匯報人：何嘉榮學(xué)號：SC13013020LIB硅基負(fù)極材料研究進展1、鋰離子電池的應(yīng)用--移動通訊、便攜式電子產(chǎn)品

目前市場上的混合動力車鋰離子電池的研究趨勢？2、鋰離子電池的研究狀況在WOS平臺檢索：主題=(lithiumionbattery)在WOS平臺檢索：主題=(“l(fā)ithiumionbattery”)5689LIB硅基負(fù)極材料的目前研究（900）3、鋰離子電池的原理與基礎(chǔ)知識3.1鋰離子電池原理鋰離子電池由正負(fù)極材料，電解液，隔膜，集流體，粘結(jié)劑，導(dǎo)電劑，電池殼，極耳等構(gòu)成，其中正負(fù)極材料和電解液是關(guān)鍵。鋰離子電池的正負(fù)極活性物質(zhì)都是可逆嵌脫鋰材料，但具有不同的電位，其實質(zhì)上是鋰離子濃差電池。鋰的相對原子質(zhì)量為6.94，在金屬元素中是最輕的；它的標(biāo)準(zhǔn)電極電位最負(fù)（-3.045V），電化學(xué)當(dāng)量最?。?.26g/Ah)，這些特性決定了它具有較高的能量密度。圖1-1鋰離子電池的充放電示意圖固體氧化物燃料電池電化學(xué)反應(yīng)過程示意圖鋰離子電池的充放電示意圖3.2鋰離子電池關(guān)鍵材料概述圖1-2鋰二次電池正負(fù)極材料的電位-比容量圖任何一個鋰離子電池主要是由正極板、負(fù)極板、有機電解液和隔膜等組成。由正極板、負(fù)極板和隔膜組成的疊片通過沖壓使它們之間能夠電接觸。正極板是一種涂有由正極活性材料、導(dǎo)電劑、PVDF粘結(jié)劑和添加劑組成的混合物的鋁集流體；負(fù)極板是一種涂有由石墨、導(dǎo)電劑、PVDF粘結(jié)劑和添加劑組成的混合物的銅集流體。電解質(zhì)是良好的離子傳導(dǎo)體和電子絕緣體，而且大部分電解質(zhì)是將無機鋰鹽溶解在含有兩種或兩種以上的有機溶劑的混合物溶液中。隔膜的作用是防止陽極與陰極之間短路并且為充放電過程中Li+的傳遞提供運輸通道。充電時，Li+從正極逸出，通過電解液，最后嵌入負(fù)極如石墨。3.3鋰離子電池的組成部分扣式電池結(jié)構(gòu)圖將稱重后的負(fù)極極片、隔膜和電池殼干燥后送入氬氣保護的手套箱中。如左圖所示，按照負(fù)極片、電解液、隔膜、電解液、金屬鋰片、墊片和彈片的順序依次放入電池殼中，組裝成扣式電池。其中，電解液的用量要完全浸潤隔膜和電極活性物質(zhì)。鋰離子電池的組裝4.1硅負(fù)極的研究意義硅是能夠替代石墨作為鋰離子電池負(fù)極材料最有吸引力的材料之一，其主要有如下優(yōu)點：①硅能夠與鋰進行化學(xué)反應(yīng)形成合金Li4.4Si，具有4200mAh·g-1的非常大的理論比容量，這一優(yōu)勢是其它鋰離子電池負(fù)極材料所無可比擬的；②硅的電壓平臺略高于石墨類負(fù)極材料，在充電時不易引起表面析鋰的現(xiàn)象發(fā)生，從而提高了電池的安全性；③硅是地殼中豐度最高的元素之一，儲量豐富、價格便宜、對環(huán)境無害。4硅基負(fù)極材料的研究概述硅負(fù)極為合金儲鋰機制，在高溫下（450℃）發(fā)生電化學(xué)合金化反應(yīng)時，鋰硅合金經(jīng)歷了多相轉(zhuǎn)變，分別形成了四個相，其充放電曲線對應(yīng)著多個電壓平臺在室溫下，硅的首次放電（鋰化）曲線并沒有顯示多個電壓平臺，而是呈現(xiàn)一個較低的長平臺（如左圖所示），對應(yīng)著晶態(tài)硅變成無定形態(tài)的鋰硅合金。硅的充放電過程如左式所示，式中c表示結(jié)晶態(tài)，a表示無定形態(tài)。4.2硅負(fù)極的儲鋰機理硅材料也有許多缺點：1、循環(huán)壽命短，在鋰合金/去合金化過程中其體積會發(fā)生巨大的膨脹(大約300%)，破壞了電極結(jié)構(gòu)的穩(wěn)定性，從而導(dǎo)致電極結(jié)構(gòu)的坍塌。2、硅是半導(dǎo)體材料，本征電導(dǎo)率低，僅有6.7×10-4Scm-1，導(dǎo)致了硅材料的利用率較低，同時也影響了硅材料的大電流循環(huán)性能。3、在現(xiàn)有電解液中，鋰鹽LiPF6分解產(chǎn)生的微量HF也會對硅造成腐蝕，導(dǎo)致硅負(fù)極的容量發(fā)生衰減。由于硅具有劇烈的體積效應(yīng)，因此在常規(guī)的LiPF6電解液中，硅表面很難形成穩(wěn)定的固體電解質(zhì)膜；在充放電循環(huán)過程中，電極結(jié)構(gòu)逐漸發(fā)生破壞暴露出新鮮的硅表面，從而不斷形成新的固體電解質(zhì)膜，降低了充放電的庫侖效率，導(dǎo)致循環(huán)性能下降。以上的因素限制了其在商業(yè)方面的應(yīng)用。4.3硅負(fù)極的技術(shù)瓶頸為了減少不可逆容量和電極容量的衰減，從而延長電極的循環(huán)壽命。硅負(fù)極的性能改善方案：（1）減少硅顆粒大小至納米化范圍，提供更好的循環(huán)行為，這是由于納米化的硅在鋰化過程中機械應(yīng)力減少的緣故，同時納米材料還具有更小的鋰離子擴散距離，能夠提高電化學(xué)反應(yīng)速率；（2）將硅均勻地分散在一個活性或非活性緩沖基體中，制備出一些復(fù)合材料，引入的基體可以緩沖硅的體積變化，從而提高了材料的循環(huán)穩(wěn)定性；（3）通過選用不同的粘結(jié)劑、改進電解液、改進集流體及電極結(jié)構(gòu)等手段可在一定程度上提高硅材料的電化學(xué)性能。4.4硅負(fù)極的性能改善方案4.4.1Thepreparationofnano-sizeactivematerials

