Atomic-Scale Control of Silicon Expansion Space as Ultrastable

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Atomic-Scale Control of Silicon Expansion Space as Ultrastable Battery Anodes Jian Zhu,†,§ Tao Wang,†,§ Fengru Fan,‡ Lin Mei,† and Bingan Lu*,† †

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry, School of Physics and Electronics, Hunan University, Changsha 410082, People’s Republic of China ‡ Department of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, People’s Republic of China S Supporting Information *

ABSTRACT: Development of electrode materials with high capability and long cycle life are central issues for lithiumion batteries (LIBs). Here, we report an architecture of three-dimensional (3D) flexible silicon and graphene/ carbon nanofibers (FSiGCNFs) with atomic-scale control of the expansion space as the binder-free anode for flexible LIBs. The FSiGCNFs with Si nanoparticles surrounded by accurate and controllable void spaces ensure excellent mechanical strength and afford sufficient space to overcome the damage caused by the volume expansion of Si nanoparticles during charge and discharge processes. This 3D porous structure possessing built-in void space between the Si and graphene/carbon matrix not only limits most solidelectrolyte interphase formation to the outer surface, instead of on the surface of individual NPs, and increases its stability but also achieves highly efficient channels for the fast transport of both electrons and lithium ions during cycling, thus offering outstanding electrochemical performance (2002 mAh g−1 at a current density of 700 mA g−1 over 1050 cycles corresponding to 3840 mAh g−1 for silicon alone and 582 mAh g−1 at the highest current density of 28 000 mA g−1). KEYWORDS: atomic scale, flexible lithium-ion batteries, graphene/carbon nanofibers, silicon, ultrastable anodes ion transport.22,23 Many works have focused on improving the electrode stability for LIBs to a few tens up to even hundreds of cycles with relatively high capacity retention.22−34 However, it is still far from the desired cycle life needed for practical applications. Three-dimensional flexible silicon and graphene/carbon nanofibers (3D FSiGCNFs) with precise control of the expansion space were prepared by atomic layer deposition (ALD) (to prepare the Si/NiO composite), electrospinning (to prepare the Si/NiO/graphene/PAN nanofiber web), calcination (to prepare the Si/NiO/graphene/carbon nanofiber web), washing and calcination (to remove NiO and form void space) as a flexible anode for high-performance LIBs. The expansion space of Si NPs was controlled to more than 3.2 times the volume of single Si NPs, indicating there is sufficient space for Si NPs to overcome the damage caused by the volume expansion of Si NPs during charge and discharge processes. The 3D FSiGCNF electrode demonstrated a reversible capacity as high as 2002 mAh g−1 at a current density of 700 mAh g−1 over 1050 cycles (corresponding to 3840 mAh g−1 for silicon

L

ithium-ion batteries (LIBs) are ubiquitous in portable personal electronics and grid storage due to their relatively high discharge voltage/energy density and good power performance.1−9 There is great interest in the pursuit of electrode materials with high theoretical capacities to replace the current state-of-the-art graphite anode. Among all, silicon-based anodes are the most appealing alternative with an outstanding theoretical capacity of 4200 mAh g−1 (forming Li4.4Si in full lithiation state) and low discharge potentials (the average delithiation voltage of Si is 0.4 V).10−21 However, electrode cycle life is mainly limited due to cracking and pulverization caused by the large volume change (as high as 311%) during charge and discharge processes.22,23 Although numerous nanostructured silicon materials including micro/ nanotubes, silicon nanoparticle/carbon composites, nanoporous silicon, and nanowires have been proposed to improve the cycle stability of silicon anodes, it is still a great challenge to fabricate Si-based electrodes with free expansion space for Si nanoparticles (NPs).10−13,17−21 Another critical factor in limiting a long cycle life of Si-based electrodes is formation of an unstable solid-electrolyte interphase (SEI) on the electrode’s surface. If the SEI layer deforms or breaks, in the next charge process, formation a fresh SEI on the electrode surface is needed and may lead to a battery with poor Coulombic efficiency, where the accumulated SEI can block Li© 2016 American Chemical Society

