon (Fig. 1b) via a galvanic displacement reaction [33]. The 3D porous Si powder (Fig. 1c) was prepared by Ag-assisted chemical etching process and the Ag nanoparticles were used as the active cata lyst. The size of pore can be controlled by changing the amount of silver nitrate, and Ag particles were distributed within the porous Si [32]. Carbon coating was used to increase the conductivity of the material. Porous Si materials were distributed in chitosan solution under acid circumstance, and chitosan acidic liquid has positive charges because of protonation of amino groups, making them able to coordinate with the SiOx on the surfaces of porous Si in an acid medium [30]. To further validate the self-assembly process, we carried out zeta potential mea surements of porous Si particles and porous Si/CTS particles, which are 37.8 mV and þ75.1 mV, respectively. It further confirms that chitosan adsorbs on the surface of porous Si. After heat treatment, the porous Si/C can be obtained (Fig. 1d). Fig. 2a exhibits the X-ray diffraction patterns of bare Si, porous Si and porous Si/C. All of the three patterns exhibit the same major
diffraction peaks of 28.5, 47.3 and 56.1� , which can be indexed to lattice plane of (111), (220) and (311) of well-crystallized silicon (JCPDS 2701402). Beyond that, a new phase can be detected in porous Si and porous Si/C samples at 38.2, 44.4, 64.5 and 77.5� , which can be indexed to lattice plane of (111), (200), (220) and (311) of cubic Ag (JCPDS 040783). Moreover, the broad diffraction peak at 21� is observed for porous Si/C, which indicates the existence of amorphous carbon derived from thermal of chitosan. To further verify the feasibility of the exper iment. The Roman spectra of bare Si, porous Si and porous Si/C samples are collected in Fig. 2b. The evident peak at 520 and 956 cm 1 can be observed in all the samples, representing the crystalline nature of Si. The two broad peaks at 1341 and 1604 cm 1 of porous Si/C and named as disorder-induced D band and the graphitic G band of carbon structure, respectively. The intensity ratio of D band and G band is about 0.98, indicating an amorphous carbon. Fig. 2c shows the TGA measure results of the porous Si, porous Si/C. It can be observed that the weight is increasing after 250 � C because of the continuous oxidation of Si and Ag 5
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Journal of Power Sources 439 (2019) 227027
Fig. 5. (a) Nitrogen adsorption and desorption isotherms, (b) pore size distribution plots of samples.
Fig. 6. Electrochemical performance of various electrodes: (a) the initial discharge and charge voltage profiles, (b) cycle voltammetry of the porous Si/C in the 3 cycles at a rate of 0.1 mV/s, (c) cycling property at 0.5 C of various samples and the coulombic efficiency of porous Si/C, (d) the high-rate cycling performance of the Si, porous Si, porous Si/C electrodes. 6
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particles in the air, and the weight loss of 12% between 500 and 680 � C is ascribed to the burning of carbon laying of porous Si/C. Combined with the previous experimental part, the Si and Ag contents in porous Si are 74 wt% and 26 wt%. Therefore the Si, Ag and C contents in porous Si/C are 65 wt%, 23 wt% and 12 wt%. Fig. 3 shows the SEM images of Si, Ag-deposited Si, porous Si and porous Si/C. It can be observed in Fig. 3a that the bare silicon particles before the reaction possess irregular morphology and an average par ticle size of 5 μm. The surface of bare Si particles can observe in Fig. 3b are smooth and few other particles. After the first step of the etching reaction, Ag-deposited Si can be observed in Fig. 3c and d which shows uniformly distributed Ag particles are deposited on the surface of the silicon, and there exhibited the agglomeration of a few Ag particles, which are nearly spherical with dimensions from several nanometers to dozens of nanometers in size. After the second step of the etching re action with Ag particles as catalyst agents, the 3D porous structure can be observed in Fig. 3e and f with a size in the mesoporous range, and Ag particles fall into the bottom of the porous Si [32]. The corresponding Transmission electron microscopy images are presented in Fig. 4a and b. Moreover, the pore size of porous Si/C as shown in Fig. 3g and h is obviously smaller than that of porous Si as shown in Fig. 3e and f, demonstrating the carbon layer successfully coats on the surface of porous Si. In addition, the porous Si and porous Si/C samples are analyzed by transmission electron microscopy (TEM) and corresponding high reso lution transmission electron microscopy (HRTEM) are presented in Fig. 4. It is shown (Fig. 4a and b) that the porous Si has a porous structure with mesoporous size. The silver nanoparticles were roughly uniformly distributed in porous Si after the chemical etching. It can be obviously observed that carbon layer has been successfully coated with porous silicon, and the pore structure still sustained inside of porous Si/ C in Fig. 4c and d. The HRTEM images in Fig. 4e and f were further conducted to illustrate the structure of porous Si/C. The well-ordered lattice spacing of 0.31 nm is agreed well with the (111) planes of cubic Si confirming the highly crystalline nature of Si, and an amor phous layer can be attributed to the amorphous