Metal fluorides as electrode materials.docx

Metal fluorides as electrode materials.docx

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Metalfluoridesaselectrodematerials

1.FIELD

Thepresentinventionrelatestorechargeableelectrochemicalenergystoragesystems,particularlytosuchsystems,suchasbatterycells,comprisingmaterialscapableofreversiblytakingupandreleasinglithiumionsasameansofstoringandsupplyingelectricalenergy.Morespecifically,theinventionrelatestotheformationandutilizationofnanostructuretransitionmetalfluoride:

carboncomposites,ornanocomposites,asactiveelectrodecomponentmaterialsinfabricatingelectrochemicalcells,suchaslithiumbatterycells,capableofexhibitinghighspecificcapacityathighrechargerates.

2.BACKGROUND

TheLi-ionbatteryisthepremierehigh-energyrechargeableenergystoragetechnologyofthepresentday.Unfortunately,itshighperformancestillfallsshortofenergydensitygoalsinapplicationsrangingfromtelecommunicationstobiomedical.Althoughanumberoffactorswithinthebatterycellcontributetothisperformanceparameter,themostcrucialonesrelatetohowmuchenergycanbestoredintheelectrodematerialsofthedevice.

Duringthecourseofdevelopmentofrechargeableelectrochemicalcells,suchaslithiumandlithium-ionbatterycellsandthelike,numerousmaterialscapableofreversiblyaccommodatinglithiumionshavebeeninvestigated.Amongthese,occlusionandintercalationmaterials,suchacarbonaceousandgraphiticcompounds,andtransitionsmetaloxidespinels,haveprovedtobeparticularlywell-suitedtosuchapplications.However,evenwhileperformingreasonablywellinsuchrecyclingelectricalstoragesystemsofsignificantcapacity,manyofthesematerialsexhibitdetrimentalproperties,suchasmarginalenvironmentalcompatibilityandsafety,whichdetractfromtheultimateacceptabilityoftherechargeablecelldevices.Inaddition,someofthemorepromisingmaterialsareavailableonlyatcoststhatlimitwidespreaduse.

Materialsofchoiceinthefabricationofrechargeablebatterycells,particularlyhighlydesirableandbroadlyimplementedLi-ioncells,haveforsomeconsiderabletimecenteredupongraphiticnegativeelectrodecompositionswhichproviderespectablecapacitylevelsintherangeof300mAh/g.Complementarypositiveelectrodematerialsinpresentcellscomprisethelesseffectivelayeredintercalationcompounds,suchasLiCoO2whichgenerallyprovidescapacitiesintherangeof150mAh/g.Alternativeintercalationmaterials,suchasLiNiO2andLiMn2O.sub.4,havemorerecentlygainedfavorintheindustry,since,althoughexhibitingnogreaterspecificcapacity,thesecompoundsareavailableatlowercostand,further,provideagreatermarginofenvironmentalacceptability.

Duetotheincreasingdemandforevermorecompactelectricalenergystorageanddeliverysystemsforallmannerofadvancingtechnologies,frombiomedicaltotelecommunications,thesearchcontinuesforbatterycellmaterialscapable,ontheonehand,ofprovidinggreaterspecificcapacityoverwiderrangesofcyclingspeeds,voltages,andoperatingtemperatureswhile,ontheotherhand,presentingfewerenvironmentalhazardsandgreateravailabilityatlowerprocessingandfabricationcosts.Searchesformoreeffectivepositiveelectrodematerialsinparticularhavebecomefar-reachingwithattentionturningmorefrequentlytotheabundantlowertoxicitytransitionmetalcompounds,whicharetypicallyaccessibleateconomicalcosts.

Inthislatterrespectcompoundsofiron,e.g.,ironoxides,attractedsomeattention.However,althoughexhibitingelectrochemicalactivity,ironoxideswerefoundtofunctionappropriatelyonlyatvoltageswhicharetoolowforpracticalimplementationinrechargeablelithiumandlithium-ionbatterycells.

Uponfurtherconsiderationoftheeconomicadvantagespossiblyattainableintransitionmetalcompounds,interestshiftedtoexaminationofthemoreactivefluoridecompounds.Investigationsintosuchuseofthesefluoridesconfirmed,however,that,whiletheopenstructuresofthetransitionmetalfluoridessupportthegoodionicconductivityessential,inpart,forusefulelectrodeperformance,thelargebandgapinducedbythehighlyioniccharacterofthemetal:

halogenbondresultsinpoorelectronicconductivity.Withoutthislatteressentialconductivepropertytocomplementprovenionicconductivity,thetransitionmetalfluorideswereconsideredvirtuallyuselessaslithiumbatteryelectrodematerials.

Despitetheinconsequentialperformanceofthetransitionmetalfluoridesintypicalrechargeablecellfabrications,thetheoreticalpromiseofoutputvoltagesintherangeof3V,duetothehighionicityofthecompoundbonds,promptedsomefurtherinvestigationsintometalhalidesforuseinelectrodecompositions.Increasingtheelectricalconductivityofirontrifluoride（FeF3）wasattemptedbyincorporatingitinanelectrodecompositioncomprisingtheadmixtureofabout25partsacetyleneblackto70partsofFeF3.Araietal.,68J.POWERSOURCES716-719（1997）.Theperformanceofsuchacell,despitetheimpracticallylowcharge/dischargeratewhichextendedovera60hourcycleperiod,wasmarginalatadischargecapacityover4.5to2.0Vofonlyabout80mAh/gvis-a-visatheoretical（letransfer）capacityof237mAh/g.Subsequentindependentfabricationandtestingofsimilarbatterycellsatmorerealistic4hourcyclerateswouldyieldnomorethanabout50mAh/g.

