applied sciences
Review Recent Progress of Metal–Air Batteries—A Mini Review
Chunlian Wang 1, Yongchao Yu 2, Jiajia Niu 3, Yaxuan Liu 2, Denzel Bridges 2, Xianqiang Liu 3, Joshi Pooran 4, Yuefei Zhang 3 and Anming Hu 1,2,*
1 Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China 2 Department of Mechanical, Aerospace and Biomedical Engineering, University of Tennessee, Knoxville, TN 37996, USA 3 Institute of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124, China 4 Oak Ridge National Lab, Oak Ridge, TN 37831, USA * Correspondence: [email protected]
Received: 3 June 2019; Accepted: 6 July 2019; Published: 11 July 2019
Featured Application: This paper can provide the basic knowledge on metal-air batteries for beginners and relevant comprehensive review for researchers.
Abstract: With the ever-increasing demand for power sources of high energy density and stability for emergent electrical vehicles and portable electronic devices, rechargeable batteries (such as lithium-ion batteries, fuel batteries, and metal–air batteries) have attracted extensive interests. Among the emerging battery technologies, metal–air batteries (MABs) are under intense research and development focus due to their high theoretical energy density and high level of safety. Although significant progress has been achieved in improving battery performance in the past decade, there are still numerous technical challenges to overcome for commercialization. Herein, this mini-review summarizes major issues vital to MABs, including progress on packaging and crucial manufacturing technologies for cathode, anode, and electrolyte. Future trends and prospects of advanced MABs by additive manufacturing and nanoengineering are also discussed.
Keywords: metal–air batteries; laser processing; 3D printing
1. Introduction
1.1. Market Demand and Technical Tendencies With the continued growth of the global economy, the demand for energy has significantly increased. Unfortunately, Earth’s conventional non-renewable energy resources, such as coal, oil, and natural gas, are limited. Hence, the development of new energy devices is important for a sustainable society. Innovative biofuel batteries, supercapacitors, and metal–air batteries are among the most suitable candidates to meet the energy storage demand [1–6]. Among the various power storage devices currently on the market, lithium-ion batteries (LIBs) have the best performance. However, 1 it is still a challenge to achieve high capacity (>200 mA h g− ) in LIBs and to meet safe energy storage requirements for electric vehicles [7,8]. Recently, MABs attracted significant attention as they can operate in an open-air atmosphere. MABs consist of metal anodes and an air cathode. The MAB cathode uses oxygen from ambient air, which leads to significant battery weight reduction, which has unprecedented advantages for many applications. Compared to other batteries, especially Lithium-ion batteries, which currently dominate the market share, MABs are cheap, because the cathode source (oxygen from air) is abundant and the anode can be made using low-cost metals, such as, Al, Zn, Fe. Figure1 shows the application of MABs as the energy storage system for various technologies. MABs
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technologies. MABs are attractive not only as compact power sources for portable electronics and areelectric attractive vehicles not onlybut also as compact as compelling power sourcesenergy fortransfer portable stations electronics or energy and electricstorage vehiclesdevices butto manage also as compellingenergy flow energy among transfer renewable stations energy or energy generators, storage su devicesch as towind manage turbines energy and flow photovoltaic among renewable panels, energyelectric generators, grids and end-users. such as wind turbines and photovoltaic panels, electric grids and end-users.
FigureFigure 1.1. ApplicationsApplications ofof metal–airmetal–air batteriesbatteries asas energyenergy sourcesource andand storagestorage systems.systems.
TheoreticalTheoretical energyenergy densitydensity isis anan importantimportant factorfactor inin evaluatingevaluating thethe performanceperformance ofof variousvarious batterybattery configurations.configurations. Figure2 2 shows shows theoreticaltheoretical energyenergy density,density, specific specific energy, energy, and and nominal nominal cell cell voltagevoltage ofof didifferentfferent metal-air metal-air batteries batteries (MABs) (MABs) [ [9].9]. AsAs oxygen,oxygen, directlydirectly suppliedsupplied fromfrom thethe surroundingsurrounding environment,environment, isis involvedinvolved inin thethe cathodecathode asas anan oxidantoxidant duringduring thethe dischargedischarge period,period, MABsMABs showshow considerablyconsiderably higherhigher energyenergy density.density. Although,Although, theoretically,theoretically, lithium–airlithium–air batteriesbatteries (LABs)(LABs) oofferffer thethe 1 bestbest combinationcombination ofof thethe highesthighest theoreticaltheoretical energyenergy densitydensity (5928(5928 WhWhkg kg−−1)) andand highhigh cellcell potentialpotential (nominally(nominally 2.962.96 V),V), iron–airiron–air batteriesbatteries (FABs)(FABs) possesspossess thethe smallestsmallest theoreticaltheoretical energyenergy densitydensity andand cellcell voltagevoltage (nominally(nominally 1.281.28 V).V). Al-,Al-, Zn-,Zn-, andand Fe–airFe–air batteriesbatteries areare alsoalso thethe researchresearch hotspotshotspots becausebecause ofof economiceconomic andand safetysafety considerations.considerations. InIn thethe presentpresent paper,paper, aluminum–airaluminum–air batteriesbatteries (AABs),(AABs), zinc–airzinc–air batteriesbatteries (ZABs),(ZABs), iron–airiron–air batteriesbatteries (FABs),(FABs), andand lithium–airlithium–air batteriesbatteries (LABs)(LABs) havehave beenbeen reviewedreviewed withwith aa focusfocus onon workingworking principleprinciple andand devicedevice configuration,configuration, andand performanceperformance progress.progress. InIn addition,addition, majormajor technologytechnology barriersbarriers havehave beenbeen identified,identified, and and possible possible solutions solution discussed.s discussed. Emerging Emerging advanced advanced manufacturing manufacturing methods, methods, such such as 3D as printing3D printing and laserand laser processing processing techniques, techniques, for the for development the development a high-performance a high-performance rechargeable rechargeable MABs, haveMABs, also have been also discussed. been discussed.
1.2. Working Principles 1.2. Working Principles TheThe workingworking principleprinciple ofof MABsMABs didiffersffers fromfrom thatthat ofof traditional traditional ionic ionic batteries. batteries. TheThe traditionaltraditional ionicionic batteriesbatteries involveinvolve thethe transformationtransformation ofof metallicmetallic ionsions fromfrom thethe anodeanode toto thethe cathode.cathode. InIn MABs,MABs, metalsmetals oror alloysalloys transformtransform toto metallicmetallic ionsions atat anodeanode andand oxygenoxygen transformstransforms toto hydroxidehydroxide ionsions atat thethe cathode.cathode. Figure Figure 33 showsshows the operation of of a a MAB MAB in in aqueous aqueous or or non-aqueous non-aqueous electrolyte electrolyte medium. medium. In Inan an aqueous aqueous electrolyte electrolyte system, system, oxygen oxygen diffuses diffuses in intoto batteries batteries through through the the gas didiffusionffusion layerlayer andand transformstransforms intointo receivingreceiving electronselectrons formingforming oxygenoxygen anions.anions. In aa non-aqueousnon-aqueous electrolyteelectrolyte system,system, oxygenoxygen receivesreceives electronselectrons andand transformstransforms intointo oxygenoxygen anion.anion. MetalsMetals releaserelease electrons,electrons, transformtransform toto Appl. Sci. 2019, 9, 2787 3 of 22 Appl.Appl. Sci.Sci. 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 33 ofof 2222 metallicmetallic ions ions andand dissolvedissolve intointo electrolytes.electrolytes. TheseThese processesprocesses willwill be be reversible reversible during during a a chargingcharging procedureprocedure ofof aa rechargeablerechargeable MAB.MAB.MAB.
FigureFigure 2.2. TheoreticalTheoretical specific specificspecific energies, energies, volumetric volumetric energy energy densities, densities, and and nominal nominal battery battery voltages voltages of of variousvarious metal–airmetal–air batteriesbattebatteriesries (MABs)(MABs) [[9].[9].9].
FigureFigure 3.3. SchematicSchematic diagrams diagrams of of MABs MABs working working principles principles for for ( (aa)) non-aqueous non-aqueous electrolyte, electrolyte, and and ( (bb)) aqueousaqueous electrolyte. electrolyte. For MABs, oxygen and metals participate in electrochemical reactions. Specific reaction formulas ForFor MABs,MABs, oxygenoxygen andand metalsmetals participateparticipate inin electrochemicalelectrochemical reactions.reactions. SpecificSpecific reactionreaction formulasformulas areare asas EquationsEquations (1)(1,2): and (2): are as Equations (1,2): n+ Anode: M M + ne− (1) Anode:Anode: MM ⇌⇌ MMn+n+ ++ nene-- (1)(1) Cathode: O2 + 2H2O + 4e− 4OH− (2)
2 2 −− ⇌ -- The reaction kinetics of FABsCathode:Cathode: in the OO alkaline2 ++ 2H2H2OO aqueous ++ 4e4e ⇌ electrolyte4OH4OH are shown in Equations(2)(2) (1) and (6)TheThe [10 reactionreaction]. kineticskinetics ofof FABsFABs inin thethe alkalinealkaline aqaqueousueous electrolyteelectrolyte areare shownshown inin EquationsEquations (1)(1) andand (6)(6) [10].[10]. Anode: Fe + 2OH− Fe(OH)2 + 2e− (3)
- - 3Fe(OH)Anode:Anode:2 + Fe2OHFe ++ 2OH2OH− -Fe ⇌⇌3 O Fe(OH)Fe(OH)4 + 4H22 O++ 2e2e+ -2e − (3)(3)(4) Appl. Sci. 2019, 9, 2787 4 of 22
Cathode: O2 + 2H2O + 4e− 4OH− (5)
Overall reaction: 2O2 + 3Fe Fe3O4 (6) The working principle of AABs in the alkaline aqueous electrolyte is shown in (9) [11].
