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nitrogen dual-doped graphene supported nano silicon as anode for Li-ion batteries
Accepted Manuscript Title: One-step synthesis of 3D sulfur/nitrogen dual-doped graphene supported nano silicon as anode for Li-ion batteries Authors: Ruihong Li, Junli Li, Kaiyu Qi, Xin Ge, Qiwei Zhang, Bangwen Zhang PII: DOI: Reference:
S0169-4332(17)32985-9 https://doi.org/10.1016/j.apsusc.2017.10.061 APSUSC 37404
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APSUSC
Received date: Revised date: Accepted date:
21-7-2017 20-9-2017 9-10-2017
Please cite this article as: Ruihong Li, Junli Li, Kaiyu Qi, Xin Ge, Qiwei Zhang, Bangwen Zhang, One-step synthesis of 3D sulfur/nitrogen dual-doped graphene supported nano silicon as anode for Li-ion batteries, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-step synthesis of 3D sulfur/nitrogen dual-doped graphene supported nano silicon as anode for Li-ion batteries Ruihong Li1, Junli Li1, Kaiyu Qi1, Xin Ge1, Qiwei, Zhang1, Bangwen Zhang1,2* 1
School of Material and Metallurgy, Inner Mongolia University of Science & Technology, Baotou 014010, China
2
Instrumental Analysis Center, Inner Mongolia University of Science & Technology, Baotou 014010, China
∗ Corresponding authors. E-mail addresses: [email protected] (B.Zhang)
Highlights
3D SN-G/Si anode composite was synthesized by one-step hydrothermal method.
The SN-G/Si composite anode exhibits high capacity, good cyclic and rate performance.
The improvement is attributed to the 3D SN-G as the superior conductive skeleton and flexible loader with well-distributed Si nanoparticles.
Abstract: Silicon is one of the most promising candidates for next-generation anode of Lithium-ion batteries. However, poor electrical conductivity and large volume change during alloying/dealloying hinder its practical use. Here we reported a three-dimensional (3D) nitrogen and sulfur codoped graphene supported silicon nanoparticles composite (SN-G/Si) through one-step hydrothermal self-assembly. The obtained SN-G/Si was investigated in term of instrumental characterizations and electrochemical properties. The results show that SN-G/Si as a freestanding anode in LIBs delivers a reversible capacity of 2020 mAh g-1 after 100 cycles with coulombic efficiency of nearly 97%. The excellent electrochemical performance is associated with the unique structure and the synergistic effect of SN-G/Si, in which SN-G provides volume buffer for nano Si as the flexible loader, short paths /fast 1
channels for electron /Li ion transport as porous skeleton, and low charge-transfer resistance.
Keywords:
Graphene; Dual-doped; nano silicon; hydrothermal self-assembly; lithium ion batteries
1. Introduction Lithium-ion batteries (LIBs) as clean source of energy have widely been applied in portable electric vehicles and hybrid electric vehicles due to their reusability, high energy density and longer life cycle [1-3]. To boost the development of pure electric vehicles and other high-tech industries, it is urgent to develop high-performance electrode materials for LIBs. Among all anode materials for LIBs, silicon (Si) stands out owing to high specific capacity (4200 mAh g-1), satisfactory potentials for lithium alloying/dealloying (0.02–0.6 V vs. Li/Li+), eco-friendliness and safety. However, the implementation of silicon anode has been hindered by its poor electrical conductivity and large volume change (~400%) during alloying/dealloying. These easily result in electrical contact fail and capacity fade of the anode [4, 5]. To overcome the above problems, two main strategies have been developed in last decade. One is the use of nanoscale Si (nanoparticles, nanowires, nanotubes, hollow nanospheres, etc.), aiming to restrict the volume expansion, and shorten the diffusion lengths of electrons and lithium ions [6-10]. Another is the coupling of Si with conductive buffers (amorphous carbon, carbon nanotubes and graphene etc.) [11-14]. Recently, graphene nanosheets, two-dimensional monolayer of graphitic carbon, have been utilized as flexible conductive enhancer of Si anode due to large surface area (2630 m2/g), high conductivity and rational Li insertion capacity. The studied results show nano Si loaded graphene composites not only could prevent the pulverization of Si and the formation of new 2
solid electrolyte interphase (SEI) [15-18], but also enhance the electronic contact within Si anode, so contribute to significant improvement in electrochemical performance [19-21]. A nanosized Si-based composite with monolayer graphene was fabricated by melt-self-assembly route with Cu film[11], which keeps a capacity of 1287 mAh g−1 with coulombic efficiency of 89% over 500 cycles. Liu et al [12] prepared carbon-coated Si nanoparticles/reduced graphene oxide multilayer with a specific capacity above 800 mAh g−1 over 350 cycles at a current density of 2.0 A g−1. Jiang et al[22] reported a free-standing Si/G paper, in which Si nanoparticles were synthesized by acid-etching Al-Si alloy powder. The freestanding electrode exhibits a reversible discharge capacity of 1500 mAh g-1 after 100 cycles at a current density of 100 mA g-1. Despite these achievements, a facile and fast access to 3D high-performance Si/graphene based anode of LIBs is challenging. Here we reported a facile synthesis of 3D S/N dual-doped graphene supported Si nanoparticles (SN-G/Si) composite through hydrothermal reduction of graphene oxide (GO) precursor mixed with nano Si using thiourea as the reductant and dopant. The obtained SN-G/Si, consists of 3D SN-G as the porous skeleton and dispersed nano Si as the electroactive species with improved interface properties, hence exhibits significant enhancement in electrochemical performance.