Received: July 7, 2016 Accepted: July 27, 2016 Published: July 27, 2016 8243

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Figure 1. Synthesis process and electrode design of the 3D FSiGCNFs. (a) Illustration of the synthesis process of the 3D FSiGCNFs. The 3D FSiGCNFs were first synthesized by ALD, followed by electrospinning, annealed at 450 °C in Ar atmosphere to obtain the Si/NiO/graphene/ carbon composite, and soaked in hydrochloric acid solution to remove the NiO to form precise and controllable expansion space and annealed at 800 °C in Ar atmosphere for 2 h. (b) Schematic diagram of the 3D FSiGCNF electrode design. Rationally designed FSiGCNFs with precise control of the expansion space by ALD, followed by electrospinning, were used as protection of Si NPs for flexible and binderfree lithium-ion batteries. The graphene/carbon matrix with excellent mechanical strength and electron transport properties not only achieves a superfast electron transfer but also provides enough space to buffer the volume changes of Si NPs during the lithium insertion and extraction reactions. In addition, the stable SEI forms outside of the graphene/carbon matrix rather than on the surface of Si NPs, which is attributed to the electrolyte being blocked by the hierarchical porous graphene/carbon matrix, while facilitating lithium transport throughout the whole structure. The 3D FSiGCNFs can keep the overall morphology, and the SEI outside the graphene/carbon matrix is not ruptured and remains thin after deep electrochemical cycles. Illustration of (c) electron transmission and (d) Li+ storage in the 3D FSiGCNF film.

center hollow spheres were generated within the graphene/ carbon matrix after carbonization in Ar atmosphere, washed in HCl solution, and calcined. When completely dried, the 3D flexible, binder-free electrodes were directly used as the working anodes for electrochemical measurements.

alone), demonstrating an average of only 0.006% decrease per cycle compared with its initial capacity. More importantly, the porous graphene/carbon matrix with excellent electron and lithium-ion transport properties offers outstanding rate performance (582 mAh g−1 at the highest current density of 28 000 mA g−1).

RESULTS AND DISCUSSION Figure 1a is a schematic illustration of the synthetic process of 3D FSiGCNFs. Si/NiO NPs with a 16 nm thick NiO layer were prepared by ALD. The volume ratio of void space and single Si NPs is calculated to be about 3.4, indicating sufficient space for Si NPs to expand without rupturing the 3D electrode structure during cycling (eqs 1−4). In a typical electrospinning procedure, a precursor of polyacrylonitrile (PAN), Si/NiO NPs, and graphene in N,N-dimethylformamide (DMF) were fabricated into a nanofiber web. PAN was carbonized, and off-

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4 4 π (a + r1)3 ≈ π (16 + 25)3 3 3

(1)

Vs =

4 3 4 πr1 ≈ π 253 3 3

(2)

Vv = Vt − Vs =

4 4 π (a + r1)3 − πr13 3 3

(3)

V − Vs Vv (16 + 25)3 − 253 = t ≈ ≈ 3.4 Vs Vs 253 8244

(4)

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Figure 2. Morphology of the 3D FSiGCNFs. (a) Digital image of the 3D FSiGCNF fabric after calcination reveals excellent flexibility of the fabric. (b,c) Top-view FESEM images of the 3D FSiGCNF film at different magnifications and (d) side-view FESEM images of the 3D FSiGCNF film, clearly showing a 3D porous cross-linked microstructure with a thickness of 50 μm. (e) TEM of the 3D FSiGCNF film. The Si NPs possessing off-center hollow spheres embed in the graphene/carbon matrix. (f) Diameter distribution of the 3D FSiGCNF film. (g−j) TEM mapping of the 3D FSiGCNF films.

