硫氮掺杂碳纳米管

Sulfur e nitrogen doped multi walled carbon nanotubes composite as a cathode material for lithium sulfur batteries

Yinchuan Li a ,Rui Mi b ,Shaomin Li b ,Xichuan Liu b ,Wei Ren b ,Hao Liu b ,*,Jun Mei a ,**,Woon-Ming Lau b

a

School of Materials Science and Engineering,Southwest University of Science and Technology,Mianyang 621010,PR China b

Chengdu Green Energy and Green Manufacturing Technology R&D Center,Chengdu Development Center of Science and Technology,China Academy of Engineering Physics,Southwest Airport Economic Development Zone,Shuangliu,Chengdu 610207,PR China

a r t i c l e i n f o

Article history:

Received 31October 2013Received in revised form 26February 2014Accepted 6April 2014Available online 11May 2014Keywords:Nitrogen doped Carbon nanotubes Lithium e sulfur batteries Sulfur distribution

a b s t r a c t

The performance of lithium sulfur (Li/S)battery was greatly improved by the employment of nitrogen doped carbon nanotubes (N-CNTs)based cathode.By manipulating its structure thereby creating more defects,N-CNTs presents better dispersion of sulfur particles on N-CNTs and higher electrical conductivity compared with their non-doped counterpart,which explain the reason why N-CNTs/S composite shows improved performance.The speci?c discharge capacity was maintained at 625mAh g à1and 513mAh g à1after 100cycles at 0.2C and 0.5C,respectively,which was about 2times as that of CNTs.This method is proved to be a promising way to develop cathode materials for lithium sulfur batteries.

Copyright a2014,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights

reserved.

Introduction

The increasing capabilities of portable electronic devices as well as the desire for long driving distances between re-charges of electric vehicles require electrical energy storage systems with high energy density [1].The Lithium/sulfur (Li/S)battery is an attractive and promising candidate among emerging battery technology.It has attracted great interest as

potential energy storage devices for electrical vehicles and other applications needing large-scale electricity storage [2].Conventional Li/S cells consist of a lithium metal anode,an organic liquid electrolyte,and a sulfur composite cathode [3].Sulfur is useful in the cathode because assuming complete reaction to Li 2S,it has a theoretical speci?c capacity of 1672mAh g à1,and energy density of 2600Wh Kg à1[4],which is signi?cantly higher than the conventional lithium-ion cathode materials [5].

*Corresponding author .Tel.:t862867076208;fax:t862867076210.**Corresponding author .Tel.:t862867076202.

E-mail addresses:mliuhao@https://www.360docs.net/doc/4213931753.html, (H.Liu),meijun12@https://www.360docs.net/doc/4213931753.html, (J.

Mei).

Available online at https://www.360docs.net/doc/4213931753.html,

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journal homepage:

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 39(2014)16073e 16080

https://www.360docs.net/doc/4213931753.html,/10.1016/j.ijhydene.2014.04.047

0360-3199/Copyright a2014,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights reserved.

Although the Li/S battery has considerable advantages when considering the energy density,cost and environmental friendliness,there are many challenges associated with its commercialization[6].For example,the insulating nature of sulfur and its reduced products leads to low utilization of active material,and the intermediate lithium polysul?des (Li2S n,2

To improve the performance of Li/S battery,researchers have found that using nanostructured sulfur e carbon composite cathodes can considerably improve reversible capacity[9],rate capability[10],and cyclic performance[11]of the battery,as well as the utilization[12]of sulfur in the battery cycle.Recent reports also suggest that carbon nanotubes(CNTs)with high mechani-cal strength,advantageous electrical properties and great chemical stability[13]are regarded as promising conductive materials that could improve the cycling performance of Li e S batteries[14e16].There is a large interface area between the CNTs and the lithium polysul?des,on which the electro-chemical reaction can take place.The three-dimensional network structures with regular pores retard the out-diffusion of the intermediate lithium polysul?des from the cathode[17]. Ahn et al.[18]reported a homogeneous and well dispersed sul-fur/CNTs composite for lithium sulfur batteries,which was synthesized by a simple direct precipitation method.The sulfur/ CNTs composite exhibits excellent performance with high spe-ci?c capacity and improved cyclic durability.

