Silicon-encapsulated hollow carbon nanofiber networks with ample space around the Si nanoparticles (hollow Si/C composites) were successfully synthesized by dip-coating phenolic resin onto the surface of electrospun Si/PVA nanofibers along with the subsequent solidification and carbonization. More importantly, the structure and Si content of hollow Si/C composite nanofibers can be effectively tuned by merely varying the concentration of dip solution. As-synthesized hollow Si/C composites show excellent electrochemical performance when they are used as binder-free anodes for Li-ion batteries (LIBs). In particular, when the concentration of resol/ethanol solution is 3.0%, the product exhibits a large capacity of 841 mAh g−1 in the first cycle, prominent cycling stability, and good rate capability. The discharge capacity retention of it was ~90%, with 745 mAh g−1 after 50 cycles. The results demonstrate that the hollow Si/C composites are very promising as alternative anode candidates for high-performance LIBs.
To meet the increasing demands of rapidly developing market from cell phone to electric vehicles for the Li-ion batteries (LIBs), new anode materials with higher capacity have attracted significant attention. Graphite, the most commonly used commercial anode material, has low theoretical specific capacity (372 mAh g−1) and poor rate capability. Silicon is considered as one of the most attractive and promising alternative anode materials to replace graphite in LIBs in the coming decades in virtue of its fascinating performance, such as relatively low working potential (~370 mV versus Li/Li+), rich abundance in earth, environmental benignity, and especially the highest theoretical capacity of 4212 mAh g−1 among the existing anode materials [
In order to address abovementioned challenges, it would be crucial to release the mechanical strains, as well as reducing the diffusion length of Li-ions in Si electrode materials during charge/discharge process. Nanostructured Si would be a good solution. So far, many nanostructured silicon and Si-based composites with various morphologies, such as nanoparticles [
More recently, to improve the cycling stability of silicon anode materials, some new nanostructures with Si nanoparticles encapsulated in continuous hollow carbon tubes have gained much attention due to the enhanced electrical conductivity and stable solid electrolyte interface (SEI). Moreover, particularly, the ample empty space inside the hollow tubes among the silicon nanoparticles allowed for silicon nanoparticles expansion freely during electrochemical cycling, supporting a stable cycling of the entire electrode as well as high charge and discharge rates [
With a motivation to further increase the cycling stability of the silicon anode materials, in this work, we put forward a facile strategy to synthesize novel silicon-encapsulated hollow carbon nanofiber networks with ample space around the Si nanoparticles (hollow Si/C composites) by dip-coating resol on the surface of electrospinning Si/PVA nanofibers along with the subsequent solidification and carbonization. These freestanding membranous hollow Si/C composites can be used directly as anodes for LIBs without adding any carbon conductors or polymer binders. The as-prepared hollow Si/C composites display a high electrochemical performance with excellent cycle stability of ~90% of discharge capacity retention after 50 cycles.
Firstly, polyvinyl alcohol (PVA, molecule weight = 80000) and deionized water were mixed in the ratio of 15 g PVA/135 g water. The mixture was stirred in water bath at 90°C for 2 h to form a homogenous solution. Meanwhile, silicon nanoparticles (n-Si, 0.04 mol, average particle size of 80 nm) were dispersed in deionized water (10 mL) with sodium linear alkylbenzenesulfonate by ultrasonication and magnetically stirred for 2 h. Then, the suspension and the former solution were mixed and agitated for 12 h to be used for the precursor of electrospinning for Si/PVA nanofibers.