where Vt, Vs, and Vv are the total volume of a single Si/NiO nanoparticle, a Si nanoparticle, and void space, respectively; a denotes the thickness of nickel oxide, and r1 refers to the radius of the Si nanoparticle. Figure 1b is an electrode design of the 3D FSiGCNFs; such a design has multiple advantages: (1) sufficient space for the Si NPs in a graphene/carbon matrix allows Si NPs to expand without rupturing the overall morphology of 3D FSiGCNFs, which greatly improves the cycle stability of the electrode during deep electrochemical cycles; (2) the graphene/carbon matrix with large interspace and porosity can limit most SEI formation to the outer surface instead of on the surface of individual NPs and limit the amount of SEI and prevent fracture of the matrix; (3) the graphene/carbon nanofiber matrix with excellent mechanical strength and electron transport properties acts as 1D conductive paths for fast transfer of electrons and achieves superfast charge transfer, facilitating the cyclic performance at high current densities (see Figure 1c); (4) the 3D porous architecture shortens the transportation pathway for Li+ diffusion, increases electrode− electrolyte contact area in this system, and allows Li+ to permeate into the innermost region of the electrode, achieving a high utilization rate of electrode materials, resulting in the large storage capacity of 3D FSiGCNF electrodes (see Figure 1d); (5) the 3D binder-free FSiGCNF electrode without polymer binders and conductive carbon additives has an intimate electric contact with the metallic current collector, achieving high-energy densities of LIBs. Digital image of the 3D FSiGCNF fabric over 20 cm2 is schematically illustrated in Figure 2a. The output of the products can be freely controlled according to our requirement because of the facile method to yield high output. Obviously, the fabric with very good flexibility can maintain perfect

integrity even under strong bending. Figure 2b,c presents the top-view field-emission scanning electron microscope (FESEM) images of the net-like 3D FSiGCNF film at different magnifications. The ultralong nanofibers with an average diameter of 130 nm cross-link to form a fibrous mat film. As seen from the side-view SEM image of the film (Figure 2d), the 3D porous cross-linked film with a thickness of about 50 μm can be clearly observed. Besides, Supplementary Figure S1 reveals the Si/carbon nanofibers (SiCNFs) and Si NPs with an average diameter of 130 and 50 nm, respectively. Transmission electron microscopy (TEM) is employed to further evaluate the microstructure of the 3D FSiGCNF film. A typical TEM image of the nanofiber structure is presented in Figure 2e. As shown in the area indicated by a red dot, the graphene nanosheets uniformly embed in the nanofiber matrix and extend to outside of the matrix. A close view of the single nanofiber from the 3D FSiGCNF sample clearly shows that the surface of the single nanofiber is generally smooth with many Si NPs possessing offcenter hollow spheres embedded in the graphene/carbon nanofiber framework. It is interesting to note that these offcenter hollow spheres derived from removal of the NiO layer play an important role in accommodating the volume change of the Si NPs and avoiding disruption of the microstructure of the 3D FSiGCNF electrode. Supplementary Figure S2 displays similar morphology of 3D FSiGCNFs possessing off-center hollow spheres with different expansion distances (0, 5, and 25 nm). The EDS mapping images of the 3D FSiGCNFs clearly show that Si NPs possessing off-center hollow spheres with an expansion distance of 16 nm are well-distributed throughout the graphene/carbon matrix (Figure 2g−j). Phase and purity of the as-prepared 3D FSiGCNFs, SiCNF films, and pure Si NPs were investigated by X-ray diffraction (XRD) patterns (see Figure 3a). Most of the peaks can be 8245

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Figure 3. Structural characterization of the 3D FSiGCNFs. (a) XRD of 3D FSiGCNFs, SiCNFs, and pure Si NPs. (b) TGA curve for the 3D FSiGCNF fabric in atmospheric environment. (c) Nitrogen adsorption/desorption isotherms and the corresponding Barrett−Joyner− Halenda distributions (inset) of 3D FSiGCNF nanocomposites. (d) Raman spectra of 3D FSiGCNFs, SiCNFs, and pure Si NPs.

