Comparative structural and electrochemical study of high density spherical and non-spherical Ni(OH)2 as cathode materials for Ni–metal hydride batteries

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Journal of Power Sources, 196, pp. 7797-7805, 2011-05-18

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Comparative structural and electrochemical study of high density

spherical and non-spherical Ni(OH)2 as cathode materials for Ni–metal

hydride batteries

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

Haijiang

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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

Comparative

structural

and

electrochemical

study

of

high

density

spherical

and

non-spherical

Ni(OH)

2

as

cathode

materials

for

Ni–metal

hydride

batteries

Enbo

Shangguan

_,

_Zhaorong

_Chang

_,

_Hongwei

_Tang

_,

_Xiao-Zi

_Yuan

_,

_Haijiang

_Wang

a_{CollegeofChemistryandEnvironmentalScience,HenanNormalUniversity,Xinxiang453007,PRChina} b_{InstituteforFuelCellInnovation,NationalResearchCouncilofCanada,Vancouver,BC,CanadaV6T1W5}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received13January2011

Receivedinrevisedform9April2011 Accepted10May2011

Available online 18 May 2011

Keywords:

Nickelhydroxide High-density

Nickel–metalhydridebatteries High-ratedischarge

Cathodematerials

a

b

s

t

r

a

c

t

Inthispaperwecomparethebehaviorofnon-sphericalandspherical␤-Ni(OH)2ascathodematerialsfor Ni–MHbatteriesinanattempttoexploretheeffectofmicrostructureandsurfacepropertiesof␤-Ni(OH)2 ontheirelectrochemicalperformances.Non-spherical␤-Ni(OH)2powderswithahigh-densityare syn-thesizedusingasimplepolyacrylamide(PAM)assistedtwo-stepdryingmethod.X-raydiffraction(XRD), infraredspectroscopy(IR),scanningelectronmicroscopy(SEM),thermogravimetric/differentialthermal analysis(TG–DTA),Brunauer–Emmett–Teller(BET)testing,laserparticlesizeanalysis,andtap-density testingareusedtocharacterizethephysicalpropertiesofthesynthesizedproducts.Electrochemical characterization,includingcyclicvoltammetry(CV),electrochemicalimpedancespectroscopy(EIS),and acharge/dischargetest,isalsoperformed.Theresultsshowthatthenon-spherical␤-Ni(OH)2 mate-rialsexhibitanirregulartabularshapeandadensesolidstructure,whichcontainsmanyoverlapped sheetnanocrystallinegrains,andhaveahighdensityofstructuraldisorderandalargespecificsurface area.Comparedwiththespherical␤-Ni(OH)2,thenon-spherical␤-Ni(OH)2materialshaveanenhanced dischargecapacity,higherdischargepotentialplateauandsuperiorcyclestability.Thisperformance improvementcanbeattributabletoahigherprotondiffusioncoefficient(4.26×10−9_cm2_s−1_),better reactionreversibility,andlowerelectrochemicalimpedanceofthesynthesizedmaterial.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

As a result of excellent electrochemical properties, nickel hydroxide(Ni(OH)2)hasbeenintensivelystudiedandwidelyused

asacathodematerialinallnickel-basedalkalinesecondarycells

[1–5].Generally,thereexisttwonickelhydroxidepolymorphs: ␣-phaseNi(OH)2 and␤-phaseNi(OH)2,which canbetransformed

into␥-phaseNiOOHand␤-phaseNiOOH,respectively,afterafull charging[5].Although␣-Ni(OH)2hasahighertheoretical

electro-chemicalcapacity,itisveryunstableinastrongalkalinemedium andeasilytransformsto␤-phaseafterafewcycles.Moreover, ␣-Ni(OH)2exhibitsalowertapdensity(<1.7gcm−3),whichgreatly

limitsitspracticaluse.Thehighertapdensitycanenhancethe addi-tionamountofpositivematerialandpromotethespecificenergy density ofNi–MH batteries further. Owingtoitshigh tap den-sity(2.1–2.2gcm−3_{)andexcellentelectrochemicalperformance,}

spherical␤-Ni(OH)2 hasbeenwidelyusedasacathodematerial

incommercialNi–MHbatteries [6,7]. However,thepreparation

∗ Correspondingauthorat:CollegeofChemistryandEnvironmentalScience, HenanNormalUniversity,Xinxiang453007,PRChina.Tel.:+863733326335; fax:+863733326336.

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

proceduresforsphericalparticlesarerathercomplexandthe pre-cipitatednickelhydroxiderequiresagingforalongtimetoobtain the desired product, which result in the high production cost. Besides,theeffectiveelectrodecapacityofthespherical␤-Ni(OH)2

samplestillhasroomtoimproveascomparedwiththetheoretical value(289mAhg−1_).

