Metal-Embedded Porous Graphitic Carbon Fibers Fabricated from

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Metal-Embedded Porous Graphitic Carbon Fibers Fabricated from Bamboo Sticks as a Novel Cathode for Lithium-Sulfur Batteries Xuqing Zhang, Yu Zhong, Xinhui Xia, Yang Xia, Donghuang Wang, Cheng'ao Zhou, Wangjia Tang, Xiuli Wang, J. B. Wu, and Jiangping Tu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02504 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Applied Materials & Interfaces

Metal-Embedded Porous Graphitic Carbon Fibers Fabricated from Bamboo Sticks as a Novel Cathode for Lithium-Sulfur Batteries

Xuqing Zhang,† Yu Zhong,† Xinhui Xia,*,† Yang Xia,‡ Donghuang Wang,† Cheng’ao Zhou,† Wangjia Tang,† Xiuli Wang,*,† J. B. Wu,§ and Jiangping Tu * †

†

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials

and Applications for Batteries of Zhejiang Province, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡

College of Materials Science and Engineering, Zhejiang University of Technology,

Hangzhou 310014, China §

School of Physics and Electronic Engineering, Taizhou University, Taizhou 318000,

China

Corresponding Authors *E-mail: [email protected] (X.W.) *E-mail: [email protected]; [email protected] (J.T.). *E-mail: [email protected] (X.X.). .

KEYWORDS: lithium-sulfur battery, bamboo, porous graphitic carbon fiber, Ni nanoparticle, electrochemical property 1

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ABSTRACT: Lithium-sulfur batteries (LSBs) are deemed to be among the most prospective next-generation advanced high-energy batteries. Advanced cathode materials fabricated from biological carbon are becoming more popular due to their unique properties. Inspired by the fibrous structure of bamboo, herein we put forward a smart strategy to convert bamboo sticks for barbecue into uniform bamboo carbon fibers (BCF) via a simple hydrothermal treatment proceeded in alkaline solution. Then NiCl2 is used to etch the fibers through a heat treatment to achieve Ni-embedded porous graphitic carbon fibers (PGCF/Ni) for LSBs. The designed PGCF/Ni/S electrode exhibits improved electrochemical performances including high initial capacity (1198 mAh g−1 at 0.2 C), prolonged cycling life (1030 mAh g−1 at 0.2 C after 200 cycles), and improved rate capability. The excellent properties are attributed to the synergistic effect of 3D porous graphitic carbon fibers with highly conductive Ni nanoparticles embedded.

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1. INTRODUCTION Recently, lithium-sulfur batteries (LSBs) have attracted ever-increasing attention by virtue of their high theoretical capacity (1675 mAh g−1) and specific energy density (2600 Wh kg−1),1-4 which are five times higher than those of lithium-ion batteries (LIBs) built on conventional insertion-host metal oxides (e.g., LiCoO2, LiFePO4, LiMn2O4,

Li(NiCoMn)O2)

cathodes.5-8

Moreover,

sulfur

possesses

many

overwhelming advantages, such as environmental friendliness, nontoxicity, natural abundance, and low cost.9-12 However, LSBs are puzzled by several drawbacks impeding their practical application. The insulating nature of sulfur and sulfides results in ultralow sulfur utilization, and the high solubility of lithium polysulfides (Li2Sn, 4≤ n≤ 8) gives rise to severe capacity diminution with cycling.13-16 To date, massive efforts have been made to overcome the above limitations of LSBs, wherein the most promising approach is to accommodate sulfur with carbon materials owing to their good electrical conductivity and high sulfur-adsorption properties.17-20 However, most researches of LSBs cathodes are confined in the chemosynthetic carbon materials, such as hierarchical porous carbon, graphene, carbon black and carbon nanotubes.21-24 In recent years, their applications are drastically weakened due to the fatal drawbacks of high cost, tedious preparation technology and uncertain performance. Learning from nature is an approach of progress in human society all the time.25 To solve the universal problems of resource waste, energy consumption and environmental pollution in the modern world, advanced functional carbon fabricated from biological materials are becoming more popular.26-28 Typically, plants 3



