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2.TypesofEESSsandtheirworkingprinciplesTheworkingmechanismofanyEESSreliesonaninherentpotentialdifferencebetweentwoelectrodesknownastheoperatingvoltage.Theoperatingvoltageofthedeviceisdictatedbythedifferencesinredoxpotentialofthepositiveandnegativeelectrode.Thepotentialdifferenceisusedtodriveelectrochemicalreactionsoneitherelectrodewhentheyareconnectedthroughanexternalcircuit.Thiscreatesaflowofelectronsfromthenegativeelectrodetothepositiveelectrode.Theflowofelectronsinducesoxidationreactionsonthenegativeelectrode(anode)andreductionreactionsonthepositiveelectrode(cathode)whendischarging.Thechargedelectrodesarebalancedbyaconcomitantflowofcounter-ions.EESSsaregroupedintoanumberofdifferentcategoriesdependingonthecompositionoftheelectrodes,thecounter-ions,andthenatureoftheredoxreactions(Fig.1).2.1Solidelectrodebatteries2.1.1Metal-ionbatteryworkingprinciple.Batteriesoperatewithaconstantvoltagedefined,approximately,bythepotentialdifferencebetweentheanodeandcathode.Becauseofthis,Inagalvanostaticcharge/dischargeexperimentthepotentialoftheelectrodeordeviceideallyremainsconstantuntiltheactivematerialhasbeenfullyreduced(oxidized).Inacyclicvoltammogramexperiment,oneobservesareversible,sharpredoxpeakwhenaredoxeventoccurs(insetFig.1a–c).Metal-ionbatteriesarethemostcommontypeofEESSs.Theyaretypicallycomposedofananode(negativeelectrode),acathode(positiveelectrode),electrolyte(eitheraqueous,organic,solid-state,18orpolymeric10,19),aseparator(topreventshortcircuiting),currentcollectors(tocollectchargeateachelectrode),andacellcasing(tokeepthecomponentstogetherandpreventexposuretotheexternalenvironment).Metal-ionbatteriesareusedforawidevarietyofbothportableandstationaryapplicationsforeitherprimaryorback-uppower.Inmetal-ionbatteriesthechargedanodesandcathodesarebalancedbythemetalionina‘rocking-chair’typemechanism(Fig.1a).Thisisastrictrequirementimposedbythedefinitionofmetal-ionbatteriesthatshouldbeclearlydistinctfromdual-ionbatteriesdescribedbelow.Metal-ionbatteriescanbeconstructedwithrelativelysmallamountsofelectrolytebecausetheionsbalancingthechargeatoneelectrodeareconstantlybeingreplenished.Additionally,metal-ionbatteriesareveryattractivecandidatesforusewithsolid-stateelectrolytesbecausethemobilityofonlyoneionneedstobeconsidered.2.1.2Dual-ionbatteryworkingprinciple.Inadual-ionbatterythechargedanodesandcathodesarebalancedbycationsandanionsrespectively(Fig.1b).Dual-ionbatteriesencompassawidevarietyofelectrolytesandelectrodes.Theanodesrangefromnegativecharge-acceptingcompoundstoreducedmetalsandinorganicmaterials.Thecathodescanalsobeawidevarietyofmaterialsaslongastheyarebalancedbyanionswhencharged.Wewilladheretothisdefinitionthroughoutthisreview,butwenotethatothershavereferredtothesesystemsasorganicbatteries,metalorganicbatteries,andradicalpolymerbatteries.14,22Althoughthesetermsmaybeusedtodescribetheelectrodes,theconventionofnamingsolidelectrodebatteriesbasedonthemobilecounter-ionsisupheldwiththisnomenclature.2.1.3Performancemetricsofsolidelectrodebatteries.Anumberofperformancemetricsneedtobeconsideredforthedevelopmentofelectrodematerialsforsolidelectrodebatteries.Theseperformancemetricscanbeusedtoestimatetheoverallperformanceofthedevice(Tables1–7).Thetheoreticalcapacitance(Ctheor)ofamaterialisthemaximumamountofchargeamaterialcanholdwithrespecttoitsmass.ItistypicallyreportedinmAhgÀ1andiscalculatedusingeqn(1):Here,nisthemaximumnumberofchargesthecompoundcanaccept(orgiveup),FisFaraday’sconstant,andMisthemolecularweightofthecompoundingmolÀ1.Typically,theCtheorisusedtoassesshowwellthematerialcouldperformunderoptimizedconditions.IftheCtheorisreached,thenitisexpectedthattheelectrodecannotacceptanymorecharge.Thespecificcapacity(Csp)isthemeasuredcapacityoftheelectrodeataspecificcurrentdensityforeitherchargingordischarging.TheCspisreportedinmAhgÀ1andbymeasuringCspatdifferentrates(usuallyreportedasaC-rate,where1CistheamountofcurrentitwouldtaketocollectthetotalchargeoftheCtheorin1hour)theratecapabilitiesoftheelectrodecanbedetermined.TheCspistypicallycalculatedfromgalvanostaticcharge/dischargecurvesusingeqn(2):Here,iisthecurrentinmilliamperes,∆tisthetimeofdischarging(charging)inseconds,andmisthemassoftheactivematerialingrams.IftheCspatlowandhighratesaresimilar,itcanbesaidthattheelectrodehashighratecapabilities.Thistypicallydependsontheelectrontransferkineticsofthecompound,andtheelectronicandionicconductivityoftheelectrodeandelectrolyte.Thecoulombicefficiency(CE)ismeasuredbydividingtheCspfordischargingbytheCspforcharging.Thisprovidesinsightintothereversibilityoftheredoxreactionsandindicateswhetheranysidereactionsoccurwiththeelectrodeandelectrolyte.TheCEisagoodindicatorofwhetherastablesolidelectrolyteinterface(SEI)isformedinthechargingcyclesandifthematerialitselfwillbestableuponextendedcycling.IftheCEislowinthefirstchargingcyclesbutincreasestoB100%afterwards,itistypicallyattributedtoSEIformation.Thecyclingstabilityisanimportantparameterthatquantifiestheretentionofcapacityuponcharginganddischargingtheelectrodemultipletimes.Usuallythismeasurementisperforme