CeF3-Doped Porous Carbon Nanofibers as Sulfur Immobilizers in

Subscriber access provided by UNIV OF DURHAM

Article

CeF3-doped porous carbon nanofibers as sulfur immobilizers in cathode material for high performance lithium-sulfur batteries Nanping Deng, Jingge Ju, Jing Yan, Xinghai Zhou, Qiqi Qin, Kai Zhang, Yueyao Liang, Quanxiang Li, Weimin Kang, and Bowen Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19746 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

CeF3-doped porous carbon nanofibers as sulfur immobilizers in cathode material for high performance lithium-sulfur batteries Nanping Deng1, 2,#, Jingge Ju1, 2,#, Jing Yan1, 2, Xinghai Zhou1, Qiqi Qin1, Kai Zhang1, 2, Yueyao Liang1, 2, Quanxiang Li3, Weimin Kang1, 2,*, Bowen Cheng1, 2,* 1. State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China. 2. School of Textiles, Tianjin Polytechnic University, Tianjin 300387, China. 3. Deakin University, Geelong, Australia, Carbon Nexus, Institute for Frontier Materials., Victoria 3216, Australia.

Abstract In this study, the CeF3-doped porous carbon nanofibers (PCNFs), prepared via electro-blown spinning technique and carbonization process, are used as sulfur immobilizers in cathodes for lithium-sulfur (Li-S) batteries for the first time. The cathode composed of CeF3-doped PCNFs, carbon nanotubes (CNTs) and S is successfully prepared through ball-milling and heating method. The formed porous structure in the PCNFs and CNTs facilitate the construction of highly electrically conductive pathways and effectively alleviate volume changes which can maintain the stability of the cathode structure and make them close contact between the electrodes. Meanwhile, the intermediate polysulfide dissolved and lost in the electrolyte can also be suppressed because of the hierarchical porous carbon nanofibers and CeF3. The Li-S battery using the cathode can display excellent electrochemical properties and stable capacity retention, presenting an initial discharge capacity of 1395.0 mAh g-1 and retaining a capacity of 901.2 mAh g-1 after 500 cycles at 0.5 C. During the rate capability tests of battery, the discharge capacity of Li-S battery with the electrode slowed down from the discharge capacity of 1284.6 mAh g-1 at 0.5 C to 1038.6 mAh g-1 at 1 C and 819.3 mAh g-1 at 2 C, respectively. It is noteworthy that the battery can still endow an outstanding discharge capacity of 1269.73 mAh g-1 with a high retention of 99.2% when the current density returns to 0.5 C. Keywords Lithium-sulfur battery; Hierarchical porous carbon nanofibers; CeF3-doped; High initial discharge capacity; Stable capacity retention

*Corresponding author. E-mail: [email protected] (W. Kang), [email protected] (B. Cheng) #Joint first authors

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

■ INTRODUCTION With enhancing demand for batteries with good safety, low spending and high power density to provide energy for electronic equipments, it is difficult for lithium ion cells in common use to meet the high requirements because of its limited gravimetric energy density (less than 200 Wh kg-1).1 Nowadays, there has been much attention paid to the nature energies including solar, water wind, etc. But it is difficult to utilize effectively and produce continuously these nature energies.2 Therefore, exploring novel electrode materials or new battery systems with higher capacity becomes more and more important. Recently, lithium-sulfur (Li-S) battery has obtained wide concern and attention due to its advantages such as outstanding specific capacity of 1675 mAh g-1 in theory, large theoretical energy density of 2600 Wh kg-1, and elemental sulfur is abundant, non-toxici and environmental friendly.3 But the commercialization process of the battery is hindered by some shortcomings including low electrical conductivity, poor ionic conductivity of S8 and the discharge product Li2S2 and Li2S, considerable volume change and associated structural collapse of electrode and especially “shuttle effect” caused by the dissolved polysulfide species (PS) between electrodes.4 As everyone knows, the “shuttle effect” of Li-S battery is one of the main reasons for the rapid decay of the battery capacity. To overcome the “shuttle effect” of Li-S battery as noted above, many researches have been carried out extensively and deeply. Significant advances have been achieved by designing and optimizing cathode materials and its structures, for example mixing or dispersing sulfur into various carbon materials5, conductive polymer materials6, metal oxide materials7, multiple sulfur-based materials8 and even metal-oxide nanoparticle-decorated carbon materials9. In this field, carbon materials are promising candidates to enhance the availability of active material and suppress “shuttle effect” of PS owing to their outstanding conductivity, electrochemical stability and high specific surface area such as conductive carbon black, aerogels carbon, graphene, carbon nanotubes, carbon nanofibers, hybrid carbon, etc.10-12 And these carbon materials usually are designed to be some porous structure which are tunable and hierarchical pore, and have a variety of bionic shapes.2 The appropriate porous structures were extremely beneficial to store sulfur, prevent migration the polysulfide intermediates and accommodate the change of volume.13 Cui et al. prepared and researched the hollow carbon nanofibers with different structural forms in the cathode of Li-S cells to improve the electrochemical properties of the battery.14 Zhang et al. made a composite nanomaterials which sulfur was embedded into hierarchical porous graphene as the battery electrode, and the structure of the prepared composite nanomaterials can be regarded as an electronic channel for the encapsulated active materials. The framework also can be acted as a “small” electrochemical reaction place. At the same time, the mesopores in the nanomaterials was favorable to render more room to load sulfur and PS in the reaction process. And it was beneficial for the microporous structure in the materials to diffuse lithium ion and prevent the “shuttle effect” of PS. And the functionalization of graphene with epoxy and hydroxyl groups can further prevent the diffusion of polysulfide to the battery anode.15 Qiu et al. prepared self-closure graphene aerogel with inbuilt baffle plates and applied it as electrode material for the Li-S cell. The graphene aerogels had a large number of cavities, providing enough space to load sulfur and relieve the volume expansion. And the inbuilt baffle plates existed in the aerogel can effectively confine the dissolution and diffusion of Li PS to reduce the “shuttle effect” of PS. So the battery with cathode presented excellent electrochemical properties and stable cycle performance.16 Cheng and his co-workers prepared a composite material including vanadium nitride nanoribbon and graphene as chemical adsorption to PS for the Li-S cell. The cathode can provide strong anchoring for PS, fast polysulfide conversion, low polarization and accelerated redox reaction kinetics due to the high conductivity of vanadium nitride and porous graphene. The initial capacity of the battery with the cathode reached 1471 mAhg-1 and the battery capacity was 1252 mAh g-1 at 0.2 C after 100 cycles.17 In addition, currently, some extremely novel and advanced method such as atomic-layer-deposition functionalized treatment18 or self-repairing functionalized layers19, molecularly imprinted polymer20 and the formation of short-chain intermediates (eg. impregnating sulfur into sulfydryl-functionalized rGO)21 were also applied in Li-S batteries, enabling the batteries to achieve remarkable electrochemical performance.


