The electrochemical behavior of nitrogen-doped carbon nanofibers derived from a polyacrylonitrile precursor in lithium sulfur batteries

YAO Shan-shan, HE Yan-ping , Arslan Majeed , ZHANG Cui-juan ,SHEN Xiang-qian , LI Tian-bao , QIN Shi-biao

（1.Insititute for Advanced Maerials, College of Materials and Engineering, Jiangsu University, Zhenjiang 212013, China;2.Hunan Engineering Laboratory of Power Cathode Materials, Changsha Research Institute of Mining and Metallurgy, Changsha 410012, China）

Abstract：A 3D assembly of nitrogen-doped carbon nanofibers (NCFs) derived from polyacrylonitrile was synthesized by a combined electrospinning/carbonization technique and was used as the positive current collector in lithium sulfur (Li-S) batteries containing a Li2S6 catholyte solution.The physical and electrochemical behavior of the NCFs were investigated and it was found that their electrochemical performances depended on the pyrolysis temperature.Of the samples carbonized at 800,900 and 1 000 °C,those carbonized at 900 °C performed best,and delivered a reversible capacity of 875 mAh?g?1 at a high sulfur loading of 4.19 mg?cm2 and retained at 707 mAh?g?1 after 250 cycles at 0.2 C.The coulombic efficiency of the NCF-900@Li2S6 electrode was almost 98.55% over the entire cycle life.In addition,the capacity retention of the electrode reached 81.53% even at a high current density of 1 C for over 150 cycles.It was found that the NCFs carbonized at 900 °C had the highest electrical conductivity,which might be the dominant factor that determined its performance for use as a positive current collector.

Key words:Polyacrylonitrile；NCFs；Pyrolysis temperatures；Electrochemical behaviors

1 Introduction

Development of efficient electrochemical energy conversion and storage technologies could promote sustainability energy.Compared with the commercial battery energy storage,lithium-sulfur (Li-S) batteries have attracted considerable attention due to their numerous advantages,such as high energy density(2 600 Wh/kg),rich raw materials,low cost,which represent the promising next-generation high energy power system[1,2].However,the poor electronic/ionic conductivity of elemental sulfur and its solid discharge products (Li2S2/Li2S) caused a low utilization of sulfur and rapid capacity fading.Additionally,the dissolution and migration of polysulfides (Li2Sn,4 ≤n≤ 8) in electrolytes cause the lithium anode corrosion[3,4].

In the past decades,plenty of research concentrated on the aforesaid hindrances has been unscrambled to improve the cycling performance of Li-S cells.The regular stratagem is the combination of sulfur with conductive carbonaceous additives,such as carbon nanofibers/nanotubes[5,6],graphene[7]and porous carbon with diverse pore sizes[8,9].Due to their high electrical conductivity accompanied by reasonably designed pore system,porous carbons show high potential to anchor sulfur,trap soluble lithium polysulfides and eliminate volume changes during charge/discharge process[10].Nevertheless,the carbonaceous hosts alleviate the migration of lithium polysulfides by physical-sorption[11].The heteroatoms doped carbonaceous materials were used as sulfur host and suppressed lithium polysulfides shuttle effect by chemisorption[12,13].

