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Accepted Manuscript
Progress in Aqueous Rechargeable Batteries
Jilei Liu, Chaohe Xu, Zhen Chen, Shibing Ni, Ze Xiang Shen
PII:
S2468-0257(17)30147-4
DOI:
10.1016/j.gee.2017.10.001
Reference:
GEE 91
To appear in:
Green Energy and Environment
Received Date: 5 September 2017
Revised Date:
12 October 2017
Accepted Date: 12 October 2017
Please cite this article as: J. Liu, C. Xu, Z. Chen, S. Ni, Z.X. Shen, Progress in Aqueous Rechargeable
Batteries, Green Energy & Environment (2017), doi: 10.1016/j.gee.2017.10.001.
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Graphic Abstract
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Progress in Aqueous Rechargeable Batteries
∥
Jilei Liu,†* Chaohe Xu,§* Zhen Chen,† Shibing Ni, and Ze Xiang Shen†
†
§
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Division of Physics and Applied Physics, School of Physical and Mathematical Sciences,
Nanyang Technological University, 637371, Singapore
College of Aerospace Engineering, Chongqing University, Chongqing 400044, P.R. China
College of Materials and Chemical Engineering, China Three Gorges University, 8 Daxue
Road, Yichang, Hubei 443002, China
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E-mail: liujilei036@163.com; liuj0058@e.ntu.edu.sg (J. Liu), xche@cqu. edu.cn (C. Xu)
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Abstract
Over the past decades, a series of aqueous rechargeable batteries (ARBs) were explored,
investigated and demonstrated. Among them, aqueous rechargeable alkali-metal ion (Li+, Na+,
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K+) batteries, aqueous rechargeable-metal ion (Zn2+, Mg2+, Ca2+, Al3+) batteries and aqueous
rechargeable hybrid batteries are standing out due to peculiar properties. In this review, we
focus on the fundamental basics of these batteries, and discuss the scientific and/or
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technological achievements and challenges. By critically reviewing state-of-the-art
technologies and the most promising results so far, we aim to analyze the benefits of ARBs
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and the critical issues to be addressed, and to promote better development of ARBs.
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Keywords: Aqueous rechargeable batteries, hybrid, fundamental basics, challenges
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1. Introduction
The growing energy demands and the increasing environmental concerns drive the
transformation of power generation from primarily fossil and nuclear sources to solely
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renewable energy sources and the search of efficient energy management systems
(conversation, storage and delivery), to achieve a secure, reliable and sustainable energy
supply.[1-4] The success is strongly dependent on the achievements in efficient
electrochemical power sources that are also safe to operate, economically viable, and
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environmental friendly. Rechargeable battery technologies including lead-acid (Pb-acid),
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nickel-cadmium (Ni-Cd), nickel-metal hydride (Ni-MH), redox flow-cells (RFCs) and
lithium-ion batteries (LIBs) have found practical applications in various areas, however, the
inherent limitations of these systems impede their applications in large-scale energy
storage.[5] In which operational safety is of prime importance along with other desirable
characteristics such as low installed cost, long cycling life, high energy efficiency and
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sustainability.[6] For example, the Pb-acid and Ni-Cd generally suffer from the limited
energy density (∼ 30 Wh kg-1)[7], in addition to the employment of environmentally
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threatened electrode materials.[8] The nickel−iron battery is challenged by the poor
charge/discharge efficiency (ca. 50-60%) and the self-discharge (20-40% per month) related
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to the corrosion and poisoning of the iron anode.[9, 10] The Ni-MH possesses higher energy
density, but delivers poor low-temperature capability, limited high-rate capability, and poor
Coulombic efficiency.[11, 12] Redox-flow cells can be easily piled up, however, the
relatively low power/energy density and the special heat/temperature control requirements
limit their widely applications.[13, 14] Lithium ion batteries hold great promise, benefiting
from higher energy density, lighter weight and longer life time.[15, 16] However, incidents
caused by the flammability of the organic electrolyte and the reactivity of the electrode
materials with the organic electrolytes in the case of overcharging or short-circuiting raise
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serious safety concerns.[17, 18] In addition, the LIBs technologies are comparatively high
cost due to materials used (organic Li salts and organic electrolytes), the special cell
designing and manufacturing processes, and auxiliary systems required for their operation.[5,
19, 20] Another challenge regarding LIBs is the limited rate capability and specific power
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that are restricted by the limited ionic conductivities of the organic electrolyte. All these,
together, intrigue the development of novel battery systems that are safe to operate,
economically viable, and environmental friendly.
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Aqueous rechargeable batteries (ARBs) are of particular attractive for large-scale
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energy storage in terms of safe, economic and sustainable : (i) inherently safe by avoiding the
usage of flammable organic electrolytes, ii) the ionic conductivities of the aqueous electrolyte
is about two orders of magnitude higher than that of nonaqueous ones, ensuring fast
charge/discharge and high round-trip efficiency, and iii) the electrolyte salt and solvent are
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benign. [5, 20-22]
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cheaper and the rigorous manufacturing requirements are avoided, and iv) environmentally
Figure 1. Fundamental electrochemistry. Schematic illustration of “Rocking chair” and “Hybrid type”
aqueous rechargeable batteries (ARBs).
In 1994, Dahn and co-workers proposed the first ARBs prototype using LiMn2O4 and
β-VO2 as positive and negative electrodes, respectively.[23] In which metal-ions are
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intercalated into or extracted from the active materials upon charge/discharge processes,
identical to that of organic systems. It is therefore referred as “rocking chair” type or
“intercalation-chemistry” type (Fig. 1a). Since then, significant progresses have been made in
this intriguing area with more electrochemical redox couples are identified, mores insights
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into fundamental chemistry are gained, and new battery chemistries are explored. More
recently, hybrid design via coupling an intercalation cathode with a metal anode (Fig. 1b) or
combining an intercalation anode with a metal oxides/sulphide (Fig. 1c), was introduced in
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ARBs with the appearance of a new class of aqueous hybrid batteries systems such as
LiMn2O4//Zn ,[24] Na0.44MnO2//Zn,[25] Na0.61Fe1.94(CN)6
0.06//Zn (
indexed as
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vacancy),[26] Ni(OH)2//TiO2,[27] and CoxNi2-xS2//TiO2 [28]. Differ from the “rocking-chair”
type batteries, these batteries operate based on two reversible electrochemical redox
processes involved in anode and cathode parts separately, and the charge/discharge
mechanism in one or two electrodes is not guest ion intercalation/de-intercalation (Figs. 1b &
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1c). Instead, it can be the reaction of Zn2+ deposition-dissolution (Fig. 1b) and or protoninduced oxidization/reduction (Fig. 1c). The electrolyte here acts as conducting ions and
cooperates with the electrodes to store energy, rather than used as the simple supporting
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media in “rocking chair” type batteries. Here were denoted them as “hybrid-chemistry” type
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batteries with abbreviation as ARHBs. These hybrid systems enrich the existing ARBs
chemistry and open up new research era for high-performance ARBs design.
In this review, we summary the latest progresses in aqueous rechargeable batteries with
an emphasis on i) “rocking chair” type ARBs based on intercalating cations such as Li+ ions
and Na+ ions, as well as multivalent ions such as Mg2+, Zn2+, Ca2+ , and Al3+ ions; and ii)
“hybrid chemistry” type ARBs such as interaction cathode//metal anode and metal oxide
and/or metal sulfide cathode//intercalation anode. By critically reviewing state-of-the-art
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technologies and the most promising results so far, we aim to analyze the benefits of ARBs
and the critical issues to be addressed, and to promote better development of ARBs.
2. Fundamental Basics of Aqueous Rechargeable Batteries
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2.1 Safety Working Window & Oxygen/Hydrogen Evolution.
Since electrochemical redox reactions involved in ARBs take place in water
environment. The electrochemical stability window is generally limited to be 1.23 V, beyond
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which H2O is electrolysed with O2 or H2 gas evolution. Thus, materials with working
potentials located between the H2 evolution potential and O2 evolution potential are
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promising electrode candidates for ARBs. In principle, electrodes with a working potential
between 3-4 V (vs. Li+/Li) can be used as cathode, and electrodes with a working potential
between 2-3 V (vs. Li+/Li) can be chosen as anode.[29] The potential diagram comparing
with the stable potential window (vs. SHE) of water and working potential (vs. Li+/Li, or
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Na+/Na) of representative electrode materials as oxides, polyanionics, and other compounds
on the same potential scale is shown in Fig. 2.[20] Note that the H2 evolution potential and O2
evolution potential are strongly dependent on pH value, special caution should be given for
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electrode materials selection to avoid water decomposition.
