Regulation of the discharge reservoir of negative electrodes in Ni–MH batteries by using Ni(OH)x (x = 2.10) and y-CoOOH

(1)

Publisher’s version / Version de l'éditeur:

Journal of Power Sources, 196, 18, pp. 7791-7796, 2011-05-19

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

NRC Publications Archive

Archives des publications du CNRC

This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.1016/j.jpowsour.2011.05.012

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Regulation of the discharge reservoir of negative electrodes in Ni–MH

batteries by using Ni(OH)x (x = 2.10) and y-CoOOH

Shangguan, Enbo; Chang, Zhaorong; Tang, Hongwei; Yuan, Xiao-Zi; Wang,

Haijiang

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=5612c497-1c84-4f34-af6f-8cc7eb94cc67

https://publications-cnrc.canada.ca/fra/voir/objet/?id=5612c497-1c84-4f34-af6f-8cc7eb94cc67

(2)

ContentslistsavailableatScienceDirect

Journal

of

Power

Sources

j ou rn a l h o m e pa g e :w w w . e l s e v i e r . c o m / l o c a t e / j p o w s o u r

Regulation

of

the

discharge

reservoir

of

negative

electrodes

in

Ni–MH

batteries

by

using

Ni(OH)

x

(x

=

2.10)

and

␥-CoOOH

Enbo

Shangguan

a

_,

_Zhaorong

_Chang

a,∗

_,

_Hongwei

_Tang

a

_,

_Xiao-Zi

_Yuan

b

_,

_Haijiang

_Wang

b

a_College_of_Chemistry_and_{Environmental}_Science,_Henan_Normal_University,_Xinxiang_453007,_PR_China b_Institute_for_Fuel_Cell_Innovation,_National_Research_Council_of_Canada,_Vancouver,_BC,_Canada_V6T_1W5

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received27September2010 Receivedinrevisedform12April2011 Accepted10May2011

Available online 19 May 2011

Keywords:

Nickelhydridebattery Thedischargereservoir Gamma-cobaltoxy-hydroxide Cyclestability

a

b

s

t

r

a

c

t

Inthispaper,anovelstrategytoregulatethedischargereservoirofnegativeelectrodesinNi–MH batter-iesisintroducedbyusingNi(OH)x(x=2.10)and␥-CoOOH.Theelectrochemicalmeasurementsofthese

batteriesdemonstratethattheuseofNi(OH)x(x=2.10)and␥-CoOOHcannotonlysuccessfullyregulate

thedischargereservoirofnegativeelectrodesinNi–MHbatteriestoanadequatequantity,butalso effec-tivelyimprovetheelectrochemicalperformanceofthebatteries.Comparedwithnormalbatteries,the in-housepreparedbatterieswithalowerdischargereservoirexhibitanenhanceddischargecapacity, improvedhigh-ratedischargeability,higherdischargepotentialplateauandsuperiorcyclestability.The effectofNi(OH)x(x=2.10)and␥-CoOOHontheelectrochemicalperformanceofnickelelectrodeisalso

investigatedbycyclicvoltammetry(CV)andelectrochemicalimpedancespectroscopy(EIS).Theresults suggestthatthenewmethodissimpleandeffectiveforcostreductionofNi–MHbatterieswithimproved electrochemicalperformance.

1. Introduction

Nickel–metalhydride(Ni–MH)batterieshavebeenintensively studied and widely used in today’s power tools and portable applications because of their high specific energy power and specific energy density, fast charge and discharge capabilities, environment-friendlycharacteristics,longcyclicstabilityandgood security[1–3].AlthoughNi–MHbatteriesarecommercially avail-able, further research is still required to improve their power performanceforapplicationsinelectricvehiclesandhydride vehi-cles[4].ItisalsonecessarytoreducethecostofNi–MHbatteriesin ordertofurtherexpandtheirapplicationfields,suchasa replace-mentfornickel–cadmium(Ni–Cd)batteriesinpowertools,because thelatterarecheaperbuthavecadmiumpollution.

