Journal ofMaterials Chemistry A

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A high performan

aDepartment of Chemistry, National Univ

Singapore. E-mail: chghs2@gmail.combSustainable Energy Laboratory, Faculty of

University of Geosciences, 388, Lumo Road,

† Electronic supplementary informa10.1039/c5ta02628h

Cite this: J. Mater. Chem. A, 2015, 3,20267

Received 11th April 2015Accepted 25th August 2015

DOI: 10.1039/c5ta02628h

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

ce polysiloxane-based single ionconducting polymeric electrolyte membrane forapplication in lithium ion batteries†

Rupesh Rohan,a Kapil Pareek,a Zhongxin Chen,a Weiwei Cai,ab Yunfeng Zhang,ab

Guodong Xu,a Zhiqiang Gaoa and Hansong Cheng*ab

We report a polysiloxane based single-ion conducting polymer electrolyte (SIPE) synthesized via a

hydrosilylation technique. Styrenesulfonyl(phenylsulfonyl)imide groups were grafted on highly flexible

polysiloxane chains followed by lithiation. The highly delocalized anionic charges in the grafted moiety

give rise to weak association with lithium ions in the polymer matrix, resulting in a lithium ion

transference number close to unity (0.89) and remarkably high ionic conductivity (7.2 � 10�4 S cm�1) at

room temperature. The high flexibility arising from polysiloxane enables the glass transition temperature

(Tg) to be below room temperature. The electrolyte membrane displays high thermal stability and a

strong mechanical strength. A coin cell assembled with the membrane comprised of the electrolyte and

poly(vinylidene-fluoride-co-hexafluoropropene) (PVDF-HFP) performs remarkably well over a wide range

of temperatures with high charge–discharge rates.

1. Introduction

Electrolytes play a vital role in battery performance and regulatebattery working life. The development of novel electrolytes withhigh conductivity in a wide temperature range with a broadelectrochemical window and strong thermal stability is essen-tial for the next generation of lithium-ion battery technologiesthat utilize high potential electrode materials at highcharge–discharge rates.1–4

The concept of single ion polymeric electrolytes (SIPEs) wasproposed with the aim to overcome most of the problemsassociated with the current electrolytes,5–22 such as severeconcentration polarization and potential safety hazard. Unfor-tunately, there are also many technical problems with SIPEs.Most notably, these materials exhibit relatively small ionicconductivity and oen present high interfacial resistance whenused in batteries.15,23 To date, few SIPE materials have beendemonstrated with satisfactory battery performance. MostSIPEs enable batteries to perform only at elevated temperatureswith relatively low charge–discharge rates.23–29 Indeed, a versa-tile SIPE capable of sustaining a high C-rate in a wide range oftemperatures remains to be developed.30,31

ersity of Singapore, 3 Science Drive 3,

Materials Science and Chemistry, China

Wuhan 430074, P. R. China

tion (ESI) available. See DOI:

hemistry 2015

It has been well demonstrated that ion migration occurspredominantly in an amorphous opulent phase where transportproperties, such as ionic conductivity, diffusivity and mechan-ical relaxation, are explicitly related to the segmental movementof polymers.30,32 Therefore, a SIPE with low or no crystallinityand low glass transition temperature with high thermal andelectrochemical stability would present a reasonable choice as abattery electrolyte.31 Unlike most of the polymers used as elec-trolytes, polysiloxanes, which are inherently amorphous innature and possess very low Tg with high thermal stability andgood mechanical strength, may potentially serve as usefulbackbone polymers for designing SIPEs.33–35

Based on this idea, we developed a methodology to synthe-size a polysiloxane based single ion conducting gel polymerelectrolyte with graed bis(sulfonyl) imide groups via a hydro-silylation process. Hydrosilylation is a catalyst assisted tech-nique of adding Si–H bonds across unsaturated bonds.36 In thispaper, we describe the synthetic procedure and characterizationof this compound. A battery fabricated with the polysiloxanebased electrolyte is shown to perform successfully in atemperature range of 25–80 �C with one of the highest charge–discharge rates reported to date for SIPEs.

2. Experimental2.1 Materials

All chemicals, poly(methylhydrogensiloxane) (PMHS) (viscosity-15–40 mPa S) (Sigma-Aldrich), chloroplatinic acid (Sigma-Aldrich), 4-styrenesulfonic acid and sodium salt (Sigma-Aldrich),oxalyl chloride (Alfa-Aesar), benzenesulfonamide (Sigma-

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Aldrich), 4-dimethylaminopyridine (DMAP) (Sigma-Aldrich),triethylamine (Sigma-Aldrich), lithium perchlorate (Sigma-Aldrich), poly(vinylidene uoride-co-hexauoropropylene) (PVDF-HFP) (Mw – 400 000 gmol�1) (Sigma-Aldrich), 2-propanol (QREC),acetonitrile (Tedia), dimethylformamide DMF (Sigma-Aldrich),dimethyl sulfoxide (QREC), tetrahydrofuran (THF) (RCI labscan),and dichloromethane (QREC), were of analytical grade and usedas received.

