Flexible NiOâGrapheneâCarbon Fiber Mats Containing Multifunctional

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Flexible NiO-graphene-carbon fiber mats containing multifunctional graphene for high stability and specific capacity lithium-ion storage Zhongqi Wang, Ming Zhang, and Ji Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01958 • Publication Date (Web): 18 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

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ACS Applied Materials & Interfaces

Flexible NiO-graphene-carbon fiber mats containing multifunctional graphene for high stability and high specific capacity lithium-ion storage Zhongqi Wang,1 Ming Zhang,2* Ji Zhou1* 1

State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China.

2

Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, School of

Physics and Electronics, State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China. KEYWORDS: flexible mats, carbon fibers, lithium-ion batteries, graphene

ABSTRACT An electrode’s conductivity, ion diffusion rate, and flexibility are critical factors in determining its performance in a lithium-ion battery. In this study, NiO-carbon fibers were modified with multifunctional graphene sheets, resulting in flexible mats. These mats displayed high conductivities, and the transformation of active NiO to inert Ni0 was effectively prevented at relatively low annealing temperatures in the presence of graphene. The mats were also highly flexible and contained large gaps for the rapid diffusion of ions, because of the addition of

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graphene sheets. The flexible NiO-graphene-carbon fiber mats achieved a reversible capacity of 750 mA h/g after 350 cycles at a current density of 500 mA/g as the binder-free anodes of lithium-ion batteries. The mats’ rate capacities were also higher than those of either the NiOcarbon fibers or the graphene-carbon fibers. This work should provide a new route towards improving the mechanical properties, conductivities, and stabilities of mats using multifunctional graphene.

Introduction Recent concern over issues related to energy and the environment have caused a dramatic demand for environmentally friendly devices for energy storage. The use of lithium-ion batteries (LIBs) that employ graphite as their anodes and LiCoO2 as their cathodes is a promising approach to achieving high energy and power densities.1-3 Because graphite has a relatively low theoretical capacity (372 mA h/g), much research has focused on the development of composites based on carbon and transition metal oxides with high specific capacities.4 For example, NiO has been used to modify a variety of carbon materials to create high performance anodes.5-9 The performance of an electrode material is highly dependent on its activation polarization, ohmic polarization, and concentration polarization.10 Traditional LIBs require binders to bond the active materials (graphite or LiCoO2) and conducting agents to the collectors. An insulating polymeric binder (e.g., polyvinylidene fluoride) can block electron transfer and limit the diffusion of ions, possibly resulting in increased concentration and ohmic polarizations. Electron transfer limitations can be overcome to some extent by binding NiO to the carbon fibers, since the use of a binder cannot be avoided.11 A previous study revealed that carbon fibers can be decorated with NiOx nanoparticles, causing them to form networks without an additional adhesive.11 The large spaces between the fibers in the networks allowed for rapid ion diffusion,


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which reduced the concentration polarization. The resulting networks also had good cycling stabilities and rate capacities. However, the brittleness of NiO made the carbon fiber networks inflexible and resulted in NiO-carbon fiber networks that were unsuitable for flexible LIBs. Generally, high temperatures are necessary to carbonize polyacrylonitrile (PAN) fibers in order to obtain highly conductive carbon fibers, which are prerequisites for the construction of LIBs with high rate capacities.12 However, these high carbonization temperatures can cause the active NiO structures to be transformed into inert Ni0, decreasing the theoretical capacities of the composites.13-14 Therefore, it remains difficult to fabricate NiO-carbon fiber composites with high conductivities while avoiding the simultaneous reduction (and inactivation) of NiO. Graphene and other common carbon materials have garnered much recent attention because of their excellent properties. For example, a precursor of graphene with a hexatomic ring can promote the carbonization and graphitization of furan resin, because of the structural similarities between graphene and graphite.15 Moreover, graphene, because of its excellent mechanical properties, can be used to improve the mechanical strengths of other materials, especially onedimensional nanofibers.16-17 Therefore, because graphene (or its precursor) promotes graphitization, which improves conductivity, and enhances a composite’s mechanical properties, graphene was used in this study to modify PAN-based carbon fibers to obtain NiO-graphenecarbon fiber mats (NGCs) that took the form of flexible mat-like structures. The effect of graphene was investigated on the flexibilities and conductivities of the carbon fiber mats. The flexible mats performed well as the binder-free anodes of LIBs. Their high specific capacities and good cycling stabilities were ascribed to the improved flexibility and conductivity of the composite, the protection provided by both the carbon fibers and the graphene, and the networklike structures of the flexible mats.