A needle, with the inner diameter of 1 mm, was connected to a high voltage DC power and vertically clamped on an insulating glass stick. A piece of graphite paper was used as the collector. The height of the needle and the distance between the needle and the collector were adjustable. Typically, 20 kV of high positive voltage was adopted and the distance between the needle and collector was 15 cm. The precursor solution was electrospun into fiber networks at a constant flow rate of 1.0 mL h−1. Then the Si/PVA nanofibers fabric was dipped into resol (thermosetting phenolic resin, molecule weight = 2080)/ethanol solution for 5 seconds to obtain the Si/PVA-Resol composites (abbreviated as Si/PVA-Resol-
The morphology and microstructure of samples were characterized using a scanning electron microscope (SEM, LEO 1530, Germany) and a transmission electron microscopy (TEM, JEOL 2010, Japan). Raman spectroscopy (Renishaw Invia RM200, England) was employed to study the structure of Si/PVA nanofibers. X-ray powder diffraction (XRD, Rigaku D/Max 2500PC, Japan) was used to characterize the crystal structures of the Si/PVA nanofibers and hollow Si/C composites. Thermogravimetric analysis (TGA) was performed to ascertain the carbon yield and Si content of samples on a TA instrument SDT-Q600 with temperature increments of 10°C min−1 in Ar and air atmosphere. The specific surface area of samples was evaluated by N2 adsorption measurement on a Belsorp Max apparatus (Japan) and determined by Brunauer-Emmett-Teller (BET) method.
The electrochemical performance was characterized by galvanostatic cycling and cyclic voltammetry at room temperature in a half cell, in which lithium foil was used as the counter electrode and 1 M LiPF6 was dissolved in a mixture of ethyl carbonate (EC) and dimethyl carbonate (DMC) (1 : 1, v/v) as the electrolyte. The samples were used as working electrode directly without adding any nonactive materials such as polymer binders or electronic conductors. Celgard 2400 was used as separator. The test cells were galvanostatically cycled between 0.01 V and 1.5 V versus Li+/Li to evaluate the electrochemical performance (LAND battery tester, Wuhan Jinnuo Electronics Co., Ltd.). Cyclic voltammetry was measured between 0.01 V and 1.50 V versus Li+/Li at a scan rate of 0.1 mV s−1 by an electrochemical workstation Im6ex (ZAHNER, Germany).
The strategy to design and fabricate the silicon-encapsulated hollow carbon nanofiber networks is schematically illustrated in Figure
(a) Schematic illustration of the formation of the hollow Si/C nanofiber networks; (b) SEM image of Si/PVA nanofibers; (c) a macrograph of the Si/PVA-Resol composites after solidification and hollow Si/C composites (inset is SEM image of Si/PVA-Resol-3).
Magnified SEM images of hollow Si/C composites, (a) H-Si/C-1, (b) H-Si/C-3, (c) H-Si/C-5, and (d) H-Si/C-10. TEM images of hollow Si/C composites, (e) H-Si/C-1, (f) H-Si/C-3, (g) H-Si/C-5, and (h) H-Si/C-10 (insets in (g) and (h) are HRTEM images of H-Si/C-5 and H-Si/C-10, resp.).
Figures
The carbon yield and silicon content of the hollow Si/C composites were characterized by TGA. As shown in Figure
The physical and electrochemical properties of different hollow Si/C composite nanostructures.
Sample | Si content (%) | SSAa (m2 g−1) | Theoretical capacity (mAh g−1) | 1st cycle reversible capacity (mAh g−1) | 1st cycle CEb (%) | 50th cycle reversible capacity (mAh g−1) |
---|---|---|---|---|---|---|
H-Si/C-1 | 17.3 | 177.3 | 1181 | 913 | 48 | 567 |
H-Si/C-3 | 13.0 | 273.7 | 1025 | 841 | 58 | 745 |
H-Si/C-5 | 9.3 | 332.8 | 889 | 626 | 50 | 652 |
H-Si/C-10 | 8.0 | 274.8 | 842 | 579 | 54 | 493 |
bThe columbic efficiency (CE).
Thermogravimetric analysis (TGA) profiles of different hollow Si/C composites in (a) Ar and (b) air atmosphere.
