nanostructured electrode materials for high-energy...

Nanostructured Electrode Materials for High-Energy Rechargeable Li,Na and Zn Batteries†

Yamin Zhang and Nian Liu*

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

ABSTRACT: Fossil fuel is the main energy resource currently.The continuous consumption of this nonrenewable resourcehas caused very serious environment problems, which hasmotivated tremendous research efforts in this century. Energystorage is critical to alleviate the current energy andenvironmental problems. Comparing to mechanical energystorage, rechargeable batteries allow energy storage with asmaller footprint. The intersection of rechargeable battery withnanomaterials has been a booming research topic recently andyielded new applications of nanomaterials as well as newsolutions to many long-lived problems in battery science andtechnology. In this Perspective, we highlight the most recent(2015−2017) examples across lithium, sodium and zinc batterychemistries, where nanoscale materials tailoring and design addresses the intrinsic problems and limitations at both the materialslevel and device level. And a few principles are generalized at the end.

■ INTRODUCTION

Fossil fuel is the main energy resource currently, but it isnonrenewable. The consumption of this resource has causedsome serious problems, such as climate change, and air andwater pollution.1 In the past few decades, researchers haveachieved much progress in sustainable energy development,including photothermal receivers, wind turbines and photo-voltaic cells. However, compared with energy generators,energy storage technology is also critical to alleviate the energyand environmental problems. Finding low-cost, high efficiencyand environmentally friendly energy storage devices isimportant for the development of economy and society,which can be used in small electronic devices (smart phones,laptops, etc.), electric vehicles and even grid energy storage.2

Among energy storage technologies, rechargeable batterieshave advantages of small footprint, fast response and location-independence, which give them great potential. The recentsuccess of entirely battery-powered electric vehicles is animportant intermediate step toward even larger scale energystorage for the grid.3 Energy density, power density, safety andcost are the most important factors for batteries, includingrechargeable lithium batteries,4,5 sodium batteries6 and zincbatteries.7

Lithium batteries are widely used currently. Lithium has thehighest energy density and it is the lightest metal on theperiodic table. However, organic electrolyte is used for lithiumbatteries, which is not safe. Besides, lithium electrodes are lessstable with limited capacity, which limits its development forvarious different applications.

Sodium is more abundant compared with lithium and it canbe a cheaper option for large-scale energy devices. Sodiumpossesses high electrochemical capacity, which is good to be acandidate for rechargeable batteries. But the ionic radius ofsodium is bigger than lithium’s ionic radius, which makes itharder to find the reversible intercalation electrodes. Also,compared with the lithium battery, the sodium battery has asmaller obtainable voltage window due to the higher standardelectrode potential of sodium.Zinc has various advantages such as abundance, low cost,

lower equilibrium electrochemical potential, a flat dischargeprofile, a long storage life and environmental benignity. Zinc isthe most active metal that is stable with water. By usingaqueous electrolyte, a zinc battery is safe and can bemanufactured in ambient condition. Also, Zn has highvolumetric capacity with two valence electrons and highdensity. However, ZnO passivation and zincate ion dissolutionlimit the reversibility of zinc battery. Besides, hydrogenevolution makes it difficult to realize sealed cell geometry.Nanomaterials have at least one dimension in the nanoscale,

which include nanoparticles, nanowires, nanorods and so on.Nanostructured materials offer unusual chemical, mechanicaland electrical properties that are absent in the bulk, and havefound applications in various energy storage devices.8 Forexample, cycling life of lithium batteries can be improved byusing nanoelectrodes thanks to the better strain accommoda-tion during lithium insertion/deinsertion steps. Also, highercontact area between electrode and electrolyte and shorter path

Received: September 11, 2017Revised: October 14, 2017Published: October 23, 2017†This Perspective is part of the Up-and-Coming series.

Perspective

pubs.acs.org/cm

© 2017 American Chemical Society 9589 DOI: 10.1021/acs.chemmater.7b03839Chem. Mater. 2017, 29, 9589−9604

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lengths of nanoelectrodes can lead to higher charge/dischargerates.9 Most importantly, nanomaterials are highly tunable to befabricated and designed for different purposes, which isbeneficial for solving complex problems in batteries.Many technical approaches have been explored to fabricate

nanomaterials, including vapor phase growth, liquid phasegrowth, solid phase formation and hybrid growth. Besides, mostcommonly used characterization methods have matured, whichinclude structural characterization methods (scanning electronmicroscopy (SEM), transmission electron microscopy (TEM),X-ray diffraction (XRD) and various scanning probe micros-copy) and chemical characterization methods (electron spec-troscopy, ion spectroscopy and optical spectroscopy). Thesemake it possible for nanomaterials to be used and evaluated inbatteries.In this Perspective, we will focus on nanostructured

electrodes for lithium, sodium and zinc batteries. Due tolimited space, we will use a few examples in each batterychemistry to illustrate the design principles. In a lithium battery,we will focus on silicon anode, lithium metal anode, sulfurcathode and oxygen cathode. In a sodium battery, we will focuson anode materials, sulfur and metal sulfide cathodes. In a zincbattery, we will focus on zinc anode, MnO2, NiOOH andoxygen cathodes. Figure 1 shows calculated energy densities ofLi, Na and Zn secondary battery systems.10

■ LITHIUM BATTERYSilicon Anodes. Lithium-ion batteries (LIBs) are widely

used in small electronic devices (smart phones, laptops, etc.)and electric vehicles.11−14 However, the increasing energystorage demands continues to request more operating cycles,better stability, higher power and energy densities. Thetheoretical specific capacity of a commercialized graphiteanode is 372 mAh g−1, which is far from the energy storagerequirement.15 There are many alloy16,17 and conversion-reaction anode materials including Si,18 Sn,19 Al,20 metaloxides,21 etc. Silicon is a very attractive material to be the anodein lithium batteries. It has a theoretical specific capacity of 4200mAh g−1, which is more than 10 times that of graphite.22,23 Asan anode, silicon could be used in traditional LIBs, recent Li−S

and Li−O2 batteries. A silicon anode is a good replacement forthe lithium metal, which has potential to form dendrite.However, during the cycling, the silicon anode has a big volumechange (∼300%), which causes the structural degradation ofsilicon, instability of solid-electrolyte interphase (SEI) andfurther limits the development of silicon anodes. Nano-structured electrode design has been proven to be an effectiveapproach to solve these challenges.For nanoparticle design, Liu et al.24 developed a hierarchical