a good crystallinity of the carbon, which facilitates fast transfer of electrons through the whole graphene/carbon matrix. Coin cells are fabricated using 3D FSiGCNF films as the anode and Li foil as the counter electrode without binder and conductive material to evaluate the electrochemical lithium storage performance. Supplementary Figure S3a shows that the nanofiber fabrics maintain perfect integrity after electrospinning (yellow), preoxidation (dark yellow), and calcination (black). The round platelets with excellent flexibility were directly used as flexible anode for lithium-ion batteries (see Supplementary Figure S3b). Figure 4a shows a cyclic voltammogram of the 3D FSiGCNF film electrode at a scan rate of 0.5 mV s−1 over a voltage range of 0.01−2.00 V versus Li/Li+, which shows typical electrochemical characteristics of Si-based electrodes with electrochemical lithiation of silicon.35,36 As presented in Figure 4b and Supplementary Figure 4Sa,b, discharge−charge profiles of 3D FSiGCNF films, SiCNFs, and Si NP electrodes at a current density of 700 mA g−1 within the voltage range of 0.01−2.00 V versus Li+/Li. The 3D FSiGCNF electrode displays the first discharge and charge capacities of 2143 and 1565 mAh g−1, with an initial Coulombic efficiency of 78.6%. In addition, an irreversible capacity is observed for the three different electrodes, which is mainly due to formation of an SEI layer, as well as the consumption of lithium by the defects in the graphene/carbon matrix (for 3D FSiGCNFs and SiCNFs).35 The structural design allows 3D FSiGCNF anodes to achieve significantly improved performance compared to previous studies.37−41 For comparison, the cyclic performances of the electrodes based on pristine SiCNFs as well as pure Si NPs (see Methods section for their fabrication details) at the current density of 700 mA g−1 are exhibited in Figure 4c, as well. A very fast capacity fading of the Si NP electrode can be observed from the beginning. The SiCNF electrode shows a slightly improved

readily indexed to crystalline silicon, suggesting the high crystal quality of the Si NPs before lithiation. Compared to pure Si NPs, a peak at 26.4° in the XRD pattern of the 3D FSiGCNF film corresponds to the interlayer distance of graphite, which indicates partially graphitized carbon and a well-ordered graphene/carbon matrix. Thermogravimetric analysis (TGA) was used to quantify the mass percentage of Si in 3D FSiGCNFs. As presented in Figure 3b, the 3D FSiGCNF film has a large mass loss of 53% mainly between 350 and 550 °C, corresponding to the oxidation of the graphene/carbon matrix in the sample. The gradually regained mass above 650 °C is attributed to oxidation of the Si NPs because Si NPs are stable up to 600 °C. The overall percentage of Si in 3D FSiGCNFs could be easily determined to be about 47%. Nitrogen adsorption and desorption isotherms were used to evaluate the surface area of the 3D FSiGCNF film (Figure 3c). The Brunauer−Emmett−Teller surface area of 3D FSiGCNFs was calculated to be 65.3 m2 g−1 with the pore diameter in the range of 0−60 nm. The high surface area of 3D FSiGCNFs suggests our synthetic strategy can also effectively improve the utilization of the electrode materials. Such a large surface area compared to ordinary nanofibers is favorable to shorten the diffusion path for the lithium to flux cross the electrolyte− electrode interface. As exhibited in Figure 3d, a clear peak at 508 cm−1 in the Raman spectra of 3D FSiGCNFs, SiCNFs, and pure Si NPs is assigned to first-order Raman scattering from optic phonons of Si−Si stretching motions of Si. The small peak at 958 cm−1 is due to the stretching mode of amorphous Si−Si. Besides, two additional characteristic peaks are located at 1337 and 1577 cm−1 in the Raman spectrum of 3D FSiGCNFs and SiCNFs compared to that of pure Si NPs, corresponding to the D band (disordered band) and G band (graphite band) of carbon, respectively. The small ratio of the peak intensity (ID/ IG) for 3D FSiGCNFs compared with that of SiCNFs indicates 8246