2. Expremental 2.1 Materials Natural flake graphite (300 mesh, Qingdao MeiliKun) was used to afford GO by a modified Hummer's method [23] and silicon nanoparticles (crystalline, Alfa Aesar) as the anode material. Concentrated sulfuric acid (96%~98%), hydrochloric acid, phosphorus pentoxide (P2O5), potassium permanganate (KMnO4), potassium persulfate (K2S2O8), H2O2 (30%), thiourea and other chemical reagents were supplied by Shanghai Aladdin Corp. All chemicals were of analytical reagent. 2.2 Synthesis of SN-G/Si composite 3
The schematic of the fabrication process for SN-G/Si composite is shown in Fig. 1. In a typical synthesis, 60 mg of Si nanoparticles were dispersed in 6 mL of 35mg mL-1 thiourea aqueous solution for 3 h. Then 5mL of 6 mg mL-1 GO solution was added and sonicated for 30 min. The resultant suspension was sealed in a Teflon-lined stainless steel autoclave and heated at 170 ºC for 10 h. Finally, the obtained hydrogel was washed repeatedly and freeze-dried to give the 3D SN-G/Si composite. It was cut into discs of about 16 mg before used as the freestanding anode of LIBs. For comparison, the G/Si sample was prepared by the same method in the absence of thiourea. Insert Fig. 1
2.3 Materials characterization The powder phases were identified by X-ray diffractometer (XRD, PANalytical, X’Pert PRO) with a Cu/Kα radiation. The elemental and bonding composition of samples was analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Analytical, Axis Ultra) with a monochromatic Al/Kα radiation and Raman spectroscopy (RS, Horiba Scientific, XploRA PLUS) with a 488nm laser. Thermal stability of samples was evaluated by a thermogravimetric analyzer (TGA, Q600). The morphology and microstructure of composites was investigated by scanning electronic microscope (SEM, HITACHI S4800) equipped with an energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscope (TEM, Tecnai G2 20). Specific surface area and micropore size were measured by a BET specific surface analyzer (Micromeritics Instrument Corp., ASAP 2020). 2.4 Electrochemical characterization To evaluate electrochemical properties, 3D SN-G/Si based CR2032 coin half-cells were assembled in a glove box filled with Ar air. For the cell, SN-G/Si discs was employed as working electrode, a 4
lithium foil as counter and reference electrode, a microporous PP film as separator, with an electrolyte of 1М LiPF6 dissolved in a mixture of ethylene carbonate, diethyl carbonate, and dimethyl carbonate at the ratio of 1:1:1 by volume. An electrochemical workstation (Chenhua, CHI 660E) was utilized to measure cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with a frequency range from 100 kHz to 10 mHz. The galvanostatic charge/discharge (GCD) tests were carried out on a multichannel battery tester (LAND, CT2001A).
3. Results and discussion The XRD patterns of nano Si, GO, SN-G and SN-G/Si were given in Fig.2a. The sharp peaks of Si and SN-G/Si samples at 28.5°, 47.52°, 56.2°, 69.1°, and 76.4° are assigned to (111), (220), (311), (400) and (331) planes of crystalline Si (JCPDS, No. 027-1402) [24]. GO and SN-G exhibit characteristic (001) and (002) peaks [20], respectively, while N, S codoping has no effect on the crystal structure of graphene. In RS spectra (Fig.2b), two sharp peaks at about 1351 and 1582 cm-1 correspond to D band and G band, respectively. In general, D band represents the defects and disorders in graphic structure, while G band indicates sp2 hybridization of carbon network [25]. The value of ID/IG (1.14) of SN-G/Si is higher than that of G/Si (0.83), suggesting that the insertion of S and N into the carbon network of graphene introduces more structural defects. 2D band located at 2678 cm-1 indicates the presence of graphene in G/Si and SN-G/Si. Further, the N2 adsorption–desorption tests were carried out to investigate the texture properties of the as-prepared G/Si (Fig.S1) and SN-G/Si (Fig.2c). A typical characteristic N2 hysteresis loop (P/P0 =0.40), corresponding to the IV-type isothermal curve, was found in the N2 adsorption–desorption isotherm of SN-G/Si, featuring a mesoporous property. The BJH pore size (the inset of Fig.2c) and BET surface area of SN-G/Si are estimated to be 41 nm and 226 m2 g-1, respectively, while the 5
corresponding values of G/Si are 26 nm and 147 m2 g-1. This indicates S and N codoping of graphene leads to the generation of more mesopores and the increase of BET surface area for the 3D SN-G/Si, which is beneficial for the diffusion of Li ion [26]. Finally, TGA was conducted for G/Si and SN-G/Si, as shown in Fig.2d. SN-G/Si exhibits a weight loss about 42% below 600°C relative to G/Si due to the pyrolysis of graphene, suggesting the load of Si in SN-G/Si is about 58wt%. Insert Fig.2