Figure 2. Electrochemical stability range of water and redox potentials for electrode materials in LIBs and
NIBs. Reprinted with permission from Ref [20]. Copyright 2014 American Chemical Society.
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2.2 Electrode Materials Chemical Stability.
Another key aspect that should be considered is the chemical stability of electrode
materials in aqueous electrolytes. According to Mckinnon and Haering,[30] the chemical
()
1 ()
(
), (1)
=
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() of intercalated Li in Lix(Host) is given as:
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where is the Gibbs free energy of 1 mol Lix(Host) in its standard (solid) state and
()
is Avagadro’s number. Integrating, the eq (2) is derived as:
=
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()
() (2)
+ Following Mckinnon and Haering,[30]the voltage, V(x), of a Li/ Lix(Host) intercalation cell
1 () − !(3)
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is given by
() = −
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is the Gibbs free energy of host in its standard state, is the chemical potential
where AC
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of Li in Li metal, given by = (
/) , , and is the magnitude of the electron charge.
Li, Mckinnon and Dahn [31]demonstrated the stability of the electrode materials in an
aqueous medium from thermodynamic equilibrium perspective, on the basis of the following
reaction
$%(%&'()*+*') + ,- . ↔ $% 0 + ., 1 + (1/2),- (4)
They illustrated the potential of the () in equilibrium with H2O at a particular pH as:
() = 3.885 − 0.1187,()(5)
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This indicates that Lix(Host) with a voltage,() vs. Li, placed in water with react, and the
resulting pH can be given by solving the above equation. Theoretically, one can determine
whether or not the Lix(Host) at a particular pH of electrolyte is stable. Equally, to ensure the
stability of a particular electrode, one can adjust the pH of aqueous electrolytes. For example,
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lithium-ion intercalation potential in Li2Mn2O4 is 2.97 V versus Li+/Li, it is theoretically
stable in aqueous LiOH solution with pH greater than about 8, provided there is no oxygen or
CO2 in the environment.
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Assuming [Li+]=[OH-] or given Lix(Host) with voltage () is in equilibrium, an
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alternative expression to eq (6) is derived as:
() = 2.23 − 289ln<Li0 ?()(6)
If() > 2.23 − 289ln<Li0 ?, no Li will deintercalate from Lix(Host), and it will be stable in
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the alkaline solution. Therefore, Li2Mn2O4 is stable in a 1M aqueous LiOH solution.
Luo and Xia also discussed the thermodynamic requirements regarding electrode
materials stability in the presence of O2, because the aqueous batteries customarily operate in
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atmosphere .[32] The eq (4) was modified, and the following reaction will occur:
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$%(%&'()*+*') + (1/2),- . + (1/4).- ↔ $% 0 + ., 1 (7)
The chemical potential of Lix(Host), (), in equilibrium with O2 and H2O at a particular pH
can be identified with eq (8):
() = 4.268 − 0.0597,()
In their case, [Li+] ≈ 2 M, and [OH-] is adjusted by the amount of LiOH. Based on eq(8) , the
equilibrium voltage is 3.85 V at pH 7, and 3.44 V at pH 14, which exceed the lithium-ion
intercalation potential of the negative electrodes for aqueous LIBs. This means that reduction
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state of all negative electrode materials would be chemical oxidized by O2 and H2O rather
than participating electrochemical redox process. And thus aqueous LIBs can not work
theoretically in the presence of O2. Therefore, eliminating O2 would be one key prerequisite
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for high-performance ARBs design.
3. “Rocking Chair” Type Aqueous Rechargeable Metal Ion Batteries
3.1 Aqueous Rechargeable Lithium Ion Batteries (ARLBs)
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ARLBs work in the same way as that of LIBs with Li+ ions shuttle between cathode
and anode upon charging/discharging. Spinel LiMn2O4//VO2 was the first reported ARLBs by
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using 5 M LiNO3 and 1mM LiOH in water as the electrolyte.[23] This prototype cell operates
at an average voltage of 1.5 V with an energy density of 75 Wh kg-1. Although attractive,
LiMn2O4 suffers from limited specific capacity and several capacity degradation. Since at a
high pH value, oxygen evolution reaction may occur during Li+ extraction;[33] and H+
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insertion reaction and/or Li+/H+ exchange may occur in a low pH electrolyte. Therefore, an
appropriate electrolyte pH value is extremely important for LiMn2O4-based ARLBs. In
addition, enhancing the structural stability of electrodes in aqueous electrolytes is another
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effective approach for optimizing the cyclic performance. Strategies include metal ions
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doping, morphological optimization, and/or introducing additives into electrolytes have been
proposed. For example, Yuan et al. synthesized a series of Al-doped LiAlxMn2-xO4 by solid
sintering at various temperatures.[34] The Al-doped LiAlxMn2-xO4 electrode is found to
deliver a cycle life of more than 4000 cycles at current density of 1000 mA g-1, superior to
the undoped LiMn2O4 samples. The improvement of cycle performance via metal ions doping
were attributed to the depressed Jahn-Teller distortion,[34, 35] which can further stabilize the
octahedral sites. Wu and co-workers highlighted that morphological optimization of LiMn2O4,
especially constructing novel nanostructures such as nanochains,[36] nanotubes[37] or porous
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architectures,[38] could greatly improve the lithium storage properties in aqueous solution.
Specifically, LiMn2O4 nanotubes exhibit excellent rate behaviour with reversible capacities
of 97.3, 80.2 and 59.3 mAh g-1 at high current rates of 120 C, 300 C and 600 C, respectively
(Figs. 3a & 3b).[37] That means that the charge process of nanotubes could be finished within
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several seconds. Fig. 3c demonstrated the LiMn2O4 nanotube//AC full cell achieved excellent
capacity retention with no definite capacity loss even after 1200 cycles. The porous LiMn2O4
synthesized by PS template process also shows extremely high rate capabilities and power
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performance (Figs. 3d, 3e & 3f). A high discharge capacity of 112 mAh g-1 is achieved, even
at a current density of 10000 mA g-1, retaining 95 % of its normal capacity (Fig. 3e).[38]
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Using AC as anode, the LiMn2O4//AC cell was construed and delivers excellent cycling
performance (Fig. 3f). This fascinating high power and long cycle stability of nanostructured
LiMn2O4 in ARLBs was attributed to (i) nanostructures could reduce the Li+ diffusion length
and increase the electrode/electrolyte interface area, (ii) nanograins compensate the strain
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surface energy.[39]
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resulting from Jahn-Teller distortion, and (iii) inhibition of Mn3+ dissolution owing to high
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Figure 3. (a) SEM image of LiMn2O4 nanotubes. Inset is TEM image. (b) charging and discharging curves
measured using Ni as the counter electrode and SCE as the reference electrode, and (c) cycling behavior of
the prepared LiMn2O4 nanotube tested by 2-electrode cells by using active carbon as the counter electrode.
Reprinted with permission from Ref [37]. Copyright 2013 American Chemical Society. (d) SEM image of
porous LiMn2O4. Inset is TEM image. (e) the discharge curves of the porous LiMn2O4 electrode at various
current densities when the current density of charge was fixed at 100 mA g-1. (f) the cycling behaviors of
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the solid and porous LiMn2O4 electrodes measured by using an activated carbon as the counter electrode.
Reprinted with permission from Ref [38]. Copyright 2013 The Royal Society of Chemistry.
