硅基负极材料的研究进展

汇报人：何嘉荣学号：SC13013020LIB硅基负极材料研究进展1、锂离子电池的应用--移动通讯、便携式电子产品

目前市场上的混合动力车锂离子电池的研究趋势？2、锂离子电池的研究状况在WOS平台检索：主题=(lithiumionbattery)在WOS平台检索：主题=(“lithiumionbattery”)5689LIB硅基负极材料的目前研究（900）3、锂离子电池的原理与基础知识3.1锂离子电池原理锂离子电池由正负极材料，电解液，隔膜，集流体，粘结剂，导电剂，电池壳，极耳等构成，其中正负极材料和电解液是关键。锂离子电池的正负极活性物质都是可逆嵌脱锂材料，但具有不同的电位，其实质上是锂离子浓差电池。锂的相对原子质量为6.94，在金属元素中是最轻的；它的标准电极电位最负（-3.045V），电化学当量最小（0.26g/Ah)，这些特性决定了它具有较高的能量密度。图1-1锂离子电池的充放电示意图固体氧化物燃料电池电化学反应过程示意图锂离子电池的充放电示意图3.2锂离子电池关键材料概述图1-2锂二次电池正负极材料的电位-比容量图任何一个锂离子电池主要是由正极板、负极板、有机电解液和隔膜等组成。由正极板、负极板和隔膜组成的叠片通过冲压使它们之间能够电接触。正极板是一种涂有由正极活性材料、导电剂、PVDF粘结剂和添加剂组成的混合物的铝集流体；负极板是一种涂有由石墨、导电剂、PVDF粘结剂和添加剂组成的混合物的铜集流体。电解质是良好的离子传导体和电子绝缘体，而且大部分电解质是将无机锂盐溶解在含有两种或两种以上的有机溶剂的混合物溶液中。隔膜的作用是防止阳极与阴极之间短路并且为充放电过程中Li+的传递提供运输通道。充电时，Li+从正极逸出，通过电解液，最后嵌入负极如石墨。3.3锂离子电池的组成部分扣式电池结构图将称重后的负极极片、隔膜和电池壳干燥后送入氩气保护的手套箱中。如左图所示，按照负极片、电解液、隔膜、电解液、金属锂片、垫片和弹片的顺序依次放入电池壳中，组装成扣式电池。其中，电解液的用量要完全浸润隔膜和电极活性物质。锂离子电池的组装4.1硅负极的研究意义硅是能够替代石墨作为锂离子电池负极材料最有吸引力的材料之一，其主要有如下优点：①硅能够与锂进行化学反应形成合金Li4.4Si，具有4200mAh·g-1的非常大的理论比容量，这一优势是其它锂离子电池负极材料所无可比拟的；②硅的电压平台略高于石墨类负极材料，在充电时不易引起表面析锂的现象发生，从而提高了电池的安全性；③硅是地壳中丰度最高的元素之一，储量丰富、价格便宜、对环境无害。4硅基负极材料的研究概述硅负极为合金储锂机制，在高温下（450℃）发生电化学合金化反应时，锂硅合金经历了多相转变，分别形成了四个相，其充放电曲线对应着多个电压平台在室温下，硅的首次放电（锂化）曲线并没有显示多个电压平台，而是呈现一个较低的长平台（如左图所示），对应着晶态硅变成无定形态的锂硅合金。硅的充放电过程如左式所示，式中c表示结晶态，a表示无定形态。4.2硅负极的储锂机理硅材料也有许多缺点：1、循环寿命短，在锂合金/去合金化过程中其体积会发生巨大的膨胀(大约300%)，破坏了电极结构的稳定性，从而导致电极结构的坍塌。2、硅是半导体材料，本征电导率低，仅有6.7×10-4Scm-1，导致了硅材料的利用率较低，同时也影响了硅材料的大电流循环性能。3、在现有电解液中，锂盐LiPF6分解产生的微量HF也会对硅造成腐蚀，导致硅负极的容量发生衰减。由于硅具有剧烈的体积效应，因此在常规的LiPF6电解液中，硅表面很难形成稳定的固体电解质膜；在充放电循环过程中，电极结构逐渐发生破坏暴露出新鲜的硅表面，从而不断形成新的固体电解质膜，降低了充放电的库仑效率，导致循环性能下降。以上的因素限制了其在商业方面的应用。4.3硅负极的技术瓶颈为了减少不可逆容量和电极容量的衰减，从而延长电极的循环寿命。硅负极的性能改善方案：（1）减少硅颗粒大小至纳米化范围，提供更好的循环行为，这是由于纳米化的硅在锂化过程中机械应力减少的缘故，同时纳米材料还具有更小的锂离子扩散距离，能够提高电化学反应速率；（2）将硅均匀地分散在一个活性或非活性缓冲基体中，制备出一些复合材料，引入的基体可以缓冲硅的体积变化，从而提高了材料的循环稳定性；（3）通过选用不同的粘结剂、改进电解液、改进集流体及电极结构等手段可在一定程度上提高硅材料的电化学性能。4.4硅负极的性能改善方案4.4.1Thepreparationofnano-sizeactivematerials