ItiswellknownthatinthesealedNi–MHbatteries,the capac-ities of thebatteries are usually positive-electrodelimited, the capacityoftheMHnegativeelectrodesisusuallargerthanthat oftheNi(OH)2 positive electrodesdue toproperrecombination

reactionsand for safety reasons. Generally,during the produc-tionprocessofNi–MHbatteries,thecapacityratioofpositiveand negativeelectrodesisusuallyadjustedat1:(1.35–1.5)toensure thesealingstateandcycliclifetime.Itisnoticedthatthe exces-sivenegativecapacity has two primaryfunctions: toprovide a

∗ Correspondingauthor.Tel.:+863733326335;fax:+863733326336.

E-mailaddress:czr 56@163.com(Z.Chang).

chargereservoir(Rch)forpreventinghighpressuregenerated

dur-ingchargeandovercharge,andtoprovideadischargereservoir (Rdis)forprotectingthenegativeelectrodefromoxidationduring

forcedover-discharge.

AsillustratedinFig.1,theresidualnon-chargedcapacityofthe negativeelectrodeunderfullychargedconditionofthepositive electrodeisreferredtoasthechargereservoir,andtheresidual chargedcapacityofthenegativeelectrodeunderfullydischarged conditionofthepositiveelectrodeisreferredtoasthedischarge reservoir.

Generally,thedischargereservoirisobtainedintwoways: (1)Cobaltanditscompounds(CoOandCo(OH)2etc.)havebeen

widelyadoptedasanadditiontothenickelhydroxideelectrode in many commercialNi–MH batteries [5–8]. Intrinsically, these divalentcobaltoxideshavenoconductivities,buttheycanbe con-vertedintoconductive␤-cobaltoxyhydroxidebyelectrochemical oxidationin theprocessofinitialcharging,forminganeffective conductivenetwork.

ForthepositiveelectrodesmixedwithCoO,Oshitanietal.[8]

reportedthatCoOcontainedintheelectrodedissolvedinalkaline electrolytetoformblueCo(II)complexionandthenprecipitated onnickelhydroxideparticlesas␤-Co(OH)2.Afterinitialcharging,

such␤-Co(OH)2 convertedto␤-CoOOH.Thereactionsthatoccur

onthepositiveandnegativeelectrodesareasfollows:

(+)electrode Co(OH)2+OH−→CoOOH+H2O+e (1) Ni(OH)2+OH−_⇄NiOOH+H₂O+e (2)

(3)

7792 E.Shangguanetal./JournalofPowerSources196 (2011) 7791–7796

Fig.1. Theschematicdiagramofthecapacitydistributionbetweenthepositive Ni(OH)2electrodeandnegativeMHelectrodeofaNi–MHbattery.

(−)electrode M+H₂O+e_⇄MH+OH− (3) whereMisthehydrogen-absorbingalloy.AsCoOOHisconsidered tobeastablecompoundthatcannotbereducedduringusual dis-chargecycles, metalhydridegeneratedduringtheformation of CoOOHisconservedandwillnotbeused[8].Theelectricalquantity storedinthenegativeelectrodeduringthisprocessbecomesapart ofthedischargereservoir(namedasRdis-1).InthecasethatCoOis

usedatarateofaboutonetenthoftheweightofnickelhydroxide, theRdis-1isaboutonetenthofthecapacityofthepositiveelectrode.

(2) Reaction (2) is not a complete reversible reaction. The valence(oxidationnumber)of nickelinnickel hydroxideis ini-tially2butrisestoapproximately3.2bychargingofthebattery,so thatnickelhydroxideischangedtoanickeloxyhydroxide.A phe-nomenonoccursinvolvingthesuspensionofdischargereactionat avalenceofapproximately2.2whennickeloxyhydroxideis con-vertedbacktonickelhydroxideduringdischarge.Accordingly,the negativeelectrodehasalwayselectricityleftthereininanamount correspondingtoavalenceof0.2.Thisremainingelectricitymakes nocontribution to battery capacity. The non-discharged nickel oxyhydroxidethusremainstogiveanotherpartofthedischarge reservoir(namedasRdis-2)ofabouttwotenthsofthecapacityof

thepositiveelectrode.

