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Electrochimica Acta 333 (2020) 135493

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Electrochimica Acta

journal homepage: www.elsevier .com/locate/electacta

Flexible free-standing sulfurized polyacrylonitrile electrode for stableLi/Na storage

Xinyue Huang a, b, 1, Jun Liu a, b, 1, Zexi Huang a, b, Xi Ke a, b, Liying Liu a, b,Naiguang Wang a, b, Jianping Liu a, b, Zaiping Guo c, **, Yong Yang d, ***, Zhicong Shi a, b, *

a Smart Energy Research Centre, School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, PR Chinab Guangdong Engineering Technology Research Center for New Energy Materials and Devices, Guangzhou, 510006, PR Chinac Institute for Superconducting & Electronic Materials, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong,Wollongong, NSW, 2522, Australiad State Key Lab for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen, 361005, PR China

a r t i c l e i n f o

Article history:Received 16 October 2019Received in revised form25 November 2019Accepted 10 December 2019Available online 18 December 2019

Keywords:Free-standing electrodeSulfurized polyacrylonitrile (SPAN)Hollow nanofibersLithium sulfur batteriesSodium sulfur batteries

* Corresponding author. Smart Energy Research CeEnergy, Guangdong University of Technology, Guangz** Corresponding author.*** Corresponding author.

E-mail addresses: zguo@uow.edu.au (Z. Guo), yzhicong@gdut.edu.cn (Z. Shi).

1 X. Huang and J. Liu contributed equally.

https://doi.org/10.1016/j.electacta.2019.1354930013-4686/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

Flexible lithium sulfur batteries are promising power sources for the next generation wearable elec-tronics, due to their high energy density and low cost. Here, we demonstrate a metal current collector-free, binder-free, flexible sulfurized polyacrylonitrile film electrode with hollow tubular nanofibers (H-SPAN), which is fabricated via coaxial electrospinning and a simple heat treatment. The all-fibrous filmsH-SPAN not only provides three-dimensional continuous electron and ion transport paths, but alsosuppresses the shuttle effect, contributing to better redox kinetics, cycling performance, and flexibility.The H-SPAN film electrode delivers a high specific lithium storage capacity of 1250 mAh g�1

sulfur or514.75 mAh g�1

electrode at 0.1C with stable cycling over 300 cycles. Additionally, when H-SPAN is appliedas a cathode for room temperature NaeS batteries, it also exhibits superb capacity and cycling stability(717 mAh g�1

sulfur or 295.2 mAh g�1electrode at 0.1C after 200 cycles). The working mechanism of H-SPAN

is well elucidated by ex situ Raman spectroscopy and ex situ X-ray Photoelectron Spectroscopy (XPS). Theexcellent flexibility of the H-SPAN film electrode lends itself well to potential applications in wearableelectronic devices.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

With the rapid growth of portable electronic devices and elec-tric vehicles, lithium (Li) ion batteries have become themost widelyadopted technology for energy storage devices and have inspired agreat deal of research activity [1]. Nevertheless, traditional lithium-ion batteries are approaching their theoretical energy density limit,so there is an urgent need to find new battery systems with higherenergy density. Lithium sulfur (LieS) batteries have attracted muchattention due to their high energy density (2600 Wh kg�1),extremely low cathode cost, and environmental inertness [2,3].

ntre, School of Materials andhou, 510006, PR China.

yang@xmu.edu.cn (Y. Yang),

Nevertheless, the commercialization of LieS batteries faces threechallenges: (1) the insulating nature of both elemental sulfur andits discharge products (Li2S) results in low utilization of the activematerials and slow redox kinetics; (2) the dissolution of lithiumpolysulfide (LiPS), an intermediate species during operation, in theliquid organic electrolyte gives rise to the well-known “shuttle ef-fect”, which results in low Coulombic efficiency, electrochemicalinstability and poor cycle life; and (3) the large volume changes ofsulfur during lithiation and delithiation often result in cathodepulverization and rapid capacity fade [4e8]. Thus, the developmentof high-quality sulfur cathodes has great significance for the reali-zation of LieS batteries with high energy density and long cycle life.