carbon layer on the surface of sample (Fig. 4e). Most of the carbon layer has a thickness of 5–10 nm, and a small number of edge regions have tens of nanometers of the carbon layer. Furthermore, the well-ordered lattice spacing of 0.23 nm is agreed well with the (111) planes of cubic Ag confirming the highly crystalline nature of Ag (Fig. 4f). In order to better observe the distribution of elements, we conducted EDS elemental mapping after grinding the porous Si/C (Fig. 4g). We are convinced that silver can be roughly evenly distributed in the silicon substrate and carbon can be coated entirely on the surface of silicon. In addition, carbon elements in the background in Fig. 4g3 are derived from carbon-support-film-coated copper mesh TEM grid. To further insight into the porous structure of the samples, the spe cific surface area measurement and pore size distribution were detected in the Fig. 5. According to the Brunauer-Emmet-Teller (BET) data (Fig. 5a), the specific surface area of Si sample with no porous structure was only 0.97 m2/g. The porous structure significantly increases the specific surface area that is 15.136 m2/g, and after carbon coating, the specific surface area turns to 6.154 m2/g. Moreover, the pore size of porous Si samples is distributed mainly from 3 to 30 (Fig. 5b). The average pore size of porous Si and porous Si/C is 12.035 and 4.077 nm according to Barrett-Joyner-Halenda (BJH) model. The measurement result is in agreement with the scanning electron microscope (SEM) images in Fig. 3. Fig. 6a shows the first charge-discharge curves of bare Si, porous Si, porous Si/C at 0.5C in the potential range of 0.01–1.5 V (versus Li/Liþ). The charge potential plateaus of the three electrodes at 0.4 V and the discharge potential plateaus below 0.1 V, in common with the charge and discharge characteristics of typical silicon materials. The potential drop of porous Si/C is faster than that of bare Si and porous Si samples during the initial discharge process, because carbon-coated layer can
Fig. 7. Nyquist plots of the Si, porous Si, porous Si/C electrodes.
conduce to electronic transmission in the interior of electrode materials. Meanwhile, the initial discharge capacity of Si, porous Si and porous Si/ C are 3665, 2747 and 2047 mAh/g and the initial charge capacity are 2764, 2283 and 1732 mAh/g, and the initial coulombic efficiency of all the three samples are 75.4, 83.1 and 84.6%. The cell capacities are calculated based on the weight of active materials, and the first cycle irreversible capacity can be attributed to the SEI is formed on the surface of the electrode. It can be detected that the porous Si/C electrode has the highest initial coulombic efficiency, which clearly demonstrates porous structure and carbon layer contribute to the formation of the more stable SEI layer and sustain better reversible capacities. Fig. 6b shows the cyclic voltammetry (CV) measurements, which are performed on half cells in the potential window of 0.05–1.5 V at a scanning rate of 0.1 mV/s. In the first cycle, it is seen that a peak at 0.67 V correspond to the formation of SEI, which disappears in the following cycles. It is the reason for the initial irreversible capacity. The cathodic peak at 0.17 V is ascribed to amorphization process of con verting crystalline Si to LixSi. The anodic displays peak at 0.33 V and 0.51 V is attributed to the phase transition reaction from amorphous LixSi to amorphous Si [1,30]. Fig. 6c shows the cycling performance and corresponding coulombic efficiency at 0.1C for the first cycle and then 0.5 C for the following cycles in the potential range of 0.01–1.5 V (versus Li/Liþ). The bare Si electrode shows the highest initial capacity, 3665 mAh/g, but its ca pacity drops rapidly to 178mAh/g after 20 cycles, because of huge volume expansion and poor electrical conductivity. In contrast to bare Si, porous Si electrode shows better cyclic performance, which delivers the initial capacity, 2634.6 mAh/g, and it drops to 241mAh/g after 200 cycles. The porous structures increase the space to accommodate volume expansion, and Ag particles improve electrical conductivity. Porous Si/C electrode exhibits the best cycling performance, which is 2055 mAh/g at the initial cycle, 1263.4 mAh/g after 50 cycles, 1002.6 mAh/g after 100 cycles, 782.1 mAh/g after 200 cycles. Beyond that, it exhibits the high coulombic efficiency (over 98% after the 3rd cycle). Meanwhile, the average capacity fading rates of bare Si, porous Si and porous Si/C are 9.5, 0.45 and 0.3%, respectively. Such obviously improved cycling performances of Porous Si/C electrode can be mainly due to the inter connected coated carbon layer and the porous structure can provide effective transmission path for Lithium ion and alleviate the volume change. The rate capability in the potential range of 0.01–1.5 V (versus Li/ Liþ) is investigated in Fig. 6d. The specific capacity of bare Si electrode decreases rapidly to 164 mAh/g, when the current density is larger than 7
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Journal of Power Sources 439 (2019) 227027
Fig. 8. The SEM surface views of the Si, porous Si, porous S/C electrode after cycle test, (a) Si electrode after 50 cycles, (b) porous Si electrode after 100 cycles, (c) porous Si/C electrode after 100 cycles and (d) porous Si/C electrode after 200 cycles.