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Experimental Section Materials Synthesis All chemical reagents, including PAN (Mw = 150 000), NiAc2, and N,N-dimethylformamide (DMF), were of analytical grade and used without further purification. Graphene oxide (GO), which was used as the graphene precursor, was prepared by a modified Hummer’s method.18-19 Mixtures of PAN (6.5 wt%), NiAc2 (1.25-2.3 wt%), and GO (0.07 mg/ml) were prepared in DMF for electrospinning. These mixtures were transferred to 3 mL syringes with stainless steel needles (i.d. ≈ 0.6 mm). The flow rates of the mixtures were maintained at approximately 0.4 mL/h using a syringe pump. A piece of aluminum foil acted as the collector for the fibers and was vertically positioned about 15 cm away from the tip of the needle. A DC voltage (13 kV to 17 kV) was applied between the needle and the collector using a high voltage power supply. This electrospinning process created a PAN-NiAc2-GO fiber mat, which was removed from the aluminum foil. The mat was pre-oxidized in air at 225 °C for 6 h. The resulting brown mat was then annealed at 550 °C to 700 °C in N2 for 2 h to carbonize the PAN, decompose the NiAc2, and reduce GO to graphene. Pure carbon fibers, graphene-carbon fibers and NiO-carbon fibers were also fabricated using the similar method. The detail chemicals usage and the annealing temperature for samples are listed in Table 1. Table 1. Chemicals and annealing temperatures for the synthesis of samples in this study.

No.

PAN (wt%)

NiAc2 (wt%)

GO (mg/mL)

Annealing temperature (°C)

1

6.5

1.95

0.07

600

A-1-600

2

6.5

1.95

0.07

650

A-1-650

3

6.5

1.95

0.07

700

A-1-700

4

6.5

1.95

NONE

650

A-2-650

ID


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5

6.5

1.25

0.07

650

B-1-650

6

6.5

2.3

0.07

650

C-1-650

7

6.5

NONE NONE

650

E-650

8

6.5

NONE 0.07

650

F-650

Characterization The microstructure and morphology of fibers were observed by a scanning electron microscope (SEM, Hitachi S-4800), and a transmission electron microscope (TEM, FEI Tecnai G2 F20) with an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) data were collected on a Netzsch STA449C thermal analyzer. The valence and ratio of the atoms in samples were detected by X-ray photoelectron spectroscopy (XPS, Surface Science Instruments S-probe spectrometer). The conductivity of the mats was tested using a semiconductor parameter instrument (Agilent 4156). The binding-energy scale of XPS characterization was calibrated by assigning the lowest binding energy of the C 1s peak to 285.0 eV. The specific surface area was determined using a micromeritics surface area and porosity analyzer (ASAP 2020 HD88). Electrochemical Tests The mats (including NGCs, carbon fibers, graphene-carbon fibers, and NiO-carbon fibers) with weights of approximate 0.8 mg were directly used as binder-free anodes for LIBs, and all of the specific capacities in this study were calculated based on the whole composites. The Celgard 2400 polypropylene membrane was used as a separator for half-cells. A solution of ethylene carbonate-dimethyl carbonate (1 : 1 by volume) containing 1 M LiPF6 was used as electrolyte in coin-cells (CR-2016). Pure lithium foils with diameters of about 12 mm were used as both counter and reference electrodes. All of the coin-cells were assembled in an Ar-filled glove-box with the moisture and oxygen levels less than 1 ppm. Discharge and charge properties were characterized using Arbin BT2000 and LAND battery testing systems with the cut-off potentials


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being 0.005 V for discharge and 3 V for charge. Electrochemical workstation (CHI-660B) was employed to collect the cyclic voltammetry curves.

