Effect of Temperature on the Morphology and Anode Performance of

Effect of Temperature on the Morphology and Anode Performance of Binder-Free Carbon Nanofiber/Nickel–Cobalt ... Publication Date (Web): October 13, ...
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Effect of Temperature on the Morphology and Anode Performance of Binder-Free Carbon Nanofiber/Nickel−Cobalt Hydroxide and Carbon Nanofiber/Nickel−Cobalt Oxide for Lithium-Ion Batteries Jia-Min Syu, Ming-Liang Hsiao, and Chieh-Tsung Lo* Department of Chemical Engineering, National Cheng Kung University, number 1, University Road, Tainan City 701, Taiwan S Supporting Information *

ABSTRACT: Hierarchical composite materials composed of electrospun carbon nanofibers and binary nickel−cobalt hydroxide (Ni−Co(OH)2) nanoflakes were synthesized for use as anodes for lithium-ion batteries. To clarify the effect of temperature on the structure and electrochemical performance of the composites, the ternary composite fibers were annealed at various temperatures. When the annealing temperature increased, the interconnected Ni−Co(OH)2 nanoflakes on the carbon surface gradually converted to Ni−Co(OH)2 nanoneedles. Subsequently, the chemical structure of Ni−Co(OH)2 was transformed into binary nickel−cobalt oxide (Ni−CoO) at 300 °C. Further annealing at 400 °C caused the aggregation of nanoneedles, forming flowerlike particles. The incorporation of Ni−Co(OH)2 nanoflakes into carbon nanofibers resulted in a substantial increase in the specific capacity compared with that of the pure carbon nanofibers. The increased specific capacity was because of the synergistic effects of the high theoretical capacity of Ni−Co(OH)2 and easily accessible charge transport in the carbon nanofiber network. The specific capacity of the composite fibers increased with the annealing temperature and reached a maximum of 645 mAh/g at a current density of 150 mA/g at 300 °C. The increased specific capacity was mainly attributed to the change in the specific surface area, mesopore volume, and electrical conductivity of the composite fibers with temperature.



INTRODUCTION To satisfy the increasing demand for multifunctional consumer electronics, electric vehicles, and renewable energy over the past few decades considerable effort has been devoted to developing high-performance energy storage systems. Lithiumion batteries are considered a promising energy storage device because of their high energy density, high working voltage, long cycle stability, and absence of a memory effect. Electrode materials are the major component of lithium-ion batteries that determine their electrochemical performance, and researchers have devoted substantial effort to search for high-performance electrode materials to meet the requirements of the next generation of lithium-ion batteries. Among the materials, transition metal hydroxides and metal oxides are promising anode candidates because they exhibit much higher theoretical capacities than graphite, as well as being low-cost and abundant materials. However, they are associated with the disadvantage of quick capacity fading caused by large volume expansion/ contraction during charging and discharging. Additionally, the aggregation of transition metal hydroxides and metal oxides during the cycling process limits the kinetics of ion diffusion, thereby reducing the performance of batteries. Strategies for improving the electrochemical performance of transition metal hydroxides and metal oxides include the manipulation of morphology and the synthesis of binary mixtures. Nanostructured anodes provide a highly accessible surface area to electrolytes, enabling efficient ion diffusion to electrodes. Additionally, three-dimensional nanostructures offer open space for volume expansion during the cycling process, © XXXX American Chemical Society

thereby promoting electrode stability. Moreover, the combination of two types of transition metal oxides or metal hydroxides, such as binary nickel−cobalt oxide (Ni−CoO)1−5 and binary nickel−cobalt hydroxide (Ni−Co(OH)2),6 may induce the synergistic effects of the two compounds and further surpass the intrinsic properties of the single material. For example, the addition of Co ions to Ni(OH)2 decreased mechanical stress during charging and discharging, preventing electrode failure.7,8 Additionally, mixed materials induced the catalytic decomposition of the solid electrolyte interphase (SEI) during charging, leading to enhanced electrochemical performance.9−11 Extensive research has been conducted for designing nanostructured Ni−CoO or Ni−Co(OH)2 for use as anodes for lithium-ion batteries. Mai et al. synthesized Co-doped NiO nanoflake arrays by using low-temperature chemical bath deposition.1 The presence of Co2+ increased the electrical conductivity of the anode, facilitating charge transport. Consequently, Co-doped NiO nanoflake arrays delivered a specific capacity of 600 mAh/g at a current density of 100 mA/ g for the 50th cycle. By contrast, the pure NiO electrode exhibited a specific capacity of only 463 mAh/g at the same current density. Similarly, Thi et al. prepared anodes composed of Co-doped NiO nanoparticles by using a solvothermal approach.4 The uniform distribution of Co in the crystal lattice Received: July 26, 2017 Revised: September 28, 2017 Published: October 13, 2017 A