2 C. The specific capacity of porous Si electrode is better than bare Si electrode but worse than porous Si/C electrode. In the current densities of 0.2, 0.5, 1, 2, 5 and 8 C, the porous Si/C electrode has an average specific capacity of 1833, 1533, 1303, 1118, 910 and 730 mAh/g, respectively. When the current density back to 0.2 C, the specific ca pacity could restored to 1046 mAh/g. It obviously supports the advan tage of the porous structure which is conducive to Liþ/electron transport, carbon layer and Ag particles improve electrical conductivity [37–39]. To further understand the difference of the electrochemical perfor mance of Si, porous Si, porous Si/C electrode, the electrochemical impedance spectra (EIS) was used to analyze the resistance evolution of all the three electrodes. As shown in Fig. 7, the EIS curves were fitted by an equivalent circuit model, where Rs represents Ohmic resistance of electrode, RSEI and CSEI are the resistance and the capacity of SEI for mation, RCT and CCT are the resistance and the capacity of charge transfer, and WO is the Warburg impedance tail which describes ion diffusion phenomena in the device. The Nyquist plots of all the three electrodes show similar trend which includes a typical semicircle, a less obvious transition semicircle and an inclined line. The typical semicircle can be observed in high frequency, the diameter of semicircle is asso ciated with the resistance at the interface of electrode and the electro lyte. Moreover, the less obvious semicircle in the medium frequency contains information about charge transfer resistance and the line in the low frequency is associated with the ion diffusion resistance. It is clearly seen that the bare Si exhibits the biggest interface resistance, charge transfer resistance and ion diffusion resistance which of the porous Si is much lower and the porous Si/C shows the smallest. Therefore, we can draw a conclusion that the 3D porous structure and carbon coating significantly improve the charge transfer performance and provide effectively ionic transfer paths. Meanwhile, the Ag particles provide high electrical conductivity and stable ionic conductivity [38]. The SEM images of bare Si, porous Si and porous Si/C electrode after cycle test can be showed in Fig. 8a–d. The electrode surface of Si with extensive shedding can be clearly observed after 50 cycles (Fig. 8a), mostly because the large volume changes unable to form a stable SEI and keep consuming the electrolyte. Moreover, the electrode pulverization
lead to the electrode and current collector lose electric contact. Compared with Si, porous Si electrode inhibits less loss of irreversible capacity to some extent, but it also had cracks after 100 cycles (Fig. 8b). Note that after 100 cycles the porous Si/C electrode exhibits a much more flat surface and after 200 cycles without large visible cracks, which indicating the porous Si/C electrode can effectively suppress volume expansion during the repeated Li-ion intercalation/deintercalation and maintain a relatively stable structure (Fig. 8c and d). Thus, the porous Si/C electrode exhibits the better electrochemical performance compared with porous Si and Si electrode. 4. Conclusion In summary, we successfully fabricate porous Si/C composite mate rials using a controllable, cost-effective and simple strategy by Agassisted chemical etching and carbon coating of the micro-sized silicon materials, which are less agglomeration and much cheaper than nano scale materials. The 3D porous structure into the micro-sized silicon materials accommodates the huge repeated volume changes and shortens the Li-ion transfer paths. After coating a thin carbon layer, the porous Si/C composites with Ag particles form a stable architecture and multiplicity conductive network. Therefore, the porous Si/C materials are used as anodes for lithium-ion batteries showed an improved initial Coulombic efficiency of 84.6% and prominent cycling stability of 782.1mAh/g at 0.5 C after 200 cycles, with only 0.3% average loss rate of per cycle. It is demonstrated that the porous Si/C electrode with high rate performance and excellent cycling stability is very promising for lithium-ion batteries with superior practicality. Acknowledgments The work was supported by Shanghai Leading Academic Discipline Project under the grant (No. S-1.670107). Financial support from the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning was also gratefully acknowl edged. The authors also acknowledged the Shanghai Science and Technology Commission for the Grant (No. 13521101202) gratefully. 8
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Novel design and synthesis of carbon-coated porous silicon particles as high-performance lithium-ion battery anodes Tianting Zhao a, b, Delun Zhu a, b, Wenrong Li b, Aijun Li a, *, Jiujun Zhang b, ** a b
School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China Institute for Sustainable Energy/College of Sciences, Shanghai University, Shanghai, 200444, China
H I G H L I G H T S
� A effectively strategy to optimize the micro-sized Si-base anodes. � Porous Si/C composite prepared by Ag-assisted chemical etching and self-assembly. � We prepared a composite with 3D porous structure and Ag/C conductive network. � This composite used as LIBs anodes exhibited remarkable rate performance. A R T I C L E I N F O
A B S T R A C T
Keywords: Porous silicon Silver nanoparticles Lithium-ion batteries Carbon coating
Porous silicon-based materials are considered promising next generation lithium-ion battery anode materials. Here, we report the rational design of multiscale recombined porous Si/C composites via a controllable and simple Ag-assisted chemical etching process with subsequent heat treatment of porous Si and chitosan com posites (porous Si/CTS) which prepared by self-assembly process. Chitosan is a green, nontoxic and biode gradable carbon source. The three-dimensional porous (3D porous) structure accommodates the huge volume changes during lithiation and delithiation, which helps form more stable solid electrolyte interface (SEI) film and significantly improves the electrochemical stability. The multiplicity conductive network is constructed by Ag nanoparticles and carbon layer, which keep conductive contact and maintain structure stability. The Li-ion battery anode based on porous Si/C exhibits a reversible high capacity of 782.1 mAh/g at 0.5 C after 200 cy cles with excellent Coulombic efficiency. This work opens a promising approach for production of highperformance micro-sized silicon-based anode materials in Li-ion battery.