改變硅基負(fù)極材料結(jié)構(gòu)SiliconnanowiresThereisgreatinterestindevelopingrechargeablelithiumbatterieswithhigherenergycapacityandlongercyclelifeforapplicationsinportableelectronicdevices,electricvehiclesandimplantablemedicaldevices.Siliconisanattractiveanodematerialforlithiumbatteriesbecauseithasalowdischargepotentialandthehighestknowntheoreticalchargecapacity(4,200mAh/g).Althoughthisismorethantentimeshigherthanexistinggraphiteanodesandmuchlargerthanvariousnitrideandoxidematerials,siliconanodeshavelimitedapplicationsbecausesilicon’svolumechangesby400%uponinsertionandextractionoflithium,whichresultsinpulverizationandcapacityfading.Here,weshowthatsiliconnanowirebatteryelectrodescircumventtheseissuesastheycanmodatelargestrainwithoutpulverization,providegoodelectroniccontactandconduction,anddisplayshortlithiuminsertiondistances.Weachievedthetheoreticalchargecapacityforsiliconanodesandmaintainedadischargecapacitycloseto75%ofthismaximum,withlittlefadingduringcycling.Sibulk?lmsandmicrometer-sizedparticles:capacityfadingandshortbatterylifetime[pulverizationandlossofelectricalcontactbetweentheactivematerialandthecurrentcollector(Fig.1a)];Sinanowire(NW)anode:1、bettermodationofthelargevolumechangeswithouttheinitiationoffracture;2、eachSiNWiselectricallyconnectedtothemetalliccurrentcollectorsothatallthenanowirescontributetothecapacity.3、theSiNWshavedirect1Delectronicpathwaysallowingforef?cientchargetransport.AfterchargingwithLi,theSiNWshadroughlytexturedsidewalls(Fig.3b),andtheaveragediameterincreasedto141nm.Despitethelargevolumechange,theSiNWsremainedintactanddidnotbreakintosmallerparticles.Theyalsoappearedtoremainincontactwiththecurrentcollector,suggestingminimalcapacityfadeduetoelectricallydisconnectedmaterialduringcycling.TheSiNWsmayalsochangetheirlengthduringthechangeinvolume.AlthoughtheNWlengthincreasedafterlithiation,theNWsremainedcontinuousandwithoutfractures,maintainingapathwayforelectronsallthewayfromthecollectortotheNWtips.Pristine,unreactedSiNWswerecrystallinewithsmoothsidewalls(Fig.3a)andhadanaveragediameterof89nm.Cross-sectionalscanningelectronmicroscopy(SEM)showedthattheSiNWsgrewoffthesubstrateandhadgoodcontactwiththestainlesssteelcurrentcollector(Fig.3a,inset).PoroussiliconnanoparticlesFigure1(a)Schematicdiagramoftheproceduretoprepareporoussiliconnanoparticles.(b)PhotographsofnonporousandporousSinanoparticles,illustratingthescalablenatureofourporoussiliconnanoparticlepreparation.1/3oftheoriginalamountofnonporoussilicon.Wenotethatthistechniqueishighlyscalabletolargequantities,asthestartingmaterial,siliconnanoparticles,iscommerciallyavailableinbulkquantities,andthedopingandetchingprocessesarealsocompatiblewithlarge-scalemanufacturing.Figure2(a)TEMimageofsiliconnanoparticlesafterborondoping.(b)TEMimageshowingporoussiliconnanoparticleswithlargeAgnanoparticlesafterelectrolessetching.(c)TEMimageofporousSinanoparticlesafterwashingwithHNO3andH2OtoremoveAgnanoparticles.(d)HighresolutionTEMimageshowingthattheporesareuniformlydistributedonparticlesurfacewithsizearound9nm.Afteretching,theredcurveinFig.3revealsthattherelativeintensityratiosofthe(400)reflectionrelativetootherreflectionswerelargerthanthestandardvaluesintheJCPDScard(No.27-1402),whichisindicativeofthepreferentialpreservationof(400)planesduetoanisotropicetchingofAg+alongthesilicon<100>orientation4.4.2Thepreparationofactive/inactivecompositematerials復(fù)合化