Nitrogen doped carbonaceous materials with pyridinic-N and pyrrolic-N active sites,which have been proved to be an efficient way for chemisorption polysulfides[14,15].Wang et al reported that the nitrogen (N)-doped graphene can improve the cycle stability for sulfur cathode and explained that N-doping facilitates Li+diffusion and thereby effectively promoting the charge transfer form polysulfide species to Ndoped substrates with Li atoms serving as bridges[16].Song et al.claimed that polysulfide species can be confined due to the enhanced affinity caused by Ndoping[17].In a combined experimental and theoretical work,Li et al.concluded that the improvement in the performance of Li-S batteries originates from the enhanced binding of polysulfide species and N-sites[18].Other experimental have also indicated that two types of pyridinic N and pyrrolic N are more effective in forming SxLi-N interactions,which result in alleviating dissolution of polysulfide species in the electrolyte and improving their redeposition process during discharge/charge[19,20].Qiu and Rao calculated the binding energies of Li2Snon N-doped graphene sheets in detail and showed that compared with pristine graphene,the binding energies of Li2Snon N-doped samples were significantly increased[21,22].As stated above,researchers usually calculated binding energies between polysulfide species and N-sites based on density functional theory (DFT).It is well-known that the polysulfide adsorption capability of nitrogen doped carbon is determined by the total amount of introduced nitrogen and its chemical state,which is associated with nitrogen precursors or heat treatment conditions.Also,the specific surface area and the porous structure are one of the important factors that determine the activity[23,24].

As we known,carbonized nanofibers produced from pyrolysis of electrospun polyacrylonitrile (PAN)precursor has received a great amount of attention due to their promising potentials[25,26].Cho et al.provided an in-depth analysis of electrospun PAN precursor followed by stabilization and carbonization of the nanofibers processed under different pyrolysis temperatures.The results showed that different pyrolysis temperature condition had a significant influence on chemical and morphological characteristics of the products[27].However,there was no electrochemical analysis conducted in the study to examine the applicability of fabricated material in real application.Meanwhile,in contrast to the traditional solid sulfur melting approach,here we used polysulfide-containing liquid catholyte,which have been often avoided in typical Li-S batteries due to the promotion of the shuttle mechanism.The liquid catholyte (Li2S6),however,can alleviate the aggregation of irreversible S or Li2S,and offer a higher utilization of active materials,comparing to the sluggish reaction of insulating solid phases[28].Several recent studies also used polysulfide-containing electrolytes as shuttle inhibitors,backups for sulfur,or active materials,but low sulfur loading or/and fast capacity fading of these batteries are still far from satisfactory[29,30].Herein,in this study,three-dimensional nitrogen-doped carbon nanofibers (NCFs) derived from PAN was successfully synthesized by a combined electrospining/carbonization technique.NCFs were employed as positive current collector containing Li2S6catholyte solution for Li-S batteries.As a result,NCFs prepared at 900℃(denoted as NCFs-900) shows a higher electronic conductivity,a larger specific area and a higher content of pyridinic-N.The free-standing NCFs-900@Li2S6electrode (sulfur loading:4.19 mg cm?2) delivers an initial capacity of 875 mAh·g?1at 0.2 C and maintains 707 mAh·g?1after 250 cycles.Even at high current density of 1 C,the capacity retention of electrode reaches 81.53% after 150 cycles,which demonstrate the improved cycling stability.

2 Experimental

2.1 Fabrication of NCFs and NCFs@Li2S6 electrode

Nitrogen doped carbon nanofibers (NCFs) were prepared by a combined electrospinning/carbonization technique.4 g of polyacrylonitrile (PAN,Mw=150 000) polymer was dispersed into 20 mL of N,Ndimethylformamdie (DMF) solvent with stirring for 24 h to get PAN/DMF solution precursor.The PAN nanofibers were formed under the electrospun condition:working voltage was 15 kV,a feeding rate 0.5 mL·h?1and aluminum foil as grounded counter electrode.The as-collected nanofibers were dried overnight under vacuum at 60 ℃ and pre-oxidization at 260 ℃ for 2 h and then carbonization for 2 h under nitrogen atmosphere.The NCFs prepared at the three different carbonization temperatures were denoted as NCFs-800,NCFs-900 and NCFs-1000,respectively.The schematic illustration of NCFs was shown in Fig.1 and NCFs membranes were sliced into the disc with the diameter of 12 mm was shown in Fig.S1.

