Highly Graphitic Carbon Nanofibers Web as a Cathode Material for Lithium Oxygen Batteries

The lithium oxygen battery is a promising energy storage system due to its high theoretical energy density and ability to use oxygen from air as a “fuel”. Although various carbonaceous materials have been widely used as a cathode material due to their high electronic conductivity and facial processability, previous studies mainly focused on the electrochemical properties associated with the materials (such as graphene and carbon nanotubes) and the electrode configuration. Recent reports demonstrated that the polarization associated with cycling could be significantly increased by lithium carbonates generated from the reaction between the carbon cathode and an electrolyte, which indicates that the physicochemical properties of the carbon cathode could play an important role on the electrochemical performances. However, there is no systematic study to understand these phenomena. Here, we systematically explore the electrochemical properties of carbon nanofibers (CNF) webs with different graphitization degree as a cathode for Li oxygen batteries. The physicochemical properties and electrochemical properties of CNF webs were carefully monitored before and after cycling. CNF webs are prepared at 1000, 1200 and 1400 ◦C. CNF web pyrolyzed at 1400 ◦C shows lowered polarization and improved cycle retention compared to those of CNF webs pyrolyzed at 1000 and 1200 ◦C.


Introduction
Development of new energy storage systems with a high energy density beyond Li ion batteries is essential to the critical applications such as large-scale storage for renewable power sources, electric vehicles, and plug-in hybrid electric vehicles [1,2].Li-O 2 batteries have received significant attention as a promising energy storage system due to their higher energy density (~3500 Wh•kg −1 ) than conventional Li ion batteries and simple concept [3,4].However, their poor electrochemical properties, caused by the electrolyte instability, a large polarization upon both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and pore clogging of cathode by the discharge product (Li 2 O 2 ), limit their practical use [5,6].
Various materials and structures on the cathode have been intensively explored to overcome these problems [7][8][9][10][11].Many kinds of carbonaceous materials, including carbon black, single/multi-walled carbon nanotubes, graphene and reduced graphene oxide, have been intensively explored as a cathode material due to their high electronic conductivity and facile processability [12][13][14][15].The pore size and volume in the carbonaceous materials based cathode were engineered to provide a large tri-phase (solid-liquid-gas) regions for oxygen reduction and prevent the pore clogging associated with Li 2 O 2 discharge product [16].Recent reports demonstrated that the carbon cathode reacts with Li 2 O 2 and this reaction results in the formation of lithium carbonates film on the surface of carbon cathode, which significantly increases the charge polarization [3,[17][18][19][20][21].To decrease polarization associated with formation/decomposition of Li 2 O 2 , various transition metal oxides and novel metals have been explored as a catalyst [5,[22][23][24].Although it is generally accepted that the electrocatalyst could enhance the kinetics associated with formation/decomposition of Li 2 O 2 , their catalytic activity should be investigated in the stable electrolyte system to understand their substantive effectiveness and underlying mechanism.Regarding the catalytic effect of carbonaceous materials itself during the cycling, it is certain that carbon materials play an important role in electrochemical performances.Therefore, it is expected that the physicochemical properties and geometry of carbon based cathode could significantly affect upon the electrochemical performances.However, there is no systematic study to understand the effect of physicochemical properties of carbon cathode on the electrochemical performances, which is critical to improve the performances of Li-oxygen batteries.
Here, we systematically study the electrochemical properties of carbon nanofibers (CNF) webs with different graphitization degree as a cathode for Li oxygen batteries.CNF webs are simply prepared using electrospinning method, and its graphitization degree was controlled by adjusting pyrolysis temperature [25,26].The three dimensional network structure of CNF web enables an efficient electron transport along one dimensional geometry and provides uniform pore structure [27].The geometrical advantages of CNF webs allow us to investigate the influence of graphitization degree on the electrochemical performances without its degradation caused by the pore clogging associated with Li 2 O 2 .The physicochemical properties and electrochemical properties of CNF webs were carefully monitored before and after cycling.The CNF web shows different electrochemical properties as a function of the graphitization degree.The CNF web with high graphitization degree exhibits the low polarization during OER/ORR reactions and improved cycle performance compared to those of the CNF webs with lower graphitization degree.These improvements could be attributed to the enhanced chemical stability came from the high graphitization degree.

