Polymer International Polym Int 56:660–665 (2007)

Preparation and characterizationof novel hybrid thermoplastic poly(etherurethane)/poly(vinylidene fluoride) elastomers,and their application as solid polymerelectrolytesYe Lin,1 Qin Qian,2,3 Feng Zeng-Guo,1∗ Zhang Xiao-Wen,1 Bai Ying2,3 andWu Feng2,3

1School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China2School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, China3Laboratory of National Development Center of Hi-Tech Green Materials, Beijing 100081, China

Abstract: A comb-like polyether, poly(3-2-[2-(2-methoxyethoxy)ethoxy]ethoxymethyl-3′-methyloxetane)(PMEOX), was reacted with hexamethylene diisocyanate and extended with butanediol in a one-pot proce-dure to give novel thermoplastic elastomeric poly(ether urethane)s (TPEUs). The corresponding hybrid solidpolymer electrolytes were fabricated through doping a mixture of TPEU and poly(vinylidene fluoride) with threekinds of lithium salts, LiClO4, LiBF4 and lithium trifluoromethanesulfonimide (LiTFSI), and were characterizedusing differential scanning calorimetry, thermogravimetric analysis and Fourier transform infrared spectroscopy.The ionic conductivity of the resulting polymer electrolytes was then assessed by means of AC impedance mea-surements, which reached 2.1 × 10−4 S cm−1 at 30 ◦C and 1.7 × 10−3 S cm−1 at 80 ◦C when LiTFSI was added at aratio of O:Li = 20. These values can be further increased to 3.5 × 10−4 S cm−1 at 30 ◦C and 2.2 × 10−3 S cm−1 at 80 ◦Cby introducing nanosized SiO2 particles into the polymer electrolytes. 2006 Society of Chemical Industry

Keywords: polyurethane; comb-like polyether; solid polymer electrolyte; ionic conductivity

INTRODUCTIONSince the pioneering work by Wright and co-workers1

and Armand and co-workers2,3 was first reported inthe 1970s, solid polymer electrolytes (SPEs) haveattracted tremendous attention for their potentialapplication in lithium ion batteries, mainly becauseof their high thermal stability compared to liquidelectrolytes. However, the early polymer electrolytes,based on poly(ethylene oxide) (PEO)–LiX complexsystems, usually exhibited very low ionic conductivity,around 10−7 S cm−1, at ambient temperature mainlybecause of their high crystallinity and poor motionperformance of the PEO backbones.

Recently, poly(ether urethane)s (PEUs)4–6 havebeen broadly explored and exploited as SPE matrices.The soft segments in PEUs can dissolve Li+ cations,while the hard segments act as reinforcing fillers toimprove the dimensional stability of the resultingpolymer electrolytes. In general, the ionic conductivityof PEU-based SPEs can reach 10−5 S cm−1.7–14

However, the soft segments in these PEU matricesusually possess a linear and regular structure,such as PEO, poly(propylene glycol) (PPG) and

poly(tetramethylene glycol) (PTMG). As a result,they inevitably experience partial crystallization in thebulk, which is often regarded as the main obstacle thatprevents further improvement of the ionic conductivityof PEU-based SPEs.

In our previous work,15 a comb-type polyether,poly(3-2-[2-(2-methoxyethoxy)ethoxy]ethoxymethyl-3′-methyloxetane) (PMEOX), was synthesized bycationic ring-opening polymerization of the monomer3-{2-[2-(2-methoxyethoxy)ethoxy]ethoxymethyl}-3′-methyloxetane (MEOX) in solution. This polyetherexhibited a relatively low Tg, about −52 ◦C, and amor-phous structure due to its flexible and longer pendantchains. It looked like a promising candidate for the softsegments of thermoplastic elastomeric poly(ether ure-thane)s (TPEUs), and is used as an SPE matrix in thepresent work. To this end, an aliphatic isocyanate, hex-amethylene diisocyanate (HDI), was utilized insteadof aromatic isocyanates 4,4′-diphenylmethane diiso-cyanate (MDI) and 2,4-tolune diisocyanate (TDI) toavoid the rigidity of the aromatic moieties, which is

