聚偏氟乙烯粘结剂溶液干燥条件对锂电池用石墨负极附着力的影响_英文_

第38卷第6期Vol.38,No.62008年6月JOU RNAL OF UNIVE RSITY OF SCIENCE AND TECHNOLOGY OF CHINAJun.2008A rt icle ID:0253 2778(2008)06 0623 05Received:2008 01 18;Revised :2008 05 05Foundation item:Supported by Nation al Natu ral Science Foundation of Chin a (50372064).Biography:SH UI J iang lan,male,born in 1979,PhD.Res earch field:Lithium batteries.E mail:shu ijl@ Corresponding author:CH EN Chun hua,Ph D/Prof.E mail:cchchen @Effect of drying condition of poly (vinylidene fluoride)binder solution on film adhesion of graphiteelectrode for lithium batteriesSH U I Jiang lan,DIN G Chu x iong,Y U Yan,CH EN Chun hua(L abor atory of A d v anced F unctional M aterials and Dev ic es,Dep artment of M ater ials Sc ienc e and Eng ine ering ,Univ ersity of S cience and T ec hnology of China ,H e f ei 230026,China)Abstract:T he dr ying conditions of a poly (v inylidene fluoride)(PVDF)solution hav e a great influence on its crystalinity and binding streng th during the preparatio n o f electro de laminates for lithium ion batteries.A peeling experiment w as desig ned to evaluate the m echanical strength o f the PVDF adhering to g raphite.X r ay diffractio n and g alvanostatic cell cycling w ere used to evaluate the influence of PVDF binder treated under different thermal co nditions.T he results indicate that for the PVDF binder that is dried and crystallized at a temperature slig htly hig her than its m elting temperature,it has hig her cr ystallinity and adhesion strength to the g raphite par ticles,w hether it is so aked in electrolyte or not.This binder related mechanical pr operty o f the electr ode can improve the cy cling performance o f the g raphite electro de altho ug h the capacity m ay under go a gr adual rising process during the first 150or so cy cles due to the partial blocking of the electrical conduction path by PV DF initially.Key words:poly (viny lidene fluoride);gr aphite;cry stallinity;adhesion;lithium batteries CLC number:O646 Document code:A聚偏氟乙烯粘结剂溶液干燥条件对锂电池用石墨负极附着力的影响水江澜,丁楚雄,余 彦,陈春华(中国科学技术大学材料科学与工程系先进功能材料与器件实验室,安徽合肥230026)摘要:聚偏氟乙烯(PVDF)溶液的干燥条件对其结晶性、粘结强度有很大影响,对制备锂离子电池的电极材料亦有影响.为此,设计了一种薄膜剥皮试验来评估PVDF 附着于石墨的机械强度;以XRD 和电池循环来测量不同热处理条件下PVDF 粘结剂的影响.结果表明,在高于PVDF 熔点的温度处理的样品,PVDF 具有更高的结晶性和附着力,从而改善了石墨电极的循环性能,还观察到前150次循环过程中的容量逐步上升现象.PVDF 的逐渐溶胀被认为是出现该现象的原因.关键词:聚偏氟乙烯;石墨;结晶性;附着力;锂电池0 IntroductionGraphite is a widely used ano de material in the lithium ion battery.In order to improv e its capacity r etention ability,various attem pts have been made to r ealize surface m odification of the g raphite particles w hich w ill influence the form ation of SEI(so lid electr olyte interphase) film.Such a treatment usually reduces the initial capacity loss(ICL)and impr oves the cycling per for mance of the battery[1~3].In addition,the m icrostr ucture o f the po rous electrode can also be eng ineered dur ing the slur ry preparation by means of apply ing pr essure to the com posite electrode to tailor the poro sity of the electr ode.Pr oper porosity is necessary for the electr ode to have g ood capacity retention ability[4~7].Som e studies m entioned that the dr ying temperature of the composite electrode film has a g reat im pact on the adhesio n streng th of the w hole electrode to the current co llector,and that crystallinity of the PVDF binder plays an important r ole in this streng th.A detailed investigatio n on the effect of elevated dr ying temperature o n the adhesion streng th of the PVDF binding g raphite particles and the corresponding electro chem ical character ization has not been r eported in literature.In this paper,w e investigated this binding str ength betw een the binder PVDF and the g raphite particles,and com pared the cycling per for mance of the co mposite electro de m anufactured w ith different dry ing conditions.It is understo od that the crystallinity of PVDF sho uld differ greatly w hen it is o btained by dry ing its solution at a tem perature either far below or higher than its melting point(about170 ).M or eov er, the cry stallinity of PVDF binder has a gr eat influence o n the adhesion str ength as mentioned above.Therefor e,we selected70 (our usually used dry ing temperature)and200 as the dr ying temperatures to perform a comparative study.We have found that the crystallinity and adhesion strength of PVDF binder to the g raphite in the sample dried at200 are much hig her than that dried at70 ,and this kind o f adhesio n streng th has a crucial influence on the cy cling performance of the graphite electrode.H igher adhesion streng th results in better capacity retention ability.1 ExperimentalThe gr aphite electrodes w ere manufactured by mix ing a graphite pow der(about1 m particle size)(90w t%)w ith10w