Journal of Energy Chemistry 25 (2016) 957–966

http://www.journals.elsevier.com/

journal-of-energy-chemistry/

Contents lists available at ScienceDirect

Journal of Energy Chemistry

journal homepage: www.elsevier.com/locate/jechem

Review

Nanocarbons and their hybrids as catalysts for non-aqueous

lithium–oxygen batteries

✩

Yunchuan Tu

a , b , c , Dehui Deng

a , b , ∗, Xinhe Bao

a

a State Key Laboratory of Catalysis , iChEM , Dalian Institute of Chemical Physics , Chinese Academy of Sciences , Dalian 116023 , Liaoning , China b Collaborative Innovation Center of Chemistry for Energy Materials , College of Chemistry and Chemical Engineering , Xiamen University , Xiamen 361005 ,

Fujian , China c University of Chinese Academy of Sciences , Beijing 10 0 039 , China

a r t i c l e i n f o

Article history:

Received 15 September 2016

Revised 12 October 2016

Accepted 14 October 2016

Available online 9 November 2016

Keywords:

Electrocatalysis

Electron transfer

Lithium–oxygen batteries

Nanocarbon materials

a b s t r a c t

Rechargeable lithium-oxygen (Li–O 2 ) batteries have been considered as the most promising candidates

for energy storage and conversion devices because of their ultra high energy density. Until now, the crit-

ical scientific challenges facing Li–O 2 batteries are the absence of advanced electrode architectures and

highly efficient electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction

(OER), which seriously hinder the commercialization of this technology. In the last few years, a number

of strategies have been devoted to exploring new catalysts with novel structures to enhance the battery

performance. Among various of oxygen electrode catalysts, carbon-based materials have triggered tremen-

dous attention as suitable cathode catalysts for Li–O 2 batteries due to the reasonable structures and the

balance of catalytic activity, durability and cost. In this review, we summarize the recent advances and

basic understandings related to the carbon-based oxygen electrode catalytic materials, including nanos-

tructured carbon materials (one-dimensional (1D) carbon nanotubes and carbon nanofibers, 2D graphene

nanosheets, 3D hierarchical architectures and their doped structures), and metal/metal oxide-nanocarbon

hybrid materials (nanocarbon supporting metal/metal oxide and nanocarbon encapsulating metal/metal

oxide). Finally, several key points and research directions of the future design for highly efficient catalysts

for practical Li–O 2 batteries are proposed based on the fundamental understandings and achievements of

this battery field.

© 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published

by Elsevier B.V. and Science Press. All rights reserved.

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. Introduction

To minimize the consumption of fossil fuels, and thereby re-

uce CO 2 emissions which consequent effects on global warming,

n indispensable strategy to electrification of road transportation

s now underway. The major technical hurdle confronting complete

lectrification transformation is the exploration of advanced energy

torage systems [1,2] . Lithium-ion battery technology has estab-

ished itself as a promising and reliable energy storage system over

he past 20 years. While after continuous development, the en-

rgy density of Li-ion battery system increased ≈15% per year and

✩ This work was supported by the Ministry of Science and Technology of China

Nos. 2016YFA0204100 and 2016YFA020 020 0), the National Natural Science Foun-

ation of China (Nos. 21321002, 21573220 and 21303191), and the strategic Priority

esearch Program of the Chinese Academy of Sciences (No. XDA09030100). ∗ Corresponding author at: Collaborative Innovation Center of Chemistry for En-

rgy Materials, College of Chemistry and Chemical Engineering, Xiamen University,

iamen 361005, China. Tel: +86 0592 2186917.

E-mail address: dhdeng@dicp.ac.cn (D. Deng).

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ttp://dx.doi.org/10.1016/j.jechem.2016.10.012

095-4956/© 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academ

ay reach the theoretical limits soon, which may severely limit

he range of electric vehicles. Therefore, a novel battery chemistry

eyond the prevailing Li-ion battery technology is urgently needed

o develop the next generation of electrical energy storage systems

3–5] .

Due to the extremely high specific capacity (3860 mAh/g) and

he low reduction potential ( −3.04 V vs standard hydrogen elec-

rode, SHE), metallic lithium is the most promising anode mate-

ial for high energy density batteries [6–8] . Furthermore, low-cost

2 from the atmosphere reacts directly with the shuttled Li + ions

n the porous electrode, resulting in a greatly increased theoreti-

al specific energy. Because of the especially high theoretical gravi-

etric energy of 3500 Wh/kg, that far outperforms that of other

vailable battery chemistries, Li–O 2 battery chemistry has triggered

remendous attention as a promising candidate to conventional Li-

on batteries [9–11] . Generally, four categories of Li–O 2 batteries

re designated by the type of the electrolyte employed in batter-

es: non-aqueous, aqueous, hybrid, and all-solid-state batteries. Be-

ause of the insufficient ionic conductivity of solid-state electrolyte

nd parasitic corrosion on Li metal anode of aqueous electrolyte,

y of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

958 Y. Tu et al. / Journal of Energy Chemistry 25 (2016) 957–966

Anode: Li ↔ Li+ + e- Cathode: 2Li+ + 2e- + O2 ↔ Li2O2 Discharge (ORR) process: 2Li+ + O2 + 2e- → Li2O2 Charge (OER) process: Li2O2 → 2Li+ + O2 + 2e-

Lithium Electrolyte O2 electrode

Fig. 1. Schematic of a typical non-aqueous Li–O 2 battery.