DOI: 10.1021/acs.jpcc.7b07412 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

preparation of carbon fibers through electrospinning can generate fibers on the nanometer scale, providing a large surface area. Additionally, electrospun carbon nanofibers form a web-type structure associated with fiber entanglement. Because of their favorable mechanical and electrical properties, anodes prepared using electrospun carbon nanofibers are binder-free and flexible. However, previous studies have described only a few examples of anodes that synergize the intrinsic properties of nickel hydroxide, cobalt hydroxide, and electrospun carbon nanofibers in fabricating the ternary anodes for lithium-ion batteries.15 In this study, we analyzed the change in the microstructure and electrochemical performance of these ternary anodes with variation of the annealing temperature. As indicated in the literature,2,5,13 thermal treatment on Ni− CoO at different annealing temperatures resulted in morphological changes. These morphologies further altered the microstructure of Ni−CoO, such as the specific surface area and pore size, and consequently affected the anode performance of Ni−CoO. Furthermore, elevated temperatures on the ternary anodes could affect the interfacial integrity between carbon nanofibers and Ni−CoO. This information is crucial for manipulating the structure of the anodes and further developing anode materials for lithium-ion batteries.

of NiO prevented particle aggregation, leading to an increased surface area of NiO. Wang and Zhang synthesized Ni−CoO/C composites by using a coprecipitation and subsequent pyrolytic carbon reduction method.2 The morphology of the composites strongly depended on the thermal treatment. Flowerlike spheres with a diameter of 500 nm were observed at a low annealing temperature, whereas a high annealing temperature resulted in the coexistence of spherical and polyhedral particles with nonuniform particle sizes. This was attributed to the intense crystallization at a high annealing temperature, which caused a destruction of the flowerlike spheres. As an anode for lithium-ion batteries, the composite annealed at a low temperature exhibited the poor cycle stability, which resulted from the poor mechanical integrity of the composite with a flowerlike structure. Zhang et al. synthesized a nanostructured anode consisting of NiO nanoplates and Co3O4 nanoparticles by using a hydrothermal method.12 The NiO nanoplates offered a large surface area, allowing short pathways for ion diffusion, and the Co3O4 nanoparticles acted as a buffer, preventing nanoplate aggregation. The Ni−CoO anode delivered a specific capacity of 633 mAh/g at a current density of 100 mA/g for the 70th cycle. Huang et al. synthesized Ni− CoO mesoporous thorn microspheres by conducting a hydrothermal reaction with urea as a template.5 The unique hierarchical structure of the Ni−CoO microspheres provided both a short diffusion path and a large active surface area for lithium ions. By varying the annealing temperature, the structure of the microspheres changed from the integrated to the stone-pillar-like thorn bundle arrays, which resulted in a substantial decrease in the specific surface area of the microspheres. Furthermore, thermal treatment at 600 °C yielded multiphase crystal structures composed of NiCo2O4, Co3O4, and NiO, which hindered side reactions of the anode with the electrolyte and further reduced the volume expansion of the anode during charging and discharging. Consequently, the Ni−CoO microspheres thermally treated at a low temperature delivered a higher capacity but a lower capacity retention when compared to those annealed at a high temperature. Xu et al. demonstrated the effect of the annealing temperature on the electrode materials composed of Ni−CoO and carbonized kapok fibers.13 Increasing the annealing temperature induced a morphological transition of the composites from the cross-linked fibrous network to nanoflakes, which subsequently enhanced the electrochemical performance of the composites. Ding et al. synthesized Ni− CoO microspheres by using a bubble-template process. The hollow structures provided not only an efficient contact area between the electrodes and electrolyte but also a large open space for accommodating volume changes.14 Consequently, the Ni−CoO hollow microspheres delivered a specific capacity of 730 mAh/g at a current density of 300 mA/g for the 140th cycle. Although Ni−CoO and Ni−Co(OH)2 exhibit excellent electrochemical performance, the addition of binders during electrode preparation is inevitable. Both the amount of binders and the mixing process should be optimized. Additionally, the as-prepared electrodes are not flexible enough to meet the requirements of some developing consumer electronics. In this study, we integrated ternary mixtures of nickel hydroxide, cobalt hydroxide, and electrospun carbon nanofibers as anodes for lithium-ion batteries. Carbon fibers exhibit unique carbon structures that contribute to the high electrical conductivity, high mechanical strength, and lightweight of the fibers, making them useful in many applications. The