It is well established that the physical properties of nickel hydroxide,suchasitsmorphology,particle-sizedistribution, tap-density, and specific surface area, chemical composition, are strongly related to performance of the positive electrodes for its practical application in Ni–MH batteries [8–10]. Given the importanceof nickelhydroxide tothebattery industry, consid-erable effort has been devoted to improving the performance of thepositive electrodes,including: enhancing thestability of the␣-phase[11],cationicsubstitution[7],additives[12],surface modification[2,13]andnanosizedmaterials[5,14–16].Itisnoticed thatextensiveresearchesareconductedonthepreparationand electrochemicalperformanceofnano-Ni(OH)2incertain

applica-tions[14–20].Thedischargecapacityofnano-Ni(OH)2electrodes

preparedbyultrasonicchemistry,hardtemplatemethodandother commonmethodscanreachashighas250–270mAhg−1_.

How-ever,nano-Ni(OH)2canhardlymeettherequirementsofindustrial

massproduction[21].Especially,thenano-particlesalwaysexhibit alowertapdensity(<1.0gcm−3_{)[14],whichwillresultinalow}

0378-7753/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

7798 E.Shangguanetal./JournalofPowerSources196 (2011) 7797–7805

Table1

ThechemicalcompositionsandphysicalpropertiesofNi(OH)2samplesA(non-spherical)andB(spherical).

Sample Chemicalcomposition(wt.%) BETsurface

area(m2_g−1₎ Mean diameter(␮m) Tap-density (gcm−3₎ Ni Co Zn A(non-spherical) 57.68 1.54 3.13 75.62 27.20 2.22 B(spherical) 57.95 1.51 3.09 10.15 9.34 2.20

volumetricspecificcapacity,thusseriouslylimitingtheenergy den-sityofNi–MHbatteries.

Recently, a polyacrylamide (PAM) assisted two-step drying methodhasbeenreportedasasimplermethodtosynthesizehigh densitynon-sphericalNi(OH)2 powders[22].Theproceduresfor non-sphericalpowdersaremuchsimplerthanthoseforspherical powders,moreeconomicalwithnohydroxide-formingstep,and moreenvironmentallyfriendlywithnoammoniuminvolved.The Ni(OH)2powderspreparedbythenewmethodhasdemonstrated

notonlyahightapdensity,butalsoexcellentelectrochemical per-formance.Buttillnow,non-sphericalNi(OH)2 hasattractedlittle

attentionasrawmaterialforthepositiveelectrodeincommercial batteriesbecausenon-sphericalpowdersalwaysshowalower tap-density(1.5–1.8gcm−3_{)[8,23].Moststudiesaboutnon-spherical}

Ni(OH)2intheliteraturemainlyfocusedoneithertheproperties

ofNi(OH)2electrodesforrechargeableCd–NiandNi–MHbatteries

orthecrystalchemistryofNi(OH)2[24–27].Inthepreviouswork

[22],wehavestudiedtheeffectoftap-densityonthe electrochemi-calperformanceofnon-sphericalNi(OH)2electrodes.Nevertheless,

itisnecessarytocomparenon-sphericalNi(OH)2withcommercial

sphericalNi(OH)2 productandimprovetheperformanceof

non-sphericalNi(OH)2beforethematerialcanbeusedincommercial

cells.Hence,thegoalofthisworkistofurtherinvestigatethe struc-turalandelectrochemicalperformance ofnon-spherical,Coand ZndopedNi(OH)2preparedbythepolyacrylamide(PAM)assisted

two-stepdryingmethodincomparisonwithsphericalcommercial Ni(OH)2productwithafocusonexploringtheeffectof

microstruc-tureandsurfacepropertiesof␤-Ni(OH)2withahightapdensityon

theirelectrochemicalperformances.

On the basis of our previouswork [22], highdensity, non-spherical, Co and Zn doped ␤-Ni(OH)2 was prepared by the

polyacrylamide (PAM) assisted two-step drying method. This studyfocusesonthecomparisonofstructuralandelectrochemical performancebetweenas-preparednon-spherical␤-Ni(OH)2 and

commercialspherical␤-Ni(OH)2ascathodematerialsforNi–MH

batteries.Effectsofthemicrostructureandsurfacepropertiesof theprepared␤-Ni(OH)2 onitselectrochemicalperformance are

discussedindetail.

2. Experimental

2.1. SynthesisofNi(OH)2powders

Synthesisof non-sphericalNi(OH)2 powderswascarried out

bythepolyacrylamide(PAM)assistedtwo-stepdryingmethodas describedpreviously[22].StoichiometricamountsofNiSO4·6H2O,

CoSO4·7H2O,ZnSO4·7H2O(molar ratioofNi:Co:Znis 100:2.5:5)

were dissolved in distilled water to create a concentration of 1.7molL−1_{.Theaqueous solutionwasprecipitatedby adding a}

NaOHsolutionof4molL−1_{withcontinuousstirringat50}◦_C.The

co-precipitationmixtureswerestirredcontinuouslyafterthe reac-tionceased,andthencoagulatedbyaddingPAM(10mlPAM(0.6%) per200mlsolution),filtrated(underapressureof20MPa),dried (at120◦_{Cfor2h),ground,washed,anddried(at120}◦_Cfor2h)to

obtainthefinalproduct.