Anode: Al + 4OH− Al(OH)4− + 3e− (7)
Cathode: O2 + 2H2O + 4e− 4OH− (8)
Overall reaction: 3O2 + 2Al Al2O3 (9) The working principle of ZABs in the alkaline aqueous electrolyte is shown in (13) [12].
2 Anode: Zn + 4OH− Zn(OH)4 − + 2e (10)
2 Zn(OH)4 − ZnO + 2OH− + H2O (11)
Cathode: O2 + 2H2O + 4e− 4OH− (12)
Overall reaction: O2 + Zn ZnO (13) The working principle of LABs in the non-aqueous electrolyte is shown in (18) [1].
+ Anode: Li Li + e− (14)
Cathode: O2 + e− O2− (15) + O2− + Li LiO2 (16) + LiO2 + Li + e− Li2O2 (17)
Overall reaction: O2 + Li Li2O2 (18)
1.3. Configuration of MABs Based on packaging and practical application requirements, MABs can be classified as traditional static batteries, flow batteries, and novel flexible batteries [13]. In this part, three kinds of batteries will be discussed briefly. The latter part is based on the analysis of solid-state batteries. Traditional static batteries: As shown in Figure3, traditional static air batteries have four main parts: cathode, separator, electrolyte, and anode. Compared to the fast kinetics of the anode reaction, oxygen reaction on cathodes is kinetically sluggish in nature. A three-phase reaction boundary of solid (catalyst)–liquid (electrolyte)–gas (oxygen) contributes to the oxygen reduction reaction (ORR). Meanwhile, a reversed oxygen evolution reaction (OER) occurs on a two-phase boundary solid (catalyst)–liquid (electrolyte) at the cathode [14]. Highly efficient bifunctional catalysts are thus required to facilitate both OER and ORR. In addition, since an active electrolyte is employed in traditional static MABs, it is challenging to completely overcome the issue of insoluble deposition of by-products on the surface of both the metal anode and air cathode during the charge–discharge cycles. These deposited by-products consequently block the electrode pores limiting the diffusion of air that eventually results in a lower battery performance [15]. Flow batteries: This type of MAB consists of an electrode, separator, electrolyte, and an electrolyte bank installed as an additional part. Usually, a pump is also integrated to drive electrolyte flow, as shown in Figure4b. Flowing-electrolyte configuration addresses some of the problems associated with the metal anode and an air cathode. For instance, in zinc–nickel batteries, a large volume of flowing electrolyte decreases the formation of dendrites and irregular shape changes of zinc and thereby, avoid passivation by improving current distribution and reducing concentration gradients [16]. However, the complicated flowing-configuration of MABs has some shortcomings, including decreased Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 22 Appl. Sci. 2019, 9, 2787 5 of 22 gradients [16]. However, the complicated flowing-configuration of MABs has some shortcomings, including decreased energy efficiency and volume density, and increased complications as additional pumpsenergy eandfficiency tubes andare needed volume to density, drive andthe flow increased of electrolyte complications during as the additional discharge. pumps and tubes are neededFlexible to drive batteries the flow: With of electrolyte the increasing during demand the discharge. for portable electronics in recent years, light and smallFlexible form-factor batteries flexible: With batteries the increasing have become demand a hot for research portable topic electronics [17,18]. in The recent main years, components light and ofsmall a flexible form-factor battery flexible include batteries a havecathode, become anode, a hot separator, research topic and [17 high,18]. Theconductivity main components electrolyte. of a Conventionalflexible battery electrolyte include a for cathode, a flexible anode, battery separator, system and is a highsolid-state conductivity electrolyt electrolyte.e. A thin metallic Conventional plate iselectrolyte used as a for metal a flexible anode battery to reduce system the battery is a solid-state weight. electrolyte. Various nano A thin comp metallicounds plate and isnanocomposites used as a metal areanode being to reduce explored the batteryas a potential weight. cathode Various material. nano compounds Specific andmaterials nanocomposites include carbon are being fibers, explored carbon as nanotubes,a potential cathodeand graphene. material. According Specific materials to the include current carbon development fibers, carbon trend, nanotubes, flexible andbatteries graphene. are undergoingAccording to an the evolution, current development from polymer trend, batteries, flexible batteriesflexible alkaline are undergoing batteries, an lithium-based evolution, from batteries polymer tobatteries, metal–air flexible batteries. alkaline Currently, batteries, ZABs lithium-based and AABs batteries are the ideal to metal–air flexible batteries.batteries due Currently, to low ZABscost, safe and operation,AABs are theand ideal high flexible energy batteries density due[19]. to low cost, safe operation, and high energy density [19].
FigureFigure 4. 4. SchematicSchematic diagram diagram of ofdiffer diffenterent MAB MAB configurations: configurations: (a) (multi-cella) multi-cell static static configuration, configuration, (b) flow(b) flow battery, battery, and and (c) flexible (c) flexible battery. battery.
1.41.4. Technical Technical barriers barriers AlthoughAlthough metal–air metal–air batteries batteries have have been been studied studied for for many many years, years, there there are are still major technical issuesissues to to address address for for practical practical applications. applications. Metallic Metallic anodes anodes face face many many challe challenges,nges, such such as as corrosion, corrosion, hydrogenhydrogen generation, forming passivation layers, dendritic formation, electrode deformation, and energyenergy loss due to self-charging. The The air air anode anode has has ma manyny obstacles, obstacles, such such as as la lackck of of efficient efficient catalysts catalysts forfor both both ORR ORR and OER, affecting affecting electrolyte stability due to impurityimpurity and dissolved gas, and and gas diffusiondiffusion blockage by side reaction products. Electrolyte Electrolyte selection, selection, which which is is an an important important component component forfor efficient efficient electrochemical electrochemical reaction, also poses some technical barriers barriers due due to to side side reaction reaction with with the the anode,anode, reaction reaction with with CO CO22 fromfrom air, air, and and low conductivity. In In th thee following sections, we will discuss thesethese issues issues and and potential potential solutions. solutions.
2. Cathodes Appl. Sci. 2019, 9, 2787 6 of 22
Appl.2. Cathodes Sci. 2019, 9, x FOR PEER REVIEW 6 of 22
2.12.1. Components Components of of the the Cathodic Cathodic Electrode Electrode On thethe cathode, cathode, chemistry chemistry reactions reactions are ORRare andORR OER. and TheOER. oxidant The isoxidant oxygen is from oxygen air atmosphere. from air atmosphere.The catalysts The are requiredcatalysts to are lower required overpotential to lower of overpotential ORR and OER. of ForORR an and aqueous OER. electrolyte,For an aqueous water electrolyte,loss should water be avoided loss should to keep be batteryavoided stability. to keep battery Hence, stability. the practical Hence, cathode the practical is composed cathode of is a composedcatalyst layer, of a gas catalyst diffusion layer, layer, gas anddiffusion current layer, collector, and current as shown collector, in Figure as 5shown. The currentin Figure collector 5. The currentcan be metalcollector and can non-metal. be metal Metaland non-metal. current collectors Metal current are a porous collectors foam-like are a porous metal, foam-like for example, metal, Ni, forCu. example, Non-metal Ni, currentCu. Non-metal collectors current are carbon-based collectors are material, carbon-based for example, material, conductive for example, carbon conductive paper, carbongraphitic paper, fiber graphitic or carbon fiber cloth. or Gas carbon diffusion cloth. layer Gas (GDL)diffusion and layer the catalysts (GDL) and are alsothe catalysts extremely are crucial also extremelyfor cathode crucial performance. for cathode performance.
