Chitosan, a new and environmental benign electrode binder

Electrochimica Acta 105 (2013) 378–383Contents lists available at SciVerse ScienceDirectElectrochimicaActaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taChitosan,a new and environmental benign electrode binder for use with graphite anode in lithium-ion batteriesLili Chai a ,Qunting Qu a ,Longfei Zhang a ,Ming Shen b ,Li Zhang a ,∗,Honghe Zheng a ,∗a School of Energy,Soochow University,Suzhou,Jiangsu 215006,China bHuasheng Chemical Corporation,Zhangjiagang,Jiangsu 215635,Chinaa r t i c l ei n f oArticle history:Received 22March 2013Received in revised form 30April 2013Accepted 6May 2013Available online 14 May 2013Keywords:Lithium-ion batteries Graphite anode Electrode binder Chitosana b s t r a c tChitosan was applied as the electrode binder material for a spherical graphite anode in lithium-ion pared to using poly (vinylidene fluoride)(PVDF)binder,the graphite anode using chitosan exhibited enhanced electrochemical performances in terms of the first Columbic efficiency,rate capabil-ity and cycling behavior.With similar specific capacity,the first Columbic efficiency of the chitosan-based anode is 95.4%compared to 89.3%of the PVDF-based anode.After 200charge–discharge cycles at 0.5C ,the capacity retention of the chitosan-based electrode showed to be significantly higher than that of the PVDF-based electrode.Electrochemical impedance spectroscopy (EIS)and scanning electron microscopy (SEM)measurements were carried out to investigate the formation and evolution of the solid electrolyte interphase (SEI)formed on the graphite electrodes.The results show that a thin,homogenous and stable SEI layer is formed on the graphite electrode surface with chitosan binder compared with that using the conventional PVDF binder© 2013 Elsevier Ltd. All rights reserved.1.IntroductionElectrode binder plays a very important role in fabricating high-performance electrodes as it holds active materials and conductive additives into a cohesive laminate and provides the adhesion between the laminate and the current collector.In the past two decades,poly (vinylidene fluoride)(PVDF)was the most widely used binder material for Li-ion battery electrodes.This is attributed to its good electrochemical stability,binding capability and ability to absorb electrolyte [1–5].However,PVDF polymer is costly and it requires the use of environmentally unfriendly N-methyl pyr-rolidone (NMP)solvent in the processing.Besides,this polymer is very sensitive to the environmental humidity.It undergoes a severe degradation of viscosity after absorbing water.As a fluorinated polymer,PVDF also shows a certain reactivity against lithium metal and lithiated graphite (Li x C 6),producing resistive LiFand C CF species on the electrode surface,especially at elevated tempera-tures [6,7].For these reasons,the development of greener,cheaper and more electrochemical stable electrode binder is considered as a goal of strategic importance for battery technologies.In recent years,many groups have been studying new electrode binder materials for the realization of greener battery processing and∗Corresponding authors.E-mail addresses:hhzheng@ (L.Zhang),zhangli81@ (H.Zheng).high performance Li-ion batteries.Among different binder systems,water-soluble binders are not only much cheaper than PVDF,but also allow electrode processing in aqueous slurries.These polymers can be easily disposed at the end of the life of the battery.Gelatin [8–11],sodium carboxy-methyl cellulose (CMC-Na)and styrene-butadiene rubber (SBR)[12–18],poly(acrylic acid)(PAA)[19–23],poly(methacrylic acid)(PMA)[24],poly(vinyl alcohol)(PVA)[25],alginate [26,27],poly(acrylamide-co-diallyldime-thylammonium chloride (AMAC)[28],poly (acrylonitrile-methyl methacrylate)(AMMA)[29],and polyimide (PI)[30],all have been reported as promising new binder systems replacing PVDF and some of them have been successfully used in commercial lithium ion batter-ies.In this paper,chitosan,as a water-soluble polymer,was adopted as a new electrode binder for a graphite anode.Chitosan is a polysaccharide composed mainly of ␤-(1,4)-linked 2-deoxy-2-amino-d -glucopyranose units.The molecular structure of it is provided in Fig.1.This polymer is one of the most plentiful nat-ural biopolymers produced from poly (N-acetyl-d -glucosamine)(chitin)[31].Chitosan is widely used in many fields including molecular separation,food packaging film,artificial skin,bone substitutes,water treatment and so on [32–34].In Li-ion battery field,chitosan has also been attempted as the template for active material synthesis [35],and carbon coating precursor [36].How-ever,to the best of our knowledge,this polymer has never been adopted as an available binder material for Li-ion batteries up to today.0013-4686/$–see front matter © 2013 Elsevier Ltd. All rights reserved./10.1016/j.electacta.2013.05.009L.Chai et al./Electrochimica Acta105 (2013) 378–383379Fig.1.The molecular structure of chitosan polymer.As the matter of fact,chitosan-based aqueous slurry possesses good viscosity,which can be considered as an effective electrode binder.Herein,acid solution-based chitosan slurry was applied as the binder for a commercial spherical graphite material.Elec-trochemical performances of the chitosan-based and PVDF-based graphite anodes were compared in terms of reversible capacity,first Coulombic efficiency,rate capabilities and long-term cycling behavior.Electrochemical impedance spectroscopy(EIS)and scan-ning electron