Electrochimica Acta 50 (2004) 301–304

Lithium ion conductivity of blend polymer electrolytesbased on borate polymers containing fluoroalkane

dicarboxylate and poly(ethylene oxide)

Takahiro Aoki∗, Akinori Konno, Tatsuo Fujinami

Department of Materials Science, Faculty of Engineering, Shizuoka University,3-5-1, Johoku, Hamamatsu 432-8561, Japan

Received 2 June 2003; received in revised form 22 November 2003; accepted 29 December 2003Available online 1 August 2004

Abstract

Single ion conducting blend polymer electrolytes were prepared from borate complex polymers containing fluoroalkane dicarboxylate andt blendingw ntent. Theo©

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1

slectOccapra

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pre-in (

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ytesO) insi-

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tlyhown

ed in

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wo methoxy [oligo(ethylene oxide)] side chains and poly(ethylene oxide) (PEO). Mechanical properties were greatly improved byith PEO. Ionic conductivity was dependent on the chain length of oligoether groups, and decreased with increasing PEO coptimized blend polymer B (n = 7.2)-PEO (80:20 wt.%) exhibited ionic conductivity of 2× 10−6 S cm−1 at 30◦C.2004 Elsevier Ltd. All rights reserved.

eywords:Polymer electrolyte; Single ion conductor; Poly(ethylene oxide); Ionic conductivity

. Introduction

Dry polymer electrolytes have been attracting interest asafer alternatives to liquid electrolytes and suitable use inithium secondary batteries[1–5]. The majority of polymerlectrolytes was bi-ion conductor, which exhibited high ioniconductivity of 10−4 S cm−1 order at room temperature butheir lithium ion transference numbers were usually low.n the other hand, single ion conductor has advantages forharge–discharge performance[6], but exhibited low ioniconductivity due to strong ion pairing between lithium ionnd anion fixed on polymer backbone. Inorganic–organicolymer electrolytes containing ate complex structure wereeported to have good performance for single ion conductionnd high ionic conductivity[7–15].

We recently reported that polymer electrolytes which con-ain fluoroalkane dicarboxylate substituted aluminate or bo-ate backbone and two methoxy [oligo(ethylene oxide)] side

∗ Corresponding author. Tel.: +81 53 478 1162; fax: +81 53 478 1162.E-mail address:r5345010@ipc.shizuoka.ac.jp (T. Aoki).

chains directly bonded to the ate complex centers werepared and borate polymers containing long ether chan= 11.8) exhibited high ionic conductivity (10−5 S cm−1 at30◦C), single ion conduction (t+ = 0.95) and high electrochemical stability[16].

In this paper, we wish report the blend polymer electrolbased on borate polymer and poly(ethylene oxide) (PEorder to obtain strong film. In addition, optimized compotion of blend polymer electrolyte for ionic conductivity amechanical property was investigated.

2. Experimental

2.1. Materials

Borate polymers B (n = 3, 7.2, 11.8) containing fluoroalkane dicarboxylate substituted borate backbonetwo methoxy [oligo(ethylene oxide)] side chains direcbonded to the ate complex centers were prepared as sin Scheme 1. Detail synthesis process has been describanother report[16].

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2003.12.067

302 T. Aoki et al. / Electrochimica Acta 50 (2004) 301–304

Scheme 1. Synthesis of borate polymers.

Triethyleneglycol monomethylether (Tokyo Kasei) wasdried by distillation under reduced pressure. Poly(ethyleneglycol) monomethylether with molecular weight 350 and 550(Aldrich) were dried by dry nitrogen bubbling under reducedpressure. PEO (Mw 5 × 106, Aldrich) was dried at 50◦C for24 h under vacuum. Lithium borohydride (LiBH4, 2.0 M so-lution in THF, Aldrich) and hexafluoroglutaric acid (Aldrich)were used as supplied. Acetonitrile was dried on calcium hy-dride and distilled before use. Unless otherwise stated, allmaterials were handled on a dry nitrogen line or in an argonglove box in order to rigorously exclude moisture.