Figure 5. Various cathodic electrode of MABs, ( A)) metal current collector with with a a gas diffusion diffusion layer and coated catalyst facing facing electrolyte, electrolyte, ( (B)) carbon-base carbon-base current current collector collector and and gas gas diffusion diffusion and and catalyst, catalyst, (C) carbon paper-current collectorcollector andand gasgas didiffusionffusion andand catalyst.catalyst. (1) Gas diffusion layer: In MABs, the GDL has multi-folded functions: supporting of catalyst (1) Gas diffusion layer: In MABs, the GDL has multi-folded functions: supporting of catalyst layer; providing oxygen diffusion channels between air and catalyst layer; preventing water getting into layer; providing oxygen diffusion channels between air and catalyst layer; preventing water getting battery and electrolyte getting out of battery. To better serve as a bridge between air and catalyst layer, into battery and electrolyte getting out of battery. To better serve as a bridge between air and catalyst the GDL should be thin, light, highly porous, and hydrophobic. Figure4b indicates that the ORR in layer, the GDL should be thin, light, highly porous, and hydrophobic. Figure 4(b) indicates that the MABs occurs at the three phase boundaries (oxygen air, liquid electrolyte, and solid catalyst). The GDL ORR in MABs occurs at the three phase boundaries (oxygen air, liquid electrolyte, and solid catalyst). can simultaneously provide hydrophilic micro-channels to the liquid electrolyte, and hydrophobic The GDL can simultaneously provide hydrophilic micro-channels to the liquid electrolyte, and layers to prevent electrolyte leakage and good properties of gaseous oxygen diffusion [20]. hydrophobic layers to prevent electrolyte leakage and good properties of gaseous oxygen diffusion (2) Catalyst layer: Since the kinetics for the oxygen reaction is naturally slow, bifunctional catalysis [20]. is required to improve ORR and OER to improve electrochemical performances of MABs. Based on (2) Catalyst layer: Since the kinetics for the oxygen reaction is naturally slow, bifunctional previous researches, platinum (Pt) [21], ruthenium (Ru) oxides, and iridium oxides (Ir) [22] showed catalysis is required to improve ORR and OER to improve electrochemical performances of MABs. excellent performance in ORR and OER. Furthermore, nanostructures of the following materials also Based on previous researches, platinum (Pt) [21], ruthenium (Ru) oxides, and iridium oxides (Ir) [22] had good catalytic activity, (a) transition metal oxides, MnO, CoO, NiO, etc. [23]; (b) transition metal showed excellent performance in ORR and OER. Furthermore, nanostructures of the following hydroxide and sulfide, NiCoFe-LDH (Layered double hydroxides) layered double hydroxides [24]; materials also had good catalytic activity, (a) transition metal oxides, MnO, CoO, NiO, etc. [23]; (b) (c) spinel compounds, such as CuCo O [25]; (d) carbon-based materials, such as nitrogen doping transition metal hydroxide and sulfide,2 4NiCoFe-LDH (Layered double hydroxides) layered double carbon [26]; (e) nanocomposite materials mixing ORR catalyst Fe-N-C and OER catalyst NiFe [27]. hydroxides [24]; (c) spinel compounds, such as CuCo2O4 [25]; (d) carbon-based materials, such as nitrogen2.2. Improving doping ORR carbon and OER [26]; (e) nanocomposite materials mixing ORR catalyst Fe-N-C and OER catalyst NiFe [27]. The appropriate catalyst should be designed and applied to maximize catalytic efficiency. 2.2.Metal-based Improving catalysts ORR and possess OER high catalytic efficiencies due to different crystal structures. Spinel-type oxide (AxB3 xO4)[28] and perovskite oxides (ABO3)[29] are widely used as bifunctional electrocatalysts The appropriate− catalyst should be designed and applied to maximize catalytic efficiency. Metal- in alkaline electrolytes. Maiyalagan et al. [30] synthesized a spinel-type lithium cobalt oxide LiCoO2 at based catalysts possess high catalytic efficiencies due to different crystal structures. Spinel-type oxide (AxB3−xO4) [28] and perovskite oxides (ABO3) [29] are widely used as bifunctional electrocatalysts in alkaline electrolytes. Maiyalagan et al. [30] synthesized a spinel-type lithium cobalt oxide LiCoO2 at high-temperature (800 °C, LiCoO2-HT) and low-temperature LiCoO2-LT at 400 °C. LiCoO2-LT adopts Appl. Sci. 2019, 9, 2787 7 of 22
Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 22 high-temperature (800 ◦C, LiCoO2-HT) and low-temperature LiCoO2-LT at 400 ◦C. LiCoO2-LT adopts 3+ 3+ aa lithiatedlithiated spinelspinel structure [Li 2]]16c16c[Co[Co2]216d]16dO4O in4 inwhich which the the Co Co ionsions occupy occupy all the all 16d the 16doctahedral octahedral sites sites(space (space group: group: Fd3m) Fd3m) [31]. [ 31As]. shown As shown in Figure in Figure 6(a),6a, LiCoO 22-HT-HT has has the the α-NaFeO2 structure (space(space ++ 33++ group:group: R3m)R3m) arrays.arrays. LiLi ions andand CoCo ionion occupy occupy on alternatealternate (111)(111) NaFeONaFeO22-style structure arrays,arrays, duedue toto thethe largelarge sizesize andand chargecharge didifferencesfferences betweenbetween thethe LiLi++ and Co3+3+ ionsions [32]. [32]. In In Figure Figure 6(b),6b, it it is is obviousobvious thatthat HT-LCoOHT-LCoO22 hashas aa betterbetter catalyticcatalytic performanceperformance thanthan LT-LiCoOLT-LiCoO22and and CoCo33OO44. CatalystsCatalysts areare preferredpreferred atat nanoscalenanoscale forfor betterbetter catalyticcatalytic behavior.behavior. ByBy usingusing aa vacuumvacuum DCDC arcarc method,method, LangLang etet al.al. [[33]33] synthesizedsynthesized aa novelnovel MnMn33O4/MnO/MnO nano spherical transition metal compound. TheThe resultsresults showedshowed thatthat thethe sizesize ofof MnMn33OO44/MnO/MnO particles was controlled at a range of 40 to 60 nm. TheThe MnMn33OO44//MnOMnO catalyst potential platform reaches toto 2.72.7 V.V.
FigureFigure 6.6. ((aa)) X-rayX-ray didiffractionffraction patternspatternsand and ( b(b)) electrochemical electrochemical behaviors behaviors of of Co Co3O3O44,, low-temperaturelow-temperature (LT)-LiCoO(LT)-LiCoO22,, high-temperaturehigh-temperature (HT)-LiCoO(HT)-LiCoO22 catalysts [[30]30] (Copyright © 2014, Springer Nature).
NanoscaleNanoscale catalystscatalysts cancan alsoalso bebe fabricatedfabricated withwith variousvarious otherother morphologies,morphologies, suchsuch asas nano-rodnano-rod LaCoO [34], 3D ordered mesoporous structure Co O [35], hollow cobalt oxide nanoparticles [36]. LaCoO33 [34], 3D ordered mesoporous structure Co33O4 [35], hollow cobalt oxide nanoparticles [36]. DifferentDifferent morphologies morphologies of of catalysts catalysts are are shown shown in Figurein Figure7. Many 7. Many low-cost low-cost and efficientand efficient catalysts catalysts have beenhave developed,been developed, including including transition transition metals andmetals nitrogen and nitrogen co-doped co-doped carbons (M-N carbons/C, M (M-N/C,=Fe or Co) M=Fe [25], or metal Co) oxides[25], metal [37], oxides transition [37], metal transition carbides metal [38], carbides nitrides [3 [398],], nitrides and metal-free [39], and heteroatom-doped metal-free heteroatom-doped carbon-based catalystscarbon-based [40]. Comparedcatalysts [40]. to metal-contained Compared to metal-contained catalysts, the heteroatom-doped catalysts, the heteroatom-doped carbon-based materials carbon- withbased N, materials S, B, and P,with can promoteN, S, B, oxygenand P, can adsorption promot one oxygen the carbon adsorption nanostructure on the since carbon these nanostructure hetero-atoms aresince more these electronegative hetero-atoms thanare more carbon, electronegative and cause neighboring than carbon, carbon and cause atoms neighboring electron deficiency carbon atoms [41]. Amongelectron them, deficiency N-doped [41]. carbons Among are extensivelythem, N-doped studied carbons due to are their extensively remarkable studied ORR catalytic due to activity. their N-dopedremarkable carbon ORR materials catalyticare activity. shown N-doped in four ways, carbon graphitic materials N, Oxidizedare shown N, in pyrrolic four ways, N, and graphitic pyridic N, N inOxidized Figure8 .N, pyrrolic N, and pyridic N in Figure 8. AlthoughAlthough severalseveral materialsmaterials havehave shownshown catalyticcatalytic activityactivity forfor oxygenoxygen reaction in MABs cathodes, thethe catalyticcatalytic effi efficiencyciency isnot is idealnot whenideal usedwhen alone. used To alone. improve To comprehensive improve comprehensive catalyst performance, catalyst compositionperformance, materials composition have materials been synthesized have been and synthesized used as catalysts and used in both as catalysts ORR and in OER. both MnO ORR2 andand RuO are single catalysts for ORR and OER, respectively. Combining MnO with RuO is used as a OER.2 MnO2 and RuO2 are single catalysts for ORR and OER, respectively.2 Combining2 MnO2 with bifunctional catalyst. Sun et al. [42] synthesized RuO nanoparticles (np-RuO /nr-MnO ) supported on RuO2 is used as a bifunctional catalyst. Sun et al. [42]2 synthesized RuO2 nanoparticles2 2 (np-RuO2/nr- MnO nanorods by a two-step hydrothermal reaction. Electrochemical characterizations are carried on MnO22) supported on MnO2 nanorods by a two-step hydrothermal reaction. Electrochemical nanocomposites np-RuO /nr-MnO as catalysts for LABs. Charge–discharge tests showed a reversible characterizations are carried2 on nanocomposites2 np-RuO2/nr-MnO2 as catalysts for LABs. Charge– 1 1 dischargedischarge capacitytests showed of 500 a mAhreversible g− for discharge 75 cycles capacity at a current of 500 density mAh ofg− 501 for mA 75 g −cycles. LABs at a with current the RuO /MnO catalyst presented much lower overpotential of 0.58 V at 50 mA g 1 than that measured density2 of 250 mA g−1. LABs with the RuO2/MnO2 catalyst presented much lower− overpotential of with0.58 V a at single 50 mA catalyst. g−1 than ORR that measured and OER electrocatalyticwith a single catalyst. activity ORR were and tested OER byelectrocatalytic using rotating activity disk electrodes. It was found that np-RuO /nr-MnO ORR limitation diffusion current was 6.01 mA cm 2, were tested by using rotating disk electrodes.2 2 It was found that np-RuO2/nr-MnO2 ORR limitation− and ORR half-wave potential (E ) was 0.158 V. These results demonstrated that an np-RuO and diffusion current was 6.01 mA cm1/2 −2, and− ORR half-wave potential (E1/2) was -0.158 V. These results2 nr-MnO combination can work as an effective catalyst for LABs with high activity while maintaining demonstrated2 that an np-RuO2 and nr-MnO2 combination can work as an effective catalyst for LABs batterieswith high stability. activity while maintaining batteries stability. Appl. Sci. 2019, 9, 2787 8 of 22 Appl.Appl. Sci.Sci. 20192019,, 99,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 2222
Figure 7. (a) SEM images of Mn3O4/MnO nanoparticles [33] (Copyright © 2019, Elsevier), (b) TEM Figure 7.7. ((aa)) SEM SEM images images of of Mn Mn3O43O/MnO4/MnO nanoparticles nanoparticles [33] [33] (Copyright (Copyright© 2019, © 2019, Elsevier), Elsevier), (b) TEM (b images) TEM images of LaCoO3 catalysts [34], (c) TEM image of honeycomb-like 3D ordered mesoporous spinel ofimages LaCoO of3 LaCoOcatalysts3 catalysts [34], (c) TEM[34], image(c) TEM of honeycomb-likeimage of honeycomb-like 3D ordered 3D mesoporous ordered mesoporous spinel Co3O spinel4 [35] Co3O4 [35] (Copyright © 2016, John Wiley and Sons), (d) TEM image of hollow cobalt oxide (CopyrightCo3O4 [35] ©(Copyright2016, John © Wiley 2016, and John Sons), Wiley (d )and TEM Sons image), (d) of hollowTEM image cobalt of oxide hollow nanoparticles cobalt oxide [36] (Copyrightnanoparticlesnanoparticles© 2019, [36][36] (Copyright(Copyright Elsevier). ©© 2019,2019, Elsevier).Elsevier).