2.2 Synthesis of potassium 4-styrenesulfonyl(phenylsulfonyl)imide

4-Styrenesulfonyl(phenylsulfonyl)imide (SPSIK) was synthe-sized following the procedure reported by Meziane andco-workers24,25 for the synthesis of 4-styrenesulfonyl(tri-uoromethylsulfonyl)imide. 40 ml of dried acetonitrile and0.087 g (1 mmol) of DMF along with 2 ml of oxalyl chloride weretaken in a two-neck ask. The solution was stirred under anargon gas atmosphere at room temperature for 5 h to form aVilsmeier–Haack complex. On completion of the reaction, alight yellow transparent solution was obtained. Subsequently, 4grams of 4-styrene sulfonic acid sodium salt was added slowly tothe solution under an argon gas atmosphere and then stirredfor 24 h. The mixture initially turned yellow and then displayeda light pink color. A NaCl precipitate was separated by ltrationand the ltrate (4-styrenesulfonyl chloride) was cooled at 0 �C.

In a two-neck ask, 8.1 ml of triethylamine (58.1 mmol),3.1 grams of benzenesulfonamide (19.4 mmol) and 2.12 gramsof 4-dimethylaminopyridine (DMAP) were added to 30 ml of dryacetonitrile. The mixture was stirred for 1 h in an argon gasatmosphere and subsequently, the 4-styrenesulfonyl chloridesolution cooled at 0 �C was added slowly to the mixture understirring for the next 16 h. The solvent was removed using arotavapour and the resulting brown colored mass was dissolvedin 50 ml of dichloromethane. The solution was washed with a4% aqueous solution of NaHCO3 (20 ml) twice followed bywashing with 20 ml of 1 M HCl. The potassium salt of 4-styr-enesulfonyl(phenylsulfonyl)imide was obtained by neutraliza-tion of the acid moiety in a molar excess aqueous solution ofK2CO3. The resulting suspension was stirred at room tempera-ture for 1 h followed by ltration and drying. The resulting lightyellow colored solid was further recrystallized in water for 24 hat ambient temperature followed by storing in a refrigerator for12 h. The water was decanted and the compound was dried in avacuum oven at 80 �C for 12 h to obtain dried potassium4-styrenesulfonyl(phenylsulfonyl)imide in 50% yield. Elementalanalysis: for C14H12NO4S2K, calculated: C, 46.52; H, 3.35; N,3.87; S, 17.74%; K, 10.82; found: C, 46.56; H, 3.41; N, 3.61; S,17.12; K, 10.25%. 1H NMR (300 MHz, DMSO-d6) d 7.68–7.58 (m,4H), 7.49–7.29 (m, 5H), 6.73 (dd, J¼ 17.7, 10.8 Hz, 1H), 5.87 (d, J¼ 17.7, 1H), and 5.33 (d, J ¼ 10.8, 1H) (Fig. S1 in the ESI†); 13CNMR (75 MHz, DMSO-d6) d 146.5, 145.8, 138.6, 135.9, 130.0,127.8, 126.5, 126.0, 125.5, and 115.9 (Fig. S2†). FTIR data:{ATR(KBr) cm�1}: n¼ 3058 (nCH aromatic ring), 1627 (nCC aromatic ring),1445 (bCH ring + nCC ring), 1397 (nasSO2

), 1300 (bCH), 1265 (nCC),1171 (nsSO2

), 1090 (nCS), 1060(nCC ring), 840 (nSN), 777(nSNS) and750 (gCH), 689 (jCCC), and 616 (aCCC), {n: stretching, d: bending,

20268 | J. Mater. Chem. A, 2015, 3, 20267–20276

a and b: in plane deformation, g and j: out of plane deforma-tion, s: symmetrical, as: asymmetrical},24,37–39 ESI (�) m/z 322.2(Fig. S6†).

2.3 Synthesis of Speier's catalyst

Speier's catalyst, which conducts hydrosilylation, was synthe-sized following the procedure adopted earlier by Chung andKim.40 50 ml of Na dried 2-propanol was taken in a dark coloredbottle in which 1 gram of chloroplatinic acid (H2PtCl6$6H2O)was dissolved. The bottle was kept for four weeks in a shelf forthe formation of Speier's catalyst.