Results and Discussion

Figure 1. (a) Specific capacities of NGCs (A-1-650, B-1-650, and C-1-650) at different currents: 1-10 cycles with a current density of 100 mA/g; after 10 cycles with a current density of 500 mA/g. (b) TGA curves of NGCs collected at a heating rate of 5 °C/min in air with a flow rate of 20 mL/min. Three of the NGCs containing different ratios of NiO (A-1-650, B-1-650, and C-1-650) were evaluated as flexible anodes in LIBs. As shown in Figure 1a, B-1-650, which contained the least amount of NiO, displayed an initial discharge capacity of 970 mA h/g at 100 mA/g, corresponding to a coulombic efficiency of 66% (Figure S1). A-1-650, which contained more


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NiO, achieved a discharge capacity of 1230 mA h/g and a coulombic efficiency of 67%, showing an improvement of specific capacity. The NGCs containing the most NiO, C-1-650, displayed the lowest discharge capacity of 808 mA h/g in its first cycle. From the 2nd through 10th cycles, the discharge capacities of A-1-650, B-1-650, and C-1-650 remained approximately constant at 810 mA h/g, 596 mA h/g, and 455 mA h/g, respectively, indicating that the NGCs had good cycling stabilities. When the current density was increased to 500 mA/g in the 11th cycle, the samples’ discharge capacities decreased. In their 200th cycles, A-1-650, B-1-650, and C-1-650 displayed discharge capacities that were approximately 1.20, 1.03, and 1.14, respectively, times larger than those from their 15th cycle, which was the cycle at which the discharge capacities were lowest. The C-1-650 with the highest weight ratio of NiO showed the lowest specific capacity suggested that the main capacity of NGCs may come from carbon and graphene in NGCs and NiO was the additive to promote the capacity of carbon and graphene. Similar results can be found in previous publication about graphene or carbon based composites for Li+ storage.20 The capacity improvement that was common to the three samples was attributed to electrolyte soakage.20-21 Similar results have been observed for other carbon fiber-based composites in LIBs.20, 22 Moreover, the discharge capacity of A-1-650 in its 361st cycle was 750 mA h/g, which was approximately 28.3% higher than its capacity in its 15th cycle. Therefore, according to the above results, the addition of a suitable amount of electrochemically active NiO was critical for improving the specific capacities of these composite materials. The weight ratios of NiO in the NGCs were measured via a TGA test as shown in Figure 1b. The weight loss below 200 °C was ascribed to the evaporation of adsorbed water. A-1-650 and C-1-650 displayed rapid weight losses between 350 °C and 450 °C, corresponding to weight residues of 9.4% and 17.1%, respectively. A rapid weight loss was observed for B-1-650


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between 450 °C and 550 °C, indicating its superior thermal stability compared to those of A-1650 and C-1-650, which contained higher weight ratios of NiO. Therefore, the most suitable NiO weight ratio in the NGCs was that of A-1-650 or approximately 9.4wt%.

Figure 2. The specific capacities of A-1-650 (NGCs), A-2-650 (NiO-carbon), E-650 (carbon fibers) and F-650 (graphene-carbon fibers) at different current density: first ten cycles at 100 mA/g and following cycles at 500 mA/g for A-1-650 and A-2-650. The electrochemical performance of A-1-650 was compared to those of A-2-650, E-650, and F-650 to determine the causes of its higher specific capacity and superior cyclic stability as shown in Figure 2. In the absence of graphene, NiO-carbon (A-2-650) displayed a gradually diminishing capacity in the first ten cycles at a current density of 100 mA/g, and its capacity was higher than the stable capacity of A-1-650. After its current density was increased to 500 mA/g, the specific capacity of A-1-650 gradually increased to 750 mA h/g by its 361st cycle, while the capacity of A-2-650 was only 670 mA h/g at its 200th cycle. These results indicated the beneficial effects of graphene on the materials’ electrochemical performances, especially their cycling stabilities. It also can be found that F-650 performed better than E-650, confirming the beneficial role of graphene in the carbon materials. However, both E-650 and F-650 performed worse than A-1-650. The theoretical capacity of A-1-650 can be calculated to be 404 mA h/g


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based on the theoretical capacities of 372 and 717 mA h/g for carbon and NiO. The sample A-1650 showed an improved specific capacity (750 mA h/g ) as compared to above theoretical value, which was also higher than the estimated value (484 mA h/g) of A-1-650 based on the measured value of 460 mA h/g for graphene-carbon (F-650) and theoretical capacity of 717 mA h/g for NiO. Therefore, the presence of both graphene and NiO in the carbon fibers was found to be a critical factor to improve the electrochemical performances of the NGCs. Their improved performances were ascribed to the synergistic effects of the graphene, NiO, and carbon fibers.