Preparation of Carbon Nanofibers Air Electrode
Carbon nanofibers were synthesized using electrospinning.Polyacrylonitrile (PAN, Mw ≈ 150,000, Sigma Aldrich, St. Louis, MO, USA) was dissolved in DMF to form a 10 wt % solution.The prepared solutions were mixed using a magnetic stirrer at 60 • C for 2 h.Then, the prepared solutions were transferred into a 20 mL syringe with an 18-gauge metal nozzle made of stainless steel.The solution was electrospun at a DC voltage of ~20 kV and a flow rate of 0.5 mL•h −1 .The metallic current collector was placed ~10 cm below the tip of the nozzle to collect the nanofibers.The as-spun nanofibers were stabilized at 250 • C for 1 h in air atmosphere to induce dimensional stability of the carbon nanofibers.Then, carbon nanofibers were obtained by the calcination of as-spun nanofibers in argon atmosphere at 1000 • C, 1200 • C, and 1400 • C for 1 h.

Evaluation of Electrochemical Properties
A Swagelok-type Li air cell was assembled using the carbon nanofibers cathode together with a Li anode and glass fibre separator (GF/D, Whatman, Maidstone, UK).1.0 M LiTFSi in TEGDME was used as the electrolyte.Assembling of the Swagelok-type cells was carried out in an argon glove box.After the sealed cells were removed from the argon glove box, the cathode side was exposed to a pressure balanced chamber filled with 1 atm dry O 2 gas (<3 ppm H 2 O).The Swagelok-type cells were cycled between 2.5 V and 4.5 V using a battery cycle tester (TOSCAT 3000, Toyo Systems, Tokyo, Japan).

Results and Discussion
Figure 1 shows electron microscope images of CNFs webs synthesized at 1000 • C (CNF 1000), 1200 • C (CNF 1200) and 1400 • C (CNF 1400).The CNFs have uniform diameter ranging from 350 nm to 450 nm regardless of the pyrolysis temperature.However, their morphologies were significantly influenced by the processing temperature.The CNFs become wavy from straight morphology as increasing pyrolysis temperature.(Figure 1a,e,i) High magnification scanning electron microscope (SEM) images clearly reveal morphological changes in the surface as well as the junction between CNFs.(Figure 1b,f,j) The CNF 1200 and CNF 1400 have smooth surface while the CNF 1000 shows a rough surface, which indicates the rearrangement of carbon atoms by the increase of pyrolysis temperature [28,29].The morphological change in the junction between CNFs is also significant.Physically touched interface changes to being merged as an increase of pyrolysis temperature.Figure 1j clearly shows fused cross junction for CNF 1400.The microstructure of CNF webs was further characterized using the transmission electron microscopy (TEM).Although the CNF 1000 shows a significant contrast in the brightness at the core and shell (Figure 1c), any noticeable difference in the brightness is not observed for both CNF 1200 and CNF 1400 (Figure 1g,k), which is associated with the rearrangement of carbon atoms at the surface.Figure 1d,h,i show the high resolution (HR) TEM images for CNF webs.An increase in pyrolysis temperature results in a transformation of the amorphous and disordered structure into the high ordered and large graphitic structure.These structural and morphological differences in the CNFs, such as graphitization degree and in junction interface, significantly affect the electrical property.The CNF 1000, 1200 and 1400 exhibit sheet resistances of 15.3, 9.6 and 5.0 ohm/sq (Figure S1).