2.4 Synthesis of lithium 4-styrenesulfonyl(phenylsulfonyl)imide graed polymethyl hydrogen siloxane (SG) electrolyte

A lithium 4-styrenesulfonyl(phenylsulfonyl)imide graed pol-y(methylhydrogensiloxane) (SG) electrolyte was synthesized viaa hydrosilylation reaction in the presence of Speier's catalyst(Scheme 1). The SPSIK molecules were graed onto thepoly(methylhydrogen siloxane) (PMHS) chains, following themethod previously reported.41–43 0.2 ml of PMHS was mixed in10 ml of THF while 1.37 grams of SPSIK was dissolved in 20 mlof DMSO. Both solutions and 0.1 ml Speier's catalyst were takenin a 2-neck round bottom ask tted with amagnetic stirrer anda condenser. To avoid moisture, all operations were performedin a glovebox. The mixture was stirred in an argon atmosphereat room temperature for 24 h followed by stirring at 80 �C foranother 24 h. The completion of the hydrosilylation reactionwas conrmed by the in situ FTIR spectrum of the product, inwhich the sharp and intense peak at 2267 cm�1, correspondingto the stretch of the Si–H bond, disappeared. The solvent wasremoved under vacuum using a freeze-dryer. To remove theunreacted moieties and other impurities, the product waswashed with THF, acetone and methanol. Subsequently, theproduct was dried at 100 �C for 12 h, producing a sticky solidmaterial. To remove the stickiness, thematerial was heated withacetonitrile at an elevated temperature followed by THF addi-tion upon cooling to retrieve the product. The material was thenisolated by ltration and dried at 100 �C. The process wasrepeated thrice to obtain a non-sticky light yellow colored solid.Finally, the product was crushed to a ne powder form with amortar and pestle and dried further at 100 �C in a vacuum oven.

The product was placed in 50 ml ethanol with stirring and,subsequently, lithium perchlorate (LiClO4) (in excess) wasadded for the exchange of potassium ions with lithium ions.The solution was heated at 50 �C for 12 h. A white precipitate ofpotassium perchlorate was ltered out and the solution wasremoved with a rotary evaporator. The excess of LiClO4 wasremoved upon solubilization of the solid in DMSOfollowed by re-precipitation with THF. The nal product wasdried at 120 �C under vacuum for 12 h. Elemental analysis: forC15H16NO5S2SiLi, calculated: C, 46.26; H, 4.14; N, 3.60; S, 16.46;Si, 7.21; Li, 1.78%; found: C, 46.99; H, 4.49; N, 3.80; S, 17.12; Si,1.01; Li, 1.61%. 1H NMR (300 MHz, DMSO-d6) d 7.62–7.08 (m,9H), 2.14 (t, J ¼ 8.0 Hz, 2H), 1.85 (t, J ¼ 7.3 Hz, 2H), and 1.18 (s,3H) (Fig. S3†); 13C NMR (75 MHz, DMSO-d6) d 146.5, 145.8,142.6, 138.9, 136.0, 130.3, 129.2, 128.0, 126.7, 126.3, 125.8,

This journal is © The Royal Society of Chemistry 2015

Scheme 1 Synthesis of the SG electrolyte.

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116.2, 48.8, 30.4, 29.2, and 17.4 (Fig. S4†); FTIR data: {ATR(KBr)cm�1}: n ¼ 2978 (nCH alkyl), 1648 (nCC aromatic ring), 1396 (nasSO2

),1267 (dCH3

), 1148 (nsSO2), 1091 (nSi–O–Si), 1039 (nCC ring) and

778 (nSNS).39

2.5 PVDF-HFP/SG membrane preparation

SG and PVDF-HFP were taken in 1 : 2 proportion and dissolvedin 5 ml of a DMF solvent at 60 �C for 6 h in a 50 ml glass beakertted with a magnetic stirrer. Subsequently, the mixture wasdrop-cast into a Teon Petri dish (6 cm diameter) and dried inan oven at 100 �C to evaporate the DMF solvent. The obtainedlm was further dried under vacuum at 100 �C for 24 h toremove a trace amount of DMF and then transferred to aglovebox. The membrane was punched to obtain a circular lmof 1.5 cm diameter. The circular lm was subsequently placedin an EC/PC (1 : 1 volume ratio) solution for soaking and thenused for further characterization and performance assessmentin a battery coin cell.