Accordingly,theNi–MHbatteryhasthetotaldischargereservoir ofaboutthreetenthsofthecapacityofthepositiveelectrode.The adequatequantityofthedischargereservoirisatmostaboutone tenthofthecapacityofthepositiveelectrode.Namelythecapacity correspondingtoabouttwotenthsofthecapacityofthepositive electrodeisexcessiveinthenegativeelectrode.Inotherwords, thepriorartbatteryincludesaspecificquantityofthehydrogen storagealloythatdoesnotcontributetocharginganddischarging. Foracommercialreason,itisofgreattechnologicalimportanceto regulatethedischargereservoirandreducethesurpluscapacityof theMHelectrodewiththemaintenanceofthebatteryproperties.

Recently,Li etal. [9]have reporteda methodtoreducethe consumptionofAB5alloyinaNi–MHbatterybyusingcobalt

oxyhy-droxidecoatednickelhydroxide.However,theRdis-1ofonlyabout

8%ofthecapacityofthepositiveelectrodewasremovedfromthe metalhydrideelectrodesbythismethodandtheRdis-2obtained

fromtheincompletereversible reaction(2) wasnot takeninto account.Hence,thegoalofthisworkistoseekanewmethodto regulatenotonlytheRdis-1butalsotheRdis-2toanoptimal

quan-tityandtodecreasetheconsumptionofalloyinMHelectrodewith improvementofthebatteryproperties.

Onthebasisofourpreviouswork[10–12],anewwayto reg-ulate thedischarge reservoir of negative electrodes for Ni–MH batteriesis introducedbyusingNi(OH)x(x=2.1)and ␥-CoOOH.

Theresults showthat thedischarge reservoir of negative elec-trodesforNi–MHbatterieshasbeensuccessfullyregulatedtoan optimumquantity.Theelectrochemicalpropertiesofthebatteries witha lowerdischargereservoirwereexaminedsystematically. TheeffectofNi(OH)x(x=2.10)and␥-CoOOHonthe

electrochem-icalperformanceofnickelelectrodewasalsoinvestigatedbyCV andEIS.

2. Experimental

2.1. SynthesisofNi(OH)x(x=2.10)and -CoOOH

SynthesisoftheNi(OH)x(x=2.10)wascarriedoutbyanew

elec-trolysismethodasdescribedpreviously[11].Ti/SnO2+Sb2O3/PbO2

electrodesand stainless steelwereusedas anodeand cathode, respectively,andtheionexchangefilmwasusedasthe separat-ingmembrane.Theelectrolytewascomposedof400mLof2MKCl (pH=10).40gsphericalNi(OH)2 powderwasaddedtothe

elec-trolyteunder stirringatambienttemperature.Afterelectrolysis foraperiodoftime,theblackNi(OH)x(x=2.10)suspension

parti-cleswereformed.Thentheobtainedprecipitatewasfiltered,rinsed withdeionizationwateranddriedina80◦_C_vacuum_environment

forover6htoproducethefinalproduct.Thevalenceofnickelin nickelhydroxidewasdeterminedbyiodometryand ethylenedi-aminetetraaceticacid(EDTA)complexometrictitration[11].The spherical␤-Ni(OH)2 usedinthis workis acommercialproduct

(China,HenanKelongCo.Ltd.).

The ␥-CoOOHwith a high valence of 3.40 wasprepared as describedinRef.[12].

