Composite Alkaline Polymer Electrolytes and Its Application To

Electrochimica Acta 51 (2006) 24542462

Composite alkaline polymer electrolytenickelmetal hydride ba

ZhaoUniver

2 May 2er 2005

Abstract

In order t H baspowders, i.e pectiveelectrolytes polymroom tempe od, anstability window of ca. 1.6 V was determined by cyclic voltammetry with stainless steel blocking electrodes. The influence of the filmcomposition such as KOH, H2O and nano-additives on ion conductivity was investigated and explained. The temperature dependence ofconductivity was also determined. In addition, polyvinyl alcohol (PVA)-sodium carboxymethyl cellulose (CMC)-KOH alkaline polymerelectrolytes were obtained using solvent casting method. The properties of the polymer electrolytes were characterized by ac impedance,cyclic voltammetry and differential thermal analysis methods. The ionic conductivity of the prepared PVA-CMC-KOH-H2O electrolytes canreach the orddemonstrate 2005 Else

Keywords: A

1. Introdu

Solid popolymers, abeen intensWright et(PEO)/saltsIn 1979, APEO/saltsat 4060 Cion batterieattracted gron Li-ion pmer electro

CorresponE-mail ad

0013-4686/$doi:10.1016/jer of 102 S cm1. The effect of CMC addition on the alkaline polymer electrolytes was also explained. The experimental resultsd that the PVA-CMC-KOH-H2O polymer electrolyte could be used in Ni/MH battery.vier Ltd. All rights reserved.

lkaline polymer electrolyte; Ionic conductivity; PEO; PVA; Nano-additives

ction

lymer electrolytes, also called ionic conductivere of a new type of solid electrolytes, which haveively studied and developed since 1970s. In 1973,al. [1,2] firstly reported that polyethylene oxide

complexes exhibited ionic conductive property.rmand et al. [3] claimed that the conductivity ofcomplexes could reach the order of 105 S cm1, and there would be a potential application in Li-s. Thereafter, R&D on polymer electrolytes haseat attention in the world and most of it focusedolymer batteries. However, the studies on poly-lytes for alkaline batteries such as Ni/MH, Ni/Cd

ding author. Tel.: +86 21 66134851; fax: +86 21 66133292.dress: [email protected] (A. Yuan).

and Ni/Zn, etc., are comparatively fewer. Solid polymer elec-trolytes can be widely used in different applications, such asall-solid batteries and supercapacitors.

Alkaline polymer electrolyte, in contrast to conventionalalkaline aqueous electrolyte (i.e., KOH), for use in Ni/MHbatteries, could provide solutions to the problems of highinternal pressure upon charging, alkaline leakage, oxida-tion of negative electrode (hydrogen storage alloy) uponcharge/discharge cycling and high self-discharge rate. Inaddition, soft packaging (using aluminum/plastic laminatedfilms) could be introduced to replace metallic packaging toimprove safety and specific energy of the batteries. Fauvar-que et al. [4] firstly investigated the PEO based alkalinepolymer electrolyte and its application to Ni/Cd secondarybatteries. Concerning the published literature, the alkalinepolymer electrolytes could be divided mainly in three types,i.e., PEO based [47], polyvinyl alcohol (PVA) based [815]and hybrid PEOPVA based [16,17] electrolytes. Because of

see front matter 2005 Elsevier Ltd. All rights reserved..electacta.2005.07.027Anbao Yuan , JunDepartment of Chemistry, College of Sciences, Shanghai

Received 15 February 2005; received in revised form 1Available online 1 Septemb

o enhance the ionic conductivity of polyethylene oxide (PEO)-KO., TiO2, -Al2O3 and SiO2 were added to PEO-KOH complex, reswere prepared. The experimental results showed that the preparedrature, typically 103 S cm1 as measured by ac impedance meths and its application totteries

sity, Shanghai 200444, China

005; accepted 15 July 2005

ed alkaline polymer electrolytes, three types of nano-ly, and the corresponding composite alkaline polymerer electrolytes exhibited higher ionic conductivities atd good electrochemical stability. The electrochemical

A. Yuan, J. Zhao / Electrochimica Acta 51 (2006) 24542462 2455

the crystallinity in PEO, the conductivity of PEO-KOH alka-line polymer electrolytes is usually low in view of practicalapplications. In order to increase the conductivity, modifica-tions by coHowever, asystem is nstudy, somwere addedto enhancecompositecharacterisac impedanysis and in

Comparthe condutrolytes isPVA-KOH102 S cmeasy to debecomes diless, additio(CMC) to Pwater retenstudy, the Pwere prepaties were inand differeNi/MH batelectrolytecation of th

2. Experim

2.1. Prepapolymer ele

PEO (Mnano--Al2tively, in aconcentratimagneticalfor some tisolution watemperaturthickness o

2.2. Prepaelectrolyte

PVA andistilled waconcentratistirred to aand no bubinto a glas

the alkaline polymer electrolyte films with a thickness of ca.0.5 mm were prepared.