3. Methods

The gel permeation chromatography (GPC) technique wasused for molecular weight and polydispersity index (PDI)determination. The GPC instrument consists of a high-performance liquid chromatography pump (Waters 515), anautosampler (Waters 2707) and a refractive index detector(Waters 2414). THF was used as an eluent at a ow rate of 0.8ml min�1. A polystyrene standard was used for calibration. Allcharacteristic infrared spectra were recorded in the400–4000 cm�1 frequency range with a Bio-Rad Excalibur

This journal is © The Royal Society of Chemistry 2015

FTIR spectrometer. NMR spectra were taken on a BrukerAvance (AV300) spectrometer at 300 MHz, with tetrame-thylsilane as the standard. Dimethyl sulfoxide-d6 was used asthe solvent for NMR characterization. The microstructures ofthe samples were determined by using the eld emissionscanning electron microscopy (FE-SEM) technique on a JEOLJSM-6701F scanning electron microscope. Platinum sputter-ing of the samples was performed under 5 � 10�2 mbar atroom temperature (20 s, 30 mA) prior to the SEM analysis. Thethermal gravimetric analysis (TGA) was executed under a N2

gas inert atmosphere (ow rate: 60 cm3 min�1) at the 10 �Cmin�1 heating rate from room temperature to 1000 �C in aThermogravimetric Analyzer (model TGA Q 50) of TA, Inst.,USA. A differential scanning calorimetry (DSC) study wasperformed on a DSC-1 (Mettler Toledo, Inst., USA) instrumentin a N2 atmosphere (ow rate: 60 cm3 min�1) from �60 �C to150 �C with a heating rate of 10 �C min�1.

Elemental analysis for the determination of C, H, N and Selements was performed using a CHNS analyser, model VarioMicrocube from Elementar Analysensysteme. Samples wereweighed (1–2 mg) in a tin capsule and combusted at 1150 �C.The combustion products such as CO2, SO2, and NO2 are carriedby helium carrier gas into a reduction tube containing copperwhere NO2 is converted to N2 and any unconsumed oxygen isremoved. N2 passes through to be detected by a conductivitydetector. The rest of the gases are retained in the adsorptioncolumn. The desorption column is heated to release each of thegases at different temperatures which are detected by using athermal conductivity detector. Calibration is performed usinghigh purity sulfanilamide. The metal content was determinedby acid digestion of the samples followed by analyses with anInductively Coupled Plasma Optical Emission Spectrometer(ICP-OES), model Optima 5300DV from Perkin-Elmer. Multielement standards from Inorganic Venture were used forcalibration.

The ionic conductivity of the electrolyte membrane wascalculated by electrochemical impedance spectroscopy (EIS)using a Zahner potentiostat–galvanostat electrochemical work-station (model PGSTAT) over a frequency range of 4 � 106 to 1Hz with an oscillating voltage of 5 mV. The electrolytemembrane was placed in a stainless steel cylindrical device of1.5 cm diameter sealed in a glovebox under an argon atmo-sphere. Before the EIS analysis, the device was heated at 80 �C toenhance the contact between the surfaces of the device platesserving as the electrodes and the electrolytic membrane. TheSimulated Impedance Measurement (SIM) soware was usedfor the tting of the EIS data.

The transference number of the electrolyte was calculated bythe method proposed by Evans and co-workers, based on thecombination of DC and AC electrical polarization.44 The value ofthe cationic transference number (tLi

+) was calculated accordingto the following equation:

tLiþ ¼ IsðDV � I0R0Þ

I0ðDV � IsRsÞwhere DV is the DC potential applied across the cell; Is and I0are the steady-state and the initial current determined by

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the DC polarization, respectively. R0 and Rs are the EISmeasured bulk resistances before and aer the DCpolarization.

To measure the battery performance, a multichannelbattery testing instrument, Arbin BT-2000, was used. Thecharge and discharge capacity of the coin cells assembledwith the electrolyte membrane was measured with differentC-rates at various temperatures. The composite cathodeconsists of LiFePO4 (75%), PVDF (10%), acetylene black(10%) and a small amount of lithium bis(4-carboxy phenylsulfonyl)imide (5%) as a supporting electrolyte. All theingredients were mixed and stirred well in a NMP solvent.The prepared ink was then cast onto an aluminum foil andsubsequently dried in a vacuum oven at 80 �C for 12 h. Thedried cathode was then cut into a circular shape for use incoin cells. The coin cell assembly was performed inside aglovebox. For the coin cell assembly, stainless-steel cups andlids of coin cells CR 2025 (20 mm diameter, 2.5 mm thick-ness) comprised of the LiFePO4 cathode, Li metal anode andEC/PC swollen single ion polymer electrolyte membrane wereused. The cathode was placed in the centre of the cup whichforms the +ve terminal of the cell and subsequently coveredwith the swollen polymer electrolyte membrane. A circularlithium metal disc of 13 mm diameter and �0.6 mm thick-ness was placed centrally upon the membrane. To ensuregood sealing and achieve proper thickness of the materials,two stainless discs (spacers) followed by a steel spring, whichformed the �ve terminal, were xed upon the Li metal disc.Finally, the coin cell was closed with the lid and nal sealingwas done with an automatic coin crimper machine. Theassembled cells were then transferred outside the gloveboxand aged/equilibrated for 12 h prior to further electro-chemical characterization.