Page 3 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Besides structural characteristics of porous carbon materials, some methods such as surface chemistry treatment and polarization of carbon materials are also significant for sulfur-carbon composites. Therefore, heteroatomic dopants (O-, B-, N-, S-, I-, P- Se-, Co-, and Si-) can well decorate the surface of carbon materials and produce the strong electrostatic role between negatively charged polysulfide anion and positively charged chemical compound. These methods can effectively prevent the polysulfide diffusing between cathode and anode and remarkably enhance the electrochemical properties of the battery. Yu et al. prepared the sandwich-structured composite electrodes including physical structure (carbon nanotube/nanofibrillated cellulose) and chemical functionalization (hetero N doping and hydroxyl groups) for the Li-S battery.22 The battery with the electrode including high areal sulfur loading exhibited excellent capacity and ultralow capacity fading. Qiu and his co-workers made N-doped hollow carbon nanorods embedded cobalt and N-doped tubular/porous carbon channels implanted on graphene to apply them as sulfur hosts for Li-S battery, respectively.23-24 In addition to the specially designed cathode which can regard as the sulfur-confined room and PS reservoirs and provide stable structure, the doping N can further trap Li PS and improve the electrical conductivity of cathode material. The battery presented delightful rate capability and outstanding cycling stability. Recently, Xu et al. made fluorinated porous triazine-based frameworks (FCTF) as electrode of the Li-S battery.25 The results demonstrated that the higher rate capabilities of FCTF-S electrode can be achieved due to the introduction of the polar and ionic C-F bond.26 Furthermore, cerium (Ce) doped material is extremely favorable to form chemical bonds with PS.27 So more work need be researched by decoration for improving greatly the electrochemical performances of battery based on the physically and chemically trapping of PS. In this work, we report CeF3-doped hierarchical porous carbon nanofibers (PCNFs) for the Li-S battery electrode applications with the enhanced discharge capacity and excellent capacity retention. The porous nanofibers can be prepared through carbonizing the fiber membranes composed of Polyvinyl Alcohol (PVA), Polytetrafluoroethylene (PTFE), boric acid (BA) and CeCl3 by applying home-made electro-blown spinning (EBS) equipment. Then the CeF3-doped hierarchical PCNFs, carbon nanotubes (CNTs) and S were used in cathode material of rechargeable Li-S batteries. The improved discharge capacity and outstanding capacity retention of the battery with the cathode material can be attributed to the below reasons: Firstly, the hierarchical porous carbon fibers and CNTs can store abundant S and PS and provide the facilitated electrically conductive passageway. Secondly, these porous structures in the PCNFs and CNTs can accommodate the enormous volume change. Thirdly, in the charge/discharge process of battery, the CeF3-doped carbon material has a catalytic role for the sulfur reduction and can effectively suppress the migration of soluble PS to the Li anode surface from physical and strong chemical interaction. Thus, we believe lithium-sulfur battery using the cathode including CeF3-doped hierarchical PCNFs will provide some guidance for the development of the battery. ■ EXPERIMENTAL SECTION A detailed information about the fabrication procedures of CeF3-doped porous PCNFs-CNTs-S composite, material characterizations, and battery fabrication and electrochemical performance measurements of the Li-S battery with the electrode is provided in the Supporting Information. ■ RESULTS AND DISCUSSION To prepare the CeF3-doped PCNFs-CNTs-S composite electrode, the PVA/PTFE/BA/CeCl3 nanofiber membrane with various CeCl3 mass fraction (0 %, 5 %, 7.5 % and 10 wt% in whole spinning solution) was produced through the self-made electro-blown spinning (EBS) technique as shown in Figure 1a. Then the obtained composite nanofiber membrane was pre-treated and carbonized to form the CeF3-doped PCNFs through the decomposition of PTFE. Subsequently, the CeF3-doped PCNFs-CNTs-S composite electrode was prepared through ball-milling and heating method in the vacuum atmosphere at 155°C for 12 h and the electrode was used in the assembled battery as shown Figure 1b. According to the “Experimental section”, the CeF3-doped PCNFs-CNTs-S composite including different



Page 4 of 23

CeF3 content originating from different CeCl3 content with 0 %, 5 %, 7.5 % and 10 wt% in whole spinning solution are abbreviated to Ce-PCNFs-0, Ce-PCNFs-1, Ce-PCNFs-2 and Ce-PCNFs-3, respectively. The morphological structure of the obtained CeF3-doped hierarchical porous PCNFs and its composite material were studied by SEM and TEM as shown in Figure 2. As presented in Figure 2a1~2c4, both the pure PCNFs and CeF3-doped PCNFs presented honeycomb-like pore structure. And as shown in Figure 2d1~2d4, these TEM images also reveal that the PCNFs and CeF3-doped PCNFs have the porous structure similar to that of a honeycomb. The morphological structure of the obtained CeF3-doped PCNFs-S composite was characterized through SEM testing. From Figure 2e1, many pores in the interconnected PCNTs were filled by S. However, there was still some void space remained, which was rather beneficial for the electron transportation and the accommodation of the volume expansion during the charging and discharging process of the battery.28 The CeF3-doped PCNFs-CNTs-S composite was characterized by SEM and TEM images in different magnification were shown in Figure 2e2~2e3. From these figures, many pores in the interconnected PCNTs were filled by S and CNTs. And, there was still some void space existed. The electron conductivities of the as-prepared CeF3-doped PCNFs with different CeF3 content and carbonization temperature are presented in Table S1. It should be noted that the electron conductivity increased with the adding of carbonization temperature29, and the electron conductivity change of the obtained CeF3-doped PCNFs were very small when the CeCl3 content was less than 7.5 wt%. However, CeF3-doped PCNFs showed higher electron conductivities when the carbonization temperature was 1200°C. Combining the complete fiber morphology, stable structure and shape preservation, and high electron conductivity, Ce-PCNFs-2 in these material is most suitable to be applied in these cathode material for the Li-S battery. To visually probe the CeF3 and S dispersion in the porous carbon framework, EDX was employed to map the distribution situation of chemical elements for the CeF3-doped PCNFs and the CeF3-doped PCNFs-CNTs-S. As shown in Figure S1a1, the EDX analysis showed the presence of CeF3 in the PCNFs, indicating the successfully incorporation of CeF3 in the fibers. And, TEM images through the elemental mapping of CeF3-doped PCNFs were obtained. The elemental mapping analysis verifies the presence and homogenous distribution of F and Ce elements in the CeF3-doped PCNFs (Figure S1a2~S1a6). And the CeF3-doped PCNFs retained their original spherical morphology, again suggesting the excellent CeF3 doping capacity in the hierarchical PCNFs. As can be seen Figure S2, the uniform distributions of F, Ce and S elemental can be observed in the CeF3-doped PCNFs-S using EDX spectrum mapping of SEM images. In order to further confirm the distribution condition of F, Ce and S element, the EDX spectrum mapping was applied in the TEM images of CeF3-doped PCNFs-CNTs-S composite hybrids. As shown in Figure 3a, some CNTs (some rectangle in Figure 3a) were attached to the surface of these carbon fibers. Meanwhile, as seen from Figure 3b~f, the F, Ce and S element were uniform distributed in the CeF3-doped PCNFs-CNTs-S. XPS is conducted to discuss the chemical component and chemical element valance of CeF3-doped PCNFs composite electrode. Figure 4a shows the XPS spectrum of the CeF3-doped PCNFs-CNTs-S composite electrode. The obvious signals of S, Ce and F in the as-obtained CeF3-doped PCNFs-CNTs-S composite further revealed the presence of CeF3 and S in the composite electrode. Oxygen element can be formed in the composite because of the process of the peroxidation reaction. In the C 1s spectrum as shown in Figure 4b, the main peak corresponds to sp2-hydridized carbon (at 284.6 eV). In addition, two weak peaks at 285.9 eV and 286.5 eV were observed, exactly confirming the formation of C-S and C=O bonds during the peroxidation and carbonization reaction.