改变硅基负极材料结构SiliconnanowiresThereisgreatinterestindevelopingrechargeablelithiumbatterieswithhigherenergycapacityandlongercyclelifeforapplicationsinportableelectronicdevices,electricvehiclesandimplantablemedicaldevices.Siliconisanattractiveanodematerialforlithiumbatteriesbecauseithasalowdischargepotentialandthehighestknowntheoreticalchargecapacity(4,200mAh/g).Althoughthisismorethantentimeshigherthanexistinggraphiteanodesandmuchlargerthanvariousnitrideandoxidematerials,siliconanodeshavelimitedapplicationsbecausesilicon’svolumechangesby400%uponinsertionandextractionoflithium,whichresultsinpulverizationandcapacityfading.Here,weshowthatsiliconnanowirebatteryelectrodescircumventtheseissuesastheycanmodatelargestrainwithoutpulverization,providegoodelectroniccontactandconduction,anddisplayshortlithiuminsertiondistances.Weachievedthetheoreticalchargecapacityforsiliconanodesandmaintainedadischargecapacitycloseto75%ofthismaximum,withlittlefadingduringcycling.Sibulkfilmsandmicrometer-sizedparticles:capacityfadingandshortbatterylifetime[pulverizationandlossofelectricalcontactbetweentheactivematerialandthecurrentcollector(Fig.1a)];Sinanowire(NW)anode:1、bettermodationofthelargevolumechangeswithouttheinitiationoffracture;2、eachSiNWiselectricallyconnectedtothemetalliccurrentcollectorsothatallthenanowirescontributetothecapacity.3、theSiNWshavedirect1Delectronicpathwaysallowingforefficientchargetransport.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复合化

硅基负极材料的复合化是指引入导电性好且体积效应小的活性或非活性基体，制备多相复合材料，从而缓冲硅的体积效应，提高硅基负极的循环性能。1、硅-碳复合材料[Si-basedcarboncomposite]2、硅-金属复合材料具有嵌锂活性的金属（如Mg、Ag,、Sn等）与硅进行复合3、其它硅基复合材料某些具有锂离子传导性能的玻璃或陶瓷也可作为硅的复合母体，以缓冲其体积效应，如硅-玻璃/陶瓷复合材料，硅基含锂玻璃态复合材料。此外，导电聚合物、金属氧化物、金属氮化物、硅氧化物等都可以作为硅基材料的复合组分，如聚吡咯-硅复合材料。硅-碳复合材料[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,astheyofferspecificcapacityanorderofmagnitudebeyondthatofconventionalgraphite.Theapplicationsofthistransformativetechnologyrequiresynthesisroutescapableofproducingsafeandeasy-to-handleanodeparticleswithlowvolumechangesandstableperformanceduringbatteryoperation.Herein,wereportalarge-scalehierarchicalbottom-upassemblyroutefortheformationofSionthenanoscale—containingrigidandrobustsphereswithirregularchannelsforrapidaccessofLiionsintotheparticlebulk.LargeSivolumechangesonLiinsertionandextractionaremodatedbytheparticle’sinternalporosity.Reversiblecapacitiesoverfivetimeshigherthanthatofthestate-of-the-artanodes(1,950mAh/g)andstableperformanceareattained.Thesynthesisprocessissimple,low-cost,safeandbroadlyapplicable,providingnewavenuesfortherationalengineeringofelectrodematerialswithenhancedconductivityandpower.Magasinski等将碳黑进行热处理得到导电骨架，在骨架上CVD沉积纳米硅颗粒，再用第二次CVD进行碳包覆并造粒，最终得到具有树枝状敞开式碳骨架的硅碳复合材料，如左图所示。这种结构便于电解液渗入，可为锂离子提供畅通的传输通道，其内部无序孔结构可为硅的体积膨胀提供空间，从而获得了优异的循环和倍率性能。图1-6两步CVD法制备硅碳纳米复合材料的示意图及其电化学性能4.4.3改进粘结剂聚偏氟乙烯（PVDF）是传统的锂离子电池电极材料的粘结剂，但它对硅负极材料的粘结性较差。研究者们探索了新型的粘结剂：1、丁苯橡胶（SBR）与羧甲基纤维素钠（CMC）的混合粘结剂，与硅表面基团存在着相互作用，具有更大的粘附力，改善了硅的循环性能。2、采用海藻酸钠作为粘结剂，也可大幅提高硅负极的循环性能，这也是得益于海藻酸钠上的羟基和羧基与硅表面存在着相互作用。3、通过对PVDF改性提高其粘性和强度，改善电接触，也可提高硅负极的电化学性能。Siliconnanostructuressuchasnanowiresandnanotubescanethepulverizationproblem,howeverthesenano-engineeredsiliconanodesusuallyinvolveveryexpensiveprocessesandhavedifficultybeingappliedincommerciallithiumionbatteries.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.电极糊浆——三位网状结构（polymerchainsadsorbedontheparticles）溶剂挥发后,电极“材料”保持保持糊浆时的形貌——聚合物链包着粒子

Fig.3aChargeanddischargecyclingperformanceSifilmspreparedbyinorganicgluemethodusingthreesizesofSiparticlesandcycledbetween0.9–0.01V.Figure3ashowsthecomparisonofthecyclingperformanceofthreeparticlesizeswhencycledwithaconstantvoltagecutoffof0.9–0.01VandrateofC/10(1C=4A/g).Aswecansee,the200nmparticlesshowedagoodcyclingstabilitywithacapacity(discharge)retentionof87%after30cycles,muchbetterthantheresultofLietal.4.4.4

改进电解液1、在低电位下，电解液中存在的锂盐和溶剂会发生电化学还原，在电极表面形成一层固体电解质（SEI）膜，其对电极嵌脱锂的动力学和循环稳定性具有重要影响;2、在LiPF6电解液中添加碳酸亚乙烯酯（VC）或氟代碳酸乙烯酯（FEC），均可以提高硅负极的循环性能，这是由于二者可以在硅表面形成较稳定的SEI膜。Electrodescomposedofsiliconnanoparticles(SiNP)werepreparedbyslurrycastingandthenelectrochemicallytestedinafluoroethylenecarbonate(FEC)-basedelectrolyte.Thecapacityretentionaftercyclingwassignificantlyimprovedcomparedto