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the non-aqueous electrolyte has dominated research efforts on

Li–O 2 batteries compared to the other electrolyte-based Li–O 2 bat-

teries [12–14] . In order to avoid the contaminants (such as H 2 O

or CO 2 ) from air, most Li-air batteries are operated in pure oxy-

gen, so it is expressed as “Li–O 2 batteries” rather than “Li–air bat-

teries”. A typical non-aqueous Li–O 2 battery ( Fig. 1 ) consists of a

metallic lithium anode, a separator saturated with Li + conduct-

ing organic electrolyte, and a porous cathode. The cathode elec-

trochemical pathway is based on the reversible redox reaction:

2Li + +2e − +O 2 ↔ Li 2 O 2 ( E 0 = 2.96 V vs Li/Li + ). During the discharge

process, O 2 is reduced (oxygen reduction reaction, ORR) in the

porous structure of the cathode, and consequent combined with

the shuttled Li + ions to form insoluble and insulated Li 2 O 2 prod-

ucts, which will sluggishly decompose to release O 2 (oxygen evo-

lution reaction, OER) in the following charge process. The slow re-

actions occurring in the cathodes give rise to a large voltage gap

(usually large than 1 V) between the charge and discharge of a Li–

O 2 battery, resulting in a much lower round-trip efficiency of < 60%

compared to the commercial Li-ion battery ( > 90%).

Table 1. Summary of carbon-based materials and their hybrids for Li-O 2 batteries.

Carbon-based material Composite Capacity depth

(mAh/g)

Curren

(mA/g

1D carbon nanotubes and

carbon nanofibers

Aligned multi-walled nanotubes

(MWNTs) [ 32 ]

10 0 0 10 0 0

Carbon fibers (CNFs) [ 42 ] 4720 43

2D graphene nanosheets Graphene nanosheets (GNSs) [ 54 ] 2332 50

3D hierarchical carbon

architectures

3D ordered carbon sphere arrays

[ 61 ]

10 0 0 50

Graphene foam [ 59 ] 5500 200

Doped carbon Vertically aligned N-doped

coral-like carbon fiber (VA-NCCF)

[ 73 ]

10 0 0 100

N-doped graphene [ 74 ] 10400 200

3D porous N-doped graphene

aerogels (NPGAs) [ 62 ]

10 0 0 300

Nanocarbon supporting

metals/metal oxides

Ru on reduced graphene oxide

(rGO) [ 90 ]

20 0 0 200

FeCo-CNTs [ 96 ] 10 0 0 100

Ir-rGO [ 92 ] 10 0 0 100

Co 3 O 4 nanofibers on graphene

nanoflakes [ 103 ]

10500 200

MnCo 2 O 4 -graphene [ 110 ] 30 0 0 200

RuO 2 -CNTs [ 143 ] 1700 10

Graphene encapsulating

metals/metal oxides

N-doped graphene encapsulating

metallic Co [ 142 ]

20 0 0 200

CNT-encapsulating Pd [ 89 ] 250 50

3D N-doped graphene

encapsulating RuO 2 [ 97 ]

10 0 0 200

A number of strategies have been devoted to reducing the se-

ere hysteresis between charge and discharge, such as the reg-

lation of temperature [15–17] , employment of redox mediator

18–23] and ionic liquid [24–29] , as well as the solar integrating

30,31] . Despite much achievements have been made, Li–O 2 bat-

ery technology is still at its infant stage. Before the commercial

pplication of this technology, fundamental questions such as Li

endrites, poor rate capability, low round-trip efficiency, electrolyte

nd carbon degradation and poor cyclability should be resolved.

mong various factors affecting Li–O 2 battery performance, the

ack of superior catalytic activity with novel structures of the oxy-

en cathode catalysts have been considered as the vital point. In

he last few years, numerous oxygen cathode catalysts have been

roposed to reduce the large voltage gap, including noble metals,

ransition-metal oxides, and carbonaceous materials. Due to the

easonable structures and the balance of catalytic activity, durabil-

ty and cost, carbon-based materials have been considered promis-

ng as suitable cathodes for rechargeable Li–O 2 batteries.

In this review, the discussions and insights provided reflect

he most recent approaches and directions for carbon-based oxy-

en electrocatalysts (including nanostructured carbon materials

nd metal/metal oxide-nanocarbon hybrid materials) developments

Table 1 ). In addition, we also review the status and challenges and

rovide a perspective of oxygen electrocatalysts for non-aqueous

i–O 2 batteries.