structured silicon anode (Figure 2A,B) which successfullytackled problems including structural degradation, unstable SEIand side reactions. In addition, the problem that nanoelectrodeusually has low volumetric capacity was also solved thanks totheir design. Their inspiration came from the structure of afruit, pomegranate. In their design, every single siliconnanoparticle was encapsulated by a carbon shell (Figure 2E),which provided enough free space for silicon anode expansionand contraction resulting from lithiation and delithiation. Thenan ensemble of these hybrid nanoparticles was furtherencapsulated by a thicker conductive carbon layer and formeda micrometer-sized pomegranate. This thick carbon layer servedas an electrolyte barrier. This nanostructured silicon anode hassuperior cycle stability with a capacity retention of 97% after1000 cycles (Figure 2F). It also shows 1270 mAh cm−3

volumetric capacity and high Coulombic efficiency of almost100%. The areal capacity of this anode can even reach 3.7 mAhcm−2, which is the areal capacity of commercial LIBs, andremained stable over 100 cycles. This was attributed to thestable and spatially confined solid-electrolyte interphase and thelower electrode−electrolyte contact area. Besides, Xu et al.25

synthesized the watermelon-inspired Si/C microspheres to getbetter electrochemical performance of densely compacted Si/Cnanostructured anodes. This densely compacted Si/C anodeexhibited superior cycle stability, as well as excellent rateperformance and Coulombic efficiency. In contrast toimpregnation coating, Lu et al.26 designed a different structurednanoelectrode, nonfilling carbon-coated porous silicon micro-particles (nC-pSiMP), and solved the problems includingstructural degradation and the low volumetric capacity. In theirdesign, many interconnected silicon nanoparticles first formed aporous silicon microparticle, then the outer surface of thismicroparticle was coated with the conductive carbon. Thiscomposite structure demonstrated high volumetric capacity(∼1000 mAh cm−3), excellent cycling stability and reversibility(∼1500 mAh g−1 capacity retention for 1000 cycles). Fornanomaterials, low tap density is another major issue needed tobe addressed. Lin et al.27 demonstrated a mechanical approachto assemble nanostructured Si secondary clusters (nano-Si SC)and produced 420 g (>95% yield) of nano-Si SC in lab. Theyachieved much denser packing of nanostructures (1.38 g cm−3)and 0.91 g cm−3 high tap density. As a result, nano-Si SCshowed excellent cycle stability with more than 95% capacityretention after 1400 cycles. As a substitute for carbon, graphenecan also be used as coating to encapsulate Si particles. Li et al.28

introduced conformal graphene cages (Figure 2C,D), whichacted as a mechanically strong and flexible buffer, onmicrometer-sized silicon particles as lithium battery anodes(SiMP@Gr). Such a cage (Figure 2G) allows the expansion andcontraction of microparticles inside the cage during the cycling.The electrical connectivity was retained as well. It showedstable cycling and high Coulombic efficiency due to a stable SEIand the minimization of irreversible consumption of lithium

Figure 1. Calculated energy densities of Li, Na and Zn secondarybattery systems.

Chemistry of Materials Perspective

DOI: 10.1021/acs.chemmater.7b03839Chem. Mater. 2017, 29, 9589−9604

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ions. Besides, silicon-nanolayer-embedded graphite also ex-hibited a high Coulombic efficiency.29

In another case, Ryu et al.30 successfully prepared Sinanosheets from inexpensive natural clays. This scalablecarbon-coated Si nanosheets anode showed a higher reversiblecapacity of 865 mAh g−1 at 1.0 A g−1 rate with 92.3% capacityretention after 500 cycles. After 200 cycles, it only showed 42%nominal volume expansion. It also delivered higher ratecapability with 60% capacity at rate of 20 A g−1. Althoughcoating the nanostructured Si with conductive carbon layer orsome other functional conductive materials can significantlyimprove the performance of silicon anode, the mechanism and

relative performance of different types of coatings areincompetently understood by researchers.Luo et al.31 tried to use thin alucone and Al2O3 as coatings to

understand the mechanism of coatings on the Si nanowires(SiNWs). They focused on the lithiation step of SiNWs andsummarized the effect of these two coatings. They found twotypes of lithiation fronts of the SiNWs through a nanoscale half-cell battery by using in situ TEM and chemo-mechanicalsimulation, including “V-shaped” and “H-shaped” fronts. The“V-shaped” front was produced by alucone coating, whereas the“H-shaped” front was leaded by Al2O3 coating. Their resultsindicate that morphological evolution during lithiation step isdetermined by the lithium diffusivity on the surface and bulk

Figure 2. Structure and performance of silicon pomegranate, SiMP@Gr and Li−rGO as Li-ion battery anodes. (A) Three-dimensional view ofstructure change of a pomegranate-like microparticle before and after battery cycling. (B) Simplified 2D cross section view of structure change of apomegranate-like microparticle before and after battery cycling. Reproduced with permission.24 Copyright 2014, Springer Nature. (C) Simplified 2Dcross section view of SiMP before and after battery cycling. (D) Simplified 2D cross section view of SiMP@Gr before and after battery cycling.Reproduced with permission.28 Copyright 2016, Springer Nature. (E) SEM image of a pomegranate-like microparticle with a 40 nm void space. (F)Reversible delithiation capacity of the silicon pomegranate anode for the first 1,000 cycles. (G) TEM image of an individual SiMP@Gr particle. (H)SEM image of pristine GO film. (I) SEM image of sparked rGO films. (J) SEM image of layered Li−rGO films. Reproduced with permission.39

Copyright 2016, Springer Nature.



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lithiation reaction rate of different coating layers on the SiNWs.Besides, the rate performance of the battery can be determinedby the coating layer as a result that the lithiation rate in thecoatings could be the limiting step for the lithiation reaction.Lithium Metal Anodes. Although nanostructured Si

materials are promising candidates as anodes in LIBs, thelithium metal anode is considered as other promising candidateanode for rechargeable LIBs thanks to the highest theoreticalcapacity (3860 mAh g−1)32 and lowest electric potential (−3.04V versus SHE) of lithium metal. However, during charge/discharge cycles, uneven lithium dendrite deposition, unstableSEI and relatively big dimension change problems, lead to low

Coulombic efficiency and serious safety concerns, which limitsits application. As a result, many efforts have been put onadvanced characterization in order to elucidate the lithiumgrowth process, and on structure design of the lithium metalanode to improve cycling performance.Coating is one of the earliest explored strategies. Nanoma-

terials coating can achieve the goal with minimum increase ofthe electrode weight. Applying thin chemical protection layersdirectly on Li metal is a promising solution to protect Li metal,such as protective polymer layer, atomic layer deposition(ALD) of 14 nm thick Al2O3 layer33 and the artificial LiF-containing protection film.34 Besides, ultrathin two-dimensional

Figure 3. Various nanostructured sulfur cathodes. (A) 3D view of CB-HNSs-PD double-shelled cathode before and after being discharged. (B)Implified 2D cross section view of CB-HNSs-PD double-shelled cathode before and after being discharged. (C) SEM image of CB-HNSs-PDcathode. (D) TEM image of CB-HNSs-PD showing CB stick on the HNSs-PD. Reproduced with permission.48 Copyright 2017, Wiley. (E)Synthesis method of the MnO2@HCF/S composites. (F) Mechanism of advantages of MnO2@HCF/S composites over HCF/S. (G) SEM andTEM images of MnO2 @HCF. (H) SEM and TEM images of MnO2@HCF/S composite. Reproduced with permission.55 Copyright 2015, Wiley.