t%of PVDF binder into a NMP dispersed slurry,follow ed by spreading the slurry using the doctor blade m ethod on a sheet o f copper foil.T he natur ally dried laminate w as further dried in an oven at70 fo r5h or200 fo r20minutes.T he dried electrode films w er e taken out fro m the oven directly to the ro om tem perature about10 .Cycling test w as carried out on coin type cells(2032)w ith the config uration o f Li/1M LiPF6in ethylene car bonate and diethy l carbonate(EC DEC,11v/v)/ graphite film,w hich w ere assem bled in an arg on filled g love box(M BRAUN LABMAST ER130) w here bo th mo isture and oxy gen levels w ere less than1ppm.A Celgard2400microporous polypropylene membrane was used as the separator.To evaluate adhesio n str ength of PVDF binder to the gr aphite par ticles,a peeling exper im ent w as desig ned to simulate the adhesio n betw een g raphite particles and binder PVDF.A12w t%PVDF so lution w as prepar ed by dissolv ing PVDF pow der into NMP(1 m ethyl 2 py rrolido ne,C5H9NO).It w as coated onto the sur face o f tw o pieces o f graphite bars(8m m wide and50mm lo ng)and then dr ied in an oven in air at70 for5h(LT film)and200 for20m inutes(H T film), respectiv ely,in order to simulate the drying conditions of the graphite electrode laminates.We peeled these tw o PVDF film s fro m the g raphite bars at roughly the same peeling strength and velocity w ith a sm all clip and spring balance. Sim ilar ly,tw o PVDF coated g raphite bars w er e made and soaked in the electro lyte solution for18624中国科学技术大学学报第38卷hours at ambient temperature.T he PVDF adhesion o n these soaked bars w ere also measur ed by the peeling method.The thermal stability of the PVDF w as measured by a thermogravimetric analyzer (TGA50,Shimadzu)in the temperatur e range betw een 20and 700 w ith a heating rate of 5 /min.T he cry stal str ucture w as analyzed w ith X r ay diffr actio n (Cu K radiation,Philips X Pert PRO SU PER).Graphite electrode lam inates w er e prepared by casting onto a copper foil a slurr y consisting of g raphite pow der (90w t%)and po ly (v iny lidene fluoride )(PVDF )(10w t%)dispersed in 1 m ethyl 2 pyrr olidinone (NM P ).The lam inatesw ere then dr ied at either 70 o r 200 for 2hours and calendar ed to o btain po rosity betw een 60%~70%.Gr aphite/Li coin cells (2032size)w ere made w ith 1M LiPF 6in ethylene carbonate (EC):diethyl carbo nate (DEC)(1 1by w eig ht)as the electrolyte.Cycling tests on these cells w ere per for med under a constant current density of 0 20mA /cm 2in the vo ltag e rang e betw een 0and 3V.2 Results and discussionT he melting po int o f PVDF is around 170 .Accor ding to the thermo gravimetric analysis (T GA )o f the PVDF pow der used in this study (Fig.1),PVDF do es not deco mpose until 450 .Fig.2show s the X ray diffraction patterns o f the PVDF co atings after drying at 200 (H T film)and 70(LT film ).T he diffraction peakobserv ed at 2 !20.8∀is assigned to the unr esolved (110)and (200)diffractions of ! phase of PVDF,and phase o f PVDF show s diffraction peaks at 2 !18.5∀and 26∀,and are assigned to (020)/(110)and (120)diffractions,respectively [8].Co mparing the XRD patterns of the H T film and the LT film,the peaks of the LT film are broader than tho se of the H T film,w hich indicates that crystallinity of the film dried at 200 is higher than that dried at 70 .In addition,some ! phase has transferred to phase w hen the dryingcondition chang es from 70 /5h to 200 /20min.Different from som e resear ches to evaluate the adhesion strength o f co mposite film to substrates,w e solely w ant to know the adhesion streng th betw een the binder and g raphite because w e believ e that the interaction of the binder to the activ e materials is ver y im po rtant for the electrode stability and electro chem ical perform ance.Thus,a peeling ex periment w as desig ned to ev aluate this streng th and rectang ular g raphite bar s w er e used as substr ates to simulate the true adhesio n of the binder to the graphite particles.Tab.1sho ws the peeling strength for the H T film and the LT film before and after being soaked in the electr olyte.It can be seen that the peeling of the H T film needs fo ur times more for ce than the peeling of the LT film.Besides,w hen w e peel the binder films from the g raphite bar so me graphite particles ar e removed off the bar so that the peeled PVDF film ex hibits a fuscous co lor (Fig.3).The H T film625第6期E ffect of drying cond ition of poly (vi nylid en e flu orid e)bind er solu tion on film adhesion of graphite electrode for lithi um batteri espresents m ore fusco us than the LT film.Because of the higher adhesion strength of the H T film,m ore g raphite particles can be r em oved off from