Deter

st alk2) wass, andrequenemicare, l, Re filmine poresistapedan

Deter

st alkstainlege reg

CHI6ycliclectrocrolyte

Differroscop

ifferenere c

ifferenof temng frocontroopic (

an Aromet

Prepaor po

eparaH)2 wlene b(PTFEobtain 2 cmed toepara

5-typl powinyl a

d aga-polymerization or blend are usually employed.ddition of inorganic nano-particles to PEO-KOHot available in literatures up to now. In the presente nano-powders, i.e., TiO2, -Al2O3 and SiO2

to PEO-KOH complexes, respectively, in orderthe ionic conductivity, and the corresponding

alkaline polymer electrolytes were prepared. Thetics of the polymer electrolytes were studied usingce, cyclic voltammetry, differential thermal anal-

frared spectroscopic analysis methods.ed with PEO based alkaline polymer electrolytes,ctivity of PVA based alkaline polymer elec-usually higher. For example, the conductivity of-H2O polymer electrolyte can reach the order of

1. However, the electrolyte film is found to be

hydrate deeply, which causes it to crimple andstorted, and the flexibility becomes bad. Neverthe-n of hydrophilic sodium carboxymethyl celluloseVA could improve the mechanical property and

tion property of the electrolyte film. Hence, in thisVA-CMC based alkaline polymer electrolyte filmsred by blend of PVA with CMC, and its proper-vestigated by ac impedance, cyclic voltammetry

ntial thermal analysis methods. Finally, a polymertery with PVA-CMC-KOH-H2O alkaline polymerwas tested in order to evaluate the practical appli-e polymer electrolyte.

ental

ration of PEO based nano-composite alkalinectrolyte lms

w 100,000) was mixed with nano-TiO2 (10 nm),O3 (1020 nm) and nano-SiO2 (50 nm), respec-mortar. Then the mixture was added to a givenon of KOH aqueous solution in a beaker andly stirred to a homogeneous solution. After restedme and no bubble could be observed, the aboves poured into a glass plate to evaporate at room

e. Thus, the alkaline polymer electrolyte films withf 0.10.3 mm were prepared.

ration of PVA based alkaline polymerlms

d CMC (with a given mass ratio) were added toter in a beaker and heated to dissolve. Then a givenon of KOH solution was added and magneticallyhomogeneous solution. After rested for some timeble could be observed, the solution was poured

s plate to evaporate at room temperature. Thus,

2.3.

Te4 cmtrodethe ftroch(wheof thalkalbulkac imaxis.

2.4.

Tetwovoltausingthe cthe eelect

2.5.spect

Dples w1A drateheatiwas

troscusingspect

2.6.liquid

PrNi(OacetyleneThe2 cmpress

Prof ABnickepolyvmixemination of ionic conductivity

aline polymer electrolyte film (with an area ofplaced between two stainless steel blocking elec-then subjected to ac impedance measurement (incy range 105 to 101 Hz) using CHI660A Elec-l Workstation. According to the formula = l/RbAb and A are the thickness, bulk resistance and area, respectively), the ionic conductivity of the testlymer electrolyte film could be calculated. Thence was determined from the cross point of thece spectrum at high frequency area and the real

mination of electrochemical stability window

aline polymer electrolyte film was placed betweenss steel blocking electrodes, then cycled in theion of1.5 to +1.5 V with a scan rate of 10 mV s160A Electrochemical Workstation to determine

voltammetric behavior of the electrolyte. Thus,hemical stability window of the alkaline polymercould be determined.

ential thermal analysis (DTA) and infraredic (IR) analysis

tial thermal analysis (DTA) curves of the sam-arried out using microcomputer controlled WCT-tial thermal analysis unit (Beijing, China). Theperature rise was controlled at 10 C min1

m 25 to 250 C, and the mass of the sampleslled in the range of 1015 mg. Infrared spec-IR) analysis for the samples were performedvatar 360 FT-IR Fourier transformation infrareder.