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(2D) atomic layers, which consists of hexagonal boron nitride(h-BN) and graphene were grown directly on Cu metal currentcollectors.35 This 2D atomic crystal structured electrodeshowed a superior protection for lithium metal and smoothlithium deposition was realized. Zheng et al.36 used amorphoushollow conductive carbon monolayers to coat the Li metal andthis anode achieved a significantly higher Coulombic efficiency(more than 99% for more than 150 cycles) than the bare one.They attribute this good performance to the carbon nano-spheres, which could help to isolate the lithium deposition andfacilitate stable SEI formation. Minimizing volume change byusing stable hosts is also a good way to improve lithium metalanode cycling performance.37 Lin et al.38 developed LixSi−Li2Oas a stable host for lithium-ion insertion/deinsertion. As aconductive framework, it blocks embedded lithium from directcontact with electrolyte and keeps constant volume in theelectrode-level. It exhibited high-power output and showed astable cycle performance. Besides, Lin et al.39 reported layeredreduced graphene oxide (rGO) as a stable host for lithium (Li−rGO, Figure 2H−J) because rGO has nanoscale interlayer gaps.Such designed electrode limited electrode thickness change toonly ∼20% during cycling, avoided Li dendrite deposition andunstable SEI problems of lithium foil.On the other hand, the heterogeneous nucleation of Li metal

has been utilized in nanoscale design. Yan et al.40 explored thenucleation of lithium on various metal substrates and foundthat nucleation barrier exists on metals showing negligiblelithium solubility, whereas no nucleation barrier is present formetals exhibiting a definite solubility. Furthermore, theydesigned a nanocapsule structure with Au nanoparticle seedsinside and hollow carbon spheres outside for the lithium metalanode. As a result, the lithium metal predominantly grew insidethe hollow carbon spheres during deposition.Introducing a new electrolyte system and additives is also an

efficient method to modify the solid-electrolyte interphaselayer41 and minimize the lithium dendrite growth. Qian et al.42

reported a highly concentrated electrolyte, which minimizedthe lithium dendrite deposition and showed a high-rate cyclingperformance with more than 99% Coulombic efficiency. Thiselectrolyte was consisted of lithium bis(fluorosulfonyl)imidesalt and ether solvents. This concentrated electrolyte canincrease both the solvent coordination and Li-ion availability inthe electrolyte.Sulfur Cathodes. Among the various electrochemical

energy storage systems, rechargeable lithium−sulfur (Li−S)battery has drawn considerable attention because of their higherspecific energy (∼2500 W h kg−1) compared to lithium-ionbatteries.43−46 However, the Li−S system has fundamentalchallenges to be solved, for example, the low electricalconductivity of S and sulfides, and solubility of intermediatelithiated sulfur compounds.47

An efficient route to enhance cycling performance of Li−Sbattery is confining soluble sulfur species by coating, includingcarbon, polymer, metal oxide and metal sulfide. Wu et al.48

reported a stable double shelled sulfur cathode, carbon blackdecorated polydopamine-coated hollow nanosulfur spheres(CB-HNSs-PD) (Figure 3A−D), with stable cycling over2500 cycles. They used polydopamine as “nano-binder” to gluecarbon black and sulfur at the nanoscale, which was differentfrom those only used additional binder to fabricate cathode inthe coating process. This approach with void space insidesuccessfully prevented carbon black’s detachment from cathodesurface during cycling, provided conductive agents for their

reutilization, trapped the polysulfides and reutilized themigrating polysulfides. Li et al.49 published a pomegranate-like carbon cluster−encapsulated sulfur as the cathode. Theirinspiration came from the design of silicon−carbon pomegran-ate-like cluster structure.24 Besides, graphene-based carbon,50

which can expectedly yield a strong interaction between S andfunctional group in graphene, is also a functional coating for Scathode.51 For example, Li et al.52 achieved ordered sulfur−graphene (S-G) nanowalls by using a simply electrochemicalassembly strategy. The S-G nanowalls were aligned ontoelectrically conductive substrates. Such a cathode had excellentcyclability and high rate performance. Specifically, graphenearrays arrange perpendicularly to the substrates and sulfurnanoparticles are homogeneously anchored between graphenelayers. This structure improves the accommodation of sulfurvolume change and diffusivity of lithium and electron.Interconnected graphene/carbon nanotube (CNT)/sulfur(denoted GCS) hybrid53 can also improve the accommodationof sulfur volume variation and the transfer of Li and electronduring cycling, which can be attributed to the conductivenetwork formed by graphene and CNT. In this structure, thegraphene/carbon nanotube has ∼70 wt % uniform loading ofsulfur. Such a GCS hybrid exhibited a capacity of 657 mAh g−1

for more than 450 cycles with 0.04% capacity decay per cycle.In addition, carbon nanofibers, carbon nanotubes, carbon

nanocages and C-wood matrix54 are also promising sulfur hostsfor Li−S batteries. Li et al.55 filled MnO2 nanosheets intohollow carbon nanofibers (MnO2@HCF, Figure 3E,G) assulfur host. Both the lithium and electron transfer can befacilitated by using this MnO2@HCF hybrid host. Besides, sucha host can prevent polysulfide dissolution (Figure 3F) byphysically entrapping polysulfides into a carbon shell andchemically binding them to the MnO2. The sulfur loading wasas high as of 71 wt % in the MnO2@HCF/S electrode (Figure3H). The MnO2@HCF/S electrode showed a good perform-ance with specific capacity of 1161 mAh g−1 at 0.05 C rate andstable cycling for more than 300 cycles at 0.5 C rate. Zhu etal.56 rationally integrated high conductive superlong carbonnanotubes (CNTs) and nanosized hollow graphene spheres(GSs) and successfully constructed a free-standing paperelectrode. The hollow GSs not only accommodate sulfurspecies and sustain the volume fluctuation during cycling butalso retard the dissolution of polysulfides and parasitic shuttle.The graphene walls of GSs and superlong CNTs synergisticallyconstructed hierarchical short-/long-range electron/ion path-ways. As a result, this cathode exhibited a high-rate capacityretention of 40% at 16.7 A g−1 and a superior capacity retentionof 89.0% over 500 cycles with a high sulfur utilization of 81%.Lyu et al.57 reported a new kind of carbon−sulfur compositeswith a high sulfur loading of 79.8 wt % by infusing sulfur intothe novel hierarchical carbon nanocages (hCNC) with highpore volume, network geometry and good conductivity. Thedesigned S@hCNC composite presented a large capacity, highcurrent density capability and long cycling life, which couldshorten the charging time for mobile devices from hours tominutes. The good performance of this cathode is also due tothe alleviation of polysulfide dissolution, the enhancement ofelectron and Li-ion diffusion. The cathode architecture withnatural, three-dimensionally (3D) aligned microchannels filledwith rGO also achieved good cycling performance.54