FigureFigure 8.8. FourFour nitrogen nitrogen doping doping configuratio configurationsconfigurationsns ofof aa graphenegraphene moleculemolecule [[23].[23].23]. 2.3. In Situ Characterization Using an Electron Microscope 2.3.2.3. InIn SituSitu CharacterizationCharacterization UsingUsing anan ElectronElectron MicroscopeMicroscope An in situ electron microscope is a promising tool for scientific research due to real-time AnAn inin situsitu electronelectron microscopemicroscope isis aa promisingpromising tooltool forfor scientificscientific researchresearch duedue toto real-timereal-time observation and plays a major role in many MAB studies, such as the catalytic mechanism, the observationobservation andand playsplays aa majormajor rolerole inin manymany MABMAB studies,studies, suchsuch asas thethe catalyticcatalytic mechanism,mechanism, thethe oxidation–reduction mechanism, the growth of nanostructures, and the deformation of electrodes. oxidation–reductionoxidation–reduction mechanism,mechanism, thethe growthgrowth ofof nananostructures,nostructures, andand thethe deformationdeformation ofof electrodes.electrodes. Based on the distinctive features of the preceding, Katharine et al. [43] found that higher Coulombic BasedBased onon thethe distinctivedistinctive featuresfeatures ofof thethe preceding,preceding, KatharineKatharine etet al.al. [43][43] foundfound thatthat higherhigher CoulombicCoulombic efficiency and more homogeneous morphology of the Li deposits in a coin-cell contributed to efficiencyefficiency andand moremore homogeneoushomogeneous morphologymorphology ofof ththee LiLi depositsdeposits inin aa coin-cellcoin-cell contributedcontributed toto thethe the presence of a compressed lithium separator interface through in situ electrochemical-scanning presencepresence ofof aa compressedcompressed lithiumlithium separatorseparator interfinterfaceace throughthrough inin situsitu electrochemical-scanningelectrochemical-scanning transmission microscope (EC-STEM), compared with a macroscale cell. In addition, Yoon et al. [44] used transmissiontransmission microscopemicroscope (EC-STEM),(EC-STEM), comparedcompared withwith aa macroscalemacroscale cell.cell. InIn addition,addition, YoonYoon etet al.al. [44][44] in situ atomic force microscope (AFM) to measure the dominant wavelength of the wrinkled surface usedused inin situsitu atomicatomic forceforce microscopemicroscope (AFM)(AFM) toto measuremeasure thethe dominantdominant wavelengthwavelength ofof thethe wrinkledwrinkled topography. The planes strain modulus of the SEI was determined from the measured wavelength. surfacesurface topography.topography. TheThe planesplanes strainstrain modulumoduluss ofof thethe SEISEI waswas determineddetermined fromfrom thethe measuredmeasured Li et al. [45] explored the reaction mechanism and unveiled that α-MoO converted to crystalline 3 3 wavelength.wavelength. LiLi etet al.al. [45][45] exploredexplored thethe reactionreaction mechanismmechanism andand unveiledunveiled thatthat αα-- MoOMoO3 convertedconverted toto Li MoO in the first stage of lithiation, and further converted to metallic Mo and amorphous Li O in 2 3 2 3 2 crystallinecrystalline LiLi2MoOMoO3 inin thethe firstfirst stagestage ofof lithiation,lithiation, andand furtherfurther convertedconverted toto metallicmetallic MoMo andand the next stage. As shown in Figure9a, with a negative potential to the Au MnO nanowires (NWs), 2 / 2 2 amorphousamorphous LiLi2OO inin thethe nextnext stage.stage. AsAs shownshown inin FigureFigure 9(9(a),a), withwith aa negativenegative potentialpotential toto thethe Au/MnOAu/MnO2 2 2 2 nanowiresnanowires (NWs),(NWs), bubble-likebubble-like NaONaO2 nucleatednucleated onon thethe contactcontact wherewhere thethe Au/MnOAu/MnO2 NWNW andand NaNa2OO intersects,intersects, thenthen growsgrows alongalong thethe NW,NW, resultingresulting inin 1818 timestimes volumevolume increase.increase. Meanwhile,Meanwhile, thethe dischargedischarge Appl. Sci. 2019, 9, 2787 9 of 22
Appl. Sci. 2019, 9, x FOR PEER REVIEW 9 of 22 bubble-like NaO2 nucleated on the contact where the Au/MnO2 NW and Na2O intersects, then grows alongproduct the shrinks NW, resulting as a result in 18 of times the volumedisproportionation increase. Meanwhile, of NaO2 theto Na discharge2O2 and product O2, confirming shrinksas the a resultoccurrence of the of disproportionation ORR [46]. Similarly, of NaOLiu et2 toal. Na [47]2O also2 and reported O2, confirming the real-time the occurrence observation of of ORR ORR [46 in]. Similarly,Figure 9(b). Liu In et this al. [research,47] also reportedCuO nanowires the real-time (NWs observation), as the air cathode, of ORR infirstly Figure converted9b. In this to Cu research,2O and CuOthen nanowiresto Cu, as a (NWs),metal catalyst as the air to cathode, accelerate firstly the disproportionation converted to Cu2O andof NaO then2 to Cu,Na2O as2 aand metal O2. catalystDuring tothe accelerate reaction process, the disproportionation the morphological of changes NaO2 to were Na2 investigatedO2 and O2. by During an electrochemical the reaction process,atomic force the morphologicalmicroscope. Liu changes et wereal. [48] investigated used EC-AFM by an electrochemical to observe atomic the forcedynamic microscope. process Liu etof al.Li [482O]2 usedgrowth/decomposition EC-AFM to observe during the dynamic the ORR/OE processR on of Lia 2goldO2 growth electrode/decomposition in Figure 9(c) during and found the ORR that/OER the onLi2O a2 gold decomposed electrode inatFigure a lower9c and potential found thatdue the to Lielectrochemically2O2 decomposed generated at a lower TTF potential+ through due toa electrochemicallyhomogeneous oxidation generated mechanism. TTF+ through a homogeneous oxidation mechanism.
FigureFigure 9.9. ((aa)) StructureStructure evolutionevolution ofof thethe NaONaO22 dischargedischarge productproduct duringduring oxygenoxygen reductionreduction reactionreaction (ORR)(ORR) [[46]46] (Copyright(Copyright© © 2019,2019, Elsevier);Elsevier); ((bb)) StructuralStructural andand phasephase characterizationcharacterization ofof aa CuOCuO nanowirenanowire
(NW)(NW) duringduring dischargingdischarging andand chargingcharging inin anan OO22 environmentenvironment [47[47]] (Copyright(Copyright ©© 2018,2018, AmericanAmerican ChemicalChemical Society);Society); ((cc)) CyclicCyclic voltammetryvoltammetry performedperformed inin electrochemical-atomicelectrochemical-atomic forceforce microscopemicroscope cellcell andand thethe resultingresulting AFMAFM imageimage afterafter CVCV reductionreduction [[48]48] (Copyright(Copyright© © 2016,2016, American ChemicalChemical Society).Society). 3. Anodes 3. Anodes The chemical activity of the metal anode determines the discharge capacity. Because of high The chemical activity of the metal anode determines the discharge capacity. Because of high metal activity, an unavoidable side reaction with various components in the electrolyte may occur. metal activity, an unavoidable side reaction with various components in the electrolyte may occur. Depending on the purity of the metal, the battery performance and the incidence of side reactions can Depending on the purity of the metal, the battery performance and the incidence of side reactions be different. can be different. 3.1. Anode Materials: High Purity Metal and Alloy 3.1. Anode Materials: High Purity Metal and Alloy Fan et al. [49] took industrial 5 N Al (99.999% high purity) and aluminum alloy (1050, 2011, 3003, Fan et al. [49] took industrial 5 N Al (99.999% high purity) and aluminum alloy (1050, 2011, 3003, 4032, 5052, 6061, 7050, and 8011) as anodes for AABs in alkaline electrolytes, using the hydrogen 4032, 5052, 6061, 7050, and 8011) as anodes for AABs in alkaline electrolytes, using the hydrogen collection method and electrochemical impedance spectroscopy (EIS) to determine the corrosion collection method and electrochemical impedance spectroscopy (EIS) to determine the corrosion behaviors, electrochemical properties, and potentiodynamic polarization. Test results of corrosion and behaviors, electrochemical properties, and potentiodynamic polarization. Test results of corrosion EIS showed the sample in 4 M KOH was more suitable than in 4 M NaOH. Al 8011 had a transfer and EIS showed the sample2 in 4 M KOH was more suitable2 than in 4 M NaOH. Al 8011 had a transfer2 resistance (Rt) of 1.247 Ω cm in 4 M NaOH and 1.108 Ω cm in 4 M KOH. 5 N Al had Rt of 2.29 Ω cm 2 2 2 resistance (Rt) of 1.247 Ω cm 2in 4 M NaOH and 1.108 Ω cm in 4 M KOH. 5 N Al had Rt of 2.29 Ω cm in 4 M NaOH and 15.3 Ω cm in 4 M KOH. 5 N Al had 1.699 V Ecorr in 4M NaOH and 1.821 V Ecorr 2 inin 4M4 M KOH. NaOH All and industrial 15.3 Ω cm Al alloyin 4 M anodes KOH. had 5 N theAl had hydrogen 1.699 V adsorption Ecorr in 4M phenomenon NaOH and 1.821 of hydrogen V Ecorr in 4M KOH. All industrial Al alloy anodes had the hydrogen adsorption phenomenon of hydrogen evolution reaction, and 8011 had relatively better performance than the others. As shown in Table 1, Al 8011 had a lower corrosion potential (Ecorr) at −1.42 V, corrosion current (Icorr), of 135 mA cm−2, and Appl. Sci. 2019, 9, 2787 10 of 22 evolution reaction, and 8011 had relatively better performance than the others. As shown in Table1, 2 Al 8011 had a lower corrosion potential (Ecorr) at 1.42 V, corrosion current (Icorr), of 135 mA cm− , and 2 − polarization resistance (Rp) of 3.628 Ωcm among industrial Al alloys. Therefore, impurity elements had different roles in the corrosion behaviors. Mg, Mn, Cr, Ti, and Zn were helpful to improve the corrosion resistance of Al alloy anodes, while Fe, Cu, and Si formed cathodic sites and lowered overpotential for hydrogen evolution reaction (HER) [49–51]. Different components Al alloys (Zn-rich and Al-rich phases) worked as abode AABs anode. Test results showed Al-rich alloys were better performance, due to lower the anodic passivation. Zn-Al alloys are promising anode materials as primary and mechanical-rechargeable Zn-air batteries [52].
