This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys.

Cite this: DOI: 10.1039/c2cp42768k

Lithium oxides precipitation in nonaqueous Li–air batteries

Junbo Hou,*aMin Yang,

aMichael W. Ellis,

bRobert B. Moore

cand Baolian Yi

d

Received 8th August 2012, Accepted 16th August 2012

DOI: 10.1039/c2cp42768k

Lithium–air/oxygen battery is a rising star in the field of electrochemical energy storage as a

promising alternative to lithium ion batteries. Nevertheless, this alluring system is still at its infant

stage, and the breakthrough of lithium–air batteries into the energy market is currently

constrained by a combination of scientific and technical challenges. Targeting at the air electrode

in nonaqueous lithium–air batteries, this review attempts to summarize the knowledge about the

fundamentals related to lithium oxides precipitation, which has been one of the vital and

attractive aspects of the research communities of science and technology.

1. Introduction

Economy, energy and environment (3E) are realized by more

and more people to be linked together with an undisputed

fact that energy, which is a requisite for a clean environment,

is required for a healthy economy. To build up greener 3E in

the future, our dependence on fossil fuels should be reduced

and high energy density systems should be developed.

Although through the last two decades lithium-ion (Li-ion)

batteries have played a vital role in the portable electronic

devices and they will continue to be important for power

tools and transportation, including plug-in hybrid and all

electric vehicles, present rechargeable Li-ion batteries do

not meet the commercial requirements like long driving

range, safe and fast charging and low cost.1 Considering their

limitations, especially the fact that they cannot store and

deliver larger energy per unit mass or volume which is a

catastrophic limitation for the applications of electric vehicles

and electricity grids, alternative energy storage technologies

need to be investigated. The Li–air battery is one such

alternative.

Unlike in Li-ion batteries, a reversible O2 reduction and

combination with Li+ occurs on the cathode of Li–air

batteries, and thus a porous cathode (positive electrode)

usually consisting of an electron pathway, an ion (Li+) path-

way and a gas (O2) pathway is introduced rather than an

a Institute for Critical Technology and Applied Science, Virginia Tech,Blacksburg, VA 24061, USA. E-mail: junbo80@vt.edu,junbo.hou@yahoo.com

bDepartment of Mechanical Engineering, Virginia Tech, Blacksburg,VA 24061, USA

cDepartment of Chemistry, Virginia Tech, Blacksburg, VA 24061,USA

dDalian Institute of Chemical Physics, Chinese Academy of Sciences,Dalian, 116023, China

Junbo Hou

Junbo Hou is currently aResearch Associate in Insti-tute for Critical Technologyand Applied science atVirginia Tech. He receivedhis PhD degree (2008) inChemical Engineering fromDalian Institute of ChemicalPhysics, Chinese Academy ofSciences, studying on fuelcell technologies and electro-chemical fundamentals. Afterworking on semiconductingmaterials synthesis and char-acterization for two years inUniversity of Leoben and

Erich Schmid Institute, Austria Academy of Science, he cameto Virginia Tech and currently works on the electrochemicalenergy conversion and storage.

Min Yang

Min Yang currently works atVirginia Tech Institute forCritical Technology andApplied Science. After shereceived her PhD degree inChemical Engineering atDalian Institute of ChemicalPhysics, Chinese Academy ofSciences in 2008, she workedas a researcher in Montanuni-versitat Leoben, Austria. Herresearch focused on oxidematerials synthesis, ceramicdevices preparation, kineticstudy of electroceramic mate-rials, electrochemical basics,

and solid oxide fuel cell technology.

PCCP Dynamic Article Links

www.rsc.org/pccp PERSPECTIVE

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intercalation material into the Li-ion battery.2 The graphite

anode (negative electrode) in the Li-ion battery is replaced by

Li metal or other pre-lithiated materials with low electrode

potential which can work as a substitute but may decrease the

specific energy. Based on the used electrolyte, Li–air batteries

can be generally classified into aqueous and nonaqueous

systems though there are other architectures like solid state

and mixed aqueous–nonaqueous systems.3 In the aqueous

Li–air battery, oxygen is reduced at reaction sites in the

cathode and combined with H2O and Li+ from the electrolyte,

and LiOH�H2O is formed.4 For the nonaqueous one, there is

no water involvement and lithium oxides like Li2O2 and Li2O

are formed. The theoretical specific energy can be calculated

based on the overall cell reaction and the values for the

aqueous and nonaqueous Li–air batteries are listed in Fig. 1

together with the schematic representation of the cell struc-

tures. Even given the oxygen involvement, the specific energiesof the nonaqueous system (3505 W h kg�1) which is compar-

able to that of a regenerative fuel cell (3663 W h kg�1), and the

aqueous system (2044 W h kg�1) are much higher than that

of the Li-ion battery (387 W h kg�1). The theoretical specific

energy can provide a benchmark comparison for the active

material itself, while the practical specific energy which

is usually about 20–45% of the theoretical one should be

considered5 depending on the cell design. If fully developed,

nonaqueous Li–air batteries may meet the criteria for the

transport and stationary applications.

This review only focuses on the air electrode in nonaqueous

Li–air systems. We start from a brief assessment of anode,

electrolyte and cathode challenges in nonaqueous Li–air

batteries which have been reviewed previously.3,5,6 Inspired by

water freezing during cold start of proton exchange membrane

(PEM) fuel cells, we will provide detailed fundamental under-

standings, novel concepts and ideas addressing Li oxides pre-

cipitation is an intrinsic issue for this type of battery. Without

solving this problem, the specific power density for nonaqueous

Li–air batteries is far too low for practical use in transport and

stationary applications.

Fig. 1 Theoretical specific energy for Li ion and Li–air batteries

together with regenerative fuel cells.

Michael W. Ellis

Michael W. Ellis is an Associ-ate Professor of MechanicalEngineering at Virginia Tech.Dr Ellis has 25 years ofexperience in engineering,research, and educationrelated to advanced energysystems. His current workfocuses on the developmentand evaluation of materialsfor PEM fuel cell membranesand diffusion media, modelingliquid transport in PEM fuelcells, and fuel cell cogenera-tion for buildings. He teachescourses on thermodynamics,

sustainable energy, and engineering design. Dr Ellis is thedirector of the Sustainable Energy Research Program in VirginiaTech’s Institute for Critical Technology and Applied Science andis chair of ASME’s Advanced Energy Systems Division.

Robert B. Moore

Robert B. Moore is theAssociate Director ofResearch at the Virginia TechInstitute for Critical Techno-logy and Applied Science(ICTAS), and a Full Profes-sor in the Department ofChemistry at Virginia Tech,with 22 years of academicexperience in the field of Poly-mer Physical Chemistry. Hisresearch is focused on the areaof ioncontaining polymers forenergy applications, withspecific interests that include:control of morphology trans-

port property relationships in proton exchange membrane fuelcell systems, tailored actuation behavior in nano-structuredmaterials, and the use of small-angle X-ray and neutronscattering methods for the characterization of morphology inion-containing polymers.