EXPERIMENTAL SECTION

Materials and Methods. Electrospun carbon nanofibers were synthesized using polyacrylonitrile (PAN, molecular weight: 150000 g/mol, Sigma-Aldrich Co.) dissolved in N,Ndimethylformamide (DMF, 99.8%) as a precursor solution. The 7 wt % precursor solution was added to a 10 mL glass syringe placed 15 cm from a collector composed of vertically placed aluminum foil. The conventional single-nozzle electrospinning process was conducted at room temperature by using a power supply of 15 kV (You-Shang Technical Corporation). The flow rate of the precursor solution was maintained at 0.7 mL/h by using a syringe pump (KDS 100, KD Scientific). After electrospinning, the as-prepared fiber mats were dried in air to completely remove the residual solvent prior to thermal treatment. The polymer nanofibers were converted to carbon nanofibers through a two-step thermal treatment performed in a tubular quartz furnace. The fiber mats were stabilized by heating in air from room temperature to 250 °C at a heating rate of 2 °C/ min, and the temperature was then maintained at 250 °C for 4 h. Subsequently, carbonization was conducted at 800 °C for 2 h in argon. The carbonized nanofibers were surface-oxidized by using 68 wt % nitric acid at 75 °C for 5 h. Carbon nanofiber/Ni−Co(OH)2 composites were synthesized using a modification of previously described procedures.14,16 In brief, the HNO3-treated carbon nanofibers were mixed with 0.2 g of NiCl2·6H2O, 0.2 g of CoCl2·6H2O, 1.2 g of cetyltrimethylammonium bromide, and 3.0 g of urea in 100 mL of deionized (DI) water. The reaction was performed at 100 °C for 11 h. The resulting carbon nanofiber/Ni−Co(OH)2 composites were subjected to purification through DI water to remove residual chemicals. Subsequently, the composites were annealed at various temperatures for 3 h in nitrogen. The nomenclature of the fibers is as follows: CNF for the carbon nanofibers prepared from neat PAN; CNF-NiCo for the carbon nanofiber/Ni−Co(OH)2 composites without annealing; and CNF−NiCo200, CNF−NiCo300, and CNF−NiCo400 for the composite fibers annealed at 200, 300, and 400 °C, respectively. B

DOI: 10.1021/acs.jpcc.7b07412 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Characterization. The surface morphology of the composite fibers was examined using scanning electron microscopy (SEM) on a Hitachi SU8010 scanning electron microscope at an accelerating voltage of 10 kV. Prior to SEM imaging, the composite fibers were sputter-coated with a thin layer of platinum by using a JEOL JFC-1600 coater. The mean fiber diameter was analyzed by observing at least 100 randomly selected fibers per sample on several SEM images. X-ray diffraction (XRD) patterns were measured using a Rigaku RINT-2000 diffractometer with a filtered Cu Kα radiation source (wavelength = 1.5406 Å) at 40 kV and 40 mA. The measurements were performed with a diffraction angle (2θ) between 4° and 70° at a scan rate of 2°/min. X-ray photoelectron spectroscopy (XPS) was conducted on a PHI 5000 VersaProbe modeled X-ray photoelectron spectrometer to record both elemental and chemical information on the composite fibers. The specific surface area and pore size distribution of the composite fibers were determined from the nitrogen sorption isotherms generated using the Brunauer− Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods, respectively, using a physisorption analyzer (ASAP2020, Micromeritics). Prior to the measurements, the composite fibers were degassed at 200 °C under vacuum to remove other adsorbed species. The four-point probe method was used to measure the electrical conductivity (σ) of the composite fibers, in which σ was calculated using the following equation: σ=

1 Rw

(1) Figure 1. Surface morphology of composite fibers. (a) CNF, (b) CNFNiCo, (c) CNF-NiCo200, (d) CNF-NiCo300, and (e) CNFNiCo400. Insets: high-magnification images of the composite fibers.