Table 1. Parameters and electrochemical impedance spectroscopy (EIS) value of different grades of Al anodes [49].
2 Ecorr (V vs. Hg/HgO) I (mA cm 2) R (Ω cm ) Grade corr − p 4 M NaOH 4 M KOH 4 M NaOH 4 M KOH 4 M NaOH 4 M KOH 1050 1.290 1.291 186 176 4.108 5.683 2011 1.410 1.420 135 142 4.124 6.674 3003 1.315 1.340 181 165 4.607 5.892 4032 1.390 1.390 145 174 4.653 2.572 5052 1.310 1.320 191 157 3.766 2.539 6061 1.370 1.380 161 168 4.766 2.655 7050 1.420 1.450 189 143 4.516 2.848 8011 1.420 1.450 135 144 3.628 2.670 5N 1.699 1.821 24.3 4.7 9.668 17.9 Grade Equivalent solution 1050 2011 3003 4032 5052 6061 7050 8011 5N elements 4 M NaOH 0.70 0.75 0.57 0.84 0.74 0.49 0.76 1.247 2.29 R (Ω cm2) t 4 M KOH 1.23 0.69 1.208 0.3351 0.78 0.36 0.60 1.108 15.3
3.2. Metal Coating and Composite Electrodes Different from previous research using Al alloy as anodes, Mutlu [53] investigated Cu coating on Al and 7075 Al alloy as anodes. Copper was deposited on the Al surface by chemical (Al or Alloy/Cu-CD) and electrochemical (Al or Alloy/Cu-ED) processes, SEM images are shown in the Figure 10a,b. Al has a lower resistance than Al-Cu alloy. EIS measurements showed that copper on the Al surface could decrease anodic potential and improve batteries performance as shown in Figure 10c,d. The solution–electrode interaction resistance (Rs) was increased by adding copper to aluminum because copper can form a protective layer against the corrosion on the aluminum–solution interface. Hang et al. [54] found that FeS can employ as an additive for the electrode to suppress hydrogen evolution and improve the cyclic performance of the Fe/C composite anode. FeS additive and the carbon component also strongly affected the redox behavior of iron. An electrode with FeS can promote the process (in the Figure 11) of Fe0 to Fe2++ and Fe2+ to Fe3+ at around 0.85 V (a ), − 1 0.65 V (a ), respectively. Furthermore, there were also additional intermediate species which appeared − 2 around 0.97 V (a ). The discharge capacity of the electrode was significantly improved with adding − 0 2 wt.% FeS, which was mainly because the incorporation of FeS in the electrode improved the adsorbed 2 capability of S − on the electrode surface, resulting in the easy breaking of the oxide layer. For HER at the anode in alkaline electrolyte, both molecular recombination and electrochemical desorption can be parallel steps in the overall process. The molecular recombination reaction will appreciably contribute to HER only when the current density is low, and the molecular hydrogen concentration at the liquid boundary layer is near to zero. Under these conditions, it has been indicated that the 2 molecular recombination reaction is affected by S − ion chemisorption. Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 22 Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 22 Appl.hydrogen Sci. 2019, 9 concentration, 2787 at the liquid boundary layer is near to zero. Under these conditions, it has11of 22 beenhydrogen indicated concentration that the molecular at the liquid recombination boundary layerreaction is near is affected to zero. byUnder S2− ion these chemisorption. conditions, it has been indicated that the molecular recombination reaction is affected by S2− ion chemisorption.
Figure 10. SEM images and EDS maps of (a) Al/Cu-CD, (b) Al/Cu-ED, (c) The cyclic voltammogram Figure 10. SEM images and EDS maps of (a) Al/Cu-CD, (b) Al/Cu-ED, (c) The cyclic voltammogram Figurein 10. 1 MSEM NaOH images of Al (pure), and EDS Al/Cu-CD, maps ofand (a Al/Cu-ED) Al/Cu-CD,, (d) EIS (b) measurements Al/Cu-ED, (c )of The anodes cyclic in 1 voltammogram M NaOH, in 1 M NaOH of Al (pure), Al/Cu-CD, and Al/Cu-ED, (d) EIS measurements of anodes in 1 M NaOH, in 1 MThe NaOH Nyquist of Al a Al (pure), (pure), Al Al/Cu-CD,/Cu-CD, Al/Cu-ED and Al/Cu-ED, [53] (Copyright (d) EIS measurements© 2018, Springer ofNature). anodes in 1 M NaOH, TheThe Nyquist Nyquist a Al a Al (pure), (pure), Al Al/Cu-CD,/Cu-CD, Al Al/Cu-ED/Cu-ED [[53]53] (Copyright(Copyright ©© 2018,2018, Springer Springer Nature). Nature).
(a) (b) (a) (b) FigureFigure 11. Cyclic 11. Cyclic voltammetry voltammetry for Fe for/C Fe/C composite composite electrodes electrodes composited composited carbon carbon nano-fibers nano-fibers (a )( withouta) andFigure (bwithout) with 11. FeSandCyclic additive(b) withvoltammetry FeS [54 additive] (Copyright for [54]Fe/C (C ©opyrightcomposite2006, Elsevier).© 2006,electrodes Elsevier). composited carbon nano-fibers (a) without and (b) with FeS additive [54] (Copyright © 2006, Elsevier). 3.3. Common3.3. Common Challenge Challenge of Metal of Metal Anode Anode 3.3.The Common commonThe common Challenge issues issues of withMetal with metallicAnode metallic anodes anodes areare corrosion, passivation, passivation, and anddendrite dendrite formation. formation. These mechanisms are displayed in Figure 12. These mechanismsThe common are issues displayed with metallic in Figure anodes 12. are corrosion, passivation, and dendrite formation. Corrosion: Corrosion is one of the major side reactions between metal and electrolyte, and its TheseCorrosion: mechanisms Corrosion are displayed is one ofin theFigure major 12. side reactions between metal and electrolyte, and its reaction can be expressed as follows: reactionCorrosion: can be expressed Corrosion as is follows: one of the major side reactions between metal and electrolyte, and its reaction can be expressed as follows:M + (2 + x)H2O ⇌ 2M(OH) + H2 (19)
M + (2 +Mx)H + H2OO ⇌ MO2M(OH)X + H2 + H2 (20) (19) M + (2 + x)H2O ⇌ 2M(OH) + H2 (19) Equation (19, 20) evaluates the corrosion rate due to hydrogen evolution reaction (HER). For MM+ +H HO2O ⇌ MOMOX + HH2 (20) (20) almost all MABs, the M/MO standard voltage2 was belowX that of2 the hydrogen revolution. Therefore, EquationshydrogenEquation (19)evolution (19, and 20) (20)evaluateswas evaluatesspontaneously the corrosion the corrosionfavore rated. dueThe rate toHER due hydrogen todecreased hydrogen evolution metal evolution reactionanode Coulombic reaction (HER). For (HER). Foralmost almostefficiency all all MABs, MABs, because the the itM/MO Mconsumed/MO standard standard electrons voltage voltage from was th wase be metal belowlow thatanode that of inthe of the hydrogen charge. hydrogen Moreover,revolution. revolution. hydrogen Therefore, Therefore, hydrogen evolution was spontaneously favored. The HER decreased metal anode Coulombic hydrogen evolution was spontaneously favored. The HER decreased metal anode Coulombic efficiency efficiency because it consumed electrons from the metal anode in the charge. Moreover, hydrogen because it consumed electrons from the metal anode in the charge. Moreover, hydrogen diffusing into electrolyte leads to the increase of internal battery pressure and could result in an explosion. Hydrogen evolution reaction: Hydrogen evolution reaction (HER) was a side reaction of metal electrodes during the charge–discharge of batteries. The specific working principle is shown in Equations (19) and (20). Metal releases electrons to the aqueous electrolyte system and hydrogen ions Appl. Sci. 2019, 9, 2787 12 of 22 replace metal ions obtaining electron reduction in hydrogen. HER in MABs thus influences the rates of metal electrodes. Hydrogen overpotential decreases on the ZnO surface since the self-discharge rate reducedAppl. with Sci. increasing 2019, 9, x FOR ZnOPEER REVIEW on the electrode surface [55]. Increasing overpotential of HER12 (decreasedof 22 HER rate) can thus improve the charging efficiency. In addition, the corrosion and oxidation of Al in diffusing into electrolyte leads to the increase of internal battery pressure and could result in an alkaline electrolytes depend on electrolyte properties, temperature, and purity [56]. Using ionic liquid explosion. or solid stateHydrogen electrolyte evolution is an ereaction:ffective Hydrogen solution toevolution reduce reaction the HER (HER) rate, was which a side has reaction been of confirmed metal in FABs [57electrodes]. An alloy during as anodesthe charge–discharge replacing pure of batteri metales. also The reducedspecific working the corrosion principle rate, is shown and additives,in such asEquation bismuth (19,20). or sulfur, Metal could releases minimize electrons the to corrosionthe aqueous of electrolyte the iron syst electrodeem and andhydrogen the evolution ions of hydrogenreplace [58]. metal ions obtaining electron reduction in hydrogen. HER in MABs thus influences the rates
FigureFigure 12. Corrosion, 12. Corrosion, passivation, passivation, and and dendrite dendrite formation formation processes processes at a atmetal a metal anode. anode.