Thespherical␤-Ni(OH)2usedinthisworkisacommercial

prod-uct(HenanKelongCo.,Ltd.,China)co-precipitatedwith1.5wt.%Co and3wt.%Zn.

2.2. CharacterizationofNi(OH)2

ThecrystallinestructurecharacterizationofNi(OH)2powders

wasperformedontheX-rayDiffractometer(D8X)withCuK␣ asirradiation,utilizing aworkvoltageof 40kV,a workcurrent of40mA,a scanningspeedof0.026◦_s−1 _{anda steptimeof 3s.}

Thescanningrangewasbetween15◦ _and70◦_.Thesample

mor-phologieswereobservedusinganSEM(SEM-6701F,JEOL,Japan) withaworkvoltageof15.00kV,andtheFouriertransforminfrared (FTIR) spectra of prepared samples was recorded by a Fourier infraredspectrometer(FIRS; Bio-Rad FTS-40,US) usingthe KBr pellet method. Thermogravimetric/differential thermal analysis (TG/DTA)wascarriedoutusingaShimadzuDT-40thermal ana-lyzer (Japan). The measurement was performed in an air flow using ␣-Al2O3 asthe reference material. To determinethe tap

densityofthesample,aJZ-1tap-densitytester(ChenduPowder TestEquipmentCo.,Ltd.,China)wasused.Themeasurementerror waslessthan1%.Thespecificsurfaceareawasevaluatedbythe BET nitrogen adsorption methodusing a 3H-2000 surface area analyzer(China).Theparticlesizedistributionofthepowder sam-pleswasobtainedusinganOMECLS-POP(III)particlesizeanalyzer (China).

2.3. Preparationofnickelelectrodesandelectrochemical measurements

Porousfoamednickelwascutinto2cm_×2cmtouseas sub-strate material. The pasted nickel electrodes were prepared as follows: 80wt.% nickel hydroxide, 5wt.% CoO, 10wt.%Ni pow-derweremixedthoroughlywithacertainamountof5wt.%PTFE solutiontoobtainahomogeneousslurrypossessingadequate rhe-ologicalproperties.Theslurrywaspouredintoafoamnickelsheet and dried at 80◦_{C for 5h. Subsequently, thepasted electrodes}

werepressedat20MPafor3mintoassuregoodelectricalcontact betweenthefoamnickelandtheactivematerial.

Electrochemicaltestswereperformedinathree-compartment electrolysiscellatambienttemperature.Twonickelribboncounter electrodeswereplacedinthesidechambersandtheworking elec-trodewaspositionedinthecenter.Thegeometricalsurfaceareaof thenickelelectrodewas0.25cm2_{.Theelectrolytewasasolutionof}

6MKOH+15gL−1 _{LiOH.AHg/HgOreferenceelectrodewasused}

viaaluggincapillarywiththesamealkalinesolutionusedinthe workingcell.CVandEISwereconductedonaSolartronSI1260 impedanceanalyzerwitha1287potentiostatinterface.TheCVtest scanratewasbetween1mVs−1_and8mVs−1_andthecell

poten-tialrangedfrom0.0Vto0.8V.ForEIS,theimpedancespectrawere recordedata5mVperturbationamplitudewithasweepfrequency rangeof10kHz–1mHz.

Testcells wereassembled usingthepreparednickel hydrox-ideelectrodeasthecathode,ahydrogenstoragealloyelectrode as the anode, and polypropylene to separate the cathode and anode.Theelectrolytewasasolutionof6MKOH+15gL−1_LiOH.

Fig.1.SEMimagesofnon-sphericalNi(OH)2(a,c,e)andsphericalNi(OH)2(b,d,f)atdifferentmagnifications.

Charge/discharge measurements were conducted using a Land CT2001Abatteryperformancetestinginstrument(WuhanJinnuo ElectronicsCo.,Ltd.,China).Foractivation,fivechargedischarge cyclesat0.1Cwereperformed,andthebatteriesweredischarged to1.0V.Thebatterieswerethenchargedata0.2Cratefor6hand separatelydischargedatrespective0.2,1,2,and5Cdischarge cur-rentrates.Thecut-offvoltagesweresetas0.9V.Inthesubsequent charge/dischargecyclingtests,thebatterieswerechargedata1C ratefor 1.2h,restedfor10min,andthen dischargedat1Crate toalimitedvoltageof1.0V.Thedischargecapacityofthenickel hydroxideinthepositiveelectrodewasbasedontheamountof activematerial(Ni(OH)2)withouttakingintoaccounttheadditives

intheelectrode.