Results and Discussion
Figure 1 shows electron microscope images of CNFs webs synthesized at 1000 °C (CNF 1000), 1200 °C (CNF 1200) and 1400 °C (CNF 1400).The CNFs have uniform diameter ranging from 350 nm to 450 nm regardless of the pyrolysis temperature.However, their morphologies were significantly influenced by the processing temperature.The CNFs become wavy from straight morphology as increasing pyrolysis temperature.(Figure 1a,e,i) High magnification scanning electron microscope (SEM) images clearly reveal morphological changes in the surface as well as the junction between CNFs.(Figure 1b,f,j) The CNF 1200 and CNF 1400 have smooth surface while the CNF 1000 shows a rough surface, which indicates the rearrangement of carbon atoms by the increase of pyrolysis temperature [28,29].The morphological change in the junction between CNFs is also significant.Physically touched interface changes to being merged as an increase of pyrolysis temperature.Figure 1j clearly shows fused cross junction for CNF 1400.The microstructure of CNF webs was further characterized using the transmission electron microscopy (TEM).Although the CNF 1000 shows a significant contrast in the brightness at the core and shell (Figure 1c), any noticeable difference in the brightness is not observed for both CNF 1200 and CNF 1400 (Figure 1g,k), which is associated with the rearrangement of carbon atoms at the surface.Figure 1d,h,i show the high resolution (HR) TEM images for CNF webs.An increase in pyrolysis temperature results in a transformation of the amorphous and disordered structure into the high ordered and large graphitic structure.These structural and morphological differences in the CNFs, such as graphitization degree and in junction interface, significantly affect the electrical property.The CNF 1000, 1200 and 1400 exhibit sheet resistances of 15.3, 9.6 and 5.0 ohm/sq (Figure S1).X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed to investigate the physicochemical properties of CNF webs.XRD patterns of CNF webs are shown in Figure 2a.The diffraction peak around the 2θ angle of 25°~26° is attributed to the (002) crystallographic plane of graphite crystallite.The intensity and sharpness of this peak gradually increased with the pyrolysis temperature.Figure 2b shows the Raman spectra for CNF webs.Two X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed to investigate the physicochemical properties of CNF webs.XRD patterns of CNF webs are shown in Figure 2a.The diffraction peak around the 2θ angle of 25 • ~26 • is attributed to the (002) crystallographic plane of graphite crystallite.The intensity and sharpness of this peak gradually increased with the pyrolysis temperature.Figure 2b shows the Raman spectra for CNF webs.Two dominant peaks are observed at the ~1340 cm −1 and ~1580 cm −1 , which are assigned to D-band and G-band, respectively.The intensity ratio of D band to G band (I D /I G ) of the CNF webs decreases with an increase of the pyrolysis temperature, which clearly implies an enhancement in the graphitization degree.The high-resolution C1s spectra of XPS for CNF webs are shown in Figure 2c.Although all CNF webs have the highest intensities at 284.6 eV, the full width at half maximum (FWHM) for C1s spectra depends on their pyrolysis temperature.The CNF 1000, 1200 and 1400 have the FWHM values of 1.5, 1.4 and 1.2 eV, respectively, which indicates CNF 1400 has the highest graphitization degree among CNF webs.In O1s spectra (Figure 2d), both CNF 1200 and CNF 1400 exhibit more symmetric and narrow spectra towards higher binding energy than those of CNF 1000, which implies a decrease in the relative proportion of C=O and C-OH groups.The CNF 1200 and 1400 do not show any noticeable N1s related peak while the obvious N1s related peak is observed in CNF 1000 (Figure S2).The N1s related peak in CNF 1000 reveals the presence of nitrogen atom from polyacrylonitrile (PAN).The N atoms might be related to the high I D /I G intensity in the Raman result and low electronic conductivity [30,31].
Appl.Sci.2018, 8, x FOR PEER REVIEW 4 of 9 dominant peaks are observed at the ~1340 cm −1 and ~1580 cm −1 , which are assigned to D-band and G-band, respectively.The intensity ratio of D band to G band (ID/IG) of the CNF webs decreases with an increase of the pyrolysis temperature, which clearly implies an enhancement in the graphitization degree.The high-resolution C1s spectra of XPS for CNF webs are shown in Figure 2c.Although all CNF webs have the highest intensities at 284.6 eV, the full width at half maximum (FWHM) for C1s spectra depends on their pyrolysis temperature.The CNF 1000, 1200 and 1400 have the FWHM values of 1.5, 1.4 and 1.2 eV, respectively, which indicates CNF 1400 has the highest graphitization degree among CNF webs.In O1s spectra (Figure 2d), both CNF 1200 and CNF 1400 exhibit more symmetric and narrow spectra towards higher binding energy than those of CNF 1000, which implies a decrease in the relative proportion of C=O and C-OH groups.The CNF 1200 and 1400 do not show any noticeable N1s related peak while the obvious N1s related peak is observed in CNF 1000 (Figure