Passivationof metal electrodes. layers: PassivationHydrogen overpotential used to describe decreases an electrodeon the ZnO that surface could since not the be self-discharge further discharged becauserate an insulatingreduced with film increasing on its surface ZnO on blocked the electrod migratione surface of [55]. the Increasing discharge overpotential product. In of MABs, HER LiOH, (decreased HER rate) can thus improve the charging efficiency. In addition, the corrosion and ZnO, and Al2O3 were passivation layers for corresponding systems. Soluble species formed at the oxidation of Al in alkaline electrolytes depend on electrolyte properties, temperature, and purity [56]. air cathode will be reduced to a non-conductive layer on the metal surface. This a non-conductive Using ionic liquid or solid state electrolyte is an effective solution to reduce the HER rate, which has layer increasesbeen confirmed the internal in FABs electrical [57]. An alloy resistance as anodes of thereplacing cell and pure prevents metal also metal reduced dissolution. the corrosion The rate, e fficient methodand is to additives, use porous such electrodesas bismuth or to sulfur, hinder could the formationminimize the of corrosion passivation of the layers. iron electrode and the Dendriticevolution formation of hydrogen and[58]. deformation: During the metal electrode cycling in an alkaline electrolyte,Passivation the metal anodelayers: releasesPassivation ions used during to describe discharge an electrode and the that metal could ions not re-deposit be further on the surface ofdischarged the anode because during an charging.insulating film As aon result, its surface the metalblocked electrode migration will of the gradually discharge change product. shape, In and MABs, LiOH, ZnO, and Al2O3 were passivation layers for corresponding systems. Soluble species its surfaceformed will at become the air cathode roughened will be with reduced uneven to a non- thicknesses.conductive Over layer severalon the me chargetal surface. and This discharge a non- cycles, the unevenconductive shape layer accumulates increases the to internal form dendrites, electrical resi causingstance of the the batterycell and prevents system metal to become dissolution. unstable or short cut.The Di efficientfferent method approaches is to use have porous been electrodes attempted to hinder to mitigate the formation dendritic of passivation formation layers. and deformation, such as coatingDendritic the zinc formation metal and and using deformation non-reaction: During additives the metal in electrode the zinc electrodecycling in oran electrolytealkaline [59]. Lithiumelectrolyte, alloying withthe metal Na [ 60anode], Mg releases [61], Al ions [62 during] has beendischarge confirmed and the to metal effectively ions re-deposit suppress on the thegrowth surface of the anode during charging. As a result, the metal electrode will gradually change shape, of dendritic Li. and its surface will become roughened with uneven thicknesses. Over several charge and discharge In summary,cycles, the uneven passivation shape layersaccumulates happen to inform AABs, dendrites, ZABs, causing and LABs,the battery dendritic system structures to become form in ZABs, andunstable LABs or short and FABs,cut. Different and corrosion approaches may have occurbeen attempted in FABs, to AABs,mitigate ZABs,dendritic and formation LABs. and Di fferent strategiesdeformation, are needed such to as address coating thesethe zinc issues. metal and using non-reaction additives in the zinc electrode or electrolyte [59]. Lithium alloying with Na [60], Mg [61], Al [62] has been confirmed to effectively 4. Electrolytessuppress the growth of dendritic Li. In summary, passivation layers happen in AABs, ZABs, and LABs, dendritic structures form in AnZABs, electrolyte and LABs is a medium and FABs, to transportand corrosion ions may and occur electrons in FABs, to ensure AABs, the ZABs, continued and LABs. oxidation–reduction Different reaction.strategies Electrolyte are needed divided to into address four these types: issues. aqueous, non-aqueous, hybrid, and solid state, as shown in Figure 13. Appl. Sci. 2019, 9, x FOR PEER REVIEW 13 of 22
4. Electrolytes An electrolyte is a medium to transport ions and electrons to ensure the continued oxidation– Appl. Sci.reduction2019, 9, 2787reaction. Electrolyte divided into four types: aqueous, non-aqueous, hybrid, and solid state,13 of 22 as shown in Figure 13.
FigureFigure 13. 13.Schematic Schematic diagrams diagrams of vari variousous electrolytes electrolytes of ofMABs. MABs.
4.1. Aqueous4.1. Aqueous Electrolyte Electrolyte AlkalineAlkaline solutions solutions(7 <(7 pH< pH ≤14): 14): AlkalineAlkaline electr electrolytesolytes are are the the most most applied applied electrolyte electrolyte in in ≤ aqueous-basedaqueous-based MABs, MABs, because because the the ORR ORR isis moremore favorablefavorable with with faster faster reaction reaction kinetics kinetics and anda lower a lower overpotential, compared to acidic electrolytes. Alkaline electrolytes have a shortcoming that CO2 overpotential, compared to acidic electrolytes. Alkaline electrolytes have a shortcoming that CO2 (from(from air atmosphere) air atmosphere) reacts reacts with with electrolyte electrolyte and an formsd forms a carbonate a carbonate surrounding surrounding the the cathode. cathode. TheThe large large amount of carbonate will block the porous structure of the positive electrode material and amount of carbonate will block the porous structure of the positive electrode material and decrease decrease cathode efficiency. cathode efficiency. Neutral salt solution (pH = 7): Al alloy–air batteries can discharge in a neutral salt solution with Neutrala lower corrosion salt solution rate and(pH higher= 7): activity Al alloy–air than in batteries an alkaline can electrolyte. discharge in a neutral salt solution with a lower corrosionAcidic solutions rate and (2 higher≤ pH < 7): activity Acidic electrolytes than in an alkalineare rarelyelectrolyte. used in aqueous-based MABs because Acidica large solutionsamount of (2H+ inpH solution< 7): Acidic could electrolytesdirectly react are with rarely metal used and in reducing aqueous-based battery MABsefficiency. because ≤ a largeSaidman amount et al. of [63] H+ reportedin solution Al-Zn could alloy directlyanode performance react with changed metal andwith reducingvarious types battery of acids effi atciency. Saidmanthe same et al. concentration, [63] reported pH, Al-Zn and alloy operating anode temperat performanceure. Electrochemical changed with test various results typesindicted of acidsAl–Zn at the samealloy concentration, in 0.5 M HCl pH, was and more operating negative temperature.than that in 0.5 Electrochemical M HAc: −1.02 V and test − results0.80 V, respectively. indicted Al–Zn alloy in 0.5 M HClHybrid was electrolyte more negative: In Figure than 14 that(b), in(c), 0.5 a novel M HAc: type of1.02 aque V andous FABs0.80 was V, respectively. equipped with an − − Hybridalkaline electrolyteanode electrolyte: In Figure (anolyte) 14b,c, and a novelan acidic type cathode of aqueous electrolyte FABs (catholyte). was equipped The anolyte with an and alkaline catholyte are separated by an alkali metallic ion (Li+ or Na+) solid-state electrolyte separator. The anode electrolyte (anolyte) and an acidic cathode electrolyte (catholyte). The anolyte and catholyte are alkali metal ion serves as an ionic mediator to sustain the redox reactions at both the anode and + + separatedcathode by [64]. an alkali metallic ion (Li or Na ) solid-state electrolyte separator. The alkali metal ion servesAppl. as Sci. an 2019 ionic, 9, x mediator FOR PEER REVIEW to sustain the redox reactions at both the anode and cathode [6414]. of 22
FigureFigure 14. ( a14.) Schematic (a) Schematic illustration illustration of theoretical of theoretical voltages voltages of Fe of airFe− batteriesair batteries operated operated with with an alkalinean − or analkaline acidic cathodeor an acidic electrolyte. cathode electrolyte. Two types Two of Fe typesair batteriesof Fe−air withbatteries a Na with+- or a aNa Li+-+ or-ion a Li solid+-ionelectrolyte, solid − for (belectrolyte,) a Fe(LiOH) for//Li-SSE (b) a// OFe(LiOH)//Li-SSE//O2(H3PO4/LiH2PO4)2(H cell,3PO and4/LiH (c)2PO a Fe(NaOH)4) cell, and//Na-SS (c) //aO 2Fe(NaOH)//Na-(H3PO4/NaH2PO4) cell. SSESS//O represents2(H3PO4/NaH solid-state2PO4) cell. electrolyteSSE represents [64 ]soli (Copyrightd-state electrolyte© 2017, [64] American (Copyright Chemical © 2017, Society).American Chemical Society).
