Lithium ion polymer batteries or more commonly lithium polymer batteries.docx

Lithium ion polymer batteries or more commonly lithium polymer batteries.docx

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Lithiumionpolymerbatteriesormorecommonlylithiumpolymerbatteries

Lithiumionpolymerbattery

Lithiumionpolymerbatteries,ormorecommonlylithiumpolymerbatteries（AbbreviatedLi-PolyorLiPo）arerechargeablebatterieswhichhavetechnologicallyevolvedfromlithiumionbatteries.Ultimately,thelithiumsaltelectrolyteisnotheldinanorganicsolventlikeintheprovenlithiumiondesign,butinasolidpolymercompositesuchasPVDForpolyacrylonitrile.Therearemanyadvantagesofthisdesignovertheclassiclithiumiondesign,includingthefactthatthesolidpolymerelectrolyteisnotflammable（unliketheorganicsolventthattheLi-Ioncelluses）;thus,thesebatteriesarelesshazardousifmistreated.Lithiumionpolymerbatteriesstartedappearinginconsumerelectronicsaround1996.

Overview

Cellssoldtodayaspolymerbatterieshaveadifferentdesignfromtheolderlithiumioncells.Unlikelithiumioncylindrical,orprismaticcells,whichhavearigidmetalcase,polymercellshaveaflexible,foil-type（polymerlaminate）case,buttheystillcontainorganicsolvent.Themaindifferencebetweencommercialpolymerandlithiumioncellsisthatinthelattercells,therigidcasepressestheelectrodesandtheseparatorontoeachother,whereasinpolymercellsthisexternalpressureisnotrequiredbecausetheelectrodesheetsandtheseparatorsheetsarelaminatedontoeachother.

Sincenometalbatterycellcasingisneeded,thebatterycanbelighteranditcanbespecificallyshapedtofitthedeviceitwillpower.Becauseofthedenserpackagingwithoutintercellspacingbetweencylindricalcellsandthelackofmetalcasing,theenergydensityofLi-Polybatteriesisover20%higherthanthatofaclassicalLi-IonbatteryandapproximatelythreetimesbetterthanNiCdandNiMHbatteries.

ThevoltageofaLi-Polycellvariesfromabout2.7V（discharged）toabout4.23V（fullycharged）,andLi-Polycellshavetobeprotectedfromoverchargebylimitingtheappliedvoltagetonomorethan4.235Vpercellusedinaseriescombination.OverchargingaLi-Polybatterywilllikelyresultinexplosionand/orfire.Duringdischargeonload,theloadhastoberemovedassoonasthevoltagedropsbelowapproximately3.0Vpercell（usedinaseriescombination）,orelsethebatterywillsubsequentlynolongeracceptacharge.

Earlyinitsdevelopment,lithiumpolymertechnologyhadproblemswithinternalresistance.Otherchallengesincludelongerchargetimesandslowermaximumdischargeratescomparedtomorematuretechnologies.Li-Pobatteriestypicallyrequiremorethananhourforafullcharge.Recentdesignimprovementshaveincreasedmaximumdischargecurrentsfromtwotimesto15oreven30timesthecellcapacity（dischargerateinamps,cellcapacityinamp-hours）.InMarch2005Toshibaannouncedanewdesignofferingamuchfaster（about1-3minutes）rateofcharge.Thesecellshaveyettoreachthemarketbutshouldhaveadramaticeffectonthepowertoolandelectricvehicleindustries,andamajoreffectonconsumerelectronics;especiallyelectricallypoweredmodelaircraft.

Whencomparedtothelithiumionbattery,Li-Polyhadagreaterlifecycledegradationrate.However,inrecentyears,manufacturershavebeendeclaringupwardsof500charge-dischargecyclesbeforethecapacitydropsto80%（seeSanyo）.AnothervariantofLi-Polycells,the"thinfilmrechargeablelithiumbattery"hasbeenshowntoprovidemorethan10,000cycles.

Applications

AcompellingadvantageofLi-Polyisthatmanufacturerscanshapethebatteryalmosthowevertheyplease,whichcanbeimportanttomobilephonemanufacturersconstantlyworkingonsmaller,thinner,andlighterphones.Anotheradvantageoflithiumpolymercellsovernickelcadmiumandnickelmetalhydridecellsisthattherateofselfdischargeismuchlower.

Li-PolybatteriesarealsogainingfavorintheworldofRadio-controlledaircraft,wheretheadvantagesofbothlowerweightandgreatlyincreasedruntimescanbesufficientjustificationfortheprice.However,lithiumpolymer-specificchargersarerequiredtoavoidfireandexplosion.Explosionscanalsooccurifthebatterywiresarecrossedunderload.Radiocontrolenthusiaststakespecialprecautionstoensuretheirbatteryleadsareproperlyconnectedandinsulated.Speciallydesignedelectronicmotorspeedcontrolsareusedtopreventexcessivedischargeandsubsequentbatterydamage.ThisisachievedusingaLowVoltageCut-off（LVC）setting,thatisadjustedtomaintaincellvoltageat（typically）3vpercell.

Li-polybatteriesarealsogaininggroundinPDAsandlaptopcomputers,suchasApple'sMacBookandsmalldigitalmusicdevicessuchasiPodsandotherMP3players,aswellasportablegamingdevicesliketheSonyPSPorNintendo'sGameBoyAdvanceSP,wheresmallformfactorsandenergydensityoutweighcostconsiderations.

