EuropeanJournalofChemistry7(2)(2016)182‐186

EuropeanJournalofChemistryISSN2153‐2249(Print)/ISSN2153‐2257(Online)2016AtlantaPublishingHouseLLC‐Allrightsreserved‐PrintedintheUSA

http://dx.doi.org/10.5155/eurjchem.7.2.182‐186.1396

EuropeanJournalofChemistryJournalwebpage:www.eurjchem.com

Influenceofpolymerbinderstructureonthepropertiesofthegraphiteanodeforlithium‐ionbatteries

MonikaOsinska‐Broniarz1,*,AgnieszkaMartyla1,LukaszMajchrzycki2,MarekNowicki3andAgnieszkaSierczynska1

1InstituteofNonFerrousMetals,DivisioninPoznan,CentralLaboratoryofBatteriesandCells,Forteczna12,61‐362Poznan,Poland2WielkopolskaCenterforAdvancedTechnologies,Grunwaldzka6,60‐780Poznan,Poland3InstituteofPhysics,PoznanUniversityofTechnology,Nieszawska13a,PL‐60‐965,Poznan,Poland

*Correspondingauthorat:InstituteofNonFerrousMetals,DivisioninPoznan,CentralLaboratoryofBatteriesandCells,Forteczna12,61‐362Poznan,Poland.Tel.:+48.61.2797803.Fax:+48.61.2797897.E‐mailaddress:monika.osinska@claio.poznan.pl(M.Osinska‐Broniarz).

ARTICLEINFORMATION ABSTRACT

DOI:10.5155/eurjchem.7.2.182‐186.1396

Received:20January2016Accepted:20February2016Publishedonline:30June2016Printed:30June2016

Thispaperdiscusses the impactof thestructureandpropertiesof threedifferentpolymerbinders:polyvinylidene fluoride, sodiumcarboxymethyl celluloseandpolyvinylalcohol,onthe electrochemical properties of spherical graphite anodes for Li‐ion batteries.Electrochemical tests indicate that the nature of polyvinylidene fluoride contributes indecreasingthecycle lifeofgraphiteelectrodesincontrasttoeffectivewater‐basedbinders.Thisstudydemonstratesthepossibilityofmanufacturinggraphite‐basedelectrodeforLi‐ionbatteries that cycle longer and use water in the processing, instead of hazardous organicsolvents like N‐methylpyrrolidone, thereby improving performance, reducing cost andprotectingtheenvironment.

KEYWORDSPVACMCPVdFAnodeLi‐ionbatteryPolymerbinder

Citethis: Eur.J.Chem.2016,7(2),182‐186

1.Introduction

In lithium‐ion batteries production process, a polymerbinder has a very important function, namely binding theactivematerialsof theelectrode inoneconductivemass.Theadhesive and chemical properties of the binder have a greatimpact on the performance of the battery. Over the past 20years, during which lithium‐ion batteries have becomeincreasingly popular,major research effort has been focusedonmaterialsforthistypeofbatteries,likeanodicandcathodicactivematerials,separatorsandelectrolytes,whichhavebeenextensivelystudiedfromyeartoyearandinthecaseofwhichtremendousprogresshasbeenachieved[1‐3].

The most conventional binder used for compositeelectrodes for lithium‐ionbatteries ispolyvinylidene fluoride(PVdF).PVdFmayhavesuperiorchemicalandelectrochemicalstability, but this does not entirely eliminate Li‐ion batteryproducers’ problems such as having to use toxic organicsolvents such as N‐methylpyrrolidone (NMP), which areindispensable for dissolving polymer. In addition, the cost ofPVdF‐basedelectrodesisstillaconcerninthebatteryindustry[4‐8]. What is more, PVdF also tends to swell in organic

solvents, which can lead to the active mass breaking awayfrom theelectrode current collectorduringbatterywork [1].Thus,alargeamountofefforthasbeenputintothecreationofanenvironment‐friendlyandcost‐effectivebinderforlithium‐ion batteries. Extensive research has been conducted into inthisfield,suchascarboxymethylcellulose(CMC),mainlyasitssodiumneutralizedderivative,andpolyacrylicacid(PAAH)[8‐11].Recently,somenewkindsofbindershavebeentestedforlithium‐ion batteries, for example acrylate polymers, such aspolyacrylic acid (PAA) containing carboxyl groups maintainexcellent cyclic retention for C‐Si‐based electrode materialsthanks to hydrogen bonding. Among them polyvinyl alcohol(PVA)hasreceivedconsiderableattentionbecauseitcontainsnumeroushydroxylgroupsinitsstructurewhichcanincreasecohesionbetweenactivematerialsandacurrentcollector[12‐19].ThechemicalstructureofPVAisshowninFigure1,alongwithPVdFandCMCstructureforcomparison.