. Nanostructured carbon catalysts

Due to the chemically tailorable surface, high specific surface

rea, high porosity, excellent electrical conductivity and low cost,

anostructured carbon materials have been considered as promis-

ng alternatives of oxygen electrodes for non-aqueous Li–O 2 bat-

eries. The carbon materials can fabricate a continuous porous

tructure in order to accommodate discharge products and pro-

ide channels for ions transfer and oxygen diffusion, they can

lso act as electrocatalysts towards ORR as well as OER in non-

queous Li–O 2 batteries. In this section, the peculiar carbon nanos-

t density

)

ƞcharge

(V)

Construction features or activity origin

∼1.3 Large specific area and the unique porous framework can

be attributed to improve the electrolyte immersion and

mass transfer process. The edges or defects carbon atoms

are the active sites for oxygen reactions.

1.15

1

1

1.04

0.2 The charge distribution and electronic states of the carbon

atoms can be affected by the dopants, making the

hetero- and carbon atoms become the active sites for

oxygen reactions. ∼1.6

1.16

0.54 Carbon serves as the support for the uniform distribution

of catalysts and the mass transfer process, while metals

or metal oxides are the active sites. 0.93

∼0.3

1.2

1.1

0.6

0.56 Carbon not only serves as the support, but also as the

active sites for catalytic reactions due to the electron

transfer from the internal metals/metal oxides. 0.3

∼0.6

Y. Tu et al. / Journal of Energy Chemistry 25 (2016) 957–966 959

Fig. 2. Cross-sectional SEM micrograph of the porous anodized aluminium oxide (AAO) filter (a) after thin film deposition using electron beam evaporation and (b) after

nanofiber growth. (c) First-discharge rate capability of CNF electrodes with galvanostatic currents corresponding to 43, 261, 578, and 10 0 0 mA/g C . (d) Gravimetric Ragone plot

comparing energy and power characteristics of CNF electrodes based on the pristine and discharge electrode weight with that of LiCoO 2 . Reproduced from Ref. [42] with

permission from The Royal Society of Chemistry.

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ructures are summarized as four main groups: one-dimensional

1D) carbon nanotubes (CNTs) and carbon nanofibers (CNFs), 2D

raphene nanosheets, 3D hierarchical architectures and their doped

tructures.

.1. Carbon nanotubes and nanofibers

Carbon nanotubes (CNTs) have recently been investigated as

athode materials for non-aqueous Li–O 2 batteries because of their

igh chemical and thermal stability, high conductivity, high tensile

trength and large surface area [32–41] . A novel carbon air elec-

rode with controlled pore structure by Lim et al. [32] . The air

lectrode was fabricated by orthogonally plying individual sheets

f aligned multi-walled nanotubes (MWNTs) without use of any

inder or solvent. The unique porous framework in these woven

NT electrodes significantly improved the cyclability of the Li–O 2

attery. This was attributed to the facile accessibility of oxygen to

he inner side of the air electrode, and preventing the clogging

f pores by the discharge product (Li 2 O 2 ). Even at an extremely

igh current density (20 0 0 mA/g), the battery can still maintain at

east 60 cycles with a fixed capacity of 10 0 0 mAh/g. Similarly, an-

ther new type of porous carbon electrode architecture was pro-

osed by Mitchell et al . [42] . Vertically aligned arrays of hollow

arbon fibers with diameters on the order of 30 nm were grown

n a porous alumina substrate, as shown in Fig. 2 (a, b). At a cur-

ent rate of 43 mA/g C , Li–O 2 batteries with CNFs discharged at an

verage voltage of 2.6 V over the entire discharge process ( Fig. 2 c).

he all carbon fiber electrodes can deliver gravimetric energies up

o 2500 Wh/kg discharged at powers up to 100 W/kg discharged ( Fig. 2 d).

o suppress undesirable side reactions between the carbon elec-

rode and electrolyte or active intermediates (such as O 2 − and

2 2 −) by preventing their direct contact, Lee et al . [36] put forward

simple surface modification strategy by applying a polyimide

oating to CNTs, the obtained cathode showed excellent cycling

erformance, without significant loss of capacity. Recent study pro-

osed by Yoo et al . showed that carbon related air electrode can

lso provide low overpotential for OER process of Li–O 2 battery us-

ng double lithium salt LiNO 3 -LiTFSI/DMSO system [43] . The charge

otential of the Li–O 2 battery using MWCNTs electrode was around

.61 V at the capacity of 10 0 0 mAh/g. And the battery with MWC-

Ts can undergo around 90 cycles without obvious losses of ca-

acity with a fixed capacity of 10 0 0 mAh/g at the current rate of

00 mA/g.