S2).The N1s related peak in CNF 1000 reveals the presence of nitrogen atom from polyacrylonitrile (PAN).The N atoms might be related to the high ID/IG intensity in the Raman result and low electronic conductivity [30,31].Figure 3 shows the voltage profiles and cycle performances for CNF 1000, CNF 1200 and CNF 1400 electrodes.The electrochemical properties were evaluated with a limited capacity of 1000 mAh/g at a current density of 50 mA/gc using 1 M LiPF6/tetra(ethylene glycol)dimethyl ether (TEGDME) as an electrolyte.As shown in Figure 3a, the CNF electrodes show the different shape in the voltage profiles depending on the pyrolysis temperature.The CNF 1000 electrode has the discharge plateau at the voltage of ~2.63 V and the charge plateau at the voltage of ~4.19 V at 1st cycle, respectively, resulting in a large polarization of ~1.56 V.It is observed that the steep potential drop during the ORR and the significant increase in the OER potential after the first cycle.Although the CNF 1200 electrode shows the consistent voltage profile during the cycling, it exhibits also large polarization.On the other hand, the CNF 1400 electrode has small polarization of ~1.16 V with OER potential under 4 V, which leads to the improvement in cycle performance.The CNF electrodes Figure 3 shows the voltage profiles and cycle performances for CNF 1000, CNF 1200 and CNF 1400 electrodes.The electrochemical properties were evaluated with a limited capacity of 1000 mAh/g at a current density of 50 mA/g c using 1 M LiPF 6 /tetra(ethylene glycol)dimethyl ether (TEGDME) as an electrolyte.As shown in Figure 3a, the CNF electrodes show the different shape in the voltage profiles depending on the pyrolysis temperature.The CNF 1000 electrode has the discharge plateau at the voltage of ~2.63 V and the charge plateau at the voltage of ~4.19 V at 1st cycle, respectively, resulting in a large polarization of ~1.56 V.It is observed that the steep potential drop during the ORR and the significant increase in the OER potential after the first cycle.Although the CNF 1200 electrode shows the consistent voltage profile during the cycling, it exhibits also large polarization.On the other hand, the CNF 1400 electrode has small polarization of ~1.16 V with OER potential under 4 V, which leads to the improvement in cycle performance.The CNF electrodes exhibit markedly different cycle performances.(Figure 3d-f) The CNF 1000 electrode works only for 5 cycles.Even though the CNF 1200 electrode exhibits slightly enhanced cycle performance compared to that of the CNF 1000 electrode, it also shows fast degradation after 9 cycles.On the other hand, the CNF 1400 electrode exhibits the stable cyclability for 50 cycles.The enhanced cyclability of the CNF 1400 electrode is attributed to the high graphitization degree, confirmed by the TEM in Figure 1, and reduced defects on the surface.In Table 1, the Li-O 2 batteries performances of various carbon electrodes are summarized.The charge and discharge voltage (V) difference of CNF webs pyrolyzed at 1000, 1200, and 1400 • C are lower than the other carbon electrodes [7,13,[32][33][34][35][36][37][38][39].
exhibit markedly different cycle performances.(Figure 3d-f) The CNF 1000 electrode works only for 5 cycles.Even though the CNF 1200 electrode exhibits slightly enhanced cycle performance compared to that of the CNF 1000 electrode, it also shows fast degradation after 9 cycles.On the other hand, the CNF 1400 electrode exhibits the stable cyclability for 50 cycles.The enhanced cyclability of the CNF 1400 electrode is attributed to the high graphitization degree, confirmed by the TEM in Figure 1, and reduced defects on the surface.In Table 1, the Li-O2 batteries performances of various carbon electrodes are summarized.The charge and discharge voltage (V) difference of CNF webs pyrolyzed at 1000, 1200, and 1400 °C are lower than the other carbon electrodes [7,13,[32][33][34][35][36][37][38][39].The morphological changes of the CNF webs were carefully monitored after the discharging and charging within a limited capacity of 1000 mAh/g.As shown in Figure 4 a,b,e,f,i,j, the discharging products were homogeneously formed on the surfaces of all CNF electrodes without any blocking and clogging.In the case of the CNF 1000 electrode, the diameter of discharging product is several tens of nanometer.However, it is observed that CNF 1200 and CNF 1400 electrodes have bigger discharging products (200-400 nm diameters) with a bead-like shape than  The morphological changes of the CNF webs were carefully monitored after the discharging and charging within a limited capacity of 1000 mAh/g.As shown in Figure 4a,b,e,f,i,j, the discharging products were homogeneously formed on the surfaces of all CNF electrodes without any blocking and clogging.In the case of the CNF 1000 electrode, the diameter of discharging product is several tens of nanometer.However, it is observed that CNF 1200 and CNF 1400 electrodes have bigger discharging products (200-400 nm diameters) with a bead-like shape than that of CNF 1000 electrode.These results suggest that the formation mechanism of lithium peroxide could be greatly affected by the graphitization degree and surface defect of carbon cathode.After charging, the noticeable discharging product was not observed in all CNF electrodes, which indicates that discharge products Li 2 O 2 were totally decomposed.