Baolian Yi

Baolian Yi is a fellow of theChinese Academy of Engi-neering, and a Professor ofChemical Engineering atDalian Institute of ChemicalPhysics, Chinese Academy ofSciences. He has been workingon the research and develop-ment of fuel cells and relatedfields since the 1970’s. Heresearched on AFC for spaceapplication in the 1970’s, andconducted fuel cell technique,aqueous solution electrolysisindustry and electrochemicalsensors in the 1980’s. During

the 1990’s, he initiated the research studies on PEMFC, MCFCand SOFC.

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2. Challenges in nonaqueous Li–air batteries

2.1 Anodes

Anodes in Li–air batteries usually employ Li metal which has

lowest electrode potential and is very chemically reactive.

These chemical and electrochemical properties make most

electrolytes unstable on the Li metal surface. Fortunately, a

passive film or a solid electrolyte interface/interphase (SEI)

will form on the Li metal surface due to the decomposition of

and reaction with the electrolyte. This film will keep fresh Li

away from the electrolyte, stop the further reaction and

stabilize the interface, though it introduces additional ohmic

loss and mass transport resistance to the anode. Unfortunately,

this SEI on the Li metal surface is not stable during the

charge–discharge cycling.7 Li dissolution when discharging

and Li deposition when charging may shift the interface or

change the anode thickness. The preformed SEI may not endure

the dramatically large change of the electrode volume and

fracture. New SEI may form at the newly-formed interface,

and cause uneven distribution of the current density making

more severe dendrite growth. Like Zn, Li favors dendrites

growth. On charging, Li dendrites may grow out from the anode

and possibly penetrate into the separator leading to Li loss and

even worse internal short-circuit. There may exist two research

directions addressing this issue: chemical and mechanical. The

anode processes on charging–discharging can be considered to be

electroplating and electropolishing of Li metals. There are many

techniques and knowledge existing in this area like current

distribution, leveling agents, surface morphology and roughness

which can be applied to the study of the anode cycling. SEI

consideration should also be incorporated in such a study.

Another way is to choose a solid Li-conducting membrane,

especially a ceramic one, and wrap or press it on the anode so

that the dendrites growth is mechanically prohibited.8 But, such a

membrane should have a high Li-ion conductivity at room

temperature, chemical and electrochemical stability and high

mechanical strength, which so far remains a challenge. Also,

it must be noted that not every electrolyte will form a stable

SEI, which not only stops further reaction of Li metals with

electrolytes but also prevents O2 diffused from an air electrode

from reacting with the anode.

2.2 Electrolyte

An electrolyte usually functions as an electronic separator and an

ionic conductor between a cathode and an anode. It may consist

of solvent, salt, separator, additive, and/or a solid ion-conducting

membrane or their combination. As in other electrochemical

devices, the electrolyte should be durable in highly reductive and

oxidative environments, highly ionic-conductive, and facilitate

electrochemical reactions in Li–air batteries. The former two

requirements can draw the experience of the electrolyte in Li-ion

batteries, especially ‘‘5V’’ Li-ion batteries due to the fact that a

large electrochemical window can cover the large separation

between charge and discharge voltages in Li–air batteries. The

greatest concern is related to the cathode or positive electrode

reaction: the O2 transport and reversible electrochemical

reaction. Since most state of the art cathodes in Li–air

batteries are electrolyte flooded, O2 needs to dissolve in and

diffuse through the solvent to reach the reaction sites. Various

solvents have been investigated to correlate the dielectric

constant, viscosity and Bunsen coefficient with oxygen solubi-

lity and diffusivity and the discharge capacity.9–11 Increasing

the solubility and diffusivity of oxygen in the solvent also

increases the possibility of oxygen accessing the anode, and

thus an additional Li-ion conducting membrane12 or a Li-ion

filtering membrane13 is needed to prevent this from happening.

However, recent findings on the solvent blends with low

volatility like propylene carbonate (PC)/tris(2,2,2-trifluoro-

ethyl) phosphate (TFP)14 and methyl nonafluorobutyl ether

(MFE) and tris(2,2,2-trifluoroethyl) phosphite (TTFP)15

cannot be correlated with the viscosity and ionic conductivity

of the solvents. The authors ascribed the reason for the

performance improvement to the increased dissolution

kinetics and solubility of oxygen in one solvent of the blend.

Improvement of O2 transport can be also achieved by choos-

ing high polarity solvents due to their lower accessibility and

low affinity to carbon pores,16 which creates O2 pathways

within the porous cathode. This can go further by avoiding the

use of a liquid electrolyte, and a totally solid-state Li–O2

battery was demonstrated to be successful till 40 charge–

discharge cycles from 0.05 to 0.25 mA cm�2.17 The electrolyte

also plays a crucial role in the reversible electrochemical

reaction at the air electrode. By using differential electro-

chemical mass spectrometry (DEMS) coupling with isotopic

labeled O2 and ex situ cathode analysis the electrochemistry of

an air electrode in carbonate and DME-based solvents was

probed.18 It was found that during cell discharge carbonates

undergo chemical and electrochemical reduction in the presence

of Li2O2 or its intermediate LiO2. Although employing DME-

based solvents can produce Li2O2 the solvent is oxidatively

unstable during charging in the presence of Li2O2. The

electrochemical irreversibility of the air electrode reaction

was also confirmed by using in situ gas chromatography/mass

spectroscopy (GC/MS), and lithium-containing carbonate

species (lithium alkyl carbonates and/or Li2CO3) were the

main discharge products.19 The density functional theory

(DFT) calculations with a Poisson Boltzmann continuum

solvent model explained the nucleophilic substitution reactions

with superoxide accounted for the mechanism of solvent

decomposition.20 In addition, some additives like crown

ethers21 and organic quaternary ammonium salts22 were found

to possibly affect the oxygen reduction reaction and could improve

the discharge capacity. Very importantly, recent findings and

exploration on electrolytes have shown that a tetra(ethylene) glycol

dimethyl ether–lithium triflate (TEGDME–LiCF3SO3) electro-

lyte23 and LiClO4 in dimethyl sulfoxide (DMSO)24 can support

highly reversible formation–decomposition of Li2O2 at the

cathode on cycling, which is very encouraging for the further

study of non-aqueous Li–air batteries.

2.3 Cathodes

The cathode reaction in Li–air batteries seems to be a limiting

factor in terms of the high discharge capacity, reasonable rate

capability and good cycle performance.25 Ideally there should

be gas pathways, ionic pathways and electronic pathways

within the porous air electrode. Although there are many

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challenges existing in anodes and electrolytes, for the cathode

itself, if these three kinds of pathways are not impeded, the

discharge capacity should be limited by the anode mass, and

the rate performance should be controlled by the electrolyte.