In addition to vanadium oxides,[40-45] molybdenum oxides,[46] TiO2,[27, 47-49]
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polyanionic compounds,[50-55] and organic compounds as anode materials[46, 56] are also
explored to couple with LiMn2O4. Wu et al. constructed an LiMn2O4 // LiV3O8 that operates at
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about 1.04 V.[57] However, it exhibits poor cycling performance with only 53.5% capacity
retention after 100 cycles. Coating conductive polypyrrole (PPy) or nanocarbons on the
surface of LiV3O8 is found to be effective in boosting the electrochemical performance in
terms of specific capacity, rate capability and cycling stability.[58] Since these approaches
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could better restrict the dissolution of vanadium ions, accommodate the crystal structure
changes and prevent amorphorization upon cycling. For example, Zhu et al. reported an
ARLB consisting of
nanocomposite of PPy@MoO3 anode (Fig. 4a) and nanochains
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LiMn2O4 cathode.[46] The thickness of PPy layer is estimated to be 8.44nm (Fig. 4a).
Typical CV curves of PPy@MoO3 anode and nanochains LiMn2O4 cathode are shown in Fig.
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4b. The overall reversible redox reactions is given as follow:
$%D&- .E + DF.G ↔ $%H1 D&- .E + $% DF.G
The charge voltage of this ARLB can be extended to 1.95 V without evident side reactions.
The designed PPy@MoO3//LiMn2O4 aqueous system exhibits very good cycling behaviour
with no apparent capacity loss after 150 cycles, high rate capability and energy density (Figs.
4c & 4d). This is because the PPy layer can well prevent the dissolution of Mo ions and
buffer the possible volume changes. In addition the unique nanochains structure of LiMn2O4
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and the conductive nature of PPy also contribute to the enhanced electrochemical
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performance.
Figure 4. (a) TEM image of the nanocomposite of MoO3 coated with PPy. (b) CV curves of the
nanocomposite of MoO3 coated with PPy and the nanochains LiMn2O4 at scan rate of 1 mV s-1. (c) the
cycling behavior at 1000 mA g-1 based on LiMn2O4, and (c) Ragone plots of the ARLBs system using 0.5
of Chemistry.
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Except for LiMn2O4–based ARLB systems, layered lithium metal oxides, which are
commonly used in conventional LIBs, such as LiCoO2 and LiNi1/3Co1/3Mn1/3O2 (NCM),[5965] cooperated with appropriate anodes were also employed to develop ARLBs with high
performances. However, for this type of materials, H+ insertion occurs preferentially over Li+
insertion with LiOH electrolyte;[60, 62] while in LiNO3 and Li2SO4 electrolytes, reversible
Li+ insertion and extraction into layered structures is possible.[66, 67] Ruffo et al. reported
that the redox potential of LiCoO2 electrode increased linearly with the electrolyte
concentration, indicating of reversible Li+ ion insertion rather than H+ insertion.[60]
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Currently, the LiCoO2 electrode cycled with relative low polarization within voltage window
of 0.55-1.15 V (vs. SHE) can deliver stable capacity retention with value of 100~120 mAh g-1,
corresponding to 0.42 Li+ in attend the electrochemical reaction. Although enlarging the
voltage window up to 1.4 V or using nano-LiCoO2 as cathode material could further promote
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the specific capacity, however, the capacity fading is serious owing to water electrolysis.[60,
61] Toki et al. first reported an ARLB based on the layered LiNi0.81Co0.19O2 cathode and
LiV3O8 anode with an average voltage of 1.5 V and energy density of 30-60 Wh kg-1 could be
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achieved,[68] which is comparable with the Pb-acid battery. However, this ARLB cell
displayed very poor cyclic stability with only 25% retention after 100 cycles. The authors
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believed that the poor stability of both anode and cathode materials leading to the finally
serious capacity fading. Chen et al. further developed a surface coated anode LixV2O5, which
can be used to improve the cyclic stability of ARLBs by employing NCM as cathode.[69] It
can be ascribe to the effective reducing the contact of active materials and electrolyte,
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stabilising the phase transition and inhibiting the dissolution of vanadium ions.
Olivine polyanionic compound was another good choice for the development of high
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performance ARLBs. Manickam et al. first introduced LiFePO4 as cathode for ARLBs using
a three-electrode setup.[70] The capacity comparable with its theoretical value was lower
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than 40%. Though forming the same product during charge, however, FePO4 cannot fully
reversible since producing LiFePO4 and Fe3O4 mixture in the discharge process, leading to
inferior capacity retention.[70] Xia et al. found that O2 and OH- could greatly deteriorate the
cycle stability of LiFePO4 in 0.5 M Li2SO4 aqueous electrolyte.[35] Detail investigation by
TEM images showed that a rough surface was observed in the presence of O2; while, no
impurities were detected in the absence of O2. Luo et al. found that the negative electrodes of
ARLBs in a discharge state can react with water and oxygen, finally resulting in capacity
fading during cycles.[71] As shown in Fig.5a, the coulombic efficiency of LiTi2(PO4)3 in
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aqueous electrolyte was 99% in the absence of O2, which was higher than that in the presence
of O2 (92%). Therefore, by eliminating O2, adjusting pH value of the electrolyte and using
carbon coated electrode materials, an ARLBs based on LiFePO4/Li2SO4/LiTi2(PO4)3 could
exhibit excellent stability with capacity retention over 90% after 1000 cycles, almost the best
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ARLB systems to date (Fig. 5b).[71] Beyond optimization of electrolyte, metal ion doping is
another effective technique to improving the electrochemical performances of ARLBs. For
example, the ARLB constructed by Mn/Ni co-doped LiFePO4 such as LiMn0.05Ni0.05Fe0.9PO4
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cathode paired with LiTi2(PO4)3 anode, can deliver approximately 87 mAh g-1 of initial
capacity with potential plateau centred at 0.9 V at 0.2 mA cm-2; after 50 cycles, ~55 mAh g-1
xPO4,[53]
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of capacity was retained.[72] Other doped polyanionic compounds such as LiFexMn1LiNiPO4,[73, 74] LiNi0.5Co0.5PO4,[73, 75] etc. could also be employed as cathode
pairing with anode for ARLBs. However, for these type materials, both optimization of
electrolyte solution, stabilization the structures, inhibition of ions dissolution, and
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improvement of interface could further enhance the electrochemical performances of ARLBs.
Figure 5. (a) Typical charge/discharge curves of LiTi2(PO4)3 at 4C rate in the presence/absence of O2. (b)
Capacity versus cycle number for cells cycled at different conditions. Line 1: in the presence of O2, pH 13;
line 2: absence of O2, pH 13 at low rate of C/8; line 3: absence of O2, pH 7; line 4: absence of O2, pH 13 at
6C rate. The battery showed better stability in the absence of O2 with a capacity retention over 90% after
1,000 cycles under 6C conditions, and of 85% after 50 cycles even at a very low current rate of C/8 under
extreme cycling conditions without relaxation between cycles. Reprinted with permission from Ref [71].
Copyright 2010 Macmillan Publishers Ltd.: Nature Chemistry.
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Prussian blue analogues have archetypal hexacyanometalate framework structures and
can be described using the general formula AxPR(CN)6, in which nitrogen coordinated
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transition metal cations (P) and hexacyanometalate complexes (R(CN)6) form a face-centered
cubic framework with large interstitial A sites.[76-79] The ionic occupancy in A sites varies
between 0 and 2 according to valence changes in either or both of P and R species.[80] These
materials were capable of intercalating a variety of ions, such as Li+, Na+, and K+ in aqueous
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solutions.[81] For example, the specific of copper hexacyanoferrate (CuHCF) and nickel
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hexacyanoferrate (NiHCF) synthesized by coprecipitation method were ~58 mAh g-1 at 0.83
C (1 C=60 mA g-1), even at current rates as high as 41.7 C, more than 60% of the capacity
obtained at 0.83 C was retained. [81, 82] Though superior electrochemical performance could
be delivered, while the full ARLB system based on AxPR(CN)6 cathode are still under
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development in recent years, which should pay more attention.
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Figure 6. (a) Schematic diagram of Li-MnO2 hybrid electrolyte system. (b) Cycle life of Li-MnO2 hybrid
electrolyte battery at a rate of 50 mA g−1 using the 2-electrode system between 3.1 and 3.7 V. The inset
shows the charge and discharge curves. Reprinted with permission from Ref [83]. Copyright 2013
Elsevier. (c, d) Electrochemical performance of the designed ARLB based on LiMn2O4 as cathode,
metallic Li as anode, and GPE/LISICON as protective electrolyte, characterized at current density of 100
mA g−1 between 3.7 and 4.25 V. Reprinted with permission from Ref [84]. Copyright 2013 Macmillan
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Publishers Ltd.: Scientific Reports.