Fig.1 Schematic illustration of NCFs

The blank electrolyte was synthesized 1 mol L?1lithium bis(trifluoromethanesulfonyl) in a 1∶1V/Va solution of 1,2-dimethoxyethane and 1,3-DOL containing LiNO3(2 wt.%).The chemical reaction of sulfur and the appropriated number of Li2S was stirred at 60 ℃ for 24 h forming 1 mol L?1Li2S6catholyte in the blank electrolyte.The proportion of the NCFs collectors were adjusted to keep the sulfur contents in all the complex cathodes around 73.3 wt%.The mass of sulfur was defined byms=Ms/A= (V×M×n×Mr)/A.Where,mswas loading of sulfur (mg cm?2),Mswas mass of sulfur for each electrode (mg),Awas the area of NCFs electrode (A= 1.13 cm?2for the disc in 12 mm diameter),Vwas the volume of added catholyte (L),Mwas the mole concentration of lithium polysulfides Li2Sn(n= 6) (M:mol·L?1),Mrwas relative molecular mass of sulfur (32 g·mol?1).24.7 μL Li2S6catholyte equal to 4.74 mg of sulfur and the final areal sulfur loading of the electrode was determined about 4.19 mg·cm?2.

2.2 Materials characterization

XRD measurements were performed on a Bruker D8 advance diffractometer using Cu Kα radiation and scanned in the range of 10o-80o.The Raman spectra were acquired on a Renishaw Raman Invia spectrometer.The electronic conductivity of NCFs membranes was characterized four point probe tester.Xray photoelectron spectroscopy (XPS,VG Multilab 2000) analysis of the NCFs was using a monochromatic Al Kα X-ray source.The morphology of the composites was measured by scanning electron microscopy (JEO-JSM-7800F) and transmission electron microscopy (TENCAI G2F30).

2.3 Electrochemical measurement

An electrode comprising NCFswas evaluated as cathode using 2032 coin cells with a Celagard 2400 membrane as the separator.The cyclic voltammogram (CV) was measured on the CHI600E electrochemical workstation in a range of 1.7?2.8 V.A VMP2 multi-channel potentiostat was used to perform electrochemical impedance tests a frequency range of 0.01-10 mHz.Cycling performance and rate capability tests were performed with LAND 2001A battery testers,setting voltage from 1.7 V to 2.8 V.

3 Results and discussion

3.1 Characterization of NCFs

The pXRD patterns of NCFs in Fig.2(a) present a broad peak at around 24.93o,corresponding to the(002) plane,a characteristic of the disordered carbon materials[31].The lattice constants of the (002) peak calculated by the Bragg's law are shown in Table 1.With increase in carbonization temperature from 800,900 to 1 000 ℃,the interlayer spacing decreases slightly from 0.358,0.357 to 0.355 nm,which is always greater than that of graphite (0.336 nm)[32,33].The Raman spectrum was measured to describe the graphitization of NCFs (Fig.2(b).The Raman spectrum of NCFs possesses two well-known main peaks at 1 343 cm?1(D-band) and 1 567 cm?1(G-band),which are related to sp3-hybridized carbon and sp2carbon-type,respectively[34,35].As shown in Table 1,the ratio ofIG/IDincreased with the carbonization temperature and represented the transformation of disordered carbon into graphitic carbon during the carbonization process.However,the NCFs-1000 membrane shows the lower electronic conductivity,which is attributed to the fracture of NCFs with a higher carbonization temperature and they cannot maintain the continuity[36].Fig.S2 presents the adsorption-desorption isotherms and pore-sized distribution of composite membrane.Table 1 shows the data.The specific surface area is 83.67,142.82 and 130.6 m2·g?1for NCFs-800,NCFs-900 and NCFs-1000,respectively,with pore volume of 0.07,0.18 and 0.27 cm3·g?1.The pore structure of NCFs-900 has effectively permeating the electrolyte and thus lithium ions to diffuse rapidly.The XPS measurement spectra of NCFs is shown in Fig.2(c),and the presence of 3 distinct peaks in NCFs at 284.0,399.5 and 530.6 eV can be attributed to C 1s,N 1s and O 1s,respectively.All 3 NCFs show two N 1s peaks and can be deconvoluted into 3 peaks at 398.2,400.7 and 401.9 eV,which are attributed to pyridinic-N,pyrrolic-N and graphitic-N,respectively(Fig.2(d-f)).Table 2 presents the nitrogen contents analysis results for NCFs.The nitrogen contents decreases with increasing carbonization temperature.It is notable that the relative amount of pyrrolic-N also decreases with increases carbonization temperature,which indicates that pyrrolic-N has been chemically transformed into other nitrogen species,such as pyridinic-N and other N-O,etc[37].The electrochemical performance,especially for rate performance and electronic/ionic conductivity,could be improved by the existence of pyridinic-N and graphitic-N structures[38,39].Fig.2(g-i) show the HRTEM images of NCFs.Turbostratic structure and rough fiber surface are observable,indicating the presence of amorphous structure and structural defects which are favorable for Li+diffusion from various orientations and can provide sufficient contact area between active materials and the electrolyte.