XPS (Fig.3) was used to unveil surface chemical state of the materials. In the survey spectra (Fig. 3a), O, C and Si elements in G/Si and SN-G/Si arise from graphene and nano Si, while the additional N1s (163eV) and S2p (400eV) in SN-G/Si can be assigned to the doped S and N in SN-G, whose content is estimated about 2.36 at.% and 5.07 at.%, respectively. Fig.3b-d give the C1s, S2p and N1s spectra of SN-G/Si. The C1s spectrum (Fig.3b) is decomposed into four components centered at 284.6eV(C=C), 285.6eV(C-O/C-S/C-N), 287.3eV(C=O) and 289eV (O=C-O) [27]. In contrast, no C-S/C-N component was resolved in the C 1s spectrum of G/Si (Fig.S2a). The S 2p spectrum (Fig.3c) can be disassembled into five peaks at 162.1eV (sulfide), 163.6eV (S-S/S-C), 164.7 eV (S-C), 165.9eV (S-O) and 168.6eV (sulfate), respectively [27, 28], suggesting the S-doping into both the plane and edge of graphene [18, 29, 30]. The N 1s spectrum (Fig.3d) can be resolved into three peaks at 398.7eV (pyridinic-N), 399.6eV (pyrrolic-N) and 401.2eV (graphitic-N) [16]. Therefore, Fig.3c-d confirm that N and S heteroatoms have been successfully codoped in the framework of graphene. This kind of codoping could improve physiochemical properties of the graphene from three aspects [31]: (a) to generate more defects and active sites favorable for the sorption of Li ion by lowering the energy barrier [32-34], (b) to enhance the bond polarity and wettability of graphene in the electrolyte 6
solution [33, 35, 36], and (c) to introduce micro- and meso- pores in graphene nanosheets, thus increase the specific surface area [32, 37, 38]. Si 2p spectrum of SN-G/Si (Fig.S2b) can be divided into four peaks at 100.25eV (Si), 101.52eV (Si-Si), 102.67eV (Si-O), and 103.77eV (O-Si-O) [19, 20]. The latter bonds suggest that there exists small amount of passivated SiOx around the nano Si cores. Insert Fig.3
Fig.4 illustrates the typical SEM images (a-b) and TEM images (c-d) of SN-G/Si composite. The SEM image (Fig.4a) shows within SN-G/Si, SN-G nanosheets interconnect into 3D porous structure (as the conductive skeleton) filled with dispersed nano Si (as the electroactive spices). At high resolution (Fig.4b), 30~80 nm of nano Si are clearly observed. They are well-distributed over (denoted by the red arrow) or below (denoted by the blue arrow) the transparent graphene, or penetrate the macropores within 3D SN-G (denoted by the yellow arrows). This kind of hybrid structure not only facilities the transport of both electrons and electrolyte ions required for the fast electrochemical reaction, but also buffers the pulverization of electrode to keep good electrical contact during Si alloying/dealloying [14, 39], thus helps to enhance the energy storage. The elemental mappings (Fig.S3) show that C, S, N and Si elements are uniformly distributed in SN-G/Si, confirming the N and S codoping in SN-G. As observed in SEM image (Fig.4b), the TEM image (Fig.4c) shows similar shape, size and dispersity of nano Si. The high resolution TEM (Fig.4d) reveals the Si nanoparticle has a distinct core coated by 2~3 layers of graphene nanosheets, and its lattice spacing is about 0.31 nm, which corresponds to the (111) plane of crystalline Si. Insert Fig.4 Fig.5 evaluates the electrochemical properties of tested electrodes. Fig. 5a shows the CV curves of 7
SN-G/Si in the first three cycles, where the anodic peak at around 0.36 V corresponds to the voltage plateau of the charge process in which Li+ deintercalate from the electrode, whereas the cathodic peak at near 0 V presents the Li+ intercalating into the electrode during the discharge process (Lix Si ↔xLi+ +Si + xe- / LiC6↔ Li+ + 6C + e-) [25, 40, 41]. In the first cycle, the cathodic peak located at about 1.1 V may be attributed to the formation of the SEI layer on the surface of the electrode. Fig. 5b gives the corresponding GCD curves of the SN-G/Si at a current density of 400mAg-1. In these curves, the low plateaus correspond to the alloying/dealloying process. The charge capacity and coulomb efficiency in the first cycle are 2256.2 mAh g-1 and 77.41%, respectively. The high charge capacity can be ascribed to the short path for electron transport due to the employment of nano Si and 3D conductive skeleton, and the improved ion diffusion by the porous graphene, as observed in Fig.4. The irreversible capacity in the first cycle was associated with the decomposition of electrolyte and the forming of SEI film [18, 19, 28]. After the initial activation, the charge/discharge behavior gets stable, as a result the potential profiles in the subsequent cycles nearly overlap, indicating that the lithiation/delithiation is reversible. Fig. 5c compares the coulombic efficiency and cycling stability of G/Si and SN-G/Si within 100 circles of discharge /charge at 400 mA g-1. It suggests that SN-G/Si can deliver a higher discharge capacity than G/Si throughout the period of circling, and the difference between the two is up to 500 mAh g-1. Again, SN-G/Si exhibits more stable discharge performance, hence achieves a high coulombic efficiency of nearly 97 % after 100 cycles, while the same value for G/Si is 92.5%. The enhanced electrochemical performance of SN-G/Si is mainly ascribed to SN-G, in which N and S codoping enhances the chemisorption of Li ions on graphene by generating more defects and active sites [31, 32, 34], and improves the diffusion kinetics by increasing the wettability and specific surface area of graphene [32, 35, 36, 38]. Further, the rate 8
performance of G/Si and SN-G/Si is evaluated in Fig.5d. It shows that SN-G/Si can discharge stably as the current increase every 10 circles from 400 mA g-1 to 2000 mA g-1, and still deliver a high reversible capacity of 2213 mAh g-1 when the current density resets back to the initial level of 400 mA g-1, while the specific capacitance and stability of G/Si is less than that of SN-G/Si. Finally, Table 1 compared SN-G/Si with the reported Si-carbonaceous anodes in term of preparation method, electrode configuration and electrochemical properties. It suggests that our work is simple, efficient and economical.