microscopy(SEM)measurements were performed to investigate the effect of binder on the formation and evolution of the solid electrolyte interphase(SEI)layer on the graphite elec-trode.Mechanisms for the higher electrochemical performance of the graphite anode with chitosan binder were discussed2.ExperimentalSpherical graphite powders(Shenzhen Beterui new energy Materials Group Co,Ltd Code No AGP-8,China)were applied as the electrode active material.Acetylene black(AB)with an average particle size of40nm was acquired from Denka Singapore Private Ltd PVDF(KF1100)binder was obtained from Kureha,Japan and chitosan(Code No C0831)was purchased from TCl Shanghai.All the materials were used as received in this study.PVDF was dissolved into NMP solvent(aladdin,anhydrous, 99.9%)with a concentration of15wt%while chitosan was dis-persed into0.5%acetic acid solution with a concentration of3wt% at room temperature.Even though the concentration of the PVDF-based solution is much higher,viscosity of the aqueous solution containing3wt%chitosan seems to be slightly higher than that of the15wt%PVDF solution.Both solutions were equilibrated for at least2h at room temperature with magnetic stirring.For the preparation of the electrodes,slurry consisting of94wt%graphite powders and6wt%chitosan was thoroughly mixed for12h and casted onto15␮m-thick copper foil(99.99%purity).As a refer-ence,the electrode using PVDF binder was prepared by casting the slurry containing88.8wt%graphite,3.2wt%super P and8wt% PVDF.The recipes were obtained in our lab as described in the lit-erature[37]for making high electrochemical performance MCMB anode.Both the two electrode laminates were controlled to have the same active material loading of∼1.8mg cm−2by varyingthedoctor blade height.The obtained electrodes were dried at120◦Cin a vacuum oven for16h and pressed to enhance the mechanicalcontact between the electrode laminate and current collector. Fig.2.SEM images of as-prepared graphite electrodes with chitosan(a and b)and PVDF binder(c and d).380L.Chai et al./Electrochimica Acta010020030 0400.00.51.01.52.02.53.0221Specific capacity (mAh.g -1)V o l t a g e v s . L i /L i + (V )1-W ater-based e lectrode 2-PVDF-based e lectrode1Fig.3.The first charge–discharge profiles of the graphite anode using (a)chitosanand (b)PVDF binder.The half cells (2032stainless-steel coin cell)were assembled in an Ar-filled glove box (OMNI-LAB,VAC).The electrolyte was 1M LiPF 6in ethylene carbonate/diethylene carbonate (EC/DEC,1:1by weight)with 10%fluoroethylene carbonate (FEC)as the addi-tive (Novolyte Technologies).The separator employed was Celgard 2400and lithium metal foil (99.9%purity)was utilized as the counter electrode.All the cells were tested on a Maccor S4000(Maccor Instruments,USA)at 303K in a Thermotron Environmen-tal Chamber.Three formation cycles at C/20charge and discharge were applied between 0.005V and 2V vs Li.Rate test of the graphite electrodes consisted of full discharges at rates of C/10,C/5,C/2,1C ,2C ,5C ,and 10C ,respectively.Long-term cycling of the cells was carried out with 0.5C charge and 0.5C discharge for 200cycles.Electrochemical impedance spectroscopy (EIS)was measured by applying an alternating voltage of 5mV over the frequency ranging from 10−2to 105Hz.All impedance measurements were carried out at the discharge state of 60%DOD (depth of discharge)at different cycling stages.Morphologies of the graphite electrodes using different binders before and after electrochemical tests were observed by scanning electron microscopy (SEM)using a Hitachi S-4700operated at 1kV and 5kV accelerating voltage.3.Results and discussionSEM images of the as-prepared graphite electrodes with chi-tosan and PVDF binders are presented in Fig.2at different magnifications.As there is no conductive carbon additive,only smooth and “clean”graphite particles are observed for the elec-trode with chitosan binder (as seen in Fig.2a,b).The graphite particle size is around 10␮m in the image.By contrast,as shown in Fig.2c,d,lots of nano-scale conductive carbon particles are seen well distributed in between the graphite particles.It has been reported that PVDF binder tends to shrink and cover the graphite powders,which blocks the migration of Li ion during electrochem-ical processes [38].Addition of conductive carbon additive within the PVDF-based electrode not only contributes to an improvement of the electronic conductivity of the laminate,but also helps to increase the electrode porosity.Fig.3shows the charge–discharge profiles of the graphite anodes using different binders in 1M LiPF 6/(EC:DEC)electrolyte containing 10%FEC.FEC is used as the electrolyte additive as it significantly improves electrochemical performances of graphite anode in the electrolyte and is now widely used in Li-ion battery R&D.For the two kinds of electrodes,the cell voltage drops rapidly from open circuit voltage down to 0.2V,corresponding to theof is at is at ofL.Chai et al./Electrochimica Acta 105 (2013) 378–38338180859095100105%/n o i t n e t e r y t i c a p a c e l b i s r e v e R Cycle nu mberFig.5.Rate capability of the graphite electrodes using (a)chitosan and (b)PVDFbinder.electrode using PVDF binder.The higher rate capability of chitosan-bonded graphite electrode seems not very identical with the highpolarization of the electrode in the first charge and this will be discussed next.Fig.6displays the cycling performance of the graphite anodes with chitosan and PVDF binders.As shown in this figure,chitosan-based graphite anode shows better