Solid State Ionics 225 (2012) 604–607

Contents lists available at SciVerse ScienceDirect

Solid State Ionics

j ourna l homepage: www.e lsev ie r .com/ locate /ss i

Gel polymer electrolyte with ionic liquid for high performance lithium sulfur battery

Jun Jin, Zhaoyin Wen ⁎, Xiao Liang, Yanming Cui, Xiangwei WuCAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China

⁎ Corresponding author. Tel.: +86 21 52411704; fax:E-mail address: zywen@mail.sic.ac.cn (Z. Wen).

0167-2738/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.ssi.2012.03.012

a b s t r a c t

a r t i c l e i n f o

Available online 9 April 2012

Keywords:Lithium sulfur batteryGel polymer electrolyteIonic liquidLithium ion battery

Poly(vinylidenefluoride-hexafluoropropylene)[P(VDF-HFP)] membrane with porous structure was prepared bya simple phase separation process. The gel polymer electrolyte (GPE) prepared by combining the porousmembrane with N-methyl-N-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid shows goodthermal stability, high anodic oxidation potential (>5.0 V vs. Li/Li+) and good interfacial stability with lithiumelectrode. The lithium sulfur battery with the GPE delivers an initial discharge capacity 1217.7 mAh g−1 andmaintains a reversible capacity of 818 mAh g−1 after 20 cycles at a current density of 50 mA g−1.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

With the development of portable electronic devices and electricvehicles, lithium ion batteries with high safety, energy density andlong cycle life have attracted much attention. Compared with recentlyavailable lithium ion battery, polymer lithium batteries offer advan-tages in terms of lighter weight, higher specific energy and higherflexibility in design. However, the presence of volatile and flammablesolvents in gel polymer electrolytes reduces the thermal stability ofthe electrolyte, which causes the battery high security risk. Roomtemperature ionic liquid has been studied as the electrolytes for theelectrochemical capacitors [1,2], lithium batteries [3–5] and fuel cells[6] for its nonvolatility, nonflammable, high thermal and electrochemi-cal stability. Moreover, the combination of ionic liquid to a polymer likethe poly(ethylene oxide)(PEO) can improve the ionic conductivity andelectrochemical stability of the polymer electrolyte [7,8]. Nevertheless,its ionic conductivity at room temperature cannot meet the require-ments of practical applications of lithium ion battery. In recent years,poly(vinylidenefluoride-hexafluoropropylene)[P(VDF-HFP)] based gelpolymer electrolytes with ionic liquid have been investigated with thelithium ion battery using LiFePO4 [9–11] and LiMn0.4Fe0.6PO4 [12]cathodes as well as lithium-air battery [13].

Lithium sulfur battery is very attractive for its high theoretical specificcapacity (1670 mAh g−1) and specific energy (2600W h kg−1) [14].Furthermore, sulfur has the advantage of abundance, low toxicity andlow cost, which are promising for next generation storage battery.However, sulfur is a natural insulating material and the polysulfidesintermediate can dissolve in organic electrolytes, which leads toinferior electrochemical reaction and rapid capacity fading. To over-come these problems, various conductive matrices were reported toabsorb the polysulfides intermediate and improve the capacity

+86 21 52413903.

rights reserved.

retention [15–18]. Recently, Li/S battery exhibits high dischargecapacity and improved cycling performance with room temperatureionic liquid electrolytes, owing to the poor solubility of polysulfidesintermediate in the ionic liquid [19,20].

In this study, we developed a simple and practical process to prepareporous P(VDF-HFP)membrane. The GPEwas obtained by incorporatingionic liquid electrolyte into the porous membrane. The electrochemicalproperties of the lithium sulfur battery were investigated with the GPE.

2. Experimental

2.1. Preparation of PVDF-HFP based GPE

A porous polymermembranewas prepared by a simple phase sepa-ration process [21]. 0.25 g P(VDF-HFP) (Aldrich, Mw=4×105) was dis-solved in a mixture of 4.75 g acetone and 0.25 g distilled water at 50 °Cunder continuous stirring for 1 h in a sealed container. The solutionwascast on a glass sheet and the solventwas evaporated at ambient temper-ature. After drying under vacuum at 100 °C for 2 h, a homogeneous freestanding membrane of 100 μm was obtained. The room temperatureionic liquid, N-methyl-N-butylpyrrolidinium bis(trifluoromethanesul-fonyl)imide (P14TFSI), was prepared by a process described in literature[22]. The GPE was finally prepared by immersing the as-preparedporous membrane in the 0.5 M solution LiTFSI in P14TFSI for 30 min at50 °C in an argon-filled glove box with H2O and O2 contents below1 ppm.