Fig. 1 The FTIR spectra of PMHS, SPSIK and SG electrolyte.

4. Results and discussion4.1 Synthesis and characterization of potassium 4-styrenesulfonyl(phenylsulfonyl)imide (SPSIK)

The SPSIK monomer was synthesized following the methodproposed by Meziane et al. with a slight difference in choiceof one of the ingredients.24,25 In this work, benzene sulfon-amide was used in place of triuoromethylsulfonamide(TFSI), which was used in the original method. The rest ofingredients including the catalyst and solvents as well as thesynthetic route were kept the same. The successful synthesisof SPSIK was conrmed by 1H NMR, elemental analysis andmass spectroscopy (ESI-HRMS). The 1H NMR signals corre-sponding to 12 H atoms (9 aromatic and 3 non-aromatic)with the peak area ratio of 9 : 1 : 1 : 1 further conrm thetarget structure. Moreover, the molecular formula of SPSIKcalculated in the negative mode of ESI-HRMS was found tobe 322.2, which matches well with the theoretical value of322.3 for the anion monomer C14H12NO4S2. Finally, theagreement between the expected and the found valuesof elemental analysis data reaffirms the successful synthesisof SPSIK.

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4.2 Synthesis and characterization of lithium4-styrenesulfonyl(phenylsulfonyl)imide graed polymethylhydrogen siloxane (SG) electrolyte and PVDF-HFP/SG blendmembrane

The SG electrolyte was synthesized by graing SPSIK on poly-methyl hydrogen siloxane (PMHS) chains (SPSIK-g-PMHS) viahydrosilylation followed by lithium ion exchange. Hydro-silylation is the most versatile platinum catalyst assisted tech-nique used for the addition of Si–H on unsaturated bonds.36,45

Speier's catalyst (solution of chloroplatinic acid in 2-proponal)facilitates the addition of unsaturated groups on Si–H accord-ing to anti-Markovnikov's rule in a three step cyclic process.36

Following the same mechanism, here, the vinylic part of SPSIKwas attached to the polysiloxane chains upon the addition of aSi–H bond.

The graing of SPSIK on PMHS chains followed by theexchange of K+ ions by Li+ ions was clearly proven to besuccessful by FTIR and NMR spectra. As can be seen in the FTIRspectra of PMHS and SG (Fig. 1), the peak at 2167 cm�1 inPMHS, corresponding to the Si–H bond, disappeared in SG,which conrms the graing of SPSIK. In addition, the peak at1627 cm�1 in SPSIK, corresponding to the vinylic group, alsodisappeared in SG, conrming the graing, which consumesthe vinylic group. Furthermore, the signature signal of Si–H,which appears at 4.68 ppm (1H NMR) in PMHS, and the threepeaks in the range of 6.78 to 5.31 ppm (1H NMR) in SPSIKcorresponding to 3 vinylic protons disappeared in SG, validatingthe graing of SP on the PMHS chains.43,46 Finally, the overallsynthesis of the SG electrolyte was conrmed by the coherencebetween the expected and the found values for all the elements,except for Si, in the elemental analysis. The deviation observedfor Si is mainly attributed to the incomplete digestion of Siduring the analysis, which was also noticed during theelemental analysis of commercial products of Si such as PMHSand poly(methylphenylsiloxane) (PMPS) (Table S1 in the ESI†).The GPC analysis of SG in the tetrahydrofuran (THF) mobilephase depicts the number average molecular weight (Mn) of84 238, weight average molecular weight (Mw) of 150 230, andpolydispersity index (PDI) of 1.78 based on the polystyrenestandard.