2.2. PreparationofelectrodesandNi–MHbatteries

Thepastenickelelectrodeswerepreparedasfollows:a mix-turecontaining92wt.%Ni(OH)2and8wt.%CoO(thetotalweight

is6.4gforelectrodeA)oramixturecontaining92wt.%Ni(OH)x

(x=2.10)and8wt.%␥-CoOOH(thetotalweightis6.1gforelectrode B)weremixedthoroughlyandamillingprocedurewasneededto ensuretheuniformityofthemixture.Followedbyanadditionof aproperamountofbinders(PTFEandCMC)anddistilledwater,a homogenouspastewithadequaterheologicalpropertieswasmade. Theslurrywaspouredintoafoamnickelsheetanddriedat80◦_C

inair.Afterwards,thepositiveelectrodeswererolledtoa thick-nessof0.6mm.ThecapacityoftheNi(OH)2electrodeswasabout

1500mAh.

AcommercialAB5-typehydrogenstoragealloywithastandard

composition MmNi3.4Co0.7Mn0.4Al0.3 wasused for thenegative Table1

Threegroupsofbatterieswithdifferentpositiveandnegativeelectrodes.

Battery Composition Positiveelectrode(mAh) Negativeelectrode(mAh) Thecapacityratioof

positiveandnegative electrodes

A 92wt.%Ni(OH)2+8wt.%CoO 1500 2100 1.4

B 92wt.%Ni(OH)2.1+8wt.%␥-CoOOH 1500 2100 1.4

(4)

electrode material. A slurry containing 94% alloy powders, 3% nickelpowderand3%PTFEbinderwaspastedintothefoamnickel substrates,andthendriedandcompressedtoobtaintheMH elec-trodes.ThecapacityoftheMHelectrodeswasabout2100mAhand 1800mAh.

A solution containing 6M KOH, 50gL−1 _NaOH _and ₁₅_g_L−1 LiOH and fluorinated polyolefinporiferous membrane (FS2216, Freudenberg,Germany)wereselectedaselectrolyteandseparator, respectively.Afterbeingsealed,theAA-typeNi–MHrechargeable batterieswithacapacityof1500mAhwereassembled.Asseenin Table1,threegroupsofbatterieswereassembledinthiswork. 2.3. TestofsealedNi–MHrechargeablebatteriesandnickel electrodes

Charge/dischargemeasurementswereconductedusingaLand CT2001Abatteryperformancetestinginstrument(China,Wuhan JinnuoElectronicsCo. Ltd).Foractivation,five chargedischarge cyclesat0.1Cwereperformed,andthebatteriesweredischarged to1.0V.Thebatterieswerethenchargedata0.2Cratefor7hand separatelydischargedatrespective1,2,and5Cdischargecurrent rates.Thecut-offvoltagesweresetas1.0,0.9,0.8V,respectively.In thesubsequentcharge–dischargecyclingtests,thebatterieswere chargedata1Cratefor1.2h,restedfor10min,andthendischarged atrespective1and5Cdischargecurrentrates.Thecut-offvoltages weresetas1.0and0.8V,respectively.Theinternalresistanceofthe batteriesat100%depthofdischarge(DOD)wasmeasuredunder alternativecurrent(AC)of1kHzbyusinganinternalresistance testinginstrument.

Rdistestsofthebatterieswereperformedasfollows:the

bat-terywaschargedata0.1Cratefor14h,restedfor20min,andthen dischargedata0.2Cratetoacut-offvoltageof1.0V.Afterwards, thesealedbatterywascutasanopencell,andimmersedintothe electrolyteinabeakerfollowedbya20minrest.Theworking elec-trodewasdischargedat0.2Ctothecut-offpotentialof−0.6V(vs. Hg/HgO)at25◦_C_and_the_R

diswasthenobtained.

Electrochemicaltestsof nickelelectrodesA andBwere per-formedinathree-compartmentelectrochemicalcellatambient temperature.Twonickelribboncounterelectrodeswereplacedin thesidechambersandtheworkingelectrodewaspositionedinthe centerchamber.AHg/HgOreferenceelectrodewasusedviaa Lug-gincapillary,whichwasmadeusingthesamealkalinesolutionfor theworkingcell.CVandEISwereconductedonaSolartronSI1260 impedanceanalyzerwitha1287potentiostatinterface.TheCVtest scanratewas2mVs−1_and_the_cell_potential_ranged_from_0.0_V_to

0.8V.ForEIS,theimpedancespectrawererecordedata5mV per-turbationamplitudewithasweepfrequency rangeof10kHzto 1mHz.