2.2. Preparation of PPy-S composite

Polypyrrole nanotube (PPy) was synthesized by a self-degradedtemplate method [23]. The PPy-S composite was prepared by heatingsublimed sulfur and PPy (1:1 w/w) at 155 °C for 5 h. The cathode wasprepared by mixing 60 wt.% S-PPy composite, 30 wt.% acetylene blackand 10 wt.% polyvinylidenefluoride with N-methyl-2-pyrrolidone(NMP) to form a slurry, and then casting the slurry onto aluminum

Fig. 2. Temperature dependence of the ionic conductivity for GPE.

605J. Jin et al. / Solid State Ionics 225 (2012) 604–607

foil. After the organic solvent evaporated, the cathode film wascut into pellets with a diameter of 14 mm and dried for 12 h undervacuum at 60 °C.

2.3. Characterization and electrochemical measurements

The surface morphology of the membrane was observed with ascanning electron microscope (SEM S-3400). Differential scanningcalorimetric (DSC) analysiswas carried out using an apparatus (NetzschSTA 409PC) from 30 °C to 170 °C at a scan rate of 10 °C min−1 underflowing N2. TG analysis was performed with the heating rate of5 °C min−1 from 50 °C to 500 °C under flowing N2. The ionic conductiv-ity of the GPE over the temperature range 0 to 80 °C was measured bythe AC impedance method using symmetrical stainless steel electrodes(2.0 cm2) with Autolab PGSTAT30 over the frequency range from1 MHz to 1 Hz at the amplitude of 10 mV. The electrochemical imped-ance spectroscopy (EIS) was measured over the frequency range of10 mHz to 1 MHz. The electrochemical stability was evaluated by linearsweep voltammetry (LSV) of SS/GPE/Li cell at a rate of 1 mV s−1.

CR2025-type coin cells were assembled by sandwiching the GPEbetween a lithium foil and PPy-S composite electrode in an Ar-filledglove box. Cyclic voltammetry (CV) of the cell was measured at ascanning rate of 0.1 mV s−1 between 1.0 and 3.0 V. The discharge–charge tests were carried out with a LAND CT2001A battery testsystem in the voltage range 1.0–3.0 V vs Li/Li+ at room temperature.

3. Results and discussion

3.1. Characterization of the GPE

The photo view and detailedmorphology of P(VDF-HFP)membraneprepared by the phase separation process were shown in Fig. 1. As seen,the white area of the membrane shows homogeneously distributedmicropores with a diameter range of 3–5 μm. The micro porous andinterconnected skeleton structure provides the membrane the abilityto absorb enough electrolytes and functions as a framework to transportLi ions.

After swelled with 0.5M LiTFSI-P14TFSI ionic liquid, the porousmembrane becomes transparent and exhibits an electrolyte uptakeof 500% by weight. The PVDF-HFP functions as porous frameworkfor absorbing electrolyte and stabilizing the matrix structure. Fig. 2shows the temperature dependence of the ionic conductivity of theGPE, which exhibits non-Arrhenius VTF behavior [13]. The ionicconductivity of the GPE was 2.54×10−4 Scm−1 at room temperature,which was similar to that of the GEP with mesoporous SiO2 fillerprepared by solution casting method [10].

The thermal properties of the P14TFSI ionic liquid and the GPEwere shown in Fig. 3. Fig. 3a exhibits the DSC curves of the P(VDF-HFP) porous membrane and GPE. An endothermic peak at 142 °C for

Fig. 1. Photograph (a) and SEM imag

P(VDF-HFP) membrane assigns to the melting of the PVDF-HFPcrystal. When the porous membrane was swelled by the ionic liquid,the obvious endothermic peak shifted to 110 °C due to the suppressionof the recrystallization process of the membrane by the interactionamong salt, ionic liquid and P(VDF-HFP), which decreases the meltingpoint of the polymer electrolyte. No weight loss for the P14TFSI andthe GPE was observed below 380 °C. The weight loss between 380and 500 °C was attributed to the decomposition of P14TFSI as shownin Fig. 3b.

Linear sweep voltammetry (LSV) curve of the GPE sandwichedbetween Li and SS electrodes is shown in Fig. 4. The electrochemicaloxidation potential of the GPE is mainly related to the absorbed liquidelectrolyte [9]. As found the GPE incorporated ionic liquid shows wideelectrochemical stability window (ESW) up to 5.0 V. The anodiccurrent corresponds to the oxidation of the TFSI− anions in the GPE[24]. A wide electrochemical stability window of the GEP is suitablefor the lithium ion batteries with high potential cathode materialssuch as LiMn2O4, LiNiO2, LiNi1/3Co1/3Mn1/3O2.

