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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: [email protected] 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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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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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.
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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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