ABSTRACT
Electrochemical energy storage technologies play a key role in the movement of the worldtowardgreenenergyandef›cientusageofelectricityinmicro-andsmartgrids.Thesetechnologiescancompensateforthegapsbetweendemandandsupply ofelectricityandbeusedinthetransportationsector,providingareliable,low-cost, andcleanenergysource.Metal-airbatteries(MABs)representthemostef›cient energy storage technology with high round-trip ef›ciency, a long life cycle, fast responseatpeakdemand/supplyofelectricity,anddecreasedweightduetotheuse ofatmosphericoxygenasoneofthemainreactants.ThemostdevelopedMABs withtechnicalpotentialandmarketperspectivesarezinc-,lithium-,aluminum-, andmagnesium-airbatteries(ZnAB,LiAB,AlAB,MgAB).1-3 However, reasonable performancewasachievedonlyforrechargeableZnABs.MgABsandAlABsare basicallyusedasprimarybatteries.MABsarenowconsideredthebestpotential substitutesforcurrentlyusedrechargeablenickel-cadmium(NiCd),nickelmetal hydride(NiMH),andLi-ionbatteriesfordifferentapplicationsduetosomelimitations in terms of the safety (Cd for NiCd batteries and Li ion), raw material resources (lithiumforLi-ionbatteries),costef›ciency,andweight(Li-ion,NiMHbatteries). MABshavehighenergydensitycomparedtootherbatteries,lowcost,possiblefuel recycling, long shelf life, no ecological issues, and »at discharge voltage.1-5
The main technical characteristics of MABs are outlined in Table 12.1.1 The comparisonofemerging(e.g.,LiAB)andestablished(e.g.,ZnAB)MABswithconventionalrechargeablebatteriessuchasLi-ion,NiCd,andNiMHshowstheadvantage ofZnABsandLiABsintermsoftheirbetterbalanceofenergydensityandspeci›c energy(Figure12.1).5 LiABs and MgABs have the maximal theoretical speci›c energydensity,13,000and6,800Wh/kg,respectively(forcomparison,gasoline, e.g.,hasanenergydensityof13,000Wh/kg,seeRef.[2]).However,thesebatteries havesomelimitations,forexample,LiABshaveunstableanodesthatreactwithan electrolyte,incompletedischargeduetodepositionofdischargedproductsinporous cathodes, low cycle life, insuf›cient electrical ef›ciency because of the higher overpotentialatthechargingmodethanthatofatthedischarging,formationoflithium carbonatesoralkylcarbonates,andsafetyissues.1,2,6-10 MgABs could be rechargeablewithlimitedcyclelife,butAlABsarenotrechargeable.Themainchallengesof
TABLE 12.1 Technical Characteristics of MABs
MgABs and AlABs are low durability of their anodes, low irreversibility, and high self-discharge rate.1-4 ZnABs have less speci›c energy density than LiABs, MgABs, and AlABs but have higher volumetric energy density. They are mostly practically used for different applications due to their lower cost, longer cycle life, absence of safety issues, and unrestricted outdoor usage.11-17
The main components of MABs are air cathode (air-breathing bifunctional electrodes [BEs]), anodes, electrolytes, and separators, which mainly determine battery cycle life and cost ef›ciency. Therefore, the main focus in this chapter will be on these components, their current status and perspectives. ZnABs also have technical issues related to a low durability of BEs at high oxygen evolution potentials, carbonation of alkaline electrolytes for aqueous batteries, zinc dendrite formation on anodes at the charging mode, nonuniform zinc dissolution, and low zinc solubility in electrolyte.1-4,11-17 The most successful solutions to these issues were recently found in the rechargeable ZnABs developed by Energy Storage (EOS),18,19 ReVolt,20,21 and Fluidic Energy22 with a long cycle life. EOS’s ZnAB Aurora with neutral aqueous electrolyte18 demonstrated stable performance during 5000 cycles without degradation of its BEs.19 It is very important that usage of the stable and cheap neutral aqueous electrolyte allowed for EOS to avoid the common problems of carbonation of the electrolyte for alkaline MABs. Fluidic Energy developed a »ow ZnAB with ionic liquid (IL) electrolyte with an energy density 11 times higher than that of a Li-ion battery at 3 times less cost.21 An IL, in contrast to an aqueous alkaline electrolyte, does not evaporate at the operation temperature of ZABs, is stable (without alkaline electrolyte carbonation issues), is conductive, and has a wider electrochemical
window (water decomposes at the potential over 1.23 V).22 ZnABs are commerciallyavailablefordifferentapplications.Thesebatteriesconsistofinexpensiveand availablefuel-zincincontrasttoexpensivenickelorlithium(Zn-US$2/kg;NiUS$21.4/kg23; and Li-US$8/kg24,25). World zinc resources are estimated to be 1.8 gigatons,with200megatonseconomicallyavailablein2008.26 The development of aircathodesinthe1970sledtosigni›cantimprovementsinZnABsperformance, resulting in low operating costs and cell weight.
