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www.nanoadv.org

Review Article

Nano Adv., 2016, 1, 17−24. www.nanoadv.org

http://dx.doi.org/10.22180/na166 Volume 1, Issue 1, 2016

Tuning Electrochemical Reactions in Li-O2 Batteries

Youwei Wang, a Beizhou Wang, ab Feng Gu, ab Zhihui Zheng ab and Jianjun Liu a*

a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese

Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China. Email: jliu@mail.sic.ac.cn b School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China.

Received Dec. 27, 2015; Revised Feb. 6, 2016

Citation: Y. Wang, B. Wang, F. Gu, Z. Zheng and J. Liu, Nano Adv., 2016, 1, 17–24.

Rechargeable lithium-O2 battery is considered as promising next-generation devices for energy storage and

conversion because of their high theoretical specific energy density. However, its application suffers from several

issues such as high overpotential, poor cycle performance, and limited rate capability. Tuning

electrochemical/chemical reactions in discharge and charge play an important role in reducing overpotential,

increasing current density, and improving reversibility of Li-O2 batteries. In this review, the fundamental principles

and complicated electrochemical and chemical reactions in electrolytes and cathodes are first discussed. Based on

these mechanisms, various strategies such as stabilizing electrode materials, selecting suitable electrolytes, adding

catalysts and mediators, and changing O2 pressure are reviewed to improve electrochemical performance by tuning

electrochemical/chemical reactions. Finally, we explore future research directions in improving electrochemical

performance of lithium-O2 battery.

KEYWORDS: Lithium-O2 battery; Electrochemical reaction; Charge/discharge process; Energy storage and

conversion

1. Introduction

In the long term, society development’s need for energy storage

such as electric vehicles (EVs) and large scale power station will

far exceed that achievable by Li-ion batteries since the delivered

energy and power densities of Li-ion batteries are not enough.1−5

In contrast to conventional Li-ion batteries, non-aqueous aprotic

Li-O2 batteries can form Li2O2 in discharge by electrochemical

and chemical reactions by Li+ and O2, which induces an

extremely large theoretical specific energy ~3500 Wh/kg and

generate an equilibrium voltage of 2.96 V according to Nernst

equation. In recent years, Li-O2 batteries have received

heightened attention because they can provide gravimetric

energy density considerably higher than Li-ion batteries.

However, to make Li-O2 batteries suitable for practical

applications, major challenges must be overcome, which include

low round-trip efficiency,3, 6−7 electrode and electrolyte

instability,8−10 and low current density,11−12 and low cycle

life.13−14

The electrochemical performance of Li-O2 batteries is mainly

regulated by the gas electrode (cathode), non-aqueous electrolyte,

O2 pressure, and additive catalysts and mediator in cathode or

electrolyte.2 Li metal is currently used as anode to provide extra

Li resource and ultimately will be replaced due to safety issue

before and deployment of Li-O2 batteries into real market. The

cathode is a main site oxygen reduction reaction (ORR) during

discharge and oxygen evolution reaction (OER) during charge.

The solid-state catalysts are composited on the cathode to

enhance kinetic rate of ORR and OER.3 Therefore, the cathode

should have a large surface area and large pore volume to store

discharge product Li2O2, while catalyst contains enough active

sites. Non-aqueous electrolyte not only functions as Li+ and O2

diffusion channel, but also provides a site of electrochemical and

chemical reactions in a molecule/ion level. During discharge, the

O2− released from oxygen reduction reaction on electrode reacts

with Li+-complex to form superoxide (LiO2), further undergoing

chemical disproportionation reaction to generate peroxide

(Li2O2). During charge, the Li+ and O2 are continuously

desorbed from Li2O2 surface and interface between Li2O2 and

electrode (catalyst). Certainly, some metastable intermediates

such as LiO2 and Li3O4 may be generated.15−17 The

above-mentioned electrochemical and chemical reactions are

summarized as following:

Discharge Reactions:

O2 + (Li+ + e−) = LiO2 (1)

2 LiO2 = LiO2 + O2 (2)

LiO2 + Li+ = Li2O2 (3)