30

Figure 4c presents a typical 3d spectrum

of Ce. The characteristic peaks of the Ce 3d5/2 and Ce 3d3/2 can be observed at 905.6.2 and 886.4 eV, respectively.31 Figure 4d presents the S 2p spectrum and the results of the deconvolution, revealing the presence of C-S and S-S bonds in the CeF3-doped PCNFs-CNTs-S composite which corresponds to S-S/S-C bonds at 163.7 eV and 164.9 eV, respectively. These results indicated that the S and a small amount of CNTs were successfully incorporated in the CeF3-doped PCNFs framework.32 FTIR testing is also used to further confirm the successful doping of CeF3 in the PCNFs (Figure 4e). Compared with pure PCNFs, some new absorption peaks were observed. The peak at about 500 cm-1 is corresponding to the stretching vibration of Ce-F bonds33, indicating that CeF3 was doped into the PCNFs successfully. As everyone knows, the G band is corresponding to the crystalline graphitic carbon at about 1335 cm-1,


Page 5 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and the D band is ascribed to the carbon with disordered structure at about 1585 cm-1.34 The PCNFs showed two characteristic Raman bands including D-band and G-band as shown in Figure 5a, which is a sign of carbon material. When the carbonized temperature was changed from 800℃ to 1200℃, the intensity rate of the D and G bands of these PCNFs declined from 1.3382 to 0.9112. The reduced ratio indicated that low crystallinity and high graphitization were formed in the carbon material35, which made the material have high electrical conductivity. And the Raman spectrum of CeF3-doped PCNFs shows an additional peak at about ~447 cm-1 appeared, attributing to the CeF3 crystals.36 The accurate sulfur content in CeF3-doped PCNFs-CNTs-S composite evaluated by thermo gravimetric analysis (TGA, Figure 5b) was about 75%. Considering the unique structures and special compositions of CeF3-doped PCNFs-CNTs-S composite, the achieved sulfur loading masses are acceptable, and moreover, these values which were comparable to the previously reported carbon with doped material were summarized in Table S2. The textural properties of the CeF3-doped PCNFs and CeF3-doped PCNFs-S materials were assessed by N2 sorption, as shown in Figure 5c. The CeF3-doped PCNFs and CeF3-doped PCNFs-S nitrogen gas isotherm presented a joint isotherms including Type I and IV, which meant that CeF3-doped PCNFs and CeF3-doped PCNFs-S both were a combined micro, meso and macro-porous material.37 The insert figure in Figure 5c shows the pore size distribution of these samples, also indicating that the pore size in CeF3-doped PCNFs included hierarchical pore structure. The detailed structural parameters of CeF3-doped PCNFs and CeF3-doped PCNFs-S composite are presented in Table S3. The surface area and pore volume of CeF3-doped PCNFs can reach 479.45 m2 g-1 and 0.4702 cm3 g-1, respectively. After the mixed powder of CeF3-doped PCNFs and S were heated, both of the surface area and pore volume of CeF3-doped PCNFs-S decreased dramatically to 11.94 m2 g-1 and 0.2185 cm3 g-1, respectively, indicating that the most nanopores of CeF3-doped PCNFs-S were occupied by the element S38 in accord with the above SEM and TEM images. The XRD patterns (Figure 5d) show the characteristic peaks of PCNFs, CeF3-doped PCNFs and CeF3-doped PCNFs-S composite. As can be revealed, the main diffraction peaks can be well indexed to CeF3 phase (PDF#:72-1436) in the CeF3-doped PCNFs. For the CeF3-doped PCNFs-S sample, several additional peaks can be observed which were corresponding to orthorhombic S (PDF#83-2283)39, further presenting the existence of sulfur in the CeF3-doped PCNFs-S composite because of a relatively large pore volume and high specific surface area. The electrochemical impedance spectroscopy (EIS) of the battery is applied to discuss the charge transfer kinetics of the PCNFs-CNTs-S and CeF3-doped PCNFs-CNTs-S cathode with different CeF3 content. Figure 6 presents the typical Nyquist plots of the PCNFs-CNTs-S and CeF3-doped PCNFs-CNTs-S cathode with different CeF3 content before and after the battery charge/discharge cycle. And the Nyquist plots include a semicircle in the high-frequency areas and an oblique line in the low-frequency regions.40 As everyone knows, the semicircle corresponds to the charge-transfer resistance and contact resistance, and the oblique line corresponds to ion diffusion of the PCNFs-CNTs-S and CeF3-doped PCNFs-CNTs-S with different CeF3 content. The pure PCNFs-CNTs-S electrode showed a larger semicircular diameter compared with the CeF3-doped PCNFs-CNTs-S electrodes, which illustrated that the CeF3-doped PCNFs-CNTs-S electrode exhibited the lower contact and charge-transfer resistance than that of PCNFs-CNTs-S due to existence of CeF3 with high ionic conductivity and more active site.41 And the contact resistance and charge-transfer resistance of the CeF3-doped PCNFs-CNTs-S electrode reduced when the CeF3 content was increased. But when CeF3 content continues increasing, the contact resistance and charge-transfer resistance of CeF3-doped PCNFs-CNTs-S electrode slowly became greater, reflecting the improved interfacial properties between electrode and electrolyte due to the addition of appropriate CeF3-doped chemical compound in the electrode. These obtained results showed that the porous CeF3-doped PCNFs can give effective conductive networks and strongly improve the electrochemical properties of the CeF3-doped PCNFs-CNTs-S electrode composite. And the used high aspect ratio and excellent conductive carbon nanotubes synergistically provided hierarchical short and long-range electron and ion pathways. Meanwhile, the slope of the oblique line at the low-frequency areas is corresponding to the diffusion of Li+.42 The Warburg impedance of the battery is smaller when the slope of the oblique line is higher. From Figure 6a, the Warburg impedance of Ce-PCNFs-2-CNTs-S composite was smaller than that of the



Page 6 of 23

pure-PCNFs-CNTs-S and other CeF3-doped PCNFs-CNTs-S composite. The reduced Warburg impedance was extremely favorable to the transportation of lithium ions. The reduced Warburg impedance could be attributed to the reduced agglomeration of S and the intricate network structure of PCNFs which is extremely helpful to the transportation of electrons and ions. But the excessive CeF3-doped chemical compound made the conductivity of porous carbon fiber reduce. The inset figure in Figure 6a presents a typical EIS equivalent circuit for the charge-transfer resistance of these electrodes. The charge-transfer resistance (Rct) value of the Ce-PCNFs-2-CNTs-S composite electrode was found to be 37.8 Ω, which was much lower than that of Ce-PCNFs-0-CNTs-S (89.5Ω), Ce-PCNFs-1-CNTs-S

(58.9

Ω)

and

Ce-PCNFs-3-CNTs-S

(80.7Ω)

composite

electrodes.