3. Resultsanddiscussion 3.1. CharacterizationofNi(OH)2 3.1.1. Physicalproperties

Criticalphysicalpropertiesofnickelhydroxidepowder, includ-ingtapdensity,particleshape,particlesize,chemicalcomposition, strongly affect the performance of the positive electrodes. The chemicalcompositionsandphysicalpropertiesofNi(OH)2samples

A(non-spherical)andB(spherical)arepresentedinTable1. Thecobaltandthezinccontentsarealmostthesameinboth samples.Thetap-densitiesofsamplesAandBareashighas2.22 and2.20gcm−3_{,respectively.Itisworthnotingthatinorderto}

Table2

FWHM,d-valuesin(001),(100),(101)diffractionlinesofNi(OH)2samplesA(non-spherical)andB(spherical).

Sample (001) (100) (101)

FWHM0 0 1(◦) d0 0 1(nm) FWHM1 0 0(◦) d1 0 0(nm) FWHM1 0 1(◦) d1 0 1(nm)

A(non-spherical) 0.970 0.46866 0.638 0.27059 1.057 0.23411

7800 E.Shangguanetal./JournalofPowerSources196 (2011) 7797–7805

Fig.2. Schematicillustrationofthesynthesisprocessof(a)non-sphericalNi(OH)2and(b)sphericalNi(OH)2.

exploretheeffectsofmicrostructureandsurfacepropertiesof ␤-Ni(OH)2onitselectrochemicalperformances,wechoosethe non-sphericalsamplewithatapdensityashighasthesphericaloneto avoidtheeffectoftapdensity,whichisnotjustaparticularcase. ThemeandiametersofsamplesAandBare27.20and9.34␮m, andBETsurfaceareasare75.62and10.15m2_g−1_{,respectively.It} isnoticedthatalthoughbothsampleshavealmostthesamehigh density,thespecificsurfaceareaofnon-sphericalpowdersisseven timeslargerthanthatofthesphericalsamples,whichshouldbe attributedtotheirregularparticleshape[8].Thehigherspecific surfaceareacanprovideahighdensityofactivesitesandthereby promotesintimateinteractionbetweentheactivematerialandthe surroundingelectrolyte.

3.1.2. SEM

Fig.1(a–f) displays SEM photographs of spherical and non-spherical Ni(OH)2 materials at different magnifications. The

powdersinFig.1a,candearesampleA(non-sphericalNi(OH)2),

andthepowdersinFig.1b,dandfaresampleB(sphericalNi(OH)2).

FromFig.1aandc,itisobservedthatsampleAappearstobe aggre-gatesofirregulartabularshapes,similartotheballmilledpowders. Theirregularshapesmayresultfromgrinding, whichleadstoa higherspecificsurfaceareaandmorestructuraldisorder,asshown inTables1and2.Thehighermagnification(30,000×)presented inFig.1e,showsthateachnon-sphericalparticlecontainsmany overlappedsheetnanocrystallinegrains,whichresultinadense solidstructure.So,Itisbelievedthatthehightap-densityof non-sphericalNi(OH)2isattributabletothedensesolidstructure.

Fig.3.XRDpatternsof(a)non-sphericalNi(OH)2and(b)sphericalNi(OH)2.

AscanbeseeninFig.1bandd,sampleBiscomposedof well-dispersedhomogeneoussphericalparticles.Eachsphericalparticle containsalargeamountofcuneiformorclaviformnanoparticlesof 50–100nminwidthand100–500nminlength.AsshowninFig.1f, thesenanocrystallitesareintercrossedandoverlapped,resulting inaradialnetworkmicrostructurewithalargenumberofpores

[28].Whenthepowdersaremixedwiththeelectrolytethesepores helptheelectrolytetosoakintotheinterspacesanddirectlycontact thecrystallinegrains,whichimprovestheelectrochemical perfor-manceofnickelhydroxide.Thus,itisconcludedthatsamplesAand Bwithhighdensityexhibitdifferentmicrostructuresandsurface propertiesduetodifferentproductionprocesses,whichmayresult indifferentelectrochemicalperformances.

Fig.2 representsa schematic diagram for theproduction of non-spherical Ni(OH)2 using the PAMassisted two-step drying

methodandspherical Ni(OH)2 preparedby theaforementioned

“controlledcrystallization”method.Experimentsduringthe syn-thesisofthenon-sphericalNi(OH)2haverevealedthattheeffectof

thecoagulatingagent(PAM)ontap-density,morphology,and sur-facepropertiesofthenon-sphericalNi(OH)2issignificant.Addition

ofanappropriateamountofPAMintotheNi(OH)2suspensioncan

greatlyacceleratefiltrationrateandreducethemoisturecontentin thepresscake,whichresultsinagglomerationsofcolloidparticles

[22].Coagulationfurtherincreasesthestructure’sdensity,asthe agglomerationssqueezeoutwater.Duetothisdensestructure,the Ni(OH)2particlesarepronetogrowandcrystallizeinthe

subse-quentdryingprocess,andthetap-densityoftheNi(OH)2thereby

remarkablyincreases.

