Advanced Batteries Materials Science Aspects Robert A. Huggins Advanced Batteries Materials Science Aspects ABC RobertA.Huggins DepartmentofMaterialsScienceandEngineering StanfordUniversity Stanford,CA94305 [email protected] ISBN:978-0-387-76423-8 e-ISBN:978-0-387-76424-5 DOI:10.1007/978-0-387-76424-5 LibraryofCongressControlNumber:2008930484 (cid:176)c SpringerScience+BusinessMedia,LLC2009 Allrightsreserved.Thisworkmaynotbetranslatedorcopiedinwholeorinpartwithoutthewritten permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY10013,USA),exceptforbriefexcerptsinconnectionwithreviewsorscholarlyanalysis.Usein connectionwithanyformofinformationstorageandretrieval,electronicadaptation,computersoftware, orbysimilarordissimilarmethodologynowknownorhereafterdevelopedisforbidden. Theuseinthispublicationoftradenames,trademarks,servicemarks,andsimilarterms,eveniftheyare notidentifiedassuch,isnottobetakenasanexpressionofopinionastowhetherornottheyaresubject toproprietaryrights Printedonacid-freepaper springer.com Preface 1 Introduction Energy is important to all of us, for a variety of reasons, but primarily because it can be useful. It can be found in a number of different forms. One readily recog- nizes the concepts of potential energy and kinetic energy, as well as the chemical energyinfuels,thermalenergyasheat,thekineticenergyinwindandmovingwa- ter, and magnetic and electrical energy in a variety of guises. But energy is often presentinoneform,whereaswewanttouseitinanotherform.Thisrequiressome kind of conversion mechanism or transducer device. Furthermore, energy may be available in amounts and at times and places that are different from those when and wherewewanttoutilizeit.Thus, methods tostoreandtransportenergy from place to place can be of great importance.This text has to do with the storage of energy, and there are a number of different ways in which this can be done. The title indicates that it is about electrochemical energy storage. This may appear to bemisleading, forthereisactuallynosuchthingaselectrochemical energy. What it actually means is that we shall deal with the general topic of the use of electro- chemicalmeanstoconvertbetweentwodifferenttypesofenergy:electricalenergy and chemical energy. This involves the use of electrochemical devices that act as transducers,fortheyconvertbetweenelectricalandchemicalquantities–energies, potentials, and fluxes. Such electrochemical transduction systems are often called galvanic cells, or in more common parlance, batteries. Electrical energy can be stored in electric or magnetic fields; mechanical energy can be stored in devices such as flywheels, and thermal energy can be stored in the form of heat. But the magnitudes of these forms of energy are all relatively small and the methods for their conversion into other forms are relatively unwieldy. Much larger amounts of energy can be present in the form of chemical species. This can be relatively at- tractive, for it can be relatively inexpensive and efficient in terms of the amount of energy stored per unit volume or weight. Thus, storage in chemical form is of- ten a useful intermediate stage, holding energy for later use in other forms, such as electrical, heat, light, or mechanical energy. Chemical reactions can be used to v vi Preface convertthischemicalenergyintomechanicalenergybytheuseofinternalcombus- tion engines, for example. Alternatively, electrochemical systems and devices can beusedtoconvertthischemicalenergyintoelectricalenergy.Amajoradvantageof electrochemicaltransductionmethodsisthattheycanoperateisothermallyandthus avoidtheso-calledCarnotlimitation.Thismakesitpossibletoachievemuchgreater efficiencies than are available by the use of thermal conversion processes.In many cases,electrochemicalcellscanalsobeoperatedinthereversedirection.Thus,itis possible to devise reversible electrochemical systems in which electrical energy is convertedtochemicalenergy(thechemicalsystemischarged),andtheprocesscan laterbereversedtogiveelectricalenergyagain(thechemicalsystemisdischarged). 