In this study, aqueous‐based PVA and CMC binders andorganic‐based PVdF for spherical structure graphite (MCMB)electrodes were investigated. The physical and chemicalproperties of the pristine binders were researched. ThenMCMB‐basedanodeswithdifferentbinderswerepreparedto

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examinetheeffectofwater‐basedbindersonelectrochemicalcharacteristics which were compared with the PVdF basedbindersystem.

PVdF CMC PVA

Figure1.ChemicalStructureofPVdF,CMC,PVA.2.Experimental

Three kinds of polymer binders were used to prepareanodes:PVdFpowder(Mw=~530000g/mol),purchasedfromFluka, sodium carboxymethyl cellulose (CMC,Mw =~700000g/mol) from Aldrich and PVA (Mw = 85‐124000 g/mol)suppliedbyAldrich.Allbinderpowderswerecharacterizedbyscanningelectronmicroscopy(SEM,QuantaFEG250ScanningElectronMicroscope). The X‐ray powder diffraction patternsof the samples were taken by means of Philips PW1050diffractometer,withNi filteredCuKα. Structural analysiswascarried out using ATR‐FT‐IR spectrometer (Bruker, TENSOR27)andsolubilitytestsweredone.

The commercial anode active material MCMB(MesoCarbon MicroBeads Graphite) was provided by MTICorporation.SuperC65conductivecarbonblackwasacquiredfrom Timcal Graphite & Carbon. The graphite MCMB wascharacterizedby scanning electronmicroscopy (SEM,QuantaFEG 250 Scanning Electron Microscope) and a detailedspecificationwasconductedtodeterminethespecificsurfaceareausingASAP2010M.

To study the effect of different polymer binders onelectrochemical performance of theMCMB anodes in lithiumionbatteries,PVdF,CMC,PVAwereutilizedtoprepareanodesconsisting of MCMB and C‐65 in a weight ratio of 10:80:10,with suitable amounts of solvent (NMP for PVdF, deionizedwaterforCMC,PVA,respectively).TheobtainedslurrieswerecoatedontoCufoilanddriedat120°Cfor24hundervacuum.Swelling properties of the electrodeswere tested by soakingthem in theelectrolyte for48hand calculating thedifferencein mass. Electrochemical measurements were performed byusingSwagelok‐typehalf‐cells.Thecarbonelectrodewasusedas a work electrode. Lithium metal was used as a counterelectrode and a reference electrode and Celgard 2400(poly(ethylene) foil, Celgard) as a separator. The electrolytewas LP30 (mixture of ethylene carbonate (EC) andethylmethylcarbonate (EMC) involumeratioofEC:DMC(1:1)with1Mlithiumhexafluorophosphate(LiPF6),Merck).Alltheoperationsonthecellsassembledwerecarriedoutinaglove‐box filled with argon gas. The cells were galvanostaticallydischargedto0.01Vandchargedto2.0Vattherateof30or50mApergramofactivemass in10cyclesand150mA/gin100 cycles. Galvanostatic experimentswere performed usingmultichannel potentiostat/galvanostat Atlas 0461 (Atlas‐Sollich,ZSE).3.Resultsanddiscussion

Infrared spectroscopywasused to characterize the chainstructureofpolymersemployedtoprepareanodeelectrodes.Figure2displaystheATR‐FT‐IRspectraofthePVA,CMCandPVdFbinders.

ForPVAbinder, themainabsorbancebandat3000‐3600cm‐1 verified the presence of a hydroxyl group ‐OH and C‐Obonding in the region of 1000‐1250 cm‐1. Also, the peak ataround2850‐2900cm‐1onIRcurveofPVAwasanindicationof CH/CH2 bonding. For CMC binder, the peaks of sodium

carboxylategroupat1600and1420cm‐1 couldbeobserved.Additionally, small vibrational bands in the range 900‐1100cm‐1 observed in the pattern were attributed to the ethergroupsfromcellulose[15,18].ForPVdFbinderthevibrationalbandsat890and~1200cm‐1,wereascribedtothestretchingfrequenciesofCF2.CrystallizationpeakswhichwereassignedtothevibrationofthecrystallizationPVdFaccordingtoPeng’sreport [17,21] were observed at range 600‐900 cm‐1. Thecrystallinities of the binders were examined by X‐raydiffraction.Figure3presentsX‐raydiffractionpatternsofthethreebinders.

Figure2.ATR‐FT‐IRanalysisofPVdF,CMCandPVAbinder.