.2. Graphene nanosheets

Graphene is a two-dimensional (2D) crystal consisting of a one-

tom-thick of sp 2 -hybridized carbon. Since Geim and Novoselov

rst exfoliated single-layer graphene from highly oriented pyrolytic

raphite (HOPG) in 2004, a wide range of applications on this

ovel material have surged with the merits of its intrinsically su-

erior thermal and electrical conductivity, large surface area, re-

arkable mechanical flexibility, and high mobility of charge car-

iers [44–51] . Recently, because of the high discharge capacity

960 Y. Tu et al. / Journal of Energy Chemistry 25 (2016) 957–966

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and superior round-trip efficiency, graphene has been employed

as a promising cathode material for non-aqueous Li–O 2 batteries

[52–54] . Li et al . [53] firstly employed graphene nanosheets (GNSs)

as oxygen electrode for non-aqueous Li–O 2 batteries. The cathode

based on GNSs delivered an excellent electrochemical performance

with a high discharge capacity (8705.9 mAh/g) at a current den-

sity of 75 mA/g, which was much higher than that of the cath-

odes with commercial carbon materials of BP-20 0 0 and Vulcan XC-

72 (1909.1 and 1053.8 mAh/g, respectively). Although the detailed

mechanism for the oxygen reduction reaction (ORR) on GNSs in

the non-aqueous electrolyte is unclear, this work revealed that this

unique morphology of GNSs can provide promising applications in

Li–O 2 batteries. At the same time, Sun et al. [54] also applied GNSs

as cathode catalysts for Li–O 2 batteries with alkyl carbonate elec-

trolyte. In comparison to the commercial Vulcan XC-72 electrode,

the GNSs electrode exhibited better cyclability and lower overpo-

tential due to the presence of carbon vacancies and defects on

GNSs surface.

2.3. Hierarchical architectures

Because of the insoluble and insulated discharged products of

Li 2 O 2 , which accumulate at the active sites of the oxygen electrode

during discharge, potentially clogging the pores and consequently

leading to the failure of batteries, it is imperative to develop the

porous structures and the hierarchical architectures of the oxy-

gen electrode for high performance of non-aqueous Li–O 2 batter-

ies. Accordingly, significant effort s have been devoted to optimiz-

ing the structure of the oxygen electrode. Xiao et al. [55] designed

a novel hierarchically porous oxygen electrode with functionalized

graphene sheets (FGSs) that contains lattice defects and functional

groups by a colloidal microemulsion approach. This unique hier-

archical morphology consists of numerous large tunnels facilitat-

ing continuous oxygen diffusion, while other interlaced nanoscale

pores providing ideal tri-phase regions with abundant reactive

sites for the oxygen reactions. DFT calculations also revealed that

the defects and functional groups on graphene nanosheets favor

the formation of isolated nanosized Li 2 O 2 particles, which can en-

sure facile oxygen diffusion during the discharge process and help

prevent oxygen blocking in the oxygen electrode. This oxygen elec-

trode design shines light on how to control the architecture and

surface chemistry of the carbon-based electrode for rechargeable

Li–O 2 batteries. Based on this insight, a number of strategies have

been devoted to improving the performance of Li–O 2 batteries

[56–63] . A free-standing hierarchically porous carbon (FHPC) de-

rived from graphene oxide (GO) gel had been proposed by Wang

et al. [56] to maximize the utilization of porous carbon parti-

cles, which facilitated a continuous oxygen flow and also provided

enough void volume for Li 2 O 2 accommodation. When employed

as a cathode, the Li–O 2 battery simultaneously exhibited a high

specific capacity (11,060 mAh/g at a current density of 280 mA/g)

and excellent rate capability. Lin et al . [57] prepared hierarchically

porous honeycomb-like carbon (HCC) as the oxygen electrode for

Li–O 2 batteries ( Fig. 3 a,b), the obtained HCC-based cathode deliv-

ered a higher specific capacity of 3233 mAh/g, which is more than

three times that of Super P ( Fig. 3 c). Due to the larger BET sur-

face area, larger pore volume and increased number of defects,

HCC-100 sample with smaller diameter pores showed a better per-

formance than HCC-400 with larger pores according to the ini-

tial discharge capacity and cycling performance ( Fig. 3 d). Guo et

al . [61] used 3D ordered mesoporous-macroporous carbon sphere

arrays (MMCSAs) as the cathode catalyst for high performance

Li–O 2 batteries. The ordered mesoporous channels could effectively

improve the electrolyte immersion and facilitate Li + diffusion and

electron-transfer process. Moreover, these macropores, surrounded

by ordered mesoporous channels in the structure, could provide

ufficient space for Li 2 O 2 accommodation/decomposition and O 2

iffusion.