Insert Fig.5 To further implore charge transfer of G/Si and SN-G/Si eletrodes, EIS measurements are carried out as the Nyquist plots are shown in Fig. 6a (G/Si) and Fig. 6b (SN-G/Si). The equivalent circuits before and after cycling are given in Fig.6c and Fig. 6d, respectively. In Fig.6a, the semicircle (black line) in the high frequency region is related to the charge-transfer resistance (Rct=60 Ω) of G/Si encountered at the interface between electrolyte and G/Si electrode, while the sloping straight line at low frequencies reflects Warburg impedance (W) within the solid electrodes. After 50 cycles, another semicircle (red line) appeared in the high frequency range, which should be caused by the SEI film formed on the surface of the active material during cycling, and the corresponding RSEI and Rct are about 30 Ω and 182 Ω, respectively. In Fig.6b, similar phenomena are observed for SN-G/Si. Before cycling the Rct is 39 Ω, which is lower than that of G/Si. After 50 cycles, the RSEI and Rct are 5 Ω and 56 Ω, respectively, still less than that of G/Si. We suggest the reduced Rct and RSEI for SN-G/Si is correlated with the improved chemisorption and wettability of SN-G, and hence responsible for the enhanced stability and rate capability of SN-G/Si as shown in Fig 5c-d. 9
Insert Fig.6 4. Conclusions In conclusion, we reported a type of 3D SN-G/Si composite and its one-step synthesis based on hydrothermal self-assembly. In the composite, the 3D SN-G exhibits hierarchical porous structure as a result of the heteroatoms codoping and hydrothermal self-assembly, and service as a conductive network for electron transport and the flexible loader of nano Si with highly accessible channels for electrolyte diffusion, while nano Si are well-distributed over graphene nanosheets, or penetrate the macropores within 3D SN-G. This unique structure enables the freestanding and the improvement in interface bonding and charge transport of SN-G/Si anode, and excellent electrochemical performances, such as high specific capacitance and good cycling stability, were obtained. Acknowledgement The authors are grateful of financial support by the National Research Foundation of China (No. 51164026), Natural Science Foundation of the Inner Mongolia autonomous region, china (Grant No.2015BS0512), and School of materials and metallurgy young talent incubator platform support project (Grant No.2014CY012). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at……
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Fig. 1. Schematic of the self-assembly process for SN-G/Si composite
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Fig. 2. (a) XRD patterns of the pure Si, GO, SN-G and SN-G/Si; (b) Raman spectra of the G/Si and SN-G/Si ; (c) N2 adsorption-desorption isotherm with the BJH pore distribution (the inset) of SN-G/Si and G/Si; (d) TGA of Si and SN-G/Si
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Fig. 3. XPS spectra (a) survey of G/Si and SN-G/Si, (b-d) C1s, S2p, and N1s of SN-G/Si
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Fig 4. (a-b) FESEM images and (c-d) TEM images of SN-G/Si
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Fig. 5. (a) Cyclic voltammograms of SN-G/Si at a scan rate of 0.3 mV/s; (b) Discharge-charge curves of the SN-G/Si at 400 mAg-1 within a voltage window of 0.01–3.0 V vs. Li + /Li; (c) Coulombic efficiency and cycling stability of G/Si and SN-G/Si at 400 mAg-1; (d) Rate capability evaluation of G/Si and SN-G/Si.
18
Fig.6. Nyquist plots of (a) G/Si and (b) SN-G/Si before (black line) and after (red line) cycling and the corresponding equivalent circuits used to fit the plot before (c) and after (d) cycling
19
Table 1 Comparison of Si-based anodes through various synthesis methods for LIBs . Cycling performances Specific capacity Products
free-standing
Current
anode
density
Method
(mAh/g)/
Cycle
Coulombic
number
Ref.
(A/g) efficiency SNP/14%CB/1%rGO
Emulsion templating method
Yes
0.2
1370/98.9%
50
[1]
Yes
0.1
425/90%
100
[10]
Vacuum filtration technique + Si/rGO/MWCNT
radio
frequency
magnetron
sputtering SNP@void@mG
CVD+Erosion
Yes
0.5
1287/89%
500
[11]
Si/rGO/C
Alternate deposition-annealing
Yes
2
800 /99.18%
350
[12]
Si-G-PANI
in-situ chemical polymerization
No
1
721.1/98
100
[13]
Si-NG
Facile spray drying
No
0.1
1152.9/98%
150
[17]
Si@rGO/CFP
Soaking and pyrolysis
Yes
1
364
500
[21]
SN-G/Si
One-step hydrothermal method
Yes
0.4
2020/97%
This 100 work Note: C (carbon), SNP (Si nanoparticle), mG(monolayer graphene), G (graphene), MWCNT (multiwalled carbon nanotube), rGO (reduced graphene oxide), NG (N-doped graphene), CB (carbon black), CFP (carbon fibers paper).