2 ApplicationsofElectrochemicalEnergyStorage Thereisagreatdealofcurrentinterestinthedevelopmentofbetterelectrochemical systems,orbetterbatteries.Thereareseveralbasicfactorsdrivingthis.Oneofthese is the concern about several important issues related to the environment in which welive. Itisnowincreasinglyclearthatthecombustionoffossilfuelsleadstotheemis- sion of species, sometimes called greenhouse gases, into the atmosphere, and that theiraccumulationleadstoasignificantamountofglobalwarming. Inaddition,theproductsofsuchcombustionprocessescanleadtoenvironmental pollutionandpoisonousatmosphericsmoginurbanareas.Inadditiontoindustrial sources, the use of combustion engines in an ever-increasing number of vehicles makes this problem worse with time. Its seriousness is not the same everywhere, being worse in areas of high population concentration and where the natural pro- cessesthatcausethemotionofairaremostrestricted.Oneofthelocationsinwhich thisproblem isparticularlysevereandinwhichthereisagreatdeal ofpublicand politicalsentimentbehindeffortstowarditsalleviationistheLosAngelesBasinin Southern California. This led to the installation of regulations that limit the emis- sionofspecificgaseousspeciesbyvehiclesinthatlocationanumberofyearsago. Suchregulationshavebecomestricterwithtime,andsimilarrequirementshavealso graduallybecomeadoptedinotherlocations. Because of the poisonous nature of the gases emitted by internal combustion engines,theiruseinsideclosedbuildingshaslongbeenprohibited. There is thus a great incentive to develop alternative methods for vehicular propulsion. One approach to this has been the electric automobile, in which the motive power is supplied by the use of a battery, or perhaps by a nonpolluting hydrogen/oxygenfuelcell,oracombinationofthetwo. In order to provide sufficient range, as well as adequate acceleration, relatively largeamountsofenergymustbeavailableinthevehicle.Thus,theamountofenergy perunitweight,thespecificenergy,isanimportantparameterinsuchapplications. Preface vii As a general rule of thumb, the useful range of a battery-propelled vehicle, in kilometers,isapproximatelytwotimesthespecificenergyofitsbatterysystem,in Whkg−1. Anotherimportantconsiderationis,ofcourse,thecost.Unfortunately,theleast expensivecurrentlyavailablebatteries,baseduponthePb/PbO system,havearel- 2 ativelylowvalueofspecificenergy,some30–40Whkg−1.Thus,electricvehicles usingsuchbatterieshaveusefulrangesofonly60–80kmundernormalconditions. Although there is a great deal of activity aimed at their improvement, alterna- tiveelectrochemicalsystemswithgreaterspecificenergiesarepresentlymuchmore expensive. Even though there has been a considerable amount of interest in the develop- mentofsuchbattery-drivenvehiclesforsomeyears,ithasbecomeobviousthatthe currentstateofbatterytechnologyisnotsufficientlyadvancedforsuchall-electric vehiclestobeprice-andperformance-competitive.Asaresult,suchvehicleshave foundonlyaratherlimitedmarkettodate. More recently, a number of automobile companies have been involved in the development of vehicles propelled by hybrid systems in which a relatively small internalcombustionengineisusedtochargeamodest-sizedelectricbattery,which thenactstoprovidethevehicularpropulsion.Thisapproachhasproventobemuch more attractive than the all-electric vehicle. Although it is only a compromise in terms of the reduction in the use of fossil fuels, the reduced size of the battery in suchsystemsgreatlydecreasesboththeirweightandtheircost. Hybrid vehicles began to be introduced in the USA market in 1999, and their sales have increased rapidly. The first two manufacturers were Toyota and Honda, anddataontheirsalesduringthefirst5yearsareshowninFig.1.Anumberofother Fig. 1 Growth in the sales of hybrid automobiles in the USA in the first few years after their introduction viii Preface manufacturershavebeguntointroducehybridautosrecently,andmorewilldothis withinthenextfewyears. In addition to the internal combustion engine–battery hybrid approach, there is currently a large development effort in several countries aimed toward use of hydrogen-driven fuel cells to provide the primary motive power in vehicles. Be- cause of the fact that the power requirements can vary greatly with time, it may be necessary that an energy storage system also be present. Thus fuel cell–battery hybridcombinationsarealsobeingconsidered. Anadditionalconsiderationariseswhenconsideringhydrogen-poweredvehicles. Thisisthequestionofhowthehydrogen,orhydrogen-producingfuelcanbecarried onboardthevehicle.Thereareseveralapproachestothisproblembeingconsidered, but one of the most desirable would be to carry the hydrogen in the vehicle in the form of a hydrogen-containing solid that would have a high hydrogen density and alsocouldprovidetherequiredhydrogenondemand.Suchmaterialsareoftencalled metal hydrides, and are closely related to the metal hydrides that are used as the reactants in the negative electrodes of the common small hydride/nickel batteries. Thismatterwillbediscussedlaterinthistext. Thistrendtowardhybridvehiclesisalsoleadingtoconsiderationofdualenergy storage systems, for in addition to the obvious desire to optimize the amount of energystoredthereisalsotheneedtohandleveryhighratesofchargeanddischarge. Itisunrealistictoexpectasingletypeoftechnologytobeoptimizedforbothofthese verydifferenttypesofapplication.Thishasledtoresearchanddevelopmentefforts aimedattheveryhighrateandhighcycle-lifeapplications,anddifferentapproaches