.4. Doped carbon materials

Carbon materials doped with heteroatoms (such as N, B, P or S)

an tune the chemical and electronic nature of carbon-based ma-

erials [64–66] . Many effort s have demonstrated that doping het-

roatoms can increase the degree of defectiveness, which act as the

ctive sites for oxygen reduction reaction (ORR) in aqueous elec-

rolyte [67–71] . Based on this insight, Li et al. [72] firstly used N-

oped GNSs for Li–O 2 batteries, and found that by nitrogen dop-

ng could introduce more defects and functional groups, leading

o a higher ORR catalytic activity than that of pure GNSs. After

hat, doping carbon materials have been widely used as an effec-

ive cathode catalyst of Li–O 2 batteries [34,62,73–79] . Dai’s group

73] demonstrated that vertically aligned nitrogen-doped coral-like

arbon fiber (VA-NCCF) arrays can act as a metal-free cathode cat-

lyst for Li–O 2 batteries ( Fig. 4 a–c). The VA-NCCF electrode ex-

ibited a discharge capacity as high as 40,0 0 0 mAh/g and ran for

ore than 150 cycles under a limited capacity of 10 0 0 mAh/g at

he current density of 500 mA/g ( Fig. 4 e). The voltage gap be-

ween the discharge and charge plateaus at a current density of

00 mA/g was only 0.3 V ( Fig. 4 d). The obtained excellent battery

erformance can be attributed to the nitrogen dopants which in-

uced the catalytic activity of carbon fiber to lower the charg-

ng overpotential, and the unique porous structure that provided

large free space for Li 2 O 2 accommodation and enhanced elec-

ron/electrolyte/reactants transport.

Theoretical calculation has also applied to understand the

RR/OER processes using heteroatoms doping graphene-based

athode catalysts for non-aqueous Li–O 2 batteries and provide use-

ul insight into the catalyst design [80–83] . Yan et al. [83] carried

ut DFT calculations towards oxygen adsorption and dissociation

n the graphene and N-doped graphene surfaces. They found that

harge transfer between the O 2 molecule and the graphene sheet

an make the adsorption energetically stable. Upon N-doping, the

dsorption of oxygen atoms can be enhanced, and the energy bar-

ier of O 2 dissociation can decrease from 2.39 eV (undoped case)

o 1.20 eV. Jiang et al . [80] systematically investigated the feasibil-

ty of graphene, N-doped, B-doped and N, B co-doped graphene as

he potential catalysts in non-aqueous Li–O 2 batteries. Their results

howed that B-doped graphene exhibits the lowest discharge and

harge overpotentials. While co-doping of N and B atoms does not

ave synergistic effects to enhance the ORR/OER catalytic activity

n the presence of lithium atoms, which can significantly change

he most stable adsorption site and adsorption energy in ORR/OER

rocess, leading to the obstruction of electron transfer between

xygen atoms and the substrates.

. Metal/metal oxide-nanocarbon hybrid catalysts

Metals especially noble metals have been widely used as ef-

ective electrocatalysts to facilitate electrochemical reactions (in-

luding ORR/OER and HER) based on their superior electrocatalytic

ctivity and high stability [84,85] . However, due to their heavy

olecular weight and the scarce reserves, which lead to low en-

rgy density and high cost of batteries, nanocarbon materials as

he lightweight and porous substrates have been extensively intro-

uced into cathodes of Li–O 2 batteries to address the problems.

.1. Nanocarbon supporting metals/metal oxides

To achieve lower charge overpotential and higher round-trip ef-

ciency, there has been a universal strategy to construct compos-

te electrode catalysts by loading metals or metal oxides, which

Y. Tu et al. / Journal of Energy Chemistry 25 (2016) 957–966 961

Fig. 3. FESEM images of (a) HCC-400 and (b) HCC-100. (c) Discharge characteristic of Li–O 2 batteries at the current density of 0.05 mA/cm

2 and (d) various current densities.

Reproduced from Ref. [57] with permission from The Royal Society of Chemistry.

Fig. 4. (a) SEM image of a VA-NCCF array grown on a piece of Si wafer by CVD. (b) TEM image of an individual VA-NCCF. (c) The sketch of Li 2 O 2 grown on a coral-like carbon

fiber. (d) Rate performance of the VA-NCCF electrode at different current densities. (e) Discharge/charge voltage profile of the VA-NCCF at a current density of 500 mA/g.

Reprinted with permission from Ref. [73] . Copyright (2014) Amercican Chemical Society.

962 Y. Tu et al. / Journal of Energy Chemistry 25 (2016) 957–966

Fig. 5. (a) TEM image of Ir-rGO composite, showing Ir nanoparticles less than 2 nm in size. The circle in (a) shows some small Ir atomic clusters. Scale bars: 2 nm. (b) Voltage

profiles of the Ir-rGO cathode. Cycle number of voltage plot is given by the color of the plotting symbol. Inset shows capacity as a function of cycle number. (c) Schematic

showing lattice match between LiO 2 and Ir 3 Li that may be responsible for the LiO 2 discharge product found on the Ir-rGO cathode. Reprinted from Ref. [92] with permission

from Nature Publishing Group.