ration and testing of Ni/MH battery withlymer electrolyte

tion of nickel electrode: a given amount ofas mixed with 5 wt.% cobalt powder and 2 wt.%lack, then a given amount of polytetrafluoroethy-) emulsion was added as binder and mixed again.ed paste was filled into a foamed nickel (with

dimensions), and then dried at 65 C and rolla thickness of ca. 0.6 mm.tion of metal hydride electrode: a given amounte hydrogen storage alloy was mixed with 7 wt.%der and 2 wt.% graphite, then a given amount oflcohol (PVA) solution was added as binder and

in. The obtained paste was filled into a foamed

2456 A. Yuan, J. Zhao / Electrochimica Acta 51 (2006) 24542462

nickel (with 2 cm 2 cm dimensions), then vacuum dried at65 C and roll pressed to a thickness of 0.35 mm.

The liquid Ni/MH battery was consisted of a positivenickel electrode and a negative hydride electrode, and itscapacity being governed by the positive electrode. The bat-tery was cycled three times with C/10 charge/discharge ratein 6 M KOH solution for activation. After that, the nickelelectrode and the hydride electrode were taken out from thesolution for dryness, and then a Ni/MH battery with poly-mer electrolyte was assembled. The battery was consistedof a piece of polymer electrolyte film (with a compositionof m(PVA):m(CMC):m(KOH):m(H2O) = 13.8:3.5:34.7:48.0and a thickness of 0.5 mm) sandwiched by an activated nickelelectrode and an activated hydride electrode. The above-assembled battery was enwrapped with a plastic cloth androll pressed again in order to get the electrode/electrolyteinterface contact well.

The charge/discharge performance of the battery wastested usingical impedCHI660A E

3. Results

3.1. Relatisystem and

The conpolymer elFig. 1. It ca(mass ratiincreases gof KOH cthe film inBesides, thnot linearm(KOH):mm(KOH)/mm(KOH):m

Fig. 1. Condm(KOH):m(P

Fig. 2. Condu

parativelyof KOH isductivity is

es aal fac

m(Kanica

er valu.4.

Relatim and

e coner elebe s

ases wal infls thanrkablyr contmes lent ised to

Inueuctivit

orderpolymer electrolytes at room temperature, three types

organic nano-powders, i.e., TiO2, -Al2O3 and SiO2added to PEO-KOH complex, respectively. The influ-of nano-additives amount on film conductivity was

ed and the tested data are listed in Table 1. KnownTable 1 that the ionic conductivity increases from

S cm1 order of magnitude for the pure PEO-KOHm to 106 to 105 S cm1 for the nano-composite

er electrolytes, i.e., increased 12 orders. It can beseen from Table 1 that when m(PEO):m(TiO2) orO):m(-Al2O3) equals 92.5:7.5, or m(PEO):m(SiO2)

ls 87.5:12.5, the conductivities exhibit the maximum val-LAND auto-cycler (China), and the electrochem-ance spectroscopy (EIS) was carried out usinglectrochemical Workstation.

and discussion

onship between conductivity of PEO-KOHKOH content

ductivity variation of PEO-KOH based alkalineectrolyte film with KOH content is presented inn be seen from Fig. 1 that with m(KOH):m(PEO)o) increasing, the conductivity of the systemradually. This is because that with increasing

ontent, the concentration of conductive ions increases, and hence the conductivity increases.e variation of log() with m(KOH):m(PEO) isbut presents different slops. When the value of(PEO) is small, the variation of conductivity with(PEO) is drastic. Nevertheless, when the value of(PEO) exceeds 0.4, the variation becomes com-

uctivity of polymer electrolyte with different ratios ofEO).

reachcruciwhenmechpropca. 0

3.2.syste

ThpolymIt canincrecruciis lesrema

watebecoconteinclin

3.3.cond

Inbasedof inwere

ence

studifrom107systepolymalsom(PEequactivity of polymer electrolyte with different wt.% of water.

mild. This indicates that when the concentrationlower, the influence of KOH content on con-considerable, and while when the concentration

given degree, the KOH content becomes not ator. Moreover, we found from experiment thatOH):m(PEO) exceeds 0.5, the homogeneity andl property of the film become bad. Hence, thee for m(KOH):m(PEO) should be controlled at

onship between conductivity of PEO-KOHwater content

ductivity variation of PEO-KOH based alkalinectrolyte film with water content is shown in Fig. 2.