As with the cycle performance, it requires reversible electro-

chemical reactions. Unfortunately, none of them have been

presently achieved. Unsuitable use of electrolytes gives the

discharge products Li2CO3, LiRCO3 and Li2O2, and the

repeated coating of different Li salts on the carbon electrode

after each discharge and charge process diminishes the electronic

conductivity of the air electrode.26 Even the desired product,

Li2O2, is generally insoluble and insulating and will block the

mass transport and electron transfer processes. The easiest

way to solve this problem is to find a solvent or an additive

that has increased solubility of lithium oxides, but still cannot

prevent lithium oxides precipitation.27–30 Regarding H2O or

CO2 in the air which may diffuse from a cathode to an anode

and react with Li metals, an O2 permeated membrane should

be used at the cathode inlet.

3. Lithium oxides precipitation

Why can we put water freezing and lithium oxides precipita-

tion together? Because with respect to the air electrode what

happens during cold start of PEM fuel cells at subzero

temperatures is electrochemically and physically very similar

to that during discharge of Li–air batteries. What is known in

the field of subzero PEM fuel cells will help in understanding

fundamentals and challenges in Li–air batteries. In the air

electrode in PEM fuel cells, the microstructure of the catalyst

layer can be depicted as catalyst-loaded carbon particles

flooded with the electrolyte form agglomerates covered with

a thin film of electrolytes. The reactant gas firstly passes

through the channels among the agglomerates, diffuses

through the ionomer thin film and thereafter in the agglo-

merates, and then reaches the reaction sites. During cold start,

the generated water due to the oxygen reduction reaction

(ORR) will freeze once the heat produced is not enough to

warm the membrane electrode assembly (MEA) above the

freezing temperature. The freezing of the generated water

covers the catalyst sites, reduces the three phase boundaries

(TPBs) and blocks the reactant gas access to the reaction

sites.31,32 The increased polarization will usually lead to the

cold start failure, as shown in Fig. 2a. For most state of the art

air electrodes in Li–air batteries, the microstructure is basically

the same as that in PEM fuel cells but it is usually flooded

with liquid electrolytes. During discharge, electrochemical

combination of oxygen and lithium ions is expected to form

Li2O2 and/or Li2O solids which are generally thought to be

insulators and hard to be dissolved in the solvents. Like water

freezing, the precipitation of lithium oxides will passivate

reaction sites, block pores, and increase mass transport resis-

tance, which limits the discharge capacity and rate capability,

as shown in Fig. 2b. For cycle performance, the volume

change due to phase transition, like water–ice transition in

the subzero PEM fuel cells may damage the electrode structure

in Li–air batteries.

3.1 Lithium oxides

The intercalation or conversion reaction at the positive electrode

in Li-ion batteries is replaced by the oxygen reduction and

combination with Li ions for the Li–air battery.37 What exactly

are the discharge products at the air electrode in nonaqueous

Li–air batteries although it was thought and is expected to be

Li2O2? To answer this question, several things must be identi-

fied: (i) the mechanism of oxygen reduction in non-aqueous

solvents in the presence of Li ions; (ii) the influence of solvents

and Li ions; (iii) the role of catalysts.

3.1.1 Electrochemistry of ORR. The kinetics andmechanisms

of oxygen reduction in acetonitrile containing four different

hexafluorophosphates (general formula APF6, where A = tetra-

butylammonium (TBA), K, Na, and Li) on glassy carbon

electrodes were firstly reported and it was found that larger

cations represented by TBA salts displayed a reversible O2/O2�

redox couple while the smaller Li cations showed an irreversible

one-electron reduction of O2 to LiO238 (see Fig. 3). The redox

peaks in Fig. 3B can be correlated with the electrochemical

steps shown below:

Ep1: O2 + Li+ + e� = LiO2 (Eo = 3.0 V vs. Li electrode)

(1)

Ep2: LiO2 + Li+ + e� = Li2O2 (Eo = 3.1 V) (2)

Ep3: Li2O2 = O2 + 2Li+ + 2e� (3)

Fig. 2 (a) Cold start failure of PEM fuel cells due to water freezing: start curve33 and SEM image of catalyst layers;34 (b) discharge behavior of

Li–air batteries with lithium oxides precipitation: discharge curve35 and SEM image of an air electrode.36

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An electrochemical–chemical (EC) process may happen

after Ep1:

2LiO2 = Li2O2 + O2 (4)

The rotating disk electrode (RDE) data in that study also

supported that the lithium oxides formed through electrochemical

and chemical reactions passivated the electrode surface, making

the process irreversible.

Later the same authors reported the influence of nonaqueous

solvents (dimethyl sulfoxide (DMSO), acetonitrile (MeCN),

dimethoxyethane (DME) and tetraethylene glycol dimethyl

ether (TEGDME)) on the ORR in the presence of Li ions.39

Reversible redox peaks related to the O2/O2� couple were

observed only in DMSO (Epc1/Epa1 in Fig. 4). Scanning the

electrode to lower potentials changed the anodic behavior in

all four solvents and Nicholson plots showed that charge

transfer number equaled 1, which indicated a stepwise fashion

of ORR to form O2�, O2

2� and O2� as products. This means

there is another possible electrochemical reaction following

eqn (1) and (2).

Li2O2 + 2Li+ + 2e� = 2Li2O (5)

Based on Pearson’s hard soft acid–base (HSAB), the stability

of the complex [Li+(solvent)n–O2�] was introduced to explain

the difference in ORR among the four solvents and accordingly

the solvent with the low donor number (DN) favored the O2�

product. But in situ surface enhanced Raman spectroscopy

(SERS) results exhibited only observation of the Li2O2 product

and the LiO2 intermediate at 2.2 V discharge potential, as

shown in Fig. 5a, and also it was found that the disproportio-

nation dominated the transformation of LiO2 to Li2O2

(eqn (4)).40 This implies that a 1-electron EC process makes

the main contribution to ORR rather than a 2-electron transfer

process (eqn (1) and (2)). In the electrocatalytic activity studies

of glassy carbon, Au and Pt, relatively complete and possible

pathways for the ORRmechanism occurring at the air electrode

were hypothesized by considering oxygen adsorption on the

catalyst surface (Fig. 5b).41,42 Unfortunately, the ORR products

following the hypothesized pathways on particular catalyst

Fig. 3 Cyclic voltammograms for the reduction of oxygen saturated (a) 0.1 M TBAPF6/MeCN and (b) 0.1 M LiPF6/MeCN on a GC electrode.38

Fig. 4 Cyclic voltammograms for the ORR in 0.1 M LiPF6/DMSO

within various electrochemical windows.39

Fig. 5 (a) In situ SERS of ORR and re-oxidation on a Au electrode in O2-saturated 0.1 M LiClO4–CH3CN;40 (b) hypothesized ORR mechanism

in nonaqueous solvent containing Li ions.41

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surfaces have not been verified yet. So far, it has been safe to

say that ORR in the presence of Li ions in nonaqueous

solvents is kinetically irreversible once Li2O2 is formed due

to either chemical or electrochemical reaction; carbon materials

can themselves facilitate the O2/O2� reversible couple which

may mask the intrinsic activity of other catalysts; the products

highly depend on the cathodic end potential or discharge depth;

the adsorption strength of oxygen on different catalyst surfaces

may influence ORR pathways; solvents may influence desolva-

tion of Li ions most importantly solvation of O2� and LiO2. We

need to point out that electrochemical quartz crystal micro-

balance (E-QCM) may be a very helpful technique to investi-

gate the electrochemistry of product precipitation.