硅基負(fù)極材料的復(fù)合化是指引入導(dǎo)電性好且體積效應(yīng)小的活性或非活性基體，制備多相復(fù)合材料，從而緩沖硅的體積效應(yīng)，提高硅基負(fù)極的循環(huán)性能。1、硅-碳復(fù)合材料[Si-basedcarboncomposite]2、硅-金屬復(fù)合材料具有嵌鋰活性的金屬（如Mg、Ag,、Sn等）與硅進行復(fù)合3、其它硅基復(fù)合材料某些具有鋰離子傳導(dǎo)性能的玻璃或陶瓷也可作為硅的復(fù)合母體，以緩沖其體積效應(yīng)，如硅-玻璃/陶瓷復(fù)合材料，硅基含鋰玻璃態(tài)復(fù)合材料。此外，導(dǎo)電聚合物、金屬氧化物、金屬氮化物、硅氧化物等都可以作為硅基材料的復(fù)合組分，如聚吡咯-硅復(fù)合材料。硅-碳復(fù)合材料[Si-basedcarboncomposite]Anew,simple,andgreenmethodologyforthesimultaneouscoatingofpreformedsiliconnanoparticlesinaone-stepprocedurewithathinlayerofSiOxandcarbonbythehydrothermalcarbonizationofglucose.FigureS6.TypicalSEMimageofSi@SiOx/Cposite.Figure1.a)XRDpatternsandb)Ramanspectraofthesiliconnano-particlesbeforeandaftercarboncoating(solidline:Si@SiOx/C,dottedline:Si).a、Therelativelowintensityandblueshiftofthebandatca.515cm-1b、Thelowgraphiticdegreeinthehydrothermalcarbonmaterial.Figure2.TEMimagesoftheSi@SiOx/Cpositeproducedbyhydrothermalcarbonizationandfurthercarbonizationat750oCunderN2.a)OverviewoftheSi@SiOx/CpositesandaTEMimageathighermagnification(intheinset)showinguniformspherelikeparticles;b)HRTEMimageclearlyshowingthecore/shellstructure;c),d)HRTEMimagedisplayingdetailsofthesiliconnanoparticlescoatedwithSiOxandcarbon.Si-basedLi-ionbatteryanodeshaverecentlyreceivedgreatattention,astheyofferspeci?ccapacityanorderofmagnitudebeyondthatofconventionalgraphite.Theapplicationsofthistransformativetechnologyrequiresynthesisroutescapableofproducingsafeandeasy-to-handleanodeparticleswithlowvolumechangesandstableperformanceduringbatteryoperation.Herein,wereportalarge-scalehierarchicalbottom-upassemblyroutefortheformationofSionthenanoscale—containingrigidandrobustsphereswithirregularchannelsforrapidaccessofLiionsintotheparticlebulk.LargeSivolumechangesonLiinsertionandextractionaremodatedbytheparticle’sinternalporosity.Reversiblecapacitiesover?vetimeshigherthanthatofthestate-of-the-artanodes(1,950mAh/g)andstableperformanceareattained.Thesynthesisprocessissimple,low-cost,safeandbroadlyapplicable,providingnewavenuesfortherationalengineeringofelectrodematerialswithenhancedconductivityandpower.Magasinski等將碳黑進行熱處理得到導(dǎo)電骨架，在骨架上CVD沉積納米硅顆粒，再用第二次CVD進行碳包覆并造粒，最終得到具有樹枝狀敞開式碳骨架的硅碳復(fù)合材料，如左圖所示。這種結(jié)構(gòu)便于電解液滲入，可為鋰離子提供暢通的傳輸通道，其內(nèi)部無序孔結(jié)構(gòu)可為硅的體積膨脹提供空間，從而獲得了優(yōu)異的循環(huán)和倍率性能。