1. Introduction
highest theoretical capacity at room temperature (3579 mAh/g), low discharge voltage (<0.5 V vs Li/Liþ), abundant crustal reserve and environment friendly [8–10]. However, the widely practical imple mentation of Si-based anode is hampered by several severe problems such as its considerable volume change (~300%) during lithiation and delithiation processes and poor electronic conductivity. The huge vol ume change causes electrode pulverization leading to loss of electrical contact between electrode materials and detachment the electrode ma terials from current collector, and producing the unstable SEI layer leading to constant consumption of electrolyte [11–13]. These disad vantages are the main reasons for serious capacity fading and low cycling performance. To overcome these aforementioned defects, several strategies have
Li-ion batteries (LIBs) are widely used in portable electric devices, electro vehicles and renewable energy resources because of high energy density, lightweight, friendly environment and long cycle performance [1–3]. So far, the current LIBs need to improve the performance of higher energy density and higher power density to apply in upgraded products and many other applications [4–6]. It is known that the elec trode materials are considered to be one of the most important factors determining the performance of LIBs. Recently, silicon-based materials as anodes have been studied as the most promising candidates to alternative commercial graphite electrode materials which have limited theoretical capacity (372 mAh/g) [7]. Silicon-based anodes are the * Corresponding author. ** Corresponding author. E-mail address: [email protected] (A. Li).
https://doi.org/10.1016/j.jpowsour.2019.227027 Received 12 May 2019; Received in revised form 28 July 2019; Accepted 16 August 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The schematic illustration of the preparation of porous Si/C composite.
been applied to optimize the Si-base anodes. One of the common methods is designing specific nanostructure, like Si nanoparticle, Si nanofiber and Si nanotube can accommodate large stress root in large volume expansion [14–20]. Compared with nanostructured silicon ma terial, micro-sized silicon particles do not need high price and rigorous preparation conditions, but they suffer from unavoidable huge volume expansion during electrochemical cycling. Recently, the studies of micro-sized Si-based materials have been attracted attention by the re searchers. It is an effective strategy to construct the 3D porous structure or the hollow structure in micro-sized Si materials, since the porous structure or the hollow structure provided the void space to accommo date the large volume expansion during electrochemical cycling. Various methods have been used for fabrication of such electrode ma terials, mainly including magnesiothermic reaction, electrochemical etching and acid etching metal-Si alloy [21–25]. For example, He et al. prepared scalable micro-sized bulk porous Si by ball milling and etching using HCl/HF as the etchant of pristine Fe–Si powder [22]. T.lkonen et al. prepared mesoporous Si by electrochemical etching in an aqueous hydrofluoric acid/ethanol electrolyte [23]. Huang et al. prepared morphological controllable hollow Si such as hollow cubes, hollow spheres and flowers shapes using templates of carbonates, which were removed by hydrochloric acid (HCl) [10]. Although micro-sized Si-based anodes with the porous structure or the hollow structure can partially accommodate the large volume expansion, the problem of poor conductivity of Si materials remains unsolved. It is a universal and effective strategy to coat conductive carbon layer on silicon-based ma terial, since the carbon layer can alleviate the mechanical stress of electrode and increase the electronic conductivity [26–30]. However, the improvement of electrical conductivity caused by carbon materials is limited. Moreover, micro-sized Si is much larger than nano-sized Si, the carbon elements only concentrated on the surface by the carbon coating, and the conductivity inside the particles needs to be further improved. In this study, we use micro-sized Si as the raw materials instead of nano-sized Si. Then, we report a controllable and simple operable method to synthesize interconnected porous Si materials by Ag-assisted chemical etching process. The pore size, depth and morphology of porous Si can be turned by the amount of catalyst, the relative compo sition of etching bath and reaction time, and Ag particles were distrib uted within the porous Si [31–34]. The tiny void space can partially buffer the huge volume change and provide a valid path of Li-ion. The Ag particles can form a conductive structure inside the silicon. In order to further restrain the large volume change and improve the conduc tivity, we produced modifying porous Si by carbon coating using the self-assembly reaction. Electrostatic self-assembly is an effective strat egy to create well-mixed composites. First, the chitosan has plenty of amino groups which will protonated in an acidic liquid. Porous Si has negative charges as a result of the existence of silicon oxide on its sur face. Then, Porous Si materials were distributed in chitosan solution, by adjusting the pH of the dispersion, porous Si adsorbs chitosan through electrostatic interaction to change the charge from negative to positive [30]. Finally, the porous Si/C was produced by heat treatment, which with 3D porous structure and Ag/C conductive network. As a result, the synthesized porous Si/C materials were used as LIBs