Metal oxide and sulfide, such as TiO2, MoS2 and LiS2,58 can

also effectively solve the insulating problem and enhance thecapacity of sulfur cathode. Sun et al.59 prepared mesoporous



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titanium dioxide submicrospheres (MS TiO2) by triggering self-assembly of primary TiO2 nanoparticles during hydrothermalsynthesis. This unique 3D structure can realize effective contactbetween host material and lithium polysulfide intermediates,physically confine the intermediates to the electrode, andaccommodate the volume change of sulfur upon cycling. As aresult, this cathode exhibited superior rate capacity and cyclingstability than that of commercial TiO2 as well as sulfurcomposite cathode. Zhang et al.60 synthesized hierarchicalMoS2/SnO2 nanocomposites by uniformly loading SnO2 ontothe surface of MoS2 nanosheets. This nanocomposite exhibitedhigh capacity (up to 900 mAh g−1 for more than 80 cycles) andcycling capability because of the inhibition of lithiumpolysulfide diffusion by the loading of SnO2 nanoparticlesand the increasing conductance of the composites due to thepresence of metallic Mo particles. Incorporating few-layernanosheets of phosphorene, which served as an immobilizerand catalyst in Li−S batteries, into a porous carbon nanofibernetwork (cathode matrix) could also obviously improve thecycling performance of Li−S batteries.61

Oxygen Cathodes. The rechargeable lithium oxygen (Li−O2) battery has drawed increasing attention recently. Theenergy density of the Li−O2 battery is 5217 Wh kg−1, which isthe highest among all types of rechargeable batteries.62−64

However, limitations of the Li−O2 battery, such as corrosion ofcell components by reactive oxygen species, large voltagehysteresis, low current density and unstable cycling perform-ance, hinder the possible applications of the Li−O2 battery.

65

As published, the discharge process of Li−O2 batteries isoxygen reduction reaction (ORR, O2 (g) + 2Li+ + 2e− →Li2O2(s)), which forms solid Li2O2 on the surface of cathode.The decomposition of the solid Li2O2 is oxygen evolutionreaction (OER, Li2O2(s) → 2Li+ + O2(g) + 2e−), whichrequires higher potentials during the recharge process.66 Suchrequired high recharge potential causes big energy loss of Li−

O2 batteries. Using a catalyst in the air cathode is an efficientway to suppress the energy loss. The designed heterogeneouselectrocatalysts should be able to decrease overpotentials ofboth O2 reduction reaction and O2 evolution reaction becauseLi2O2 is an electronic insulator and has low solubility in organicelectrolytes.61,67 Nanostructured electrocatalysts have receivedmuch interest because of their high surface area and excellentcatalyst performance. Here we will discuss nanostructuredmetal oxide, carbon and carbon-based electrocatalysts.Metal oxide catalysts in the form of powder can be used in

cathode by mixing with carbon black and binder. For example,manganese oxides (MnOx) have attracted intensive attentiondue to their abundance in the earth’s crust, low-price,nontoxicity and high catalytic activity toward oxygen reductionreaction. Jang et al.68 synthesized sea urchin shaped α-MnO2/RuO2 mixed oxides as the air cathode. MnO2 and RuO2nanoparticles homogeneously distributed and formed severalstraight and radially grown nanorods. This nanostructureshowed excellent stable cycling performance by decreasingthe overpotential as air cathode catalyst. Huang et al.69

prepared Ag nanoparticles decorated β-MnO2 nanorods (Ag/β-MnO2) as the cathode catalyst for rechargeable Li−O2batteries, which showed superior rate capability and cyclingstability. Furthermore, the results showed that the electro-chemical performance of β-MnO2 was greatly enhanced by thepresence of Ag nanoparticles. Co3O4 also has relatively bettercatalytic activity toward both ORR and OER than othertransition metal oxides. Liu et al.70 synthesized hierarchicalCo3O4 porous nanowires (NWs, Figure 4A) as the cathodecatalyst, which showed improved efficiency and stable cyclingperformance in organic Li−O2 batteries. All the nanostructuredcatalysts shown above exhibited excellent bifunctional electro-catalytic activity for ORR and OER, which accounted for thesuperior performance when they served as the catalysts in thecathode.

Figure 4. Various nanostructured metal oxide for Li−O2 cathode. (A) SEM and TEM images of porous Co3O4 NWs. Reproduced with permission.70

Copyright 2015, Springer. (B) SEM image of MCCs. Reproduced with permission.72 Copyright 2015, Wiley. (C) Schematic of the fabrication of Au/Ni NW. (D) SEM image of Au/Ni electrode. Reproduced with permission.74 Copyright 2015, Wiley.



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As effective electrocatalysts for the ORR, carbon and carbon-based nanostructured materials have also attracted muchattention in Li−O2 batteries.71 Sun et al.72 reported themesoporous carbon nanocubes (MCCs, Figure 4B) as thecatalyst for Li−O2 batteries, which delivered discharge capacityof 26 100 mAh g−1 at rate of 200 mA g−1. Compared withcommercial carbon black catalysts, this value is promising.MCCs have a lot of hierarchical mesopores and macropores,which could increase O2 diffusion rate and give space foraccommodation of insoluble Li2O2. As a result, MCCs could beused as a conductive host for other highly efficient catalysts. Forexample, Ru functionalized MCCs showed excellent OERcatalytic activity. Guo et al.73 synthesized uniform orderedmesoporous carbon nanofiber arrays coated with RuO2, whichalso displayed good cycling performance, such as high capacity,high rate capability and long-term cycling. The explanation forthe high performance of this material is similar to that of aboveRu functionalized mesoporous carbon nanocubes. Mesoporesfacilitated both the electron and ion transfer, while themacropores provided void space to accommodate O2/Li2O2and improve O2 diffusion. Moreover, the RuO2 coatingalleviated the decomposition of electrolyte and enhanced theelectronic conductivity of surface.As reviewed above, many catalysts can be used in the cathode

to improve battery performance. However, there are drawbacksbecause catalysts were usually loaded on carbon substrates. Thedischarge product Li2O2 is a strong oxidizer. The carboncathode can be attacked by Li2O2, which is thermodynamicallyfavored.