∗ Correspondence to: Feng Zeng-Guo, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, ChinaE-mail: sainfeng@bit.edu.cnContract/grant sponsor: National Key Projects on Basic Research and Development; contract/grant number: 2002CB211800(Received 24 March 2006; revised version received 2 August 2006; accepted 3 October 2006)Published online 5 December 2006; DOI: 10.1002/pi.2188

2006 Society of Chemical Industry. Polym Int 0959–8103/2006/$30.00

Novel hybrid thermoplastic PEU/PVDF elastomers

detrimental to both chain flexibility and ionic con-ductivity. Meanwhile, in order to fabricate robust self-standing polymer electrolyte films, poly(vinylidene flu-oride) (PVDF) was added to reinforce the as-preparedTPEUs. Furthermore, nanosized SiO2 particles werealso introduced by ultrasonic mixing to prepare com-posite polymer electrolytes (CPEs). It was anticipatedthat the nano-effect of SiO2 would benefit the iontransport of CPEs as reported in the literature6,16,17

and increase the ionic conductivity.

EXPERIMENTALMaterials and measurementsTin 2-ethylhexanoate, PVDF, lithium perchlorate(LiClO4), lithium tetrafluroborate (LiBF4) andlithium trifluoromethanesulfonimide (LiTFSI) werebought from Aldrich, USA, and all were used asreceived. 1,4-Butanediol (BDO) and boron trifluorideetherate (BF3 · Et2O) were obtained from ShanghaiChemical Reagent Company and used without furthertreatment. Nanosize silicon oxide with average particlediameter of about 20 nm was received from Ming-RiGroup Company, Zhejiang, China. HDI was kindlyprovided by Liming Institute of Chemical Industries,Henan, China. CH2Cl2 and N,N-dimethylacetamide(DMA) were purchased from Beijing Chemical Fac-tory, dried over CaH2 and distilled under nitrogenatmosphere before use.

Fourier transform infrared (FTIR) spectra weremeasured with a Shimadzu IR Prestige-21 instrument.1H NMR spectra were recorded using a Bruker ARX400 instrument with CDCl3 as solvent, containingTMS as internal standard. Elemental analysis wascarried out using Vario EL III apparatus. NetzschPC-200 equipment was used for the analysis of thethermal behavior of the polymers at a heating rate of10 ◦C min−1. The polymer samples (6–10 mg) wereheated from −100 to +100 ◦C in stainless steel pans.The results of the second run were used for theglass transition temperature determination. TGA wasconducted with a TA 2000 thermogravimeter undernitrogen atmosphere at a heating rate of 20 ◦C min−1.

Synthesis of comb-type polyethersSynthesis of the MEOX monomer was described inour previous work.15 The structure was characterizedas follows.

MEOX, FTIR, ν/cm−1: 2870 (CH2), 1112 (linearC–O–C), 969 (ring C–O–C); 1H NMR, δ/ppm:1.31 (3H, CH3), 3.38 (3H, CH3O), 3.54–3.56 (2H,CH2 –OCH3), 3.57 (2H, C–CH2 –O), 3.64–3.67(10H, O–CH2 –CH2, CH2 –CH2 –O–CH3), 4.34–4.53 (4H, ring’s CH2); elemental analysis (%):calculated, C 58.06, H 9.68; experimental, C 57.83,H 9.61.

The cationic ring-opening polymerization of MEOXwas carried out in solution at 0–5 ◦C. CH2Cl2 (20 mL)was firstly added into a dry round flask under nitrogenatmosphere, to which 0.568 g BF3 · Et2O and 0.180 g

BDO were added. After stirring for 0.5 h, 8 g monomerwas introduced into the flask. The polymerizationcontinued for 6 h, and was then terminated by addingtriethylamine. The resulting polyether was purified byprecipitation in hexane three times, and dried in vacuo.