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Figure 4. Electrochemical properties of the 3D FSiGCNFs: All the specific capacity values reported in this paper are based on the total mass of the 3D FSiGCNFs without additional explanation. (a) CV curves of the 3D FSiGCNF fabric electrode obtained at the first, second, fifth, and 10th cycles. (b) Charge and discharge curves of the 3D FSiGCNF electrode from 2.0 to 0.01 V (vs Li+/Li) obtained at a current density of 700 mA g−1 in the first, second, 100th, 200th, and 500th cycles. (c) Cycle performance of 3D FSiGCNFs, SiCNFs, and pure Si NP electrodes at a current density of 700 mA g−1. Coulombic efficiency for the 3D FSiGCNF electrode is exhibited, as well. Cycle performance of the 3D FSiGCNF electrode normalized by the mass of Si NPs at a current density of 700 mA g−1 is also presented. (d) Cycle performance of the 3D FSiGCNF electrode as well as Coulombic efficiency at high current density of 1400 mA g−1. Cycle stability of the 3D FSiGCNF electrode normalized by mass of Si NPs at a current density of 1400 mA g−1 is also displayed. (e) Rate performances of 3D FSiGCNF and SiCNF electrodes at different current densities.

cycle stability with a reversible capacity of 436 mA h g−1 after 230 cycles, which may be mainly associated with a graphitic carbon matrix capable of enhancing the electrical conductivity of the whole electrode. It is worth noting that the resulting 3D FSiGCNF electrode delivers a significant cycle stability (0.006% decay per cycle) for nearly 8 months and high reversible capacity of 2002 mAh g−1 at a current density of 700 mA g−1 after 1050 cycles, corresponding to a quite high capacity (3840 mAh g−1) for silicon alone. The large storage capacity for silicon alone may be attributed to extra space for lithium intercalation/deintercalation provided by the nanopores in the graphene/carbon matrix and the interior nanocavities.42 It worth noting that the capacitance experienced an increase and then was kept relatively stable, which should be partially attributed to the change in temperature during the 8 month cyclic test. It should be mentioned that Coulombic efficiency for our 3D FSiGCNF anode was more than 99% from the fourth cycle with an average Coulombic efficiency about

99.98% during the 1050 deep electrochemical cycles. What’s more, the 3D FSiGCNFs present an impressive cycle life with high reversible capacity of 1201 mAh g−1 at a high current density of 1400 mA g−1 over 2800 cycles, corresponding to 2135 mAh g−1 for silicon alone (see Figure 4d). The excellent cycle stability can be attributed to the structure containing off-center void space which mitigates both chemical and mechanical degradation during charge/discharge process, with the stable SEI layer forming out of the porous graphene/ carbon matrix. The stable SEI on the surface promotes the cycle life of 3D FSiGCNF electrodes. Usually, the SEI layer deforms and breaks when the Si NPs expand and contract, which is coupled to the formation of fresh SEI on the freshly exposed Si NP surface (see Supplementary Figure S5a,b). In general, SEI accumulates gradually and eventually prevents Li+ transport, leading to poor Coulombic efficiency. In this work, the well-defined internal void space derived from the removal of the NiO layer allows the Si NPs to expand without rupturing 8247

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Table 1. Summary of Capacitive Parameters of Previous Lithium-Ion Batteries Based on Si Compared to What Is Achieved in This Work electrode materials connected Cu−Si alloy nanotube a-Si/carbon nanofiber NW array double-walled Si−SiOx nanotube Si/SiOx nanowires Cu−Li2O@a-Si core−shell array Cu−Si−Al2O3 nanocable array interconnected Si NW Si NPs@CNF fabric nanostructured Si secondary clusters carbon-coated mesoporous Si porous 3D Si/C nanonet Si@C NWs Si NP@C Si NW/G/RGO Si@graphene 3D FSiGCNFs

conductive materials

current density (mA g−1)

capacity (mA h g−1)