+ + → Δ = − −GLi O C12

O Li CO 561.2 kJ mol2 2 2 2 31

+ → + Δ = − −G2Li O C Li O Li CO 552.3 kJ mol2 2 2 2 31

For this reason, the cathodes with carbon have short-timecycling performance, which will cause ambiguity if investigatorswant to study the stability of electrolyte when Li2O2 is present.As a result, it is very important to investigate carbon-free andbinder-free electrodes. Kim et al.74 coated a Ni nanowire

substrate with <30 nm Au nanoparticles (Figure 4C,D) as theelectrode (Au/Ni NW) for Li−O2 batteries. This Au/Ni NWshowed a high capacity and exhibited superior cycling stabilityfor more than 200 cycles. In addition, Luo et al.75 preparedporous AgPd−Pd composite nanotubes (NTs) as a bifunctionalcatalyst for both ORR and OER in Li−O2 batteries. TheAgPd−Pd composite NTs showed excellent cycling perform-ance because this structure facilitated oxygen and electrolytediffusion and provided numerous catalytic active sites.

■ SODIUM BATTERY

Anode Materials. Li-based batteries have been widely usedin many energy storage systems in the past few years and arewell-known for superior performance in comparison with otherrechargeable batteries in the matter of energy density and cyclelife. Unfortunately, lithium is scarce on earth. Na is in the samecolumn as Li in the periodic table. These two elements havemany relatively similar chemical properties. Therefore, manyefforts have been focused on sodium-ion batteries (SIBs),which are promising to be abundant and economical.51 Earlyevidence indicates that Li-intercalation compound materialsmay be not suitable for Na-intercalation, because the potentialand diameter of them are different.6 For instance, Na is hard tointercalate into graphite, which is used for anode in thecommercial LIBs. Hence suitable host materials for SIBs needto be developed. In this section, we summarize below nanoscaledesign for SIB electrode materials.On the anode side, phosphorus (P) has the highest

theoretical capacity for sodium storage. The capacity is about2600 mAh g−1 if the calculation assumption is that P istransformed to Na3P. However, P has low electricalconductivity and large volume expansion (∼300%). Zhu etal.76 fabricated a red phosphorus-single-walled carbon nanotube(red P-SWCNT) composite by distributing red P into tangledSWCNTs bundles. The red P-SWCNT showed a superioroverall sodium capacity, long-time cycling stability and high ratecapability, which can be attributed to the improved conductivityand stable SEI. Sun et al.77 further made a hybrid material andsolved the pulverization problem by sandwiching a few

Figure 5. Structure and electrochemical performance of phosphorus−graphene anode for sodium battery. (A) Structural evolution of the sandwichedphosphorene−graphene structure during sodiation. (B) Reversible desodiation capacity and Coulombic efficiency for the first 100 galvanostaticcycles of the phosphorene/graphene (48.3 wt % P) anode tested under different current densities. Reproduced with permission.77 Copyright 2015,Springer Nature.



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phosphorene layers between graphene layers (Figure 5A). Thisanode exhibited a specific capacity as high as of 2440 mAh g−1Pat 0.05 A g−1 rate and an >80% capacity retention for more than100 cycles (Figure 5B). The large capacity of their anode couldbe attributed to the structure of the hybrid material. On the oneside, Na can be intercalated along the x axis of the P layers toform Na3P. On the other side, graphene layers serve aselectrical highway and provided buffer space to accommodateexpansion on the y/z axis.Transition metal oxides, including V2O5,

78 MnO279 and

TiO2,80 have also been used as anode materials in SIBs. For

example, TiO2 is chemically stable, abundant and nontoxic,which makes it a possible candidate to be an anode for SIBs.However, TiO2 is less conductive, which limits its ratecapability and therefore its electrochemical performance. Yeoet al.81 proposed a functional TiO2 composite with goodperformance. In their design, TiO2 nanofibers were wrapped byrGO (rGO@TiO2 NFs). This composite exhibited asignificantly high initial capacity. Besides, it showed morethan 80% capacity retention over 200 cycles and >90%Coulombic efficiency. Comparing with pristine TiO2 NFs,this excellent performance could be attributed to the highlyconductive properties of the rGO anchored to the TiO2 NFs.Highly ordered 3D Ni-TiO2 core−shell nanoarrays fabricatedby Xu et al.82 also delivered a high reversible capacity, fast ionand electron transfer, and good electrode integrity as an anodematerial.Materials with layered structure have been successful as hosts

for Li-ion batteries, because of facile ion intercalation and theinterlayer spacing accommodates volume expansion. They havebeen explored for SIB electrodes as well. Tin disulfide (SnS2)has high theoretical capacity, big interlayer distance and is notexpensive. Conventionally, SnS2 nanomaterials were usuallyhydrothermally synthesized and took a lot of time, which ishard to utilize on a big scale. Wang et al.83 developed astraightforward solid-state reaction method to synthesize acarbon-coated SnS2 (SnS2/C) nanosphere as the anode, whichshowed only 0.14% decay per cycle and exhibited excellentcycling performance with a high reversible capacity. MoS2 has alarge interlayer distance along the c-axis, which is good for

accommodation of sodium ions. Su et al.84 successfullysynthesized few-layer ultrathin MoS2 nanosheets (∼10 nm),which exhibited a superior cycling performance. This is becausethat MoS2 is ultrathin and has large interlayer space, whichprovide a short ion diffusion path. Xiong et al.85 fabricated aflexible membrane by encapsulating MoS2 with interconnectedcarbon nanofibers as a binder-free electrode. Such a membranedemonstrated superior performances, such as high reversiblecapacity as well as superior rate capability, showing promise aslong-life SIB anode.Wang et al.86 developed a new anode electrode with

pseudocapacitive charge storage to enhance the energy andpower of SIBs. The nanosheet compound MXene Ti2C showedpseudocapacitance, which accounted for the higher specificcapacity relative to double-layer capacitor electrode and higherrate capability relative to ion intercalation electrode. Also, it canbe operated even at a relatively high voltage of 2.4 V, which wasnot possible for conventional energy storage systems.

Sulfur and Metal Sulfide Cathodes. Sodium−sulfur(Na−S) batteries are triggering tremendous research interestas candidates to power our future society as a result of the hightheoretical capacity (1672 mAh g−1, based on the formation ofNa2S) of a Na−S battery. The Na−S battery at roomtemperature is safe, simple, low-cost and potentially offershigh energy densities.87,88 However, operating the Na−Sbattery at room temperature is more challenging than for Li−S batteries. Na−S batteries are limited by the low conductivityof sulfur, large volumetric variation (≈170%), slow reactionkinetics between S and Na, and high reactivity or solubility ofpolysulfides in most common liquid electrolytes which leads topoor cycle stability. In this section, we focus mostly on thenanostructured sulfur cathode of the Na−S battery.Sodium polysulfides have high reactivity and solubility in

common liquid electrolytes such as carbonates or glycols,respectively, which leads to short cycle life. Qiang et al.89

addressed this problem by demonstrating a highly dopedsulfur/carbon nanoporous cathode to inhibit reactivity of thesulfides with carbonate electrolytes and also inhibit thediffusion of polysulfides dissolved in tetraethylene glycoldimethyl ether (TEGDME). This electrode showed ultrahigh

Figure 6. Schematic and electrochemical performance of nanostructured metal sulfide for sodium battery cathodes. (A) FeS2 nanoparticle and itsconversion upon sodiation. (B) Cycling performance and charge efficiency of bulk and ultrafine FeS2 nanoparticle cathodes, upon galvanostaticcycling at 0.1 A g−1. Reproduced with permission.92 Copyright 2015, American Chemical Society. (C) Schematic of the synthesis of yolk−shellFeS@C as SIB cathode. Reproduced with permission.93 Copyright 2015, Springer Nature.