20
S0169-4332(17)32985-9 https://doi.org/10.1016/j.apsusc.2017.10.061 APSUSC 37404
To appear in:
APSUSC
Received date: Revised date: Accepted date:
21-7-2017 20-9-2017 9-10-2017
Please cite this article as: Ruihong Li, Junli Li, Kaiyu Qi, Xin Ge, Qiwei Zhang, Bangwen Zhang, One-step synthesis of 3D sulfur/nitrogen dual-doped graphene supported nano silicon as anode for Li-ion batteries, Applied Surface Science https://doi.org/10.1016/j.apsusc.2017.10.061 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-step synthesis of 3D sulfur/nitrogen dual-doped graphene supported nano silicon as anode for Li-ion batteries Ruihong Li1, Junli Li1, Kaiyu Qi1, Xin Ge1, Qiwei, Zhang1, Bangwen Zhang1,2* 1
School of Material and Metallurgy, Inner Mongolia University of Science & Technology, Baotou 014010, China
2
Instrumental Analysis Center, Inner Mongolia University of Science & Technology, Baotou 014010, China
∗ Corresponding authors. E-mail addresses: [email protected] (B.Zhang)
Highlights
3D SN-G/Si anode composite was synthesized by one-step hydrothermal method.
The SN-G/Si composite anode exhibits high capacity, good cyclic and rate performance.
The improvement is attributed to the 3D SN-G as the superior conductive skeleton and flexible loader with well-distributed Si nanoparticles.
Abstract: Silicon is one of the most promising candidates for next-generation anode of Lithium-ion batteries. However, poor electrical conductivity and large volume change during alloying/dealloying hinder its practical use. Here we reported a three-dimensional (3D) nitrogen and sulfur codoped graphene supported silicon nanoparticles composite (SN-G/Si) through one-step hydrothermal self-assembly. The obtained SN-G/Si was investigated in term of instrumental characterizations and electrochemical properties. The results show that SN-G/Si as a freestanding anode in LIBs delivers a reversible capacity of 2020 mAh g-1 after 100 cycles with coulombic efficiency of nearly 97%. The excellent electrochemical performance is associated with the unique structure and the synergistic effect of SN-G/Si, in which SN-G provides volume buffer for nano Si as the flexible loader, short paths /fast 1
channels for electron /Li ion transport as porous skeleton, and low charge-transfer resistance.
Keywords:
Graphene; Dual-doped; nano silicon; hydrothermal self-assembly; lithium ion batteries
1. Introduction Lithium-ion batteries (LIBs) as clean source of energy have widely been applied in portable electric vehicles and hybrid electric vehicles due to their reusability, high energy density and longer life cycle [1-3]. To boost the development of pure electric vehicles and other high-tech industries, it is urgent to develop high-performance electrode materials for LIBs. Among all anode materials for LIBs, silicon (Si) stands out owing to high specific capacity (4200 mAh g-1), satisfactory potentials for lithium alloying/dealloying (0.02–0.6 V vs. Li/Li+), eco-friendliness and safety. However, the implementation of silicon anode has been hindered by its poor electrical conductivity and large volume change (~400%) during alloying/dealloying. These easily result in electrical contact fail and capacity fade of the anode [4, 5]. To overcome the above problems, two main strategies have been developed in last decade. One is the use of nanoscale Si (nanoparticles, nanowires, nanotubes, hollow nanospheres, etc.), aiming to restrict the volume expansion, and shorten the diffusion lengths of electrons and lithium ions [6-10]. Another is the coupling of Si with conductive buffers (amorphous carbon, carbon nanotubes and graphene etc.) [11-14]. Recently, graphene nanosheets, two-dimensional monolayer of graphitic carbon, have been utilized as flexible conductive enhancer of Si anode due to large surface area (2630 m2/g), high conductivity and rational Li insertion capacity. The studied results show nano Si loaded graphene composites not only could prevent the pulverization of Si and the formation of new 2
solid electrolyte interphase (SEI) [15-18], but also enhance the electronic contact within Si anode, so contribute to significant improvement in electrochemical performance [19-21]. A nanosized Si-based composite with monolayer graphene was fabricated by melt-self-assembly route with Cu film[11], which keeps a capacity of 1287 mAh g−1 with coulombic efficiency of 89% over 500 cycles. Liu et al [12] prepared carbon-coated Si nanoparticles/reduced graphene oxide multilayer with a specific capacity above 800 mAh g−1 over 350 cycles at a current density of 2.0 A g−1. Jiang et al[22] reported a free-standing Si/G paper, in which Si nanoparticles were synthesized by acid-etching Al-Si alloy powder. The freestanding electrode exhibits a reversible discharge capacity of 1500 mAh g-1 after 100 cycles at a current density of 100 mA g-1. Despite these achievements, a facile and fast access to 3D high-performance Si/graphene based anode of LIBs is challenging. Here we reported a facile synthesis of 3D S/N dual-doped graphene supported Si nanoparticles (SN-G/Si) composite through hydrothermal reduction of graphene oxide (GO) precursor mixed with nano Si using thiourea as the reductant and dopant. The obtained SN-G/Si, consists of 3D SN-G as the porous skeleton and dispersed nano Si as the electroactive species with improved interface properties, hence exhibits significant enhancement in electrochemical performance.