3.1.2 Influence of solvent. Given the high oxidability of

O2�, LiO2 and even Li2O2 as well as the potential range of the

air electrode, the solvent may not be chemically and electro-

chemically stable. It was found that employing carbonate-

based solvents (propylene carbonate (PC), ethylene carbonate

(EC) and dimethyl carbonate (DMC)) resulted in decomposi-

tion of carbonates to form LiCO3, Li alkyl carbonates and a

small amount of Li2O2 in the cathode, which decomposed to

CO2 upon cell charging. Pure dimethoxyethane (DME) can

lead to Li2O2 formation but its decomposition may occur at

high potential during charge.18 The products formed by using

a combination of MS, X-ray diffraction (XRD), Fourier

transform infrared spectroscopy (FTIR) and SERS during

discharge in PC solvent were lithium propyl dicarbonate,

C3H6(OCO2Li)2, Li2CO3, HCO2Li, CH3CO2Li, CO2, and

H2O.43 In the electrolyte of EC–DMC with LiPF6 as the salt,

thick coatings of reaction products were found on both carbon

and MnO2 coated carbon cathodes (see Fig. 6), and the

products were mainly composed of Li, F, C, O and P.44 A

dimethylformamide (DMF)-based electrolyte has also been

confirmed to be not suitable for Li–O2 batteries due to its

instability at the cathode side.45 DFT calculations confirmed

that nucleophilic substitution with superoxide is a common

mechanism of nonaqueous solvents decomposition, and

chemical functionalities including N-alkyl substituted amides,

lactams, nitriles, and ethers were found to be stable against

nucleophilic substitution.20 Experimental and DFT computational

studies of an electrolyte based on a tri(ethylene glycol)-

substituted trimethylsilane (1NM3) provide evidence that the

ethers are more stable toward oxygen reduction discharge

species.46 Although the ethers are more stable than the organic

carbonates, their decomposition in Li–air batteries has also

been found.47,48 With this in mind, it is not safe to use and

compare the charge data using carbonate based solvents,

which is commonly done in the literature and should be

revised. We will limit our discussions below on the discharge

process since it can be valuable and acceptable to some extent

even using carbonate solvents when only the relation of the

structure and current or capacity within the same electrode is

considered.

3.2 Location

Where are lithium oxides located? It depends on where the

ORR reaction occurs, and accordingly can be ascribed to

triple phase boundaries (TPBs) (O2, electrolyte and catalyst)

or double phase boundaries (DPBs) (electrolyte and catalyst),

the latter of which is reasonable due to the fact that oxygen

dissolves in the nonaqueous electrolyte. A combination of

TPBs and DPBs is the most likely case in the present air

electrode, as indicated and confirmed by the correlation of

discharge capacity with oxygen transport properties of organic

electrolytes,49 wetting property and affinity to carbon pores of

the solvents,16 effect of oxygen pressures on the electrochemical

profile,50 20% pore volume occupation by the discharge

product,51 formation of extra pores in the Ketjen black (KB)

electrode,35 increased oxygen solubility in the blended solvents14

and increased oxygen concentration by using perfluorinated

compounds as oxygen carriers.52 Further, TPBs are essential

and critical to the discharge capacity regarding O2 mass trans-

port.53,54 It thus can be expected that lithium oxides (i) may

deposit on and passivate the carbon surface (see Fig. 7a) and (ii)

may be stored in the pores within the carbon agglomerates

and among agglomerates (see Fig. 7b, four agglomerates form

the two types of pores).

3.2.1 Surface passivation. In the former case, it is important

to understand the lithium oxides precipitation, since a simplified

flat electrode model can be used rather than the porous

electrode one. Understanding surface passivation helps eluci-

date the relationship between film growth and the capacity

limitation. Surface passivation was indicated by the RDE data

in that increasing rotation rate actually decreased the disk

current.38 Later, morphology of the discharge product film

and discharge curves for flat glassy carbon were obtained,

Fig. 6 SEM images of graphite foam (top left), the carbon veil (top

right), discharge products on graphite foam (lower left), and discharge

products on the carbon veil (lower right).44Fig. 7 Schematic showing the location of lithium oxides formation:

(a) surface passivation; (b) pore blockage and clogging.

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and simulation results based on a continuum-scale cell model

indicated that the discharge product film with a thickness of

tens of nanometers was highly electronically resistive and was

the main contribution to the capacity limitation in the Li–air

battery.55 A galvanostatic discharge at 0.6 mA cm�2 generates

a product film with thicknesses of 40 to 70 nm (see Fig. 8a).

Surprisingly, the discharge capacity shows rate dependence

even on a flat electrode as shown in Fig. 8b. The questions that

arise here: why does discharge at higher current density give

smaller capacity? If passivation happens at a critical thickness

of the product film, why does this critical thickness depend on

the discharge rate? Almost the same authors later designed a

clever and rigorous experiment using the ferrocene/ferroce-

nium redox couple to probe charge transport through films of

Li2O2 on a glassy carbon electrode. By combining the electro-

chemical results and the metal–insulator–metal (MIM) model

they concluded that the charge transport within the product

film (Li2O2) determined the discharge capacity or ‘‘sudden

death’’.56 The critical thickness is determined by the tunneling

of holes within the passivation film and is calculated to be

about 5–10 nm. To answer the questions mentioned above and

explain the discrepancy in critical thickness, two fundamental

things need to be clarified: (i) the mechanism of Li2O2 film

growth; (ii) the destination of LiO2 intermediate. The Li2O2

growth may follow a discrete spiral growth mechanism rather

than layer by layer growth, and this is plausible as the crystal

nucleation prefers the kink and step sites on the surface.

Discrete spiral growth at the initial state may also favor the

charge transport along the Li2O2 surface which has been

proved to be metallic57,58 not through the bulk Li2O2. If so,

the critical thickness of 5–10 nm may be underestimated and it

can explain the thick discharge film that was observed

(Fig. 8a). But, it may need further investigations coupling

electrochemical tests with SEM or atomic force microscopy

(AFM). The LiO2 intermediate may detach from the reaction

sites as indicated in RDE tests,41,59 and its movement and

solvation within the double layer might be governed by the

overpotential. This can explain the dependence of discharge

capacity on the discharge rate, but still needs further study to

confirm validity. The solvated LiO2 may disproportionate

to Li2O2 on the preformed Li2O2 surfaces, which means

discharge Coulombs would not be directly correlated with

the surface coverage. If intermediates can detach from the

reaction sites even under the stationary conditions, as it is

believed, this will account for the pore clogging. Otherwise, it

is hard to image there would be extra lithium oxides blocking

the pores either within or between agglomerates because the

passivation film with 5–10 nm thickness will stop the ORR. At

the same time, we need to be aware that the blocked pores

must be electrolyte-wetted considering TPBs or DPBs and the

movement of the LiO2 intermediate.