Figure 3. SEM images of samples: (a) and (b) for A-1-650; (c) and (d) for A-2-650; (e) for E650; (f) for F-650. Figure 3 shows SEM images of the various fibers. Figure 3a shows a low magnification SEM image of the smooth surface of A-1-650. A-2-650 was found to contain some bright particles as shown in Figure 3b. The higher magnification SEM images in Figures 3c and 3d provide more morphological details of A-1-650 and A-2-650. A-1-650 contained fibers with diameters of 140 nm containing ultra-small nanoparticles, while the A-2-650 fibers were about 190 nm in diameter and were modified internally and externally with 20 nm nanoparticles. The smaller size of the


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NiO particles in A-1-650 was attributed to the presence of GO, because its many oxygencontaining groups likely affected the nucleation and growth of the nanoparticles.23 Another significant difference between A-1-650 and A-2-650 was that the NiO nanoparticles on A-1-650 were completely covered by carbon. Figures 3e and 3f show the morphologies of E-650 and F650. Their diameters are approximately 160 nm without any nanoparticles, indicating that graphene had little effect on the diameters and morphologies of the carbon fibers. Therefore, the addition of graphene to the carbon fibers greatly decreased the diameters of the NiO nanoparticles and altered their distributions.

Figure 4. (a), (b) and (c) display the TEM images of A-1-650. The arrows in (b) mark the holes in the carbon fiber. (d) and (e) show the TEM images of A-2-650; The images from (f) to (k) provide the digital photos to show the flexibility and the integrity of A-1-650 during and after flexible tests. The mat is about 6mm×6mm. The fibers’ microstructures were imaged using TEM as shown in Figure 4. Nanoparticles were found inside A-1-650 that were approximately 150 nm in diameter. The amplified TEM image in Figure 4b confirmed that the 20 nm- to 30 nm-sized nanoparticles were embedded discretely in the carbon fibers. In addition, the carbon fibers contained some holes, which are marked by


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arrows in the figure. These holes might have resulted from the corrosion of NiO.14 The highresolution TEM images in Figure 4c indicated the presence of distinct lattice fringes at 0.241 nm, which were indexed to the (111) plane of NiO (JCPDS#47-1049), further confirming that the nanoparticles were composed of NiO. A TEM image of A-2-650 is shown in Figure 4d. Clearly, it has a different structure than A-1-650, and its nanoparticles were distributed both inside and outside of its fibers. The nanoparticles on the outside of the fibers were larger than those inside the fibers, indicating that the carbon fibers suppressed nanoparticle growth. The smaller size of the nanoparticles in A-2-650 compared to those in A-1-650 was attributed to the limited nucleation caused by GO.23 The lattice fringes in Figure 4e matched well with the (200) plane of NiO (JCPDS#47-1049), again confirming the crystal structure of these nanoparticles. Moreover, A-2-650 contained very few holes, suggesting that the smaller nanoparticles were less able to form pores. The addition of graphene to the carbon fibers resulted in the complete encapsulation of the NiO in a carbon matrix, and the carbon fibers in these materials contained holes with relatively large diameters because of the effects of NiO. The flexibility of an A-1-650 mat was tested during an unfold-fold-unfold process as shown in Figure 4f-4k. The mats were folded from 0° to 180° and returned to 0° without fracturing. After returning to their initial flat orientation, the mats did not contain crack, indicating their excellent flexibility. X-ray photoelectron spectroscopy (XPS) was performed with a Surface Science Instruments Sprobe spectrometer to determine the elements and their bond states in the fibers. Binding energies were calibrated by assigning the lowest binding energy C 1s peak to 285.0 eV.


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Figure 5. XPS data of A-1-650 and A-2-650. (a) displays the full spectrum of A-1-650; (b)-(e) are the fine spectra of C 1s, N 1s, O 1s and Ni 2p in A-1-650, respectively. (f) is the fine spectrum of Ni 2p in A-1-700, suggesting the presence of Ni0. The full spectrum of the A-1-650 mats (Figure 5a) revealed the presence of carbon, nitrogen, oxygen, and nickel. The fine C 1s spectrum (Figure 5b) was deconvoluted to reveal five peaks, which included peaks at 285 eV for graphitized carbon, 286.5 eV for the carbon in phenolic and alcohol groups, 288 eV for the carbon in carbonyl or quinine groups, 289.2 eV for the carbon in carboxyl or ester groups, and 290.4 eV for the carbon in adsorbed CO and CO2.17 These results were similar to those obtained in a previous study of PAN-based carbon nanofibers, suggesting the presence of some oxygen-containing groups on the fibers. The N 1s peak in Figure 5c was deconvoluted into three peaks at 398.3 eV, 399.2 eV, and 400.9 eV, which were assigned to pyridine, pyrrole, and conjugated nitrogen.24-25 The presence of these nitrogen species, especially the pyridine nitrogen, improves Li+ storage. Figure 5d shows the complex O 1s peak, which was deconvoluted to reveal five peaks using Gaussian fitting. The weak peak at 529.4 eV was