Figure3. X‐raydiffractionpatternsofthethreebinders.X‐raydiffractionanalysisshowedcharacteristic reflection

peaks at 20 (2Θ) for all polymers, where the peak for PVdFwasmuchsharperthanfortheothertwopolymers.Accordingto Han’s report [15,20], we probably observed the conse‐quencesofahigherdegreeofPVdFcrystallinitystructurethanCMC and PVA,which is known as a semicrystalline polymer.This conclusion can also be confirmedby the SEM images ofthethreebinderpristinesolidpowders(Figure4).

The spherical structure of the grains of PVdF withtendencytocreateocclusiveagglomerateswasobserved.Thisspherical, regular shape of particles is one of the features ofcrystal.ForCMCandPVA,weobservedonlyanirregularshapeof particles, which confirmed their rather amorphouscharacter.Thesedifferencesinthestructuralofpolymersmaygreatly influence the morphologies of polymer films formedafter being dissolved in solvent and dried in the next stepduring the preparation of the electrode. Because of thesemicrystalinestructureofthePVdF,wecanexpectnumerouspores in the film of this polymer and, consequently, thestructure of electrode with this kind of binder havingnumerous pores in contrast to amorphous polymer‐basedfilmslikeCMCorverylowcrystallitePVA[17,23‐25].

184 Osinska‐Broniarzetal./EuropeanJournalofChemistry7(2)(2016)182‐186

Figure4.SEMimagesofPVdF,CMC,PVApristinesolidpowders,respectively.

Figure5. SEMimagesofMCMBgraphitepowder.

Figure6.SEMimagesoftheMCMBelectrodeswiththreedifferentpolymericbinders.A typical SEM image of the MCMB graphite particles is

shown in Figure 5. We can notice the spherical microbeadshapedparticlesofthisgraphite.DuringASAPanalysisspecificdataof thismaterialwerecollected.Thespecificsurfacearea(BET) of theMCMB graphite stands at 1.54m2/g, total porevolume2.11cm3/gandaverageparticlesizeofthegraphite’sgrainsis10µm.

The morphology of the electrodes in this study wascharacterized by using SEM mapping techniques. Figure 6shows the SEM images of MCMB‐based electrodes withdifferent polymer binders. Composite structure of theelectrodes was observed, where the spherical structure ofgraphitewasthemostvisibleincomparisonwithwater‐basedbinders composite electrodes. Electrodes for the preparation

of which PVdF binder was used have shown a veryhomogenousstructure.

Swelling properties of the electrodes were examined bysoaking them in the electrolyte LP30 solution at roomtemperature for 48 h. After that time, the electrodes weretaken out to weigh the mass (m1) after removing theelectrolyte on the surface with the filter paper. The initialweight of the electrodes before test was letter m0. Theincreasedmasspercentageofthepolymerfilmswascalculatedaccording to the equation: (m1‐m0)/m0. The differences inmass of electrodes before and after contact with liquidelectrolyte were between 2‐3% of their mass, similar to allthree binders. Probably 10% of the binder provides enoughadhesion to prevent the electrode from decomposing afterbeingincontactwiththeelectrolytefor48hours.

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Table1.Basicelectrochemicalparametersofthegraphiteanodesintheexaminedcellsdeterminedonthegalvanostaticcharge/dischargeexperiments:Q1ch‐chargecapacityinthe1stcycle,Q1dis‐dischargecapacityinthe1stcycle,Qirr‐irreversiblecapacity(Qirr=Q1ch‐Q1dis),Eff‐coulombicefficiencyofcharginginthe1stcycle.Polymerbinderinelectrode Currentdensityofcycling(mAg‐1) Q1ch (mAhg‐1) Q1dis(mAhg‐1) Qirr(mAhg‐1) Eff.(%)PVdF 30/50/150 504/485/413 326/322/339 178/163/74 54/51/82CMC 30/50/150 377/334/172 354/303/161 23/31/11 94/91/94PVA 30/50/150 410/444/178 306/284/130 104/160/48 75/64/73

Anotherexperimentregardingswellingwasperformedbysoaking the three different binders directly in the liquidelectrolyte,simulatingasimilarconditionas insidea lithium‐ion battery. Figure 7 records the beginning and the endsituation(after48h).

Figure7. Swelling properties of the polymer binders in contact to liquidelectrolyte.