Charge Reactions:

Li2O2 = LiO2 + (Li+ + e−) (4)

Review Article Nano Advances

Nano Adv., 2016, 1, 17−24. doi: 10.22180/na166

LiO2 = O2 + (Li+ + e–) (5)

Although there is no general agreement on mechanism, it is

sure that the complicated electrochemical and chemical reactions

may occur on electrolyte and electrode. The deep discharge or

strong ORR catalyst may result in the oxide Li2O which is

irreversible in electrochemical environment.18−19 The presence of

O2, O2–, LiO2, and Li2O2 species and makes a practical Li-O2

battery more complicated than expectation.20−21 They may lead

to electrolyte and electrode decomposition during discharge.

Some irreversible species such as Li2CO3 and Li2C2O4 also may

be produced by some side reactions.8−9

It is of significant importance to tune these electrochemical

and chemical reactions to avoid irreversible species to be formed

and promote reversible and low-charge-potential species such as

LiO2/Li2O2. In this review, our discussion will concentrate on

the main challenges of Li-O2 batteries and how to improve

battery performance by tuning electrochemical and chemical

reactions.

2. Non-aqueous electrolytes

Although non-aqueous electrolytes have been studied and

developed for decades and successfully applied in

commercialized Li-ion batteries, they cannot be directly

employed in Li-O2 batteries. Electrolyte molecules coordinate

with Li+ ions and support fast Li+ ion transport through the cells.

This interaction has been demonstrated to be strongly related to

cyclic performance of Li-ion and Li-O2 batteries. In addition, the

properties of formulated electrolytes are crucial to the interfacial

structure between electrodes, O2 gas, and electrolyte and

accordingly regulate the performance of Li-O2 batteries. Very

recently, some additives and mediators have been added into

electrolyte to tune electrochemical and chemical reactions which

are directly related to battery performance.22−26

Because some highly oxidative intermediates are generated

during discharge and charge of Li-O2 batteries, some possible

polar organic compounds containing functional groups of

carbonyl (CO), nitrile (C≡N), sulfonyl (SO), and ether

(O).27 Based on these principles, the aprotic electrolytes such

as carbonate, dimethyl sulfoxide (DMSO), phosphates, nitriles,

and glymes have been attempted. In the following sections, we

concentrate on the influence of electrolyte stability, additives

and catalysts on electrochemical and chemical reactions in

electrolyte in order to reveal the possible tuning mechanism in

Li-O2 batteries.

2.1 Electrolyte stability

As mentioned above, the oxygen reduction and evolution

processes lead to the formation and decomposition of Li2O2 via a

sequence of highly active intermediates such as O2–, LiO2, O2

2–,

and LiO2–.20−21 The early attempts to operate Li-O2 batteries

based on organic carbonate failed after few cycles as the overall

process becomes dominated by electrolyte decomposition.28

Peter Bruce et al. used mass spectrometry (MS) and in situ

Surface Enhanced Raman Spectroscopy (SERS) techniques to

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

H2O during discharge and charge.29 Several groups reported

instability of carbonate electrolytes.30−32 More importantly, these

generated species are irreversible in electrochemical processes

and accumulate in the cathode on cycling, resulting in capacity

fading and final cell failure. Such a failure indicates organic

carbonates are not suitable as electrolyte of Li-O2 batteries.

Luntz et al. from IBM research group also observed CO2

evolution from electrochemical reaction on carbonate electrolyte,

while Dimethoxyethane (DME)-electrolyte favours Li2O2

discharge product.10 T. Laino and A. Curioni performed

molecular dynamic simulation (CPMD) to study decomposition

mechanism of propylene carbonate under the presence of LiO2

and Li2O2.20 They found that LiO2 plays an important role in

electrolyte decomposition rather than Li2O2.