Although ARLBs demonstrating series of advantages such as safety, high power
density, low cost, and environmental benignity, while, the major obstacle for using as power
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sources is the relatively low energy density restricting by intrinsically low voltage window of
water electrolysis.[80, 85] Thus, how to broaden the operating voltage larger than 1.23 V is
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extremely important to further improve the energy density of ARLBs. The use of metallic
lithium as anodes in aqueous systems had been proved to be an effective strategy. Chou et al.
first reported Li//MnO2 ARLBs by using hybrid electrolyte (organic/solid/aqueous electrolyte
system) (Fig. 6a).[83] The lithium anode was separated from the aqueous solution by a solid
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electrolyte, and organic electrolyte was used to improve the interfacial contact between Li
and solid electrolyte. As a result, the designed ARLBs could deliver a specific capacity of 50
mAh g-1 and an average voltage of 3.4 V (vs. Li+/Li). The cycle life could further extend to
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more than 2000 cycles at a 1 C rate with 85 % retention (Fig. 6b). However, a large IR drop
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was observed at high rate, which can be ascribed to the relative low lithium diffusion
coefficient of the used solid electrolyte. Wang et al. employed gel polymer electrolyte (GPE)
and a LISICON film to protect lithium metal anode for ARLBs.[84] After paired with 0.5 M
Li2SO4 aqueous electrolyte and LiMn2O4 cathode, the constructed cell showed two anodic
peaks at 4.04/4.18 V (vs. Li+/Li), and cathodic peaks at 3.94/4.07 V (vs. Li+/Li) at current
density of 100 mA g-1, similar with the voltage peaks in conventional electrolytes (Figs. 6c &
6d). The reversible capacity was about 115 mAh g-1, and retained over 30 cycles.[84]
Specifically, the energy density of this hybrid cell was 446 Wh kg-1 based on the total mass of
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electrode materials, which was 10-fold higher than that of the previous ARLBs. Though great
progress, however, there are still several points need to call for further solution: (i)
development of solid electrolyte with ultra-high ions conductivity; (ii) reduction of the
interfacial resistance of metal anodes and solid electrolytes; (iii) efficient packaging
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techniques. The above points could also be suitable for the development of sodium or
potassium batteries based on the hybrid cell structure, which will not be discussed in below.
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3.2 Aqueous Rechargeable Sodium and Potassium Ion Batteries.
Aqueous rechargeable sodium (ARSBs) and potassium ion batteries (ARKBs) are now
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actively developed as next generation of electric energy storage technology due to their
advantages of resources abundance, low cost and environmental compatibility, especially for
large scale stationary energy storage. Similar to ARLBs, the redox potentials should be
located in the voltage range of O2 and H2 evolution to avoid water electrolysis.[86-88] Also,
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the active materials need to be chemically stable at the operating pH of the aqueous
electrolyte; metal ions dissolution and side reactions should be inhibited for long-term battery
system.[85, 87] Considered from above concerns, various materials systems could be paired
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as cathode or anode both for ARSBs and ARKBs.
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Among various sodium insertion/extraction compounds, orthorhombic Na0.44MnO2
based ARSBs were the most widely investigated.[89-94] The sodium tunnels contain rich of
vacancies along c axis which enable rapid Na diffusion within the structures. Sauvage et al.
first introduced Na0.44MnO2 into ARSBs, and identified that the NaxMnO2 cathode works via
a multiphase reaction within an x range of 0.18< x<0.65 with at least six intermediate phases.
This was further proved by first principle calculations, PITT and in-situ XRD analyses.[90,
91, 93] They found that the insertion process within the NaxMnO2 system is fully reversible
over the 0.25< x<0.65 composition range and presents some degree of irreversibility as
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values of x below 0.25 are reached.[91] Li et al. recently proposed a Na0.44MnO2 based ARSB
full cell combined with NaTi2(PO4)3 as anode.[89] The capacity of the cell could reach 40
mAh g-1, an energy density of ~33 Wh kg-1 at 0.6 C, and retained 60 % of the initial capacity
after 700 cycles, with rate performance comparable to those of supercapacitor. Whitacre et al.
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constructed a Na0.44MnO2//NaTi2(PO4)3 based ARSBs and studied the effect of electrolyte
concentration on the electrochemical performance.[95] They found that the capacity could
increase 38 % by increasing the salt concentration at 1.5 C, as well as the oxygen-related self-
rate
performance.
Zhang
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discharge phenomenon was diminished, thus leading to ARSBs with superior stability and
and
co-workers
recently
developed
a
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Na0.44MnO2//NaTi2(PO4)3@CNT based ARSBs which could deliver an average voltage of 1.1
V, a high energy density of 58.7 Wh kg-1 and relative good cyclic stability at 2 C rate with
coulombic efficiency as high as 95 %.[96] The optimized electrochemical performance of
this ARSB was mainly owing to the enhanced electronic conductivity by CNT doping and the
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designed nanostructures.
Polyanionic compounds, such as Na2FeP2O7 and NASICON-type Na3V2(PO4)3 or
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NaVPO4V,[72, 97-101] are another series of promising materials for using as low-cost
cathode for ARSBs. Jung et al. observed that Na2FeP2O7 as cathode for ARSBs delivered
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improved rate and cycle properties in 1 M Na2SO4 aqueous electrolyte solution compared to
that of nonaqueous system, owing to the faster kinetics nature in aqueous devices.[98] This
material could achieve near theoretical capacities at current rate of 1 and 5 C in voltage of 0.654 to 0.576 V vs SCE with relative good charge/discharge stabilities more than 300 cycles.
Yu
and
co-workers
demonstrated
a
carbon-coated
NASICON-type
Na3V2(PO4)3
nanocomposite as a cathode material for ARSBs.[102] Owing to carbon coating layer could
improve the structure stability and enhance the electronic conductivity, it delivered a
discharge capacity of 94.5, 90.5 and 71.7 mAh g-1 at current rate of 10, 15 and 20 C,
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respectively; which was the best rate performance for ARSBs. For ARSBs system, Okada et
al. reported Na2FeP2O7//NaTi2(PO4)3 full cell and systematically investigated the electrolyte
dependence of its electrochemical performances.[97] They found that the energy storage
properties in 2 M Na2SO4 or 4 M NaClO4 aqueous electrolyte were better than with the
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organic electrolyte, similar to that of material in half cell setup. Very recent, Li and coworkers constructed an all NASICON ARSB based on NaTi2(PO4)3 served as anode,
Na3V2(PO4)3 served as cathode, and aqueous Na2SO4 solution as electrolyte.[100] The
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designed full cell could deliver an average voltage of 1.2 V and well maintain its performance
at high rates. This cell configuration enabled a discharge capacity of 58 mAh g-1 even at 10 A
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g-1; a high energy density of 29 Wh kg-1 at a power density of 5145 W kg-1 was achieved.
Though great progress, while, the cyclic performance is still needed to call for more
attentions on polyanionic compounds based ARSBs, such as electrolyte optimization, surface
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coating, or ions doping, etc.
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Figure 7. (a) NiHCF has the Prussian Blue crystal structure in which transition metal cations such as Fe
and Ni are bound by bridging CN ligands, forming a face-centered cubic structure. In the case of NiHCF,
Fe is 6-fold carbon coordinated, while Ni is 6-fold nitrogen coordinated. (b) Typical SEM image of the assynthesized NiHCF powder with size of 20-50 nm. (c, d) The potential profiles of NiHCF during
galvanostatic cycling of Na+ and K+ at various rates are shown. (e) The capacity of NiHCF during
galvanostatic cycling of Na+ and K+ at various rates is shown. (f) NiHCF shows no capacity loss after 5000
cycles of Na+ insertion at a 8.3C rate. However, during K+ cycling, NiHCF is stable for only about 1000
cycles, after which its capacity decays at an approximate rate of 1.75%/1000 cycles. Reprinted with
permission from Ref [77]. Copyright 2011 American Chemical Society.