2. Expremental 2.1 Materials Natural flake graphite (300 mesh, Qingdao MeiliKun) was used to afford GO by a modified Hummer's method [23] and silicon nanoparticles (crystalline, Alfa Aesar) as the anode material. Concentrated sulfuric acid (96%~98%), hydrochloric acid, phosphorus pentoxide (P2O5), potassium permanganate (KMnO4), potassium persulfate (K2S2O8), H2O2 (30%), thiourea and other chemical reagents were supplied by Shanghai Aladdin Corp. All chemicals were of analytical reagent. 2.2 Synthesis of SN-G/Si composite 3
The schematic of the fabrication process for SN-G/Si composite is shown in Fig. 1. In a typical synthesis, 60 mg of Si nanoparticles were dispersed in 6 mL of 35mg mL-1 thiourea aqueous solution for 3 h. Then 5mL of 6 mg mL-1 GO solution was added and sonicated for 30 min. The resultant suspension was sealed in a Teflon-lined stainless steel autoclave and heated at 170 ºC for 10 h. Finally, the obtained hydrogel was washed repeatedly and freeze-dried to give the 3D SN-G/Si composite. It was cut into discs of about 16 mg before used as the freestanding anode of LIBs. For comparison, the G/Si sample was prepared by the same method in the absence of thiourea. Insert Fig. 1
2.3 Materials characterization The powder phases were identified by X-ray diffractometer (XRD, PANalytical, X’Pert PRO) with a Cu/Kα radiation. The elemental and bonding composition of samples was analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Analytical, Axis Ultra) with a monochromatic Al/Kα radiation and Raman spectroscopy (RS, Horiba Scientific, XploRA PLUS) with a 488nm laser. Thermal stability of samples was evaluated by a thermogravimetric analyzer (TGA, Q600). The morphology and microstructure of composites was investigated by scanning electronic microscope (SEM, HITACHI S4800) equipped with an energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscope (TEM, Tecnai G2 20). Specific surface area and micropore size were measured by a BET specific surface analyzer (Micromeritics Instrument Corp., ASAP 2020). 2.4 Electrochemical characterization To evaluate electrochemical properties, 3D SN-G/Si based CR2032 coin half-cells were assembled in a glove box filled with Ar air. For the cell, SN-G/Si discs was employed as working electrode, a 4
lithium foil as counter and reference electrode, a microporous PP film as separator, with an electrolyte of 1М LiPF6 dissolved in a mixture of ethylene carbonate, diethyl carbonate, and dimethyl carbonate at the ratio of 1:1:1 by volume. An electrochemical workstation (Chenhua, CHI 660E) was utilized to measure cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with a frequency range from 100 kHz to 10 mHz. The galvanostatic charge/discharge (GCD) tests were carried out on a multichannel battery tester (LAND, CT2001A).
3. Results and discussion The XRD patterns of nano Si, GO, SN-G and SN-G/Si were given in Fig.2a. The sharp peaks of Si and SN-G/Si samples at 28.5°, 47.52°, 56.2°, 69.1°, and 76.4° are assigned to (111), (220), (311), (400) and (331) planes of crystalline Si (JCPDS, No. 027-1402) [24]. GO and SN-G exhibit characteristic (001) and (002) peaks [20], respectively, while N, S codoping has no effect on the crystal structure of graphene. In RS spectra (Fig.2b), two sharp peaks at about 1351 and 1582 cm-1 correspond to D band and G band, respectively. In general, D band represents the defects and disorders in graphic structure, while G band indicates sp2 hybridization of carbon network [25]. The value of ID/IG (1.14) of SN-G/Si is higher than that of G/Si (0.83), suggesting that the insertion of S and N into the carbon network of graphene introduces more structural defects. 2D band located at 2678 cm-1 indicates the presence of graphene in G/Si and SN-G/Si. Further, the N2 adsorption–desorption tests were carried out to investigate the texture properties of the as-prepared G/Si (Fig.S1) and SN-G/Si (Fig.2c). A typical characteristic N2 hysteresis loop (P/P0 =0.40), corresponding to the IV-type isothermal curve, was found in the N2 adsorption–desorption isotherm of SN-G/Si, featuring a mesoporous property. The BJH pore size (the inset of Fig.2c) and BET surface area of SN-G/Si are estimated to be 41 nm and 226 m2 g-1, respectively, while the 5
corresponding values of G/Si are 26 nm and 147 m2 g-1. This indicates S and N codoping of graphene leads to the generation of more mesopores and the increase of BET surface area for the 3D SN-G/Si, which is beneficial for the diffusion of Li ion [26]. Finally, TGA was conducted for G/Si and SN-G/Si, as shown in Fig.2d. SN-G/Si exhibits a weight loss about 42% below 600°C relative to G/Si due to the pyrolysis of graphene, suggesting the load of Si in SN-G/Si is about 58wt%. Insert Fig.2