anodes which exhibited high specific capacity, including a specific ca pacity of 2055 mAh/g at the initial cycle, 1263.4 mAh/g after 50 cycles, 1002.6 mAh/g after 100 cycles, 782.1 mAh/g after 200 cycles at 0.5C and remarkable rate performance, including an average specific capac ity of 1833, 1533, 1303, 1118, 910 and 730 mAh/g in the current densities of 0.2, 0.5, 1, 2, 5 and 8 C, which has no significant gap compared to other nano-sized Si-based materials. For example, Zhang et al. prepared a composite of nano-si with carbon coating layer, which performed a storage capacity of 2031 mAh/g at the initial cycle, 960 mAh/g over 100 cycles at 1 A/g, and an average specific capacity of 989, 911 and 780 mAh/g at the current densities of 1.0, 2.0, 3.0 A/g [14]. Li et al. fabricated a ball-milling-silicon@carbon/reduced-graphene-oxide (bmSi@C/rGO) composite, which performed a specific capacity of 2126.8 mAh/g at the initial cycle, 935.77 mAh/g after 100 cycles, and an average specific capacity of 908.2, 706.6 and 464.3 mAh/g at the current densities of 2.0, 5.0, 10.0 A/g [30]. Zhou et al. prepared a nanocomposite of silicon nanoparticles encapsulated in graphene (Si-NP@G), which performed a storage capacity of 2920 mAh/g at the initial cycle, about 1300 mAh/g after 100 cycles under a current density of 100 mA/g, and an average specific capacity of 1452, 1320 and 990 mAh/g at the current densities of 400, 800, 1600 mA/g [1]. This easy-handling and controllable process of micro-sized porous Si/C ma terials should contribute to practical application. 2. Experimental 2.1. Materials Commercially available Si powder was purchased from Shanghai Chaowei nano technology Co.Ltd. (5 μm, 99.9%). Ethanol (AR), hydro chloric acid (AR, 38 wt%), silver nitrate (AR), hydrofluoric acid (AR, 40 wt%), hydrogen peroxide (AR, 30 wt%), chitosan were purchased from Sinopharm chemical reagent Co.Ltd. Acetylene black (AR), carboxyl methyl cellulose (CMC), electrolyte including LiPF6, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), fluoroethylene carbonate (FEC) were purchased from Hongjing new energy Co.Ltd. All the chemical reagents were directly used without to any special handing. 2.2. Preparation of porous Si particles Commercially available Si powder was dispersed into absolute ethyl alcohol by ultrasonication for 30 min and dried under vacuum condi tion. This step is to remove grease and impurities from the silicon sur face. The porous Si was prepared by two steps, as shown in Fig. 1. Firstly, 1 g Si powder (Fig. 1a) was added into a solution of 2 M hydrofluoric acid (HF) and 15 mM silver nitrate (AgNO3) with stirring for 2 min. After washing with deionized water (DI) for 3 times, we obtained about 1.19 g Ag nanoparticles decorated Si powders (Fig. 1b). The ratio of Si to Ag is 21:4. Then, 1 g Ag-decorated Si particles were evenly dispersed into a solution of 0.05 M hydrogen peroxide (H2O2) and 0.03 M hydrofluoric acid (HF) at 30 � C for 30 min. We obtained about 0.62 g porous Si par ticles (Fig. 1c) by washing and drying. The weight loss is mainly due to 2
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Fig. 2. (a) XRD patterns of the Si, porous Si, porous Si/C, (b) Raman spectra of the Si, porous Si, porous Si/C, (c) TGA measure results of the porous Si, porous Si/C.
the consumption of Si, and the silver is almost not lost as a catalyst. Therefore the Si and Ag contents in porous Si are 74 wt% and 26 wt%.
2.4. Characterization X-ray diffraction patterns (XRD) of as-prepared Si, porous Si, porous Si/C samples were obtained on a Rigaku D/Max2500 Vþ/PC using the Kα radiation of Cu. The all the three samples were also characterized by Raman spectroscopy (inVia, RENISHAW (HONG KONG, England). Zeta potential of porous Si particles and porous Si/CTS particles were ob tained on Potential analyzer (ZETASIZER 3000HS). Besides, the mor phologies as well as particle size of samples were observed by the fieldemission scanning electron microscopy (SEM) (Hitachi S-8100) and transmission electron microscopy (TEM) (JEM 2100F, JEOL, Japan) with energy dispersive X-ray spectroscopy (EDS). Porosity and specific surface areas were carried out on ASAP-2020 (Micromeritics, America) were according to Brunauer-Emmett-Teller method (BET). Thermogra vimetric (TG) analysis was carried out on an STA449 F3 (NETZSCH Instruments, Germany) using a heating rate of 10 � C min 1 in air.
2.3. Preparation of porous Si/C composite Firstly, 1 g chitosan powder was putted into 50 ml of 4% (v/v) acetic acid aqueous solution by stirring for 10 min to prepare chitosan solution. Then, the porous Si particles were distributed in deionized water by ultrasonication. The 30 ml chitosan solution was injected into the porous Si aqueous solution and the pH value of the suspension was kept to 3 by adding a few 1 M hydrochloric acid aqueous solution (HCl). After washing and centrifuging with deionized water for 3 times to remove the untightly absorbed chitosan, we obtained the porous Si/CTS. Chitosan was chosen as the carbon source because it has several advantages that it is green, non-toxic, biodegradable and the second most abundant polysaccharide in nature [35,36]. Finally, the porous Si/CTS was annealed in a tube furnace at 750 � C for 2 h under an argon atmosphere to form an evenly carbon-coating on the porous Si powder, we prepared the porous Si/C composite materials (Fig. 1d).
2.5. Electrochemical test The working electrode was obtained by mixing the porous Si/C as active materials with acetylene black (AR) as the conductive additive and carboxyl methyl cellulose (CMC) as the binder with mass ratio of 3
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Fig. 3. Scanning electron microscopy (SEM) images of Si (a and b), Ag-deposited Si (c and d), porous Si (e and f), porous Si/C (g and h).