PVdF powder easily swelled, forming a viscous, gel‐like

fluid product, which was in accordance with other reports[17,26‐27]. PVA swells but only in small proportions. Incontrast, CMC, a water‐based binder, does not swell in thesame experimental conditions in contact with liquidelectrolyte. The swelling of binders is one of the mostimportant factors for reversible cycling of the electrode in abattery.PVdF‐basedelectrodesseemtobe themostunstableduring long cycling, which was confirmed during ourexperimental work. The half‐cells with graphite electrodeswerecycledgalvanostaticallyat:30and50mA/gin10cyclesand 150mA/g in 100 cycles in 25 °C. Galvanostatic charge/discharge tests have shown how and how long lithium ionshave been reversibly intercalated intoMCMB‐based graphiteelectrodesandwhatthehighestvalueofthecapacityisforthiskind of electrode materials depending on the binder’sstructure.Table1summarizesnumericalelectrochemicaldatacalculatedonthebasisofgalvanostaticcharacteristics.

Importantly, graphite anode charged/discharged inelectrodewithwater‐basedbindersexhibitsasmallervalueofirreversible capacity in the first cycle for all cycling tests incontrast to electrodes with PVdF. It is probably due toconstruction of the electrodes, where surfaces of electrodeswith PVA and CMC are much more heterogeneous incomparison with PVdF‐based electrodes. Consequently, theworking surface area of the electrode is a little bit moreexpandedbut,most importantly,water basedbindersdonotdissolveinelectrolyteincontrasttoPVdF.

Nevertheless,numericaldatainTable1indicatethatinthefirst cycle for all cycling the both Q1ch and Q1dis values arehigher forMCMB electrodeswith PVdF. These dependenciesare true for the studied systems up to 10 cycles. In systemscycling over 10 times an opposite trend can be observed(Figure8).

Dischargecapacityover10thcycleupto100thishigherforelectrodeswithwater‐basedbinders (ca. 270mAh/g). In thecaseofQdisforelectrodeswithPVdFweobserveadecreaseasthenumberofcharge/dischargecyclesincreases.Inthe100thcycleQdisisequaltoca.150mAh/gforthistypeofelectrodes,whichisalmosttwiceaslowasinthe1stcycle.Itmaysuggestthatafterabout10cyclesduringthetestwithcurrentdensity150mA/gthePVdFbasedgraphiteelectrodeisdecomposedtoquitealargeextent.Whatismore,aftercyclingtests,thePVdFcomposite electrode could be detached from the cuprumcurrent collector easily,while the other electrodeswith CMC

andPVAbinderstightlystucktothecurrentcollectors,ascanbeseeninFigure9.

This evidence also supports our conclusion that thegraphite electrodes with water based binders like CMC andPVAhavebetterphysicalandelectrochemicalstability.

Figure8.ThecyclingpropertiesofMCMBelectrodespreparedusingPVdF,CMCandPVAasabinder:(a)currentdensity:30mA/g,(b)currentdensity:50mA/g,(c)currentdensity:150mA/g.

186 Osinska‐Broniarzetal./EuropeanJournalofChemistry7(2)(2016)182‐186

Figure9.Picturesofgraphiteelectrodesafter100charge/dischargecycles(currentdensity150mA/g).4.Conclusion

The paper describes how the water‐based binders (CMCandPVA)wereappliedaseffectivebindersforgraphiteanodewithMCMBforLi‐ionbatteriesincomparisonwiththeorganicsolvent‐basedbinderPVdF.

Based on this study, the following conclusions can bedrawn:

‐ Thesurfaceof graphite electrodeswithMCMBshows ahigherhomogeneitylevelforcompoundspreparedwithwater based CMC and PVA polymers, compared withcompounds with PVdF polymer dissolved in organicsolvents. The surface of the electrodes with CMC andPVAismuchhigherthanthatofelectrodeswithPVdF.

‐ The structure of polymer binders and their ability toswell in liquid electrolyte highly influences themecha‐nical properties of graphite electrodes, especially theadhesion strength between active materials and thecurrent collector. During research work the mostsatisfying adhesion was observed for all graphiteelectrodeswithwater‐basedbinders:CMCandPVA.

‐ The influence of the binder structure on the electro‐chemical properties of electrodes is considerable. Foranode half cells with MCMB graphite and investigatedbinders,thereversiblecapacityvalueinthefirstcyclesissatisfying for electrodes based on PVdF, and is almostcomparable with the theoretical value of graphitecapacity,what ismore, thePVdF‐basedelectrodeswithMCMBgraphite showhigher capacity in the anode halfcellswithresearchedgraphiteelectrodesupto10cycles.After about 10 cycles of charging and dischargingwithcurrent density 150 mA/g discharge capacity forelectrodemassdropsdrastically.This happensbecausetheelectrodemassseparatesfromthecurrentcollector,asitislaminatedwithPVdFbasedelectrodemass,proneto swelling in a liquid electrolyte, which results indecreasingadhesion.

Acknowledgement

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