Therefore,

Ce-PCNFs-2-CNTs-S composite electrode had better reversible electrochemical properties than that of the other CeF3-doped PCNFs-CNTs-S composite electrodes. The reduced charge-transfer resistance can be ascribed to the reduced agglomeration of sulfur and the intricate network structure of PCNFs, which was extremely helpful to the transportation and diffusion of ions and electrons. And, the PCNFs adhered to CNTs with high conductivity own larger surface area for sulfur loading and provide enough space to accommodate PS.43 Meanwhile, CeF3 has rather high ionic conductivity which is prominently helpful to increase ion diffusion and reduce the contact resistance. The impedance spectra of these electrodes were changed, presenting there are two obvious semicircles in Figure 6b after 500 cycles for these batteries. The additional semicircle at the high-frequency areas is ascribed to the formation of Li2S and Li2S2 in the cycle processes of the battery. From Figure 6b, after the first cycle of charge/discharge process, these composite electrodes exhibited obviously decreased resistances compared with the corresponding fresh cells. The reduced impedance is ascribed to the rearrangement of the active material which is very beneficial to provide more electrochemically favorable position.44 The result also implied a closer contact and overlap among the active material, separator and collector after the first cycle of charge/discharge process for these batteries. It was also extremely obvious that the Rct of the Ce-PCNFs-2-CNTs-S composite electrode (8.98 Ω) composite cathode were lower than those of the Ce-PCNFs-0-CNTs-S composite electrode (26.12 Ω), the Ce-PCNFs-1-CNTs-S composite electrode (18.02 Ω) and the Ce-PCNFs-3-CNTs-S composite electrode (23.78 Ω) composite cathodes after 500 cycles from Figure 6b (inset), indicating the excellent conductivity and outstanding reversible electrochemical performances for the CeF3-doped PCNFs-CNTs-S composite electrodes in the Li-S batteries. And this impedance results can coordinate well with the outstanding electrochemical performances with the high reversible specific capacity.45 Figure 6c shows the cyclic voltammograms of Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S, Ce-PCNFs-2-CNTs-S and Ce-PCNFs-3-CNTs-S composite electrode. As shown in Figure 6c, all these four electrodes showed two representative cathodic peaks in the initial charge/discharge potential profiles, which were observed at about 2.0 and about 2.3 V, and anodic peak was seen at about 2.4 V which exhibited the oxidation reaction of Li2S and Li2S2 to final oxidation products of S. The cathodic peaks are corresponding to formation of long-chain PS (Li2Sn, 3≤n≤8) by S8 reduction and Li2S2 or Li2S by short-chain polysulfide reduction.46 And, the result in Figure 6c shows that the reduction peak current of Ce-PCNFs-2-CNTs-S composite electrode also was higher than those of CNTs-S and PCNFs-S composite electrode, indicating that S in the PCNFs was easily oxidized to lithium sulfide. The result was mainly attributed to the redistribution of S species in the cathode with the hierarchical porous carbon nanofibers, which made the electrochemical reversibility improved significantly.47 The result also meant that CeF3-doped PCNFs-CNTs-S composite electrode can provide more electrons and ions to the insulating discharge products which was higher than that of pure PCNFs-CNTs-S composite electrode. Therefore, the transition between S8 and Li2S2 or Li2S in the PCNFs and CNTs easily occurred when comparing with that of CNTs-S and PCNFs-S electrode.48 Otherwise, this also could be attributed to the strong electro catalytic effect of polysulfide, improved polysulfide redox kinetics and enhanced electrochemically active sites from the CeF3-doped chemical compound.49-50 In order to confirm the adsorption capabilities of CNTs, PCNFs and CeF3-doped PCNFs, these carbon materials were added into the identical amount of Li2S6 solution to finish the adsorption testing. As shown in Figure 6d, the color of the Li2S6 solution after the addition of CeF3-doped PCNFs and PCNFs is much lighter than that of CNTs, illustrating


Page 7 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stronger chemical or physical binding interactions of CeF3-doped PCNFs and PCNFs to PS. And the doped CeF3 makes the role of adsorption further enhance to PS. However, the color of Li2S6 solution after adding the CNTs is nearly similar to the color of raw Li2S6 solution, reflecting the weak combination between the CNTs and Li2S6. The direct visual contrast adsorption testing about CNTs, PCNFs and CeF3-doped PCNFs to Li2S6 also further explained the reasons that the intermediate polysulfide dissolved and lost in electrolyte can also be suppressed because of the hierarchical porous carbon nanofibers and doped-CeF3. The cycle performances of these electrodes were tested as shown in Figure S3 and Figure 7. Firstly, the electrochemical properties of these electrodes with CNTs-S, PCNFs-S and PCNFs-CNTs-S cathode were tested. As can be seen in Figure S3, the discharge capacity of PCNFs-CNTs-S electrode is the extremely higher and more stable than the CNTs-S and PCNFs-S electrode. And Figure 7a shows that the Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S, Ce-PCNFs-2-CNTs-S and Ce-PCNFs-3-CNTs-S composite electrode delivered an initial discharge capacity of 1332.9, 1332.9, 1395.0 and 1365.4 mAh g-1, respectively. Meanwhile, the capacity of the Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S, Ce-PCNFs-2-CNTs-S and Ce-PCNFs-3-CNTs-S composite electrode was retained at 604.6, 832.8, 901.2 and 697.6 mAh g-1 after 500 cycles with 96.64 %, 97.89 %, 98.78% and 97.78% of columbic efficiency, respectively. These results also presented that the discharge capacity of Ce-PCNFs-2-CNTs-S electrode was the extremely