(e.g., bamboo, wood, cotton, loofah sponge) with unique microstructures are brought into focus of excellent carbon sources for the fabrication of high-performance energy storage materials.29-32 Bamboo is the vernacular for members of a particular taxonomic group of large woody grasses, constantly playing a role as a multipurpose problem-solver of suiting a broad range of applications.33 Many species of bamboo have strong, light and flexible woody stems, which serve as construction materials for domestic building, furniture, musical instruments, and so on. Due to its anatomically fibrous structure, bamboo is considerably ideal for paper and textiles production industrially. Additionally, bamboo fibers are artificially converted into carbon fibers for energy storage applications, such as LIBs and supercapacitors.34-35 Unfortunately, there is still few reports on the LSBs application for bamboo to date. Hence, we report a smart strategy to convert bamboo sticks for barbecue into uniform carbon fibers for LSBs application. The separated and dispersed cellulose fibers are well prefabricated through a simple hydrothermal treatment proceeded in alkaline solution. Then the cotton-like cellulose fibers are calcined to be carbonized. The yielded carbon fibers still cannot satisfy the demand of the high-performance LSBs, and need to be modified elaborately. In most cases, alkalis (e.g., K2CO3, KOH) serve as pore-forming agents to react with carbon to obtain porous structure.36-37 In addition, metal salts and oxides (e.g., ZnCl2, ZnO) are also adopted as pore-forming agents to achieve porous nanostructure by embedding metal nanoparticles into the carbon matrix through a conversion reaction, yielding integral porous carbon/metal 4


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composites.38-39

Large

specific

surface

area

caused

by

the

introduced

micro/mesopores provides adequate space to accommodate sulfur, in addition, high electrical conductivity and active sites arouse from metal nanoparticles are beneficial to enhance reaction kinetics and hinder the dissolution of lithium polysulfides. On account of this deliberation, we develop a facile and effective pore-forming strategy by utilizing nickel chloride to etch the bamboo carbon fibers to achieve Ni-embedded porous graphitic carbon fibers and explore their potentials in LSBs application.

2. EXPERIMENTAL SECTION 2.1 Preparation of PGCF/Ni composite The bamboo sticks for barbecue produced by Zhejiang Suncha Bamboo and Wood Co., LTD were used as raw materials. The dried bamboo sticks were originally whittled into bamboo shavings using a utility knife. Then 2 g bamboo shavings were put into a stainless-steel autoclave within a Teflon liner containing 70 mL KOH solution (3 mol L−1). The autoclave was sealed and held at 150 °C for 12 h. After that, the samples were collected by vacuum filtration, washed by deionized water for several times and dried at 60 °C for 24 h. Next, the cotton-shaped products were soaked in 0.25 mol L−1 NiCl2 alcoholic solution for 30 min and fetched out to dried naturally. Last, the products were annealed at 800 °C for 2 h in Ar atmosphere to form the final PGCF/Ni composite. For comparison, BCF was also prepared as the same process without NiCl2 treatment.

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2.2 Preparation of PGCF/Ni/S Composite The PGCF/Ni/S composite was prepared via supercritical CO2 fluid method. In details, 1.0 g PGCF/Ni composite and 3.0 g sulfur powder were put into a 100 mL stainless-steel milling jar. Then CO2 was pumped into the milling jar until the pressure reached 8.5 MPa. The milling jar operated on a planetary ball mill at 250 rpm in the constant ambient temperature of 32 °C. The CO2 was released immediately and the as-prepared composite was collected after milling for 12 h. For comparison, the BCF/S composite was prepared under the same conditions. 2.3 Material characterization The morphologies of samples were observed using a field emission scanning electron microscope (SEM, Hitachi S4800) and a field emission transmission electron microscope (TEM, JEOL 2100F). X-ray diffraction (XRD) patterns were characterized by using a Rigaku D/Max-2550PC X-ray diffractometer (Cu Kα radiation). Raman spectra were performed by using an argon laser Raman spectroscope (Laber Raman Series, HR-800) with 514 nm excitation laser. X-ray photoelectron spectroscopy (XPS) was conducted by using an ESCALAB 250Xi photoelectron

spectrometer

(Al

Kα

X-ray).