Whatisneededareelectricalenergy-storageanddeliverysystemsthatprovidehighspecificcapacityoverwidecycling-speedranges,voltages,andoperatingtemperatures;areenvironmentallyfriendly;andalsoarereadilyavailableatpracticalprocessingandfabricationcosts.

3.SUMMARY

Inoneembodiment,theinventionprovidesthemeansforrealizingthepotentialimprovementinrechargeableelectrochemicalbatterycellsystemswhichtakesadvantageofthelowcostanddesirableenvironmentalcompatibilityofmetalfluoride-basedsystemstoachievecellsprovidingstableandsurprisinglyhighcapacityatrapidcycleratesoverbroadvoltageranges.

Inoneembodimentoftheinvention,theheretoforeunacceptablylowlevelofelectronicconductivityexhibitedbyelectrochemicalcellelectrodecompositionscomprisingmetalfluorideshasbeenresolvedbyuseofcarbonmetalfluoridenanocompositesreferredtohereinas"carbonmetalfluoridenanocomposites"（"CMFNCs"）inthepositiveelectrodeoftheelectrochemicalcell,suchasinrechargeablebatteries.

Inanotherembodiment,theinventionisdirectedtonanocompositescomprisingalithiumfluoridecompoundhereinreferredtoas"lithiumfluoridecompoundnanocomposites"（"LFCNCs"）.LFCNCsareusefulasthepositiveelectrodematerialofelectrochemicalcells,suchasrechargeablebatteries.Inoneembodiment,thelithiumfluoridecompoundnanocompositesoftheinventionoptionallycompriseanelementalmetal.Inanotherembodiment,thelithiumfluoridecompoundnanocompositesoftheinventionoptionallycompriseelementalcarbon.Instillanotherembodiment,thelithiumfluoridecompoundnanocompositesoftheinventioncomprisebothanelementalmetalandelementalcarbon.

4.BRIEFDESCRIPTIONOFTHEFIGURES

Theseandotherfeatures,aspects,andadvantagesofthepresentinventionwillbecomebetterunderstoodwithregardtothefollowingdescription,appendedclaims,andaccompanyingfigureswhere:

FIG.1depictsoverlaidrepresentationsofXRDtracesofcarbonmetalfluoridenanocompositesamplesobtainedfromvaryingdurationsofhighenergyimpactmilling;

FIG.2depictsasectionthetracesofFIG.1inexpandedscalehighlightingthecharacteristicbroadeningofthemajortracepeakasafunctionofdurationofsuchmilling;

FIG.3isagraphplottingthevariationincrystallitesizeofcarbonmetalfluoridenanocompositematerialasafunctionofdurationofsuchmilling;

FIG.4isagraphplottingthecharacteristicprofileofrecyclingvoltagebetween4.5Vand2.0Vat22°C.overacyclingperiodofabout70hoursinacellhavingapositiveelectrodecomprisingasimple,unmilledmechanicalmixtureofnanostructuretransitionmetalfluorideandcarbonparticles;

FIG.5isanoverlaygraphplottingthecharacteristicprofilesofrecyclingvoltagebetween4.5Vand2.0Vat22°C.overacyclingperiodofabout70hoursincellsembodyingthepresentinventionandcomprisingcarbonmetalfluoridenanocompositematerialobtainedfromhighenergyimpactmillingfor10and30minutes,respectively;

FIG.6isanoverlaygraphplottingthecharacteristicprofilesofrecyclingvoltagebetween4.5Vand2.0Vat22°C.overacyclingperiodofabout70hoursincellsembodyingthepresentinventionandcomprisingcarbonmetalfluoridenanocompositematerialobtainedfromhighenergyimpactmillingfor120and240minutes,respectively;

FIG.7isanoverlaygraphplottingthevariationincellspecificcapacityovertheindicatedcyclingperiodinthecellembodimentsofFIGS.4-6;

FIG.8isanoverlaygraphplottingthecomparativevariationsincarbonmetalfluoridenanocompositecrystallitesizeandcellspecificcapacityasafunctionofhighenergyimpactmillingtimeincellscomprisingoneembodimentofthepresentinvention;

FIG.9isanoverlaygraphplottingthecomparativevariationsincarbonmetalfluoridenanocompositecrystallitesizeasafunctionofhighenergyimpactmillingtimefornanocompositescomprisingvariouscarbontypes;

FIG.10isanoverlaygraphplottingthecomparativevariationsincellspecificcapacityasafunctionofhighenergyimpactmillingtimeofcarbonmetalfluoridenanocompositecellelectrodematerialcomprisingvariouscarbontypes;

FIG.11isanoverlaygraphplottingthecomparativevariationsincellspecificcapacityovertheindicatednumberofcyclesasafunctionofcyclingrateincellsembodyin