where R is the electrical resistance and w is the thickness of the specimen. The electrochemical performance of the composite fibers was analyzed at room temperature by using 2032 coin cells as the lithium-ion batteries (Ubiq Technology Co., Ltd.). The synthesized composite fibers were directly used as a binderfree working electrode, and lithium metal (99.9%, Hongda energy) was used for both the counter and reference electrodes. A PP−PE−PP trilayer (Celgard M824, 12 μm) served as the separator. The electrolyte was prepared by dissolving LiPF6 in a mixture of ethylene carbonate/diethyl carbonate/dimethyl carbonate (vol/vol/vol = 1/1/1) with a concentration of 1 M. The cells were assembled inside an argon-filled glovebox. Cyclic voltammetry (CV) measurements were performed to examine the electrochemical reactivity of the composite fibers by using a CHI-627D potentiostat with a potential window of 0.01−3.50 V at a scan rate of 0.3 mV/s. Galvanostatic charge− discharge tests were performed at potentials ranging from 0.01 to 2.80 V versus Li/Li+ at a constant current density of 150− 900 mA/g by using a BAT750B battery test system (Acutech Systems Co., Ltd.). A total of 100 cycles at a current density of 200 mA/g were performed for cycling stability testing. For the full cell, the cathode was prepared by casting a mixture of LiFePO4, carbon black, and polyvinylidine fluoride in Nmethyl-2-pyrrolidone solution with the weight ratio of 7.5:1.5:1 on aluminum foil. The cycling performance of the full cell was carried out at a constant current density of 50 mA/g.

exhibited defect-free and bead-free fibrous morphology, with a mean fiber diameter of 234 ± 53 nm. The fibers were randomly oriented, forming a netlike structure. When Ni and Co were coprecipitated in the presence of urea, urea dissociated with water to form hydroxyl ions and ammonium ions. The hydroxyl ions further reacted with Ni and Co ions, forming Ni(OH)2 and Co(OH)2. The chemical reactions are as follows:14 CO(NH 2)2 + 3H 2O → 2NH4 + + CO2 + 2OH−

(2)

M2 + + 2OH− → M(OH)2

(3)

(M = Ni/Co)

In Figure 1b, after the coprecipitation of Ni(OH)2 and Co(OH)2 on the fiber surface, Ni−Co(OH)2 exhibited a flakelike, vertically interconnected structure on the surface of the carbon nanofibers. This close connection between Ni− Co(OH)2 and the carbon nanofibers provided excellent electron-diffusion paths. The attachment of Ni−Co(OH)2 to the surface of the carbon nanofibers considerably increased the fiber diameter to 347 ± 64 nm. Furthermore, the as-prepared carbon nanofiber/Ni−Co(OH)2 composites were thermally annealed at various temperatures in nitrogen. After annealing, the composite fibers retained their hierarchical structure; with increasing temperature, the Ni−Co(OH)2 nanoflakes gradually converted to a needlelike structure (Figure 1, panels c and d). When the annealing temperature reached 400 °C, the nanoneedles aggregated to form flowerlike particles randomly dispersed on the fiber surface (Figure 1e). A thermogravimetric analysis (TGA) thermogram (Figure S1) indicated that when Ni(OH)2 and Co(OH)2 were thermally annealed at a



RESULTS AND DISCUSSION Physicochemical Properties of Composite Fibers. Figure 1 shows the representative SEM images of electrospun carbon nanofibers and their composites with Ni−Co(OH)2 annealed at various temperatures. The CNF sample (Figure 1a) C

DOI: 10.1021/acs.jpcc.7b07412 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Properties of Composite Fibers sample CNF CNF-NiCo CNF-NiCo200 CNF-NiCo300 CNF-NiCo400

D (nm)a

SA (m2/g)b

VT (cm3/g)c

Vmeso (cm3/g)d

Vmicro (cm3/g)e

σ (S/m)f

± ± ± ± ±

15.3 18.4 22.8 50.3 34.1

0.037 0.047 0.054 0.067 0.067

0.035 0.047 0.054 0.060 0.065

0.002 0 0 0.007 0.002

295 76 82 134 187

234 347 366 302 258

53 64 56 50 54

a D: mean fiber diameter. bSA: specific surface area. cVT: total pore volume. dVmeso: pore volume at a pore size of 1.7−300 nm. eVmicro: pore volume at a pore size of 650 mAh/g in the first cycle to 95% at 100 mA/g 77.5% at 300 mA/g ∼75% at 100 mA/g not reported

this study [1] [2] [3] [4] [12] [14] [15] [42]

K

DOI: 10.1021/acs.jpcc.7b07412 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b07412 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b07412 J. Phys. Chem. C XXXX, XXX, XXX−XXX