3.2.2 Pore clogging. As with the pore clogging and block-

age, the effects of pore size, pore size distribution, pore volume

and surface area should be emphasized along with the pores

wetted or not wetted by the electrolyte. The single point

Brunauer–Emmett–Teller (BET) surface areas for Black Pearls

2000 (BP2000), Shawinigan Black acetylene black (SAB) and

Super P were reported to be 1475, 75 and 62 m2 g�1,

respectively, and the specific capacity for these three carbon

black powders did not follow the sequence of BET surface

areas.9 These results suggested that wetting of the carbon

black is an important factor in determining discharge capacity,

and the BET surface area cannot be used to predict the trend

in discharge capacity. In the same study, by comparing

discharge capacity of the PVDF and PTFE electrodes which

have identical pore volume (73%), the discharge capacity was

found to be correlated with the available specific pore volume

(mL per gram of carbon black). Later, discharge capacity was

actually found to be related to the mesopore volume of the

carbon material,60 but the evidence is not strong as the carbon

material with large mesopore volume also showed large total

pore volume in that study. By deliberately controlling the

porosity of carbon aerogels, i.e. preparing carbons with similar

pore sizes and pore size distributions but different pore

volumes and preparing carbons with similar pore volumes

but different pore diameters, mesopores were firmly proved to

be important to improve the discharge capacity, and large pore

volume and wide pore size exhibited high discharge capacity.61

This is possibly due to (i) better accessibility of electrolytes to

the carbon surface; (ii) better diffusion of oxygen at the

carbon–electrolyte interface; (iii) larger storage volume for

discharge. The critical role of mesopore volume in determining

discharge capacity was also confirmed by comparing different

carbon based electrodes i.e. BP2000, Calgon, Denka, Ketjen

black (KB), milled KB and home-made mesopore carbon

(JMC).35 However, we need to point out that only porosity

of carbon powder itself and not that of the air electrode has

Fig. 8 (a) SEM image of the discharge film (40–70 nm thickness) produced at the end of a 0.6 mA cm�2 discharge on a flat glassy carbon electrode;

(b) discharge curves of flat-electrode cells at three current densities.55

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been related to the discharge capacity, so care must be taken in

considering these results because the porosity of the air

electrode may differ from that of carbon powder. As shown

in Fig. 7b, carbon particles may form agglomerates during

electrode preparation especially in the presence of binders.

There are at least two factors affecting the porosity of the air

electrode: (i) porosity from carbon powder itself; (ii) stacking

state. Taking Fig. 7b as an example, most likely there are

micropores and/or mesopores on the carbon surface, meso-

pores within agglomerates, and mesopores or even macropores

among agglomerates. If any binder is used in the electrode

preparation, the porosity will change. It was reported that a

PVDF binder blocked the majority of the pores with a diameter

below 30 nm, causing a decrease in discharge capacity.62

Bimodal pore distribution of the air electrode in PEM fuel

cells has been widely accepted and experimentally proved.63 It

seems that this is not widely known in the field of lithium–air

batteries given the very few reported pore size distributions on

air electrodes so far (see Fig. 9a).64 Since the synthesized

mesocellular carbon foam (MCF-C) has bimodal pore distri-

bution, it is hard to say that the same situation exists for other

carbon powder based electrodes. In the same article, MCF-C

showed a higher discharge capacity, about 40% increased

capacity compared to that of several commercial carbon

blacks. The enhanced performance was ascribed to the large

pore volume and ultra-large mesoporous structure, which

allowed more lithium oxides to be deposited during the

discharge process. One needs to note that after discharge the

characteristic peaks in the pore distribution curve disappear.

But, still this does not confirm that lithium oxides fill in the

pores or clog the open area of the pores. A lithium oxides

accommodation model was built by considering the electro-

chemical discharge and porosimetry,54 as shown in Fig. 9b. It

was thought that micropores and some of the mesopores

would be blocked by Li oxides at the beginning of discharge,

and Li oxides mainly reside inside the large mesopores. The

density of the oxides increases as the reduction proceeds until

the density is high enough to completely block mass transfer.

For this model, two things need clarification as we discussed

above: the mechanism of Li2O2 film growth and movement of

LiO2 intermediates, because very thin passivation film would

stop the reaction, which contradicts such a model. We have to

say that the failure mechanism in Li–air batteries is actually

not similar to that of alkaline fuel cells as the authors claimed

the same in that study, due to chemical reaction of alkaline

electrolytes and CO2 in the air for alkaline fuel cells and

electrochemical reaction requiring TPBs or DPBs in Li–air

batteries.

3.2.3 Novel design addressing surface passivation and pore

clogging. If the lithium oxides can dissolve in the solvents,

there should not be issues like surface passivation and pore

clogging. Unfortunately, there is no practical solution to

satisfactorily increase the solubility of lithium oxides in the

solvents. Regarding pore clogging and blockage, a dual pore

system can be helpful to address this and has been shown to be

effective by a model,53 which is composed of two inter-

connected porous systems. This idea is similar to the hydro-

phobic and hydrophilic pore system in PEM fuel cells, in

which the hydrophobic pores mainly supply O2 pathways and

the hydrophilic pores provide water pathways. In Li–air

batteries, a two pore system may require that one plays the

role of a reaction region and the other ensures transport of

oxygen to the air electrode even when the reaction region

becomes blocked by reaction precipitates. This can be realized

in two ways. The first is simply mixing porous non-carbon

material with porous carbon material decorated with or without

catalysts. The second is making electrodes partially wet-proof to

the electrolyte. With respect to the surface passivation, increasing

the conductivity of the product film by introducing Li vacancies65

cannot completely guarantee a continuous reaction. Even if

the film is conductive by doping, the electrochemical reaction

also requires this film to be electroactive towards the ORR

because the carbon surface has already been covered by the

passivation film. The possible choice may be to make the

passivation film less dense or migrate the product away from

the surface or inhibit the continuous film growth, so that the

‘‘sudden death’’ during discharge can be delayed. Carbon

surface modification by fluoroaliphatic polyoxyethylene

improved the discharge capacity 8 times higher.66 Although

the authors ascribed this result to the adsorbed molecules on

the carbon surface altering the distribution of potential at the

interface, causing the distribution of the double-layer and

the surface concentration of the reactive species to change,

the detailed mechanism needs further investigation. Another

high-impact research area addressing pore clogging and

Fig. 9 (a) Pore size distribution curves of carbon powder, electrode before and after discharge at 0.1 mA cm�2;64 (b) discharge time and specific

capacity as a function of average pore diameter and the inset shows lithium oxides accommodation in pores.54