Table 1 Microstructure parameters of NCFs.

Table 2 The nitrogen functional groups of NCFs obtained from XPS peak analysis.

Fig.2 (a) XRD patterns,(b) Raman spectra of NCFs; (c) N1s XPS spectra of (d) NCFs-800; (e) NCFs-900 and (f) NCFs-1000;HRTEM images of (g) NCFs-800; (h) NCFs-900 and (i) NCFs-1000.

The morphologies of the PAN nanofibers and NCFs were performed by SEM.Fig.3(a) shows the SEM image of the PAN nanofibers and the average diameter is about 450 nm,which are shrinked after carbonization.Fig.3(b-d) shows the typical SEM images of NCFs,which are carbonized at 800,900 and 1 000 ℃ with the precursor PAN nanofibers.The morphology of the NCFs-800,NCFs-900 and NCFs-1000 is similar to the PAN nanofibers with the average diameters at about 270,200 and 170 nm,respectively.The diameter of NCFs was decreased with increasing the carbonization temperature due to the large amount of small organic molecules releasing during higher carbonization temperature[40,41].Nevertheless,the fracture of NCFs-1000 with a higher carbonization temperature and cannot maintain the continuity,which reduces the electrochemical properties in electrodes.The cross section of NCFs-900 membrane was about 100 μm.This three-dimensional structure of NCFs could be performed as free-standing electrode materials.

Fig.3 SEM images of (a) PAN nanofibers; (b) NCFs-800; (c) NCFs-900 and (d) NCFs-1000; (e) Cross section of NCFs-900.