In 2012, Jung et al. found that glyme-based electrolyte may

operate effectively to form many cycles with capacity and rate

values as high as 5000 mAh/g and 3 A/g, respectively.33 Indeed,

Ji-Guang Zhang et al. also observed that a large amount of Li2O2

was generated in the air-electrode discharged in glyme-based

electrolytes. DME was reported to be stable during cycling by

McClosky et al.10 Further, in situ quantitative gas-phase mass

spectrometry and XRD confirmed Li2O2 was the main discharge

product in the electrolyte of DME.34 However, the inconsistency

between O2 consumption and evolution amounts suggests there

may exist a reaction between Li2O2 and DME. As shown in

Figure 1, Li2O2 both decomposed to evolve oxygen and oxidized

DME at high potential upon cell charge. At present, DME is still

actively applied in Li-O2 batteries.

Recently, some traditional electrolytes such as acetonitrile

(AcN) and DMSO have been applied in Li-O2 batteries.14, 21, 35−38

O2 electrochemistry in AcN and DMSO follows a stepwise

fashion from O2− to O2

2− and O22− to O2

− and then O2.35 When

AcN was employed as electrolyte, the Li-O2 battery showed the

highest reversibility of OER/ORR of 0.9. 14 DMSO as

Figure 1. Gas evolution and current versus cell voltage during a 0.075 mV/s

linear oxidative potential scan of a discharged DME-based cell. The cell

was discharged at −0.1 mA for 10 h under 16O2 prior to the scan.

Reproduced from Ref. 8 with permission of 2011 American Chemical

Society.

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Nano Adv., 2016, 1, 17−24. doi: 10.22180/na166

electrolyte and nonporous golden (NPG) or foam Ni as electrode

have been applied in Li-O2 batteries. It was found that DMSO is

stable within a voltage window from 2 to 4.35 V.38 The cycling

performance showed dependence on the discharge depth and

decayed with cycling. A higher charge potential leads to Li2CO3

and HCO2Li formation. Apart from AcN and DMSO, some other

electrolytes such as NMP,5 triethyl phosphate,30 DMF,39

methoxybenzene40 have been attempted. The reversible

formation of Li2O2 could be observed in the initial few cycles,

the capacity decayed quickly with cycling. Therefore,

developing a stable electrolyte is still a long way in future.

Very recently, Peter Bruce et al. established

electrolyte-dependent discharge product formation mechanisms,

in solvent or on electrode surface.24 A united mechanism of the

pathway of O2 reduction to form Li2O2 called as solution

pathway or electrode surface pathway was reported in their paper.

They found the morphology of Li2O2 has been related to the

solvent donor number (DN). In the intermediate-DN ethers, both

electrode surface and solution pathway contribute significantly

and simultaneously to Li2O2 formation at high voltages, leading

to significant Li2O2 surface films and particles in solution.

Low-DN solvents lead to Li2O2 film growth, decaying rate, low

capacity and early cell death.

2.2 Solvating additive in electrolyte

Recently, several research groups found the trace amount of

electrolyte additives such as H2O and acid can enhance the

formation of Li2O2 toroids and result in significant improvement

in capacity.22−23, 25−26 Luntz et al. combined experimental

measurements with theoretical modelling to study two different

discharge mechanisms:25 one is surface electrochemical

mechanism that produces conformal Li2O2 and the other is

solution-mediated electrochemical process driven by LiO2 partial

solubility, where O2− acts as redox mediator and ultimately

enhances the growth of Li2O2 toroids at low currents. Their

studies strongly supported the second mechanism, as shown in

Figure 2. The addition of water triggers the dissolution process

of LiO2* Li (sol) + O2− (sol). These soluble O2

− undergoes

subsequent reaction on a growing Li2O2 toroid through the

generic mechanism of 2 Li+ (sol) + 2 O2− (sol) = Li2O2 (s) + O2

(g). However, O2− is known to undergo disproportionation to

form H2O2 which further performs a slow dissociation reaction.

This step, along with a reaction between H2O and Li metal,

slowly consumes the H2O and eventually this reduction of water

reduces the overall dissolution rate of LiO2 and ultimately

terminates the solution growth mechanism. The similar reaction

mechanism was also reported in Na-O2 batteries by Nazar’s

research group.