Thesebatteriesmayalsopowerthenextgenerationofbatteryelectricvehicles.Thecostofanelectriccarofthistypeisprohibitive,butproponentsarguethatwithincreasedproduction,thecostofLi-Polybatterieswillgodown.

Technology

Therearecurrentlytwocommercializedtechnologies,bothlithium-ion-polymer（where"polymer"standsfor"polymerelectrolyte/separator"）.Theyarecalled"polymerelectrolytebatteries".Theideaistouseanion-conductingpolymerinsteadofthetraditionalcombinationofamicroporousseparatorandaliquidelectrolyte.Thispromisesnotonlybettersafety,aspolymerelectrolytedoesnotburnaseasily,butalsothepossibilitytomakebatterycellsverythin,astheydon'trequirepressureappliedto"sandwich"cathode+anodetogether.Polymerelectrolytesealsbothelectrodestogetherlikeaglue.

Thedesignis:

anode（Liorcarbon-Liintercalationcompound）/conductingpolymerelectrolyte-separator/cathode（LiCoO2orLiMn2O4）

Typicalreaction:

∙Anode:

carbon-Li（x）-xLi+-xe−

∙Separator:

Li+conduction

∙Cathode:

Li（1-x）CoO2+xLi++xe−

Polymerelectrolyte/separatorcanberealsolidpolymer（polyethyleneoxide,PEO）plusLiPF6orotherconductingsaltplusSiO2orotherfillerforbettermechanicalproperties（suchsystemsarenotavailablecommerciallyyet）.SomeusemetallicLiastheanode,whereasotherswanttogowiththeprovensafecarbonintercalationanode.

BothcurrentlycommercializedtechnologiesusePVdF（apolymer）gelledwithconventionalsolventsandsalts,likeEC/DMC/DECetc.Thedifferencebetweenthetwotechnologiesisthatone（Bellcore/Telcordiatechnology）usesLiMn2O4asthecathode,andtheother,moreconventionalLiCoO2.

FailedTechnologies:

Other,moreexotic（althoughnotyetcommerciallyavailable）Li-polymerbatteriesuseapolymercathode.Forexample,Moltechisdevelopingabatterywithaplasticconductingcarbon-sulfurcathode.However,asof2005thistechnologyseemstohaveproblemswithself-dischargeandmanufacturingcost.

Yetanotherproposalistouseorganicsulfurcontainingcompoundsforthecathodeincombinationwithanelectricallyconductingpolymersuchaspolyaniline.Thisapproachpromiseshighpowercapability（i.e.lowinternalresistance）andhighdischargecapacity,buthasproblemswithcycleabilityandcost.

PromisingTechnologies:

Thenextgenerationrechargeablebattery—lithium-ionpolymerbattery（LiPB）hasarisentomeetthedemandforincreasedpowerdensityandsmallersizewithsafetyandcostforemostinmind.Untilnow,commonlyusedbatteriesincludethenickel-cadmium（NiCd）andthenickel-metalhydride（NiMH）designtypicalincellularphonesoreventhenewerlithium-ion（Li-ion）batteries,poweringhigher-enddevices.Lithium-ionpolymerbatteries,whenhigh-volumesuppliesbecomeavailable,areexpectedtochallengeandreplaceNiMHandLi-ionbatteriesasthepreferredpowersourceforportableelectronicdevices.LiPBtechnologyaddressesthemostimportantissuesoftherechargeablebatterymarket:

powerdensity,size,designflexibilityandsafety.

Figure2.AtypeofLiPBbatteryschematic

LiPBtechnologyrevealsanexcitingnewbatterystructure,discardingtheconventionalmetal-canassembly.ThemoststrikingdifferencebetweenLiPBandLi-ionisthattheformerusesasolidpolymerelectrolyteratherthanaliquidelectrolytesolution.Thesolidpolymerelectrolyteisphysicallyasolidbutappearstotheionsasaliquidthattheycanpassthrough.Withnoliquidtoescape,thesolidelectrolyteissimplysandwichedbetweenelectrodes

（theanodeandcathodesformedoutofthinsheets）.Thenthecelliscontainedinlaminatedfoilandsealedattheedgestoformanentirebattery.Theresultingcellisextremelythinandasflexibleasarubbermat.Usingasolidelectrolyteisquiteappealingbecauseitisnaturallyspillproof,moreresilientunderpressure,andcapableofbeingengineeredintomostanyshapebecauseathinlaminated

Figure2.LiPBbatterycharge/dischargeprofiles

foilmaterial,ratherthananunyieldingmetalcan,houseseachcell.Therefore,lithium-

ionpolymerbatteriesarelighter,thinner,flexibleandleakresistant—thussafer.

LiPBCharacteristics

ThinandLightweight

LiPBtechnologyoffersthegreatestenergytosize/weightratioforrechargeablebatteries.Batteriesasthinasa0.4mmcreditcardwiththesamepowercapabilitiesasstandardbatterypacksoncellularphoneshaveappeared.Lithium'shighelectrochemicalreactivityhasmadeitdifficulttoworkwithinthepast,yetalsowhyithasheldsomuchpotential.Therefore,LithiumbatteriesaremadefromLithiumionsratherthanLithiuminitsnaturalstate.ThehighenergydensityofLithiummeanslessphysicalmaterialneedstobeused,thusreducingbatterysizeandweight,andmakingitidealforuseincompactelectronicde