een from Fig. 2 that the conductivity of the filmith water content increasing. Water content has auence on film conductivity. When water content8 wt.%, the logarithm of conductivity increaseswith water content increasing. However, when

ent is larger than 10 wt.%, the slop of the curvess steep relatively. This indicates that when waterlarger, the influence of it on film conductivity isweaken.

nce of inorganic nano-additives on they of PEO-KOH system

to enhance the ionic conductivity of PEO-KOH


Table 1Conductivity of polymer electrolytes with different ratios ofm(PEO):m(nano-powder) (water content: 3 wt.%, m(PEO):m(KOH) = 10:4)m(PEO):m(nano-powder)(in mass ratio)

Conductivity, (106 S cm1)TiO2(25 C)

-Al2O3(20 C)

SiO2(20 C)

100:0 0.50 0.40 0.4097.5:2.5 1.70 0.53 0.5595:5 5.67 1.10 0.5992.5:7.5 33.0 5.64 0.6590:10 27.8 4.01 1.1087.5:12.5 18.9 2.81 1.9085:15 6.03 0.66 1.3082.5:17.5 2.05 0.61 0.8580:20 1.91 0.59 0.83

ues. However, if the electrical and mechanical properties areconsideredbe of 87.5:

The DTand PEO-KFig. 3(ac)(about 62 (about 57 PEO meltinperatures aof the meltKOH-SiO2cially for tthat the adddecrease ththe conducand c) (arotion.

The infrpolymer erespectiveltures preseC O stretc1344 cm1ing at 2887found fromin Fig. 4(b)

Fig. 4. IR spectra of PEO and PEO-KOH-SiO2-H2O polymer electrolyte.(a) PEO and (b) PEO-KOH-SiO2-H2O polymer electrolyte.

67 cmion istion,m1 ai in thhing i(a) to

ests th-KOH

thatole ofonds tystallie DTA

Tempe-KOH

g. 5 sr the

2O3 anally w60 ) 1/

enius e

Fig. 3. DTAKOH-SiO2-Hsimultaneously, the most appropriate ratio should12.5.A curves of pure PEO powders, PEO-KOH-H2OOH-SiO2-H2O polymer electrolytes are shown in, respectively. The endothermic peak in Fig. 3(a)C) and the first endothermic peak in Fig. 3(b)C) or Fig. 3(c) (about 49 C) are corresponding tog. Compared with the pure PEO, the melting tem-

nd the peak areas (relating to the enthalpy changesing processes) for the PEO-KOH-H2O and PEO--H2O polymer electrolytes are decreased, espe-he PEO-KOH-SiO2-H2O system. This suggestsition of nano-SiO2 to PEO-KOH-H2O system cane crystallinity in PEO [18,19], and hence increasestivity. The second endothermic peak in Fig. 3(bund 80 C) should be ascribed to water evapora-

ared spectra of PEO and PEO-KOH-SiO2-H2Olectrolyte are presented in Fig. 4(a and b),y. The most important of the absorption fea-nted both in Fig. 4(a and b) appears to be [18]hing at 1640 cm1, CH2 asymmetric bending at, C O C stretching at 1115 cm1, CH2 stretch-cm1 and OH stretching at 3449 cm1. It can beFig. 4 that the absorption features presented only

, i.e., the peaks at 1007 cm1, 703 cm1, 663 cm1

and 4positvibra467 cand SstretcFig. 4suggPEOposedthe rgen bof crabov

3.4.PEO

Fiity fo-Algraduof 20log(Arrhcurves of pure PEO and polymer electrolytes. (a) pure PEO; (b) PEO-KOH-H2O2O (m(PEO):m(KOH) = 10:4, m(PEO):m(SiO2) = 87.5:12.5, water content: 10 wt.%1 are absent in Fig. 4(a). Where the 1007 cm1likely to be ascribed to Si O C or Si O Si

while the positions at 703 cm1, 663 cm1 andre not clear, but possibly are associated with K

e composite system. Besides, the frequency of OHs decreased from the acute peak at 3449 cm1 inthe obtuse peak at 3438 cm1 in Fig. 4(b), which

at the hydrogen bonds in PEO are weakened in the-SiO2-H2O polymer electrolyte. It could be sup-the addition of nano-SiO2 to PEO complex playsisolation, which hinders the formation of hydro-o a certain degree, and hence decreases the degreenity in PEO. This result is in agreement with theresult.

rature dependence of conductivity forbased nano-composite polymer electrolytes

hows the temperature dependence of conductiv-polymer electrolytes with addition of nano-TiO2,d SiO2, respectively. The conductivity increasesith temperature rise in the temperature range

C, and an approximately linear relationship forT could be obtained, which is consistent with thequation.(m(PEO):m(KOH) = 10:4, water content: 10 wt.%); (c) PEO-).