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Prussian blue analogues are other promising candidates for using both in ARSBs and
ARKBs due to their open framework crystal structures.[76-79, 82, 87, 88, 103] It can
accommodate various large cations such as Na+ and K+ with almost no lattice distortion.[77]
Thus, ARBs based on this type of electrode can always exhibit long-term stability even more
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than thousands of continuous cycles. The typical redox reaction was confirmed as follow
(using ARSBs as examples, redox potential is about ~0.3 V vs. SCE) :[77, 80]
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I%J KKK (L)M + *0 + 1 ↔ I- %J KK (L)M (I = *F(N)
As an example, the theoretical specific capacity of NiCHF is approximately ~85 mAh g-1 for
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using in ARSBs. Figs 7a-7e shows that the NiCHF nanostructured electrode in 1 M NaNO3
aqueous electrolyte delivered ~60 mAh g-1 at 0.83 C with a redox potential of 0.59 V vs. SHE;
while operating in 1 M KNO3 aqueous electrolyte, a specific capacity of 59 mAh g-1 was
delivered at 0.83 C with a redox potential of 0.69 V vs. SHE. At a high current rate of 8.3 C,
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it can achieve a cycle life of over 5000 cycles, and the lattice strain was only 0.18% during
charge/discharge cycles.[77] However, during K+ cycling, NiHCF is stable for only about
1000 cycles, after which its capacity decays at an approximate rate of 1.75%/1000 cycles
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(Fig.7f). Wessells et al. also reported CuNiHCF synthesized by solid-solution method, which
could also stable cycled more than 2000 cycles with good rate performance, further proved
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that Prussian blue analogues are great promising materials system for ARSBs or ARKBs.[104]
However, Prussian blue analogues must pair with Na or K containing anodes for full cell
assembly. Yang et al. employed NaTi2(PO4)3 as anode, Na2NiCHF as cathode, Na2SO4
aqueous solution as electrolyte, and constructed a full cell to study the electrochemical
performances.[105, 106] This ARSB delivers an average voltage of 1.27 V, an energy density
of 42.5 Wh kg-1, with capacity retention as high as 90% at 10 C rate.[105] Using Na2CuCHF
as cathode, the voltage of the full cell can broaden even to 1.4 V and achieved a rate
capability up to 100 C with long cycle life.[106] For using in ARKBs, Honda and Hayashi
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designed a symmetrical cell where Prussian blue analogues served as both anode and
cathode.[107] Two redox couples via disproportionation reaction resulted in a cell voltage of
0.68 V. In order to further increasing the operational potential, asymmetrical ARKBs,
combined with a more negative anode and more positive cathode were selected from series of
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transition metal HFC; for example, a 1.5 V ARKBs could be achieved by using a
KCrIIICrII(CN)6 anode and fully discharged KCrIIICrII(CN)6 as cathode.[108] Cui et al.
demonstrated in recent that the operation voltage can be adjusted by the ratio of transition
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metal from 0.7 to 0.95 V, such as K0.71Cu[Fe(CN)6]0.72·3.7H2O with theoretical capacity of
62 mAh g-1.[82] These electrodes show extremely high rate performance and excellent cycle
cycles at 8.3 C were achieved.
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performance, a capacity of 40 mAh g-1 at 83 C and capacity retention above 80% after 40000
Similar to ARLBs, the development of ARSBs and ARKBs also suffer from serious
challenges, such as narrow potential windows, poor cyclic stabilities and low energy densities,
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though great progress achieved in recent years. Further efforts are greatly required to restrict
the electrolyte decomposition, side reactions between active materials and electrolyte,
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materials dissolution, as well as proton co-insertion. An alternative strategy is to employ
metallic metal as anode and organic/solid/aqueous hybrid electrolyte for cell configuration
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application, which may greatly enhance the operating voltage windows and energy density.
Apart from the above points, further searching for suitable and efficient cathodes, anodes or
electrolytes for packaging applications, for example, a fascinating anode materials paired
with Prussian blue analogues cathodes, are always highly demanded.
3.3 Aqueous Rechargeable Multivalent Metal Ion Batteries.
Rechargeable batteries based on multivalent metal ions insertion/extraction in aqueous
solution, such as Mg2+, Ca2+, Zn2+, and Al3+, are considered to be one of the most promising
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ARB systems due to potential 2-3 fold high energy density than monovalent ARBs. Studies
had verified that water molecules can effectively shield the electrostatic repulsion of
multivalent ions and lower the activation energy for charge transfer at electrode/electrolyte
interface compared to organic solution.[109, 110] Thus, the multivalent ARBs could deliver
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better electrochemical properties than organic ones in most of the case.
Manganese oxide was considered to be a promising host material to accommodate these
multivalent guest ions.[111, 112] For example, Zn2+ ion insertion into α-MnO2 synthesized
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by microemulsion can deliver a capacity of ~210 mAh g-1 at a current density of 0.2 A g-1
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with ~100% coulombic efficiency.[112] Cheng et al. reported that the tunnel structured MnO2
polymorphs (α, β, γ) undergo a phase transition to layered zinc-buserite on the first discharge
by employing an aqueous mild-acidic zinc triflate electrolyte (Zn(CF3SO3)2), thus allowing
subsequent intercalation of zinc cations in the latter structure.[113] Based on this
electrochemical mechanism, the constructed aqueous Zn-MnO2 in triflate electrolyte enables
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the formation of a protective porous manganese oxide layer. And hence the cathode exhibits a
high reversible capacity of 225 mAh g−1 and long-term cyclability with 94% capacity
retention over 2000 cycles. Remarkably, the pouch zinc-manganese dioxide battery delivers a
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total energy density of 75.2 Wh kg−1. A nonstoichiometric ZnMn2O4/carbon composite was
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also reported as a new Zn-insertion cathode material in aqueous Zn(CF3SO3)2 electrolyte. It
enables ~100% Zn plating/striping efficiency with long-term stability and suppresses Mn
dissolution, much better than that of ZnSO4 electrolyte (as shown in Figs. 8a & 8b).[114] The
ions storage examination exhibits a reversible capacity of 150 mAh g-1 and capacity retention
of 94% over 500 cycles at a high rate of 500 mA g-1(Figs. 8c & 8d). The improved
performances could be attributed to the small particle size, abundant cation vacancies, and
also carbon component in the composites (Fig. 8).[114] Apart from these Mn-based materials,
vanadium oxide bronze pillared by interlayer Zn2+ ions and water molecules
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(Zn0.25V2O5·nH2O) synthesized by Nazar group could also be employed as the positive
electrode materials for Zn-ions cell.[115] They demonstrated that a reversible Zn2+ ions
(de)intercalation storage process at fast rates with more than one Zn2+ per formula unit (up to
300 mAh g−1) could be achieved. The Zn-ions cell offers an energy density of ~450Wh l−1
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and exhibits capacity retention of more than 80% over 1,000 cycles, with no dendrite
formation at the Zn electrode. The fascinating properties were attributed to the interlayer
water molecules which can reversibly expand and contract the layered galleries of Zn0.25V2O5
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to allow Zn2+ ingress/egress, leading to good kinetics and high rate performance; the
indigenous Zn ions stabilize the layered structure, thereby ensuring long-term cycling
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stability.
Figure 8. (a) Galvanostatic cycling of Zn/Zn symmetrical cells at 0.1mA cm−2 in 3 M ZnSO4 and 3 M
Zn(CF3SO3)2 electrolytes. Insets enlarge the voltage profiles of the first and 25th cycles. (b) Rate
performance and long-term cycleability of Zn/Zn symmetrical cells in 3 M ZnSO4 and 3 M Zn(CF3SO3)2
electrolytes. (c) CVs of ZMO/C electrode scanning at 0.2 mV s−1 in 3 M Zn(CF3SO3)2 electrolyte using
Zn-ZMO/C coin cells. (d) Cycling performance of ZMO/C at 500 mA g−1. Reprinted with permission from
Ref [114]. Copyright 2016 American Chemical Society.
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MgMn2O4 converted from LiMn2O4 could be used as host material of Mg2+ ions and
showed reversible insertion/extraction cycles in 1 M Mg(NO3)2 aqueous electrolyte.[116]
However, the specific capacity of this MgMn2O4 is only in the range of 35 ~ 42 mAh g-1 at a
1 C rate. Yuan et al. recently reported that γ-MnO2 synthesized by spinel LiMn2O4 acid
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leaching can deliver better Mg2+ ions storage properties.[111] The initial discharge capacities
of the γ-MnO2 electrode in 1 M Mg(NO3)2 and MgCl2 aqueous electrolytes were as high as
473 and 478.4 mAh g-1 at a current density of 13.6 mA g-1, respectively; and this cell can
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maintain a specific capacity of 155.6 mAh g-1 even after 300 cycles with Columbic efficiency
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of ~100%.