cycling performance than the PVDF-based electrode.Upon deep galvanostatic cycling for 200cycles,the capacity retention of the chitosan-based electrode is no less than 91%while the capacity retention of the PVDF-based electrode is no more than 75%at the same cycling condition.The improved cycling capability of the electrode by using Chitosan binder illustrates better electrochemical stability of the electrode during electrochemical cycles.In order to explain the higher rate performance and cycling capability of the graphite anode using chitosan binder,elec-trochemical impedance spectroscopy (EIS)measurements of the graphite anode using different binders at different cycling stages were performed at 60%DOD and the results are displayed in Fig.7.All the sets consist of two semicircles [39].The high-frequency semicircle is believed to be related to the resistance of passivating SEI film (R SEI )or active material particle-to-particle interfacial con-tact resistance.The intermediate-frequency semicircle is attributed to the charge–transfer resistance (R ct ).For the chitosan-based20406080100%/n o i t n e t e r y t i c a p a c e l b i s r e v e R Cycle numberFig.6.Cycling performance of the graphite electrodes using chitosan (a)and PVDF (b)binder at 0.5C charge and 0.5C discharge.51015202530-Z '' / O h mZ' / Ohm10203040-Z '' / O h mZ' / OhmFig.7.Nyquist plots of graphite electrodes using chitosan (a)and PVDF (b)binder at different cycling stages at 60%DOD.electrode,higher impedance after formation cycles are observed compared to that of electrode using PVDF binder.The high resistance is most likely to be related to the graphite particle-to-particle interfacial contact resistance as there is no conductive carbon within the electrode matrix.The high impedance explains the higher polarization of the electrode in the first charge as shown in Fig.4.However,after the rate test,it is clear to see that impedance of chitosan-based electrode exhibits a remarkable decrease and the impedance seems to be even lower than that of PVDF-based electrode at this stage.The decrease of electrode impedance with electrochemical cycle implies an activation process for chitosan-bonded electrode.Along with the repeating lithiation/delithiation processes,more ion transport and charge–transfer sites may be created at the electrode/electrolyte interface,which leads to the electrode impedance decreases.Actually,this phenomenon always occurs for graphite anode with water soluble binders [28].The lower impedance for the chitosan-based electrode explains the high rate performance at this stage.After 200cycles,impedance of the chitosan-based anode is well maintained and even shows a continuous decrease of R ct .By comparison,a slight increase of R ct for the PVDF-based electrode is observed.Again,this result confirms that the surface condition of electrode using chitosan binder is relatively stable with electrochemical cycles.For the PVDF-based anode,the increase of electrode impedance indicates continuous growth of the surface components during the long-term cycling test.According to these analyses,chitosan presents obvious advantages in fabricating high-performance anode compared with conventional PVDF binder.382L.Chai et al./Electrochimica Acta 105 (2013) 378–383Fig.8.SEM images of graphite electrodes after formation and rate capability test,chitosan binder (a),(b);PVDF binder (c),(d).To clearly see the surface condition of the graphite anode with different binders after electrochemical cycles,scanning elec-tron microscopy (SEM)measurements were carried out.Fig.8presents the SEM microscope images of graphite anode with dif-ferent binders after formation cycles and after the rate capability test.After the formation cycles,many alien particles and fragments are seen dispersed on the graphite surface of the PVDF-based anode (Fig.8c)while few alien particles can be detected on the graphite surface with chitosan binder (Fig.8a).This agrees with the lower irreversible capacity loss of the electrode in the first cycle.After the rate capability tests,the graphite surface of the PVDF-based elec-trode became even more rough and thicker,with a lot of agglom-erate granules and fragments staying on the electrode surface (see Fig.8d).This evidence also supports our conclusion that the elec-trode with chitosan binder has better electrochemical stability.4.ConclusionChitosan was applied as an effective binder for graphite anode in Li-ion pared to the electrode using conventional PVDF binder,the first Columbic efficiency,rate capability and cycling behavior are considerably improved EIS studies showed that the electrode resistance of the chitosan-based electrode under-goes a distinct decrease after the initial several cycles and the electrode impedance is well maintained after long-term cycling tests.Meanwhile a continuous growth of electrode resistance with electrochemical cycles is observed for the electrode with PVDF binder.This is consistent with the continuous growth of the surface species with increasing electrochemical cycles.At last,it should be noted that many key physical parameters involved in making chitosan-based electrode are not optimized in this study.Given the chitosan content and electrode processing technologies are optimized,the results may be even more optimistic.AcknowledgementsThe authors are greatly indebted to the funding of Natural Sci-ence Foundation of China (NSFC,contract no 21073129,51272168,21203132,and 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