PMEOX, FTIR, ν/cm−1: 3317(O–H), 2903 (CH2),1069(C–O–C); 1H NMR, δ/ppm: 0.90 (CH3 inside chain), 3.19 (C–CH2 –O in main chain), 3.30(C–CH2 –O in side chain), 3.38 (OCH3 in side chain),3.55–3.57 (O–CH2CH2 –O, middle ethoxy group inside chain), 3.61–3.66 (O–CH2CH2 –O, other twoethoxy groups in side chain).

Preparation of TPEUThe number average molecular weight of the as-prepared comb-type polyether PMEOX was deter-mined as 1500 g mol−1 from 1H NMR. BDO was usedas chain extender and DMA as solvent. The molarratio of PMEOX/BDO/HDI was 1/2/3. The prepara-tion process is outlined in Scheme 1. The solution ofPMEOX in DMA was added slowly into a round-bottom flask filled with the solution of HDI in DMAunder nitrogen atmosphere. The resulting solutionwas stirred for 3 h at 40 ◦C, then several drops oftin 2-ethylhexanoate were added into the flask. Then,BDO in DMA was added into the reactor in 0.5 h, andcontinued to react for 8 h. The reactant solution wasprecipitated into water and the precipitate was dippedin ethanol for 1 day. The product was dried in vacuoat 60 ◦C for 48 h.

Fabrication of polymer electrolyte filmTPEU, PVDF and lithium salts were dissolved inDMA in a dry plugged flask and mixed ultrasonicallyfor 0.5 h. Then, the mixture was coated onto apolytetrafluoroethylene (PTFE) plate and dried invacuo for 48 h to give polymer electrolyte films.

As for CPEs, nanosize SiO2 particles were addedto the mixture of TPEU, PVDF and lithium salts inDMA in a dry plugged flask and mixed ultrasonicallyfor 2 h. The mixtures were then coated onto PTFEplates and dried in vacuo for 48 h to form CPE films.

The ionic conductivity of the hybrid SPEs wasmeasured via AC impedance analysis with anelectrochemical cell consisting of the electrolyte filmsandwiched between two blocks of stainless steel. TheAC impedance analysis was performed using a CHI660A Electrochemistry Station made by the ChenHua Instrument Company, Shanghai, China. Themeasurements were carried out from 30 to 80 ◦Cwith intervals of 5 ◦C. The conductivity was calculatedusing the following equation:18

σ = l/(Rb × A) (1)

where Rb (in �) is the bulk resistance from ACimpedance, l (in cm) is the film thickness and A(in cm2) is the surface area of electrode.

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HO OH

O

O

3

n

PMEOX

+OCN

NCO

40 °C

HDI

O O

HN

O

O3

nNH

H2C

HN

O

6

O HN

H2C

NCO

O

6CH2

OCN

6

O

HOOH

BDO

OC2H4O

HN

H2C

O

6

O

HN

O O

O

O3

n

HN

H2C

O

6

O

HN

HN

H2C

O

6

O

HNO O

O

O3

n

HN

H2C

O

6

O

HN

TPEU

Scheme 1. Synthetic route to TPEU based on PMEOX, HDI and BDO.

RESULTS AND DISCUSSIONIonic conductivity measurementsThe addition of PVDF can substantially improve thedimensional stability of TPEU-based electrolytes toyield strong self-standing films. However, this certainlydepresses the ionic conductivity because PVDF isgenerally believed to be inert towards complexationwith lithium ions to contribute to ionic transport. Inorder to optimize the proportion of PVDF added tohybrid PEU/PVDF electrolyte systems, three samplescontaining 10, 20 and 25 wt% PVDF were prepared.The sample containing 10 wt% PVDF cannot forma tough free-standing film. In fact, the two othersamples were robust enough to give rise to self-standing films and their ionic conductivity plots areshown in Fig. 1. It can be seen that the samplehaving a lower amount of PVDF indeed exhibits ahigher ionic conductivity than that of the samplewith a higher amount of PVDF. The added PVDFpredominantly benefits the film mechanical integritybut hardly contributes to the ionic conductivity of themixture; therefore, all the hybrid samples studied inthis work contained the same proportion of PVDF,i.e. 20 wt%.