70%:20%:10%, using deionized water as a solvent. The above mixed paste was spread with a thickness of 50 μm and pressed onto a copper foil as the current collector. The geometric area of all the three elec trodes are 1.44 cm2. The electrode was dried overnight in vacuum at 70 � C. The electrochemical test was measured in CR2032-type cells with above prepared working electrode and pure Li metal as reference elec trode. Celgard2500 polyethylene was used as the separator and the electrolyte was a commercial solution of 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC) (1:1:1by weight) added another 5% fluoroethylene carbonate (FEC) as additives. Galvanostatic charge/discharge cycling, as well as rate performance, was carried out by the LANHE-CT2001A instrument in a fixed voltage
range between 0.01 and 1.5 V, and the rest time between charging/ discharging steps is 1 min. Cyclic voltammetry (CV) measurements were performed on a CHI760E electrochemical workstation with a scanning rate of 0.1 mV s 1. Electrochemical impedance spectroscopy (EIS) tests were carried out on an electrochemical workstation (CHI760E) with the frequency range from 100 kHz to 10 mHz and an ac-oscillation of 10 mV. All the electrochemical measurements were obtained at room temperature. 3. Results and discussion Fig. 1 schematically illustrates the preparation process of porous Si/ C composite. Note that the silver nanoparticles were deposited on the 4
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Fig. 4. Transmission electron microscopy (TEM) images of porous Si (a and b), porous Si/C (c and d), High resolution transmission electron microscopy (HRTEM) images of porous Si/C (e and f), (g) EDS elemental mapping results of porous Si/C: (g1) Ag, (g2) Si and (g3) C elements.
surface of silicon (Fig. 1b) via a galvanic displacement reaction [33]. The 3D porous Si powder (Fig. 1c) was prepared by Ag-assisted chemical etching process and the Ag nanoparticles were used as the active cata lyst. The size of pore can be controlled by changing the amount of silver nitrate, and Ag particles were distributed within the porous Si [32]. Carbon coating was used to increase the conductivity of the material. Porous Si materials were distributed in chitosan solution under acid circumstance, and chitosan acidic liquid has positive charges because of protonation of amino groups, making them able to coordinate with the SiOx on the surfaces of porous Si in an acid medium [30]. To further validate the self-assembly process, we carried out zeta potential mea surements of porous Si particles and porous Si/CTS particles, which are 37.8 mV and þ75.1 mV, respectively. It further confirms that chitosan adsorbs on the surface of porous Si. After heat treatment, the porous Si/C can be obtained (Fig. 1d). Fig. 2a exhibits the X-ray diffraction patterns of bare Si, porous Si and porous Si/C. All of the three patterns exhibit the same major
diffraction peaks of 28.5, 47.3 and 56.1� , which can be indexed to lattice plane of (111), (220) and (311) of well-crystallized silicon (JCPDS 2701402). Beyond that, a new phase can be detected in porous Si and porous Si/C samples at 38.2, 44.4, 64.5 and 77.5� , which can be indexed to lattice plane of (111), (200), (220) and (311) of cubic Ag (JCPDS 040783). Moreover, the broad diffraction peak at 21� is observed for porous Si/C, which indicates the existence of amorphous carbon derived from thermal of chitosan. To further verify the feasibility of the exper iment. The Roman spectra of bare Si, porous Si and porous Si/C samples are collected in Fig. 2b. The evident peak at 520 and 956 cm 1 can be observed in all the samples, representing the crystalline nature of Si. The two broad peaks at 1341 and 1604 cm 1 of porous Si/C and named as disorder-induced D band and the graphitic G band of carbon structure, respectively. The intensity ratio of D band and G band is about 0.98, indicating an amorphous carbon. Fig. 2c shows the TGA measure results of the porous Si, porous Si/C. It can be observed that the weight is increasing after 250 � C because of the continuous oxidation of Si and Ag 5
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Fig. 5. (a) Nitrogen adsorption and desorption isotherms, (b) pore size distribution plots of samples.