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act as the catalytic active structures, on graphene and other car-

bon materials. Up to now, most of the research on precious metal

based composites focused on Ru [63,86–90] , Pt [89–91] , Ir [92] , Au

[89,91] , and Pd [89,90,93–95] . Lu et al. [91] have systematically in-

vestigated the catalytic activity of Pt and Au on ORR and OER in

Li–O 2 batteries. They found surface Pt atoms were catalytically ef-

fective towards OER while Au atoms were responsible for ORR. In

order to exert both merits, they combined Au and Pt on Vulcan

XC-72 support, the PtAu/C hybrid catalyst demonstrated the low

charging voltage and high round-trip efficiency of Li–O 2 batteries.

The OER catalytic activities of noble metals such as Pt, Pd, Ru sup-

ported on reduced graphene oxide (rGO) in Li–O 2 battery cathodes

have been systematically investigated by Sun’s group [90] . All the

noble metals explored could lower the charge overpotentials, and

among them, Ru-rGO hybrid exhibited the lowest charge overpo-

tential and the most stable cycling performance without electrolyte

decomposition, whereas Pt- and Pd-rGO hybrids showed fluctu-

ating potential profiles and electrolyte instability during cycling.

Most recently, Lu et al. [92] reported a stable one-electron pro-

cess that forms LiO 2 without conventional Li 2 O 2 as the only dis-

charge product using a suitable Ir-rGO cathode in Li–O 2 batteries

( Fig. 5 ). This one-electron process that LiO 2 formed can maintain

stable during cycling with a very low charge plateau (about 3.2 V).

They proposed a novel templating growth mechanism that Ir 3 Li

intermetallic compound with an orthorhombic lattice may be re-

sponsible for the growth of crystalline LiO 2 by the results of TEM

and DFT calculations. This significant discovery will lead to meth-

ods of synthesizing and stabilizing LiO 2 , which could open up a

way to high-energy-density Li–O 2 batteries based on one-electron

process. Quite different from the precious metals, the studies on

non-precious metals used in Li–O 2 batteries are rarely reported,

probably because of their readily oxidation exposing in oxygen at-

mosphere during battery operation. Kwak et al . [96] prepared FeCo

imetal nanoparticles coating on CNTs as an efficient catalyst in

oth ORR and OER for Li–O 2 battery. The charge potential of FeCo-

NTs (3.89 V) is lower than that of CNTs (4.31 V) with the capacity

imitation of 10 0 0 mAh/g at the rate of 100 mA/g.

Apart from the nanocarbon-metal composite catalysts men-

ioned above, functional carbon based metal oxides compos-

te have been widely investigated as oxygen electrocatalysts for

i–O 2 batteries. Noble metal oxide especially ruthenium oxide

RuO 2 ) [97–101] can serve as an excellent OER catalyst in Li–O 2

atteries. Yin et al. [98] proposed a binder-free and self-standing

ierarchical macroporous active carbon fiber (MACF) electrode via

facile and scalable strategy. With RuO 2 decorating (R-MACF), the

pecific capacity of the R-MACF cathode can reach as large as

3,290 mAh/g even at a high current rate of 10 0 0 mA/g, which is

igher than those of 11,150, 6810, 5307 and 1737 mAh/g for MACF,

NTs, macroporous active carbon (MAC) and active carbon fibers

ACFs), respectively. Even after 29 cycles, the charge overpotential

sing R-MACF cathode was as low as 0.66 V, with the energy con-

ersion efficiency of 78%. In consideration of the scarcity and high

ost of noble metals, transition-metal oxides have dominated most

f the research on Li–O 2 batteries, including single-metal oxides

nd multi-metal oxides [102–114] . Among different single-metal

xides, manganese oxides and cobalt oxides are the most exten-

ively studied catalyst for Li–O 2 batteries. Ryu et al. [103] pro-

osed 1D Co 3 O 4 nanofibers immobilized on both sides of the 2D

onoxidized graphene nanoflakes as an efficient oxygen electrode

atalyst for Li–O 2 batteries ( Fig. 6 a–c). The catalyst exhibited a

igh discharge capacity of 10,500 mAh/g at the current density of

00 mA/g and stable cyclability for 80 cycles ( Fig. 6 d,e). This excel-

ent performance was mainly attributed to the improved bifunc-

ional catalytic activity of Co 3 O 4 nanofibers for both ORR and OER,

nd the large surface area which could provide sufficient space

or easy O 2 diffusion and facile electron transport. Multi transi-

Y. Tu et al. / Journal of Energy Chemistry 25 (2016) 957–966 963

Fig. 6. (a,b) TEM images and (c) SAED patterns of the Co 3 O 4 NF/GNF composite. (d) Initial charge/discharge curves and (e) cycle performance of different electrodes at a

current density of 200 mA/g. Reprinted with permission from Ref. [103] . Copyright (2013) Amercican Chemical Society.