Fig. 5. Conductivity of polymer electrolytes with temperature variation.() m(PEO):m(TiO2):m(KOH):m(H2O) = 57.4:8.1:23.0:11.5; () m(PEO):m(Al2O3):m(KOH):m(H2O) = 51.9:7.3:28.5:12.3; () m(PEO):m(SiO2):m(KOH):m(H2O) = 52.2:7.5:27.8:12.5.

3.5. Electrnano-comp

For pracies, not onlstability wgate the elepolymer eldetail in Sein Fig. 6. Itis nearly zthat the eleis about 1.6

3.6. Differand PVA-C

The DTCMC-KOHand b), resfor pure PVcorrespondendothermH2O polymperature rato water evto the melthe pure PV

Fig. 7. DTA curves of pure PVA and PVA-CMC-KOH-H2O polymerelectrolyte. (a) Pure PVA and (b) m(PVA):m(CMC):m(KOH):m(H2O) =13.8:3.5:34.7:48.0 (in mass ratio).

. CondH):m(P

allinitvity im

RelatiCMC-KOH-H2O system and KOH content

e relationship between conductivity of PVA-CMC-polymer electrolyte film and KOH content is

nted in Fig. 8. The conductivity increases withH):m(PVA + CMC) ratio increasing. When the value

(KOH):m(PVA + CMC) changes from 0 to 0.5, the con-vity increases sharply from 107 to 102 S cm1 orderagnitude. However, when m(KOH):m(PVA + CMC)

nd 0.5, The variation of conductivity becomescomparatively and the variation of log() with

Fig. 6. Cycli23.0:11.5; (b)ochemical stability of alkalineosite polymer electrolytes

tical application of polymer electrolytes in batter-y high conductivity but also wide electrochemicalindow should be required. In order to investi-ctrochemical stability of alkaline nano-compositeectrolytes, cyclic voltammetry method (describedction 2.4) was employed. The results are showncan be seen from Fig. 6 that the faradaic current

ero at ca. 0.8 to +0.8 V region. This indicatesctrochemical stability window of the electrolytesV.

ential thermal analysis (DTA) of pure PVAMC-KOH-H2O polymer electrolyte

A curves of pure PVA (dried powder) and PVA--H2O polymer electrolyte are shown in Fig. 7(a

pectively. There is an endothermic peak (226 C)A in the temperature range of 25250 C, whiching to the melting of PVA. Whereas, there are twoic peaks (99 and 195 C) for PVA-CMC-KOH-er electrolyte can be observed in the same tem-

nge. The former peak (99 C) could be attributedaporation and the later (195 C) corresponding

ting of the polymer electrolyte. Compared with

Fig. 8m(KO

crystducti

3.7.PVA-

ThKOHpresem(KOof mductiof mbeyomildA, the decline of melting point suggests that the m(KOH):m

c voltammograph for polymer electrolytes at room temperature (scan rate: 10m(PEO):m(Al2O3):m(KOH):m(H2O) = 51.9:7.3:28.5:12.3; (c) m(PEO):m(SiO2):muctivity of polymer electrolytes with different ratios ofVA + CMC).

y in PVA is decreased, which is favourable to con-provement of the polymer electrolyte.

onship between conductivity of(PVA + CMC) presents nearly linear relationship.

mV s1). (a) m(PEO):m(TiO2):m(KOH):m(H2O) = 57.4:8.1:(KOH):m(H2O) = 52.2:7.5:27.8:12.5.