Fig. 6. Electrochemical performance of various electrodes: (a) the initial discharge and charge voltage profiles, (b) cycle voltammetry of the porous Si/C in the 3 cycles at a rate of 0.1 mV/s, (c) cycling property at 0.5 C of various samples and the coulombic efficiency of porous Si/C, (d) the high-rate cycling performance of the Si, porous Si, porous Si/C electrodes. 6
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particles in the air, and the weight loss of 12% between 500 and 680 � C is ascribed to the burning of carbon laying of porous Si/C. Combined with the previous experimental part, the Si and Ag contents in porous Si are 74 wt% and 26 wt%. Therefore the Si, Ag and C contents in porous Si/C are 65 wt%, 23 wt% and 12 wt%. Fig. 3 shows the SEM images of Si, Ag-deposited Si, porous Si and porous Si/C. It can be observed in Fig. 3a that the bare silicon particles before the reaction possess irregular morphology and an average par ticle size of 5 μm. The surface of bare Si particles can observe in Fig. 3b are smooth and few other particles. After the first step of the etching reaction, Ag-deposited Si can be observed in Fig. 3c and d which shows uniformly distributed Ag particles are deposited on the surface of the silicon, and there exhibited the agglomeration of a few Ag particles, which are nearly spherical with dimensions from several nanometers to dozens of nanometers in size. After the second step of the etching re action with Ag particles as catalyst agents, the 3D porous structure can be observed in Fig. 3e and f with a size in the mesoporous range, and Ag particles fall into the bottom of the porous Si [32]. The corresponding Transmission electron microscopy images are presented in Fig. 4a and b. Moreover, the pore size of porous Si/C as shown in Fig. 3g and h is obviously smaller than that of porous Si as shown in Fig. 3e and f, demonstrating the carbon layer successfully coats on the surface of porous Si. In addition, the porous Si and porous Si/C samples are analyzed by transmission electron microscopy (TEM) and corresponding high reso lution transmission electron microscopy (HRTEM) are presented in Fig. 4. It is shown (Fig. 4a and b) that the porous Si has a porous structure with mesoporous size. The silver nanoparticles were roughly uniformly distributed in porous Si after the chemical etching. It can be obviously observed that carbon layer has been successfully coated with porous silicon, and the pore structure still sustained inside of porous Si/ C in Fig. 4c and d. The HRTEM images in Fig. 4e and f were further conducted to illustrate the structure of porous Si/C. The well-ordered lattice spacing of 0.31 nm is agreed well with the (111) planes of cubic Si confirming the highly crystalline nature of Si, and an amor phous layer can be attributed to the amorphous carbon layer on the surface of sample (Fig. 4e). Most of the carbon layer has a thickness of 5–10 nm, and a small number of edge regions have tens of nanometers of the carbon layer. Furthermore, the well-ordered lattice spacing of 0.23 nm is agreed well with the (111) planes of cubic Ag confirming the highly crystalline nature of Ag (Fig. 4f). In order to better observe the distribution of elements, we conducted EDS elemental mapping after grinding the porous Si/C (Fig. 4g). We are convinced that silver can be roughly evenly distributed in the silicon substrate and carbon can be coated entirely on the surface of silicon. In addition, carbon elements in the background in Fig. 4g3 are derived from carbon-support-film-coated copper mesh TEM grid. To further insight into the porous structure of the samples, the spe cific surface area measurement and pore size distribution were detected in the Fig. 5. According to the Brunauer-Emmet-Teller (BET) data (Fig. 5a), the specific surface area of Si sample with no porous structure was only 0.97 m2/g. The porous structure significantly increases the specific surface area that is 15.136 m2/g, and after carbon coating, the specific surface area turns to 6.154 m2/g. Moreover, the pore size of porous Si samples is distributed mainly from 3 to 30 (Fig. 5b). The average pore size of porous Si and porous Si/C is 12.035 and 4.077 nm according to Barrett-Joyner-Halenda (BJH) model. The measurement result is in agreement with the scanning electron microscope (SEM) images in Fig. 3. Fig. 6a shows the first charge-discharge curves of bare Si, porous Si, porous Si/C at 0.5C in the potential range of 0.01–1.5 V (versus Li/Liþ). The charge potential plateaus of the three electrodes at 0.4 V and the discharge potential plateaus below 0.1 V, in common with the charge and discharge characteristics of typical silicon materials. The potential drop of porous Si/C is faster than that of bare Si and porous Si samples during the initial discharge process, because carbon-coated layer can
Fig. 7. Nyquist plots of the Si, porous Si, porous Si/C electrodes.
conduce to electronic transmission in the interior of electrode materials. Meanwhile, the initial discharge capacity of Si, porous Si and porous Si/ C are 3665, 2747 and 2047 mAh/g and the initial charge capacity are 2764, 2283 and 1732 mAh/g, and the initial coulombic efficiency of all the three samples are 75.4, 83.1 and 84.6%. The cell capacities are calculated based on the weight of active materials, and the first cycle irreversible capacity can be attributed to the SEI is formed on the surface of the electrode. It can be detected that the porous Si/C electrode has the highest initial coulombic efficiency, which clearly demonstrates porous structure and carbon layer contribute to the formation of the more stable SEI layer and sustain better reversible capacities. Fig. 6b shows the cyclic voltammetry (CV) measurements, which are performed on half cells in the potential window of 0.05–1.5 V at a scanning rate of 0.1 mV/s. In the first cycle, it is seen that a peak at 0.67 V correspond to the formation of SEI, which disappears in the following cycles. It is the reason for the initial irreversible capacity. The cathodic peak at 0.17 V is ascribed to amorphization process of con verting crystalline Si to LixSi. The anodic displays peak at 0.33 V and 0.51 V is attributed to the phase transition reaction from amorphous LixSi to amorphous Si [1,30]. Fig. 6c shows the cycling performance and corresponding coulombic efficiency at 0.1C for the first cycle and then 0.5 C for the following cycles in the potential range of 0.01–1.5 V (versus Li/Liþ). The bare Si electrode shows the highest initial capacity, 3665 mAh/g, but its ca pacity drops rapidly to 178mAh/g after 20 cycles, because of huge volume expansion and poor electrical conductivity. In contrast to bare Si, porous Si electrode shows better cyclic performance, which delivers the initial capacity, 2634.6 mAh/g, and it drops to 241mAh/g after 200 cycles. The porous structures increase the space to accommodate volume expansion, and Ag particles improve electrical conductivity. Porous Si/C electrode exhibits the best cycling performance, which is 2055 mAh/g at the initial cycle, 1263.4 mAh/g after 50 cycles, 1002.6 mAh/g after 100 cycles, 782.1 mAh/g after 200 cycles. Beyond that, it exhibits the high coulombic efficiency (over 98% after the 3rd cycle). Meanwhile, the average capacity fading rates of bare Si, porous Si and porous Si/C are 9.5, 0.45 and 0.3%, respectively. Such obviously improved cycling performances of Porous Si/C electrode can be mainly due to the inter connected coated carbon layer and the porous structure can provide effective transmission path for Lithium ion and alleviate the volume change. The rate capability in the potential range of 0.01–1.5 V (versus Li/ Liþ) is investigated in Fig. 6d. The specific capacity of bare Si electrode decreases rapidly to 164 mAh/g, when the current density is larger than 7
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Journal of Power Sources 439 (2019) 227027
Fig. 8. The SEM surface views of the Si, porous Si, porous S/C electrode after cycle test, (a) Si electrode after 50 cycles, (b) porous Si electrode after 100 cycles, (c) porous Si/C electrode after 100 cycles and (d) porous Si/C electrode after 200 cycles.