Many strategies have been developed to address these short-comings, such as, employing conductive carbonaceousmaterials forsulfur/C composites [2,9,10], modifying separators [11,12], anddeveloping porous metal�organic frameworks and metal oxides[13e17]. The use of sulfurized polyacrylonitrile (SPAN) as thecathode for LieS batteries is one promising strategy, first reportedby J.Wang et al., in 2002 [18], which has continuingly receivedwide

X. Huang et al. / Electrochimica Acta 333 (2020) 1354932

attention due to its excellent electrochemical properties [19e25].For example, Wang et al. reported a polyacrylonitrile/graphenecomposite as a precursor to produce sulfur-based cathode materialfor high-rate rechargeable LieS batteries [26]. Yang et al. reported aNiS2-modified sulfurized polyacrylonitrile composite as a highperformance cathode material [27]. Although great progress hadbeenmade in the past several decades, the mass loading of sulfur inSPAN is usually less than 50 wt%, which is further reduced whenconsidering the mass of the metal current collector, conductivecarbon and binding agents.

Designing a free-standing electrode which is free from bindingagents, conductive carbon, and current collectors directly improvesthe proportion of active material in the electrode. Moreover, thefabrication of the free-standing electrode reduces environmentalimpact by avoiding the use of the conventional toxic organic sol-vent (N-methylpyrrolidone). A robust, flexible freestanding elec-trode also has potential applications in increasingly wearableconsumer and medical electronics. The most popular method tofabricate freestanding electrodes in the field of energy storage iselectrospinning, due to its simplicity, high efficiency and easycontrol over fiber morphology [28e30].

Herein, we have prepared a flexible sulfurized polyacrylonitrilefilm with hollow tubular nanofibers (H-SPAN) by coaxial electro-spinning and simple heat treatment, which can be directly used asthe cathode for lithium-sulfur batteries. Benefiting from its uniquehollow tubular nanofiber structure, there is a greater contact areabetween the electrolyte and the electrode, and the utilization of theelectrode is improved. At the same time, the energy density of thebattery is increased by eliminating the metal current collector,conductive carbon black, and binder. As a result, the battery basedon the H-SPAN cathode exhibited excellent electrochemical per-formancewith a reversible capacity of 1250mAh g�1

sulfur or 514.75mAh g�1

electrode and good capacity retention (300 cycles withalmost no capacity loss). When cycled at 0.1 C in room-temperature(RT) NaeS batteries, a high capacity of 717 mAh g�1

sulfur or 295.2mAh g�1

electrode was achieved after 200 cycles with high Coulombicefficiency.

2. Experimental section

2.1. Material synthesis

Polyacrylonitrile (PAN, Mw ¼ 150000, Sigma-Aldrich, USA),polyethylene oxide (PEO, Mw ¼ 400000 Sigma-Aldrich, USA), andN,N-dimethylformamide (DMF, Mw ¼ 79.12, Sigma-Aldrich, USA)were used without any further purification.

To prepare the precursor solution for coaxial electrospinning,0.5 g PAN was dissolved in 4.5 g DMF and vigorously stirred for12 h at room temperature. A PEO solution was prepared similarly.The PEO solution was intended for the cores and the PAN solutionfor the shells via the coaxial electrostatic spinning to fabricate thePEO-PAN core/shell nanofibers on aluminium foil. The flow rate ofthe precursor solutions was 10 mL/min, and the high voltage of15 kV was applied to the coaxial needle. The distance between thecoaxial needle and the collector was 15 cm. Under these conditions,the PEO/PAN core-shell nanofibers were generated and formed afilm. In order to remove the PEO cores from the PAN sheaths, thefilms were ultrasonically processed for 10 min in deionized water.After drying for 24 h at 60 �C, the PAN film (punched to 12mmdiscsin diameter) and commercial S8 powders (1:3, w/w) were sealed ina glass vessel and heated at 390 �C for 8 hwith a heating rate of 5 �Cmin�1 under argon atmosphere. To remove the extra S8, the samplewas put in an alumina boat and heated at 350 �C for 2 h in flowingargon atmosphere. Then, the H-SPAN films were used directly ascathodes for batteries. The SPAN powder was prepared by the same

method as a control sample.