3.2 Electrochemical behaviors of NCFs

Electrochemical performances of Li-S cells with NCFs@Li2S6were then investigated by CV (Fig.4(a)).The NCFs@Li2S6composite electrodes were at the first circulated at 0.05 C,and then the cells were in the activation and fully charged at 0.2 C.As depicted in Fig.4(a) ,there are two reduction peaks and two oxidation peaks in the CV curves[42].Two reduction peaks at 2.29?2.31 V and 1.97?2.01 V,respectively,correspond to the transformation of long-chain polysulfide(Li2Sn,4 ≤n≤ 8) and further to Li2S2/Li2S[43,44].During the following charge,two overlapping anodic peaks were observed around 2.36 and 2.42 V are attributed to the oxidation of solid Li2S2/Li2S to soluble polysulfides finally to elemental sulfur[45].Obviously,there are almost same CV curves of three cells.While the NCFs-900@Li2S6possesses a higher current intensity,which suggest an increase in capacity.Additionally,the NCFs-900 can lower the polarization(ΔV= 0.40 V) and enhance the electrochemical reaction kinetics.In Fig.4(b) shows the discharge curves of three kinds of electrodes,which comprises of three well-defined discharge plateaus and are in agreement with the CV curves.To further analyze the excellent capacity characteristics,the values ofQa(the capacity of the higher discharge plateau),Qb(the capacity of the lower discharge plateau) and the ratio ofQb/Qaare presented[46,47].NCFs-900@Li2S6electrode has the highestQb/Qa(2.42) than NCFs-800@Li2S6(2.37) and NCFs-1000@Li2S6(2.25) electrodes,indicating that NCFs-900 contributes to accelerate the reactions kinetic.The rate performance of Li-S cells with NCFs@Li2S6(correspond sulfur loading:4.19 mg·cm?2) is shown in Fig.4(c).For the cell with NCFs-900@Li2S6electrode,it achieves discharge capacities of 947,875,773,721 and 623 mAh·g?1at 0.1,0.2,0.5,1 and 2 C,respectively.And when the rate returns to 0.1 C,the capacity was recovered to 923 mAh·g?1,suggesting the excellent stability of the NCFs-900@Li2S6electrode.Cycling performances and coulombic efficiency were tested at 0.2 C (Fig.4(d)).NCFs-900@Li2S6electrode exhibits a capacity of 875 mAh·g?1and maintains at 707 mAh·g?1at 0.2 C over 250 cycles.While,the capacities of NCFs-800@Li2S6and NCFs-1000@Li2S6electrodes faded to 531 and 394 mAh·g?1,respectively.In addition,the Coulombic efficiency of NCFs-900@Li2S6electrode is almost 98.55% over the entire cycling.Even at 1 C (Fig.S3),the capacity retention of NCFs-900@Li2S6electrode reaches 81.53% after 150 cycles,behaving improved cycling stability.Among all 3 NCFs,NCFs-900 displays the excellent electrochemical performance,mainly owing to its high conductivity (12.19 s·cm?1)and large surface area (142.82 m2·g?1),which is greater than those of NCFs-800 (0.26 S·cm?1; 83.67 m2·g?1)and NCFs-1000 (2.78 S·cm?1; 130.60 m2·g?1),respectively.Especially,the coexistence of both pyridinc-N and gaphitic-N in the NCFs-900 has been used to enhance the electrochemical performance[48,49].The Ndoped carbon hosts have attracted great attention for sulfur cathodes because these matrices not only enhance the conductivity of sulfur but also increase the interaction of polar N sites and anchor polysulfides,which can alleviate polysulfides shuttling through physical and chemical confinement using the porous structure and chemisorption.Similar result has also been found by Guo and Nazar et al.in their study on N-doped carbon materials in Li-S batteries[50,51].Therefore,good electrochemical properties of NCFs can be obtained by optimizing the thermal treatment conditions,such as treatment temperature and time,and balancing the structural parameters such as pore structure of unique nanofibers,etc[52].Generally,the superior electrochemical performance of NCFs-900 is also attributed to the unique nanofibrou structure with appropriate surface area.As illustrated in Fig.4 (e),first of all,the unique nanofibrous carbon structure formed by continuous N-CNFs ensures an efficient and uninterrupted electron transport.Secondly,the doping nitrogen in NCFs was to be polarized and more active in anchoring polysulfides.Based on the synergistic effect of nitrogen doping and the mesoporous conductive pathway,the as-fabricated NCFs-900 has displayed excellent electrochemical performance.

Fig.4 Electrochemical performances of NCFs@Li2S6 composite electrodes:(a) CV profiles at 0.1 mV s?1 between 1.7-2.8 V;(b) Discharge profiles at 0.2 C; (c) Rate performances of NCFs@Li2S6 composite electrodes at various current densities from 0.1 to 2 C;(d) Long cycling performance of NCFs@Li2S6 composite electrodes at 0.2 C; (e) Schematic of Li2S6 adsorption in the NCFs.