4.2. Non-Aqueous Electrolyte Solid-state electrolyte: Solid-state electrolytes are different from aqueous electrolytes in dual characteristics of wettability and ion conduction. For MABs, aqueous electrolytes with an excellent wetting property at three-phase boundaries could be in full contact with the cathode. For a solid- based electrolyte, the three-phase interface reaction can be restricted by the poor wetting property of the “immobilized” electrolyte, thereby, interfacial transporting resistance of OH− may be remarkably higher than that of an aqueous system [65]. Alkaline gel electrolytes (AGEs), consisting of low molecular weight polymer and alkaline solutions have been developed to mitigate these issues for primary lithium–air batteries [66]. Ionic liquid electrolyte: Ionic liquids are non-aqueous liquid electrolytes, including two types of cations: large organic cations with organic/inorganic anions and alkali metal ions in an organic solvent, such as organic carbonates, ethers, and esters [67,68]. Lithium salt, such as LiPF6, LiAsF6, LiN(SO2CF3)2, and LiSO3CF3 are commonly used in LABs [69]. PYR14TFSI–TEGDME–LiCF3SO3 are also employed in LABs [70], consisting of LiCF3SO3 in tetraethylene glycol dimethyl ether (TEGDME) and pure PYR14TFSI. Ionic liquid electrolytes also face challenges because of the formation of carbonates, which consume the electrolyte and block the electrode pores. What is more, the understanding of the oxygen reaction in an ionic liquid is very limited. These hinder the practical application of ionic liquid electrolytes. In summary, neutral salt solution, acidic solutions, and hybrid electrolyte are rarely used in industrial application. Alkaline solutions electrolytes and ionic liquid electrolyte are usually employed. Aqueous electrolytes do not match LABs. Furthermore, solid-state electrolytes can work in all MABs.
5. Advanced manufacturing of MABs Advanced manufacturing techniques for electrodes and batteries is composed of printing and laser processing. Printing technology has various advantages in microstructure controlling and large batch low-cost fabrication. Printing includes screen printing [71], spray printing [72], direct ink writing processing [73], and roll-to-roll fabrication, as shown in Figure 15. Figure 16 shows printed 1- to 4D structures. 3D and 4D printing are achieved by layer-to-layer printing of functional micro/nanomaterials in geometric and temporal complexes. Screen printing is one popular and simple method among printing technologies. During screen printing, the ink is pressed through a patterned screen onto the substrate using a roller and forms a film with the structures defined by the patterned screen. Spray printing injects particles from a solution and can easily fabricate large area sheets with a non-contact mode. Meanwhile, direct ink writing using a nozzle can feasibly form a 2D–3D structure with a certain thickness on the substrate. Therefore, different printing technologies can be applied to various battery electrodes. Appl. Sci. 2019, 9, 2787 14 of 22
4.2. Non-Aqueous Electrolyte Solid-state electrolyte: Solid-state electrolytes are different from aqueous electrolytes in dual characteristics of wettability and ion conduction. For MABs, aqueous electrolytes with an excellent wetting property at three-phase boundaries could be in full contact with the cathode. For a solid-based electrolyte, the three-phase interface reaction can be restricted by the poor wetting property of the “immobilized” electrolyte, thereby, interfacial transporting resistance of OH− may be remarkably higher than that of an aqueous system [65]. Alkaline gel electrolytes (AGEs), consisting of low molecular weight polymer and alkaline solutions have been developed to mitigate these issues for primary lithium–air batteries [66]. Ionic liquid electrolyte: Ionic liquids are non-aqueous liquid electrolytes, including two types of cations: large organic cations with organic/inorganic anions and alkali metal ions in an organic solvent, such as organic carbonates, ethers, and esters [67,68]. Lithium salt, such as LiPF6, LiAsF6, LiN(SO2CF3)2, and LiSO3CF3 are commonly used in LABs [69]. PYR14TFSI–TEGDME–LiCF3SO3 are also employed in LABs [70], consisting of LiCF3SO3 in tetraethylene glycol dimethyl ether (TEGDME) and pure PYR14TFSI. Ionic liquid electrolytes also face challenges because of the formation of carbonates, which consume the electrolyte and block the electrode pores. What is more, the understanding of the oxygen reaction in an ionic liquid is very limited. These hinder the practical application of ionic liquid electrolytes. In summary, neutral salt solution, acidic solutions, and hybrid electrolyte are rarely used in industrial application. Alkaline solutions electrolytes and ionic liquid electrolyte are usually employed. Aqueous electrolytes do not match LABs. Furthermore, solid-state electrolytes can work in all MABs.
5. Advanced Manufacturing of MABs Advanced manufacturing techniques for electrodes and batteries is composed of printing and laser processing. Printing technology has various advantages in microstructure controlling and large batch low-cost fabrication. Printing includes screen printing [71], spray printing [72], direct ink writing processing [73], and roll-to-roll fabrication, as shown in Figure 15. Figure 16 shows printed 1- to 4D structures. 3D and 4D printing are achieved by layer-to-layer printing of functional micro/nanomaterials in geometricAppl. Sci. 2019 and, 9, x temporalFOR PEER REVIEW complexes. 15 of 22
FigureFigure 15. 15.Schematics Schematics of of (a ()a a) a screen screen printing printing [ 71[71]] (Copyright(Copyright © 2017,2017, RSC RSC Pub), Pub), (b (b) spray) spray printing printing [72] [72 ] (Copyright(Copyright© ©2015, 2015, John John Wiley and and Sons), Sons), (c) (c) direct direct ink ink writing writing processes processes [73] (Copyright [73] (Copyright © 2017,© John2017, JohnWiley Wiley and and Sons). Sons).
Figure 16. of 1-, 2-, 3-, and 4D concepts. A 4D structure is a structure (x, y, z) made by 3D changes over time (t). Arrows indicate the direction of change with respect to time.
5.1. Spray Coating and 2D Printing Spray coating is a traditional coating method to fabricate composites by depositing particles onto the substrate surface. According to the working mechanism, spray can be divided into two types, cold spray and thermal spray. Thermal spray delivers melted metal drops or non-metal particles at high temperatures and forms a coat on the substrate [74]. Thermal spray has relatively wide applications in metal or alloy materials processing, such as surface coating and corrosion resistance. In contrast to thermal spray, cold spray without heating is a coating process to accelerate particles using the supersonic driving gas passing through a convergent–divergent nozzle and subsequently ejected onto a substrate in high speed [75]. Cold spray enables the delivery of various materials, including high melting point metal materials, low melting point polymer materials, even biomaterials. Helfritch [76] reviewed 24 new applications of cold spray, such as medical devices, electronics, microdevices, and so on. Printing is another advanced manufacturing method, including inkjet printing, lithography, 3D printing, 4D printing. In this section, we will first discuss lithography and inkjet 2D printing. Inkjet printing [77,78] is additive manufacturing and appears after screen-printing and spin coating. The principle of inkjet printing consists of five stages: drop ejection, drop flight, drop impact, drop spreading, and drop solidification. Inkjet printing has been used in depositing functional inks onto various substrates for numerical devices, to specific, sensors, micro-batteries, solar cell, and other Appl. Sci. 2019, 9, x FOR PEER REVIEW 15 of 22
Appl. Sci. 2019, 9, 2787 15 of 22
Screen printing is one popular and simple method among printing technologies. During screen printing, the ink is pressed through a patterned screen onto the substrate using a roller and forms a film with the structures defined by the patterned screen. Spray printing injects particles from a solution andFigure can easily 15. Schematics fabricate of large (a) a screen area sheets printing with [71] a(Copyright non-contact © 2017, mode. RSC Meanwhile,Pub), (b) spray direct printing ink [72] writing using(Copyright a nozzle © can 2015, feasibly John Wiley form and a 2D–3D Sons), structure(c) direct ink with writing a certain processes thickness [73] (Copyright on the substrate. © 2017, John Therefore, differentWiley printingand Sons). technologies can be applied to various battery electrodes.