tothisproblemnowgenerallycarrythelabelssupercapacitorsorultracapacitors. There is an additional factor that is pushing the development of more effective batteries for use in vehicles that relates to the more traditional application of bat- teriesforprovidingpowerforstarting,lighting,andignition–theSLIapplications. Thisisthetrendtowardtheuseofmoreandmoreelectricallypoweredfunctionsin vehicles,suchaselectricwindows,electricallyoperatedseats,radios,etc.Thishas led the automobile manufacturers to move toward higher voltage systems. Some yearsagoautobatteriesprovided6V.Nowtheyprovide12V.Withinthenextfew years auto electrical circuits will operate at 36 V (the batteries will be charged at 42 V). This will be more than just placing three of the current 12-V batteries in series, and changing to quite different chemistries and systems is being actively considered. In addition to the environmentally driven issues that have been discussed here, thereisgrowingawarenessthattheavailabilityoffossilfuelsintheearthislimited. Thishasledtoincreasingpressuretowardthedevelopmentoftechnologiesthatcan reduce the needs for fossil energy. In addition to energy-saving methods, there is increasing interest in methods to enable the effective use of alternate sources of energy, such as solar energy and wind energy in order to reduce the dependence uponfossilfuels. There is another feature of energy supply and consumption that also has to be takenintoconsideration.Thisisthatboththeutilizationandthesourcesofenergy areoftennotuniformwithtime.Asanexample,heatingandairconditioning,aswell Preface ix aslighting,energyrequirementsgenerallyvarywiththetimeofdayaswellaswith weatherconditionsandthetimeoftheyear.Inaddition,itisobviousthattheenergy production of some alternative energy sources, solar and wind energy devices, is intermittent.Theresultisthatthereisaneedformechanismstocoordinatethetime dependenceoftheoutputandthecostofenergysourceswiththatoftheirneeds.The termsloadlevelingandpeakshavingareoftenusedinthisconnection,andwhatis needed,ofcourse,isamechanismforlarge-scalereversibleenergystorageinorder tobettermatchthetimedependenceofenergysuppliesandtheiruse. In addition to these environmental and economic issues, another major driving force that has greatly increased the push toward more effective energy storage de- vicesismarket-driven.Itisthefactthatthenumberofportableelectronicproducts isincreasingveryrapidly;theseincludethefourCs:computers,cellulartelephones, cordlesspowertools,andcamcorders. Itiswellrecognizedthattheirperformanceisalmostalwayslimitedbytheavail- ablepowersource,whichistypicallyasmallbattery.Somesuchbatteriesareelectri- callyrechargeable,whereasothersarenot.Forapplicationinsuchportabledevices theamountofenergyperunitvolume,theenergydensity,andtheamountofpower available per unit volume, the power density, are often the most important criteria insteadoftheweight-relatedspecificenergyandpower. Thesepowersourcesaretypicallymuchsmallerthanthosenecessaryforvehicle propulsion,andthereforethepriceisnotsuchadecisivefactor.Instead,theamount ofenergythatcanbestoredinagivensizeandshapeisoftenparamount.Methodsof fabricationthatcanproducecellsofsmall,andespecially,thinshapeshavebecome increasinglyimportantinthistypeofapplication.Thetermformfactorisoftenused inthisconnection. Also, smaller device sizes make technological problems significantly easier to solve. These factors, as well as the ready marketability of such products, have led toagreatdealofactivityinrecentyearsinanumberofcountries,withlarge-scale production of the more advanced systems occurring first in Japan, but Korea and Chinahavenowbecomemajorplayers.Thecurrentmarketisverylarge–billions ofsmallbatteriesperyear. A further general area of application that provides an increasing market for electrochemical energy storage devices and systems relates to security and safety. There are a number of critical electrically powered systems in which backup en- ergy sources are necessary in order to prevent major problems if the primary, or normal,sourceofenergyisinterrupted.Theseincludelargecomputersystems,data networks,telephonesystems,andemergencylighting. 