Brunauer-Emmett-Teller

(BET,

Autosorb-1-C) tests were performed to calculate the specific surface area and pore size. Thermogravimetric analysis (TGA) measurements were carried out on a Netzsch STA 449C thermal analyser from room temperature to 500 °C at a heating rate of 10 °C min−1 in N2 atmosphere.

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2.4 Electrochemical characterization The electrode slurry was prepared by mixing PGCF/Ni/S composite with polyvinylidene difluoride (PVDF) binder and conductive carbon black (Cabot 2000) at the weight ratio of 8: 1: 1 in N-methyl-2-pyrrolidone (NMP). Then the slurry was coated onto aluminium foil and dried in a vacuum oven at 60 °C for 24 h. CR-2025 coin-type cells were assembled to perform electrochemical measurements in an argon-filled glove box. The electrolyte was 1.0 M bis-(trifluoromethane) sulfonimide lithium salt (LiTFSI) in a 1: 1 (V/V) mixture of 1,2-dimethoxyehane (DME) and 1,3-dioxolane (DOL) containing 1.0 wt.% LiNO3. Lithium metal foil was used as the counter and reference electrodes, and polypropylene microporous film (Cellgard 2300) was used as the separator. The mass loading of sulfur on each electrode was about 1.5 mg cm−2. And the ratio of electrolyte to sulfur (E/S) is 20 µL mg-1. Cyclic voltammetry (CV) tests were performed on a CHI 660D electrochemical workstation (Shanghai, China) in the potential range of 1.7−2.8 V (vs. Li/Li+) at a scan rate of 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were conducted on a PARSTAT MC potentiostat/galvanostat (Princeton Applied Research, Co., LTD) in the frequency range from 100 kHz to 10 mHz by applying an AC signal of 5 mV. The discharge/charge profiles and cyclability were measured on a LAND battery program-control test system (Wuhan, China) in a voltage range of 1.7−2.8 V.

3. RESULTS AND DISCUSSION Bamboo with unique fibrous structure usually serves as good carbon source for the 7



fabrication of high-performance active material suiting all-purpose applications.35 Whereas, microscopic observations on bamboo carbon reveal that numerous oriented cellulose fibers are bound together with the ligneous matrix (Figure S1), leading to their limited use. Herein, we employ a facile and valid strategy to develop bamboo sticks for barbecue into separated BCF, and the entire fabrication process of BCF is illustrated by Figure 1a schematically. Bamboo sticks are initially dried and peeled into shavings followed by hydrothermal treatment in alkaline solution for delignification. The cotton-like cellulose fibers are further heat-treated to obtain individual carbon fibers. The optical photographs veritably exhibit the macroscopic features of bamboo sticks, bamboo shavings, cellulose fibers and carbon fibers (Figure 1b). Figure S2 reveals the diameter distribution for a representative set of yielded carbon fibers, mostly ranging from 2.5 to 7 µm with the largest distribution between 4 and 5 µm in the BCF. The SEM images demonstrate that the fibrous samples are homogeneously dispersed (Figure 1c, d), possessing a wrinkled solid texture and an ultrahigh aspect ratio to hundreds or even thousands. The average diameter of the single fiber is merely several microns whereas its maximum length can reach a few millimeters. And the cross section of the fiber shows considerably smooth and flat. The TEM image further verifies the solid texture of the BCF with no holes or cavities (Figure 1e). In addition, the amorphous character of the BCF is further verified by the SAED pattern (the inset in Figure 1e). Next, we adopt a simple and effective pore-forming method by utilizing NiCl2 to etch the BCF to achieve Ni-embedded porous carbon fibers (PGCF/Ni). After metal 8