Prussian blue analogues were widely explored as host materials for multivalent guest
ions owing to the open-framework structure. Cui et al. reported that nanosized NiHCF
allowed for the reversible insertion of aqueous alkaline earth divalent ions,[76] including
Mg2+, Ca2+, Sr2+, and Ba2+ with unprecedented long cycle life more than 2000 cycles and high
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rate performance. They found that the crystal increases at full discharge state in strain by 1.3,
1.1, and 0.9 % for K+, Mg2+, and Ba2+, respectively. This lattice increase is primarily due to
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the increase in diameter of Fe(CN)64- upon oxidation to Fe(CN)63-, which can also explain
why there is little difference in strain with cycles during different ions insertions.[76] The
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open A sites in the structure are large enough to contain a variety of ions without
distortion.[80] They proposed that the strain upon full insertion of ions is very low (~1%),
resulting in mechanical stability during charge/discharge cycles.
Besides, vanadium pentoxides could also be employed as host materials for Mg2+
storage.[117, 118] The electrochemical performance in a Mg(NO3)2 aqueous solution was
characterized with a Pt as counter electrode.[117] The redox peaks are around -0.25/0.63 and
-0.25/-0.1 V (vs SCE), with a capacity of 107 mAh g-1 delivered. Nanostructured anatase
TiO2 could also be used as cathode materials to store Al3+ ions in 1 M AlCl3 aqueous
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electrolyte, with a discharge capacity of 75 mAh g-1 delivered.[109] Another work further
demonstrated that black mesoporous TiO2 could greatly enhance the electrochemical
activities with Al3+ insertion.[119] The specific capacity was as high as 278.1 mAh g-1,
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corresponding to Al0.27TiO2 at 50 mA g-1.
For ARB based on multivalence metal ions insertion/extraction, though great progress
achieved, however, the mechanism of these multivalent inserted into or extracted from these
host materials should be disclosed in further studies. Another points should be highlighted is
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that most of these host materials do not contain the multivalent guest ions, which will
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obstacle the full cell applications. At last, appropriate negative materials should be
successfully developed for multivalent ions insertion/extraction reversibly.
Table 1. Recent progresses of various “Rocking Chair” type ARBs.
Cell type
Electrolyte
Working
Potential (V)
Capacity
retention (%)
Initial capacity
(mAh g-1)
Ref.
110
[36]
110
[37]
~115
[38]
~88
[46]
90
[46]
N.A.
[71]
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Aqueous Rechargeable Lithium Ion Batteries
0.5 M Li2SO4
0~1.8
0.5 M Li2SO4
0~1.8
Porous LiMn2O4//AC
0.5 M Li2SO4
0~1.8
0.5 M Li2SO4
0~1.95
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LiMn2O4
nanochains//AC
LiMn2O4 nanotubes//AC
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LiMn2O4
nanochains//PPy@MoO3
LiFePO4@C//LiV3O8
9 M LiNO3
0~0.8
0~1.4
MnO2//Li
0.5 M Li2SO4 + 0.1
M LiOH, absence O2
1.0 M Li2SO4
3.1~3.7
LiMn2O4//Li
0.5 M Li2SO4
3.7~4.25
LiCoO2//Li
0.5 M Li2SO4
3.5~4.3
LiFePO4//LiTi2(PO4)3
~100% (200)
at 4.5C
~112% (1200)
at 4.5C a
93% (10000)
at 9C
~90% (150) at
4.5C
~91.8% (100)
at 10C
90% (1000) at
6C
85% (2400) at
50 mA g-1
~100% (30) at
100 mA g-1
N.A.
170 Wh kg-1
[83]
115 mAh g-1
or 446 Wh kg-1
~465 Wh kg-1
[84]
>1000 cycles
~127 Wh L-1
[89]
[46]
Aqueous Rechargeable Sodium and Potassium Ion Batteries
Na0.44MnO2//NaTi2(PO4)
1.0 M Na2SO4
0.1~1.4
3
at 7C
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0.2~1.4
1.0 M Na2SO4
or 4.0 M NaClO4
1.0 M Na2SO4
0.4~1.8
1.0 M Na2SO4
0.5~1.6
1.0 M Na2SO4
0.2~1.6
1.0 M Na2SO4
0.2~1.8
0~1.4
Aqueous Rechargeable Multivalent Metal Ion Batteries
β-MnO2//Zn
Zn0.25V2O5·nH2O//Zn
1.0 M ZnSO4
α-MnO2//Zn
1.0 M ZnSO4
todorokite MnO2//Zn
1.0 M ZnSO4
0.8~1.9
0.5~1.4
1.0~1.9
0.7~2.0
128
[96]
~45
~45
56.5
[97]
[46]
58
[100]
89.7 mAh g-1
or 42.5 Wh kg-1
85
[105]
150
[113]
85
[114]
260
[115]
~100
[120]
98
[121]
94 % (2000) at
6.5C
94% (500) at
500 mA g-1
80% (1000) at
8C
~100% (100)
at 6C
stable up to 50
cycles
[106]
Specific value are estimated from the electrochemical curves.
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a
0.8~1.9
98% (100) at
1C
50% (50) at 10
A g-1
88% (250) at
5C
88% (1000) at
10C
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ZnMn2O4@C//Zn
3.0 M Zn(CF3SO3)2 +
0.1 M Mn(CF3SO3)2
3.0 M Zn(CF3SO3)2
37% (300) at
2C
2.0 mA cm-1
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Na3MnTi(PO4)3//
Na3MnTi(PO4)3
Na3V2(PO4)3//NaTi2(PO4
)3
Na2NiFe(CN)6//NaTi2(P
O 4) 3
Na2CuFe(CN)6//
NaTi2(PO4)3
1.0 M Na2SO4
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Na0.44MnO2//NaTi2(PO4)
3@MWCNTs
Na2FeP2O7//NaTi2(PO4)3
“Hybrid Chemistry” Type Aqueous Rechargeable Hybrid Batteries
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4.1 Intercalation Cathode-Zn Metal Anode Battery
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The first aqueous rechargeable hybrid battery (ARHBs) using LiMn2O4 cathode and Zn
metal anode was proposed by Chen in 2012 (Fig. 9a).[24] Metallic Zn is a promising anode
candidate for aqueous batteries because of i) its low equilibrium potential (-0.762 V vs. SHE),
ii) high specific energy density (825 mAh g-1), and iii) abundance and low toxicity.[122]
Different from the “rocking chair” type batteries, exchange of Li+ and Zn2+ ions in mild
acidic aqueous electrolyte occurs upon charging/discharging (Fig. 9a). The electrolyte here
acts as conducting ions and cooperates with the electrodes to store energy, rather than as the
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simple supporting media in “rocking chair” type batteries. The electrochemical reaction
between the LiMn2O4 cathode and Zn metal anode can be expressed as follows:
O&-0 + 2 ↔ O&
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$%D&- .E ↔ $%H1 D&- .E + $% 0 + 1
In which two reversible redox chemistry types, Li+ ion de-intercalation/intercalation into
spinal LiMn2O4 and deposition/dissolution of Zn, occur at about 1.76 V-1.9 V (vs. Zn2+/Zn)
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in cathode and 0 V (vs. Zn2+/Zn) in anode parts, respectively (Fig. 9b). XRD analysis reveals
these reversible reactions (Figs. 9c & 9d). Specifically, the characteristic peaks of the spinel
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LiMn2O4 shift toward higher 2θ values upon charging, indicating the extraction of Li+ ions
and formation of Mn2O4. This process is reverse upon discharging. The reversible Zn2+
dissolution and deposition are also identified (Fig. 9d). The LiMn2O4//Zn cell operates at
about 2 V and delivers acceptable energy density (50 - 80 Wh kg-1) and good cycling
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feasibility.