This journal is © The Royal Society of Chemistry 2015

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Since it was not possible to form a free standing membraneof the SG electrolyte alone, PVDF-HFP was used as a binder. Thereason for choosing PVDF-HFP was based on the followingconsiderations. Although polyethylene oxide (PEO) is one of themost extensively utilized binders for polymer electrolytes, it maycause severe passivation upon contact with a lithium metalanode. On the other hand, PVDF, a homo-polymer, exhibitsstrong electrochemical stability arising from the strong electronwithdrawing uorine atoms in the compound. It has a highdielectric constant of 8.4, which helps in greater ionization oflithium salts. However, due to its crystalline nature, PVDF maycause separation of solvent liquids from the membrane surface(known as syneresis), which may result in a leakage of a solventand thus become a safety hazard when used in batteries. As theadvanced version of a PVDF based polymer, PVDF-HFP, thecopolymer of vinylidene uoride and hexauoropropylene,performs better than PVDF mainly due to the presence of anamorphous domain of hexauoro propylene, which overcomesthe problem of syneresis.47

Fig. 2 depicts the thermal degradation of the SG electrolyteand SG/PVDF-HFP membrane, in a N2 gas atmosphere. For theSG electrolyte, aer the 5% initial weight loss till 180 �C, whichis attributed to the loss of moisture and a trace amount of thesolvents, no degradation occurs till 410 �C. Two-stage degra-dation starts aer 410 �C and ultimately diminishes at 1000 �Cwith ca. 10% of char. The results conrm the high thermalstability of the SG electrolyte at an elevated temperature, whichis a typical feature of polysiloxanes. The residual weight of thesample, on completion of the TGA analysis, corresponds to thetotal Si and Li contents of the material. For the SG/PVDF-HFPmembrane, the TGA thermogram shows stability up to 450 �Cbefore showing single step degradation. The high thermalstability of the membrane is mainly attributed to the thermalstability of both the SG electrolyte and PVDF-HFP (degradationtemperature > 460 �C).48 The TGA suggests that the SG electro-lyte and the SG/PVDF-HFPmembrane should also be potentiallyuseful for high temperature applications in batteries.

The DSC thermograms of the SG electrolyte and SG/PVDF-HFP membrane are shown in Fig. 3. The glass transitiontemperature (Tg) of SG was found to be 10 �C, fairly below roomtemperature and the lowest among the SIPEs reported to

Fig. 2 The TGA thermogram of the SG electrolyte and PVDF-HFP/SGmembrane (N2, 10 �C min�1, RT to 1000 �C).

This journal is © The Royal Society of Chemistry 2015

date.23,24,27,49,50 The low Tg arises mainly due to PMHS, the parentpolysiloxane chain. Polysiloxane is known for its very low glasstransition temperature (��100 �C) due to the presence of highlyexible Si–O–Si bonds. This is the main reason for PMHS beingselected as the base polymer. The graing of the bulky SPSIKgroup on PMHS increases the Tg of the resulting SG electrolyte;however, the Tg still remains well below room temperature. Theabsence of a melting peak below 150 �C, which was expecteddue to the inherent amorphous nature of PMHS, conrms theamorphous nature of the SG electrolyte. The low temperatureexibility of the SG electrolyte as well as of PVDF-HFP (Tg ��35 �C) works synergistically for the SG/PVDF-HFP membraneand as a result, the Tg of the membrane was found to be�17 �C,which ensures the high exibility of the membrane andconsequently, decent performance of the membrane.51

Furthermore, the low crystallinity of the membrane coupledwith low Tg makes the membrane well suited as a single ionconducting electrolyte membrane for lithium ion batteries.

The SEM image of the SG electrolyte is shown in Fig. 4a. Thepowder forms irregularly shaped aggregates, which are attrib-uted to the high polarization of the particles in view of thepresence of high polar groups. The morphology of the PVDF-HFP/SG blend membrane is depicted in its SEM images (Fig. 4band c). There are some blind pores found on the surface of themembrane (Fig. 4b). However, from the cross-section of themembrane (Fig. 4c), no pores are visible, indicating the densenature of the membrane. The blind pores may be formed due tothe evaporation of the solvent from the surface. The tensilestrength of the membrane was found to be 5.8 MPa with anelongation at break of 18% (Fig. 4d), inferring its sufficientlyhigh mechanical strength. The membrane plays a dual role as aseparator and as an electrolyte for battery operation. To assessthe mechanical strength of the membrane further, it wasclamped between two clips against the gravity and a 15 gramstainless steel cylinder was kept on the membrane (Fig. 4e). Themembrane withstood the load of the cylinder successfullywithout showing a sign of mechanical failure, which conrmsits suitability as an electrolyte membrane for battery assembly.In addition, the membrane displays high exibility (Fig. 4f),

Fig. 3 The DSC thermogram of the SG electrolyte and PVDF-HFP/SGmembrane (N2, 10 �C min�1, �30 to 180 �C).

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Fig. 4 (a) SEM image of the SG electrolyte, (b) SEM image of thesurface view of the PVDF-HFP/SG membrane, (c) SEM image of thecross-sectional view of the PVDF-HFP/SG membrane, (d) tensilestrength vs. tensile strain graph of the PVDF-HFP/SG membrane, (e)photograph of the clamped PVDF-HFP/SG membrane holding astainless steel cylinder of 15 grams of weight, and (f) photograph of theflexible nature of the PVDF-HFP/SG membrane.