Appl.Sci.2018, 8, x FOR PEER REVIEW 6 of 9 that of CNF 1000 electrode.These results suggest that the formation mechanism of lithium peroxide could be greatly affected by the graphitization degree and surface defect of carbon cathode.After charging, the noticeable discharging product was not observed in all CNF electrodes, which indicates that discharge products Li2O2 were totally decomposed.The morphological change of CNF electrodes after the fully discharging (cut-off voltage: 2.3 V) was further investigated (Figure S3).The toroidal shaped lithium peroxides formed on the surface of CNF electrodes without clogging even after full discharge, as shown in Figure S3.Interestingly, the CNF electrodes show the different Li2O2 morphology as a function of the pyrolysis temperature.The CNF 1000 electrode with low graphitization degree and many defects shows film like Li2O2 formation with small beads of ~200 nm.In contrast, CNF 1200 and 1400 electrodes with higher graphitization and lower defect amount compared to CNF 1000 favor large toroidal Li2O2 with 500 nm and 1 μm, respectively.These results imply that the graphitization degree and functional groups on the surface play important roles for the formation shape of Li2O2.Previous reports represented that the current density effects on the morphology evolution of the discharge product [21,[40][41][42].Our results suggest further study is required to understand the dominant factors governing the shape of discharge product, which is critical to improve the electrochemical properties of Li-O2 batteries including the rate capability, polarization and round-trip efficiency.
Figure 5 shows X-ray diffraction (XRD) of the CNF electrodes after discharging and the Raman results of the CNF electrode after 1 cycle.The XRD results of CNF electrodes in Figure 5a reveal the formation of lithium peroxide (Li2O2) without Li2O and LiOH [7,43].The Li2Co3 of byproducts peak observed from XRD pattern [44,45].In the Raman spectra (Figure 5b), the intensity ratios of the D-band to the G-band for CNF electrodes after 1 cycle increase significantly compared to those of pristine CNF electrodes.The ID/IG of CNF 1000 electrode after 1 cycle is 1.6, which is 64.9% and 55.3% higher than those of CNF 1200 (ID/IG ~ 1.03) and 1400 (ID/IG ~ 0.97) electrodes after 1 cycle.The intensity ratio of ID/IG of the CNF webs after 1 cycle show similar trend to those of the pristine CNF webs.And notably, the values of ID/IG of the CNF webs after 1 cycle are increased compared those of pristine CNF webs, which results from increasing defectivity after the electrochemical cycling [46].The morphological change of CNF electrodes after the fully discharging (cut-off voltage: 2.3 V) was further investigated (Figure S3).The toroidal shaped lithium peroxides formed on the surface of CNF electrodes without clogging even after full discharge, as shown in Figure S3.Interestingly, the CNF electrodes show the different Li 2 O 2 morphology as a function of the pyrolysis temperature.The CNF 1000 electrode with low graphitization degree and many defects shows film like Li 2 O 2 formation with small beads of ~200 nm.In contrast, CNF 1200 and 1400 electrodes with higher graphitization and lower defect amount compared to CNF 1000 favor large toroidal Li 2 O 2 with 500 nm and 1 µm, respectively.These results imply that the graphitization degree and functional groups on the surface play important roles for the formation shape of Li 2 O 2 .Previous reports represented that the current density effects on the morphology evolution of the discharge product [21,[40][41][42].Our results suggest further study is required to understand the dominant factors governing the shape of discharge product, which is critical to improve the electrochemical properties of Li-O 2 batteries including the rate capability, polarization and round-trip efficiency.
Figure 5 shows X-ray diffraction (XRD) of the CNF electrodes after discharging and the Raman results of the CNF electrode after 1 cycle.The XRD results of CNF electrodes in Figure 5a reveal the formation of lithium peroxide (Li 2 O 2 ) without Li 2 O and LiOH [7,43].The Li 2 Co 3 of byproducts peak observed from XRD pattern [44,45].In the Raman spectra (Figure 5b), the intensity ratios of the D-band to the G-band for CNF electrodes after 1 cycle increase significantly compared to those of pristine CNF electrodes.The I D /I G of CNF 1000 electrode after 1 cycle is 1.6, which is 64.9% and 55.3% higher than those of CNF 1200 (I D /I G ~1.03) and 1400 (I D /I G ~0.97) electrodes after 1 cycle.The intensity ratio of I D /I G of the CNF webs after 1 cycle show similar trend to those of the pristine CNF webs.And notably, the values of I D /I G of the CNF webs after 1 cycle are increased compared those of pristine CNF webs, which results from increasing defectivity after the electrochemical cycling [46].

