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Lithium oxides precipitation in nonaqueous Li–air batteries Junbo Hou,*a Min Yang,a Michael W. Ellis,b Robert B. Moorec and Baolian Yid

Downloaded by Dalhousie University on 14 September 2012 Published on 16 August 2012 on http://pubs.rsc.org | doi:10.1039/C2CP42768K

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) a

Institute for Critical Technology and Applied Science, Virginia Tech, Blacksburg, VA 24061, USA. E-mail: [email protected], [email protected] b Department of Mechanical Engineering, Virginia Tech, Blacksburg, VA 24061, USA c Department of Chemistry, Virginia Tech, Blacksburg, VA 24061, USA d Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China

Junbo Hou is currently a Research Associate in Institute for Critical Technology and Applied science at Virginia Tech. He received his PhD degree (2008) in Chemical Engineering from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, studying on fuel cell technologies and electrochemical fundamentals. After working on semiconducting materials synthesis and characterization for two years in Junbo Hou University of Leoben and Erich Schmid Institute, Austria Academy of Science, he came to Virginia Tech and currently works on the electrochemical energy conversion and storage. This journal is

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

Min Yang currently works at Virginia Tech Institute for Critical Technology and Applied Science. After she received her PhD degree in Chemical Engineering at Dalian Institute of Chemical Physics, Chinese Academy of Sciences in 2008, she worked as a researcher in Montanuniversita¨t Leoben, Austria. Her research focused on oxide materials synthesis, ceramic devices preparation, kinetic study of electroceramic mateMin Yang rials, electrochemical basics, and solid oxide fuel cell technology.

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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 LiOHH2O 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 structures. Even given the oxygen involvement, the specific energies

Michael W. Ellis is an Associate Professor of Mechanical Engineering at Virginia Tech. Dr Ellis has 25 years of experience in engineering, research, and education related to advanced energy systems. His current work focuses on the development and evaluation of materials for PEM fuel cell membranes and diffusion media, modeling liquid transport in PEM fuel cells, and fuel cell cogeneration for buildings. He teaches Michael W. Ellis courses on thermodynamics, sustainable energy, and engineering design. Dr Ellis is the director of the Sustainable Energy Research Program in Virginia Tech’s Institute for Critical Technology and Applied Science and is chair of ASME’s Advanced Energy Systems Division.

Robert B. Moore is the Associate Director of Research at the Virginia Tech Institute for Critical Technology and Applied Science (ICTAS), and a Full Professor in the Department of Chemistry at Virginia Tech, with 22 years of academic experience in the field of Polymer Physical Chemistry. His research is focused on the area of ioncontaining polymers for energy applications, with specific interests that include: Robert B. Moore control of morphology transport property relationships in proton exchange membrane fuel cell systems, tailored actuation behavior in nano-structured materials, and the use of small-angle X-ray and neutron scattering methods for the characterization of morphology in ion-containing polymers. Phys. Chem. Chem. Phys.

Fig. 1 Theoretical specific energy for Li ion and Li–air batteries together with regenerative fuel cells.

of the nonaqueous system (3505 W h kg 1) which is comparable 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 understandings, novel concepts and ideas addressing Li oxides precipitation 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.

Baolian Yi is a fellow of the Chinese Academy of Engineering, and a Professor of Chemical Engineering at Dalian Institute of Chemical Physics, Chinese Academy of Sciences. He has been working on the research and development of fuel cells and related fields since the 1970’s. He researched on AFC for space application in the 1970’s, and conducted fuel cell technique, aqueous solution electrolysis industry and electrochemical Baolian Yi sensors in the 1980’s. During the 1990’s, he initiated the research studies on PEMFC, MCFC and 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 This journal is

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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 solubility 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-trifluoroethyl) 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 choosing 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 electrochemical 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 DMEbased 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) electrolyte23 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 Phys. Chem. Chem. Phys.