圖1-6兩步CVD法制備硅碳納米復(fù)合材料的示意圖及其電化學(xué)性能4.4.3改進粘結(jié)劑聚偏氟乙烯（PVDF）是傳統(tǒng)的鋰離子電池電極材料的粘結(jié)劑，但它對硅負(fù)極材料的粘結(jié)性較差。研究者們探索了新型的粘結(jié)劑：1、丁苯橡膠（SBR）與羧甲基纖維素鈉（CMC）的混合粘結(jié)劑，與硅表面基團存在著相互作用，具有更大的粘附力，改善了硅的循環(huán)性能。2、采用海藻酸鈉作為粘結(jié)劑，也可大幅提高硅負(fù)極的循環(huán)性能，這也是得益于海藻酸鈉上的羥基和羧基與硅表面存在著相互作用。3、通過對PVDF改性提高其粘性和強度，改善電接觸，也可提高硅負(fù)極的電化學(xué)性能。Siliconnanostructuressuchasnanowiresandnanotubescanethepulverizationproblem,howeverthesenano-engineeredsiliconanodesusuallyinvolveveryexpensiveprocessesandhavedif?cultybeingappliedincommerciallithiumionbatteries.Inthisstudy,wereportanovelmethodusingamorphoussiliconasinorganicgluereplacingconventionalpolymerbinder.Thisinorganicgluemethodcansolvethelossofcontactissueinconventionalsiliconparticleanodeandenablessuccessfulcyclingofvarioussizesofsiliconparticles,bothnano-particlesandmicronparticles.Withalimitedcapacityof800mAh/g,relativelylargesiliconmicron-particlescanbestablycycledover200cycles.Theverycheapproductionofthesesiliconparticleanodesmakesourmethodpromisingandcompetitiveinlithiumionbatteryindustry.Fig.ThechargeofLi+intotheSianodeleadstoswellingoftheelectrodelayer,inwhichcarbonblackpowderhadbeenaddedtocompensateforthepoorconductivityofSiparticles.DuringtheLi+dischargeperiod,Siparticlescontractasaresultofthede-alloyingreaction,buttheelectrodelayerstillremainsswollenbecausethepolymerchainisnotelasticenough.Thenetresultaftercharge–dischargecyclingisabreakdownoftheconductivenetworkbetweentheSiandthecarbonparticles,whichleadstoanincreaseintheinternalresistance.Inthisreport,wereplacethepolymerbinderwithamorphousSi(a-Si)inorganicandconductingglue,whichisconformallycoatedontoSiparticles.Thea-SilayercanalsoalloywithLiandexpandsandcontractsatthesameratiowithSiparticles,effectivelyconnect-ingalltheSiparticleselectricallywiththecurrentcollectorduringLi+intercalation.電極糊漿——三位網(wǎng)狀結(jié)構(gòu)（polymerchainsadsorbedontheparticles）溶劑揮發(fā)后,電極“材料”保持保持糊漿時的形貌——聚合物鏈包著粒子

Fig.3aChargeanddischargecyclingperformanceSi?lmspreparedbyinorganicgluemethodusingthreesizesofSiparticlesandcycledbetween0.9–0.01V.Figure3ashowsthecomparisonofthecyclingperformanceofthreeparticlesizeswhencycledwithaconstantvoltagecutoffof0.9–0.01VandrateofC/10(1C=4A/g).Aswecansee,the200nmparticlesshowedagoodcyclingstabilitywithacapacity(discharge)retentionof87%after30cycles,muchbetterthantheresultofLietal.4.4.4

改進電解液1、在低電位下，電解液中存在的鋰鹽和溶劑會發(fā)生電化學(xué)還原，在電極表面形成一層固體電解質(zhì)（SEI）膜，其對電極嵌脫鋰的動力學(xué)和循環(huán)穩(wěn)定性具有重要影響;2、在LiPF6電解液中添加碳酸亞乙烯酯（VC）或氟代碳酸乙烯酯（FEC），均可以提高硅負(fù)極的循環(huán)性能，這是由于二者可以在硅表面形成較穩(wěn)定的SEI膜。Electrodescomposedofsiliconnanoparticles(SiNP)werepreparedbyslurrycastingandthenelectrochemicallytestedina?uoroethylenecarbonate(FEC)-basedelectrolyte.Thecapacityretentionaftercyclingwassigni?cantlyimprovedcomparedto