free sodium alginate binder free PVDF free free free free PVDF

free carbon black free carbon black free free free free carbon black

36000 10000 24000 7200 4200 28000 21000 8400 16800

360 458 600 282 500 about 400 about 500 500 500

18 30 31 41 44 45 46 47 48

styrene butadiene rubber/sodium carboxymethyl cellulose PVDF free free PVDF CMC free

carbon black

17000

568

49

carbon black free free carbon black free free

21000 8000 9750 8400 12000 28000 14000 7000

358 700 721 500 700 582 761 886

50 51 52 53 54 this work

binders

refs

power tools. Figure 4e shows capacities versus cycle number at various charge/discharge current rates ranging from 280 to 28000 mA g−1. The 3D FSiGCNFs exhibit excellent rate capability and deliver a reversible capacity of 1862, 1583, 1240, 1028, 886, and 761 mAh g−1 with increasing current densities of 280, 700, 1400, 2800, 7000, and 14 000 mA g−1, respectively. Particularly noteworthy is that reversible capacity of the electrodes can also reach 582 mAh g−1 (nearly 1.6 times the theoretical capacity of commercial graphite) when the highest current density reached 28 000 mA g−1. Furthermore, the electrode can also deliver a specific discharge capacity of about 1830 mA h g−1 after the current rate returns to 280 mA g−1. Excellent conductivity of 3D FSiGCNFs and low polarization resulted in high utilization of materials and excellent rate capability. As a result, the 3D FSiGCNF electrode displayed continuous capacities higher than that of SiCNFs at various current densities. The small D/G band peak intensity ratios of 3D FSiGCNFs compared with that of SiCNFs in the Raman spectra explains the excellent electrical conductivity of the electrode materials (see Figure 3d). In addition, the smaller resistance of the electrochemical system and diffusion resistance of the 3D FSiGCNF electrode compared to that of SiCNFs and pure Si NP electrodes suggest the dramatic performance of the electrode materials (see Supplementary Figure S9). Table 1 is the summary of the capacitive parameters of the 3D FSiGCNF electrode in comparison to the date in the available literature, indicating that the specific capacity under high-rate operation is one of the most challenging for promoting high-performance Si-based LIB applications.

the whole electrode structure. The entire secondary particle was encapsulated by the graphene/carbon matrix (see Figure 1b). Most SEI formed to the outer carbon surface instead of on individual NPs, limiting the amount of SEI and increasing its stability. As presented in Supplementary Figure S6a, the 3D FSiGCNF electrodes maintain a complete 3D structure without obvious morphology changes before and after 1050 cycles. Supplementary Figure S6b is the TEM image of the 3D FSiGCNF electrode after 1050 cycles. Apparently, the stable SEI with a thickness of about 35 nm encapsulates the whole nanofiber containing off-center void space between Si NPs and the graphene/carbon matrix, indicating the controlled volume expansion and pulverization of Si NPs. As shown in Supplementary Figure S7, the cycle performance of 3D FSiGCNF electrodes possessing off-center hollow spheres with different expansion distances is also investigated. They display worse cycle stabilities or lower specific capacities than 3D FSiGCNFs with a 16 nm expansion distance, indicating that the atomic-scale control of silicon expansion space is important to achieve ultrastable and high-capacity Si-based anodes for lithium-ion batteries. Besides, the transportation pathway for Li+ diffusion is shortened as a result of the 3D porous architecture. Numerous efforts have been devoted to the development of electrode materials with porous structure to meet the demand for energy storage.43 Electrode−electrolyte contact area is increased in this system, allowing Li+ to permeate into the innermost region of the electrode, leading to a high utilization rate of electrode materials. The 3D FSiGCNFs have a specific surface area of 65 m2 g−1 with a pore distribution range from 0 to 60 nm, including a micropore, a mesopore, and macropore (see Figure 3c). The 3D binderfree FSiGCNF electrode also afforded an intimate electric contact with the metallic current collector, leading to a sustained capacity higher than that of the pure Si/NiO electrode and 3D FSiGCNF electrode coated on copper foil (see Supplementary Figure s8a,b). Rate performance is an attractive battery feature, as it is important in practical applications of LIBs in electric vehicles or