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indexed to the oxygen in NiO.6, 26 The relatively low intensity of this peak was caused by the encapsulation of NiO in the carbon matrix, which was confirmed in the TEM images. The other peaks at 532.1 eV, 533.1 eV, and 534.1 eV were ascribed to C=O, C-OH (and/or C-O-C), and chemically adsorbed oxygen and adsorbed water.27-28 The additional peak at 530.9 eV was attributed to the presence of Ni-O-C bonds, because the peaks for the possible Ni-C bonds are located at approximately 853 eV or 285 eV.6 The Ni 2p peak was deconvoluted into two peaks by Gaussian fitting as shown in Figure 5e. The strong peak at 855.1 eV and its associated weaker peak at 860.9 eV were attributed to the Ni 2p 3/2 spin-orbit levels of NiO.6,

29

The Ni 2p

spectrum of A-1-700 shown in Figure 5f not only contained the typical Ni 2p 3/2 peaks of NiO but also a peak at 852.3 eV, implying the presence of Ni0.30 This observation suggested that the annealing temperature of 700 °C resulted in the transformation of the electrochemically active NiO into Ni0 which was inert towards the storage of Li+.

Figure 6. CV curves of fiber mats in the first four cycles: (a) E-650 (pure carbon fibers); (b) F650 (graphene-carbon fibers); (c) A-2-650 (NiO-carbon); (d) A-1-650 (NGCs). Cyclic voltammetry (CV) was performed in the voltage range from 0 V to 3 V (vs. Li+/Li) to investigate the electrochemical mechanism of Li+ storage by the fiber mats. The CV curve of the


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pure carbon fibers (E-650) displayed three peaks in the 1st cycle as shown in Figure 6a. The cathodic peak at 0.4 V was attributed to the irreversible formation of solid electrolyte interface (SEI) films.31 The less intense anodic peak at 0.2 V in the first cycle was related to the delithiation of graphite, while the larger anodic peak at 1.3 V arose from the extraction of Li+ from defect sites or holes on the carbon fibers.32-34 The CV curves in the 2nd and 4th cycles were nearly identical, indicating the excellent cycling stability of E-650. Figure 6b shows the CV curves of the graphene-carbon fibers (F-650). In the 1st cycle, F-650 produced a similar CV curve to that of E-650 with the addition of graphene having little impact on the electrochemical mechanism of the carbon fibers. The CV curves for F-650 were also similar in their 2nd and 4th cycles, confirming the good cycling stability of this composite. There was obvious plateau in charge-discharge curves of F-650 in Figure S2, which were typical lithiation/delithiation curves for carbon obtained at a relative low temperature.34 Although both A-1-650 and A-2-650 displayed good cycling stabilities, A-2-650 produced a unique CV curve compared to those of E650 and F-650, suggesting that NiO altered the electrochemical mechanism of Li+ storage. The anodic peak of A-2-650 in the 1st cycle shifted to a lower potential (approximately 1 V) compared to the anodic peaks of E-650 and F-650, suggesting that Li+ extraction was easier at a decreased overpotential for A-2-650.35 In addition, the anodic peaks at 1 V in A-1-650 were larger than those of A-2-650. Previous studies have shown that the typical cathodic and anodic peaks of NiO occur at approximately 1.1 V and 2.2 V and that SEI films can be partially decomposed in the first few cycles.36-38 Therefore, the anodic peaks at 1 V were attributed to the delitiation of defect sites and micropores in agreement with previous reports on Li+ storage in porous carbon materials and graphene.17, 34, 39 CV peaks related to NiO were not observed in Figure 6c and 6d. Moreover, there was no typical plateau relative to NiO in the charge-discharge