2.2. Preparation of polymer electrolytes based on boratepolymer and PEO

Borate polymer and PEO were dissolved in acetonitrileby magnetic stirring to form homogeneous solution. The sol-vent was removed under reduced pressure at 70◦C for 24 hand blend polymer electrolytes based on borate polymer andPEO were obtained. The composition of the blend polymerelectrolytes is summarized inTable 1.

2.3. Measurements

Ionic conductivity was determined by ac impedancemeasurement in the frequency 1 MHz–0.1 Hz (signal

TP olyme

S Bo e

B 10B erBBB mB mB m

B 10B erBBB mB m

B 10B erBB

amplitude 10 mV) using a Solartron 1260 frequencyresponse analyzer and 1287 electrochemical interface.Polymer electrolyte films were obtained by hot press-ing between PTFE disks using a PTFE spacer to controlfilm thickness (0.4 mm). Cells were consisted of poly-mer electrolyte films sandwiched between stainless steelelectrodes.

3. Results and discussion

3.1. Characterization

Single ion conducting polymers containing ate com-plex structure were obtained by incorporating methoxy[oligo(ethylene oxide)] groups and fluoroalkane dicarboxy-late groups. The results of theoretical calculations showedthat partial negative charge was dispersed on oxygen atomsaround ate complex center and lithium ion formed ionpairing with the oxygen atoms as shown inFig. 1. Thepartial charge on oxygen atoms in borate polymers issmaller than that in aluminate polymers. Therefore weakerion pairing between lithium ion and oxygen atoms isestimated for borate polymers and it results in the en-hancement of the ionic conductivity[16]. Borate poly-mers B (n = 3, 7.2 and 11.8) were waxy materials. Fora ter-i x-h rtieso ms d onb im-p lytefi ob-t ong

able 1hysical appearance of blend polymer electrolytes based on borate p

ample Chain length ofoligoether chain (n)

(n = 3) 3(n = 3)-PEO (90:10 wt.%)(n = 3)-PEO (80:20 wt.%)(n = 3)-PEO (70:30 wt.%)(n = 3)-PEO (60:40 wt.%)(n = 3)-PEO (50:50 wt.%)(n = 3)-PEO (40:60 wt.%)

(n = 7.2) 7.2 7.2(n = 7.2)-PEO (90:10 wt.%)(n = 7.2)-PEO (80:20 wt.%)(n = 7.2)-PEO (70:30 wt.%)(n = 7.2)-PEO (60:40 wt.%)(n = 7.2)-PEO (50:50 wt.%)

(n = 11.8) 11.8 11.8(n = 11.8)-PEO (90:10 wt.%)(n = 11.8)-PEO (80:20 wt.%)(n = 11.8)-PEO (70:30 wt.%)a EO: CH2CH2O.

rs and poly(ethylene oxide)

rate polymer:PEO (wt.%) EO:Li+a Appearanc

0:0 6:1 Soft wax90:10 8:1 Soft rubb80:20 9:1 Weak film70:30 12:1 Weak film60:40 15:1 Strong fil50:50 19:1 Strong fil40:60 26:1 Strong fil

0:0 14:1 Sticky wax90:10 17:1 Soft rubb80:20 20:1 Weak film70:30 24:1 Weak film60:40 29:1 Strong fil50:50 36:1 Strong fil

0:0 24:1 Sticky wax90:10 27:1 Soft rubb80:20 31:1 Weak film70:30 37:1 Weak film

pplication in all solid state lithium secondary bates, it is important for polymer electrolyte films to eibit good mechanical properties. Mechanical propef borate polymers were insufficient for use in lithiuecondary batteries. Blend polymer electrolytes baseorate polymers and PEO were prepared in order torove mechanical properties. Blend polymer electrolms were obtained by hot pressing. Weak films wereained by incorporation of 20, 30 wt.% PEO and str

T. Aoki et al. / Electrochimica Acta 50 (2004) 301–304 303

Fig. 1. Ion pairing between lithium ion and oxygen atoms in borate structure.

films were obtained by incorporation of 40, 50, 60 wt.%PEO. However self standing films were not obtained whenblend polymer electrolytes contained less than 20 wt.%PEO.