The compatibility of lithium with the GPE was also analyzed byimpedance plots of the symmetrical Li/GPE/Li cell as shown in Fig. 5.The plots consist of a semicircle at high frequency corresponding tothe GPE bulk resistance (Rb) and another semicircle at lower frequencydenoting the electrode/electrolyte interfacial resistance (Rf). The Rf wasfound varying from648Ω to 1371Ω after 48 h storage and shows slightincrease from 96 h to 120 h, indicating a stable interface between theGPE and lithiummetal was formed after 96 h.

3.2. Electrochemical properties of Li–S battery

The cyclic voltammograms of the PPy-S composite electrode in theGPE are shown in Fig. 6. During the first scan, it gives a broad cathodicpeak at potential region of 2.2–1.5 V and an anodic peak at 2.56 V. The

e (b) of P(VDF-HFP) membrane.

Fig. 3. DSC curves of P(VDF-HFP)membrane and GPE (a) and TGA of P14TFSI and GPE (b).

Fig. 5. Impedance plots for the symmetrical Li/GPE/Li cell under open circuit conditionsas a function of time.

606 J. Jin et al. / Solid State Ionics 225 (2012) 604–607

cathodic peak shifts to the positive direction and divides into twopeaks at 1.87 V and 2.19 V, and the anodic peak shifts to the negativedirection at 2.5 V in the following scans.

The broad negative potential peak in the first discharge processcould be attributed to the fact that the electrochemical reaction duringthe discharge process needs to overcome the absorbing energy betweenS and conductive matrix [18] as well as the poor dissolvability of thepolysulfide in the ionic liquid [19]. However, during the second scan,the two cathodic peaks at 2.19 V and 1.87 V are lower than that of thesulfur electrode in organic electrolyte [25], which is similar to electro-chemical behavior of sulfur in N-methyl-N-butyl-piperidiniumbis(trifluoromethanesulfonyl) imide(PP14TFSI) ionic liquid electro-lyte [19,20]. The shift of anodic peak potential from 2.56 V to 2.5 Vand the increase of peak current indicate the decrease of polarization

Fig. 4. LSV curve of the GPE at a scan rate of 1 mV s−1 from open-circuit voltage to 6 V.

of the PPy-S composite cathode and the improvement of reversibilityof the battery.

Fig. 7 shows the galvanostatic charge–discharge curves and cyclingperformance of the Li/GPE/S battery. As seen in Fig. 7a, the firstdischarge curve shows two plateaus, which may be attributed to theformation of polysulfides (Li2S8, Li2S6, Li2S4) and Li2S. However, thedrop of potential between 2.25 and 2.0 V is a slash, which is differentfrom the potential plateau in the second cycle. This phenomenon indi-cates a potential hysteresis in the first discharge process, which demon-strates the obvious change of cathodic peaks in Fig. 6. The Li/S batterydelivers an initial discharge capacity of 1217.7 mAh g−1, about 73% ofthe theoretical capacity based on sulfur. However, the low initialcharge–discharge efficiency (86%) indicates an incomplete conversionof the first cycle discharge product deposited on the surface of conduc-tive polymer. Anyway, the PPy-S composite cathode exhibits goodcycling performance as shown in Fig. 7b. The discharge capacity main-tained at 818 mAh g−1 after 20 cycles. The improvement of the capacityretention may be attributed to the reduced solubility of the intermedi-ate polysulfides in the GPE with ionic liquid electrolyte, which reducesthe loss of the active sulfur during the charge–discharge process.

4. Conclusions

The gel polymer electrolyte was prepared by combining the porousP(VDF-HFP) membrane with P14TFSI ionic liquid. And it shows goodthermal stability, high anodic oxidation potential (>5.0 V vs. Li/Li+)and good interfacial stability with lithium electrode. The lithium sulfurbattery with the GPE delivers high initial discharge capacity and main-tains a reversible capacity of 818 mAh g−1 after 20 cycles. The GPE notonly ensures good safety, but also reduces solubility of lithium

Fig. 6. CV profiles of Li/GPE/S battery at room temperature. Scan rate: 0.1 mV s−1, voltagerange: 1.0–3.0 V.

Fig. 7. Charge–discharge curves (a) and cycling performance (b) of Li/GPE/S battery ata current density of 50 mA g−1.

607J. Jin et al. / Solid State Ionics 225 (2012) 604–607

polysulfides in IL-based electrolytes and improves the cycling perfor-mance of lithium sulfur battery.