2 C. The specific capacity of porous Si electrode is better than bare Si electrode but worse than porous Si/C electrode. In the current densities of 0.2, 0.5, 1, 2, 5 and 8 C, the porous Si/C electrode has an average specific capacity of 1833, 1533, 1303, 1118, 910 and 730 mAh/g, respectively. When the current density back to 0.2 C, the specific ca pacity could restored to 1046 mAh/g. It obviously supports the advan tage of the porous structure which is conducive to Liþ/electron transport, carbon layer and Ag particles improve electrical conductivity [37–39]. To further understand the difference of the electrochemical perfor mance of Si, porous Si, porous Si/C electrode, the electrochemical impedance spectra (EIS) was used to analyze the resistance evolution of all the three electrodes. As shown in Fig. 7, the EIS curves were fitted by an equivalent circuit model, where Rs represents Ohmic resistance of electrode, RSEI and CSEI are the resistance and the capacity of SEI for mation, RCT and CCT are the resistance and the capacity of charge transfer, and WO is the Warburg impedance tail which describes ion diffusion phenomena in the device. The Nyquist plots of all the three electrodes show similar trend which includes a typical semicircle, a less obvious transition semicircle and an inclined line. The typical semicircle can be observed in high frequency, the diameter of semicircle is asso ciated with the resistance at the interface of electrode and the electro lyte. Moreover, the less obvious semicircle in the medium frequency contains information about charge transfer resistance and the line in the low frequency is associated with the ion diffusion resistance. It is clearly seen that the bare Si exhibits the biggest interface resistance, charge transfer resistance and ion diffusion resistance which of the porous Si is much lower and the porous Si/C shows the smallest. Therefore, we can draw a conclusion that the 3D porous structure and carbon coating significantly improve the charge transfer performance and provide effectively ionic transfer paths. Meanwhile, the Ag particles provide high electrical conductivity and stable ionic conductivity [38]. The SEM images of bare Si, porous Si and porous Si/C electrode after cycle test can be showed in Fig. 8a–d. The electrode surface of Si with extensive shedding can be clearly observed after 50 cycles (Fig. 8a), mostly because the large volume changes unable to form a stable SEI and keep consuming the electrolyte. Moreover, the electrode pulverization
lead to the electrode and current collector lose electric contact. Compared with Si, porous Si electrode inhibits less loss of irreversible capacity to some extent, but it also had cracks after 100 cycles (Fig. 8b). Note that after 100 cycles the porous Si/C electrode exhibits a much more flat surface and after 200 cycles without large visible cracks, which indicating the porous Si/C electrode can effectively suppress volume expansion during the repeated Li-ion intercalation/deintercalation and maintain a relatively stable structure (Fig. 8c and d). Thus, the porous Si/C electrode exhibits the better electrochemical performance compared with porous Si and Si electrode. 4. Conclusion In summary, we successfully fabricate porous Si/C composite mate rials using a controllable, cost-effective and simple strategy by Agassisted chemical etching and carbon coating of the micro-sized silicon materials, which are less agglomeration and much cheaper than nano scale materials. The 3D porous structure into the micro-sized silicon materials accommodates the huge repeated volume changes and shortens the Li-ion transfer paths. After coating a thin carbon layer, the porous Si/C composites with Ag particles form a stable architecture and multiplicity conductive network. Therefore, the porous Si/C materials are used as anodes for lithium-ion batteries showed an improved initial Coulombic efficiency of 84.6% and prominent cycling stability of 782.1mAh/g at 0.5 C after 200 cycles, with only 0.3% average loss rate of per cycle. It is demonstrated that the porous Si/C electrode with high rate performance and excellent cycling stability is very promising for lithium-ion batteries with superior practicality. Acknowledgments The work was supported by Shanghai Leading Academic Discipline Project under the grant (No. S-1.670107). Financial support from the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning was also gratefully acknowl edged. The authors also acknowledged the Shanghai Science and Technology Commission for the Grant (No. 13521101202) gratefully. 8
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