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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 electrochemical 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 precipitation 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 agglomerates, 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 resistance, 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 identified: (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 and mechanisms of oxygen reduction in acetonitrile containing four different hexafluorophosphates (general formula APF6, where A = tetrabutylammonium (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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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

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

Fig. 4 Cyclic voltammograms for the ORR in 0.1 M LiPF6/DMSO within various electrochemical windows.39

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 , O22 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 disproportionation 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 ORR mechanism 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. 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 desolvation of Li ions most importantly solvation of O2 and LiO2. We need to point out that electrochemical quartz crystal microbalance (E-QCM) may be a very helpful technique to investigate the electrochemistry of product precipitation.

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.

3.1.2 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 electrochemically stable. It was found that employing carbonatebased solvents (propylene carbonate (PC), ethylene carbonate (EC) and dimethyl carbonate (DMC)) resulted in decomposition 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

3.2 Location

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).44


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 transport.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 elucidate 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. 7 Schematic showing the location of lithium oxides formation: (a) surface passivation; (b) pore blockage and clogging.

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Fig. 8 (a) SEM image of the discharge film (40–70 nm thickness) produced at the end of a 0.6 mA cm (b) discharge curves of flat-electrode cells at three current densities.55

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/ferrocenium redox couple to probe charge transport through films of Li2O2 on a glassy carbon electrode. By combining the electrochemical 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 This journal is

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discharge on a flat glassy carbon electrode;

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 blockage, 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 Phys. Chem. Chem. Phys.


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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, mesopores 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 distribution, 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 electrochemical 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 interconnected porous systems. This idea is similar to the hydrophobic 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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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

surface passivation is designing novel electrode structures. MnO2 nanoflakes were uniformly coated on multi-walled carbon nanotubes (MWNTs) by immersing MWNTs 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 allcarbon-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 electrochemical energy conversion and storage.70 A very high discharge capacity (15 000 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. 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 distribution since the electrochemical interface either TPBs or DPBs which at least requires a liquid–solid interface is essential for the ORR.

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

3.3.1 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 bifunctional (ORR and OER) electrocatalysts,77 H2O2 decomposition 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 Au 4 GC 4 Pt in PC + DME41 while the trend follows the

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


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Li5FeO4 (5Li2OFe2O3) and Li2MnO3LiFeO2 ([Li2OMnO2] [Li2OFe2O3])95 and even the hybrid with insertion materials,96 would decrease the specific energy density.


Fig. 14 Evolution of discharge product morphology and insets showing the corresponding discharge voltage profile.36

are rich. Referring back to the heterogeneous metal- or oxideloaded 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 distribution and morphology of the discharge product. For example, without the catalyst, the air electrode was covered by a filmlike 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 present 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

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 electrolytefilling 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 capacity.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 quantitative 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 electrolyte distribution. Volatile liquid within 3-D electrodes

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 simultaneously 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 electrochemical process on the oxide surface films as the charge is stored capacitively across the oxide barrier film on the electrode. The parallel connection of charge transfer resistance (Rct) and capacitance (Cm), which contains the double layer capacitance at the carbon–electrolyte interface and pseudocapacitance from MnO2 catalysts, induces the second semicircle. 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 specifically 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 utilization 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 straightforward 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 electrolytefilled pores across the electrode thickness based on the transport properties of 0.1 M LiClO4 DME (see Fig. 17).103

Fig. 16 SEM micrographs 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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of volume change in solid systems may use the techniques employed in the anode of lithium ion batteries for reference.


4. Summary and outlook

Fig. 17 Dimensionless oxygen concentration in a flooded porous electrode: e = 0.73 and e = 0.3 correspond to the electrode porosity at the onset of discharge and a discharge state of B2500 mA h g 1 at a rate of j = 0.8 mA cm 2.103

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 electrochemical 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

In this review, we have shown detailed fundamental understandings, 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 gratefully acknowledged. The proof-reading by Jessica Wright is appreciated.

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 performance of Li–air batteries. The research on the accommodation This journal is

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