Figure 16. Of 1-, 2-, 3-, and 4D concepts. A 4D structure is a structure (x, y, z) made by 3D changes over Figuretime (t). 16. Arrows of 1-, 2-, indicate 3-, and the 4D direction concepts. of A change 4D structure with respect is a structure to time. (x, y, z) made by 3D changes over time (t). Arrows indicate the direction of change with respect to time. 5.1. Spray Coating and 2D Printing 5.1. SpraySpray Coating coating and is 2D a traditional Printing coating method to fabricate composites by depositing particles onto theSpray substrate coating surface. is a traditional According coating to the method working to mechanism, fabricate composites spray can by be depositing divided intoparticles two types,onto thecold substrate spray and surface. thermal According spray. Thermal to the spray working delivers mechanism, melted metal spray drops can orbenon-metal divided into particles two types, at high coldtemperatures spray and andthermal forms spray. a coat Thermal on the substrate spray deliver [74].s Thermal melted metal spray drops has relatively or non-metal wide applicationsparticles at highin metal temperatures or alloy materials and forms processing, a coat on such the as subs surfacetrate coating [74]. Thermal and corrosion spray resistance.has relatively In contrast wide applicationsto thermal spray,in metal cold or alloy spray materials without heatingprocessing, is a such coating as surface process coating to accelerate and corrosion particles resistance. using the Insupersonic contrast to driving thermal gas spray, passing cold through spray awithout convergent–divergent heating is a coating nozzle process and subsequently to accelerate ejected particles onto usinga substrate the supersonic in high speed driving [75 gas]. Cold passing spray through enables a theconvergent–divergent delivery of various nozzle materials, and includingsubsequently high ejectedmelting onto point a metalsubstrate materials, in high low speed melting [75]. point Cold polymer spray enables materials, the evendelivery biomaterials. of various Helfritch materials, [76 ] includingreviewed 24high new melting applications point of metal cold spray,materials, such aslow medical melting devices, point electronics, polymer microdevices,materials, even and biomaterials.so on. Helfritch [76] reviewed 24 new applications of cold spray, such as medical devices, electronics,Printing microdevices, is another advancedand so on. manufacturing method, including inkjet printing, lithography, 3D printing,Printing is 4D another printing. advanced In this manufacturing section, we will method, first discuss including lithography inkjet printing, and inkjet lithography, 2D printing. 3D printing,Inkjet printing 4D printing. [77,78 ]In is this additive section, manufacturing we will first anddiscuss appears lithography after screen-printing and inkjet 2D andprinting. spin coating. Inkjet printingThe principle [77,78] of is inkjetadditive printing manufacturing consists of and five appears stages: dropafter ejection,screen-printing drop flight, and dropspin coating. impact, dropThe principlespreading, of andinkjet drop printing solidification. consists Inkjetof five printing stages: hasdrop been ejection, used in drop depositing flight, functionaldrop impact, inks drop onto spreading,various substrates and drop for solidification. numerical devices,Inkjet printing to specific, has been sensors, used micro-batteries, in depositing functional solar cell, inks and onto other variousconductive substrates parts offor cells numerical [79–82]. devices, Lim [83 ]to thoroughly specific, sensors, reviewed micro-batteries, technology issues solar and cell, influence and other on different substrates for printed capacitive sensors. Furthermore, since Mirkin [84] reported “Dip-pen nanolithography” (DPN) in science, lithography became the focus of contemporary microfabrication. A DPN system is composed of an atomic force microscope tip as a “nib”, solid-state substrate as “paper”, and molecules with a chemical affinity for the solid-state substrate “ink”. By controlling the AFM tip, the nib directly writes controlled patterns on the substrate materials. Lithography contributes to microfabrication in nanomaterials and micro-devices, such as micro-reactors and sensors, micro-optical system [85–88]. Shao [89] reported nanoimprint lithography in the processing of flexible electronics, conductive electrodes, optoelectronic devices, flexible microlens, and flexible sensors. Certainly, it is feasible to print electrodes with a thin-film structure for metal–air batteries. Appl. Sci. 2019, 9, x FOR PEER REVIEW 16 of 22
conductive parts of cells [79–82]. Lim [83] thoroughly reviewed technology issues and influence on different substrates for printed capacitive sensors. Furthermore, since Mirkin [84] reported “Dip-pen nanolithography” (DPN) in science, lithography became the focus of contemporary microfabrication. A DPN system is composed of an atomic force microscope tip as a “nib”, solid-state substrate as “paper”, and molecules with a chemical affinity for the solid-state substrate “ink”. By controlling the AFM tip, the nib directly writes controlled patterns on the substrate materials. Lithography contributes to microfabrication in nanomaterials and micro-devices, such as micro-reactors and sensors, micro-optical system [85–88]. Shao [89] reported nanoimprint lithography in the processing of flexible electronics, conductive electrodes, optoelectronic devices, flexible microlens, and flexible
Appl.sensors. Sci. 2019 Certainly,, 9, 2787 it is feasible to print electrodes with a thin-film structure for metal–air batteries.16 of 22
5.2. Laser Processing 5.2. Laser Processing Laser processing has gained more and more attention in recent years. Laser ablation, laser cutting,Laser laser processing welding, has laser gained sintering, more andlaser more direct attention writing, in and recent other years. laser-a Laserssisted ablation, synthesis laser cutting,process laserare powerful welding, lasertools sintering, for precise laser manufacturing direct writing, [90,91]. and other For laser-assisted microfabrication, synthesis laser process ablation are is powerful used to toolsfabricate for precise porous manufacturing graphene and [90 graphene,91]. For microfabrication,quantum dots. Laser laser ablationpower, isspot used size to fabricatediameter, porous hatch graphenedistance, andscanning graphene speed, quantum wavele dots.ngth, Laser had power, an influence spot size on diameter, the formation hatch distance, of nanomaterials scanning speed, and wavelength,nanostructures had an[92,93]. influence Lasers on thecan formation be applied of nanomaterialsfor sintering andvarious nanostructures materials [including92,93]. Lasers metals, can beceramics, applied and for sinteringpolymers various [94–96]. materials In recent includingyears, lase metals,r processing ceramics, has been and polymersemployed [ 94in –the96]. fabrication In recent years,of electrodes laser processing [97], supercapacitor has been employeds [98,99], in even the fabrication full batteries of electrodes[100]. Successful [97], supercapacitors micromanufacturing [98,99], evenincludes full batterieslaser-drilling [100 ].of Successful microhole micromanufacturings in LiFePO4 cathode includes for Li-ion laser-drilling batteries of[101] microholes and laser in LiFePOcarbonization4 cathode anode for Li-ion(graphene) batteries for an [101 interdigital] and laser film carbonization battery [102]. anode Li [6] (graphene) reported femtosecond for an interdigital laser- filmreduced battery nano [102 joined]. Li [ 6graphene] reported oxide/Au femtosecond conductive laser-reduced network nano as micro-supercapacitors joined graphene oxide electrodes./Au conductive Pröll network[103] reported as micro-supercapacitors femtosecond-laser structuring electrodes. of PröllLiMn [2103O4 composite] reported cathodes femtosecond-laser for Li-ion micro-batteries. structuring of LiMnYu2O4 [3]composite reported cathodeslaser sintering for Li-ion of printed micro-batteries. anodes for AABs. Results indicated that laser sintering can removeYu [3] reported the organic laser solvent sintering from of the printed printed anodes Al nanoparticle for AABs. Results slurry and indicated increase that the laser conductivity sintering canof the remove printed the organicanode. solventElectrochemical from the printedcharacterization Al nanoparticle demonstrated slurry and laser increase power the conductivityof 10 W for ofsintering the printed for better anode. performance, Electrochemical and characterization 3-layer printed demonstrated anode with a laser bigger power discharge of 10 W capacity. for sintering A 3- forlayer better battery performance, cell can yield and 3-layera 239 mAh printed g−1 anodedischarge with capacity a bigger at discharge an operation capacity. voltage A 3-layer of 0.95 battery V, as 1 cellshown can yieldin Figure a 239 17. mAh g− discharge capacity at an operation voltage of 0.95 V, as shown in Figure 17.
FigureFigure 17.17. ((aa)) SchematicSchematic ofof aluminum–airaluminum–air batteriesbatteries (AAB)(AAB) workingworking principle,principle, ((bb)) PhotoPhoto ofof aa packagedpackaged batterybattery cell,cell, ((cc)) TheThe firstfirst cycle cycle discharge discharge capacity capacity with with di differentfferent laser laser sintering sintering powers. powers. ((dd)) DischargeDischarge capacitycapacity for for 3D 3D printed printed anode anode and and relationship relationship between between anode anode thickness thickness and and capacity capacity [3] [3] (Copyright (Copyright© 2018,© 2018, Electrochemical Electrochemical Society). Society). 5.3. 3D Printing Traditional thin film 2D batteries have suffered from limited energy capacity. 3D printing of flexible micro-batteries with nanostructures can overcome this weakness. Currently, the printed parts of batteries can be electrodes, current collector, solid-state separator, and catalyst in metal–air batteries [104]. Zhou [105] reviewed 3D printing energy storage devices with a sandwich-type and in-plane architecture and demonstrated that the electrochemical energy storage systems can be greatly promoted with 3D printing. Lewis et al. [106,107] reported 3D fully printed electrodes for Li-ion batteries. Li4Ti5O12 (LTO) and LiFePO4 (LFP) were separately employed as anode and cathode materials. Electrode material inks were printed onto the substrate, forming multilayer electrodes and an anode and cathode in an interdigitated structure. The results showed that the charge and discharge of 8-layer full cell delivered Appl. Sci. 2019, 9, 2787 17 of 22
2 1.2 mAh cm− at a rate of 0.5 C. EIS test revealed the thicker electrode had higher resistance. Meanwhile, CV testing showed the thin wall displayed broader redox peaks. Furthermore, both thin and thick 2 2 electrodes exhibit excellent Coulombic efficiencies. 3D printed LIBs had 4.45 mAh cm− at 0.14 mA cm− , 2 2 corresponding a full cell delivering 14.5 mAh cm− at 0.2 mA cm− . The same printed technology can also be employed for MABs. For MABs, screen printing has been used in catalysts [108]. A remarkable shortcoming is the narrow choice of suitable materials for printing. In addition, expensive equipment also limits application. However, 3D printing is the destructive technology in MAB manufacturing due to unprecedented designing freedom, high precision, and cost-effective processing.
6. Summary and Outlook In summary, this paper briefly reviewed the recent advances in the studies of the metal–air batteries. Better batteries should be an excellent combination of cathode, anode, and electrolyte., however, there are still some problems to be solved, such as anode side reaction, impure gas CO2 release, electrolyte instability, and so on. Essentially, improving ORR and OER are quite important to the cathode. Crystallographic structure, materials size, materials morphology, carbon-based materials-doped, and composites can influence different activities of catalyst, which is required to improve both ORR and OER in the cathode. Nanocomposites and doped-carbon materials are good choices for catalysts in ORR and OER. Research indicates that compared with traditional alloy as an anode, alloys with nanocomposites can reduce the side reaction and improve discharge capacity. While various electrolytes have different advantages more efficient solid-state electrolyte is required in rechargeable metal–air batteries. Integration of advanced manufacturing, especially 3D printing and laser processing, opens new horizons for MABs. These manufacturing processes allow a better strategy for the systematic combination of the best performance of anodes, cathodes, and electrolyte for improved energy density, efficiency, and cycling stabilities. Although many issues still exist, the further development of MABs, as a compelling alternative to LIBs, holds great promise to address emergent needs of portable electronics, electrical vehicles, and IoTs.
Author Contributions: C.W.is responsible for the reference survey,analysis, figure preparation and manuscript drafting. Y.Y.contributes to the laser processing section. J.N., X.L., Y.Z. contribute to the section of “in situ characterization using an electron microscope”. Y.L., D.B., J.P. are responsible for the revision of the paper. A.H. advises the review structures, revises the logic order of the chapters and polishes the final manuscript. Funding: This work is partially supported by the National Natural Science Foundation of China (51575016) and NSFC-DFG joint project (51761135129). Conflicts of Interest: The authors declare no conflict of interest.
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