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performance (95% of capacity retention after 4000 cycles)[24] , demonstrating great practical
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Figure 9. (a) Schematic illustration of the working mechanism of LiMn2O4-Zn Hybrid Battery. (b) Cyclic
Voltammerty curves of metallic Zn anode, LiMn2O4 cathode, and the electrochemical stability window of
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electrolyte. Structural evolution of electrodes upon cycling. (c) in-situ XRD patterns of LiMn2O4 cathode at
different charge/discharge states during the first cycles. (d) ex-situ XRD patterns of stainless steel node
current collector before and after charge. Reprinted with permission from Ref.[24] Copyright 2012
Elsevier B.V. (e) Comparative CV curves of LiMn2O4-Zn Hybrid Battery in electrolytes with and without
0.14 wt% thiuourea. Reprinted with permission from Ref.[123] Copyright 2015 Elsevier B.V.(f)
Comparative galvanostatic charge/discharge profiles of LiMn2O4-Zn cell in different electrolytes.
Reprinted with permission from Ref. [124] Copyright 2016 Elsevier B.V.
Further improvements in electrochemical performance were achieved via modifying the
electrolyte composition.[123-125] The electrochemical measurements were performed in a
three-electrode system using 2M ZnSO4+1M Li2SO4 electrolytes with and/or without
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thiourea (0.14 wt%).[123] Two distinct redox peaks with a much narrower peak separation
were clearly resolved in battery with thiourea (TU) in contrast to that of bare one (Fig. 9e),
indicative of a smaller polarization and faster response. Moreover, the cell with thiourea
exhibits much larger reduction peaks in area, suggesting that the TU additive could improve
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the Coulombic efficiency.[123] All these, together, contribute to the enhanced
electrochemical performance. Similar to other aqueous battery systems, these hybrid batteries
suffer from rapidly capacity degradation resulting from the change in concentration and pH
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of the electrolyte during long period cycling. Lu and co-workers demonstrated that the
introduce of hydrophilic silica nanoparticels into electrolyte can help maintain the chemical
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state of aqueous electrolytes, and thus boost the electrochemical performance in terms of
specific capacity, rate capability and cycling stability.[124] They prepared a silica containing
gel electrolyte by mixing 2M ZnSO4+1M Li2SO4 with as-received silica nanoparticles and
conducted various electrochemical tests to illustrate the role of silica nanoparticles. Fig. 9f
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shows the charge/discharge profiles of the LiMn2O4//Zn battery in electrolytes without SiO2
(green line), 5% SiO2 (black line) and 10% SiO2 (red line), respectively. The cells operating
in SiO2-doped electrolytes deliver higher specific capacities and smaller polarization than
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those of the bare cell. The highest specific capacity of 137.5 mAh g-1 is achieved for the cell
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with 5% SiO2-doping, approaching to the theoretical value of LiMn2O4 (148 mAh g-1). The
enhanced wettability between LiMn2O4 cathode and the SiO2-doped electrolyte and the
accumulation of Li+ ions near the LiMn2O4 cathode resulting from the Coulombic interaction
between hydrophilic SiO2 and Li+ ions, contribute to the good use of LiMn2O4.[124, 126]
Furthermore, improvements in rate capability were identified in all C-rates, benefiting from
the SiO2-doping in electrolytes that provide enhanced diffusion pathways for guest ions, and
thus reduce the concentration polarization upon charging/discharging.[124] Moreover, the
SiO2-doped electrolytes ensure much more stable open circuit voltages (OCVs) and much
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lower float charge current densities. They suggested that the SiO2 dopant would i) protect the
surface of the Zn metal anode and suppress dendrite formation, and ii) mitigate the
dissolution of manganese ions in LiMn2O4, and thus reduce the float charging and selfdischarging.[124] Although attractive, the SiO2-doped gel electrolytes is not practical from
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technical perspective because of i) the short gelling time (order of seconds) when introducing
the electrolyte into the absorbed glass mat (AGM) separator with large-size, and ii) the high
silica loading (5 wt%-10 wt%) that would increase the cost and reduce the ion concentration.
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Hoang and co-workers proposed a new hybrid gelling system, the mixture of β–cyclodextrin
(CD) and fumed silica (FS), to address these challenges.[125] Both CD and FS possess
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multiple hydroxyl groups on the surface (Fig. 10a), which initiate gelation with aqueous
electrolytes through the Coulombic interaction and hydrogen boding formation with the water
component of the electrolyte. These features, together, contribute to the enhanced water
retention capability in the gel electrolytes. The gel electrolytes (with only 3% - 5% of fumed
, while water in the AGM piece containing aqueous
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silica) can hold water up to 100
electrolyte evaporates almost completely below 70
(Fig. 10b). In addition, the gelling time
extends from 4h for gel (50, 5% FS) to 17h for gel (41, 4%FS+1%CD), and to 33h for gel (32,
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3% FS+2%CD), making the introduce of gel electrolyte into the industry-scaled application
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be possible. Moreover, there is nearly no sacrifice in ionic conductivity upon changing the
CD concentration from 0 to 2 wt%. Benefiting from these, the mixed gel electrolyte-based
batteries deliver about 10% higher specific capacity than those of (50) and (00) cells at the 4C
rate (Fig. 10c). The mixed gel electrolytes is also found to suppress the Zn dendrite formation,
giving rise to the improved cycling stability and Coulombic efficiency (Fig. 10d).[125]
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Figure 10. (a) Structure of β–Cyclodextrin (CD) and a typical surface of amorphous silica (FS). (b) TGA
profiles of pristine (00), the 5% FS (50), the 4% FS+1% CD (41),and the 3% FS+3% CD (32) electrolytes
loaded in the absorbed glass mat (AGM) separator. (C) Corresponding cycling stability at 4C rate. (D)
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Corresponding SEM images of Zn metal anode after 1000 charge/discharge cycles. Reprinted with
permission from Ref. [125]. Copyright 2017 American Chemical Society.
Considerable efforts have also been devoted to suppress side reactions including corrosion
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and dendrite formation in Zn metal anode.[123, 127, 128] Wu and Chen et al found that the
surface state of Zn metal anode influences the Zn deposition-dessolution efficiency and the
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overall Couboumbic efficiency. The capacity retention of polished Zn is much higher than
that of unpolished Zn, probably due to the removal of holes and/or strips on the surface.[123]
Porous Zn metal anode is advantages over planar Zn because it possesses higher surface area
and is accessible to electrolyte easily.[129, 130] However, the higher surface area also leads
to the passivation, and thus degrades the battery performance.[131] Tao et al. demonstrated
that the adding of carbon additives into porous Zn anode could optimize its anodic
behaviour.[127] Three different kinds of carbon additives, acetylene black (AC), carbon
nanotube and active carbon, were used to prepare porous Zn anode composites for
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Zn/ZnSO4+Li2SO4/LiMn2O4 aqueous hybrid batteries. Experimental results show that, in all
cases the carbon additives can improve the discharge capacity (Fig. 11a) as well as the
cycling stability of the Zn anode (Fig. 11b). [127] Specifically, the porous Zn/AC composite
anode displays an initial discharge specific capacity of 150 mAh g-1, which is higher than that
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of pure Zn anode (114 mAh g-1). Moreover, a specific capacity of 60 mAh g-1 is achieved for
Zn/AC composite anode even after 210 cycles, while the capacity of pure Zn anode decreases
to 59 mAh g-1 after 36 cycles. This is attributed to the carbon coating of the Zn particle
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surface that prevents the direct contact of the Zn anode with electrolyte, and thus the
corrosion of the active Zn particle was restrained. [127] In addition, the pores of activated
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carbon can accommodate the deposition of Zn dendrites and insoluble anodic products,
giving an increase in cycling stability[130]. Organic additives were also employed to
suppress the dendrite formation and corrosion of Zn anode upon cycling. Fig. 11c shows the
float current of batteries using the synthesized zinc anodes with and without organic additives
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and the commercialized zinc foil. Generally, the higher float charge currents means more
energy is required to keep the batteries at 100% state-of-charge.[128] The float charge current
decreases in the order of commercial Zn, Zn/TU, Zn with additive, Zn/CTAB, Zn/PEG, and
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Zn/SDS. This indicates that side reactions are effectively suppressed on Zn/SDS anode and
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Zn/PEG, and the modification using CIAB and TU are not effective enough. These results
were further supported by the corrosion data, in which the corrosion rates of Zn/SDS,
Zn/PEG, and Zn/CTAB follow the same decreasing trend as the float current results (Zn/SDS
< Zn/PEG < Zn/CTAB). However, the Zn/anode delivers the smallest corrosion rate and
highest float current, suggesting that it is able to prevent anode corrosion but can not address
the hydrogen evolution when the battery is in the charge state. As a result, average capacity
retention of 79, 76 and 80% are obtained for Zn/SDS, Zn/PEG and Zn-TU after 1000 cycles,
which are much higher than that of commercial Zn foil (67%) (Fig. 11d). On the basis of
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experimental results, they suggested that the cyrstallographic properties and morphology of
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the Zn surface can be tuned using different additives.[128]
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Figure 11. Comparative the first discharge curves (a) and cycling performance (b) of batteries using pure
porous Zn, Zn/CNT, Zn/AC, and Zn/AB anode, respectively. Reprinted with permission from Ref.[127]
Copyright 2016 Elsevier B.V. Comparative float currents (c) and cycling performance (d) of batteries
using zinc anode with and without organic additives and commercial Zn foil. Reprinted with permission
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from Ref.[128] Copyright 2017 American Chemical Society.