Conclusions
We have explored the graphitization effect of the carbon based electrode on the performances of Li-O2 batteries through the systematic approach using CNFs prepared by different pyrolysis temperature.The CNFs electrode with high graphitization degree and low surface defects shows lower polarization and improved cyclability compared to those of CNFs electrode with low graphitization degree and high surface defect.Electrochemical properties and analysis results imply that the graphitization degree and surface defect of carbon based air electrode play an important role on the electrochemical properties.We plan to synthesize the highly graphitic carbon nanofibers with metal oxide catalyst, including Co3O4, MnO2, NiO, RuO2, etc., as a cathode electrode for Li-O2 batteries and evaluate their electrochemical properties in future research.Further study on the relationship between physicochemical properties of carbon electrode and its electrochemical properties should be carried out to understand the fundamental insight.

Supplementary Materials:
The following are available online at http://www.mdpi.com/s1,Table S1: Some typical achievements for lithium oxygen cells in cathode materials, electrolytes, difference of charge and discharge voltage, and current density,

Conclusions
We have explored the graphitization effect of the carbon based electrode on the performances of Li-O 2 batteries through the systematic approach using CNFs prepared by different pyrolysis temperature.The CNFs electrode with high graphitization degree and low surface defects shows lower polarization and improved cyclability compared to those of CNFs electrode with low graphitization degree and high surface defect.Electrochemical properties and analysis results imply that the graphitization degree and surface defect of carbon based air electrode play an important role on the electrochemical properties.We plan to synthesize the highly graphitic carbon nanofibers with metal oxide catalyst, including Co 3 O 4 , MnO 2 , NiO, RuO 2 , etc., as a cathode electrode for Li-O 2 batteries and evaluate their electrochemical properties in future research.Further study on the relationship between physicochemical properties of carbon electrode and its electrochemical properties should be carried out to understand the fundamental insight.

Table 1 .
Some typical achievements for lithium oxygen cells in cathode materials, electrolytes, difference of charge and discharge voltage, and current density.

Table 1 .
Some typical achievements for lithium oxygen cells in cathode materials, electrolytes, difference of charge and discharge voltage, and current density.