higher

and

more

stable

than

that

of

the

Ce-PCNFs-0-CNTs-S,

Ce-PCNFs-1-CNTs-S

and

Ce-PCNFs-3-CNTs-S composite electrode. The more stable voltage plateaus and smaller capacity loss illustrated a favorable adsorption or captured ability of PCNFs-CNTs and CeF3-doped chemical compound to inhibit PS diffusion when compared with the pure PCNFs-CNTs electrode. However, the excessive CeF3-doped chemical compound made the electron conductivities and specific surface areas of electrodes drop rapidly, which was unfavorable to keep the discharge capacity stable. The average columbic efficiency of Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S, Ce-PCNFs-2-CNTs-S and Ce-PCNFs-3-CNTs-S composite electrodes in the 500 cycles all were over 97.5% and less than 99.1%. The excellent columbic efficiency of the electrodes were attributed to three-dimensionally hierarchical porous carbon nanofibrous network structure ensuring the electron and Li+-facilitating properties, CeF3-doped chemical compound and the addition of LiNO3 in electrolyte which can improve polysulfide redox kinetics, increase the electrochemically active sites and prevent the “shuttle effect” of polysulfide. And the columbic efficiency of cells with more than 100% in some numbers of cycling process within a certain range was because that the lithium stored in the “overhang” diffused from the active areas during the process of battery charge51-52, strong electrostatic interaction between fluorine and lithium in the high concentration region lithium ion can quicken the conversion of polysulfides to Li2S2/Li2S25-26 or the only Li2S8 produced under the upon charge because of the enormous utilization of elemental sulfur during the discharge process53. And the lithiation voltage curves (Figure 7b) for the battery with Ce-PCNFs-2-CNTs-S composite electrode showed a smaller plateau at about 2.30 V and a larger one at about 2.05 V when comparing with other CeF3-doped PCNFs-CNTs-S composite electrode, which have been assigned to the reduction reaction from sulfur to high molecular weight PS (Li2Sn (4≤n≤8)) and the formation of Li2S2/Li2S from soluble polysulfide species, respectively.49 The delithiation voltage curves presented charge plateau at about 2.41 V corresponding to the oxidation of Li2S2/Li2S to lithium PS and sulfur. The lithiation and delithiation processes were consistent with the above CV curve. And with the battery cycling, the charge-discharge plateau and discharge capacity were more and more stable in the 500 cycles than the starting 200th cycle as shown in Figure 7b. In order to better illustrate the rate performance of these electrodes, the testing current density was changed from 0.5 to 2 C shown in Figure 7c. The average discharge capacities for the PCNFs-CNTs-S electrodes at 0.5, 1, 2, 1 and 0.5 C rate were 1149.1, 910.7, 725.4, 908.6 and 1120.7 mAh g-1, respectively. This indicated good stability of the PCNFs-CNTs-S electrodes. However, the obtained results of PCNFs-CNTs-S electrodes were also significantly lower than that of CeF3-doped PCNFs-CNTs-S composite electrode at 0.5, 1, 2, 1 and 0.5 C. And the average discharge capacities for the Ce-PCNFs-2-CNTs-S electrode at 0.5, 1, 2, 1 and 0.5 C was corresponding to 1279.9, 1043.2, 819.3, 1038.6 and 1269.7 mAh g-1, respectively, which were also higher when compared to Ce-PCNFs-1-CNTs-S electrode



Page 8 of 23

and Ce-PCNFs-3-CNTs-S electrode at 0.5, 1, 2, 1 and 0.5 C. Meanwhile, the average discharge capacities of the Ce-PCNFs-1-CNTs-S electrode about were 1237.9, 999.6, 779.4, 981.1 and 1216.5 mAh g-1, respectively, which were higher when compared with these of the Ce-PCNFs-3-CNTs-S electrode corresponding to 1181.2, 950.7, 763.8, 948.6 and 1170.9 mAh g-1, respectively, These testing results presented that the doping CeF3 made the PCNFs have strong electro catalytic effect of polysulfide, improve polysulfide redox kinetics and increase the electrochemically active sites, which can improve the initial discharge capacity and stable capacity retention. But the excessive CeF3-doped chemical compound was unfavorable to enhance the electrochemical properties. And as shown in Figure 7d, when the current density of testing battery were increased to 1 C and 2 C, the Ce-PCNFs-2-CNTs-S electrode achieved capacities of 1174.2 and 1169.1 mAh g−1 in the initial cycle, respectively. And the batteries with these electrodes also showed outstanding cycling performance including the capacity retention ratio of 48.3% and 46.8% after 500 cycles at 1 and 2 C, respectively. This result also presented that the Ce-PCNFs-2-CNTs-S electrode can withstand various high current charge/discharge process. The excellent electrochemical performances could be attributed to the three-dimensionally hierarchical porous carbon nanofibers network structure which can be regarded as channels for the transportation of electron. And the network shape and structure of carbon material in the electrode material also endows a successive roadway for electron transportation. And many pores in the PCNFs can regard as a reservoir to accommodate S and PS, effectively prevent “shuttle effect” of the PS and add the utilization of active material when the battery is going the charge/discharge process. And the large volume of micropores and mesopores in PCNFs electrode can effectively alleviate the large volume changes. Especially, the CeF3-doped material is extremely favorable to form chemical bonds with PS. The schematic illustrations of Li-S batteries show the effects of the unique 3D network structure sulfur hosts with doping CeF3 on electrochemical performance (Figure 8). The Ce-PCNFs-2-CNTs-S electrode is a three-dimensionally network sulfur host structure which affords advantages of enough storing sulfur space, continuous pathway for electron transport, enhanced availability of active material, adequate space and sites for the electrochemical reaction, reduced the transportion length of Li+ ions and effective adsorption to PS compared to conventional hybrid sulfur hosts. ■ CONCLUSIONS In conclusion, combining the aforementioned characteristics and advantages of carbon nanofibers with hierarchical porous nanostructure and doping CeF3 chemical compound, the CeF3-doped porous carbon nanofibers (PCNFs)-carbon nanotubes (CNTs)-sulfur(S) composite electrode was successfully designed and prepared through a simple ball-milling and heating method, which the CeF3-doped PCNFs were carbonized from the fibers in the membrane composed of Polyvinyl Alcohol (PVA), Polytetrafluoroethylene (PTFE) boric acid (BA) and CeCl3 by electro-blown spinning (EBS) technology. The CeF3-doped PCNFs-CNTs-S composite electrodes were used as sulfur immobilizers in cathodes for high performance Li-S cells for the first time. The constructed porous structure and carbon nanotubes in the cathode material facilitated the construction of a high electrically conductive pathway, capture more sulfur and polysulfides and effectively alleviate volume changes in battery cycling process. And the intermediate polysulfide dissolved and lost in electrolyte can also be suppressed from physical and strong chemical interaction and adsorption because of PVA based hierarchical porous carbon nanofibers and doping CeF3. The Li-S cells using CeF3-doped PCNFs as the cathode can exhibit excellent electrochemical performance stable capacity retention with a high initial capacity as high as 1395.0 mAh g-1 and retain a capacity of 901.2 mAh g-1 after 500 cycles at a current density of 0.5 C. The discharge capacity of Li-S cells with the CeF3-doped PCNFs-CNTs-S composite electrode slowed down from the capacity of 1284.6 mAh g-1 at 0.5 C to 1038.6 mAh g-1 at 1 C and 819.3 mAh g-1 at 2 C, respectively. It was noteworthy that these cells can still show the outstanding capacity of 1269.73 mAh g-1 when it was back to 0.5 C. The present work demonstrates the significant potential in practical application of high-performance and long-life Li-S cells. ■ ASSOCIATED CONTENT Supporting Information: Experimental preparation methods and electrochemical measurements; the EDX spectrum of