As is well known, the characteristics of the lithiumsalts used play an important role in promoting theionic conductivity of SPEs.19 Consequently, threelithium salts, LiClO4, LiBF4 and LiTFSI, were tested.As reported in our previous work,15 the comb-like polyether PMEOX, after termination with aceticanhydride, showed a maximum ionic conductivity at

2.8 2.9 3.0 3.1 3.2 3.3

-4.4

-4.3

-4.2

-4.1

-4.0

-3.9

-3.8

-3.7

-3.6

-3.5

-3.4

-3.3

-3.2

log

σ (S

cm

-1)

103 T-1 (K-1)

25% PVDF in blend20% PVDF in blend

Figure 1. Ionic conductivity versus temperature for mixtures withdifferent PVDF contents (O:Li = 20 and LiClO4).

a molar ratio of O:Li = 20:1. Taking into account thefact that the ionic transport takes place predominantlywithin the soft blocks of the TPEU-based SPEs, theconcentration of lithium salts added was also kept atthe same O:Li ratio. Figure 2 reveals that the hybridpolymer electrolyte film doped with LiTFSI reached amaximum ionic conductivity of 2.1 × 10−4 S cm−1 at30 ◦C and 1.7 × 10−3 S cm−1 at 80 ◦C. The higherionic conductivity imparted by this lithium saltcan most likely be attributed to its unique anioniccharacter. In general, a larger anion radius benefits

662 Polym Int 56:660–665 (2007)DOI: 10.1002/pi

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2.8 2.9 3.0 3.1 3.2 3.3-4.4

-4.2

-4.0

-3.8

-3.6

-3.4

-3.2

-3.0

-2.8

-2.6lo

g σ

(S c

m-1

)

103 T-1 (K-1)

LiClO4LiBF4

LiTFSI

Figure 2. Influence of different lithium salts on the ionic conductivityof SPEs (O:Li = 20) versus temperature.

the ionic conductivity, since it is easier to ionize inthe polymer matrix and to set off more free lithiumcations that promote ion conductivity. Among thethree lithium salts studied, LiTFSI has the largestanion radius and thus exhibits the highest ionicconductivity.

Also, the ionic conductivity is further related to twoother factors, i.e. the amount of free lithium ions andthe polymer segment motion. The segment motionability of the polymers can be evaluated by means ofDSC measurements. However, since it is difficult todetect the exact amount of free Li+ ions existing in thepolymer matrix, only indirect evidence can be deducedfrom FTIR and TGA analysis, which is discussed inthe following sections.

DSC measurementsThe thermal behavior of hybrid TPEU/PVDF elec-trolytes is shown in Fig. 3. At first, no crystal melting

-50 0 50 100

Tgs -52.9°C

Tgs -59.0°C

Tgs -62.6°C

Tgh53.0°C

Tgh51.9°C

Tgh49.1°C

3

2

1

Temp/°C

End

othe

rm

Figure 3. DSC plots of SPEs with different lithium salts at a ratio ofO:Li = 20 (1, LiClO4; 2, LiBF4; 3, LiTFSI). (Tgs and Tgh refer to Tg ofsoft and hard segments, respectively).

peak emerged in the DSC curves, indicating that thiskind of TPEU-based SPE possesses a completelyamorphous structure; two glass transition temper-atures related to the soft and hard segments wereobserved in all the polymer electrolyte samples. TheTg value around −60 ◦C is attributed to the transitionof the soft segments from glassy to rubbery state.4,13