F((

3.2. Ionic conductivity

Relationships between PEO content and ionic conductiv-ity for blend polymer electrolytes B (n = 3, 7.2 and 11.8)-PEO are shown inFig. 2a–c respectively. For all systems,ionic conductivity decreased with increasing PEO contentand it can be ascribed to the crystallization of PEO. Thesame tendency was commonly observed in PEO–Li salt sys-tems and PEO–ate complex polymer electrolytes systems[13]. The optimized composition polymer electrolytes forionic conductivity and mechanical property were B (n = 3)-PEO (80:20 wt.%), B (n = 7.2)-PEO (80:20 wt.%) and B (n= 11.8)-PEO (80:20 wt.%) which exhibited ionic conductiv-ities of 4× 10−7, 2 × 10−6 and 5× 10−7 S cm−1 at 30◦C,respectively. The maximum conductivity is often observedin the ration of EO:Li+ = 16 ∼ 20:1 for poly(ether) basedelectrolytes[17,18]. The EO:Li+ values of B (n = 3)-PEO(80:20 wt.%), B (n = 7.2)-PEO (80:20 wt.%) and B (n =11.8)-PEO (80:20 wt.%) are 9:1, 20:1 and 31:1 as shown inTable 1. For B (n = 3)-PEO (80:20 wt.%), low ionic conduc-tivity was observed due to increased pseudo cross-linking ofether chains with high content of lithium ion. For B (n =11.8)-PEO (80:20 wt.%), ionic conductivity decreased withlow lithium ion concentration. Therefore, B (n = 7.2)-PEO(80:20 wt.%) with the optimized EO:Li+ values exhibited them

ig. 2. Relationships between PEO content and ionic conductivity for (a) Bn = 3)-PEO, (b) B (n = 7.2)-PEO, (c) B (n = 11.8)-PEO systems at 80◦C�), 60◦C (�), 40◦C (+), 30◦C (�).

Fi(

aximum ionic conductivity.

ig. 3. Relationships between the chain length of oligoether groups andonic conductivity for (a) borate polymers B and (b) blend polymers B-PEO80:20 wt.%) at 80◦C (�), 60◦C (�), 40◦C (+), 30◦C (�).

304 T. Aoki et al. / Electrochimica Acta 50 (2004) 301–304

Relationships between the chain length of oligoethergroups and ionic conductivity are shown inFig. 3a for bo-rate polymers andFig. 3b for blend polymer electrolytes with20 wt.% PEO. For borate polymers, an enhancement in theionic conductivity was observed by incorporation of longerether chain fromn = 3 to 11.8 and B (n = 11.8) exhibitedhigh ionic conductivity of 1× 10−5 S cm−1 at 30◦C and8 × 10−5 S cm−1 at 80◦C. This can be ascribed to opti-mized EO:Li+ values for B (n = 11.8). For blend polymerelectrolytes with 20 wt.% PEO, optimized ionic conductiv-ity was observed for B (n = 7.2)-PEO (80:20 wt.%) as de-scribed previously and exhibited 2× 10−6 S cm−1 at 30◦Cand 8× 10−5 S cm−1 at 80◦C. Optimized length of the etherside chains in borate polymers was shifted to shorter in PEOblended borate polymers. Therefore maximum conductivitywas observed EO:Li+ ration was 20∼ 24:1 for blend polymerelectrolytes.

4. Conclusion

Single ion conducting blend polymer electrolytes based onborate polymers containing ate complex structure and PEOwas prepared. Mechanical properties were greatly improvedby blending with PEO. The single ionic conductivity of theoptimized blend polymer B (n = 7.2)-PEO (80:20 wt.%) ex-h8 ctedb

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