Another research direction is to use nitrogen as a primary avenue for enhancing CNTs properties[19].Previous theoret-ical calculations and experiments show that substituting ni-trogen into sp2carbon structures could enhance their electronic conductivity signi?cantly because the nitrogen atoms provide additional free electrons to the conduction band[20].And doping with nitrogen leads to more disorders through an increase in the number of defects[21].Further electrochemical characterization shows that Li storage per-formance of CNTs is enhanced by nitrogen doping[22]. Although it has been proved that nitrogen enriched meso-porous carbon materials show improved performance in Li e S batteries compared with their non-doped counterpart[23],to the best of our knowledge,there were no studies on the application of N-CNTs in Li e S batteries.Our group did some related works about controlling the structure and morphology of aligned N-CNTs by varying the nitrogen content[24],and found that more defects in the structure would lead to better dispersion of nanoparticles[25].This inspired us to investigate the effect of N-CNTs/sulfur composite cathode on the elec-trochemical properties in Li e S batteries.

In this present work,N-CNTs/S composites were proposed as cathode materials for Li e S batteries.Our work demon-strates that nitrogen doping into CNTs not only increases the discharge capacity but also enhances the reversibility in the charge/discharge process.By manipulating its structure thereby creating more defects,N-CNTs promises better dispersion of sulfur particles on N-CNTs and higher electrical conductivity which explain the reason why N-CNTs/S composite show improved performance.Our work thus pro-vides a promising way to develop cathode materials for lithium sulfur batteries.Further research to increase the cycling performance is being carried on in our group.

2.Experimental

2.1.Preparation of N-CNTs

CNTs with diameters of20e30nm were purchased from Chengdu timesnano,China.N-CNTs were synthesized through an injection chemical vapor deposition(CVD)method with a tubular furnace.1g of imidazole(C3H4N2)and150mg of ferrocene(Fe(C5H5)2)were added into20ml of acetonitrile (CH3CN)and the mixture was subsequently ultrasonicated for 5min to obtain a homogeneous solution.Before the furnace was heated,argon(99.999%in purity)was introduced into the quartz tube at a?ow rate of500sccm for15min to exhaust the air in the tube.Then the system was heated to950 C at a rate of 30 C/min.Once the furnace reached the desirable tempera-ture,5ml of the solution prepared as above was injected into the tube at a rate of0.5ml/min.Then those gasi?ed droplets were carried into the center of the furnace by the argon?ow. After the exhaust of the solution,the furnace was turned off and cooled down to room temperature in the?owing argon gas.

2.2.Preparation of sulfur/carbon composites

Sublimed sulfur(99.5%)and N-CNTs were dried at60 C for 12h before use.Sublimed sulfur was then mixed with N-CNTs in the weigh ration of2:7.The mixture was ground for uni-formity and then heated at155 C for6h in a quartz tube?lled with argon gas.At this temperature,the melted sulfur has the lowest viscosity and can integrate well with N-CNTs[12].The temperature was then increased to300 C and was main-tained for2.75h to vaporize super?uous sulfur covering the surface of N-CNTs.The sulfur content in the composite, roughly estimated by the weight loss,was60%.For compari-son,the sulfur/CNTs composite with the similar sulfur con-tent was prepared by the same method.

2.3.Material characterization

Morphological and structural information were obtained from scanning electron microscopy(SEM,Hitachi S-5200),X-ray diffraction(XRD,D/max2200/PC,Rigaku,40KV,20mA,Cu K a radiation).X-ray photoelectron spectroscopy(XPS)analysis was collected at a XSAM800spectrometer using mono-chromatized Al K a-radiation at14KV.Raman spectroscopy was performed using a micro-Raman2000system(Renishaw, Britain)with a10mW helium e neon laser excitation source of wavelength633nm.Thermal Gravimetric Analysis(TGA Netzsch STA449C)was carried out under nitrogen?ow of 50mL minà1with a heating rate of20 C minà1.

2.4.Electrochemical measurements

The electrochemical properties of the obtained samples were tested using a two-electrode electrochemical cell.Working

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electrodes were prepared by casting the slurry homoge-neously on an Al foil as a current collector.The slurry was produced by uniformly mixing the active material(CNTs or N-CNTs,80wt.%on dry solids basis),carbonaceous additive (acetylene black,10wt.%)and a poly vinylidene di?uoride (PVDF,10wt.%on dry solids basis)in N-methylpyrrolidone (NMP)solvent.The loading of all sample are around 0.45e0.55mg cmà2.The electrodes were dried in a vacuum oven at60 C for20h.Lithium foil was used as the counter electrode.The electrolyte was composed of1M lithium bis(-tri?uoromethane sulfone)imide(LiTFSI)dissolved in1,3-dioxolane(DOL)/dimethoxy ethane(DME)in a1:1(volume) mixture and Celgard2400microporous polypropylene?lm was used as a separator.The CR-2032-type coin cells were assembled in an argon-?lled glove box(moisture and oxygen concentration<0.1ppm).The cells were charged and dis-charged over a voltage range of1.5e2.6V(vs Lit/Li)at different rates using an Arbin BT-2000Battery Test System.Electro-chemical impedance spectroscopy(EIS)measurements were performed using CHI760D electrochemical workstation.The impedance spectra were obtained by applying an AC voltage of5.0mV over the frequency range from0.1to100KHz at room temperature.Cyclic voltammetry measurements were carried out on the CHI760D electrochemical workstation over the potential range 1.5e2.6vs.Lit/Li at a scan rate of 0.1mV sà1