Fig.5. TGandDTAcurvesfor(a)non-sphericalNi(OH)2and(b)sphericalNi(OH)2.

AsshowninFig.2b,theproductionprocessofsphericalNi(OH)2

isdifferentfromthatofnon-sphericalpowders,whichinvolvesa fewsteps:nucleationtoformtinyparticles,growthoftiny par-ticlestoformsheetsorplates,andself-assemblyandgrowthto formradialnetworkstructure[28].Althoughthesphericalshape canresultinhigh-density,thecrystalgrowthprocessforspherical particlesrequiresalongtime,thequalityoftheproductcannotbe guaranteedeasilyandtheproductioncostishigh.Consequently, theproceduresfornon-sphericalNi(OH)2powdersaremuch

sim-plerandmoreeconomicalbecausethereisnohydroxide-forming step,andaremoreenvironmentallyfriendlybecauseno ammo-niumisinvolved.

3.1.3. XRD

TheXRDpatternsofNi(OH)2samplesA(non-spherical)andB

(spherical)arepresentedinFig.3.TheXRDpatternsshowthatthese twosamplesexhibitthe␤-Ni(OH)2 phasecharacteristicswitha

brucite-typestructureandahexagonalunit,comparedwiththe standard(JCPDScardNo.14-0117).Nocharacteristicpeaks

corre-Table3

ResultsofweightlossmeasurementsforNi(OH)2samplesA(non-spherical)andB

(spherical). Weightloss region(◦_C) Reasonof weightloss Sample A (non-spherical) B (spherical) 20–200 Dehydration 1.22% 0.96% 200–350 Dehydroxylation 17.15% 17.28% 350–600 Removalof intercalatedanions 0.53% 0.29%

Fig. 6.Cyclic voltammograms of(a)non-spherical Ni(OH)2 and (b) spherical

Ni(OH)2atvariousscanrates.(c)Linearrelationshipbetweenthecathodicpeak

cur-rent(Ip)andsquarerootofscanratefor(a)non-sphericalNi(OH)2and(b)spherical

Ni(OH)2.

spondingtootherphasesareobserved,butthewidthofthepeaksis different.Thebroadeningofsomediffractionpeaks[e.g.(001)and (hk0)]in␤-Ni(OH)2XRDpatternsisdirectlyrelatedtothe crystal-litesize[29].Thecrystallitesizeperpendiculartovariousdiffraction planescanbeestimatedfromtheXRDlinesusingtheScherrer for-mula[30,31].TheFWHMsandthed-valuesoftherespectivesample inthe(001),(100),and(101)diffractionlinesarelistedinTable2.

7802 E.Shangguanetal./JournalofPowerSources196 (2011) 7797–7805

Fig.7. Nyquistplotsof(a)non-sphericalNi(OH)2and(b)sphericalNi(OH)2.

AspresentedinTable2,theFWHMsofthe(001)and(100) diffrac-tionlinesofsampleAarelargerthanthoseofsampleB,indicating thatthecrystallitesizeofsampleAissmallerthanthatof sam-pleB.However,thisresultisnotinaccordancewiththeparticle sizeresultsshowninTable1,whichmayresultfromthedifferent particleshapes.

It shouldbe noticedthat abnormal broadening of the(10l) reflectionlines(l =/ 0)cannotbeattributedtothecrystallitesize alone.Structuraldisorderplaysanimportantroleinthis broaden-ing[32,33].StructuraldisorderinNi(OH)2canprovideaneasier

pathforthediffusionofprotonswithintheNiOlayersandcanhelp lowerthefreeenergybyincreasingtheentropy,whichcaninturn increasetheelectrochemicalreactionrate[10,12].Previousreports haveindicatedthatthepeak(10l)wasespeciallybroadwhenthe nickelhydroxidewasmoreactive[34,35].Itisobviousthatsample AshowsabroaderFWHMofthe(101)diffractionlineof1.057, whereasthevalueforsphericalsampleBis0.782,indicatingthat non-sphericalpowderspossessahigherdensityofstructural disor-derandabetterelectrochemicalactivityinspiteofitslargerparticle size.