CONCLUSIONS In conclusion, a silicon/graphene/carbon anode with atomic control of the expansion space has been designed by ALD and subsequent electrospinning as a flexible electrode material for superior Li-ion batteries. This 3D hybrid structure possesses built-in void space between Si and graphene/carbon components and porous structure throughout the whole graphene/carbon matrix, which limits the amount of SEI and 8248

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coated on copper foil and dried in a vacuum oven at 80 °C overnight. Lithium ribbon (Aldrich) and 1 M LiPF6 in ethylene carbonate and diethyl carbonate (1:1 v/v) were used as the counter electrode and electrolyte, respectively. Standard half-cells (CR2016-type coin cells) were assembled inside a glovebox with water and oxygen contents of less than 0.5 ppm with a Separion S240 P25 (Degussa) separator. Electrochemical experiments were performed in the voltage range of 0.01−2.0 V at room temperature with a computer-controlled battery tester system (Arbin BT-2000). An electrochemical workstation (Chen Hua Shanghai Corp., China) was used to perform cyclic voltammetry measurements over the potential range of 0.01−2.00 V (vs Li+/Li) at a scanning rate of 0.5 mV s−1. An ac voltage of 5 mV amplitude at a bias of 0 V was employed to conduct electrochemical impedance spectroscopy measurements at a frequency range of 0.01 Hz to 100 kHz.

increases its stability, allows Si NPs to expand freely without rupturing the whole electrode structure, and achieves highly efficient channels for the fast transport of both electrons and lithium ions during cycling. The rational and dimensional engineering combined with outstanding battery performance of the 3D FSiGCNF electrode, such as remarkable cycling stability (2002 mAh g−1 at current density of 700 mA g−1 after 1050 cycles, 1201 mAh g−1 at 1400 mA g−1 after 2800 cycles) and rate capability (582 mAh g−1 at 28 000 mA g−1), establishes this material as a very promising prospect in the field of energy storage.