3.Results and discussion

3.1.Morphological and nitrogen doping characterization

Typical SEM images of CNTs and N-CNTs are shown in Fig.1. Both samples have uniform distributions in diameters.The

diameter of CNTs and N-CNTs are in the same range.The bamboo-like structure in N-CNTs indicates that nitrogen atoms were introduced into the carbon network[26].

Fig.2shows XRD patterns of CNTs and N-CNTs as well as N-CNTs/S composite.Both CNTs and N-CNTs samples exhibit

a broad(002)diffraction peak at2q around26 and a weak(1

00)diffraction peak around43 in the hexagonal graphitic carbon structure(Fig.2(a)).Both peaks of N-CNTs slightly shift to lower2q values compared with those of CNTs.The(100) line shift is assigned to the relaxation and distortions caused by the introduction of C e N bond(shorter than C e C bond) within the sp2carbon layer[27].The(002)peak shift was associated with the expansion of the interlayer distance be-tween the two graphitic layers to relax the distortion caused by nitrogen doping in the sp2carbon layer[28].The XRD pat-terns given in Fig.2(b)con?rm the nanostructure of the N-CNTs/S.The re?ections of the sulfur are consistent with Fddd orthorhombic https://www.360docs.net/doc/4213931753.html,pared with the pattern of the raw elemental sulfur,the XRD spectrum of the N-CNTs/S did not exhibit many changes except for the appearance N-CNTs peaks(Fig.2(a)),indicating that no phase transformation occurred during heat treatment and the crystal structure of sulfur still remains a Fddd orthorhombic structure.The weight loss of CNTs and N-CNTs after sulfur incorporation was recorded by TGA(Fig.3).The sulfur contents of CNTs/S and N-CNTs/S were around60%.

The XPS spectra shown in Fig.4further con?rm the incorporation of nitrogen in N-CNTs.A full scan spectrum of N-CNTs is illustrated in Fig.4(a).Three strong peaks at290, 401,and530eV are attributed to C1s,N1s and O1s,respec-tively.The atomic concentration of N can be estimated by the area ratio of N peak to the sum of C and N peaks.In this work, the nitrogen content in N-CNTs is2.34at.%.The position of the main C1s peak at290eV con?rms the graphite structure of carbon which corresponds to sp2C e C bond[29,30].Deconvo-lution of the N1s peak was carried out to understand the bonding environment of nitrogen atoms incorporated in N-CNTs.As shown in Fig.4(b),the peak at398.8eV was attrib-uted to the pyridine-like nitrogen which bonds with two sp2 carbons,while the peak at400.9eV could correspond to the graphite-like nitrogen which bonds with three sp2carbons mostly located inside the graphitic carbon plane[31].The peak located at405.1eV could be ascribed to the chemisorbed ni-trogen oxide on the graphite layers[32].

3.2.Electrochemical performance

The cyclic voltammograms(CV)of the CNTs/S and N-CNTs/S composite electrodes are shown in Fig.5(a)and(b).Two reduction peaks are observed from both electrodes.The?rst peak around2.3V is attributable to the reduction of sulfur

to Fig.1e SEM micrographs of(a)pristine CNTs and(b)N-CNTs.

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lithium polysul?des and the second peak around 2.05V to the reduction of longer-chain polysul?des to lithium sul?des [33].Compared with CNTs/S electrode,the cathodic and anodic peaks of N-CNTs/S electrode show a complete overlapping

through cycles,suggesting an effective retention of capacity and prevention of the shuttle mechanism [34].Fig.5(c)and (d)show the discharge/charge pro?les of CNTs/S and N-CNTs/S electrodes at different current densities.The N-CNTs/S composite-based cells deliver a higher capacity in the initial cycle at 0.1C (168mA g à1),0.2C (336mA g à1),0.5C (840mA g à1),respectively.

Cycling performance of N-CNTs/S cathode at 0.2C and 0.5C is presented in Fig.6,together with that of the CNTs/S without nitrogen doping.A reversible capacity of around 625mAh/g was observed after 100cycles of charge and discharge.The discharge capacity at 0.5C also shows good cycling stability,and the reversible capacity was around 513mAh/g after 100cycles.N-CNTs/S electrode also exhibits higher coulombic ef?ciencies during cycling processes at both rates.Obviously,these results show improved performance in speci?c capacity as compared to CNTs/S composite.