Fig. 9. Condu

3.8. RelatiPVA-CMC

Fig. 9 sPVA-CMCtent. The cwater continfluence o30 wt.%, thslowly withtent exceedwater conteas water copossibly asphase cantrolyte. Whcannot betent reacheincreases wconductivit

3.9. InuePVA-CMC

Accordim(CMC):mtively, therPVA. Therelectrolytestively, in thmance ofwas investity and CMratio of m(tion (i.e., thmass ratioKOH-H2Othat the coof 103 S cwith CMC

0. ConC):m(P

ncreaaximufilm sexcee

ut, anof m

film cis tolve mously,ncreait hig

mongC):m

vity. Wvity ofexperPVA-sphereever, texibilderab

Storarolytectivity of polymer electrolytes with different wt.% of water.

onship between conductivity of-KOH-H2O system and water content

hows the relationship between conductivity of-KOH-H2O polymer electrolyte and water con-onductivity of the film increases gradually withent increasing, and water content has a crucialn conductivity. When water content is withine conductivity of the film is lower and increaseswater content increasing. While, when water con-

s 30 wt.%, the conductivity increases rapidly withnt increasing, and can reaches 8 102 S cm1ntent increases to 50 wt.%. This can be explained

follows. Conductive phase and nonconductivecoexist in PVA-CMC-KOH-H2O polymer elec-en water content is lower, the conductive phase

formed perfectly, and whereas, when water con-s a certain extent, the fraction of conductive phaseith water content increasing, and hence the ionicy of the film increases rapidly.

nce of CMC on performance of-KOH-H2O polymer electrolyte lm

ng to the literature [20], when the mass ratio of(PVA) being 20:80, 50:50 and 80:20, respec-

e would be a good compatibility of CMC with

Fig. 1m(CM

say, ithe mH2Oratiolize ovalueH2OThatdissoprevitent iexhib

Am(CMductiductifromtion (atmoHowand flconsi

3.10.electefore, three types of PVA-CMC based polymerwith the above three ratios were prepared, respec-is study, and the influence of CMC on perfor-

PVA-CMC-KOH-H2O polymer electrolyte filmigated. The correlation between film conductiv-C content is shown in Fig. 10, wherein the massKOH):m(PVA) for the film without CMC addi-e PVA-KOH-H2O film) being 1.5:1, whereas theof m(KOH):m(PVA + CMC) for the PVA-CMC-films being 2:1. It can be seen from Fig. 10

nductivity of the film without CMC addition ism1 order, whereas the conductivity of the filmsaddition are all of 102 S cm1 order. That is to

In ordePVA-CMCelectrolytem(KOH):ment periodsshows the105 to 1024 h, 7 dayFig. 11 thatsomewhat.method de(24 h, 7 day5.2 102ductivity of polymer electrolytes with different ratios ofVA + CMC).

sed one order of magnitude. This is because thatm value of m(KOH):m(PVA) for the PVA-KOH-

hould be controlled within 1.5. Otherwise, if theds this value, the KOH would prefer to crystal-d hence make the film brittle [10]. Whereas, the(KOH):m(PVA+CMC) for the PVA-CMC-KOH-an be increased to 2 without KOH crystallization.say, the electrolyte film with CMC addition canore KOH in it. According to the results describedthe film conductivity increases with KOH con-

sing, and hence the PVA-CMC-KOH-H2O filmsher ionic conductivity.the three polymer electrolytes, the one with(PVA + CMC) = 0.2 exhibits the maximum con-hen CMC content further increasing, the con-the film decreases slightly. In addition, we found

iment that the electrolyte film without CMC addi-KOH-H2O film) is easy to lose water deeply in the

of air, and hence to crinkle and become brittle.he water retention property, mechanical strengthity of the film with CMC addition can be improvedly.

ge stability of PVA-CMC-KOH-H2O polymerr to investigate the storage stability of the-KOH-H2O polymer electrolyte, an as-preparedfilm with a composition of m(PVA):m(CMC):(H2O) = 13.8:3.5:34.7:48.0 was stored for differ-and examined by ac impedance method. Fig. 11ac impedance spectra (in the frequency range

1 Hz) of the polymer electrolyte being stored fors and 2 months, respectively. It can be seen frombeing stored for 2 months, the spectrum changedBut the calculated conductivity (according to thescribed in section 2.3) for the three situationss and 2 months) are 6.2 102, 5.7 102 andS cm1, respectively. This suggests that the varia-


Fig. 11. The ac impedance spectra of the polymer electrolyte being storedfor different periods (measured at 25 C).

Fig. 12. Cycl

tion of the cIn other wo

3.11. Elecpolymer el

Electroctrolyte witm(H2O) = 1voltammetcan be seen

rent response in the voltage region of ca. 0.8 V to +0.8 V.So, the electrochemical stability window of the polymer elec-trolyte is about 1.6 V, which could meet the requirement ofpractical application in Ni/MH battery.