Acknowledgments

Thiswork isfinancially supported by theNatural Science Foundationof China (NSFC, Projects No. 50730001 and No. 50973127) and 973

Project of China No. 2007CB209700, as well as Research Projects fromthe Science and Technology Commission of Shanghai Municipality No.08DZ2210900 and 09PT1410800.

References

[1] M. Ue, M. Takeda, T. Takahashi, M. Takehara, Electrochem. Solid State Lett. 5(2002) A119–A121.

[2] A. Balducci, U. Bardi, S. Caporali, M.Mastragostino, F. Soavi, Electrochem. Commun. 6(2004) 566–570.

[3] H. Nakagawa, S. Izuchi, K. Kuwana, T. Nukuda, Y. Aihara, J. Electrochem. Soc. 150(2003) A695–A700.

[4] H. Sakaebe, H. Matsumoto, Electrochem. Commun. 5 (2003) 594–598.[5] C. Tizzani, G.B. Appetecchi, M. Carewska, G.T. Kim, S. Passerini, Aust. J. Chem. 60

(2007) 47–50.[6] R.F. de Souza, J.C. Padilha, R.S. Goncalves, J. Dupont, Electrochem. Commun. 5

(2003) 728–731.[7] J.H. Shin, W.A. Henderson, S. Passerini, J. Electrochem. Soc. 152 (2005)

A978–A983.[8] C.B. Zhu, H. Cheng, Y. Yang, J. Electrochem. Soc. 155 (2008) A569–A575.[9] G. Cheruvally, J.K. Kim, J.W. Choi, J.H. Ahn, Y.J. Shin, J. Manuel, P. Raghavan, K.W.

Kim, H.J. Ahn, D.S. Choi, C.E. Song, J. Power Sources 172 (2007) 863–869.[10] S. Ferrari, E. Quartarone, P. Mustarelli, A. Magistris, M. Fagnoni, S. Protti, C.

Gerbaldi, A. Spinella, J. Power Sources 195 (2010) 559–566.[11] P. Raghavan, X.H. Zhao, J. Manuel, G.S. Chauhan, J.H. Ahn, H.S. Ryu, H.J. Ahn, K.W.

Kim, C. Nah, Electrochim. Acta 55 (2010) 1347–1354.[12] J.K. Kim, J. Manuel, G.S. Chauhan, J.H. Ahn, H.S. Ryu, Electrochim. Acta 55 (2010)

1366–1372.[13] H. Ye, J. Huang, J.J. Xu, A. Khalfan, S.G. Greenbaum, J. Electrochem. Soc. 154 (2007)

A1048–A1057.[14] B.H. Jeon, J.H. Yeon, K.M. Kim, I.J. Chung, J. Power Sources 109 (2002) 89–97.[15] C. Lai, X.P. Gao, B. Zhang, T.Y. Yan, Z. Zhou, J. Phys. Chem. C 113 (2009)

4712–4716.[16] S.C. Wei, H. Zhang, Y.Q. Huang, W.K. Wang, Y.Z. Xia, Z.B. Yu, Energy Environ. Sci. 4

(2011) 736–740.[17] X.L. Ji, K.T. Lee, L.F. Nazar, Nat. Mater. 8 (2009) 500–506.[18] B. Zhang, X. Qin, G.R. Li, X.P. Gao, Energy Environ. Sci. 3 (2010) 1531–1537.[19] J. Wang, S.Y. Chew, Z.W. Zhao, S. Ashraf, D. Wexler, J. Chen, S.H. Ng, S.L. Chou, H.K.

Liu, Carbon 46 (2008) 229–235.[20] L.X. Yuan, J.K. Feng, X.P. Ai, Y.L. Cao, S.L. Chen, H.X. Yang, Electrochem. Commun.

8 (2006) 610–614.[21] J.L. Wang, J. Yang, J.Y. Xie, N.X. Xu, Y. Li, Electrochem. Commun. 4 (2002)

499–502.[22] P.C. Howlett, D.R. MacFarlane, A.F. Hollenkamp, Electrochem. Solid State Lett. 7

(2004) A97–A101.[23] X. Liang, Z.Y. Wen, Y. Liu, X.Y. Wang, H. Zhang, M.F. Wu, L.Z. Huang, Solid State

Ionics 192 (2011) 347–350.[24] G.B. Appetecchi, S. Scaccia, C. Tizzani, F. Alessandrini, S. Passerini, J. Electrochem.

Soc. 153 (2006) A1685–A1691.[25] S.C. Han, M.S. Song, H. Lee, H.S. Kim, H.J. Ahn, J.Y. Lee, J. Electrochem. Soc. 150

(2003) A889–A893.