Very recently, Zhou et al. developed a state-of-the-art Li-O2

battery in which ruthenium and manganese dioxide nanoparticles

supported on carbon black super P (Ru/MnO2/SP) by applying a

trace amount of water in electrolyte to catalyze the cathode

reactions.26 They reported a charge potential of 3.5 V that 0.7 V

higher than discharge potential, and superior cycling stability of

200 cycles. Further a new mechanism was proposed as

conversion reaction of Li2O2 into LiOH catalyzed by trace

amount of H2O,22 Li2O2 + H2O 2 LiOH + H2O2. The stable

H2O2 is MnO2-catalyzed to dissociate into H2O and O2. The

resultant LiOH has a lower charge potential than Li2O2 due to a

weaker electrostatic bonding between Li+ and OH−. Therefore,

H2O plays an important role in converting Li2O2 into LiOH.

There are two possible catalysts added in Li-O2 batteries,

solid-state catalyst on electrode surface and liquid-state redox

mediator in electrolyte in order to reduce OER overpotential and

improve cycling performance. The incorporated redox mediator

(M) in electrolyte is oxidized directly at the electrode surface to

generate Mox, followed by oxidizing the Li2O2 particles and

reducing itself back to M.23 In essence, the redox mediator acts

as an electro-hole transfer agent between electrode and Li2O2.

The strategy is helpful to solve poor conductivity of Li2O2 and

speed up slow kinetics of Li2O decomposition. In 2013, Peter

Bruce et al. firstly studied tetrathiafulvalene (TTF) as a redox

mediator, proving far more effective oxidization of Li2O2 as

compared with its absence.23 The charge potential is reduced to

Figure 2. Proposed mechanism for the growth of Li2O2 toroids in the presence of water. The deposited Li2O2 is shown schematically to

proceed via a surface electrochemical growth process that occurs on a nucleated film of Li2O2 through the sequential transfer of solvated

Li+ and an electron (e–) to the intermediate species LiO2*, and eventually forming Li2O2. Reproduced from Ref. 25 with permission of 2012

NPG publisher.

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Nano Adv., 2016, 1, 17−24. doi: 10.22180/na166

3.5 V with a complete reversibility of 100 cycles.

In 2014, Kang Kisuk et al. reported a Li-O2 battery system

using a soluble catalyst (LiI) combined with hierarchical

nanoporous air electrode, achieving high reversibility and good

energy efficiency.41 The cell can deliver a high reversible

capacity (1000 mAh/g) up to 900 cycles with reduced charge

potential of less than 3.5 V. Schematic illustration of the role of

redox mediator in Li-O2 battery are presented in Figure 3.

Recently, Yunhui Huang et al. used the

organic-electrolyte-dissolved iron phthalocyanine (FePc) as

redox mediator to shuttle O2− species and electrons between

Li2O2 and electrode, achieving an excellent electrochemical

performance.42

3. Cathodes

3.1 Metal and non-metal cathode materials

The solid Li2O2 formed on discharge must be stored a porous

conducting matrix, which in practice has to combine sufficiently

high conductivity and surface area with a low cost and ease of

fabrication as a porous electrode materials. In addition, structural

stability of electrode materials, especially with discharge and

charge reactions, must be considered as a vital requirement.

Carbon has extensively applied as cathode materials of Li-ion

batteries due to its conductivity and large surface area. Several

studies examined that carbon electrode appeared relatively stable.

However, some side reactions were observed and attributed to

carbon decomposition. Peter Bruce et al. studied carbon

electrode using acid treatment and Fenton’s reagent by

differential electrochemical mass spectroscopy (DEMS) and

FTIR.9 As shown in Figure 4, they observed that carbon is

relatively stable below 3.5 V on discharge and charge. Above

3.5 V, carbon may experience an oxidizing decomposition to

form Li2CO3. But direct chemical reaction between carbon

electrode and Li2O2 hardly take place during discharge and

charge processes. However, discharged Li2CO3 was measured to

have a relatively high charge potential, resulting in electrode

passivation and capacity fading and irreversibility.