3.1.4. IRspectra

Fig.4presentstheinfraredspectraoftheas-preparedsamples A(non-spherical)andB(spherical).BothIRspectrashowsimilar characteristics.Thestrong,sharpbandcenteredat3640cm−1

cor-respondstothe

Thebroadbandat 3300cm−1 _{is characteristic ofthestretching}

vibrationofthehydroxylgroupextensivelyhydrogenbondedto watermoleculesandthebandat1650cm−1 _{correspondstothe}

bendingvibration ofwatermolecules[37].Thecouplebandsat about1470cm−1_and1380cm−1_{inbothIRspectracanbeassigned}

to

systemusedforsynthesis[38].Thebandatabout1125cm−1

cor-respondstothevibrationofSO42− [39].Atlow wavenumbers,

thebandsat460cm−1_and520cm−1_{areassignedtothebending}

vibrationof

[40].Notably,forsampleA,thetypicalIRspectracharacteristicof PAMarenotfound,indicatingthatPAMhavebeenremovedinthe

Fig.8.Schematicdrawingofproton/electrondiffusionin(a)non-sphericalNi(OH)2

and(b)sphericalNi(OH)2.

washinganddryingprocessanddonotaffectthequalitiesofthe products.

3.1.5. TG–DTA

TheTGandDTAcurvesforNi(OH)2samplesA(non-spherical)

andB(spherical)areshowninFig.5withdetaileddatalistedin

Table3.BothsamplesA andBshowthree weightlossregions. The first region is below 200◦_{C, where the adsorbed water is}

removed.Thesecondisbetween200◦_Cand350◦_C,wherethe

sam-pledecomposestoNiO.Thepracticalweightlosscorrespondingto thisreaction,whichcanbeestimatedfromthesecondweightloss stepontheTGcurves,is17.15%and17.29%forsamplesAandB, respectively.Thethirdis between350◦_{C and600}◦_C,wherethe

Table4

ParametersfromCVandEISmeasurementsfornon-sphericalandsphericalparticlesofNi(OH)2pasteelectrode.

Sample Ea(mV) Ec(mV) Ea,c(mV) EOER(mV) EOER−Ea(mV) EOER−Ec(mV) Rc(cm2) W(cm2) CPE(Fcm2)

A(non-spherical) 266 465 199 622 356 157 0.1428 0.4237 0.002767

Fig.9. Dischargecurvesoftheelectrodespreparedwith(a)non-sphericalNi(OH)2

and(b)sphericalNi(OH)2atdifferentcurrentrates.

intercalatedanionsareremoved.Theweightlossdueto intercala-tionofanionswasobservedtobe0.53%forsampleA,and0.29%for sampleB.

InFig.5,itcanbeseenthatthetemperaturescorresponding tothedecompositionreactionpeaksontheDTAcurvesgradually decline,withreadingsof267.76and259.97◦_{CforsamplesAand}

B,respectively.Thisindicatesthatthesynthesizednon-spherical Ni(OH)2 powdersarethermallymorestablethanspherical

pow-ders, asreflected inthelower decompositionreaction rateand higherdecompositiontemperature.Non-sphericalNi(OH)2

pow-dershaveahigherthermalstabilitythansphericalpowdersdueto thedensesolidstructureandlargeparticlesize,althoughtheyhave thesametap-density.

3.2. Electrochemicalinvestigationofˇ-Ni(OH)2 3.2.1. Cyclicvoltammetry

Fig.6illustratestypicalCVcurvesforsamplesA(non-spherical) andB(spherical)atvariousscan ratesand 25◦_{C. Obviously,for}

bothnickelelectrodes,oneanodicnickelhydroxideoxidationpeak andonecathodicoxyhydroxidereductionpeakareobservableon theCVcurves.TocomparetheCVcharacteristicsofnon-spherical andsphericalparticles,theresultsofCVmeasurementsatascan rateof1mVs−1_{aretabulatedinTable4.Thepotentialdifference}

(Ea,c)betweentheanodic(Ea)andcathodic(Ec)peakpotentials

istakenasanestimateofreversibilityoftheredoxreaction:the higherthereversibility,thesmallerEa,c is[41,42].TheEa,cof

Fig.10.Cyclicperformanceoftheelectrodespreparedwith(a)non-spherical Ni(OH)2and(b)sphericalNi(OH)2ata1Ccharge/dischargerate.

non-sphericalNi(OH)2ismuchsmallerthanthatofsphericalone,

indicatingbetterreversibilityoftheelectrochemicalredoxreaction inthenon-sphericalparticles,thusmoreactivematerialcanbe uti-lizedduringthecharge/dischargeprocess,whichisprovenbythe charge/dischargeresultsshowninFig.9.Thechargeprocessofthe Ni(OH)2 electrodeusuallyoccursincompetitionwithanoxygen

evolutionreaction(OER),whichlimitstheelectrochemical perfor-manceofthenickelhydroxideelectrodes.AsshowninTable4,the potentialdifferencebetweentheoxygenevolutionpotentialand theoxidationpotential(EOER−Ec)forthenon-sphericalsampleis

largerthanthatforsphericalone.Thelargevalueallowsthe non-sphericalsampletobechargedfullybeforeoxygenevolution,which canefficientlyrestraintheoxygenevolutionreactionandimprove thechargeefficiency[42,43].