3. Resultsanddiscussion

Onthebasisoftheabovediscussionaboutthedischarge reser-voirofnegativeelectrodesforNi–MHbatteries,theregulationof thedischargereservoircanbecarriedoutintwoaspects.Onthe onehand,Rdis-1iscompletelyremovedfromthemetalhydride

elec-trodesbytheuseof␥-CoOOH,similartothatofNi(OH)2coatedwith

CoOOH[9].ForthebatteriesmixedwithCoO,theRdis-1equalsthe

electricquantityconsumedduringtheformationofCoOOH.Thus, theRdis-1canbecalculatedfromtheformulaas:

Rdis-1=

(mCoO×26.8×1×1000) 74.9

wheremCoOisthequantityofCoOinthepositiveelectrode.Onthe

otherhand,Rdis-2isreducedtoanadequatequantityofaboutone

tenthofthecapacityofthepositiveelectrodebyusingpre-oxidized Ni(OH)2.ThevalenceofnickelinNi(OH)2waspre-oxidizedfrom2

Fig.2.DischargecurvesofbatteriesA(a),B(b),andC(c)atdifferentdischargerates.

to2.1.Theresultofregulationofthedischargereservoirofnegative electrodesforNi–MHbatteriesA,B,andCispresentedinTable2.

Obviously,itcanbeseenthatthebatteriesBandCshowalower dischargereservoirof159mAhand144mAh,whereasthevalues forthenormalbatteryAis419mAh,indicatingthatthedischarge reservoirofbatteriesBandCissuccessfullyreducedtoanadequate quantityofaboutonetenthofthecapacityofthepositiveelectrode bytheuseofNi(OH)x(x=2.10)and␥-CoOOH.Especially,batteryC

exhibitsaremarkablylowersurpluscapacityoftheMHelectrodes thanthatofbatteriesAandBandholdsanadequatequantityof

(5)

7794 E.Shangguanetal./JournalofPowerSources196 (2011) 7791–7796

Table2

TheresultofregulationofthedischargereservoirofnegativeelectrodesforNi–MHbatteriesA,B,andC.

Battery SurpluscapacityoftheMHelectrodes(mAh)

Totala _Charge_reservoirb_(R

ch) Dischargereservoir(Rdis)

Total Rdis-1 Rdis-2

A 598 179 419 183 236

B 590 431 159 0 159

C 294 150 144 0 144

a_Total_reservoir_(the_surplus_capacity_of_the_MH_electrode)₌_the_capacity_of_the_MH_electrode-_the_discharge_capacity_of_the_Ni(OH)

2electrode(0.2C). b _Charge_reservoir_(R

ch)=totalreservoir−dischargereservoir(Rdis).

chargereservoirasmuchasaboutonetenthofthecapacityofthe positiveelectrode.

Fig.2depictsthetypicaldischargecurvesofNi–MHbatteries A,BandCatdifferentdischargerates.Theexperimentalresults aresummarizedinTable3.Obviously,batteriesBandCexhibita largerdischargecapacityandhigherdischargepotentialplateau thanbatteryAatthesamedischargerate(0.2,1,2and5C).As thedischargerateincreasesfrom0.2to5C,thespecificdischarge capacityofbatteryCdecreasesfrom268.9to243.2mAhg−1_,

show-ingthatatsuchahighdischargerate90.4%capacityisretained.At a5Crate,batteriesBandCshowahigherdischargecapacityof 244.5mAhg−1_and_243.2_mAh_g−1_,_whereas_the_value_for_battery_A

is187.8mAhg−1_,_indicating_that_the_high-rate_performance_of

bat-teriesBandCismuchbetterthanthatofbatteryA.Table3also showsthatthespecificcapacityofNi(OH)2 oftheelectrodewith

CoOisobviouslylowerthantheelectrodewiththesamecontent

Fig.3.CyclicperformanceofbatteriesA,B,andCat1Cand5Crates.

of␥-CoOOH.TheutilityofNi(OH)2isgreatlyimprovedby

substi-tuting␥-CoOOHfornormalCoO,whichisinaccordancewithRef.