Page 9 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the CeF3-doped PCNFs; the STEM image and its corresponding EDX elemental mapping images of C; O; F and Ce; the SEM image of CeF3-doped PCNFs-S composite and its corresponding EDX elemental mapping images of F; Ce and S; Cycling performances of CNTs-S without the addition of LiNO3 in the electrolyte, PCNFs-S without the addition of LiNO3 in the electrolyte, PCNFs-CNTs-S without the addition of LiNO3 in the electrolyte and PCNFs-CNTs-S with the addition of LiNO3 in the electrolyte; the electric conductivities of PCNFs with different CeF3 content; the performance

comparison of CeF3-doped PCNFs-CNTs-S composite with other representative sulfur host materials for Li-S batteries in the literatures; the structural parameters of CeF3-doped PCNFs and CeF3-doped PCNFs-S. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (W.K.). *E-mail: [email protected] (B.W.). ORCID Weimin Kang: 0000-0003-3862-3361 Bowen Cheng: 0000-0001-7470-8077 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors are very grateful to their support: Weimin Kang and Bowen Cheng received funding from the National Natural Science Foundation of China (51173131, 51678411 and 51673148). Bowen Cheng received funding from the Science and Technology Plans of Tianjin (15PTSYJC00230 and 16JCTPJC45600). Weimin Kang and Bowen Cheng received funding from the Fund Project for Transformation of Scientific and Technological Achievements from Jiangsu Province（BA2015182）for their support. ■ REFERENCES [1] Poizot, P.; Laruelle, S.; Grugeon, S.; Dupont, L.; Tarascon, J. M. Nano-sized transition-metal oxides as negative-electrode materials for lithium-ion batteries. Nature, 2000, 407, 496-499. [2] Kang, W. M.; Deng, N. P.; Ju, J. G.; Li, Q. X.; Wu, D. Y.; Ma, X. M.; Li, L.; Naebe, M.; Cheng, B. W. A review of recent developments in rechargeable lithium-sulfur batteries. Nanoscale, 2016, 8, 16541-16588. [3] Lochala, J.; Liu, D. Y.; Wu, B. B.; Robinson, C.; Xiao, J. Research Progress toward the Practical Applications of Lithium-Sulfur Batteries. ACS Appl. Mater. Inter., 2017, 9, 24407-24421 [4] Pang, Q.; Kundu, D.; Cuisinier, M.; Nazar, L. F. Surface-enhanced redox chemistry of polysulphides on a metallic and polar host for lithium-sulphur batteries. Nat. Commun., 2014, 5, 4759. [5] Lv, W.; Li, Z. J.; Deng, Y. Q.; Yang, Q. H.; Kang, F. Y. Graphene-based materials for electrochemical energy storage devices: Opportunities and challenges. Energy Storage Mater., 2016, 2, 107-138. [6] Yang, Y.; Yu, G. H.; Cha, J. J.; Wu, H.; Vosgueritchian, M.; Yao, Y.; Bao, Z.; Cui, Y. Improving the performance of lithium-sulfur batteries by conductive polymer coating. Acs Nano, 2011, 5, 9187-9193.



Page 10 of 23

[7] Liu, X.; Huang, J. Q.; Zhang, Q.; Mai, L. Nanostructured Metal Oxides and Sulfides for Lithium-Sulfur Batteries. Adv. Mater., 2017, 29, 1601759. [8] Wang, J.; He, Y.; Yang, J. Sulfur-based composite cathode materials for high-energy rechargeable lithium batteries. Adv. Mater., 2015, 27, 569-575. [9] Tao, X. Y.; Wang, J. G.; Liu, C.; Wang, H. T.; Yao, H. B.; Zheng, G. Y.; Seh, Z. W.; Cai, Q. X.; Li, W. Y.; Zhou, G. M.; Zu, C. X.; Cui, Y. Balancing surface adsorption and diffusion of lithium-polysulfides on nonconductive oxides for lithium-sulfur battery design. Nat. Commun., 2016, 7, 11203. [10] Guo, J.; Xu, Y.; Wang, C. Sulfur-Impregnated Disordered Carbon Nanotubes Cathode for Lithium-Sulfur Batteries. Nano Lett., 2011, 11, 4288-4294. [11] Su, D.; Cortie, M.; Wang, G. Fabrication of N-doped Graphene-Carbon Nanotube Hybrids from Prussian Blue for Lithium-Sulfur Batteries. Adv. Energy Mater., 2017, 7, 1602014. [12] Ji, X.; Lee, K. T.; Nazar, L. F. A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries. Nat. Mater., 2009, 8, 500-506. [13] He, G.; Evers, S.; Liang, X.; Cuisinier, M.; Garsuch, A.; Nazar, L. F.; Tailoring Porosity in Carbon Nanospheres for Lithium-Sulfur Battery Cathodes. ACS Nano, 2013, 7, 10920-10930. [14] Zheng, G. Y.; Zhang, Q. F.; Cha, J. J.; Yang, Y.; Li, W. Y.; Seh, Z. W.; Cui, Y. Amphiphilic Surface Modification of Hollow Carbon Nanofibers for Improved Cycle Life of Lithium Sulfur Batteries. Nano Lett. 2013, 13, 1265-1270. [15] Huang, J. Q.; Liu, X. F.; Zhang, Q.; Chen, C. M.; Zhao, M. Q.; Zhang, S. M.; Zhu, W. C.; Qian, W. Z.; Wei, F. Entrapment of sulfur in hierarchical porous graphene for lithium-sulfur batteries with high rate performance from -40 to 60 °C. Nano Energy, 2013, 2, 314-321. [16] Zhao, C. T.; Yu, C.; Zhang, M. D.; Yang, J.; Liu, S. H.; Li, M. Y.; Han, X. T.; Dong, Y. F.; Qiu, J. S. Tailor-made graphene aerogels with inbuilt baffle plates by charge-induced template-directed assembly for high-performance Li-S batteries. J. Mater. Chem. A, 2015, 3, 21842-21848. [17] Sun, Z. H.; Zhang, J. Q.; Yin, L. C.; Hu, G. J.; Fang, R. P.; Cheng, H. M.; Li, F. Conductive porous vanadium nitride/graphene composite as chemical anchor of polysulfides for lithium-sulfur batteries. Nat. Commun., 2017, 8, 14627. [18] Luo, C.; Zhu, H. L.; Luo, W.; Shen, F.; Fan, X. L.; Dai, J. Q.; Liang, Y. J.; Wang, C. S.; Hu, L. B. Atomic-Layer -Deposition Functionalized Carbonized Mesoporous Wood Fiber for High Sulfur Loading Lithium Sulfur Batteries. ACS Appl. Mater. Inter., 2017, 9, 14801-14807.


Page 11 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[19] Ao, X.; Wen, Z.; Wu, X.; Wu, T.; Wu, M. The self-repairing function of Ni3S2 layer on Ni particles in the Na/NiCl2 cells with the addition of sulfur in the catholyte. ACS Appl. Mater. Inter., 2017, 9, 21234. [20] Liu, J.; Qian, T.; Wang, M. F.; Liu, X. J.; Xu, N.; You, Y. Z.; Yan, C. L. Molecularly Imprinted Polymer Enables High-E

ciency Recognition and Trapping Lithium Polysulfides for Stable Lithium Sulfur Battery. Nano Lett.