Asisknown,theelectrochemicalreactionprocessofanickel hydroxideelectrodeislimitedbytheprotondiffusionthroughthe lattice[5,44].Therefore,itisofgreatimportancetostudythenickel electrode’sprotondiffusioncoefficient.Theprotondiffusion coeffi-cientoftheNi(OH)2electrodewasestimatedbasedontheCVtests.

Fig.6cshowstherelationshipbetweenthecathodicpeakcurrent (ip)andthesquarerootofthescanrate(1/2)forbothnickel

elec-trodes.Thegoodlinearrelationshipbetweenipand1/2suggests

thattheoxidationofnickelhydroxideisdiffusionlimited.Incase ofsemi-infinitediffusion,thepeakcurrentimaybeexpressedby theRandles–Sevcikequation.At25◦_{C,thepeakcurrent,ip,inthe}

cyclicvoltammogramcanbeexpressedas

ip=2.69×105×n3/2×A×D1/2×C0×1/2 (1) wherenistheelectronnumberofthereaction(≈1for␤-Ni(OH)2),

Aisthegeometricalsurfaceareaoftheelectrodeincm2_,Disthe

diffusioncoefficientofH+_incm2_s−1_,

v

andC0istheinitialconcentrationofthereactantinmolcm−3.For

anNi(OH)2electrode,

C0= 

M (2)

whereandMare,respectively,thedensityandthemolarmass (92.7gmol−1_{)ofNi(OH)2.Usingthisequationandtheslopeofthe}

fittedlineinFig.6c,theprotondiffusioncoefficientforsampleAis calculatedtobe4.26_{× 10}−9_cm2_s−1_{,whichislargerthanforsample}

B(3.13×10−9_cm2_s−1_).

3.2.2. Electrochemicalimpedancespectroscopy(EIS)

Fig.7presentstheelectrochemicalimpedancespectrafor elec-trodesAandBatasteadystate.ItcanbeseenthattheNyquist

7804 E.Shangguanetal./JournalofPowerSources196 (2011) 7797–7805

plotsofbothelectrodesdisplayasemicircleresultingfromcharge transferresistanceinthehigh-frequencyregion,andasloperelated toWarburg impedance in thelow-frequency region[45,46]. In general,theslopeinthelow-frequencyregionisregardedasthe Warburgslope,anempiricalparameterrelatedqualitativelyrather thanquantitativelytothediffusionresistance[45].Ahighslope signifiesaslowrateofdiffusion,andalowslopearapidrateof diffusion.Thecharacteristicsoftheelectrochemicalsystemcanbe representedbytheelectricalequivalentcircuitshownintheinset ofFig.7,whereRsisthetotalresistanceofthesolution,CPEisthe

constantphaseelementrelatedtothedoublelayercapacity,Rcis

thecharge-transferresistanceoftheelectrode,andWisthe gener-alizedfiniteWarburgimpedance(ZW)ofthesolidphasediffusion

[5].Thesimulatedvaluesoftheelementsbasedontheequivalent circuitarelistedinTable4,whereitcanbeseenthattheRcandW

valuesofelectrodeA(Fig.7a)aremarkedlylowerthanthoseof elec-trodeB(Fig.7b),whichimpliesthattheelectrochemicalreactionon electrodeAproceedsmoreeasilythanonelectrodeB.Nevertheless, theCPEvaluesofnon-sphericalparticlesarehigherthan spheri-calparticles,whichmaybeduetothefactthattheas-prepared non-sphericalNi(OH)2hasalargereffectiveactivesurfaceareafor

theelectrochemicalreactions.Thus,itprovesthatnon-spherical particleshavealowerresistancetocharge-transferandaremore efficientforprotondiffusioncomparedwithsphericalparticles dur-ingtheelectrochemicalreactions,andagreeswiththeresultsfrom CVmeasurements.

The highproton diffusion coefficientand low charge trans-ferresistanceofnon-sphericalNi(OH)2isattributabletoitshigh

structuraldisorderdensity, highspecificsurface areaand com-pact structure. First, thehigh densityof structural disorder for nickel hydroxide powder facilitates the solid-state proton dif-fusion in the Ni(OH)2 lattice and helps diminishconcentration

polarizationoftheprotonsduringthecharge/dischargeprocess, leadingtoabettercharge/dischargecyclingbehavior[47].Second, largespecificsurfaceareaofnon-sphericalNi(OH)2 canprovide

agoodinterconnectivitynetworkbetweentheNi(OH)2particles,

andmorechancesfortheparticlestocontacttheelectrolyte solu-tion,therefore, protondiffusionisenhanced, whichwillinturn acceleratetheelectrodereaction.Lastbutnotleast,thehigh pro-tondiffusioncoefficientofnon-sphericalNi(OH)2mayberelatedto

thedifferentproton/electrondiffusionroutesincomparisonwith sphericalNi(OH)2particles.Aschematicdrawingtoillustratethe

proton/electrondiffusionroutesin(a)non-sphericalNi(OH)2and

(b)sphericalNi(OH)2[48]ispresentedinFig.8.