2.2. Materials characterization

The elemental contents of the H-SPAN and SPAN powder ma-terials were determined by elemental analysis (CHNS, EA-3000Italy, Eurovector). The X-ray diffraction (XRD) patterns of thesamples weremeasured on an X-ray diffractometer (Rigaku D/max-2200/PC) using a Cu Ka radiation source at a wavelength of0.1541 nm. The structures and morphologies of the samples werecharacterized by field-emission scanning electron microscopy(FESEM; Se3400 N) and transmission electron microscopy (TEM;JEOL, JEM-2010). Elemental mappings were recorded by EDXspectroscopy attached to TEM (JEOL, JEM-2100). The detailedcharacteristics of the organic functional groups were characterizedwith attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectrometer (Nicolet IS50). The chemical bonds were alsocharacterized by Raman spectra (Horiba Jobin Yvon, l ¼ 532 nm).The thermal stability was assessed by thermogravimetric analysis(TGA) in the temperature range from 30 to 900 �C in Ar atmospherewith a heat rate of 10 �C min�1. The X-ray photoelectron spec-troscopy (XPS) analysis of H-SPAN materials is conducted with aThermo Fisher Scientific ESCALAB 250Xi. The chemical bonding andgroups of all samples at certain charged/discharged states arecharacterized by ex situ Raman spectroscopy and ex situ X-rayPhotoelectron Spectroscopy (XPS). Before characterization, coincells were disassembled in a glovebox to obtain electrodes, whichwere immersed into 1,3-dioxolane/dimethoxyethane (DOL/DME)solvents to remove residual electrolyte and lithium salt, and thendried for 30 min.

2.3. Electrochemical measurements

The electrochemical performances of H-SPAN and SPAN powderfor lithium storage were characterized in CR2032-type coin cellsassembled in an argon-filled glove box. The SPAN powder electrodewas prepared by casting a slurry of 80 wt% active material, 10 wt%acetylene black, and 10 wt% polyvinylidene difluoride (PVDF) (asthe binder) on Al foil with a carbon coating. The electrodes weredried at 60 �C under vacuum for 24 h and then were cut into discs12 mm in diameter. The H-SPAN electrodes had cathode compositeloadings of ~2.2 mg cm�2, corresponding to the sulfur load of0.91 mg cm�2. The mass loading of sulfur on the SPAN powderelectrodes was 0.7 mg cm�2.

The H-SPAN films were directly used as working electrodes aftervacuum drying at 60 �C overnight. For LieS batteries, Li metal wasapplied as the anode, and Celgard 2400 membrane was applied asthe separator. The electrolyte was 1.0 M lithium bis(tri-fluoromethanesulfonyl)imide (LiTFSI) in 1,3-dioxolane (DOL) anddimethoxymethane (DME) (1:1,v/v) with 1 wt % LiNO3. For RT NaeSbatteries, Na metal was applied as the anode and Whatman glassfiber membrane as the separator. The electrolyte was 1.0 M NaClO4in ethylene carbonate (EC) and propylene carbonate (PC) (1:1, v/v).Electrolyte is added to each coin cell with the amount of 40 mLduring assembling. The galvanostatic charge/discharge measure-ments for the LieS batteries were performed in a voltage cut-offwindow of 1e3 V (vs. Li/Liþ) using a LAND CT2001A multi-channel battery test system at room temperature, while for theRT NaeS batteries, the voltage range was 0.8e2.8 V. To test theirlong-term cycling performance, the Li/H-SPAN batteries were firstcycled once at 0.05 C for activation and then cycled at 0.1 C. Thecurrent density was calculated based on the sulfur mass of theactive materials (1675 mAh g�1 ¼ 1 C). Cyclic voltammetry mea-surements were carried out on an electrochemical workstation(Solartron 1287/1260) over the potential range of 1e3 V at a scan

X. Huang et al. / Electrochimica Acta 333 (2020) 135493 3

rate of 0.1 mV/s, while for the RT NaeS batteries, the voltage rangewas 0.8e2.8V. The electrochemical impedance spectrum (EIS) testswere also carried out on an electrochemical workstation (Solartron1287/1260) at a frequency from 10 K Hz to 0.1 Hz.