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etching, the PGCF/Ni composite favorably maintains fibrous morphology (Figure 2a). The entire fibers are uniformly decorated by vast Ni nanoparticles ranging from 20 to 50 nm and numerous interconnected micro/ mesopores both inside and outside (Figure 2b). Compared with the bare BCF, the cross section of the PGCF/Ni composite becomes more rugged (Figure 2c). The microstructure of the PGCF/Ni composite is further characterized by the TEM observations (Figure 2d, e). The product is filled with interconnected pores, and Ni particles with diameters of 10−50 nm are firmly embedding in the porous carbon frameworks, in accordance with the SEM analysis. The high-resolution TEM image forcefully illustrates the excellent crystallinity of Ni nanoparticles (Figure 2f). The distinct lattice spacing (~ 0.20 nm) is corresponding to the (111) crystal face of fcc Ni.40 The above plausible pore formation may be expounded as follows: 10nNiCl2 (s) + 2(C6H10O5)n (s) → 10nNi (s) + 7nC (s) + 20nHCl (g) + 5nCO2 (g).41 When NiCl2 is reacting with (C6H10O5)n (cellulose) in the high temperature condition, partial carbon is consumed to yield CO2 to fluff out the host, meanwhile vast corrosive gas HCl is generated to further etch the cellulose. Ultimately, plenty of interconnected cavities within reductive Ni nanoparticles are left behind. The porosities of PGCF/Ni and BCF are performed by BET measurements in Figure S3. No hysteresis exists in the nitrogen adsorption and desorption isotherms of the bare BCF. In contrast, the PGCF/Ni composite exhibits a typical type-IV curve with an obvious hysteresis, indicating the presence of mesoporous pores. The pore size distribution peak of the PGCF/Ni composite is at approximate 3.8 nm, and the 9



interconnected nanopores increase the specific surface area to 170.5 m2 g−1, superior to the BCF counterpart (11.7 m2 g−1). The high porosity and large surface area in the PGCF/Ni composite not only supply more space of sulfur loading but also benefit to rapid ion transfer during the electrochemical reactions. The detailed phases and crystallographic structures of the PGCF/Ni and BCF powders are verified by XRD and Raman spectrum. In Figure 3a, the XRD pattern of the BCF composite includes a strong broad peak at 23° and a weak one at 43°, corresponding to the (002) and (100) crystal planes of carbon, respectively.42 As a contrast, the (002) peak of carbon in the PGCF/Ni composite becomes much narrower and shift right, revealing the graphitization degree of the inner carbon is much higher. In addition, strong characteristic peaks of Ni are observed in the PGCF/Ni composite.43 When the carbohydrates in bamboo fibers decompose into amorphous carbon, the amorphous carbon around the Ni particles is easily transformed into graphitic carbon owing to the catalytic effect of the Ni. A graphitic G band at 1577 cm−1 and a disordered D band at 1347 cm−1 are found in the BCF (Figure 3b), demonstrating that the amorphous carbon fibers are partially graphitized.44 In the PGCF/Ni composite, both G and D bands keep the same positions, but the integrated intensity IG/ID ratio of 2.05 is almost twice of that of the BCF (IG/ID = 1.03), indicating the higher graphitization degree of the inner carbon,45 in accordance with the XRD result. The surface compositions of the samples are investigated by XPS analysis. The survey scan spectrum of the PGCF/Ni composite reveals the existence of Ni, C and O elements (Figure 3c). As shown in Figure 3d, the Ni 2p spectrum 10


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contains one peak of Ni (853.3 eV), indicating the presence of metal nickel.41 For comparison, only C and O peaks are clearly observed in the BCF. As shown in Figure 3e, the C 1s spectrum of the BCF can be deconvoluted into sp2-C (284.6 eV) and sp3-C (285.3 eV), while that of the PGCF/Ni composite only contains sp2-C, ascribed to the higher order degree of the graphitic carbon.14 The O 1s spectra of the both samples include C−O (531.7 eV) and C=O (532.8 eV) (Figure 3f), indicating the presence of oxygen functionalities.46 Moreover, dilute HCl solution was used to remove the Ni nanoparticles in the PGCF/Ni composite via a hydrothermal method. According to the decrement of weight, the content of Ni in PGCF/Ni is calculated to be ~15 wt.%. In our case, sulfur is poured into the PGCF/Ni and BCF via a facile supercritical CO2 fluid method to work as cathodes for LSBs. The PGCF/Ni/S composite still presents the morphology of uniform fibers and the sulfur is well dispersed in the Ni-embedded porous materials (Figure 4a, b). In addition, the energy dispersive spectrometer (EDS) elemental mapping further confirms the homogeneous distribution of S in the PGCF/Ni/S composite (Figure 4c). The TEM image reveals that the micro/mesopores in the PGCF/Ni/S composite are fully filled with active sulfur as well (Figure 4d). Whereas, in the BCF/S composite (Figure S4), the carbon fibers and sulfur are thoroughly separated and the sulfur even assembles to big blocks, and the surface of carbon fibers is quite smooth without sulfur loading. For the PGCF/Ni/S composite (Figure 4e), the peaks of S remain the same positions, moreover, none of new phases has formed during the synthesis process.47 Meanwhile, 11