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surface passivation is designing novel electrode structures. MnO2

nanoflakes were uniformly coated on multi-walled carbon nano-

tubes (MWNTs) by immersingMWNTs into an aqueous KMnO4

solution, as shown in Fig. 10a. Direct use of the MnO2/MWNT

composites (containing 40 wt% MWNTs) as lithium–air battery

electrodes enhances the kinetics of the oxygen reduction.67 From

Fig. 10b it is inferred that, hollow carbon fibers with diameters on

the order of 30 nm were grown on a porous alumina substrate and

were used as the oxygen electrode in Li–O2 batteries. These all-

carbon-fiber (binder-free) electrodes were found to yield high

discharge capacities of up to 7200 mA h g�1 carbon even at 63

mA g�1 carbon and high gravimetric energies of up to 2500 W h

kg�1.36 As shown in Fig. 11, a novel free-standing type electrode

composed of only a Co3O4 catalyst and a Ni foam current

collector was designed and constructed.68 The new air electrode

was found to yield noticeably higher specific capacity and

improved cycle efficiency over the conventional carbon supported

electrode with almost the highest discharge voltage (2.95 V), the

lowest charge voltage (3.44 V), the highest specific capacity

(4000 mA h g�1 cathode) and the minimum capacity fading.

Another example is the construction of 3-D hierarchically

bi-modal porous air electrodes with functionalized graphene sheets

(FGSs) (see Fig. 12)69 which are hot research materials in electro-

chemical energy conversion and storage.70 A very high discharge

capacity (15000 mA h g�1 carbon) was obtained, which results

from facilitated oxygen transport and unique lithium oxide

growth. SEM images (a and b) in Fig. 12 show that the

graphene-based air electrode contains numerous large tunnels

which facilitate continuous oxygen flow into the air electrode

while other small ‘‘pores’’ provide ideal triphase regions for the

oxygen reduction. DFT calculations show that Li2O2 prefers to

nucleate and grow near functionalized lattice defect sites on

graphene. Also, SEM and TEM images (c to e in Fig. 12, indicated

by white arrows) show that the deposited Li2O2 forms the isolated

nanosized ‘‘islands’’ on FGS, avoiding the surface passivation

and pore clogging and ensuring smooth oxygen transport

during the discharge process.

3.3 Distribution

How do the lithium oxides distribute on the surface and within

the electrode? Of course distribution may need to be considered

Fig. 10 (a) TEM image of the MnO2–MWNT nanocomposites;67 (b) SEM image of a carbon nanofibers carpet based on the porous anodized

aluminium oxide template.36

Fig. 11 The schematic diagram of the free-standing-catalyst based electrode during cycling in the Li–O2 battery.

Fig. 12 The discharge curve of a Li–O2 cell using functionalized graphene sheets and morphologies of the graphene-based air electrode.69

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in two scales: (i) carbon particles or fiber surfaces and (ii) an

interface or a reaction zone within the whole electrode. For the

former, if the catalysts are decorated on the carbon surface

how do they influence the product morphology? Since the

carbon itself is electroactive to the ORR what is the role of the

catalyst? The latter is highly related to the lithium oxide

distribution across the electrode and may be correlated with

the oxygen profile along the depth of the electrode and highly

associated with the rate capacity, especially when the electrode

is thick. Another important factor is the electrolyte distribu-

tion since the electrochemical interface either TPBs or DPBs

which at least requires a liquid–solid interface is essential for

the ORR.

3.3.1 Role of catalysts. Since the use of electrolytic manganese

dioxide (EMD) was demonstrated in a rechargeable Li–O2

battery,71 the exploration of the role of catalysts in ORR and

oxygen evolution reaction (OER related to the charge process)

in nonaqueous systems becomes more and more common. For

example, screening of materials including metal (Pt), perovskite

(La0.8Sr0.2MnO3), metal oxides (Fe2O3, NiO, Fe3O4, Co3O4,

CuO and CoFe2O4),72 exploration of various manganese oxides

(a-MnO2 in bulk and nanowire form, b-MnO2 in bulk and

nanowire form, g-MnO2, l-MnO2, Mn2O3, and Mn3O4),73

synthesis of carbon supported manganese oxides (MnOx/C)

and its application in the air electrode,74,75 influence of C, Au/

C, and Pt/C catalysts on the charge and discharge voltages,76

PtAu nanoparticles supported on carbon (PtAu/C) as bifunc-

tional (ORR and OER) electrocatalysts,77 H2O2 decomposi-

tion reaction as a selecting tool for choosing catalysts,78

comparison of metals and metal oxides79 and a heat-treated

metal phthalocyanine complex as the catalyst.80 As discussed

before, ORR in the nonaqueous Li–air cathode is not a

catalytically sensitive reaction because the carbon possesses

catalytic activity. Further, the electrode performance in the

electrochemical devices like PEM fuel cells does not directly

indicate the intrinsic activity of a catalyst because of the

electrode structure effect, especially the surface passivation,

pore blockage and the use of high mass ratio of carbon

materials in Li–air batteries. To more accurately determine

the catalytic activity, testing the materials on a thin film

electrode rather than in a single cell may be more appropriate.

We also need to point out three things. First, even when the

thin film electrode is used to quantify the catalytic activity,

complication may arise from the unstable solvents. For

example, the ORR activity trend ranks in the descending order

of Au4GC4 Pt in PC+DME41 while the trend follows the

sequence of Pt 4 Au 4 GC in DME.42 Second, the normal-

ized current or activity based on the true surface area of

catalyst or catalyst mass may not be as critical as in the

PEM fuel cells, especially when non-novel catalysts are used

in Li–air batteries. The focus should be on the large surface

area particularly electrochemical effective surface area, but

this does not mean smaller particle size implies better ORR

activity. Once the product film covers a catalyst particle, it

would not work as the reaction site any more. The morphology

of the catalyst, like nanorods, nanowires, dendrites and random

clusters81 or hollow sphere,82 may impact the ORR activity.

Third, novel metals should be finally avoided in Li–air batteries83

to prevent cost issues similar to PEM fuel cells.