METHODS Preparation of Si/NiO Nanocomposites. Si/NiO with a 16 nm thick NiO layer was obtained by spreading the Si nanoparticles with a size of about 50 nm into the reaction chamber and treated with ALD. The Si NPs were coated by NiO layers (5, 16, and 25 nm) in an ALD reactor with Ni(dmamp)2 and ozone as reactants. One NiO ALD cycle was defined as sequential exposure to Ni(dmamp)2 and ozone at 280 °C. Preparation of Si/NiO/Graphene/Carbon Nanocomposites. A mixture of PAN, Si/NiO nanocomposites, DMF, and graphene with a mass ratio of 50:30:500:1 was stirred with a magnetic stir bar overnight at 60 °C. For comparison, PAN, Si NPs, and DMF with a mass ratio of 50:20:500 were stirred overnight at 60 °C to prepare the precursor solution for Si/carbon nanofibers. The solution was sonicated for 120 min before being electrospun to obtain a homogeneous suspension. The precursor solutions were pumped through a 5 mL syringe and electrospun by applying a voltage of 15 kV using high-voltage dc power at a flow rate of 2 mL h−1 with a distance of 15 cm between the stainless steel nozzle (inner diameter is 0.5 mm) and the collector. The as-obtained films were dried for 24 h under vacuum conditions, followed by calcination at 450 °C (to prepare Si/ NiO/graphene/carbon) or 800 °C (to prepare SiCNFs) for 2 h in flowing argon environment at a ramp rate of 2 °C min−1. Preparation of the 3D FSiGCNFs. The resulting Si/NiO/ graphene/carbon sample was soaked in a 3 M HCl solution for 3 h to completely dissolve NiO components. Then, the film was washed with deionized water three times and dried under vacuum conditions at 80 °C for 2 h. The 3D FSiGCNFs were obtained by calcination at 800 °C for 2 h in flowing argon environment at a ramp rate of 2 °C min−1. Material Characterization. Morphology of the samples was characterized by FESEM (Hitachi S-4800, 5 kV) and a field-emission transmission electron microscope (JEM-2100F, 200 kV) operated in a scanning mode with a nominal analytical beam size of 0.5 nm. X-ray diffraction patterns of samples were characterized on a Rigaku D/MAX 2000 PC diffractometer operating at 40 kV and 25 mA using Cu Kα radiation (λ = 1.5406 Å). Raman spectrometry of samples was conducted by employing a LabRAM HR800 (HORIBA Jobin Yvon) confocal Raman spectrometer with an excitation laser wavelength of 514 nm. TGA was performed with a thermogravimetric analysis tool (PerkinElmer, Diamond TG/DTA) in air with a heating rate of 4 °C min−1 at a temperatures ranging from 120 to 850 °C. The 3D FSiGCNFs were first dried at 120 °C for 2 h to remove the moisture in the sample. Specific surface area and pore size distribution of the sample were measured by Brunauer−Emmett−Teller nitrogen adsorption−desorption (NOVA 2200e, Quanthachrome, USA) at 77 K and the adsorption branch of the nitrogen adsorption−desorption isotherm via the Barrett−Joyner−Halenda formula. Electrochemical Measurements. The obtained FSiGCNF and SiCNF films cut into round pieces with a diameter of 13 mm and weighed in a high-precision analytical balance (Sartorius, max weight 5100 mg, d = 0.001 mg) were used directly as the working electrode without any polymeric binder or conductive additives. As contrasts, the Si NPs, Si/NiO, and FSiGCNF films were mixed with poly(vinylidene fluoride) and carbon black in a mass ratio of 80:15:5. Then, the three different samples were milled for 30 min to form slurries which were

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b04522. Additional detals and supplementary figures (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions §

J.Z. and T.W. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (No. 21303046), the Research Fund for the Doctoral Program of Higher Education (No. 20130161120014), China Scholarship Council (File No. 201308430178), Hunan University Fund for Multidisciplinary Developing (No. 531107040762), and Hunan Provincial Innovation Foundation for Postgraduate (No. 521293041). REFERENCES (1) Bruce, P. G.; Scrosati, B.; Tarascon, J. M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem., Int. Ed. 2008, 47, 2930−2946. (2) Dudney, N. J.; Li, J. Using All Energy in a Battery. Science 2015, 347, 131−132. (3) Kang, B.; Ceder, G. Battery Materials for Ultrafast Charging and Discharging. Nature 2009, 458, 190−193. (4) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359−367. (5) Whittingham, M. S. Lithium Batteries and Cathode Materials. Chem. Rev. 2004, 104, 4271−4302. (6) Nam, K. T.; Kim, D. W.; Yoo, P. J.; Chiang, C. Y.; Meethong, N.; Hammond, P. T.; Chiang, Y. M.; Belcher, A. M. Virus-Enabled Synthesis and Assembly of Nanowires for Lithium Ion Battery Electrodes. Science 2006, 312, 885−888. (7) Lee, Y. J.; Yi, H.; Kim, W. J.; Kang, K.; Yun, D. S.; Strano, M. S.; Ceder, G.; Belcher, A. M. Fabricating Genetically Engineered HighPower Lithium-Ion Batteries Using Multiple Virus Genes. Science 2009, 324, 1051−1055. (8) Idota, Y.; Kubota, T.; Matsufuji, A.; Maekawa, Y.; Miyasaka, T. Tin-Based Amorphous Oxide: A High-Capacity Lithium-Ion-Storage Material. Science 1997, 276, 1395−1397. (9) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928−935. 8249

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DOI: 10.1021/acsnano.6b04522 ACS Nano 2016, 10, 8243−8251

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DOI: 10.1021/acsnano.6b04522 ACS Nano 2016, 10, 8243−8251