In 2012, Peng and Bruce et al. proposed a nanoporous gold

and DMSO as air electrode and electrolyte, which generated

excellent electrochemical performance with 95% capacity

maintainence in 100 cycles.37 The electrode design with pure

metal prevents carbon decomposition to block active sites of

electrode. Following by this pioneering work, Wen et al.

Figure 3. Schematic illustration of the role of the redox mediator (RM) in a Li-O2 battery system using hierarchical CNT fibril electrode.

Reproduced from Ref. 41 with permission of John Wiley & Sons, Inc.

Figure 4. Schematic illustration of electrochemical reaction products and cell voltages of Li-O2 battery with carbon as electrode.

Reproduced from Ref. 9 with permission of 2011 American Chemical Society.

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Nano Adv., 2016, 1, 17−24. doi: 10.22180/na166

designed a free-standing electrode of foam Ni nanocomposited

with Co3O4 electrocatalyst, which has a high discharge voltage

of 2.95 V, a low charge voltage of 3.44 V, and high specific

capacity of 4000 mAh/g, and low capacity fading.43 The

electrode design principle and synthesized nanostructure are

presented in Figure 5. They attributed these superior

electrochemical performance to abundant available catalytic sites

on electrode surface, the intimate contact of discharge product

with the catalyst, the effective suppression of the volume

expansion in the electrode during discharge and charge processes,

the good adhesion of the catalyst to current collector, and the

open pore system for unrestricted access of the reactant

molecules to and from active sites of the catalysts.

Very recently, Larry Curtiss et al. constructed a

nanocomposited electrode of iridium/reduced graphene oxide

(Ir/RGO) and observed LiO2 as alone discharge product, which

was called as Li-O2 battery based on superoxide.44 This is

different from their previous studies that Li2O2 and LiO2 have

been found with some different cathodes and electrolyte. They

explained the existence of LiO2 may be attributed to several

possible factors. Firstly, Ir/RGO was used as electrode materials

to favor fast deposition and crystal growth of LiO2 due to a good

lattice match between IrLi3 and LiO2, which is favorable to

metastable LiO2 growth. It indicates fast deposition of LiO2

decreases soluble LiO2 concentration and partially prevents

disproportionation reaction into Li2O2. Without Ir incorporated

into RGO, a different deposition and growth of discharge

products may occur to lead to LiO2 and Li2O2 mixture. Secondly,

they attributed LiO2 formation to O2 desorption kinetics and

interfacial effect between electrolyte and electrode. The ab initio

molecular dynamic (AIMD) simulations indicated that O2 had a

lower desorption barrier from LiO2 at the interface between LiO2

and electrolyte, which greatly enhanced LiO2 formation. Thirdly,

a possible OER catalysis of Ir/RGO for Li2O2 dissociation into

LiO2 is not excluded. However, the complicated electrochemical

and chemical reactions took place in solution and on electrode.

By combining advanced experimental characterization

techniques and quantum chemical computational methods,

exploring these reaction mechanisms are very necessary to

optimize electrochemical performance and design novel

electrode materials of Li-O2 batteries.

3.2 Solid-state catalysts

One of large challenges faced by Li-O2 battery is the round-trip

efficiency which is because of high overpotential or polarization

of cathode reactions. A high charge voltage of 4.5 V and low

discharge voltage of 2.7 V are measured, which leads to a low

efficiency. An important strategy of improving this efficiency is

applying effective catalysts on cathode. Although the previous

experimental report by Luntz et al. doubted efficacy of

electrocatalysis due to decomposition of carbonate electrolyte,10

a large amount of theoretical and experimental studies indicated

that by applying electrocatalysts, discharge voltages could be

increased and charge voltages could be decreased, which

generated a higher round-trip efficiency with a longer cycling

life and larger energy capacity.5−7, 33, 45−50 Therefore, we would

review here several catalysts such as carbon, transition metal

oxides, and noble metals.