XPS (Fig.3) was used to unveil surface chemical state of the materials. In the survey spectra (Fig. 3a), O, C and Si elements in G/Si and SN-G/Si arise from graphene and nano Si, while the additional N1s (163eV) and S2p (400eV) in SN-G/Si can be assigned to the doped S and N in SN-G, whose content is estimated about 2.36 at.% and 5.07 at.%, respectively. Fig.3b-d give the C1s, S2p and N1s spectra of SN-G/Si. The C1s spectrum (Fig.3b) is decomposed into four components centered at 284.6eV(C=C), 285.6eV(C-O/C-S/C-N), 287.3eV(C=O) and 289eV (O=C-O) [27]. In contrast, no C-S/C-N component was resolved in the C 1s spectrum of G/Si (Fig.S2a). The S 2p spectrum (Fig.3c) can be disassembled into five peaks at 162.1eV (sulfide), 163.6eV (S-S/S-C), 164.7 eV (S-C), 165.9eV (S-O) and 168.6eV (sulfate), respectively [27, 28], suggesting the S-doping into both the plane and edge of graphene [18, 29, 30]. The N 1s spectrum (Fig.3d) can be resolved into three peaks at 398.7eV (pyridinic-N), 399.6eV (pyrrolic-N) and 401.2eV (graphitic-N) [16]. Therefore, Fig.3c-d confirm that N and S heteroatoms have been successfully codoped in the framework of graphene. This kind of codoping could improve physiochemical properties of the graphene from three aspects [31]: (a) to generate more defects and active sites favorable for the sorption of Li ion by lowering the energy barrier [32-34], (b) to enhance the bond polarity and wettability of graphene in the electrolyte 6
solution [33, 35, 36], and (c) to introduce micro- and meso- pores in graphene nanosheets, thus increase the specific surface area [32, 37, 38]. Si 2p spectrum of SN-G/Si (Fig.S2b) can be divided into four peaks at 100.25eV (Si), 101.52eV (Si-Si), 102.67eV (Si-O), and 103.77eV (O-Si-O) [19, 20]. The latter bonds suggest that there exists small amount of passivated SiOx around the nano Si cores. Insert Fig.3
Fig.4 illustrates the typical SEM images (a-b) and TEM images (c-d) of SN-G/Si composite. The SEM image (Fig.4a) shows within SN-G/Si, SN-G nanosheets interconnect into 3D porous structure (as the conductive skeleton) filled with dispersed nano Si (as the electroactive spices). At high resolution (Fig.4b), 30~80 nm of nano Si are clearly observed. They are well-distributed over (denoted by the red arrow) or below (denoted by the blue arrow) the transparent graphene, or penetrate the macropores within 3D SN-G (denoted by the yellow arrows). This kind of hybrid structure not only facilities the transport of both electrons and electrolyte ions required for the fast electrochemical reaction, but also buffers the pulverization of electrode to keep good electrical contact during Si alloying/dealloying [14, 39], thus helps to enhance the energy storage. The elemental mappings (Fig.S3) show that C, S, N and Si elements are uniformly distributed in SN-G/Si, confirming the N and S codoping in SN-G. As observed in SEM image (Fig.4b), the TEM image (Fig.4c) shows similar shape, size and dispersity of nano Si. The high resolution TEM (Fig.4d) reveals the Si nanoparticle has a distinct core coated by 2~3 layers of graphene nanosheets, and its lattice spacing is about 0.31 nm, which corresponds to the (111) plane of crystalline Si. Insert Fig.4 Fig.5 evaluates the electrochemical properties of tested electrodes. Fig. 5a shows the CV curves of 7
SN-G/Si in the first three cycles, where the anodic peak at around 0.36 V corresponds to the voltage plateau of the charge process in which Li+ deintercalate from the electrode, whereas the cathodic peak at near 0 V presents the Li+ intercalating into the electrode during the discharge process (Lix Si ↔xLi+ +Si + xe- / LiC6↔ Li+ + 6C + e-) [25, 40, 41]. In the first cycle, the cathodic peak located at about 1.1 V may be attributed to the formation of the SEI layer on the surface of the electrode. Fig. 5b gives the corresponding GCD curves of the SN-G/Si at a current density of 400mAg-1. In these curves, the low plateaus correspond to the alloying/dealloying process. The charge capacity and coulomb efficiency in the first cycle are 2256.2 mAh g-1 and 77.41%, respectively. The high charge capacity can be ascribed to the short path for electron transport due to the employment of nano Si and 3D conductive skeleton, and the improved ion diffusion by the porous graphene, as observed in Fig.4. The irreversible capacity in the first cycle was associated with the decomposition of electrolyte and the forming of SEI film [18, 19, 28]. After the initial activation, the charge/discharge behavior gets stable, as a result the potential profiles in the subsequent cycles nearly overlap, indicating that the lithiation/delithiation is reversible. Fig. 5c compares the coulombic efficiency and cycling stability of G/Si and SN-G/Si within 100 circles of discharge /charge at 400 mA g-1. It suggests that SN-G/Si can deliver a higher discharge capacity than G/Si throughout the period of circling, and the difference between the two is up to 500 mAh g-1. Again, SN-G/Si exhibits more stable discharge performance, hence achieves a high coulombic efficiency of nearly 97 % after 100 cycles, while the same value for G/Si is 92.5%. The enhanced electrochemical performance of SN-G/Si is mainly ascribed to SN-G, in which N and S codoping enhances the chemisorption of Li ions on graphene by generating more defects and active sites [31, 32, 34], and improves the diffusion kinetics by increasing the wettability and specific surface area of graphene [32, 35, 36, 38]. Further, the rate 8
performance of G/Si and SN-G/Si is evaluated in Fig.5d. It shows that SN-G/Si can discharge stably as the current increase every 10 circles from 400 mA g-1 to 2000 mA g-1, and still deliver a high reversible capacity of 2213 mAh g-1 when the current density resets back to the initial level of 400 mA g-1, while the specific capacitance and stability of G/Si is less than that of SN-G/Si. Finally, Table 1 compared SN-G/Si with the reported Si-carbonaceous anodes in term of preparation method, electrode configuration and electrochemical properties. It suggests that our work is simple, efficient and economical.