the gr aphite bar.After being soaked in the po lar or ganic electro lyte for 18hours at ambient temperature of about 20 ,the adhesion str ength of both the H T film and the LT film all decreased a little,but the sequence is still H T >LT.The decrease in the adhesion strength after soaking in the electroly te may be due to the sw elling of the amo rphous portion of PVDF w hen the electr olyte solvent is absor bed by the film.As repor ted by Yoo and Despotopoulou et.al [9,10],the crystallinity is an important factor in determ ining the adhesion strength:the higher crystallinity the higher adhesion str ength betw een com posite film and cur rent collector.H ence abov e r esults,combined w ith XRD results,clearly indicate that PVDF m ay crystallize at a tem perature slightly hig her than its m elting point.Co nsequently ,thus r esulted higher crystallinity leads to much higher adhesion strength of PVDF binder to the surface of graphite particles.Tab.1 Peel strength of dried PVDF film from graphite barspeel strength /(N #mm -1)70 /5hpeel stren gth /(N #mm -1)200 /20minbefore electrolyte soakin g 0.110.49after soaking in electrolytefor 18hours0.080.45Th e PVDF film s w ere dried at 70 /4h (a)and 200 /20min (b),respectivelyFig.3 Photo of the peeled PVDF film from graphite barsFig.4sho w s the capacity retention ability oftw o Li/graphite cells w ith the graphite electrodesmentioned above.T he cell w ith electro de dr ied at 70 /5h show s some capacity loss during the first 100cy cles fr om 365to 330m Ah/g,and the capacity fading accelerates after 125cycles.On the other hand,the cell w ith electro de dried at 200 /20min show s very g ood capacity retention ability.The capacity no t only keeps stable but also increases somew hat from 336to 367m Ah/g during the first 165cy cles,about 0 19mAh/g per cy cle.Though the capacity increases initially ,it finally decreases after r eaching the peak value (367mAh/g),that is still below the theo retical capacity o f graphite (372mAh/g ),dur ing a pr olongedcy cling.Li/com posite graphite (drying condition:70 /4h (a)and 200 /20min (b)).Th e pas sing current w as 0.2m A/cm 2Fig.4 Cycling performance of cellsThe lithium insertion extraction in a g raphite electr ode involv es several pro cesses including ∃periodic volume changing in the g raphite flakes,w ith the amo unt of activ e m aterial not decreasing [11],%surface film fo rmation o n the graphite par ticles [1~3,11],and &the sw elling o f po lymer binder PVDF [9,10].It should be mentio ned here that coulo mbic efficiencies o f these tw o kinds of cells are alm ost the same,abo ut 80%.Thus,the reaso n for the cycling performance change in Fig.4sho uld not be only due to the for mation o f SEI,w hich has been intensively investigated [1~4],although some gro ups repo rted that the br eaking o f SEI film can cause the capacity bining the abo ve facto rs ∃,%and &,w e speculate that the contact among gr aphite particles may be deteriorated w ith the incom pact electrode626中国科学技术大学学报第38卷upo n pr olonged cy cling.The loose particle contact can lead to a poor electrical conduction among the par ticles,and to the form ation of surface films w hich are m ore resistiv e.This may certainly cause the capacity loss of the battery.H ig her adhesion str ength betw een the PVDF binder and graphite can alleviate the po ssible detachm ent betw een the PVDF binder and the sur face of the graphite par ticles during the periodic vo lum e chang es o f the g raphite particles.On the other hand,some g raphite particles in the H T electrode may not be utilized fully initially because so me particles are covered closely by the PVDF binder w ith higher crystallinity so that the electronic conduction path is blocked and only a fraction(<100%)o f the g raphite surface is accessible to the electro lyte.H ence the w hole capacity can not be utilized fully at first,but the capacity can rise during prolo ng ed cycling due to the g radual binder swelling by the electroly te solution,as observed in the case of H T electrode(Fig.4).3 ConclusionWe have co mpared lam inated graphite electrodes and pure PVDF films m anufactured by different dry ing pro cesses,i.e.heating at either 70 /5h o r200 /20min.The cell w ith the 200 dried electr ode show s much better capacity retention ability,and the capacity ev en underg oes a little rise fo r the first165cy cles.PV DF dr ied at temperature slightly higher than its m elting po int w ill cause higher crystallinity w hich co nsequently enhances its adhesion strength as a binder w ith the g raphite particles and the stability of the electro de str ucture.Better adhesion o f the electrode can alleviate 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