Due to the controllable pore size, carbon materials have

extensively applied as the air electrode of Li-O2 batteries. Xia et

al. synthesized mesocellular carbon material with narrow pore

size distribution (30 nm) through nanocasting method by using

nanosheets aggregated into loosely packed structures with large

interconnected channels which are favorable to supply oxygen

into the interior of the electrode using the discharge reactions.51

As shown in Figure 6, the structure can deliver the high energy

Figure 5. The top figure exhibits schematic diagram of the free-standing-catalyst based electrode during cycling in the Li-O2 battery.

The below figure is the SEM images of Co3O4@Ni foam and TEM image and SAED patterns of the Co3O4 nanorods. Reproduced

from Ref. 43 with permission of 2011 Royal Society of Chemistry.

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Nano Adv., 2016, 1, 17−24. doi: 10.22180/na166

capacity of 15000 mAh/g.

In addition to the carbon porosity and structure, the carbon

nature also affects the catalysis in Li-O2 battery. The N-doped

carbon draws much attention because conjugation between

lone-pair electron of N and graphene -electron.52−53 The

electronic properties play an important role in improving oxygen

reaction activity. However, doping carbon materials used as

oxygen evolution catalysts have not been reported in experiment.

Ren et al. performed the first-principles thermodynamic

calculations to study catalytic activity of X-doped graphene (X =

B, N, Al, Si, and P) materials as potential cathodes to enhance

charge reaction in Lithium-air battery.54−55 Among these

materials, P-doped graphene exhibits the highest catalytic

activity in reducing the charge voltage by 0.25 V, while B-doped

graphene has the highest catalytic activity in decreasing the

oxygen evolution barrier by 0.12 eV. By combing these two

catalytic effects, B, P-codoped graphene was demonstrated to

have an enhanced catalytic activity in reducing the O2 evolution

barrier by 0.70 eV and the charge voltage by 0.13 V. B-doped

graphene interacts with Li2O2 by Li-sited adsorption in which

the electron-withdrawing center can enhance charge transfer

from Li2O2 to the substrate, facilitating reduction of O2 evolution

barrier. In contract, X-doped graphene (X = N, Al, Si, and P)

prefers O-sited adsorption toward Li2O2, forming a X−O22−Li+

interface structure between X−O22− and rich Li+ layer. The active

structure of X−O22− can weaken the surrounding Li−O2 bonds

and significantly reduce Li+ desorption energy at the interface.

Tremendous research efforts in experiment and theory have

been made to address high overpotential of charge reactions by

incorporating transition metal compounds (TMC, oxide, carbide,

nitride) in cathode to enhance OER kinetics. However, it is still

controversial whether TMC can improve electrochemical

performance of Li-O2 battery. Many TMCs were determined to

have little, even not, catalytic effects in reducing overpotential

and improving current density. In contrast, some transition metal

oxide with novel nanostructures, doping metal, and conductive

substrate21, 27, 43, 56, 57 were experimentally found to have catalytic

activity for electrochemical reaction in Li-O2 battery. Among all

applied metal oxides, spinel Co3O4 with a mixed oxidation states

of Co2+ and Co3+ is promising as it can significantly reduce OER

overpotential and improve cyclic performance of Li-O2 battery.

After studying several metal oxides as cathode catalysts, Débart

et al. found that Co3O4 supported on carbon gives the lowest

charging voltage of ~4.0 V and maintains a relative good

discharging capacity.6−7 In 2012, the electrochemical studies for

an innovatively designed Co3O4@Ni cathode demonstrated a

higher rechargeable capacity and much lower charging voltage

(3.5 V) than noble metal Pt/Au as cathode catalyst.43 However,

the detailed catalytic mechanism is unclear. Recently, Black et al.

studied the electrochemical performance of Co3O4 grown on

reduced graphene oxide (Co3O4@RGO) and observed kinetic

improvement of mass transport for both OER and ORR.58 Zhu et

al. performed first-principles calculation to elucidate that the

O-rich Co3O4 (111) with a relatively low surface energy in high

O2 concentration has a high catalytic activity in reducing

overpotential and O2 desorption barrier due to the electron

transfer from the Li2O2 layer to the underlying surface.49 Further

they found that P-type doping of Co3O4 (111) exhibits

significant catalysis in decreasing both charging overpotential

and O2 desorption barrier. Further Zhu et al. performed the

first-principles calculations based on interfacial model were

performed to study the OER mechanism of Li2O2 supported on

active surfaces of TMC. They found that the O2 evolution and

Li+ desorption energies show linear and volcano relationship

with surface acidity of catalysts, respectively.48 Therefore, the

charging voltage and desorption energies of Li+ and O2 over

TMC could correlate with their corresponding surface acidity.