Fig.5(a) is the Nyquist plots of the cells with NCFs@Li2S6composite electrodes.The EIS plots after fitting based on the equivalent circuit of the composite electrodes consist of a high-frequency semicircle and an inclined line at low-frequency region.Moreover,the kinetic values of the two electrodes are listed in Table S1.Obviously,the charge transfer resistance (Rct) of the cell using NCFs-900 (23.53 Ω) is the lower than that of the cell using NCFs-800(27.28 Ω).It is important to note that although the NCFs-1000 has the lowestRct(20.02 Ω),which is due to a higher electronic conductivity.However,the fracture of NCFs-1000 caused unstable structure could reduce the migration of lithium ions.A linear relationship between the resistance in real part and angular velocity to invers square root during the low-frequency region is also displayed in Fig.5(b)[53,54].TheDLi+values according to the Equation S1 and S2 are 8.36 ×10?10,4.08×10?9and 3.11×10?10cm2·s?1for NCFs-800@Li2S6,NCFs-900@Li2S6and NCFs-1000@Li2S6composite electrode,which further proved as above results.

Fig.5 (a) EIS of the Li-S batteries with NCFs@Li2S6 electrode; (b) The dependence of Zre on the reciprocal square root of the frequency ω-0.5 in the low frequency region for composite electrodes

To further understand the effect of NCFs on polysulfides,XPS and TEM were used to analyze the composition and morphologies of the cycled electrode.Fig.S4 shows the high resolution spectrum of S2p,indicating that the binding energy of 162.7 eV and 164.3 eV represents Li–S and S–S bond,and the binding energy of 166.1 eV and 169.2 eV corresponding to SO32-and S–O.In fact,the former Li-S bond and S-S bond are attributed to elemental sulfur and deposition of Li2S.The presence of latter SO32-and S–O bond are due to sulfate and the decomposition of the electrolyte used to impregnate the electrode during charging and discharging[55,56].Additionally,the NCFs after cycling were characterized by HRTEM.Due to the high vacuum and high voltage HRTEM testing environment and the generationof high temperature on the surface as the electrons pass through the sample,the deposited products (such as sulfur or polysulfide species) were sublimated.A video was performed to exhibit the changing of sublimation process as shown in supplementary material.Fig.6(a-d)shows several screen shots of abovementioned video at different times,respectively.Compared with the smooth surface of pristine NCFs (Fig.S5),the deposited products on the NCFs gradually disappeared because of the electrons irradiation,which indicate that the NCFs have capability of anchoring polysulfides and improving the utilization of active material.Fig.6(e) is an EDS elemental mapping of NCFs that shows the homogenous distribution of carbon,nitrogen and sulfur.

Fig.6 TEM images of NCFs-900 at different electron irradiation times:(a) 0 s; (b) 5 s; (c) 10 s and (d) 15 s; (e) The corresponding elemental mapping images of NCFs-900@Li2S6 electrode at charge state after 250 cycles.

4 Conclusion

In summary,the fibrous NCFs membrane was prepared by a combined electrospinning/carbonization technique.The result indicated that carbonization processes of PAN webs influenced the chemical and morphological characteristics as well as the thermal properties according to the pyrolysis temperature.The continuous conducting framework of NCFs is connected to the current collector and contained Li2S6catholyte solution for Li-S batteries.Especially for NCFs-900,it shows the initial discharge capacity of 875 mAh·g?1with 4.19 mg·cm?2sulfur loading and retains 707 mAh·g?1at 0.2 C over 250 cycles.Even at high current density of 1 C,the capacity retention of electrode reaches 81.53% after 150 cycles,which behaving improvement cycling stability.It may be suggested that appropriate control of carbonization processes probably plays a role in monitoring the characteristics of NCFs for energy storage applications.

Acknowledgments

Financial support from National Natural Science Foundation of China (51874146),China Postdoctoral Science Foundation (2018T110551 and 2017M621640),Six Talent Peaks Project of Jiangsu Province (XCL-125),Start-up Foundation of Jiangsu University for Senior Talents (15JDG014) are gratefully acknowledged.