the broad peaks of C and the characteristic peaks of Ni in the PGCF/Ni/S composite also stay the same as those in the PGCF/Ni composite. The XPS analysis of the PGCF/Ni/S composite also verifies the metallic state of the Ni nanoparticles (Figure S5). Ultimately, thermogravimetric analysis (TGA) curves are applied to calculate the actual contents of sulfur in the samples (Figure 4f). Obviously, we can calculate that the sulfur contents of the PGCF/Ni/S and BCF/S composite are 68 and 60 wt.%, respectively. When the CO2 is released immediately in the supercritical CO2 fluid method of loading sulfur, the BCF composite can’t supply enough space to accommodate sulfur because of its non-porous structure, with much sulfur adhering to the milling jar. Note that the existence of numerous interconnected pores in the PGCF/Ni composite enhances the loading content of sulfur effectively. The electrochemical properties of PGCF/Ni/S cathode are thoroughly evaluated, and the BCF/S composite is also measured in comparison. Figure 5a exhibits the CV curves of the PGCF/Ni/S electrode for the first three cycles at a scan rate of 0.1 mV s−1 between 1.7 and 2.8 V (vs. Li/Li+). In the cathodic scans, two reduction peaks at 2.27 and 2.05 V are induced by the specific transformations of sulfur to long-chain lithium polysulfides (Li2Sn, 4≤ n≤ 8) and then to short-chain polysulfides (Li2S2, Li2S). In the anodic scans, one oxidation peak at 2.39 V corresponds to the reverse processes.48-50 Compared with the BCF/S electrode (Figure 5b), the PGCF/Ni/S electrode delivers larger peak intensities, overlap ratio and smaller peak separation, indicating higher utilization of sulfur, better reaction reversibility and faster reaction kinetics due to the metal-embedded graphitic porous structure with high electrical 12


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conductivity. Meanwhile, the CV curves of the BCF/S electrode express apparent decline and broadening, which is caused by the loss of active material and serious polarization in the early cycles. Figure 5c shows the charge/discharge profiles of the PGCF/Ni/S and BCF/S electrodes for the first cycle at 0.2 C. Both the electrodes reveal typical charge and discharge plateaus in accordance with the CV analysis. However, the voltage interval of 0.15 V between the charge/discharge plateaus of the PGCF/Ni/S electrode is lower than that of the BCF/S electrode (0.22 V), demonstrating better activity and less polarization of PGCF/Ni/S cell. Subsequently, rate capability of the PGCF/Ni/S electrode is tested in a voltage range of 1.7−2.8 V, and the BCF/S electrode is also measured under the same conditions for comparison (Figure 5d). On cycling at different current densities of 0.2, 0.5, 1, 2 and 5 C, the PGCF/Ni/S electrode delivers average discharge capacities of 1128, 1006, 906, 842 and 746 mAh g−1, respectively. When the current density is switched to 0.2 C abruptly, the discharge capacity is still recovered to 1068 mAh g−1. As a contrast, the BCF/S electrode reveals inferior rate capability with rapid capacity fading (1019 mAh g−1 to 774 mA h g−1) in the first 10 cycles at 0.2 C and miserably maintains 188 mAh g−1 at 5 C. Unfortunately, the capacity continues to descend fastly after the current density returns back to 0.2 C. Impressively, after pore-forming by NiCl2, the PGCF/Ni/S cathode exhibits much high specific capacity and capacity retention, especially at high-rate charge/discharge process. The promotions on PGCF/Ni/S electrode are further supported by the EIS analysis (Figure 5g). Based on the semicircle and intercept on the X-axis in the Nyquist plots, the PGCF/Ni/S 13