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stability at room temperature with <3% degradation ofdischarge capacity for more than 8000 cycles. This combinationof low cost and excellent cycle stability is promising forstationary, grid-level energy storage. Wang et al.88 also reportedan interconnected mesoporous hollow carbon nanospheres as asulfur host to realize superior electrochemical cycling perform-ance of Na−S battery. In this structure, the C backbone wascontinuous and interlaced, which ensured high tap density, highstructural intimacy, and the integrity of the nanocomposite afterS loading. Besides, the inner hollow nanospaces encapsulated ahigh sulfur ratio and could tolerate the volume changes of theinterior sulfur. The outer carbon nanoshells could confine thepolysulfides, thereby inhibiting the shuttle effect. Therefore, themesoporous carbon shells not only served as a highlyconductive network for S and electron transport but alsosupplied open active diffusion channels. Furthermore, theyproposed the mechanism of their Na/S battery, with reversibleconversion between S8 and Na2S4. Zheng et al.90 successfullyloaded 10% of Cu nanoparticles into high-surface-areamesoporous carbon (HSMC) cathode, which can effectivelystabilize the sulfur (50%) loaded in HSMC−Cu−S. The specialstructured HSMC−Cu−S composite showed a Coulombicefficiency of 100%, which was attributed to the synergisticaleffects of Cu nanoinclusions and HSMC host.Nanoscale metal sulfides have reversible conversion reactions

with lithium or sodium. FeS2 is promising because of its earthabundance, low toxicity and low raw material cost, but has notbeen widely studied for secondary SIBs.91 Douglas et al.92

demonstrated that FeS2 nanoparticles possessed ultrafine sizes(Figure 6A), which were favorable to maintain reversiblereactions compared to bulk-like electrode materials in sodiumion batteries. This is a result that nanoparticle size was similarto or smaller than the diffusion length of Fe. The ultrafine FeS2nanoparticles showed improved cycling, rate capability andreversible capacity over 500 mAh g−1 for sodium storage(Figure 6B). FeS also has high theoretical capacity and is lowcost, environmental friendly and abundant. However, lowelectrical conductivity, inactive kinetics and severe volumechange during sodiation/desodiation hinder its application. Tosolve these problems, Wang et al.93 designed yolk−shell FeS@carbon nanospheres (FeS@C) as the cathode (Figure 6C),which achieved high capacity of ∼545 mAh g−1 over 100 cyclesat 0.2C (100 mA g−1), and delivered high energy density of∼438Wh kg−1. Such nanostructural design, including nanosizedFeS yolks (∼170 nm) with porous conductive carbon shells(∼30 nm) and buffer space (∼20 nm), has been widely used toget satisfactory cycle stability. Na2S could be directly used as acathode material in the discharged state, yet it has a lowconductivity and sodium ion diffusivity. To overcome theseproblems, Yu et al.94 wrapped Na2S particles with multiwalledcarbon nanotube (MWCNT) and then spread them ontoMWCNT fabrics. This unique cathode provided a high capacityof 500 mAh g−1 over 50 cycles.

■ ZINC BATTERYElectric vehicles (EVs) are expected to replace internalcombustion engine vehicles in the coming years. However,Most EVs today use lithium-ion batteries, which have high costand concerns regarding both their safety and the supply oflithium and cobalt. These factors have led to intense researchon alternative rechargeable battery technologies. Comparedwith lithium, zinc is more stable, inexpensive, and compatiblewith aqueous electrolytes.95 There are various energy storage

systems including Zn−MnO2 batteries, Zn−Ni batteries96 andZn−air batteries, among which Zn−air has comparablevolumetric energy density as Li−S batteries. However, theirreversibility of Zn anode limit all of their cycle life. Also, thepower performance of Zn−air battery has historically been adrawback due to sluggish oxygen reduction and evolution at theair electrode. Recently, progress has been made on usingnanostructured electrode materials to improve performance ofZn-based batteries.

Zn Metal and Zinc Oxide Anodes. Although zinc hasrelatively high specific energy and can be charged moreefficiently in aqueous electrolytes, the performance of a zincelectrode is limited by dendrite (defined as sharp, needle-likemetallic protrusions) growth, shape change, passivation andinternal resistance, and hydrogen evolution. A multitude ofstrategies have been investigated to increase the performance ofzinc electrodes, such as high surface area/3D electrodestructure design and electrode additives. Nanomaterials havethe potential to minimize the anode passivation and dendritegrowth problems. Besides, shape change and hydrogenevolution problems can also be solved with the properlynanostructured design. The electrode design can be startedwith Zn or fully discharged state ZnO.Zn foil has widely been used as the anode. However, the low

utilization of Zn foil and poor rechargeability due to dendriteformation have prevented its wider practical applications. Veryrecently, Parker et al.97 designed a monolithic, porous,aperiodic Zn anode structure. Even at a deep discharge, theinner core of Zn is still remained. This 3D microstructuredesign minimizes the dendrite formation problem due to thelarge surface area and porous property. They demonstrated thatthe microstructure 3D zinc (Figure 7) elevated the performance

of Ni−Zn battery in the following fields: (i) more than 90%theoretical depth of discharge (DODZn) in primary cells, (ii)more than 100 high-rate cycles at 40% DODZn and (iii) morethan ten thousand duty cycles required for start−stopmicrohybrid vehicles.Nanostructured ZnO also has attracted much attention. Liu

et al.98 developed ZnO nanoparticles deposited on hierarchical

Figure 7. Schematic of the effect of 3D Zn sponge compared withconventional powder zinc anodes. Reproduced with permission.97

Copyright 2017, AAAS.