During the ionic transportation processes, the Li+ ionsmove along with the movement of the soft segmentsthrough the continual formation and rupture of tran-sient cross-linkages between Li+ ions and ether oxygenatoms of the soft segments. Since a lower Tg meansbetter motion performance of the polymer segments,the lower the Tg, the higher the ionic conductivity.As observed, the hybrid polymer electrolyte dopedwith LiTFSI had a lower Tg, whereas that doped withLiClO4 had a higher Tg. Consequently, the former hasa higher ionic conductivity than the latter.

FTIR analysisAn FTIR spectrum of pure TPEU film is shownin Fig. 4. The peak at 3362 cm−1 indicates N–Hstretching vibration and the peak at 1699 cm−1 impliesthat of carbonyl groups, which result from the reactionof the end-capping hydroxyl groups of PMEOX andBDO with the isocyanate groups of HDI. Additionally,the peak assigned to the ether bond of the soft segmentsappears at 1108 cm−1.

It was reported13,20 that adding lithium salts toa polymer matrix can weaken the strength of etherbonds because of the interactions between Li+ ionsand ether oxygen atoms, which reduce the electroncloud density of the C–O bond. The wavenumber ofan IR absorption peak is associated with the strengthof a bond according to the following equation:

ν = (2πC)−1 × (k/µ)0.5 (2)

where µ is a reduced atom mass related to the bondedatoms and k is a parameter associated with the strength

3000 2000 100020

40

60

80

100

Tra

ns/%

C-O stretching

C=O stretching

N-H stretching

Wavenumber/cm-1

Figure 4. FTIR spectrum of TPEU prepared from PMEOX, HDI andBDO.

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2000 1500 1000

3

2

1

Tra

ns%

Wavenumber/cm-1

Figure 5. FTIR spectra of polymer electrolytes with different lithiumsalts at a ratio of O:Li = 20 (1, LiTFSI; 2, LiBF4; 3, LiClO4).

of the bond. When the bond strength weakens, thevalue of k decreases. As a result, this leads to a redshift of the absorption peak. As observed in Fig. 5, thelargest red shift occurs for the peak of the ether bondof the polymer electrolyte film doped with LiTFSI,whereas the smallest one occurs for that doped withLiClO4. This indirectly revealed that LiTFSI sets offmore free ions compared with LiBF4 and LiClO4 atthe same concentration. This is most likely due tothe fact that LiTFSI has the biggest anion amongthese lithium salts, so that it is easier to ionize inthe polymer matrix to liberate more free lithiumcations, thus inducing the largest red shift of theabsorption peak. These IR analytical results are ingood agreement with those of the ionic conductivitymeasurements.

Thermal stabilityThe thermal decomposition processes of the resul-tant hybrid polymer electrolytes are depicted inFig. 6. All the samples start to lose weight at

0 100 200 300 400 5000

20

40

60

80

100

321

Temp/°C

Wei

ght/%

1 LiTFSI2 LiBF4

3 LiClO4

Figure 6. TGA plots of SPEs with different lithium salts at a ratio ofO:Li = 20.

about 300 ◦C, but the process ends when the tem-perature gets over 400 ◦C. This means that theweight-loss temperature of these kinds of SPEs issubstantially higher than that of liquid electrolytescurrently used in lithium ion batteries. In addi-tion, the weight-loss process of the TPEU-basedSPEs is more gradual. Accordingly, if these kindsof hybrid electrolytes are used in lithium ion bat-teries, they are more thermally stable than liquidelectrolytes.21

It can also be seen that the weight loss of TPEU-based electrolytes doped with LiTFSI takes place ata lower temperature range compared with the twoother lithium salts. The weight-loss process of thepolymer doped with LiClO4 takes place at a highertemperature range. It was reported22 that the weight-loss curve moving to a lower temperature rangeis possibly due to the decrease of the strength ofC–O ether bonds caused by the decrease of theelectron cloud density resulting from the interactionsof Li+ ions with ether oxygen atoms. Since LiTFSIliberates more free ions than LiBF4 and LiClO4 atthe same concentration, the strength of the etherbond is weakened more by LiTFSI. Consequently,it loses weight mostly in the lowest temperaturerange. This result is also consistent with that ofthe ionic conductivity measurements and the FTIRmeasurements discussed above.