2017, 17, 5064-5070. [21] Xu, N.; Qian, T.; Liu, X, J.; Liu, J.; Chen, Y.; Yan, C. L. Greatly Suppressed Shuttle E

ect for Improved Lithium

Sulfur Battery Performance through Short Chain Intermediates. Nano Lett. 2017, 17, 538-543. [22] Yu, M. P.; J. S.; Ma, M.; Xie, H. Q.; Song, F. Y.; Tian, S. S.; Xu, Y.; Zhou, Li, B.; Wu, D.; Qiu, H.; Wang, R. M. Freestanding and Sandwich-Structured Electrode Material with High Areal Mass Loading for Long-Life Lithium-Sulfur Batteries. Adv. Energy Mater., 2017, 7, 1602347. [23] Zhang, M. D.; Yu, C.; Zhao, C. T.; Song, X. D.; Han, X. T.; Liu, S. H.; Hao, C.; Qiu, J. S. Cobalt-embedded nitrogen-doped hollow carbon nanorods for synergistically immobilizing the discharge products in lithium-sulfur battery. Energy Storage Mater., 2016, 5, 223-229. [24] Zhang, M. D.; Yu, C.; Yang, J.; Zhao, C. T.; Yang, J.; Qiu, J. S. Nitrogen-doped tubular/porous carbon channels implanted on graphene frameworks for multiple confinement of sulfur and polysulfides. J. Mater. Chem. A, 2017, 5, 10380-10386. [25] Xu, F.; Yang, S. H.; Jiang, G. S.; Ye, Q.; Wei, B. Q.; Wang, H. Q.; Fluorinated, Sulfur-Rich, Covalent Triazine Frameworks for Enhanced Confinement of Polysulfides in Lithium-Sulfur Batteries. ACS Appl. Mater. Inter., 2017, 9, 37731-37738. [26] Waller, P. J.; Gandara, F.; Yaghi, O. M. Chemistry of Covalent Organic Frameworks. Acc. Chem. Res., 2015, 48, 3053-3063.

[27] Yuan, Z.; Peng, H. J.; Hou, T. Z.; Huang, J. Q.; Chen, C. M.; Wang, D. W.; Cheng, X. B.; Wei, F.; Zhang, Q. Powering Lithium-Sulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts. Nano Lett., 2016, 16, 519-527. [28] Zhang, M. D.; Yu, C.; C. T.; Zhao, X. D. Song, X. T. Han, S. H. Liu, C. Hao, J. S. Qiu, Cobalt-embedded nitrogen-doped hollow carbon nanorods for synergistically immobilizing the discharge products in lithium-sulfur battery. Energy Storage Mater., 2016, 5, 223-229.



Page 12 of 23

[29] Yu, Z. T.; Fang, X.; Fan, L. W.; Wang, X.; Xiao, Y. Q.; Zheng, Y.; Xu, X.; Hu, Y.-C.; Cen, K.-F. Increased thermal conductivity of liquid paraffin-based suspensions in the presence of carbon nano-additives of various sizes and shapes. Carbon, 2013, 53, 277-285. [30] Lei, T. Y.; Chen, W.; Huang, J. W.; Yan, C. Y.; Sun, H. X.; Wang, C.; Zhang, W. L.; Li, Y. R.; Xiong, J. Multi-Functional Layered WS2 Nanosheets for Enhancing the Performance of Lithium-Sulfur Batteries. Adv. Energy Mater. 2016, 7, 1601843. DOI: 10.1002/aenm.201601843. [31] Jia, W. D. N.; García-Melchor, M.; Bajdich, M.; Chakthranont, P.; Kirk, C.; Vojvodic, A.; Jaramillo, T. F. Gold-supported cerium-doped NiOx catalysts for water oxidation. Nat. Energy, 2016, 1, 16053. [32] Zhou, G.; Paek, E.; Hwang, G. S.; Manthiram, A. Long-life Li/polysulphide batteries with high sulphur loading enabled by lightweight three-dimensional nitrogen/sulphur-codoped graphene sponge. Nat. Commun., 2015, 6, 7760. [33] Wang, L.; Zhang, M.; Wang, X.; Liu, W. The preparation of CeF3 nanocluster capped with oleic acid by extraction method and application to lithium grease. Mater. Res. Bull., 2008, 43, 2220-2227. [34] Guerin, H.; Poche, H. L.; Pohle, R.; Bernard, L. S.; Buitrago, E.; Ramos, R.; Dijon, J.; Ionescu, A. M. High-yield, in-situ fabrication and integration of horizontal carbon nanotube arrays at the wafer scale for robust ammonia sensors. Carbon, 2014, 78, 326-338. [35] Fatema, U. K.; Tomizawa, C.; Harada, M.; Gotoh Y.;, Iodine-aided fabrication of hollow carbon fibers from solid poly(vinyl alcohol)fibers. Carbon, 2011, 49, 2158-2161. [36] Xie, Y.; Gao, D.; Zhang, L. L.; Chen, J. J.; Cheng, S.; Xiang, H. F.; CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries. Ceram. Int., 2016, 42, 14587-14594. [37] He, J.; Lv, W.; Chen, Y.; Wen, K.; Xu, C.; Zhang, W.; Li Y.; Qin W., He W. Tellurium-impregnated porous cobalt-doped carbon polyhedra as superior cathodes for lithium-tellurium batteries. Acs Nano, 2017, 11, 8144-8152. [38] Luo, C.; Zhu H. L.; Luo W.; Shen F.; Fan, X. L.; Dai J. Q.; Liang Y. J.; Wang, C. S.; Hu, L. B.


Page 13 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Atomic-Layer-Deposition Functionalized Carbonized Mesoporous Wood Fiber for High Sulfur Loading Lithium Sulfur Batteries. ACS Appl. Mater. Inter., 2017, 9, 14801-14807. [39] Fujimori, T.; Morelos-Gómez, Zhu, A.; Z.; Muramatsu, H.; Futamura, R.; Urita, K.; Terrones, M.; Hayashi, T.; Endo, M.; Hong, S. Y.; Choi, Y. C.; Tománek, D.; Kaneko, K. Conducting linear chains of sulphur inside carbon nanotubes, Nat. Commun., 2013, 4, 2162. [40] Zhou, G. M.; Pei, S. F.; Li, L.; Wang, D. W.; Wang, S. G.; Huang, K.; Yin, L. C.; Li, F.; Cheng, H. M. A Graphene–Pure-Sulfur Sandwich Structure for Ultrafast, Long-Life Lithium-Sulfur Batteries. Adv. Mater., 2014, 26, 625-631. [41] Xie, Y.; Gao, D.; Zhang, L. L.; Chen, J. J.; Cheng, S.; Xiang, H. F.; CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries. Ceram. Int., 2016, 42, 14587-14594. [42] Karuppasamy, K.; Reddy, P. A.; Srinivas, G.; Tewari, A.; Sharma, R.; Shajan, X. S.; Gupta, D. Electrochemical and cycling performances of novel nonafluorobutanesulfonate (nonaflate) ionic liquid based ternary gel polymer electrolyte membranes for rechargeable lithium ion batteries. J. Membrane Sci., 2016, 514, 350-357. [43] Hu, G.; Sun, Z.; Shi, C.; Fang, R.; Chen, J.; Hou, P.; Liu C.; Cheng H.; Li F. A Sulfur-Rich Copolymer@CNT Hybrid Cathode with Dual-Confinement of Polysulfides for High-Performance Lithium-Sulfur Batteries. Adv. Mater. 2017, 29, 1603835. [44] Zeng, L. C.; Pan, F. S.; Li, W. H.; Jiang, Y.; Zhong, X. W.; Yu, Y. Free-standing porous carbon nanofibers-sulfur composite for flexible Li-S battery cathode. Nanoscale, 2014, 6, 9579-9587. [45] Li, Z. Q.; Li, C. X.; Ge, X. L.; Ma, J. Y.; Zhang, Z. W.; Li, Q.; Wang, C. X.; Yin, L. W.; Reduced graphene oxide wrapped MOFs-derived cobalt-doped porous carbon polyhedrons as sulfur immobilizers as cathodes for high performance lithium sulfur batteries. Nano Energy, 2016, 23, 15-26. [46] Fang, R. P.; Zhao, S. Y.; Hou, P. X.; Cheng, M.; Wang, S. G.; Cheng, H. M.; Liu, C.; Li, F. 3D Interconnected Electrode Materials with Ultrahigh Areal Sulfur Loading for Li-S Batteries. Adv. Mater., 2016, 28, 3374-3382.