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ion metal oxides with spinel [110–112,115] , perovskite [113,116–

19] , or pyrochlore [120] structure are widely used as excellent

RR and OER electrocatalysts for Li–O 2 batteries. The transition-

etal atoms have been substituted by other metal atoms in these

ulti-metal oxides, modulating the electronic structure which can

ontribute to the catalytic activity for ORR/OER. Wang et al .

110] employed MnCo 2 O 4 -graphene hybrid material as the cath-

de catalyst. This uniformly distribution of oxide nanoparticles on

raphene substrate showed an excellent catalytic activity, leading

o lower overpotentials and longer lives of Li–O 2 batteries than

ther catalysts including noble metals such as platinum. Xu et al .

116] firstly employed perovskite-based porous La 0.75 Sr 0.25 MnO 3

anotubes (PNT-LSM) as an excellent oxygen electrocatalyst in

onaqueous Li–O 2 batteries. Compared to the conventional Ket-

enblack (KB) carbon cathode, the mixed PNT-LSM/KB could sig-

ificantly suppress the ORR and especially OER overpotentials and

hus improve the round-trip efficiency. Furthermore, the battery

ith a high specific capacity, superior rate capability and good cy-

le stability can be attributed to the synergistic effect of the high

atalytic activity and the unique hollow channel structure of the

NT-LSM catalyst.

.2. Graphene encapsulating metals/metal oxides

Non-precious metals have been widely studied as promising

lternatives to noble metals in many catalytic redox reactions

121–126] . While, some catalytic reactions usually carried out un-

er harsh conditions such as strong acid or base, oxidation and

igh temperature, the exposed non-precious metal catalysts can-

ot be stable in these harsh environments, leading to the deacti-

ation of catalysts. How to protect the non-precious metals from

eactivation and maintain their intrinsic activities in the harsh

tmosphere turned into a challenging and significant point to

esign low-cost catalysts. Different from aforementioned prevalent

pproach, Deng et al. [127] proposed a novel strategy to encap-

ulate a series 3d transition metals (TMs) within graphene lay-

rs, converting the carbon surface into the active sites for catalytic

eactions due to the electron transfer from the internal metals.

his penetrated electron can increase the density of states (DOS)

f graphene around the Fermi level and decrease the local work

unction of the graphene surface, resulting in a significantly en-

anced catalytic activity. More importantly, owing to the protec-

ion of the graphene “chainmail”, the metal nanoparticles can ef-

ectively avoid the corrosion and poisoning without direct con-

acting with the solutions, reaction molecules or poisons during

eaction. This “chainmail” construction strategy has been widely

nvestigated in different catalytic systems including fuel cells

128–132] and water electrolysis [133–137] , triiodide reduction

eaction (IRR) in dye-sensitized solar cells (DSSCs) [138,139] ,

nd other heterogeneous catalytic oxidation and reduction reac-

ions [140,141] . Moreover, reducing the layer number of graphene

atrix and introducing the heteroatoms (such as nitrogen)

nto the graphene matrix will significantly promote the elec-

ron transfer thus enhance the catalytic activity. It was found

hat N-doped single-layer graphene encapsulating FeNi alloys

howed excellent activity and durability of electrocatalytic oxy-

en evolution reaction (OER), even exceeding that of a com-

ercial IrO 2 catalyst. According to the concept of 2D crystal

chainmail’ for catalysts, Tu et al. firstly systematically investi-

ated the nitrogen-doped single layer graphene shell encapsu-

ating non-precious metals (M@NC) as the highly efficient cat-

lysts for Li–O 2 batteries ( Fig. 7 a) [142] . The Co-based catalyst

Co@NC) showed significantly enhanced OER catalytic activity, with

charge overpotential of 0.58 V, which was remarkably lower

ompared with the corresponding N-free graphene encapsulat-

ng metal, metal oxide and metal-free carbon materials ( Fig. 7 b).

964 Y. Tu et al. / Journal of Energy Chemistry 25 (2016) 957–966

Fig. 7. (a) HRTEM images of Co@NC. The inset shows the (111) crystal plane of the Co particle. (b) Discharge/charge profiles of the Li–O 2 batteries with different cathodes at

a current density of 200 mA/g with the fixed capacity of 2000 mAh/g. (c) Calculated free energy diagram for the oxygen electrode reactions on the surface of Co@NC. (d) A

schematic representation of the oxygen reaction process using Co@NC based oxygen electrode for Li–O 2 batteries. (e) Cycling stability and the terminal discharge voltage as

a function of cycle number for the graphene, CNTs and Co@NC based cathodes for Li–O 2 batteries. Reprinted from Ref. [142] with permission from Elsevier.

4

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d

Moreover, the Co@NC based electrode showed a better cycling

performance than graphene and CNTs based electrodes ( Fig. 7 e).