3.3.2 Distribution on the carbon surface. As mentioned

before, carbon materials themselves are electroactive towards

the ORR in nonaqueous Li–air batteries. All-carbon materials

without heterogeneous catalysts have been demonstrated as

the active materials for the air electrodes, i.e. a free standing

carbon nanotube–nanofiber mixed bulky paper,84 nitrogen

doped KB and Calgon activated (CA) carbon,85 nitrogen

doped carbon nanotubes,86 nanostructured diamond-like

carbon thin films87 and graphene nanosheets.88 Although

experimental results show enhanced discharge capacity before

and after nitrogen doping85,86 or by comparing graphene with

Vulcan XC72,88 mechanistic understanding of the action of

carbon in the Li–air cathode and details of molecular-level

surface chemistry interaction with lithium oxides are not

fully explored. DFT calculations have been performed to

examine the ORR on several model carbon structures including

the g(0001) basal plane (including graphene), the (8,0) single-

walled nanotubes (SWNTs) to represent curvature, the

armchair-type edge (henceforth referred to as an armchair

edge) of a graphene nanoribbon (GNR) to represent the edge

of graphite, and a di-vacancy in graphene to represent point

vacancies (see Fig. 13).89 The basal plane and the curved

surface of the SWNT do not well stabilize the key intermediate

and limits the complete O2 reduction. The armchair edge and

di-vacancy are highly reactive and can form oxidized carbon

structures (COx) which will serve as the active sites for

catalyzing O2 reduction. And further, LiO2 can be chelated

and stabilized by neighboring oxygen ligands. From the

experimental evidence in Fig. 12, the deposited Li2O2 would

form isolated nanosized ‘‘islands’’ on the FGS defect sites.

Also from Fig. 14,36 it is evident that the nuclei formation and

particle growth of lithium oxides prefer some sites on the

carbon fibers where probably the defects and functionalities

Fig. 13 Structural models in top (upper panels) and side (lower panels) views for: (a) graphene; (b) 3-layer g(0001); (c) (8,0) SWNT; (d) GNRwith

armchair edge; (e) graphene with a di-vacancy.89

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are rich. Referring back to the heterogeneous metal- or oxide-

loaded carbon, such metal or oxide particles to some extent

can be considered to increase the defects or functionalities on

the carbon surface, which will definitely influence the distribu-

tion and morphology of the discharge product. For example,

without the catalyst, the air electrode was covered by a film-

like discharge product, while with the MnO2 catalyst, some

granulated type voids within the discharge product were

observed which would allow oxygen diffusion.90 One needs

to be aware that (i) the decoration of heterogeneous catalysts

would consume the defects and functionalities originally pre-

sent on the carbon surface; (ii) the preference of lithium oxides

on the heterogeneous catalysts and carbon is different.

Although the oxidized carbon structure can favor the ORR

by DFT calculations (discussed above), we must consider the

oxidation of carbon in the air electrode. If decorated with

catalyst particles on the carbon, the oxidation may collapse

carbon and cause catalyst loss, as found in the PEM fuel cells.

Further, COx species formed from the carbon oxidation may

combine with Li species, resulting in carbonate-like products

and thus less charge–discharge efficacy.91 The carbon electrode

oxidation should be distinguished from that of nonaqueous

solvents and the electrochemical window in the stable solvents

needs to be determined. Avoiding the use of carbon, i.e.

exploitations of the mixture of Pd and MnO2,92 mesoporous

a-MnO2/Pd,93 mesoporous b-MnO2/Pd,94 Li2O rich phases

Li5FeO4 (5Li2O�Fe2O3) and Li2MnO3�LiFeO2 ([Li2O�MnO2]�[Li2O�Fe2O3])

95 and even the hybrid with insertion materials,96

would decrease the specific energy density.

3.3.3 Electrolyte distribution. Electrolyte distribution within

the air electrode directly affects the electrochemical interface

and probably impacts O2 mass transport. Fig. 15a shows

discharge curves of three Li–O2 cells with different electrolyte-

filling status. With insufficient electrolyte, more discharge

products were observed by SEM deposition on the separator

side, while in the excess electrolyte case a denser deposition on

the air side was found. When the amount of the electrolyte is

appropriate the discharge products can be evenly deposited

throughout the air electrode, resulting in high specific capa-

city.97 This was explained by the authors using fast O2 mass

transport within non-electrolyte-occupied pores and slow

transport in the liquid electrolyte. Although the words

‘‘excess’’, ‘‘insufficient’’ and ‘‘appropriate’’ are not quantita-

tive and it is believed that the cell with insufficient electrolyte

might show lower discharge capacity than the one with excess

electrolyte once the electrochemical interface is too small,

Fig. 15a gives an example of the effects of the electrolyte

amount and distribution on the cell performance and thus the

product of lithium oxides. To precisely depict the relation of

electrolyte distribution with product formation within the air

electrode or give an ideal electrolyte distribution, a schematic

map presenting partly and fully wetted electrode structure is

demonstrated in Fig. 15b.98 If the electrolyte can evenly

distribute along the inner wall of the pore, the oxygen can

diffuse easily through the pores in the cathode and then

penetrate the thin layer of the electrolyte. The advantages

of the fully wetted electrode structure are obvious: large

electrochemical interface or effective surface area, facilitated

gas mass transport and possible avoidance of pore clogging by

products. One question that remains now is how to quickly

and effectively measure or quantitatively determine the elec-

trolyte distribution. Volatile liquid within 3-D electrodes

Fig. 14 Evolution of discharge product morphology and insets show-

ing the corresponding discharge voltage profile.36

Fig. 15 (a) Discharge curves of three Li–O2 cells with different electrolyte-filling status, which were recorded at 0.2 mA cm�2;97 (b) partly (above)

and fully (below) wetted electrode structure;98 (c) equivalent circuit used for the analysis of the lithium oxides precipitated air electrode;99 (d) the

finite transmission-line equivalent circuit including the double layer and the charge transfer resistance.100

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makes general imaging techniques unsuitable, but fortunately

the liquid electrolyte and carbon solids form an electrochemical

interface, which is important for the ORR. The electrochemical

impedance spectroscopy (EIS) technique proves to be a good

diagnostic tool which is able to separate the responses of the

charge transfer and mass transport processes occurring simulta-

neously on the electrode. The capacitance of the double layer is

a direct index of the area of the interface which is covered by the

electrolyte. Although the mass transport resistance can be

deconvoluted from the EIS results, it cannot be used to measure

the thickness of the electrolyte film covering the interface

because the mass transport resistance for the partially and fully

wetted electrodes as shown in Fig. 15b may be the same. EIS

was demonstrated to be a powerful technique for studying the

capacity loss due to the interfacial changes occurring at the air

cathode of a lithium oxygen battery.99 The equivalent circuit

used in that study is shown in Fig. 15c. The first semicircle

consisting of parallel connection of interfacial resistance (Rint)

and interfacial capacitance (Cint) can be assigned to the electro-

chemical process on the oxide surface films as the charge is

stored capacitively across the oxide barrier film on the elec-

trode. The parallel connection of charge transfer resistance

(Rct) and capacitance (Cm), which contains the double layer

capacitance at the carbon–electrolyte interface and pseudo-

capacitance from MnO2 catalysts, induces the second semi-

circle. Warburg impedance may include the effects of the

limited diffusion of Li ions and O2 mass transport. Therefore,

EIS can be used to determine the electrolyte distribution and

even the product. When the impedance response exhibits a

Warburg-like straight line at about 451 in the high frequency

region, this region is dominated by the charging process of

the double layer coupled with the ionic transport in the

CL, and it can be depicted by the finite transmission-line

equivalent circuit (Fig. 15d).100 The ionic resistance or its

profile can be another measure of the electrolyte distribution,

which may need further investigation and confirmation in

Li–air electrodes.