3. Results and discussion

The SPAN hollow tubular nanofibers were prepared by a facilemethod, and a schematic illustration of the synthesis process isshown in Scheme S1 in the Supporting Information. Firstly, twoprecursor solutions of PAN-DMF and PEO-DMFwere fabricated intoa film with a core-shell structure by coaxial electrospinning. Thefibers consisted of a core layer of PEO sheathed in a PAN shell. Toremove the PEO core fiber, the prepared sample was ultrasonicatedfor 10 min in deionized water at room temperature. The resultingflexible films were punched into circular films with diameters of12 mm, as shown in Fig. S1a. Then, commercial sulfur powder andthe circular PAN films were heated together in a sealed glasscontainer at 390 �C for 8 h. As proven in previous reports, elementalsulfur could bondwith the carbon atoms of the PAN-derived carbonmatrix at elevated temperature [31e33]. After complete drying, theresulting binder-free H-SPAN (Fig. S1b) was directly used as thecathode for electrochemical performance measurements.

The prepared nanofibers maintained a one-dimensional (1D)morphology (Fig. 1a and b) with an extremely large aspect ratio.Fig. 1c shows that the diameters of the fibers are regular, in therange of 500e600 nm, and appear to be well-structured fibers.Therewas no change in the morphology of the nanofibers, and theymaintained their one-dimensional structure (Fig.1d), even after thePAN film was treated by ultrasonic processing in deionized waterand subsequent sulfurization. A clear hollow tubular structure canbe seen from the cross-section of the fiber (Fig. 1e, Fig. S2), which

Fig. 1. Morphology and structure characterizations. (aec) SEM images of the hollow PANnanofibers. (f) Digital photographs of H-SPAN films showing flexibility. (gei) TEM images a

was further confirmed by high resolution TEM (HRTEM) (Fig. 1g). Itcan be expected that the hollow tubular fiber structure will havethe following advantages: (1) It has a large specific surface area thatprovides more reaction sites during charge/discharge reactions andsulfurization (reacting with S8), improving the utilization of sulfur.(2) It can mediate the volume expansion from the repeated con-version of sulfur to Li2S during charge/discharge and stabilize theelectrode structure. (3) The capillary action of nanofiber film fa-cilitates the permeation of electrolyte into the electrode. (4) Theinterconnection of the fibers accelerates lithium ion transport byshortening the diffusion paths. The detailed morphology andstructure of the H-SPAN were further characterized by TEM, asshown in Fig. 1h. The H-SPAN composite shows an amorphousphase, indicating that the sulfur was bonded to the PAN backbone[34]. Furthermore, sulfur particles were not observed on the surfaceof the PAN framework. Elemental mappings of the H-SPAN sampleshowed that all of the elements (carbon, nitrogen, sulfur) are uni-formly distributed within the H-SPAN framework, which indicatesthat the sulfurization reaction proceeded to completion (Fig. 1i).The flexibility of the H-SPAN film was tested via bending experi-ments. As shown in Fig. 1f, the film did not show any cracks when itwas bent, confirming the good flexibility and robustness of H-SPANfilms. In Video S1, the H-SPAN film can be bent, folded, and restoredto its original state. Its demonstrated mechanical strength indicatesthe potential application value of H-SPAN films in wearable elec-tronic devices.

Supplementary video related to this article can be found athttps://doi.org/10.1016/j.electacta.2019.135493.

XRD was used to evaluate the crystal phase of the samples.Fig. 2a shows the XRD patterns of the above samples. Despite thepresence of a substantial amount of sulfur (41.19 wt %), the signa-ture peaks of elemental sulfur (S8) were not observed for H-SPAN.

nanofibers. (d) SEM image of H-SPAN. (e) Typical cross-sectional image of H-SPANnd corresponding elemental mappings.

Fig. 2. The spectral characterization of all samples. (a) XRD patterns of hollow SPAN, hollow PAN and pure S. (b, c) ATR-FTIR and Raman spectra of hollow SPAN and hollow PAN. (d)TGA profiles of hollow SPAN, pyrolyzed PAN and powder SPAN. XPS spectra of (e) C1s and (f) S2p for H-SPAN.