DFT calculations revealed that the nitrogen dopants and enclosed

metal clusters can synergistically modulate the electronic prop-

erties of the graphene surface, resulting in a dramatic reduction

of the overpotentials ( Fig. 7 c, d). Similar configurations have also

been applied to noble metals/metal oxides. Recent study by Huang

et al. [89] found that the encapsulation of noble metal nanoparti-

cles (Pd, Pt, Ru and Au) inside end-opened CNTs by wet impreg-

nation followed by thermal annealing, can significantly strengthen

the electron density of the carbon surface, serving the entire sur-

face of carbon shells as catalytic active regions for Li 2 O 2 accom-

modation and decomposition, thus effectively reducing the high

charge overpotential. Notably, the charge overpotential can be as

low as 0.3 V when Pd-CNTs sample is used as the cathode. Guo

et al. [97] indicated that although RuO 2 nanoparticles were com-

pletely within graphene and broken off the direct contact with the

electrolyte and Li 2 O 2 , the encapsulated nanoparticles still showed

high catalytic activities toward oxygen reactions. These works pro-

vide a novel electrode design platform on how to control the elec-

tronic structure and surface chemistry of the carbon-based elec-

trodes for high efficiency Li–O 2 batteries by enhancing the ORR as

well as OER kinetics.

. Conclusion and outlook

Li–O 2 batteries are becoming one of the most promising energy

torage and conversion technologies for applications that require

igh-energy density. Although substantial progress has been made

n the past few years, rechargeable non-aqueous Li–O 2 batteries

re still investigated on a lab-scale, further fundamental under-

tanding of oxygen reaction (involving ORR and OER) mechanisms

uring battery operation, parasitic reaction mechanisms of elec-

rolyte/electrode reacting with the highly active intermediates, and

he dynamic transportation information of electron and lithium

ons are the key points to explore new catalysts with novel archi-

ectures for highly efficient and long-life Li–O 2 batteries.

Rational design and development of chemically stable and re-

ersible oxygen electrodes as well as the catalysts is indispens-

ble needed to improve the performance of Li–O 2 battery. Pre-

ise control over the structural and electronic properties of the

urface and interface of catalysts, which efficiently affects the ad-

orption and desorption of the intermediates during reaction pro-

esses that closely related to the performance of batteries, can be

most promising mean to improve the energy conversion effi-

iency. The electrochemical reactions in Li–O 2 batteries involve the

iffusion of oxygen and the precipitation of discharge products.

Y. Tu et al. / Journal of Energy Chemistry 25 (2016) 957–966 965

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R

he porous structure and the catalyst distribution in the oxygen

lectrode require careful optimization to realize the rapid mass

ransportation of all reactants and electrolyte wetting. Carbon ma-

erials suffer corrosion challenges at the high operation potentials

pon charging in the high concentration oxygen atmosphere of Li–

2 batteries. Recently, it has been demonstrated that carbon is un-

table upon charging above 3.5 V in the presence of Li 2 O 2 , under-

oing oxidative decomposition to form Li 2 CO 3 [144] . In this regard,

he carbon supported catalysts and the corresponding composites

ith optimized porous structure and uniform catalysts distribution

re worthy of further optimization in terms of both catalytic activ-

ty and stability.

In-situ techniques, as well as the theoretical calculations, can

irectly probe and predict the oxygen electrochemical reaction

rocesses that occurred at the surface or interface of the catalytic

aterials, which are quite vital for in-depth understanding of fun-

amental electrochemical reaction mechanisms of Li–O 2 batteries.

sing in situ SERS and DEMS can detect the tri-phase reaction

roducts and intermediates, as well as elucidate the origins of par-

sitic reactions of electrolyte/electrode reacting with intermediates

pon battery operation. Moreover, in situ TEM can reveal signif-

cant details into the accommodation and decomposition of Li 2 O 2

n the active sites of catalysts with high spatial and temporal reso-

ution observations. In addition, theoretical calculations have been

eveloped quickly in the field of Li–O 2 battery, but there still re-

ains a big gap between the results of calculations and realistic

eaction processes. Combining the fundamental understanding of

eaction mechanisms and cutting edge computational techniques

hat closed to realistic conditions with materials innovation can

uide us to rationally design stable and efficient catalyst for prac-

ical rechargeable Li–O 2 battery.

The key points of the future design for a high efficient catalyst

or practical Li–O 2 batteries should be summarized as follows:

(1) Low cost and non-precious bifunctional electrocatalysts for

both ORR and OER are required. Electrocatalysts should

not only promote the formation/decomposition of discharge

products, but also suppress the side reactions which seri-

ously affect the cycling stabilities of the batteries. Moreover,

exploring a suitable electrocatalysts that can stabilize one-

electron process to form LiO 2 as the only discharge product

is also a promising strategy for Li–O 2 batteries.

(2) The electrocatalysts should not only significantly lower the

overpotentials, but also improve the capacities, as well as

stabilities of the batteries.

(3) Since the 3D structures can provide abundant space for ac-

commodation and decomposition of the discharge products

and electrolyte immersion, as well as mass transfer process.

Electrocatalysts with advanced 3D architecture or uniformly

loaded on 3D interlaced skeleton is preferred.

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