Seite - 112 - in Emerging Technologies for Electric and Hybrid Vehicles

Energies 2017,10, 1314 3.1. State of theArt—Anode Twentyyearsagographite (372mAh/g[34])wasfirst commercialized[35]asanodematerial ina lithium-ionbatteryanduptonowit is stillbeingused inmost lithium-ionbatteries. Its lowcost,good electrochemicalperformance, lowvolumeexpansionduringcharginganddischargingaswellas that it isabundantlyavailable,explains thewidelyaccepteduseofgraphiteasanodematerial [33,35,36]. Manyresearcheffortsallowedtooptimize thismaterial resulting it isalmost reaching itsmaximum theoretical capacityandonly incremental improvementscanbeexpected[29].However,byadding small amountsofmetalswithhigh theoretical energydensities, suchas silicon (4200mAh/g [37]), the overall energydensity can be increased [38]. Addinghigh concentrations of these additional components causenumerousproblemssuchasvolumeexpansionsup to300%aswell as reduced lifetimedespite thecurrentnumerousresearchefforts, forexampleusingsiliconasnano-particlessuch as in [37]. OtheroftenusedanodesmaterialsusedarelithiumalloyedmetalswithasmostpopularLi4Ti5O12 LTO(175mAh/g[35]).Morenoblemetalsareusedresulting inahigherprice thangraphite.Other disadvantagesare its lowerenergycapacityandreducedcellvoltagecomparedtographite.However itsexceptionalgoodstabilityover its lifetimemakes it the idealanodes inspecificcasesexplaining its wideusage. Anoverviewof the twomostusedanodematerials is showninTable3 [34,35]. Table3.Anodematerials—Overviewincludingspecificenergydensity, costandlifetime. AnodeMaterial EnergyDensity Cost Lifetime(mAh/g) Graphite 372 Medium Medium Li4Ti5O12 (LTO) 175 High High 3.2. State of theArt—Cathode Theselectionof themostsuitedcathodematerial is stronglydependentontheapplication itself. Aselectionhas tobemadeofwhichkeyproperty is themost important foranapplication. Thekey propertiesofabatteryare: energydensity,powerdensity, costandlifetime.Anoverviewof themost usedcathodematerialscanbefoundinTable4 [29,33,36,38–45]. Table4.Cathodematerials—Overviewincludingenergydensitycostandlifetime. CathodeMaterial EnergyDensity Cost Lifetime(Wh/kg) LiCoO2 (LCO) 546 Medium Medium LiMn2O4 (LMO) 410–492 Low Low LiNiMnCoO2 (NMC) 610–650 High High LiFePO4 (LFP) 518–587 Medium High LiNiCoAlO2 (NCA) 680–760 High Medium TheoldestcommerciallyusedelectrodesareLiMn2O4 (LMO)dueto the lowcost,however the lifetime is limitedwhich is considered tobe thebiggest disadvantagebut they are still frequently used. LiCoO2 (LCO)anotheroldelectrode,characterizedwithamediumcostandhighenergy,has somesafetydrawbacksbut is stillusedfrequently. LiNiMnCoO2 (NMC),acombinationofLCO,LMO andnickel, isgainingpopularitydueto itshigh lifetimeaswellas itshighenergydensity. Theexact mixtureofNi,MnandCowill define thepropertyof the cathodeofwhichavariety exist suchas NMC(1:1:1),NMC(5:3:2), ... The trend is touseNi richNMCsince thisgivesan increasedenergy density. It ismainlyusedwherecost is less important. LiFePO4 (LFP)hasexcellent lifetimeproperties andis frequentlyused incombinationwithanLTOanodetogetanexcellentoverall lifetimeof the 112