X. Huang et al. / Electrochimica Acta 333 (2020) 1354934

This suggests that the sulfur exists in an amorphous phase or as anorganosulfide bonded to the carbon skeleton instead of crystallinephase S8. Attenuated total reflection (ATR) was used to reveal thetransformation of organic functional groups from PAN to SPAN(Fig. 2b). After heating commercial S8 powder at 390 �C, the eCH(2936 cm�1), eCH2, and eCN (2242 cm�1) groups on Hollow PANdisappeared, while some new bonds/groups for C]C (1488 cm�1),CeS (938 and 667 cm�1), and aromatic ring/cyclic structures (1421and 802 cm�1) emerged [35], indicating the cyclization of PAN toform a polypyridine ring containing C]C and C]N during thedehydrogenation reaction [36]. Raman spectra were also used toanalyze the chemical bonding in H-SPAN and H-PAN (Fig. 2c). Thecharacteristic peaks at 466 and 925 cm�1 are attributed to the SeSbond, while the H-PAN has no similar characteristic signal. More-over, the two major peaks at 1340 and 1538 cm�1 are ascribed tothe D-band (disordered) and G-band (graphite), respectively [37].To determine the content of free and chemical bound sulfur in H-SPAN nanofibers, TGA and elemental analysis (EA) were performed.The EA results show that H-SPAN consists of 13.58 wt % of nitrogen,39.79 wt % of carbon, 41.19 wt % of sulfur and 0.55 wt % hydrogen.By contrast, the SPAN powder consists of 13.60 wt % of nitrogen,36.55 wt % of carbon, 36.79 wt % of sulfur and 1.17 wt % hydrogen(Table S1). Therefore, these hollow fibers can host 41.19 wt % of Swithin the framework, which is clearly higher than that of the SPANpowder (36.79 wt %). The thermal stability of H-SPAN, pyrolyzedPAN and powder SPAN is assessed by TGA in the temperature rangefrom 30 to 900 �C (Fig. 2d). For all of the above materials, no sig-nificant weight loss is observed before 350 �C. It reflects the strongbonding between carbon and sulfur, and no free element sulfurstored in the above tested samples. To further investigate thechemical state of S in SPAN, XPS experiment was carried out. Thehigh-resolution C1s spectrum (Fig. 2e) reveals the chemical bondsbetween S and C. S2p spectrum can be divided into five peaks(Fig. 2f). The S2p3/2 peak and S2p1/2 peak at 161.54eV and 162.39eVis assigned to the terminal CeS bond, respectively [27,37,38]. Themain S2p peak at 163.48eV is consistent with the CeSx (2 � X � 4)bond of short-chain organosulfide bonding to the carbon skeletonand the peak at 164.70eV could be attributed to SeS bonds in thecomposite. The other peak at around 167.5 eV is the satellite peak

[22,39]. The above results confirm the formation of sulfurizedpolyacrylonitrile nanofibers with a hollow tubular structure.

In order to evaluate its electrochemical performance for lithiumstorage, C2032 coin-type cells were assembled using H-SPAN filmas the cathode and lithiummetal as the anode. As a comparison, theSPAN powder was mixed with a binder and conductive carbonblack, and spread onto Al foil with the traditional blade-castingmethod. The electrochemical properties are shown in Fig. 3. Theelectrochemical behavior of SPAN was tested using cyclic voltam-metry (CV), as shown in Fig. 3a. The cathodic peak of H-SPAN filmappeared at 1.38 V during the first reduction reaction, compared tothe cathodic peak at 1.28 V of the SPAN powder (Fig. S3). This resultindicates that the hollow tubular fiber structure (even withoutadditional conductive agent) increased the redox dynamics of thesulfurized polyacrylonitrile cathode material. In the subsequentscans, the oxidation peak of H-SPAN was located at 2.25 V, and thereduction peak was at about 1.75 V. It was also found that theoxidation-reduction potential difference of H-SPAN was smallerthan that of the SPAN powder, which indicates that the H-SPANcathode had good redox reversibility. On comparing the galvano-static discharge-charge voltage profiles of H-SPAN and SPANpowder at 0.1C (Fig. S4), the average discharge voltage plateau of H-SPAN cathode is obviously higher than that of the SPAN powdercathode, agreeing with the results from CV. Fig. S5 and Table S2show the EIS and fitting results of H-SPAN and powder SPAN,respectively. In the equivalent circuit, R1 represents the resistanceof the electrolyte. The CPE1/R2 element is attributed to theelectrode-electrolyte interface impedance at high frequency, CPE2/R3 is assigned charge transfer impedance at medium frequency andW is the Warburg impedance [27,33,40]. The fitting results showthat the charge transfer impedance of the H-SPAN is smaller thanthat of the powder SPAN cell. This indicates that the H-SPAN bat-teries showed a smaller polarization, which is in good agreementwith the results of the CV test and the discharge/charge curve. Thus,the H-SPAN cathode exhibits higher specific capacity than SPANpowder cathode. In addition, the H-SPAN cathode exhibits higherspecific capacity, which can be attributed to the unique hollowtubular structure of the electrode, allowing for greater contactbetween electrode and electrolyte. Therefore, the H-SPAN electrode