[12].Inaddition,itcanbeseenthatbatteriesBandCat100%DOD exhibitalowerinnerresistanceof11.9and11.7m,whereasthe valueforthenormalbatteryAis13.2m.Thiscanbeattributed tothepreferableconductivityoftheaddingcompoundsinthe pos-itiveelectrode.Itmayalsoresultfromadecreaseintheinternal resistanceoftheMHelectrodewiththereductionofRdis,sincethe

electricconductivityofthehydrogen-absorbingalloy(M)ismuch higherthanthatofmetalhydride(MH)[9].ComparedbatteryA withB,Itcanbeconcludedthattheelectrochemicalperformance improvementofthedischargecapacity,high-ratedischarge abil-ity,andhighdischargepotentialplateauisduetotheadditionof Ni(OH)x(x=2.10)and␥-CoOOH.

Fig.4. Variationofthemeandischargevoltagewiththecharge–dischargecycleat 1Cand5Crates.

(6)

Table3

PerformancesofNi–MHbatteriesA,B,andCatdifferentcurrentrates.

Battery Batterycapacity

(mAh)

Specificcapacity (mAhg−1₎

BatteryinternalResistance at100%DOD(m)

0.2C 1C 2C 5C 0.2C 1C 2C 5C

A 1502 1351 1251 1106 255.1 229.4 212.5 187.8 13.2

B 1518 1465 1453 1372 270.5 261.0 258.9 244.5 11.9

C 1509 1450 1442 1365 268.9 258.4 256.9 243.2 11.7

Fig.5.CVcurvesofelectrodesAandBatascanrateof2mVs−1_.

Fig.3illustratesthecyclicperformanceofNi–MHbatteriesA,B, andCat1Cand5Crates.Duringthesecycleprocesses,batteries BandCshowahigherspecificcapacityandbettercyclingstability thanbatteryA.ThecapacityfadingofbatteriesBandCarerestricted toaverylowlevelafterlong-termcycling,whilethecapacityof batteryAdiminishesquicklyalthoughitcontainsmorehydrogen storagealloy.Thisindicatesthattheperformancedeteriorationof theNi–MHbatteryduringcharge-discharge processesismainly caused by thepositive electrode which determinesthe battery capacity.Ingeneral,thecapacityfadingofthepositiveelectrode fornickelbasedbatteriesisassociatedwiththevolumeexpansion orswellingoftheNi(OH)2electrodebecauseoftheformationof

␥-NiOOH,whichinterfereswitheffectivecontactsbetweenactive materialparticlesanddamagestheconductivenetworkbetween

Fig.6. ElectrochemicalimpedancespectraofelectrodesAandB.

Ni(OH)2particlesandsubstrates[13,14].Toquantitatively

charac-terizethecyclicstabilityofthebatteries,thedeteriorationrate(Rd)

isused,whereRdisequalto(Cm−Ci)/Cm(Cm:maximumcapacity;

Ci:capacityatacertaincycle).AsseeninFig.3,theRdatthe500th

cycle(1C)forbatteriesA,B,andCare41.21%,25.94%and24.33% andtheRdatthe300thcycle(5C)are38.21%,21.56%and20.80%,

respectively, whichindicatesthat batteries BandC have much bettercyclicstabilityandhigherdischargecapacitythanbattery A.Especially,batteryCexhibitsexcellentelectrochemical perfor-mance,thoughthecapacity ofitsnegativeelectrode isreduced comparedwiththatofbatteriesAandB.Moreover,thehighpower performanceofNi–MHbatteriesnotonlydependsonthehigh-rate dischargecurrent,butalsodependsontheworkingvoltage.High meandischargevoltagemeanshigherdischargepotentialandmore energythatthebatteriescanstoreandrelease[4].