electrode has much lower charge-transfer resistance and Warburg impedance than the BCF/S electrode. The much smaller semicircle at the high frequency region reveals that the Ni nanoparticles embedded porous graphitic carbon fibers improve the charge transfer at the electrode-electrolyte interface and reduce the inner resistance of the PGCF/Ni/S electrode.51 At the low frequency region, the more vertical straight line implies better and faster ion diffusion paths of the PGCF/Ni/S electrode.52 Additionly, long cycle performance of the PGCF/Ni/S and BCF/S electrodes is evaluated (Figure 5e). An intial discharge capacity of 1198 mAh g−1 is obtained at 0.2 C for the PGCF/Ni/S electrode, which is much larger than that of the BCF/S electrode (1014 mAh g−1). Along with the following cycles, the capacity of the PGCF/Ni/S electrode degrades slightly and retains 1030 mAh g−1 (~86%) after 200 cycles, corresponding to a low degradation rate of 0.07% per cycle. Whereas, the contrast electrode only delivers the capacity of 447 mAh g−1 (~40%). Moreover, high-rate long cycling life of the PGCF/Ni/S and BCF/S electrodes are investigated (Figure 5f). After activation, the PGCF/Ni/S electrode delivers an initial discharge capacity of 856 mAh g−1 at 1 C, and the value retains 709 mAh g−1 (~83%) after 300 cycles with fairly low degradation. By contrast, the BCF/S electrode achieves much lower initial discharge capacity (656 mAh g−1), even worse, the capacity declines sharply after 100 cycles due to the inactive sulfur blocks and poor conductivity. Finally, the capacity retention is merely ~33% (215 mAh g−1) after 300 cycles. Such superior electrochemical properties of the PGCF/Ni/S electrodes are chiefly ascribed to the synergistic effect of 3D porous graphitic carbon fibers with highly 14


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conductive Ni nanoparticles embedded. Figure 6 vividly illustrates the mechanism of the PGCF/Ni/S electrode during cycling prcesses, compared with the BCF/S one. In details, firstly, the 3D porous structure can supply sufficient interspace of sulfur loading and firm physical confinement of lithium polysulfides, also buffer well against the large volume change during the reactions. Secondly, the 3D porous structure can homogeneously accommodate sulfur and maintain a large contact surface area between the active materials and electrolyte, supporting short ion diffusion channels. Thirdly, the metal-catalyzed graphitic carbon and embedded Ni nanoparticles can construct a high conductive network, affording rapid electrons transfer channels.

4. CONCLUSIONS In summary, we have developed a smart and facile method to convert bamboo sticks into carbon fibers for LSBs application. The bamboo cellulose fibers can be separated and dispersed completely through a simple delignification process conducted in alkaline solution. Then the metal nanoparticles are embedded in the BCF, high-quality PGCF/Ni composite is obtained with attractive properties of excellent electrical conductivity, high porosity and large specific surface area. Due to the synergistic effect of 3D porous graphitic carbon fibers and highly conductive Ni nanoparticles embedded in the composite, the designed PGCF/Ni/S electrode exhibits improved electrochemical performances including high initial capacity (1198 mAh g−1 at 0.2 C), prolonged cycling life (1030 mAh g−1 at 0.2 C after 200 cycles), and 15



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improved rate capability. Our research paves the way to develop biological materials and nickel salt pore-forming for the fabrication of high-performance sulfur cathodes, which is expected to be widely applied in LSBs.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXX. SEM images of bamboo carbon without delignification; Diameter distribution curve of BCF; BET analysis of PGCF/Ni and BCF; SEM images and EDS elemental mapping of BCF/S; XPS spectra of PGCF/Ni/S.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant. Nos. 51502263 and 51728204) and the program for Innovative Research Team in University of Ministry of Education of China (IRT13037), Fundamental

Research

Funds

for

the

Central

Universities

(No.

2015XZZX010-02), Qianjiang Talents Plan D (QJD1602029) and Startup Foundation for Hundred-Talent Program of Zhejiang University.