Page 14 of 23

[47] Xie, K.; You, Y.; Yuan, K.; Lu, W.; Zhang, K.; Xu, F.; Ye M.; Ke S.; Shen, C.; Zeng, X.; Fan X.; Wei B. Ferroelectric-Enhanced Polysulfide Trapping for Lithium-Sulfur Battery Improvement. Adv. Mater. 2017, 29, 1604724. [48] Pan, H.; Cheng, Z.; Xiao, Z.; Li, X.; Wang, R. The Fusion of Imidazolium-Based Ionic Polymer and Carbon Nanotubes: One Type of New Heteroatom-Doped Carbon Precursors for High-Performance Lithium-Sulfur Batteries, Adv. Funct. Mater., 2017, 27, 1703936. [49] L. B.; Ma, R. P.; Chen, G. Y.; Zhu, Y.; Hu, Y. R.; Wang, T.; Chen, J.; Liu, Jin, Z. Cerium Oxide Nanocrystal Embedded Bimodal Micromesoporous Nitrogen-Rich Carbon Nanospheres as Effective Sulfur Host for Lithium-Sulfur Batteries. ACS Nano, 2017, 11, 7274-7283. [50] Lu, C.; Wu, H.; Zhang, Y.; Liu, H.; Chen, B. J.; Wu, N. T.; Wang, S. Cerium fluoride coated layered oxide Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials with improved electrochemical performance for lithium ion batteries. J, Power Sources, 2014, 267, 682-691. [51] Weng, W.; Pol, V. G.; Amine K. Ultrasound Assisted Design of Sulfur/Carbon Cathodes with Partially Fluorinated Ether Electrolytes for Highly Efficient Li/S Batteries. Adv. Mater., 2013, 25, 1608-1615. [52] Gyenes, B.; Stevens, D. A.; Chevrier, V. L.; Dahn, J. R. Understanding Anomalous Behavior in Coulombic Efficiency Measurements on Li-Ion Batteries. J. Electrochem. Soc., 2015, 162, A278-A283. [53] Elazari, R.; Salitra, G.; Garsuch A.; Panchenko A.; Aurbach D. Sulfur-Impregnated Activated Carbon Fiber Cloth as a Binder-Free Cathode for Rechargeable Li-S Batteries. Adv. Mater., 2011, 23, 5641-5644.


Page 15 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure & Figure captions

Figure 1 (a) The preparation processes of PTFE/PVA/BA/CeCl3 composite nanofiber membrane through the self-made electro-blown spinning (EBS) technology; (b) Synthesis processes of the CeF3-doped PCNFs-CNTs-S composite electrode and battery assembly with the electrode.



Page 16 of 23

a1

a2

a3

a4

b1

b2

b3

b4

c1

c2

c3

c4

d1

d2

d3

d4

e1

e3

e2

Figure 2 SEM images of CPNFs with different carbonization temperature and CeCl3 content: (a1): 800°C and 0 wt%, (a2): 800°C and 5wt%, (a3): 800°C and 7.5wt%, (a4): 800°C and 10wt%; (b1): 1000°C and 0 wt%, (b2): 1000°C and 5wt%, (b3): 1000°C and 7.5wt%, (b4): 1000°C and 10wt%; (c1): 1200°C and 0 wt%, (c2): 1200°C and 5wt%, (c3): 1200°C and 7.5wt%, (c4): 1200°C and 10wt%. TEM images of (d1,d3) CPNFs and (d2,d4) CeF3-doped PCNFs; SEM images (e1) and TEM images with different magnification (e2~e3) of CeF3-doped PCNFs-CNTs-S.


Page 17 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

c

d

e

f

Figure 3 TEM images of CeF3-doped PCNFs-CNTs (a) (CNTs: red rectangle in Fig. 3(a)); STEM image of CeF3-doped PCNFs-CNTs-S (b) and its corresponding EDX elemental mapping images of S (c); C (d); Ce (e) and F (f).



Page 18 of 23

a

b

c

d

e

f

Figure 4 XPS spectra of the surface chemical composition of CeF3-doped PCNFs-CNTs-S composite (a), C 1s XPS spectra (b); Ce 3d XPS spectra; (c) S 2p XPS spectra (d) and F 1s XPS spectra (e) of the CeF3-doped PCNFs-CNTs-S composite; The FTIR spectra of pure PCNFs and CeF3-doped PCNFs (f).


Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

c

d

Figure 5 (a) Raman spectra of PCNFs with different the carbonization temperature and CeF3-doped PCNFs; (b) TGA curves of the CeF3-doped PCNFs-CNTs-S composite and S; (c) The nitrogen sorption isotherms; inset shows the pore size distribution of CeF3-doped PCNFs; (d) The XRD spectrum of CeF3, PCNFs, CeF3-doped PCNFs and CeF3-doped PCNFs-S.



a

b

c

d

Page 20 of 23

Figure 6 Nyquist plots of Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S, Ce-PCNFs-2-CNTs-S and Ce-PCNFs-3-CNTs-S composite electrode before charge/discharge cycle (a) and after 500 cycles (b) within the frequency range of 100 kHz to 100 mHz; The CV curves of Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S, Ce-PCNFs-2-CNTs-S and Ce-PCNFs-3-CNTs-S composite electrode (c); Photograph of the Li2S6 adsorption by di erent powders in DOL/DME (1:1, v/v) solution (1#: Li2S6 solution; 2#: CNTs; 3#: PCNFs; 4#: CeF3-doped PCNFs) (d).


Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

c

b

d

Figure 7 Battery cycling performance with (a) Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S, Ce-PCNFs-2-CNTs-S and Ce-PCNFs-3-CNTs-S composite electrode at rate of 0.5 C; (b) The charge/discharge potential profiles of Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S, Ce-PCNFs-2-CNTs-S and Ce-PCNFs-3-CNTs-S composite electrode; (c) The battery rate capability of the Ce-PCNFs-0-CNTs-S, Ce-PCNFs-1-CNTs-S,

Ce-PCNFs-2-CNTs-S

and

Ce-PCNFs-3-CNTs-S

composite

electrode;

Ce-PCNFs-2-CNTs-S at various rates of 1 C and 2 C.


(d)

Battery

cycling

performance

of


Page 22 of 23

a

S

CeF3

Li2Sx

b

Figure 8 Schematic illustration of CeF 3-doped porous carbon nanofibers and conventional carbon nanofibers as sulfur hosts for improving the performance of lithium-sulfur batteries (a) and the adsorption mechanism for polysulfide (b).


Page 23 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CeF3-Doped Porous Carbon Nanofibers as Sulfur Immobilizers in

Recommend Documents