3.3.4 Distribution across electrodes. Reaction zone or more

accurately utilization efficiency of the active sites within the

electrode, usually associated with oxygen mass transport, ionic

resistance loss and kinetics of electrochemical reaction, may be

another important factor leading to the ‘‘sudden death’’

during the discharge process in Li–air batteries. Generally in

PEM fuel cells,101 for a non-porous active layer, diffusion is

the rate limiting step with respect to ionic resistance drop and

best performance is obtained for catalyst particles located

close to the gas diffusion layer side; for a porous active layer,

ionic ohmic drop becomes the rate limiting step and the

performance is improved when the catalyst particles are

located close to the proton exchange membrane side; for

porous electrodes, at high current density the reaction zone

should shift to the region close to the membrane or the

utilization efficiency should be higher in the region close to

the membrane than that close to a gas diffusion layer side.

Referring to the Li–air battery, as the electrolyte distribution is

previously discussed (Fig. 15a), an excess electrolyte makes the

air electrode more like a non-porous electrode and introduces

O2 mass transport limitation, and an insufficient electrolyte

results in the limitation of Li ions migration. These are

consistent with the SEM results97 and similar to the situation

in a PEM fuel cell. A little more detail about the effect of

electrochemical reaction kinetics or current density or specifi-

cally rate capability in Li–air electrodes on the reaction zone

will be given. Fig. 16 shows SEM images of the air side of the

electrode (left) and at the center of the electrode (right) when

the electrodes are discharged at three rates.9 After discharge at

0.05 mA cm�2, the electrode surface contains spheres of

lithium oxides with diameters of 150–200 nm. After discharge

at 0.2 mA cm�2, the spheres on the surface appear to be larger

at 300 nm and at 1.0 mA cm�2 the deposit appears to be more

of a film. The deposit at the center of the electrode is not

visible at 1.0 mA cm�2 and fills the pores at 0.05 mA cm�2.

This phenomenon is also confirmed by a recent report,102

which indicated that when discharging at low rate the utiliza-

tion efficiency is uniform throughout the electrode and the

products distribute evenly within the air electrode, and when

discharging at high rate the reaction zone shifts to the region

close to the air side, and the products block oxygen access

to the electrode. This may be because Li–air batteries

usually show smaller discharge capacity at higher discharge

rate and only at very small discharge rate there is a straight-

forward relationship between discharge capacity and electrode

porosity. Interestingly, by using a thin electrode (20 mm) it

was found that the reduction at the discharge capacity at high

rates is not a result of the depletion of O2 in the electrolyte-

filled pores across the electrode thickness based on the

transport properties of 0.1 M LiClO4 DME (see Fig. 17).103

Fig. 16 SEMmicrographs of PTFE/Super P air cathodes (air side of electrode: left; center of electrode: right): (a) undischarged and discharged at

(b) 0.05 mA cm�2; (c) 0.2 mA cm�2, and (d) 1.0 mA cm�2.9

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The reduction in the discharge voltage and capacity with

increasing rates was attributed to the resistance associated

with solid-state Li+ diffusion in the lithium peroxide. We need

to be careful with the profile of O2 concentration within the

electrode, as shown in Fig. 17. Although there is a certain

amount of O2 at a particular position of the electrode which

can be calculated by considering the consumption in electro-

chemical reaction, it does not mean that O2 can access the

reaction sites once there is a solid product film blocking the gas

pathway. This is very similar to the case of water freezing in

PEM fuel cells even if the catalyst layer is thin (o20 mm).104

Therefore, we can only say that the depletion of O2 will

definitely cause the ‘‘sudden death’’, but non-depletion of O2

in the electrode does not guarantee a electrochemical reaction,

especially if surface passivation and pore clogging by the

products exist. Numerical modeling may be a very useful tool

in this field to explore the fundamentals behind this. Recently,

a report on three-dimensional spatial distribution of lithium

products in electrochemically discharged lithium–air cathodes

proved that neutron tomographic imaging will be a very power

technique for clarifying lithium oxides distribution.105

3.4 Amount

We care about the amount of lithium oxide products because

we worry about the volume change. The formation of a new

solid phase in the air electrode once discharging and its

disappearance accompanied by gas evolution when charging

may (i) cause stress on the electrode or even the whole cell and

(ii) destroy the porous electrode structure. The former concern

has been addressed in a previous review.6 The creation of a

new phase in the air electrode indicates that an electrolyte will

be replaced unless the new phase displaces a gas phase.

Balanced volume changes at a Li electrode and an air electrode

may be possible if the densities of the products and reactants

are matched, but this is not the case in Li–air cells: the ratio of

the discharged volume to the charged one equals 0.7. For the

latter concern, whether the volume change due to the phase

transition would negatively influence the porous electrode

structure like water/ice situation in PEM fuel cells106,107

should be clarified, which directly impacts the cycle perfor-

mance of Li–air batteries. The research on the accommodation

of volume change in solid systems may use the techniques

employed in the anode of lithium ion batteries for reference.

4. Summary and outlook

In this review, we have shown detailed fundamental under-

standings, novel concepts and ideas related to the air electrode

in nonaqueous lithium–air batteries, and correlated the chemistry

and physics of lithium oxides precipitation with electrochemistry

and material science of the electrode. To clarify the exact

discharge products at the air electrode in nonaqueous Li–air

batteries, the mechanism of oxygen reduction in non-aqueous

solvents in the presence of Li ions, the influence of solvents

and Li ions and the role of catalysts have been discussed. The

reversible electrochemistry happening in the air electrode is an

urgent research topic. TPBs and DPBs determine the location

of lithium oxides, and surface passivation and pore clogging

are responsible for the sudden death of the batteries.

The design of materials and electrode structure may be hot

research areas, and at the same time the physics of product

film growth and chemistry of LiO2 intermediates also need

further study. The distribution of lithium oxides has been

discussed in two scales like on the carbon particles or fiber

surface and within the whole electrode. The roles of defects,

functionalities and nano-catalysts on the carbon surface may

need systematic investigation. Electrolyte distribution in the

electrode and utilization efficiency of active sites contribute to

the product profile within the electrode. In these studies, EIS

has been shown to be very useful, and numerical modeling and

image technology would be helpful. The possible degradation

induced by phase transition and the related migration strategies

need to be considered in the future study.

Acknowledgements

Support from American Electric Power and the Virginia Tech

Institute for Critical Technology and Applied Science is grate-

fully acknowledged. The proof-reading by Jessica Wright is

appreciated.

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