The electrochemical behaviors of hollow Si/C composites were evaluated using deep galvanostatic charge/discharge cycles and cyclic voltammetry (CV) from 0.01 to 1.50 V. The collected free-standing silicon-encapsulated hollow carbon nanofiber networks were directly used as anodes in LIBs, without using copper current collector and adding any other polymeric binders or conductive additives. It could remarkably facilitate the high-speed electron transport, hold great potential to enhance the electrochemical performance, simplify the preparation process of electrode, and reduce the cost. The cycling performance of hollow Si/C composites at a charge/discharge current density of 100 mA g−1 is illustrated in Figure
(a) Cycling performance of hollow Si/C composites; (b) cyclic voltammograms, (c) charge/discharge curves, and (d) rate performance of H-Si/C-3.
These results indicate that hollow Si/C composites, especially H-Si/C-3, arouse the electrochemical potential of Si efficiently as well as improving the structural stability of the Si-containing anode materials. H-Si/C-1 shows the highest theoretical capacity and initial reversible capacity but a worse cycling performance than H-Si/C-3, H-Si/C-5, and H-Si/C-10. The 50th cycle discharge capacity of H-Si/C-1 was 567 mAh g−1, yielding capacity retention only about 62%. It is because the tubular nanostructure of H-Si/C-1 was crumpled up due to lack of resol, becoming solid Si/C nanofibers, thus no residual space around the silicon nanoparticles for expansion during charge-discharge process. In a word, the concentration of dip solution has an optimum value for hollow Si/C composites. If the concentration of dip solution is too high, it will cause the decrease of Si weight percentage and the increase of the carbon wall thickness accordingly. On the contrary, the decrease of the dip solution concentration will lead to the collapse of the tubular nanostructure due to the lack of enough resol. Therefore, we can effectively coordinate the structure and Si content of hollow Si/C composites by merely varying the concentration of dip solution to obtain good electrochemical performance.
Figure
The outstanding electrochemical properties of silicon-encapsulated hollow carbon nanofiber networks can be attributed to its unique 3D interconnected tubular hollow nanostructure encapsulated silicon nanoparticles with ambient empty space. Firstly, the hollow Si/C composites electrode contained plenty of void spaces around Si nanoparticle, which can effectively prevent damage of whole electrode by allowing Si nanoparticle to expand freely without mechanical constraint during lithiation process, enabling a stable electrochemical cycle performance and excellent rate performance. Secondly, the triaxial interconnected conducting nanofibrous networks constituted by carbon tubes and lack of insulating binders can remarkably enhance the conductivity of electrode, which will ensure a fast electron transfer for rapid Faradic reaction, and shorten the ionic transport length as well. Lastly, the encapsulation with hollow carbon nanofibers defends silicon nanoparticles against direct contact with the electrolyte; thus stable SEI can be obtained during cycling.
Silicon-encapsulated hollow carbon nanofiber networks with plenty of space around the Si nanoparticles were successfully prepared by dip-coating resol on the surface of electrospinning Si/PVA nanofibers and the subsequent solidification and carbonization processes. By simply varying the concentration of dip solution, we can effectively tune the structure and Si content of hollow Si/C composites and acquire prominent electrochemical performance. As binder-free anodes for LIBs, H-Si/C-3 shows a high capacity of 841 mAh g−1 in the first cycle, prominent rate capability, and excellent cycling stability. The discharge capacity retention of H-Si/C-3 was ~90%, with 745 mAh g−1 after 50 cycles. It demonstrates that the hollow Si/C composites are very promising as alternative anode candidates for high-performance LIBs. Furthermore, this study also provides a new insight that the triaxial interconnected tubular hollow nanostructure is a good solution for other LIBs electrodes that suffer from volume expansion or unstable interfacial properties with electrolytes, such as Sn and metal oxides anodes.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank the financial supports from the National Natural Science Foundation of China (no. 51232005) and 973 program of China (no. 2014CB932401). This work is also supported by Ministry of Science and Technology (MOST) of China under Grant 2010DFA72760, Collaboration on cutting-edge technology development of electric vehicle.