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curves of A-1-650 (Figure S2). A possible reason may be the low crystallinity of the NiO in these samples as determined from XRD patterns (Figure S3). Besides, the charge-discharge curve of A-1-650 in the second cycle almost overlapped with that in the tenth cycle, showing the improved cycling stability of A-1-650. The effect of annealing temperature was determined on the electrochemical performances of the NGCs as shown in Figure 7a. The specific capacities of the NGCs increased from 260 mA h/g to 810 mA h/g as the annealing temperature was increased from 550 °C to 650 °C at a current density of 100 mA/g. Meanwhile, A-1-700 only had a capacity of 643 mA h/g. When the current density was increased to 500 mA/g, the specific capacity of A-1-650 reached its highest value of 750 mAh/g, which was maintained even after 360 cycles, suggesting that 650 °C was the best annealing temperature for the NGCs. An annealing temperature of 650 °C perhaps enabled the formation of carbon fibers with high conductivities while avoiding the chemical transformation of NiO to inert Ni0.34 All of the NGCs displayed coulombic efficiencies of nearly 100% in each cycle except the first (Figure 7b), again confirming the good cycling stabilities of these materials. The rate capacities of the NGCs were further characterized as shown in Figure 7c. A-1-650 delivered reversible capacities of 834 mA h/g, 738 mA h/g, 585 mA h/g, 489 mA h/g, and 380 mA h/g at current densities of 0.1 A/g, 0.2 A/g, 0.5 A/g, 1 A/g, and 2 A/g, respectively. When the current density was then decreased to 0.1 A/g, its capacity quickly recovered to 918 mA h/g, indicating a good rate accommodation. The improved capacity of 918 mA h/g was ascribed to the activation of the NiO embedded in the carbon matrix and the soakage of the electrolyte into the material. Both of these phenomena have been observed in previous reports on carbon fiber composites for LIBs.11, 22, 40 Clearly, the capacity of A-1-650 was higher than that of both A-2-


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650 and F-650 at the same current density, indicating that the addition of both graphene and NiO improved the composites’ rate capacities.

Figure 7. (a) compares the properties of NGCs (A-1-X) obtained at different temperatures and current densities (from 1st to 10th cycle at 100 mA/g; from 11th to following cycles at 500 mA/g). (b) displays the corresponding coulombic efficiencies. (c) shows the rate capacities of A-1-650 (NGCs), A-2-650 (NiO-carbon), and F-650 (graphene-carbon fibers). (d) are the AC impedance curves and corresponding fitting results. (d) shows the pore distribution of A-1-650 after removing the NiO using a BJH method. (f) displays a digital photo of fiber mats for conductivity measurement and the I-V curves of conductivity measurement. AC impedance experiments were performed after the rate capacity tests to investigate the source of the excellent performance of A-1-650. As shown in Figure 7d, the Nyquist plots were composed of two partially overlapping semicircles in the high- and medium-frequency regions and an inclined line in the low frequency region, which was associated with the Warburg impedance.41-42 These data were fit to an equivalent circuit (Figure S4), and the intercepts on the real impedance (Z) axis indicated the electrolyte resistance (Re). These intercepts were similar for


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all of the composites, indicating a uniform value of Re. Cf and Rf were the capacitances and resistances of the SEI films (semicircles in the high-frequency region), and Cdl and Rct were the double-layer capacitance and charge-transfer resistance (semicircles in the medium-frequency region). According to the equivalent circuit model, Rf was 77.2 Ω, 64.8 Ω, and 32.9 Ω for A-2650, F-650, and A-1-650. The Nyquist plot of A-1-650 also contained a smaller semicircle than did that of A-2-650, indicating graphene’s ability to enhance electron transfer. Since the cycling tests and CV results indicated that the presence of micropores and mesopores may have contributed to the materials’ Li+ storage abilities, BET tests were performed on the A1-650 carbon fibers (Figure S5). The specific surface area of theses carbon fibers was 268 m2/g, according to the multipoint BET method. As shown in Figure 7e, the percentage of mesopores (2 nm to 50 nm in size) in the carbon fibers was relatively high, although a large number of micropores (< 2 mm in size) were also present. Micropores 0.7 nm to 0.9 nm in size have been reported to irreversibly store Li+, while micropores larger than 0.9 nm and mesopores allow for the more facile and reversible storage and diffusion of Li+.43 Therefore, the carbon fibers presented in this work contained pore sizes that are known to be beneficial for Li+ storage. To determine the effects of graphene and annealing temperature on the conductivity of these composites, current-potential (I-V) curves were obtained for A-1-650, A-1-700, and A-2-650 using a semiconductor parameter instrument.44 The conductivity of the NGCs and NiO-carbon mats were measured via a two-point probe method. Before the tests, the mats were cut into 6 mm squares. The detailed method for calculating the conductivities of these mats is shown in the Supporting Information, and the conductivities of these three samples are shown in Table S1. As shown in Figure 7f, the slope of the I-V curve for A-1-600 was lower than that of the I-V curve for A-1-650, indicating the lower resistance of A-1-650 and confirming that a higher annealing