Fig. 3. Electrochemical performances for Li storage. (a) CV curves of H-SPAN at 0.1 mV/s. (b) Discharge/charge profiles during the cycling test. (c) Discharge/charge profiles at variouscurrent densities from 0.1 to 1 C of H-SPAN. (d) Rate capabilities from 0.1 to 1 C. (e) Long-term cycling performance and Coulombic efficiency of H-SPAN at a current density of 0.1 C.

X. Huang et al. / Electrochimica Acta 333 (2020) 135493 5

delivers a capacity of 522.8 mAh g�1electrode, corresponding to a

higher utilization ratio (75.8%, theoretical capacity: 1675 mAhg�1

sulfur) (Fig. S6). In contrast, the SPAN powder electrode onlydelivers a capacity of 244.6 mAh g�1

electrode, corresponding to autilization ratio of 50.26%. Fig. 3b shows the charge and dischargecurves of H-SPAN, which are similar to the charge-discharge curvesof the SPAN powder. Previous reports noted that there are certainirreversible and partial capacity losses in the first cycle [38,41,42].The H-SPAN electrode delivers a capacity of 1782.4 mAh g�1

sulfur inthe initial discharge, and then the reversible capacity of the cellstabilized at around 1250 mAh g�1

sulfur in the subsequent cycles. Itshould be noted that the initial discharge capacity of the H-SPANsurpasses the theoretical capacity of sulfur 1675 mAh g�1

sulfur. Thiscan be ascribed to the irreversible reaction of lithium with the

surface functional groups on the H-SPAN nanofibers and the con-sumption of lithium during the formation of the solid electrolyteinterphase (SEI) layer [27,41]. The first cathodic voltage peak ofSPAN and H-SPAN locates lower potential compared with thesubsequent ones. This change can be attributed to the structurechanges of SPAN during the first lithiation/delithiation process asreported [36,43]. Fig. 3e shows the long-term cycling performanceof H-SPAN electrode at a current density of 0.1C. In the first 10cycles, the discharge capacity of H-SPAN was observed to increase,indicating an electrochemical activation process in the cathode.Similar phenomena were also observed in previous reports[33,34,44e46]. After 300 cycles, the H-SPAN electrode still shows ahigh specific capacity of 1236mAh g�1

sulfur with almost no capacityloss. In comparison, the SPAN powder cathode delivers a stable but

X. Huang et al. / Electrochimica Acta 333 (2020) 1354936

lower specific capacity of 800 mAh g�1sulfur (Fig. S8). The superior

performance of H-SPAN can be attributed to the hollow tubularstructure of the nanofibers, which restrain volume expansion andeliminate the shuttle effect of the SPANmaterial itself. Additionally,the rate performance of H-SPAN cathodes was also investigated, asshown in Fig. 3d. When the battery was cycled at current densitiesof 0.1, 0.2, 0.5, and 1C, the H-SPAN electrode delivered capacities of1259, 1171, 995, and 499 mAh g�1

sulfur, respectively. When thecurrent density was returned back to 0.1 C after cycling at variousrates, the discharge capacity recovered to 1251 mAh g�1

sulfur again,suggesting great electrode stability from H-SPAN. A detailed com-parison of H-SPAN electrode with other works reported in theliterature is shown in Table S3. At the electrode level, the free-standing H-SPAN electrode contains 41.19 wt% S, while all theother reported works have less than 40 wt% S in the electrode. TheH-SPAN electrode also exhibits outstanding cycling stability. All ofthe above results demonstrate the excellent electrochemicalproperties of the H-SPAN electrode.