Insert Fig.5 To further implore charge transfer of G/Si and SN-G/Si eletrodes, EIS measurements are carried out as the Nyquist plots are shown in Fig. 6a (G/Si) and Fig. 6b (SN-G/Si). The equivalent circuits before and after cycling are given in Fig.6c and Fig. 6d, respectively. In Fig.6a, the semicircle (black line) in the high frequency region is related to the charge-transfer resistance (Rct=60 Ω) of G/Si encountered at the interface between electrolyte and G/Si electrode, while the sloping straight line at low frequencies reflects Warburg impedance (W) within the solid electrodes. After 50 cycles, another semicircle (red line) appeared in the high frequency range, which should be caused by the SEI film formed on the surface of the active material during cycling, and the corresponding RSEI and Rct are about 30 Ω and 182 Ω, respectively. In Fig.6b, similar phenomena are observed for SN-G/Si. Before cycling the Rct is 39 Ω, which is lower than that of G/Si. After 50 cycles, the RSEI and Rct are 5 Ω and 56 Ω, respectively, still less than that of G/Si. We suggest the reduced Rct and RSEI for SN-G/Si is correlated with the improved chemisorption and wettability of SN-G, and hence responsible for the enhanced stability and rate capability of SN-G/Si as shown in Fig 5c-d. 9
Insert Fig.6 4. Conclusions In conclusion, we reported a type of 3D SN-G/Si composite and its one-step synthesis based on hydrothermal self-assembly. In the composite, the 3D SN-G exhibits hierarchical porous structure as a result of the heteroatoms codoping and hydrothermal self-assembly, and service as a conductive network for electron transport and the flexible loader of nano Si with highly accessible channels for electrolyte diffusion, while nano Si are well-distributed over graphene nanosheets, or penetrate the macropores within 3D SN-G. This unique structure enables the freestanding and the improvement in interface bonding and charge transport of SN-G/Si anode, and excellent electrochemical performances, such as high specific capacitance and good cycling stability, were obtained. Acknowledgement The authors are grateful of financial support by the National Research Foundation of China (No. 51164026), Natural Science Foundation of the Inner Mongolia autonomous region, china (Grant No.2015BS0512), and School of materials and metallurgy young talent incubator platform support project (Grant No.2014CY012). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at……
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Fig. 1. Schematic of the self-assembly process for SN-G/Si composite
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Fig. 2. (a) XRD patterns of the pure Si, GO, SN-G and SN-G/Si; (b) Raman spectra of the G/Si and SN-G/Si ; (c) N2 adsorption-desorption isotherm with the BJH pore distribution (the inset) of SN-G/Si and G/Si; (d) TGA of Si and SN-G/Si
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Fig. 3. XPS spectra (a) survey of G/Si and SN-G/Si, (b-d) C1s, S2p, and N1s of SN-G/Si
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Fig 4. (a-b) FESEM images and (c-d) TEM images of SN-G/Si
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Fig. 5. (a) Cyclic voltammograms of SN-G/Si at a scan rate of 0.3 mV/s; (b) Discharge-charge curves of the SN-G/Si at 400 mAg-1 within a voltage window of 0.01–3.0 V vs. Li + /Li; (c) Coulombic efficiency and cycling stability of G/Si and SN-G/Si at 400 mAg-1; (d) Rate capability evaluation of G/Si and SN-G/Si.
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Fig.6. Nyquist plots of (a) G/Si and (b) SN-G/Si before (black line) and after (red line) cycling and the corresponding equivalent circuits used to fit the plot before (c) and after (d) cycling
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Table 1 Comparison of Si-based anodes through various synthesis methods for LIBs . Cycling performances Specific capacity Products
free-standing
Current
anode
density
Method
(mAh/g)/
Cycle
Coulombic
number
Ref.
(A/g) efficiency SNP/14%CB/1%rGO
Emulsion templating method
Yes
0.2
1370/98.9%
50
[1]
Yes
0.1
425/90%
100
[10]
Vacuum filtration technique + Si/rGO/MWCNT
radio
frequency
magnetron
sputtering SNP@void@mG
CVD+Erosion
Yes
0.5
1287/89%
500
[11]
Si/rGO/C
Alternate deposition-annealing
Yes
2
800 /99.18%
350
[12]
Si-G-PANI
in-situ chemical polymerization
No
1
721.1/98
100
[13]
Si-NG
Facile spray drying
No
0.1
1152.9/98%
150
[17]
Si@rGO/CFP
Soaking and pyrolysis
Yes
1
364
500
[21]
SN-G/Si
One-step hydrothermal method
Yes
0.4
2020/97%
This 100 work Note: C (carbon), SNP (Si nanoparticle), mG(monolayer graphene), G (graphene), MWCNT (multiwalled carbon nanotube), rGO (reduced graphene oxide), NG (N-doped graphene), CB (carbon black), CFP (carbon fibers paper).
20