Nanocomposite SPEsIt has been shown6,19,20 that adding inorganicnanoparticles can greatly improve the ionic conductiv-ity of polyether-based SPEs. In this work, nanosizedsilicon oxide particles were added to TPEU/PVDFmixtures to fabricate composite polymer electrolytesusing ultrasonic treatment. Figure 7 shows the ionicconductivity as a function of nanoparticles added.For both LiClO4 and LiTFSI, this value increasedby 40–70% at a molar ratio of O:Li = 20. As anexample, the addition of nanoparticles increased theionic conductivity of the sample doped with LiClO4 to

2.8 2.9 3.0 3.1 3.2 3.3-4.4

-4.2

-4.0

-3.8

-3.6

-3.4

-3.2

-3.0

-2.8

-2.6

log

σ (S

cm

-1)

103 T-1 (K-1)

0% SiO2-LiClO4

10% SiO2-LiClO4

0% SiO2-LiTFSI10% SiO2-LiTFSI

Figure 7. Ionic conductivity versus temperature for different nanosizesilicon oxide particles in hybrid TPEU/PVDF polymer electrolytes(O:Li = 20).

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7.4 × 10−5 S cm−1 at 30 ◦C and to 9.3 × 10−4 S cm−1

at 80 ◦C, while the control values were only 5.4 × 10−5

and 5.4 × 10−4 S cm−1, respectively. Meanwhile, amaximum ionic conductivity of 3.5 × 10−4 S cm−1 at30 ◦C and 2.2 × 10−3 S cm−1 at 80 ◦C was obtainedwhen LiTFSI was used with these nanoparticles.Actually, the relative enhancement in the ionic con-ductivity after adding nanosize SiO2 was not as evidentas expected. As is well known, the traditional poly-mer matrix used in SPEs is almost crystalline orsemicrystalline, and adding nanosize SiO2 can sub-stantially improve the ionic conductivity by effectivelyminimizing the polymer crystallization tendency.16,23

Because the TPEU-based electrolyte studied hereis completely amorphous, this may offset partiallythe effect of the nanoparticles on the ionic con-ductivity. The positive influence of the nanoparti-cle adding on amorphous polymer may involve theimprovement of the soft segment motion perfor-mance. The added particles can compete with Li+ion to interact with ether oxygen atom of the softsegment leading to the decrease of Tg of the softsegment.

CONCLUSIONSA novel hybrid SPE has been prepared by dopinga mixture of TPEU and PVDF with lithiumsalts. The sample containing 20 wt% PVDF wasfound to form tough free-standing hybrid polymerelectrolyte films. The ionic conductivity reached2.1 × 10−4 S cm−1 at 30 ◦C and 1.7 × 10−3 S cm−1

at 80 ◦C, when LiTFSI was used to dope thepolymer electrolyte at a ratio of O:Li = 20. Theaddition of nanosize SiO2 particles substantiallybenefited the ionic conductivity of the CPEs. Asa result, the value of the corresponding ionicconductivity further increased to 3.5 × 10−4 S cm−1

at 30 ◦C and 2.2 × 10−3 S cm−1 at 80 ◦C. Hence,this composite polymer system is highly promisingas a candidate for SPEs in rechargeable lithium ionbatteries.

ACKNOWLEDGMENTSThis research was funded by the National Key Projectson Basic Research and Development (973 Program,Grant Number 2002CB211800).

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