Fig.4comparesthemeandischargevoltagecurvesofbatteries A,B,andCat1Cand5Crates.AsshowninFig.4,theadditionof Ni(OH)x(x=2.10)and␥-CoOOHtothepositiveelectrodehasgreatly

enhancedthemeandischargevoltage,especiallyatahighdischarge rate.Consequently,theoverallelectrochemicalperformanceof bat-teriesBandCwithalowerdischargereservoirisobviouslysuperior tobatteryA.AlltheresultsillustratethatusingNi(OH)x(x=2.10)

and␥-CoOOHcannotonlyeffectivelyenhancethecyclingstability, butalsoremarkablyextendthelifespanoftheNi–MHbatteryata highdischargerate.

In order to further confirm the effect of Ni(OH)x (x=2.10)

and␥-CoOOHontheelectrochemicalperformanceofnickel elec-trodes,theelectrochemicalperformanceofelectrodeB(mixedwith 92wt.%Ni(OH)2.1+8wt.%␥-CoOOH)isfurtherinvestigatedin

com-parisonwithelectrodeA(mixedwith92wt.%Ni(OH)2+8wt.%CoO)

byCVandEIS.

ThecyclicvoltammogramsofelectrodesAandBafterfull acti-vationwithascan rateof 2mVs−1 _are_illustrated_in _Fig.₅_._The

featuresofthevoltammogramsaretabulatedinTable4.Obviously, theoxidationpotentialpeakofelectrodeBshiftstoamore neg-ativepositionat504mVandthereductionpotentialpeakmoves toamorepositivesiteat262mVincomparisonwiththatof elec-trodeA,correspondingtoahigherdischargepotentialplateauon thedischargecurves forelectrode Bshown inFig.2.Generally, theaverageofthecathodicandanodicpeakpotentials(Erev)can

betakenasanestimateofthereversiblepotentialfornickel elec-trodes,andthepotentialdifference(Ea,c)betweentheanodic(Ea)

andcathodic(Ec)peakpotentialsisameasureofthereversibility

oftheredoxreaction[15,16].Theexperimentaldatashowthatthe redoxreactionsarequasi-reversibleforbothelectrodes,as indi-catedbytherelativelylargeEa,c.However,theEa,cofelectrode

Bisonly242mV,whichissmallerthanthatofelectrodeA.This indicatesthatthechargeanddischargeprocessesofelectrodeBare

Table4

PotentialvaluesofCVfeaturesforelectrodesAandB.

Electrode Potential(mV)

Ea Ec Ea,c Erev

A 246 553 307 399

(7)

7796 E.Shangguanetal./JournalofPowerSources196 (2011) 7791–7796 morereversible,andthereforemoreactivematerialcanbeutilized

duringtheseprocesses,whichaccordswiththecharge–discharge resultsshowninFig.2.

Fig.6demonstratestheelectrochemicalimpedancespectrafor electrodesAandBatsteadystate.Theimpedancespectraofboth electrodesdisplaya depressed semicircleresulting fromcharge transferresistanceinthehigh-frequencyregion,andasloperelated toWarburgimpedanceinthelow-frequencyregion[17,18].The impedanceofelectrodeBismuchsmallerthanthatofelectrode A,whichimpliesthattheelectrochemicalreactiononelectrodeB proceedsmoreeasilythanonelectrodeA.Thisisinagreementwith theresultsoftheaboveCVtesting.

Onthebasisofthesefacts,it canbeconcludedthatNi(OH)x

(x=2.10)and␥-CoOOHisakeyfactorforimprovingthe electro-chemicalperformanceoftheNi–MHbattery,suchasthedischarge capacity,high-ratedischargeability,andcyclestability.Theresults alsoillustratethatusingNi(OH)x(x=2.10)and␥-CoOOHcannot

only regulate the dischargereservoir of the negativeelectrode toan adequatequantity, but alsoenhance thecycling stability andextendthelifespanoftheNi–MHbatteryatahighdischarge rate.Inotherwords,thenewmethodmakesitpossibleto pro-videa battery withexcellent electrochemicalproperties, which willnotbedeterioratedbythereductioninthedischarge reser-voir,andhelpsthebatteryholdanadequatequantityofcharge reservoir.