To understand the improvement in Li/S batteries’perfor-mance coming from nitrogen doping,further characterization to CNTs/S and N-CNTs/S composites were carried out.The energy dispersive spectroscopy (EDS)mapping in Fig.7pre-sents homogenous distribution of sulfur and carbon in the NCNTs/S composite,indicating that sulfur forms highly-dispersed nano

particle.

Fig.3e TGA of CNTs/S and N-CNTs/S

composites.

Fig.4e XPS full scan spectra of CNTs and N-CNTs (a),XPS N1s spectra of N-CNTs

(b).

Fig.2e XRD patterns of (a)CNTs and N-CNTs including a magni?ed view in the range of 23e 31 inserted,(b)N-CNTs/S composite.

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In addition,Raman spectroscopy was carried out to study the reason why nitrogen doping improved dispersion of sulfur on N-CNTs.Fig.8shows the Raman spectra of CNTs and N-CNTs.Both samples exhibit two obvious peaks at w 1345and w 1570cm à1,corresponding to the D and G bands,respec-tively.The D band denotes the disordered graphite structure,whereas the G band indicates the presence of crystalline graphitic carbon [35].The intensity ratio of D to G bands (I D /I G )is used to evaluate the disorder in carbon materials [36].The

I D /I G ratios of CNTs and N-CNTs are 0.99and 1.16,respectively.The higher I D /I G ratio implies more defects [37]which facilitate the distribution of sulfur on N-CNTs.

The impedance spectra of CNTs/S and N-CNTs/S electrodes were analyzed and ?tted to the equivalent circuit as shown in Fig.9.In the equivalent circuit,Rs is the total resistance of electrolyte,electrode and separator.Rct and CPE1are the resistance and capacitance of the ?lm formed on the electrode surface,which is related to the formation of SEI.Zw is known as the Warburg resistance and is related to the frequency dependence of ion diffusion/transportation in the electrolyte to the electrode surface [38].The ?tting values from the equivalent circuit are presented in Table 1.It is evident that the N-CNTs based electrode has a lower resistance (Rs ?4.35,Rct ?13.03)than those of the CNTs based electrode (Rs ?5.03,Rct ?14.13).It indicates that the N-CNTs/S electrode pos-sesses faster charge-transfer kinetics [23].Hence,the improved dispersion of S on N-CNTs and the reduced impedance of N-CNTs give rise to higher reversible speci?c capacity with cycling.

4.

Conclusions

The performance of Li/S battery was greatly improved by the employment of N-CNTs based cathode,which was synthesized using an injection CVD method and

the

Fig.5e Cyclic voltammograms of CNTs/S (a)and N-CNTs/S (b)electrodes at a scan rate of 0.1mV s L 1in a voltage range of 1.5e 2.6V.Initial discharge/charge pro?les of CNTs/S (c)and N-CNTs/S (d)electrodes at different

rates.

Fig.6e Cycle performance at 0.2C (336mA g L 1

)and 0.5C (840mA g L 1)of CNTs/S and N-CNTs/S electrodes.

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addition of sulfur by heat treatment.The speci?c discharge capacity was maintained at 625mAh g à1and 513mAh g à1after 100cycles at 0.2C and 0.5C,respectively,which was about 2times as that of CNTs.This improved performance is attributed to the enhanced electronic conductivity of electrode and more uniform sulfur dispersion on the nanotubes resulting from nitrogen doping,which were proved by reduced resistance examined from EIS and highly

dispersed sulfur particles observed from EDS,separately.Although the achieved capacity is not the highest,consid-ering the straightforward composition method of N-CNTs and sulfur,this method is proved to be a promising way to enhance the performance of carbon nanotubes based cathode materials of Li e S

batteries.

Fig.7e EDS mapping of CNTs/S (a)and N-CNTs/S

(b).

Fig.8e Raman spectra of CNTs and

N-CNTs.

Fig.9e EIS of CNTs/S and N-CNTs/S electrodes after 30cycles of CV measurements at a scan rate of 0.5mV s L 1;insert:Randles equivalent circuit used for analysis of impedance spectra.

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Acknowledgments

The authors appreciate the support of the Science and Technology Foundation of China Academy of Engineering Physics(No.2012B0302041).We are indebted to Margaret Yau,Jiahui Lin,Kun Luo,Cong Wang,Changzhen Man,Jie Chen and Yongzheng Zhang for their kind help and fruitful discussions.

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