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FIGURE CAPTIONS Figure 1. (a) Schematic illustration of BCF fabrication. (b) Optical images of BCF synthesis process. (c, d) SEM images and (e) TEM image of BCF. The inset in (e) is the SAED pattern of BCF. Figure 2. (a-c) SEM images, (d-f) TEM images and high-resolution TEM image of PGCF/Ni. Figure 3. (a) XRD patterns and (b) Raman spectra of BCF/Ni and BCF. XPS spectra of PGCF/Ni and BCF: (c) survey, (d) Ni 2p, (e) C 1s, and (f) O1s. Figure 4. (a, b) SEM images, (c) EDS elemental mapping, and (d) TEM image of PGCF/Ni/S. (e) XRD patterns of PGCF/Ni/S and S. (f) TGA curves of PGCF/Ni/S and BCF/S. Figure 5. CV curves of (a) PGCF/Ni/S and (b) BCF/S electrodes for the first three cycles. (c) Discharge/charge profiles at 0.2 C, (d) rate capabilities, cycling performances at (e) 0.2 C, (f) 1 C, and (g) Nyquist plots before cycling of PGCF/Ni/S and BCF/S electrodes. Figure 6. Schematic illustration of the mechanism of the PGCF/Ni/S and BCF/S cycle processes.

23



Figure 1

24


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Figure 2

25



b

G band

Intensity (a.u.)

Ni (208)

Ni (200)

C (100)

C (002)

Intensity (a.u.)

Ni (111)

a

PGCF/Ni

D band

PGCF/Ni

BCF 10

c

20

30 40 50 60 2Theta (degree)

BCF

70

80

800 1000 1200 1400 1600-1 1800 2000 Raman shift (cm )

d

XPS Survey

Ni 2p

C 1s

PGCF/Ni

PGCF/Ni

860 858 856 854 852 850 848 Binding energy (eV)

f

C 1s

Ni 2p3/2 Satellite

BCF

BCF 800 600 400 200 0 Binding energy (eV) 2

sp -C 284.6

O 1s C-O 531.7

Intensity (a.u.)

1000

e

O 1s

Ni 2p

Intensity (a.u.)

Intensity (a.u.)

Ni 2p3/2 853.3

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

PGCF/Ni 3

sp -C 285.3

C=O 532.8

PGCF/Ni

BCF

BCF



Figure 3

26


Page 27 of 30

e

f

S: PDF#08-0247

10

20

30 40 50 60 2Theta (degree)

70

PGCF/Ni/S BCF/S

100

Weight retention (%)

Ni (208)

Ni (111)

Ni (200)

C (100)

C (002)

PGCF/Ni/S

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60

68%

60%

40 20 0 0

80

100 200 300 400 Temperature (°C)

Figure 4

27


500


ic od An

0

-1 d tho Ca

-1

0.

0 .2

5C 1C

0 0

2C

C

5C

PGCF/Ni/S BCF/S

10

20 30 40 Cycle Number

ic od An

0

50

60

s es oc pr

dic tho Ca

-1

1.8

e 1600

C

800 400

2.8

1

1200

-1

Capacity (mAh g )

s es oc pr

2.0 2.2 2.4 2.6 + Potential (V vs. Li/Li )

0 0

0

200 400 600 800 1000 1200 -1 Capacity (mAh g )

80 60

800

40

400

PGCF/Ni/S BCF/S

0 0

50

20

100 Cycle Number

80

@1C

60 40 PGCF/Ni/S BCF/S

20 100

0.22 V

1.8

100

50

0.15 V

2.0

100

600

200

@ 0.2 C

2.2

2.8

800

400

2.4

@ 0.2 C

f 1200 1000

PGCF/Ni/S BCF/S

2.6

1st 2nd 3rd

150 Cycle Number

200

250

0 300

0 200

150

g 200 150

PGCF/Ni/S BCF/S

100 50 0 0

50

100

Z' (ohm)

Figure 5

28


Coulombic Efficiency (%)

0 .2

ic

s es oc pr

2.0 2.2 2.4 2.6 + Potential (V vs. Li/Li )

d 1600 -1

s es oc pr

c 2.8 BCF/S

-Z'' (ohm)

1

2

Voltage (V)

2

1200

Current density (A g )

1st 2nd 3rd

1.8

Capacity (mAh g )

b

PGCF/Ni/S

-1

3

Capacity (mAh g )

-1

Current density (A g )

a

Coulombic Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

150

200

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Figure 6

29



TABLE OF CONTENTS (TOC)

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Metal-Embedded Porous Graphitic Carbon Fibers Fabricated from

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