The reaction mechanism of H-SPAN during the first discharge/charge processes was probed by ex situ Raman spectra (Fig. 4a). Thecharacteristic Raman peak of SeS (466 cm�1 and 925 cm�1) havesignificant changes and nearly disappears at 1.0 V. The fully dis-charged product is Li2S which impair the strength of SeS peaksignal. When charging to 3.0 V, the SeS peaks in Raman spectrareappear at the fully charged stated. To further confirm the reactionmechanism of H-SPAN cathode, ex situ XPS was performed on H-SPAN at different discharged and charged states. Fig. 4b shows the

Fig. 4. (a) Ex-situ Raman spectra for H-SPAN at different discharge/charge states. (b, c)

high-resolution S 2p spectra of H-SPAN during the first dischargingprocess. The SeS peaks decrease gradually with discharging anddisappear after discharging to 1.0 V, with a new peak at 160 eVascribed to the Li2S formation [22,41]. Moreover, the formation ofsulfated species can be judged by the intensity increment of S 2p3/2

peak at 168.8 eV. The transformation of S 2p spectra during firstchargewas shown in Fig. 4c. Comparedwith pristine H-SPAN, in thecharging process, as the degree of lithiation increases, the peaks ofSeS bond and C-Sx (2 � X � 4) reappear and increase gradually,which suggest that the H-SPAN has good reversibility and stability[22,25,37].

To further demonstrate the potential applications of H-SPANelectrode, a flexible pouch cell was fabricated by using a H-SPANfilm as cathode, Li metal on Cu foil as anode, and Celgard 2400membrane as separator (Fig. 5e). As shown in Fig. 5 a-f, the pouchcell continuously discharged at various bending degrees withoutany apparent change in the luminescence of the bulb. The flexibilityof the pouch cell can be further confirmed by Video S2, the bulb canstill work well when the pouch cell was subjected to dynamicbending which indicating excellent flexibility of the H-SPAN elec-trode. The cycle performances of flexible pouch cell were tested andshown in Fig. 5h. It can deliver a high initial specific capacity of1700mA h g�1 andmaintain at 1100mA h g�1 in the next 50 cycles.Besides, the charge and discharge tests were carried out at differentdegrees of bending at 0.6C rate. The performances are not obviouslyaffected by the bending angles of the flexible pouch cell (Fig. S9). Tofurther demonstrate the stability and practicability of the flexible

High-resolution XPS spectra of S2p for H-SPAN at different discharge/charge states.

Fig. 5. (aef) Photographs for flexible pouch cell based on H-SPAN electrode in various folding angles to light up an LED illumination. (g) The Schematic diagrams of as-assembledflexible pouch cell. (h) The cycle performance of pouch cell at 0.1C current density.

X. Huang et al. / Electrochimica Acta 333 (2020) 135493 7

pouch cell, we test the electrochemical performance after bendingthe devices for 100 times. The discharge curves are not affected bythe bending of the pouch cell (Fig. S10a). Moreover, the pouch cellcan operate stably in next 10 cycles. The H-SPAN shows goodflexibility and lends itself well to potential applications as powersource in wearable electronic devices.

Supplementary video related to this article can be found athttps://doi.org/10.1016/j.electacta.2019.135493.

Because of the similarity in electrochemical behavior betweenLieS and NaeS, RT NaeS batteries have attracted attention since2006 [47e49]. Compared to traditional sodium-sulfur batteries, RTNaeS batteries not only reduce costs, but also improve the safety.Since H-SPAN electrodes are also electrochemically active for so-dium storage, we further characterized the electrochemical per-formance of H-SPAN in RT NaeS batteries. Fig. 6a presents typicalcharge and discharge curves of H-SPAN for Na storage. By using H-SPAN as the cathode, the RT Na//H-SPAN battery delivers a highcapacity of 1455.2 mAh g�1