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carbon cloth−carbon nanofiber (CC−CF) (Figure 8A) as theanode. The conformally assembled ZnO nanoparticles(replacing traditional Zn plate) are essential to buffer theshape change and suppress the Zn dendrite growth. This anodeenables fast electron transport and rapid ion diffusion due tothe 3D highly conductive and flexible current collector and theintimate deposition of nanosized ZnO. The established full-celldevice delivered high capacity (Figure 10B), high energy andpower densities, and superior cycling stability with absence ofZn dendrite when integrated with polymer electrolyte. Theirwork shed light on the development of next-generation flexiblesolid-state electrochemical devices. Unlike 3D structure design,Liu et al.99 successfully increased the performance by addingdischarge-trapping electrode additives. As shown in Figure 8B,they synthesized Zn/Al layered double oxides (LDOs) fromZn/Al layered double hydroxides (LDHs). The LDHs weredecomposed to ZnO and aluminum species were dispersed inthe lattice. Such an anode showed good electrochemical cycling

performance with good reversible capability and low polar-ization (Figure 8C). Adding heavy metal additives is also aneffective way to improve the performance of anode. Yuan etal.100 achieved Bi-based compound film-coated ZnO withimproving electrochemical activity of ZnO. Bi-based compoundfilm was composed of Bi6(NO3)4(OH)2O6, BiO and Bi2O3

nanoparticles, which contained many micropores. Theimprovement of cycling performance was attributed to indirectcontact between ZnO/Zn with the electrolyte, suppression ofZnO/Zn dissolution, and adequate diffusion of H2O, OH

− andzincates ions.

MnO2 Cathodes. Zinc/manganese oxide batteries have lowcost and high safety, and they have been commercial as primarybatteries. Making them rechargeable is challenging yetimportant for wider applications. The overall reaction of Zn−MnO2 battery is 2MnO2 + Zn + H2O → 2MnOOH +Zn(OH)2. The MnO2 cathode suffers significant capacity fading

Figure 8. Structure, schematic and electrochemical performance of nanostructured ZnO for zinc battery anodes. (A) SEM image of ALD ZnOcoated CFs. Reproduced with permission.98 Copyright 2016, Wiley. (B) Schematic of the transformation process of ZnAl-LDHs precursor to LDOsmaterial. (C) Cycle performance of calcined LDH materials, precursor LDHs and commercial ZnO. Reproduced with permission.99 Copyright 2015,Elsevier.

Figure 9. Structure and electrochemical performance of α-MnO2 cathode for Zn-MnO2 battery. (A) TEM image of α-MnO2 nanowires. (B)Electrochemical performance of Zn-MnO2 batteries in ZnSO4 aqueous electrolyte with MnSO4 additive. Reproduced with permission.103 Copyright2016, Springer Nature.



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during the initial 20 cycles, and their utilization remainslow.101,102

Recently, Pan et al.103 demonstrated a highly reversible zinc/manganese oxide system with a capacity of 285 mAh g−1MnO2,and capacity retention of 92% for more than 5,000 cycles(Figure 9B). α-MnO2 nanowires (Figure 9A) were used as thecathode and ZnSO4-based aqueous solution was used as theelectrolyte. This system showed that a conversion reactionbetween α-MnO2 and H+ was mainly responsible for the goodperformance of the system. Tang et al.104 successfullysynthesized single-crystalline α-MnO2 nanorods with surfacearea ∼95.2 m2 g−1 and diameters ranging from 10 to 20 nm,which showed improving discharge capacity compared withcommercial MnO2.Flexible/bendable electronics are favorable currently. Flexible

energy storage devices are critical to develop that type ofelectronics. The relatively inexpensive MWCNTs is promisingto be used in flexible MnO2 composite cathode compared withgraphite. Wang et al.105 successfully demonstrated that theimplementation of MWCNTs and conductive polymer poly-(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PE-DOT:PSS) can help to develop the highly conductive MnO2composite electrode for flexible Zn/MnO2 batteries. It is worthnoting that more dispersible, carboxylated MWCNTs woulddecrease the cycling performance of Zn/MnO2 batteries byincreasing the resistance of the electrode.Nickel Cathodes. Rechargeable Zn−Ni batteries are

promising to become a substitute for LIBs due to their highenergy density and stable cycling performance in aqueouselectrolyte.96 However, Ni oxides and Ni hydroxides have somedrawbacks, including low conductivity, high rate unaffordabilityand bad cycling performance. In recent decades, many

investigators have demonstrated a lot of solutions to improvethe performance of Zn−Ni batteries.The reversible reactions of two electrodes are shown below.Cathode reaction:

+ + ⇔ +− −NiOOH e H O Ni(OH) OH2 2

Anode reaction:

+ ⇔ +− − −Zn 4OH Zn(OH) 2e42

In the cathode reaction, there are two main redox pairs,including α-Ni(OH)2/γ-NiOOH and β-Ni(OH)2/β-NiOOH,respectively. The α-Ni(OH)2/γ-NiOOH couple has highertheoretical capacity than the other one because the oxidationstate of nickel in γ-NiOOH can be larger than 3 with theexistence of Ni4+ defect. However, α-Ni(OH)2 has the potentialto be transformed to β-Ni(OH)2 and it is not stable in aqueouselectrolyte. Ion and electron transfer is another problem neededto be solved. There are two approaches can be used toovercome these problems: (1) nanostructured design tostabilize α-Ni(OH)2; and (2) combine Ni oxides or Nihydroxides with other conductive materials. Kiani et al.106

compared the performance of nanostructured β-Ni(OH)2 andmicrostructured β-Ni(OH)2. Cyclic voltammetric (CV) resultsshowed that nanostructured β-Ni(OH)2 has larger protondiffusivity and higher OER potential compared with micro-structured β-Ni(OH)2. Besides, nanostructured β-Ni(OH)2exhibited an excellent cycling performance with highreversibility and capacity. Liu et al.98 combined two approachesand deposited porous ultrathin NiO nanoflakes conformally onhierarchical CC−CF (Figure 10A) as the cathode, whichenabled fast electron transport and rapid ion diffusion anddelivered high capacity, high energy and power densities as well

Figure 10. Nanostructured nickel and oxygen cathodes for rechargeable Zn batteries. (A) Schematic illustration of the flexible Ni−Zn battery using3D hierarchical metal oxide (NiO and ZnO) coated N-doped CFs on CC as the electrodes. (B) Cycling test of flexible quasi-solid-state Ni−Znbattery. Reproduced with permission.98 Copyright 2016, Wiley. (C) SEM image of AgNW-GA. Reproduced with permission.108 Copyright 2017,Wiley.