ForsphericalNi(OH)2,Gilleetal.[48]reportedthatduringthe

charge/dischargeprocess,althoughthediffusionrouteis geometri-callynottheshortestway,theproton/electronpairstendtotravel alongthegrainboundaries,surfacesorotherareasofhighatomicor latticedisorderduetodislocationsorstackingfaultsintheactive materialofNi(OH)2/NiOOH,wherethediffusionvelocityof

pro-tonsisassumedtobehigherthanthatintheperfectlattice.For non-sphericalNi(OH)2,webelievethattheprotondiffusionroute

isgeometricallyshorterbecauseoftheirregularparticleshapeand thedensesolidstructure,andtheprotondiffusionvelocityislarger duetothehighdensityofstructuraldisorderintheactive mate-rialofNi(OH)2/NiOOH,which resultsin ahighprotondiffusion

coefficient.

3.2.3. Charge/dischargetests

Fig.9showsthedischargecurvesofnickelelectrodesA (non-spherical)andB(spherical)atratesof0.2,1,2,and5C,respectively. Obviously,sampleAexhibitsalargerdischargecapacityandhigher dischargepotentialplateauthansampleBatthesamedischarge rate(0.2C,1C,2Cand5C).Forexample,atarateof0.2C,the dis-chargecapacityofsampleAis276.1mAhg−1_{incomparisonwith}

259.3mAhg−1_{forsampleB.Asthedischargerateincreasesfrom}

0.2to5C,thedischargecapacityofsampleAdecreasesfrom276.1 to196.8mAhg−1_{,showingthatatsuchahighdischargerate71%of}

capacityisretained.Further,ata5Crate,thedischargecapacityof sampleAis196.8mAhg−1_{,whereasthevalueforsampleBisonly}

174.6mAhg−1_{,indicatingthatthehigh-rateperformanceof}

non-sphericalNi(OH)2ismuchbetterthanthatofsphericalNi(OH)2.

ThecyclicperformanceofnickelelectrodesA(non-spherical) andB(spherical)ata1CrateisillustratedinFig.10.Duringthe processofthecharge/dischargecycles,electrodeAshowsahigher specificcapacityandbettercyclingstability.Duringthecycling,the specificdischargecapacityofelectrodeAincreases,whichreaches a maximum value after 152 cyclesand then slowly decreases, whereas the specificdischarge capacity of electrode B remains constant up tothe maximum tested cycle (300th cycle). Espe-cially,thereisahighercapacity(11.5%morethancorresponding spherical-Ni(OH)2)fornon-sphericalparticlesinallcycles.

In summary, theoverall electrochemicalperformance ofthe non-spherical, Coand Zndoped Ni(OH)2 prepared by thenew

methodisobviouslysuperiortothecommercialsphericalNi(OH)2.

Thiselectrochemicalperformanceimprovementisattributableto thecompactsolid microstructure,highstructuraldisorder den-sity,largespecificsurfaceareaofthenon-sphericalNi(OH)2,thus

resultinginabetterreactionreversibility,higherprotondiffusion coefficient,andlowerelectrochemicalimpedanceofthematerial, asindicatedbyCVandEIS.

4. Conclusions

Thispaperdescribestheeffectofthemicrostructureand sur-face properties of Ni(OH)2 onits electrochemical performance.

Non-spherical,CoandZndopedNi(OH)2 withahightap-density

wassynthesized bythe PAMassisted two-step drying method. Compared withcommercial sphericalNi(OH)2, theas-prepared

Ni(OH)2 particlesexhibitanirregulartabularshape andadense

solidmicrostructure,whichcontainsmanyoverlappedsheetnano crystallinegrains,andhaveahighdensityofstructuraldisorder anda largespecificsurfacearea. Theas-preparednon-spherical samplesexhibitexcellentelectrochemicalperformance,whichis markedlysuperiortothespherical␤-Ni(OH)2,suchasbetter

reac-tionreversibility,higherprotondiffusioncoefficient,lowercharge transferresistance,higherspecificcapacity,andbettercyclic sta-bility.Themeasuredvalueoftheprotondiffusioncoefficientfor thenon-spherical␤-Ni(OH)2 is4.26×10−9_cm2_s−1_,andthe

dis-chargecapacity ofthesample reaches276mAhg−1 _{at0.2C and}

about196mAhg−1 _{at5C.Therefore,itisbelievedthat the}

non-spherical␤-Ni(OH)2synthesizedbythenewmethodisapromising

positiveelectrodeactivematerialforNi–MHbatteries. Acknowledgements

ThisworkisfinanciallysupportedbytheNaturalScience Foun-dationofChinaunderapprovalNo.21071046,andHenanProvincial Department of Science and Technology’s Key Research Project underapprovalNo.080102270013.

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