3.12. Performance of Ni/MH battery withPVA-CMC-KOH-H2O polymer electrolyte

The as-prepared Ni/MH battery was cycled three timeswith C/10 charge/discharge rate in 6 M KOH solution foractivation, then was subjected to ac impedance test. Afterthat, the nickel electrode and the hydride electrode weretaken out from the solution for dryness, and subsequentlya polymer Ni/MH battery with PVA-CMC-KOH-H2O poly-mer electrolyte instead of 6 M KOH solution was assembledand subjected to ac impedance test. The Nyquist plots for theliquid and polymer Ni/MH batteries are presented in Fig. 13,

ctiveldischfrequ

ew ofFig. 1quid aFig.

danceittle baluesipliedrespeies atin theup vifromthe lifromimpeis a lthe vmultic voltammograph of polymer electrolyte at room temperature.

onductivity with storage time is not considerable.rds, the film presents good storage stability.

trochemical stability of PVA-CMC-KOH-H2Oectrolyte

hemical stability of the alkaline polymer elec-h a composition of m(PVA):m(CMC):m(KOH):3.8:3.5:34.7:48.0 was evaluated by cyclic

ry method. The result is presented in Fig. 12. Itfrom Fig. 12 that there is almost no faradaic cur-

trode). Thithe polymeelectrolyte.

Fig. 14 sof the polyvoltage incplateau voltively, corrdischarge vand the disrespectivel

In orderNi/MH batcharged at

Fig. 13. Nyquist plots for the liquid and polymer Ni/MH bay. The ac impedance test carried out on the batter-arged state (0% SOC) and room temperature, andency range 105 to 103 Hz. Fig. 13(b) is the closeFig. 13(a) at high frequency area. It can be seen3(a) that there is no distinct difference betweennd the polymer Ni/MH batteries. But we can see

13(b) that the ohmic resistance (the real part ofat high frequency) of the polymer Ni/MH batteryig than that of the liquid Ni/MH battery (whereof Z and Z are the product of tested resistanceby the mass of nickel hydroxide in positive elec-s result reflects the fact that the conductivity ofr electrolyte is a little lower than that of the liquid

hows the typical charge/discharge characteristicsmer Ni/MH battery at different rates. The chargereases with charge rate increasing, and the chargetages are about 1.4 V, 1.45 V and 1.55 V, respec-esponding to C/10, C/5 and C/2 rates. While theoltage decreases with discharge rate increasing,

charge plateau voltages are about 1.2, 1.1 and 1 V,y, corresponding to C/10, C/5 and C/2 rates.

to evaluate the dischargeability of the polymertery, the battery was charged at C/10 rate and dis-C/10, C/5, C/2 and 1C rates, respectively. The

tteries at discharged state.


Fig. 14. Charge/discharge curves of the polymer Ni/MH battery at variousdischarge rates.

Fig. 15. Disch

results arethe dischardischarge r1.25, 1.2, 1C/5, C/2 athat whenbehavior isbattery. Whcharge voltwith conve

The chashown in Fthe chargecan be seenage decreais not remaresult indicdischarged

6. Discharge specific capacity of the polymer Ni/MH battery chargedous rate

7. Cycle life of the polymer Ni/MH battery at C/5 charge/dischargearge specific capacity of the polymer Ni/MH at different rates.

shown in Fig. 15, from which we can see thatge voltage and specific capacity decrease withate increasing. The discharge plateaus are about.1 and 1 V, respectively, corresponding to C/10,

nd 1C discharge rates. This result demonstrates

Fig. 1at vari

Fig. 1rate.the discharge rate is not too high, the dischargecomparable with commonly used liquid Ni/MHile when the discharge rate exceeds C/2, the dis-age and specific capacity are too low to comparentional liquid Ni/MH battery.rge-rate dependence of discharge capacity areig. 16, wherein the discharge rate is C/10 andrates are C/10, C/5, C/2 and 1C, respectively. Itfrom Fig. 16 that the discharge capacity and volt-

se with charge rate increasing, but the influencerkable compared with that of discharge rate. Thisates that the battery can be charged but cannot beat higher rates.