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as superior cycling stability (Figure 10B) in full cells. Besides,the device can also work well even with severe bending, whichmakes it possible for flexible batteries.Apart from Ni(OH)2 and NiOOH, Ni3S2 can also be the

cathode material to pair with the Zn anode. Hu et al.107

synthesized self-supported Ni3S2 ultrathin nanosheets grown onthe surface of nickel foam. This Ni3S2/Ni composite showed ahigh capacity and slight capacity decay for more than 100 cyclesas cathode material for Zn−Ni battery, and showedextraordinary rate capability and high operating voltage.Oxygen Cathodes. Zinc−air batteries have the highest

specific energy among all Zn-based batteries.109−112 Comparingto Li−air batteries, Zn−air is tolerant to moisture in air and ispotentially a simpler system. Moreover, the solid dischargeproduct of Zn−air, ZnO or Zn(OH)2, forms on the anode side,and will not clog the air cathode. Below are the reactions ofZn−air batteries.The air electrode reaction:

+ + ⇔ =− − EO 2H O 4e 4OH 0.40 V vs SHE2 2

The zinc electrode reaction:

+ ⇔ + +

= −

− −

E

Zn 2OH ZnO H O 2e

1.26 V vs SHE2

The overall reaction:

+ ⇔ =E2Zn O 2ZnO 1.66 V vs SHE2

Among many challenges for the development of Zn−airbatteries, it is crucial to develop highly efficient and low costcatalysts for oxygen evolution reaction (OER) and oxygenreduction reaction (ORR) in the air electrode because both theORR and the OER show considerably high overpotentials.113

Up to now, Pt and Pt-based alloys are known as the mostefficient catalysts. However, the high cost and poor durability ofPt-based catalysts hinder their broad applications.114 Dhavaleand Kurungot111 prepared well organized Cu−Pt nanocageintermetallic structures, which have high density of active sitesto facilitate dissociative oxygen adsorption, and improve ORRactivity compared to the commercially available 20 wt % Pt/C.Nanostructured catalysts are favorable due to their unique

properties; however, most of them show poor conductivity andsuffer from agglomeration when used in batteries.115

Composition and structure control would help to increasethe stability and activity of catalysts.One main approach is to grow electrocatalysts on graphene

nanosheets. For example, metal nanoparticles, metal oxide andmagnetic nanoparticles. Zhou et al.116 synthesized an efficientand economical hybrid electrocatalyst: cobalt nanoparticlesencapsulated in N-doped graphene nanoshells. It presentedequivalent performance to the commercial Pt/C electrocatalystin alkaline system but it was synthesized with cheaper and moreabundant raw materials. Also, it was robust to allow thereplacement of Zn anode and electrolyte periodically, whichcould be an ideal cathode electrocatalyst for EVs. Xie et al.117

reported a catalyst consisting of Co3O4 nanoparticlesencapsulated in a graphene supported carbon matrix(Co3O4@C/graphene) with higher electrocatalytic activityand better stability compared with commercial 20 wt % Pt/C.They proved the synergistic coupling between Co3O4 andnitrogen-doped graphene. The carbon encapsulation structurewould be more important on the performance of catalyst if theCo3O4 nanoparticle sizes become smaller. Also, Prabu118

synthesized CoMn2O4 nanoparticles supported on N-dopedreduced graphene oxide (CoMn2O4/N-rGO), which showedimproved and excellent zinc−air battery cycle performance.They attributed the improved battery performance to thecoupling effect between CoMn2O4 and graphene hybridmaterials as well. Most recently, a 3D graphene aerogel catalystengineered with a silver nanowire network (AgNW-GA, Figure10C) was presented by Hu et al.,108 which showed high ORRperformance and superior stability. Due to its 3D spongestructure, the electrocatalyst loading was increased and themass transport was effectively enhanced. Anchoring catalysts onthe other hosts has also been reported, such as nickel−cobaltnanoparticles on porous fibrous carbon aerogels119 and cobalt−manganese oxide on N-doped carbon nanotubes.120

Apart from anchoring electrocatalyst on graphene to improvethe kinetics, another approach is decorating the electrocatalystwith other catalysts. Goh et al.121 presented an inexpensive yetefficient bifunctional catalyst consisting of MnO2 nanorodsdecorated with Ag nanocrystals (∼11 nm) (Ag−MnO2) as aircathode. In aqueous alkaline electrolyte, it was successfullycycled for 270 cycles and showed improved ORR and OERcatalytic activity. The ORR mechanism of Ag−MnO2 is a direct4e− reaction, which was proved by RDE results that 3.7electrons were transferred. This was an evidence for the highORR kinetics and catalytic performance of Ag−MnO2. Core−shell nanostructure alloys have received considerable attentionrecently because of high electron conductivity.122 Zhang etal.123 produced core−shell AgCu bimetallic nanoparticles. Thiscatalyst showed enhanced ORR activity, highest workingpotential and lowest overpotential.

■ CONCLUSION AND OUTLOOK

In this Perspective, we summarized the recent progress ofnanostructured electrode materials for reversible Li, Na and Znbatteries. Nanostructured materials and their composite possessunique properties, including high surface area and shorterpathways for ions and electrons, and could provide additionalknobs to break the intrinsic limitation of bulk materials, such aspoor electrical conductivity, dissolution and volume expansionof electrode.However, there are still some unsolved problems with

nanostructured electrodes. First, high surface area of ananostructured electrode can enhance electrochemical perform-ance but also leads to severe side reactions, which isunfavorable for electrode stability and reversibility. Second,the morphology and structure of nanostructured electrode aresometimes hard to maintain over cycling, due to weakmechanical and chemical characteristics. Third, advancednanoscale characterization tools are to be developed tounderstand electrochemical phenomena at the nanoscale,guide and verify the design of nanostructured electrodematerials. High fabrication cost and low volumetric energydensity also need to be solved.Looking forward, difficult challenges in the battery field

should continue to be attempted with nanostructure design ofelectrode materials. Meanwhile, new nanomaterials should betested in battery applications. Finally, new tools should bedeveloped to allow mechanistic studies of the electrochemicalreactions and phenomena at the nanoscale. Collectively, thesecould further the application of nanostructured electrodematerials in batteries.



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■ AUTHOR INFORMATIONCorresponding Author*N. Liu. E-mail: [email protected] Liu: 0000-0002-5966-0244Author ContributionsYamin Zhang and Nian Liu cowrote the paper.NotesThe authors declare no competing financial interest.Biographies

Yamin Zhang is pursuing her Ph.D. in chemical engineering under thesupervision of Prof. Nian Liu at Georgia Institute of Technology,School of Chemical and Biomolecular Engineering (since Aug. 2016).Her current research interests include the development of advancednanostructured electrode materials and membrane separators forrechargeable zinc batteries. She received her B.S. from TianjinUniversity (China), where she specialized in nanostructured catalysts.

Nian Liu is an assistant professor at Georgia Institute of Technology,School of Chemical and Biomolecular Engineering (since Jan. 2017).His research group focuses on materials engineering for reversiblehigh-energy chemical reactions, and understanding their fundamentalscience using advanced light microscopy. Liu received his B.S. fromFudan University (China), and Ph.D. from Stanford University, wherehe worked with Prof. Yi Cui on the structure design for Si anodes forhigh-energy Li-ion batteries. He then worked with Prof. Steven Chu atStanford University as a postdoc, where he developed in situ opticalmicroscopy to probe beam-sensitive battery reactions.

■ ACKNOWLEDGMENTSN.L. acknowledges support from faculty startup funds from theGeorgia Institute of Technology.

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