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which enables signicant reduction of interfacial resistancebetween electrodes and the membrane and thus enhancesbattery performance.

4.3 Electrochemical properties

The Electrochemical Impedance Spectroscopy (EIS) response ofthe PVDF-HFP/SG (soaked in an EC/PC solution) was recordedat different temperatures and the results are plotted in Nyquist

Fig. 5 The EIS plot of the PVDF-HFP/SG membrane at varioustemperatures. The inset is the corresponding equivalent circuit.

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coordinates (Z0 vs. Z00), where Z0 and Z0 0 are the real and imagi-nary parts of the impedance, respectively (Fig. 5). The equivalentcircuit used for the tting is also depicted in the inset of Fig. 5,where R1, R2, CPE and W stand for the bulk resistance, theinterface resistance, the constant phase element and the War-burg resistance, respectively.

The ionic conductivity of the membrane at differenttemperatures was derived based on the tting results of the EISresponses obtained at the given temperatures.27 At roomtemperature, the ionic conductivity of the membrane wascalculated to be 7.2� 10�4 S cm�1, which is slightly higher thanthe conductivity exhibited by most single ion gel polymericelectrolytes reported to date.6,8,26,52,53 The high conductivity ofthe electrolyte mainly arises from the electron delocalization ofthe N atom of the bisulfonyl imide group via the strong electronwithdrawing functionality of the adjacent S]O groups. Theelectron delocalization is further enhanced in the presence ofthe two benzene rings at both ends. The delocalization thusgives rise to a weak association between cations and anions. Thecharge separation in the membrane enables high Li ionmobility, which is further supported by the high exibility of themain chains with Si–O–Si bonds.

To analyze the temperature-dependency of the ionicconductivity of the PVDF-HFP/SG membrane, a graph is plottedbetween the log values of the ionic conductivity of themembrane vs. the inverse of absolute temperatures (Fig. 6), forthe testing temperature range from 90 �C to room temperaturedownwards. As expected, the measured log value of the ionicconductivity increases linearly with temperature, exhibitingcoherence with a typical Arrhenius curve but not as steep as inwhat was observed with dual-ion based small molecular gelpolymer electrolytes.54,55 The meek increment in the ionicconductivity may be attributed to the mechanical couplingbetween ion transport and polymer host mobility at a giventemperature according to the free volume law.7 The highestconductivity at 90 �C was found to be 1.13 � 10�3 S cm�1.

To calculate the lithium-ion transference number (tLi+), the

Li|PVDF-HFP/SG membrane |Li cell was assembled, where themembrane was slotted in between two non-blocking lithium

Fig. 6 The Arrhenius plot of log(ionic conductivity) versus inverseabsolute temperature.

This journal is © The Royal Society of Chemistry 2015

Table 1 Parameters for calculation of lithium ion transferencenumbers (tLi

+)

DV I0 (mA) Is (mA) R0 (U) Rs (U)tLi

þ ¼ IsðDV � I0R0ÞI0ðDV � IsRsÞ

0.01 8.68 7.72 49.30 50.35 0.89

Fig. 8 The discharge profiles of the Li|PVDF-HFP/SG membrane(EC/PC)| LiFePO4 battery coin cell obtained at room temperature,60 �C and 80 �C at 0.1C rate.

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metal electrodes.25,56 The values of variables required for thecalculation are depicted in Table 1. The tLi

+ value was calculatedto be 0.89 at room temperature, which infers the single-ionconducting behaviour of the SG electrolyte as presumed.

To evaluate the electrochemical stability of the PVDF-HFP/SGmembrane, a cyclic voltammetry (CV) study was carried outusing the Li|PVDF-HFP/SG membrane|stainless-steel cell. CVwas performed at room temperature between 0 and 6 V (versusLi+/Li) at a scan rate of 1 mV s�1 and the response is plotted as acurrent vs. voltage curve, as depicted in Fig. 7. There is nosignicant uctuation observed in the current below 4.1 V atroom temperature, which infers the electrochemical suitabilityof the SG/PVDF-HFPmembrane with the most available cathodematerials.