sulfur or 599.2 mAh g�1electrode at 0.05 C

in the first discharge, and then its capacity stabilizes at 1046.1 mAhg�1

sulfur or 430.8mAh g�1electrode during the second cycle. Similar to

the electrochemical behavior of H-SPAN for Li storage, the voltageplateau of the second discharge is higher than that of the initialdischarge. Since the standard electrode potential of sodium (2.71 Vvs. standard hydrogen electrode (SHE)) is higher than that oflithium (3.04 V vs. SHE), Na//H-SPAN has a lower working voltagethan Li//H-SPAN. Fig. 6c shows the long-term cycling performanceof H-SPAN at a current density of 0.1 C. The battery is activated at0.02 C for three cycles and subsequently cycled at 0.1 C. After 200cycles, a high reversible capacity of 717 mAh g�1

sulfur or 295.2 mAh

g�1electrode can be maintained. Based on these results, H-SPAN ex-

hibits stable cycling performance for sodium storage. Compared toprevious studies of RT NaeS batteries [48,50e56], this work showsremarkable improvements in terms of both sulfur content andoverall whole cathode capacity (Fig. 6b).

To investigate the structural and chemical stability of H-SPANelectrode, the cycled CR2032 coin cells (Li/H-SPAN) were dis-assembled in an argon-filled glove box for further characterization.After 300 cycles, the H-SPAN electrode maintains a similarmorphology to the fresh one, with the one-dimensionalmorphology of the fibers well-maintained (Fig. 7c and d). Thestability can also be verified by the digital photograph of dis-assembled cells (Fig. S11). The H-SPAN cathode displayed goodstructural integrity, reconfirming the structural stability of theelectrode. The H-SPAN based cell exhibited a clean separator sur-face and no anodic corrosion, which confirms the absence of pol-ysulfide dissolution and the shuttle effect in the Li/H-SPAN battery.In comparison, the surface of the powder SPAN electrode exhibitscracking of the active material (Fig. 7a), which could be ascribed tothe volume expansion effect of SPAN powder. Consequently, thehollow nanofiber design controlled the volume expansion of sulfurand improved the electrochemical performance.

4. Conclusion

Flexible, free-standing H-SPAN electrodes were prepared by asimple coaxial electrospinning and heat-treatment method. Theynot only inherit the advantages of the SPAN powder materials toprevent the lithium polysulfide shuttle effect, but also improve the

Fig. 6. Electrochemical performance for Na storage. (a) Charge/discharge curves of H-SPAN at current density of 0.05C. (b) Capacity vs. S content for this work and reports in theliterature. (c) Prolonged cycling performance of H-SPAN for Na storage.

Fig. 7. SEM images of fresh (a) and cycled (b) powder SPAN electrode, and of cycled H-SPAN electrode (c, d).

X. Huang et al. / Electrochimica Acta 333 (2020) 1354938

energy density of the whole electrode. When assembled withlithium metal to form a lithium-sulfur battery, the H-SPAN fiberfilm exhibits excellent electrochemical performance (a high specific

capacity of 1250 mAh g�1sulfur with almost no capacity loss after

300 cycles). When used in RT NaeS batteries, H-SPAN delivers ahigh capacity of 717 mAh g�1

sulfur or 295.2 mAh g�1electrode after

X. Huang et al. / Electrochimica Acta 333 (2020) 135493 9

200 cycles at 0.1C. In addition, the flexibility and robustness of H-SPAN approve its potential applications in wearable medical andconsumer electronics.

Author contribution

Xinyue Huang: Conceptualization, Investigation, Validation,Data Curation, Writing-Original Draft. Jun Liu: Conceptualization,Writing-Review & Editing. Zexi Huang: Investigation. Xi Ke:Formal analysis. Liying Liu: Conceptualization. Naiguang Wang:Writing-Review & Editing. Jianping Liu: Formal analysis andWriting-Review & Editing. Zaiping Guo: Conceptualization. YongYang: Conceptualization, Zhicong Shi: Conceptualization, Re-sources, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare no competing financial interest.

Acknowledgment

This work was supported by the National Key R&D Program ofChina (2018YFB0104202), the National Natural Science Foundationof China (Nos. 21673051 and 51604086), the Department of Scienceand Technology, Guangdong Province, China (No.2017B010119003), the “One-hundred Talents Plan” (No.220418056).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.electacta.2019.135493.

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