4. Conclusion

Thesurpluscapacity oftheMHelectrodes inNi–MH batter-ieswassuccessfully reducedbytheuseof Ni(OH)x(x=2.1)and

␥-CoOOH.Theelectrochemicalmeasurementsoftheprepared bat-teriesdemonstrate that Ni(OH)x (x=2.1) and ␥-CoOOHcan not

onlysuccessfullyregulatethedischargereservoirofthenegative electrodesinNi–MHbatteries toanadequatequantity,butalso effectivelyimprovetheelectrochemicalperformanceofthe bat-teries.The batteries with a lower dischargereservoir exhibit a

highdischargecapacity,enhancedhigh-ratedischargeabilityand improvedcycle stability. Therefore, it is believed that thenew methodis promising and effective for costreduction and elec-trochemical performanceimprovement of alkalinerechargeable Ni–MHbatteries.

Acknowledgements

ThisworkisfinanciallysupportedbytheNaturalScience Foun-dation of China under approval no. 21071046, and by Henan ProvincialDepartment ofScienceand TechnologyKey Research Projectunderapprovalno.080102270013.

References

[1] M.A.Fetcenko,S.R.Ovshinsky,B.Reichman,K.Young,C.Fierro,J.Koch,A.Zallen, W.Mays,T.Ouchi,J.PowerSources165(2007)544–551.

[2]S.Yasuoka,Y.Magari,T.Murata,T.Tanaka,J.Ishida,H.Nakamura,T.Nohma, M.Kihara,Y.Baba,H.Teraoka,J.PowerSources156(2006)662–666. [3]U.Köhler,C.Antonius,P.Bäuerlein,J.PowerSources127(2004)45–52. [4] J.B.Wu,J.P.Tu,T.A.Han,Y.Z.Yang,W.K.Zhang,X.B.Zhao,J.PowerSources156

(2006)667–672.

[5]Z.R.Chang,Y.J.Zhao,Y.C.Ding,J.PowerSources77(1999)69–73. [6]J.Wu,J.Tu,X.Wang,W.Zhang,J.AlloysCompd.431(2007)321–332. [7] P.Elumalai, H.N. Vasan,N. Munichandraiah,J. Power Sources 93(2001)

201–208.

[8]M.Oshitani,H.Yufu,K.Takashima,S.Tsuji,Y.Matsumaru,J.Electrochem.Soc. 136(1989)1590–1595.

[9] X.F.Li,T.C.Xia,J.Y.Li,J.AlloysCompd.477(2009)836–839.

[10]Z.R.Chang,E.B.Shangguan,D.Cheng,Y.J.Xu,Chin.J.Battery37(2007)367–369. [11]E.B.Shangguan,Z.R.Chang,H.W.Tang,X.Z.Yuan,H.J.Wang,Int.J.Hydrogen

Energy35(2010)3214–3220.

[12] Z.R.Chang,H.J.Li,H.W.Tang,X.Z.Yuan,H.J.Wang,Int.J.HydrogenEnergy34 (2009)2435–2439.

[13]D.Singh,J.Electrochem.Soc.145(1998)116–120.

[14]W.K.Hu,M.M.Geng,X.P.Gao,T.Burchardt,Z.X.Gong,J.PowerSources159 (2006)1478–1483.

[15] W.H.Zhu,J.J.Ke,H.M.Yu,D.J.Zhang,J.PowerSources56(1995)75–79. [16]B.Liu,H.T.Yuan,Y.S.Zhang,Z.X.Zhou,D.Y.Song,J.PowerSources79(1999)

277–280.

[17] V.Mancier,A.Métrot,P.Willmann,Electrochim.Acta41(1996)1259–1265. [18] B.Liu,H.T.Yuan,Y.S.Zhang,Int.J.HydrogenEnergy29(2004)453–458.