3 ChangesThatHaveTakenPlaceinRecentYears The application of batteries in these different market segments has been growing ataveryhighrate.Theirrequirementshavebeenputtingever-increasingincentives on the development of better, and lower cost, energy storage devices and systems. x Preface Asortofchicken-and-eggtypeofsituationhasdeveloped,withthemarketdemand- ing better properties, and when this happens it also opens up the opportunity for additionalnewmarkets. Thishasledtoalotofresearchanddevelopmentactivity,andtherehavebeena numberofimportanttechnologicalchangesinrecentyears.Anumberofthesehave notbeenjustincrementalimprovementsinalready-knownareas,butinvolvetheuse ofnewconcepts,newmaterials,andnewapproaches. An important reason for this progress has been the fact that such things as the discoveryoffastionicconductioninsolidsandthepossibilityofsolidelectrolytes, the concept of the use of materials with insertion reactions as high-capacity elec- trodes,andthediscoveryofmaterialsthatcanproducelithium-basedbatterieswith unusuallyhighvoltageshavecausedanumberofpeoplewithbackgroundsinother areasofscienceandtechnologytobedrawnintothisarea.Theresulthasbeenthe infusionofnewmaterials,concepts,andtechniquesintobatteryresearchanddevel- opment,whichusedtobeconsideredonlyasapartofelectrochemistry. Hereisashortpartiallistofrecentdevelopments: • Metalhydrides • Lithium-carbonalloys • Intermetallicalloys • Lithium-transitionmetaloxides • Polymericcomponents,inbothelectrolytesandelectrodes • Liquidelectrodes • Bothcrystallineandamorphoussolidelectrolytes • Organicsolventelectrolytes • Mixed-conductormatrices • Protectivesolidelectrolyteinterfacesinorganicelectrolytesystems • Useofsoftchemistrytoproducenonequilibriumelectrodecompositions • Newfabricationmethods,andnewcellshapesandsizes • Fabricationoflithiumbatteriesinthedischargedstate 4 ObjectivesofThisBook This book deals with the Materials Science principles that underlie the behavior of advanced electrochemical storage systems, i.e., batteries. It focuses upon basic principlesandhasatutorialflavor,andthusisquitedifferentfromtheseveralother available books that deal with battery systems, which generally are more directed towardthedetaileddescriptionofcurrentbatteriesandtheirproperties. To be most useful to those interested in either understanding or contributing to thedevelopmentofthisrapidlymovingareaoftechnology,emphasisisplacedupon underlyingprinciplesthatareapplicableacrossthespectrumofmaterialsthatareof interestaselectrochemicallyactivecomponentsinadvancedbatteries:theelectrodes andtheelectrolyte. Preface xi It will be readily recognized that the content and approach in this book, based uponmaterialsscience,isalsoquitedifferentfromwhatisfoundinmanybookson electrochemistry.Insomeareasitisbasedupondifferentconcepts,andutilizesdif- ferentthinkingtools.Insometopicareastheterminologythatwillbeusedwillalso bedifferentfromthatwhichischaracteristicofmuchofthecurrentelectrochemical literature. Inaccordancewiththedesiretofocusongenericprinciplesapplicabletoavari- etyofthemoreadvancedmaterialsandsystems,noattemptwillbemadetoprovide acompletedescriptionofallcurrentmaterialsandbatteries.Indeed,severalmajor batterytypeswilldeliberatelynotbediscussed.Anobviousexampleisthecommon lead-acidbattery,whichactuallyhasthelargestmarketofallbatterysystemsatthe presenttime.Inadditiontotheomissionofthistraditionalbatterysystem,littleor no attention will be given to topics such as electron transfer kinetics at interfaces, andthenatureoftheelectricaldoublelayerattheinterfacebetween aliquidelec- trolyteandasolidelectrode.Thesearebothtopicsofcentralinterestinconventional electrochemistry,buttheyhaveverylittlerelevancetothekeyquestionsuponwhich thistextwillfocus. Significant changes and developments have occurred in recent years. An espe- ciallyimportantchangeinthinkinginthisareahasbeentherecognitionoftherole playedbyphenomenainsidesolidelectrodematerials,andtheirinfluenceuponthe interfacialconditions,andthusupontheelectrochemicalpotentialoftheelectrode. In many cases solid-state reactions play a critical role in determining two of the mostimportantparametersofanelectrochemicalenergystoragesystem:thepoten- tialsandthecapacitiesofelectrodes.Thekineticbehaviorofsolid-statereactionsis oftenveryimportantindeterminingthekineticparametersoftheelectrodereactions andthusoftheobservedoverallcellbehavior. This recognition of the importance of solid-state properties and behavior upon the characteristics of electrochemical cells, and the major changes in battery tech- nologyinrecentyearshaveledtotheentryofpeopleintothisareawithbackgrounds andapproachesbaseduponmaterialssciencethatarequitedifferentfromthoseof traditionalelectrochemistry. Sincethismonographisprimarilyintendedtoserveaneducationalroletherehas been no attempt to provide a thorough review of the research literature, although references to some of the early and definitive work are cited in some chapters. Reference is also made to review papers in the literature that may be helpful in somecases. 5 ThinkingTools Itisoftenveryusefultohavesimpletoolsorothermethodsavailablethatprovide insight,perhapsonlyqualitativeinsomecases,intotheinfluenceofvariousfactors in complex materials systems. In some cases, these involve graphical representa- tionsofparametersthatsummarizetheresultsofeitherexperimentalortheoretical