Fig. 17 sat C/5 chargity is 171 mincreased toa charge/diof the polymthis is favoever, fromto declineloss in polythe polymebut not beewith the ac

Fig. 18. The ac impedance spectra of the polymer Ni/MH battery cycleds.hows the cycle life of the polymer Ni/MH batterye/discharge rate. The first-cycle discharge capac-Ah g1, and the second-cycle discharge capacity188 mAh g1. This is because that going through

scharge cycle, the electrode/electrolyte interfaceer Ni/MH battery becomes more compatible and

rable to the subsequent electrode reaction. How-the fourth cycle, the discharge capacity beginsgradually. This is mainly because of the watermer electrolyte upon cycling. In our experiment,r battery was only enwrapped with a plastic clothn sealed strictly. This can be further illuminatedimpedance evolution displayed in Fig. 18 (in the

at C/5 rate for two and seven cycles.


frequency range 105 to 103 Hz), where Fig. 18(b) is theclose up view of Fig. 18(a) at high frequency region. It canbe seen from Fig. 18(a) that the impedance spectra are allcomposed of a semicircle and a straight line. The semicir-cle at the higher frequency region should be attributed tothe charge transfer resistance of electrode/electrolyte inter-face, and the straight line at the lower frequency regioncorresponds to the Warburg diffusion impedance. Under-gone seven charge/discharge cycles, the charge transfer resis-tance increased obviously. Additionally, it can be seen fromFig. 18(b) that the ohmic resistance of the battery increasedalso, which reflects the increase of bulk resistance of the poly-mer electrolyte. Water loss upon cycling not only induces thebulk resistance increase for the polymer electrolyte but alsoinduces the charge transfer resistance increase for the elec-trode/electrolyte interface.

4. Conclu

In this selectrolytesconductivitand H2O cPEO-KOHbased polyinfrared spadded to Pcrystallinitof the systcan make tand henceaddition capropertiestemperaturpolymer elmer electromagnitude,window ofexperiment

CMC-KOH-H2O alkaline polymer electrolyte demonstratedthat the polymer electrolyte could be applied to Ni/MH bat-teries, especially under conditions of lower discharge rates.

Acknowledgement

The authors gratefully acknowledge the financial supportof the Shanghai Educational Development Foundation (GrantNo. 02AK50) given by the Shanghai Educational Committeeof China.

References

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y of the polymer electrolytes increase with KOHontent increasing. Addition of nano-additives tosystem can increase the conductivity of PEO

mer electrolyte. Differential thermal analysis andectral analysis results revealed that nano-SiO2EO based polymer electrolyte could decrease they in PEO and hence increase the conductivityem. While, CMC addition to PVA-KOH systemhe polymer complex to accommodate more KOHto increase the ionic conductivity. Besides, CMCn improve the water-retention and mechanicalof PVA based polymer electrolyte. The roome conductivity of the PEO based nano-compositeectrolytes and the PVA-CMC-KOH-H2O poly-lytes can reach 103 and 102 S cm1 orders ofrespectively, and the electrochemical stability

the alkaline polymer electrolytes is ca. 1.6 V. Theal results of the polymer Ni/MH battery with PVA-

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Composite alkaline polymer electrolytes and its application to nickel-metal hydride batteriesIntroductionExperimentalPreparation of PEO based nano-composite alkaline polymer electrolyte filmsPreparation of PVA based alkaline polymer electrolyte filmsDetermination of ionic conductivityDetermination of electrochemical stability windowDifferential thermal analysis (DTA) and infrared spectroscopic (IR) analysisPreparation and testing of Ni/MH battery with liquid or polymer electrolyte

Results and discussionRelationship between conductivity of PEO-KOH system and KOH contentRelationship between conductivity of PEO-KOH system and water contentInfluence of inorganic nano-additives on the conductivity of PEO-KOH systemTemperature dependence of conductivity for PEO-KOH based nano-composite polymer electrolytesElectrochemical stability of alkaline nano-composite polymer electrolytesDifferential thermal analysis (DTA) of pure PVA and PVA-CMC-KOH-H2O polymer electrolyteRelationship between conductivity of PVA-CMC-KOH-H2O system and KOH contentRelationship between conductivity of PVA-CMC-KOH-H2O system and water contentInfluence of CMC on performance of PVA-CMC-KOH-H2O polymer electrolyte filmStorage stability of PVA-CMC-KOH-H2O polymer electrolyteElectrochemical stability of PVA-CMC-KOH-H2O polymer electrolytePerformance of Ni/MH battery with PVA-CMC-KOH-H2O polymer electrolyte

ConclusionsAcknowledgementReferences

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Composite Alkaline Polymer Electrolytes and Its Application To