In 2011, Shao-Horn et al. reported the intrinsic oxygen

reduction reaction (ORR) activity of polycrystalline Pd, Pt, Ru,

Au, and glass carbon surfaces in 0.1 M LiCoO4, 1,

2-dimethoxyethane via rotating disk electrode measurements.46

The Li+-ORR activity of these surfaces primarily correlates to

oxygen adsorption energy, generating a volcano-type trend. The

activity trend found on the polycrystalline surfaces was in good

agreement with the trend in the discharge voltage of Li-O2 cells

catalyzed by nanoparticle catalysts. In comparison, Xu et al.

calculated catalytic activity of Au, Ag, Pt, Pd, Ir, and Ru for

catalyzing the Li-ORR.18 As shown in Figure 7, they predicted

Figure 6. SEM images of as-prepared functional graphene nanosheets (FGSs) air electrodes at different magnifications and

electrochemical measurement. Reproduced from Ref. 51 with permission of 2011 American Chemical Society.

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Li-ORR has the smallest overpotential on Pt and Pd. The

catalytic activity exhibits a volcano-like trend with respect to the

adsorption energy of atomic oxygen, which is in close agreement

with the experimentally observed trend reported by Shao-Horn et

al.46

4. Conclusions and outlooks

The non-aqueous Li-O2 batteries have attracted a great deal of

attention as potential energy-storage systems for future electric

vehicle applications. However, it is still in developing stages and

there are many technological problems to solve before becoming

commercial applications. At present, controlling complicated

electrochemical reaction processes and understanding these

fundamental mechanisms become much important. Numerical

systematic and detailed studies on materials and chemicals are

still required to enhance discharge and charge reaction kinetics

and regulate discharge products.

Several factors such as stability, catalytic activity, electronic

and Li+ conductivity, and O2 diffusivity and adsorption must be

considered in terms of designing novel electrode materials. In

comparison, carbon electrode is relatively unstable because some

reactions intermediates such as O2− and O2

2− can attack the

defective carbon to form Li2CO3 which is irreversible and has a

relatively high charge potential. Metal electrodes combining

with catalysts may have good electric conductivity and stability.

Therefore, the development of non-carbon-based supporting

materials such as metal and metal oxides with novel

nanostructure (nanowire, nanotube, and nanosheet) is of much

importance for developing high-performance Li-O2 barriers.

Electrolyte plays an important role in regulating

electrochemical reactions because the reduced O2− enters into

electrolyte to combine with Li+ complex. Some electrolytes such

as carbonate are unstable with reactive intermediates, resulting

in Li2CO3 formation to cover active electrode and capacity

fading. Ethers and DMSO electrolytes were found to have a

good electrochemical performance and expected to be

extensively applied in future. Some additives such as H2O and

liquid-catalyst are mixed with electrolyte to control

electrochemical reactions. However, their tuning mechanisms

are not very clear.

Acknowledgements

This work is financially supported by “One-Hundred-Talent

Project”, “the Key Research Program (Grant

No. KGZD-EW-T06)” of the Chinese Academy of Sciences,

National Natural Science Foundation of Chinese, NSFC

(51432010, 21573272), and the research grant (No.

14DZ2261200) from Shanghai government.

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Figure 7. Ueq of the first e-transfer step in the Li-ORR on the six metals plotted against the adsorption energy of atomic O relative to

that on Pt, on the close-packed and step edge surfaces, respectively. Reproduced from Ref. 18 with permission of 2011 American

Chemical Society.

23

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How to cite this article: Y. Wang, B. Wang, F. Gu, Z. Zheng

and J. Liu, Nano Adv., 2016, 1, 17–24; doi: 10.22180/na166.

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