The galvanostatic charge–discharge cyclic performance ofthe PVDF-HFP/SG membrane was investigated in the voltagewindow of 2.4–3.8 V at various C-rates. The PVDF-HFP/SGmembrane was sandwiched between a lithiummetal anode anda LiFePO4 composite cathode (theoretical discharge capacity:170 mA h g�1) in a coin cell.57 Fig. 8 displays the characteristicevolution of the battery voltage of the cell Li|PVDF-HFP/SGmembrane (EC/PC)| LiFePO4 as a function of its dischargecapacity at room temperature, 60 �C and 80 �C, respectively. Thedischarge curves exhibit a typical three step plateau prole ofthe LiFePO4 cathode material with initial discharge capacitiesof 141, 159 and 154 mA h g�1 at room temperature, 60 �C and80 �C, respectively, at 0.1C rate.23,29,52 Correspondingly, theinitial coulombic efficiencies were found to be 87, 88 and 93%.The slight increment of the plateau voltage at the elevatedtemperatures may be attributed to the presence of the extra Li

Fig. 7 Cyclic voltammetry of the PVDF-HFP/SG membrane at roomtemperature with the scan rate of 1 mV s (cathodic part only).

This journal is © The Royal Society of Chemistry 2015

ions at the cathode-PVDF-HFP/SG interface. The high Limobility in the electrolyte membrane enables Li ions to bereleased more readily at an elevated temperature. The ions arecollected at the interface and offer some extra resistance. As aconsequence, the voltage increases.58

The discharge capacity vs. cycle number of the cell is depic-ted in Fig. 9. The discharge capacity of the cell at roomtemperature was found to be 141mA h g�1 at 0.1C and 135mA hg�1 at 0.2C. In particular, the battery performs very well at 60 �Cwith a C-rate up to 3C. At this temperature, the dischargecapacity was found to be near 160 mA h g�1 for low C-rates anddecrease gradually as the rate increases. However, the capacitystill remains above 110 mA h g�1 at 2C. The battery alsoperforms very well at 80 �C up to 2C but becomes slightlyinferior to the performance obtained at 60 �C, whichmay be dueto the reduction of electrochemical stability at the elevatedtemperature. The reason for the good battery performance isattributed to (i) the low Tg of the electrolyte which makes themembrane more exible, reduces the interfacial resistancebetween the electrodes and the electrolyte and supports facile

Fig. 9 The discharge capacity vs. cycle index of the Li|PVDF-HFP/SGmembrane (EC/PC)| LiFePO4 battery coin cell obtained at roomtemperature, 60 �C and 80 �C at various C-rates.

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transport of Li ions, and (ii) the high lithium transferencenumber, which limits the concentration polarization effect.59

5. Summary

A novel approach for synthesizing a new class of single-ionpolymer electrolytes based on a polysiloxane polymer and ahydrosilylation technique is demonstrated in the present work.A lithium styrenesulfonyl(phenylsulfonyl)imide graed poly-methyl hydrogen siloxane (SG) electrolyte was successfullysynthesized. The chemical structures and properties of theelectrolyte and its composite membrane with PVDF-HFP werethoroughly characterized via a range of spectroscopic tech-niques including FTIR, NMR, TGA, DSC analysis, EIS and CV.The highly exible nature of the SG electrolyte (Tg 10 �C) enablessignicant reduction of interfacial resistance between theelectrodes and the electrolyte membrane. As a consequence, thebattery cell incorporated with the electrolyte membraneperforms very well in a wide range of temperatures. The polymerelectrolyte exhibits good thermal stability up to 410 �C andelectrochemical stability up to 4.1 V with a remarkably highionic conductivity of 7.2 � 10�4 S cm�1 at room temperature.The high ion conduction is attributed to the extensive electrondelocalization through the bis sulfonyl imide groups, leading tosignicant enhancement of the mobility of lithium ions. Themeasured lithium ion transference number is close to unity(0.89), substantially higher than the value of typical dual-ionbased electrolytes. The reasonably wide electrochemicalwindow enables the electrolyte membrane to be potentiallyuseful for batteries with a range of cathode materials. Further-more, the battery performance of the SIPE membrane wasevaluated with an assembled coin cell of Li|PVDF-HFP/SGmembrane (EC/PC)| LiFePO4. The battery displays high chargeand discharge rates with good columbic efficiency at elevatedtemperatures. The successful demonstration of the electrolytemembrane may pave the way to develop new SIPEs with highexibility at low temperature, high ionic conductivity, and highthermal and electrochemical stability. Finally, the appliedsynthetic route may serve as a useful protocol for graingdesired ionic conductive moieties in Si–H containing polymerswith tailored properties.

Acknowledgements

The authors gratefully acknowledge support of a Start-up grantfrom the NUS, a POC grant from the National Research Foun-dation of Singapore, a Tier 1 grant from the Singapore Ministryof Education, a DSTA grant and the National Natural ScienceFoundation of China (No. 21233006 and 21473164).

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