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temperature resulted in a mat with a higher conductivity. The slope of the I-V curve for A-1-650 was also higher than that of A-2-650, which lacked graphene, suggesting that graphene improved the conductivity of the carbon fibers. The addition of graphene and the use of a higher annealing temperature greatly enhanced the conductivities of the mats.45 The high conductivity, porous structure, high specific capacity, high rate capacity, and good cycling stability of A-1-650 were attributed to the observations in the following. (1) The presence of graphene improved the flexibility of the mats, resulting in improved structural stabilities. (2) The high conductivity of graphene improved the conductivity of the mats and enabled more rapid electron transfer. (3) The added graphene encapsulated the NiO inside a carbon fiber matrix, enabling the composites’ excellent cycling stabilities (Figure S6). (4) The porous nature of the fibers contributed to improving the capacities of the mats. (5) The use of the relatively high annealing temperature of 650 °C balanced the competing processes of generating high conductivity carbon fibers while avoiding the transformation of NiO to inert Ni0. (6) The gaps between adjacent fibers allowed for the rapid diffusion of Li+, reducing polarization and maintaining a high rate capacity.46

Conclusions The incorporation of multifunctional graphene sheets in NiO-carbon fiber mats resulted in flexible NiO-graphene-carbon fiber mats, which had high specific capacities and good long-term cycling stabilities as binder-free anodes for Li+ storage. In the presence of graphene, the use of a suitable annealing temperature avoided the transformation of NiO to inert Ni0 and simultaneously produced fibers with relatively high conductivities. As the binder-free anodes of LIBs, the flexible mats with the optimal ratios of NiO and graphene delivered a discharge capacity of 750 mA h/g after 350 cycles at a current density of 500 mA/g. They also achieved a


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reversible capacity of 380 mA h/g at the higher current density of 2000 mA/g. The excellent performances of the flexible mats for Li+ storage were attributed to the presence of multifunctional graphene, which improved the mechanical stabilities and conductivities of the mats. The use of a relatively high annealing temperature created high conductivity carbon fiber in the presence of NiO, while the porous fibers effectively stored Li+. The excellent performance of the composite resulted particularly from the fibers having been woven into mats, which enabled rapid electron transport and contained spaces between the adjacent fibers for the rapid diffusion of Li+. This study provides a new strategy for the design and preparation of binder-free electrodes for high-performance batteries, supercapacitors, and conductive films.

ASSOCIATED CONTENT Supporting Information. Coulombic efficiency of A-1-650, B-1-650, and C-1-650; XRD patterns of A-1-650; chargedischarge curves of A-1-650 and F-650; equivalent circuit to fit the Nyquist plots; nitrogen adsorption-desorption isotherms of the carbon fibers arising from A-1-650; TEM images of A-1650 after cyclic tests, and the calculating method of conductivity. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

Email: [email protected] (Ming Zhang), [email protected] (Ji Zhou) Author Contributions


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Ming Zhang and Ji Zhou designed the experiments. Zhongqi Wang and Ming Zhang performed the experiments. All authors analyzed and wrote the manuscript, and have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This research work has been financially supported in part by the National Natural Science Foundation of China (51404103, 11274198 and 51532004) and Hunan University Fund for Multidisciplinary Developing (2015JCA04).

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Table of Contents

Graphene improve the flexibility and conductivity of carbon fibers, control the size and distribution of NiO, resulting in NiO-graphene-carbon fiber flexible mats with good properties as binder-free anodes for lithium-ion batteries.


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Flexible NiOâGrapheneâCarbon Fiber Mats Containing Multifunctional

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