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In addition to LiMn2O4, other electrode materials such as layered LiNi1/3Co1/3Mn1/3O2
(NCM)[132] and LiFePO4 (LFP)[133, 134] were introduced as promising cathode for
aqueous rechargeable hybrid batteries (Li-Zn Cell). Fig.12a shows CV curve of LPF
electrodes in aqueous electrolytes with CH3COOLi and Zn(CH3COO)2 in pH 7. A welldefined redox pair is identified with anodic peak and cathode peak locate at 1.65 V and 1.0 V
(vs. Zn2+/Zn), respectively. This corresponds to the following reaction:
$%JP.E + 1/2O&-0 ↔ JP.E $% 0 1/2O&
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Upon charging, LFP is oxidized to FP with the Li+ ion de-intercalates from the bulk matrix of
LPF and dissolves into the electrolyte , and Zn2+ is driven to the surface of the anode, accept
electrons, and reduced to metallic zinc. The discharge process involves the reverse reactions.
Then concentrations of Li+ and Zn2+ in the electrolyte change during charge/discharge process
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(Fig. 12b). Take Zn2+ as an example, its concentration decreases gradually upon charging due
to the reaction of Zn2++2e-→Zn. During discharging, Zn is oxidized and dissolves into the
electrolytes in the form of Zn2+, giving an increase in its concentration (Fig. 12b). The high
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reversible change of Zn2+ concentration translates into good rate performance (Fig. 12c) and
high Coulombic efficiency (Fig. 12d). They suggested that the employment of neutral and/or
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alkaline aqueous electrolytes instead of acidic Li2SO4 electrolyte and the carbon-coating of
LiFePO4 ensure the good electrochemical performance.[133] Yuan et al. prepared graphenecoated LFP and evaluated its electrochemical properties using Zn metal as anode in 0.5 M
CH3COOLi and 0.5 M Zn(CH3COO)2 mixture. The composite electrode delivers a high
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discharge capacity of 145.8 mAh g-1 at 0.2 C with a coulombic efficiency of 99%.[134] The
enhanced electrochemical performance was also demonstrated in LiNi1/3Co1/3Mn1/3O2 (NCM)
/RGO/CNT composite electrode,[132] probably due to enhanced electronic conductivity and
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additive.[135]
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smaller charge transfer resistance, benefiting from the introduce of carbon allotropes
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Figure 12. (a) Cyclic voltammetry curve of LiFePO4-Zn aqueous hybrid batteries at a sweep rates of 1
mV/s. (b) Change in Zn2+ concentration during charge/discharge process (at 1C rate). (c) and (d) Rate
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capability and Coulombic efficiency of LiFePO4-Zn aqueous hybrid batteries. Reprinted with permission
from Ref.[133] Copyright 2013 The Royal Society of Chemistry.
Shifting aqueous hybrid batteries from Li-Zn to Na-Zn is intrinsically attractive because
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the Na salts used in aqueous systems such as Na2SO4, CH3COONa, NaCl, and NaNO3 are
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cost-effective due to their natural abundance. Wu and co-workers demonstrated the first NaZn prototype in 2013 using Na0.95MnO2 cathode and metallic Zn anode.[136] Fig. 13a
illustrates the CV curves of Na0.95MnO2 cathode and metallic Zn anode in 0.5M
CH3COONa+0.5 M Zn(CH3COO)2 aqueous electrolyte. The electrochemical measurements
were performed in a three-electrode system using platinum and metallic zinc as the counter
and reference electrodes, respectively. For rod-like Na0.95MnO2, a distinct redox couple with
anodic peak at 1.7 V and cathodic peak at 1.4 V (vs. Zn2+/Zn) is identified, corresponding to
the de-intercalation and interaction of Na+ ions. For Zn anode, there is a clear oxidization
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peak at 0.13 V (vs. Zn2+/Zn), which is assigned to the dissolution of Zn. While the plating of
Zn occurs at potential below 0 V (vs. Zn2+/Zn). The over electrochemistry is identical to that
of Li-Zn, and the reaction can be described as follows:
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*.QR D&.- /2O&-0 ↔ *.QR1 D&.- + *0 + /2O&
This Na0.95MnO2-Zn cell delivers a reversible capacity of 60 mAh g-1 at the charge rate of 2C,
and 12 mAh g-1 was maintained up to 15 C (Fig .13b). Fig. 13c shows the stable cycle
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performance of Na0.95MnO2-Zn cell until 1000 cycles with only 8% capacity loss at 4C. [136]
Wu et al. investigated the effect of cut-off potential on electrochemical performance of
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Na0.44MnO2-Zn cell.[25] They found that the stable working window limited to 2 V (vs.
Zn2+/Zn), exceed which the oxygen evolution will occur. And the highest specific capacity is
achieved in potential range of 0.5-2.0 V (vs. Zn2+/Zn).[25] Li and Huang et al. prepared quasi
spherical Na3V2(PO4)3 nanoparticles embedded in carbon matrix (Fig. 13d), and designed
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Na3V2(PO4)3 –Zn hybrid batteries.[137] The rGO sheets provide interconnected conducting
scaffolds to promote the charger transfer and also act as the heterogeneous nucleation sites to
facilitate the uniform growth of Na3V2(PO4)3 nanoparticles. Fig. 13e shows the typical CV
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curve of Na3V2(PO4)3 –Zn hybrid cell. In which a distinct redox couple at around 1.52 and
1.37 V (vs. Zn2+/Zn) is indentified, corresponding to V3+/V4+ and Zn2+/Zn redox reactions.
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Particularly, the subsequent CV curves overlap with the 1st one, demonstrating good cycling
reversibility and high Coulombic efficiency. Galavanostatic charge/discharge profiles deliver
well-defined plateaus at 1.42 and 1.47 V (vs. Zn2+/Zn) at 0.5C, agree well with CV results.
Even at a high current rate of 15C, the Na3V2(PO4)3 –Zn hybrid cell can deliver a discharge
capacity of 65 mAh g-1, while flat charge/discharge voltage plateaus remain (Fig. 13f).[137]
The energy density of this hybrid cell is calculated to be 67 Wh kg-1 (considering the active
materials account for 60 wt% in the whole battery), which is higher than that of lead acid and
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low-voltage ARLBs and ARSBs. These results demonstrate the high power capability and
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practical feasibility of Na3V2(PO4)3 –Zn hybrid cell.
Figure 13. (a) SEM image of rod-like Na0.95MnO2.(b) aqueous hybrid batteries at a sweep rates of 1 mV/s.
(b) Cyclic voltammetry curves of like Na0.95MnO2 cathode and Zn anode at a sweep rate of 0.5 mV/s. (c)
Cycling performance of the Na0.95MnO2 –Zn cell at 4 C rate with a fixed full charge capacity of 40 mAh g-
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1 (based on Na0.95MnO2). Reprinted with permission from Ref.[136] Copyright 2014 The Royal Society of
Chemistry. (d) SEM image of quais spherical Na3V2(PO4)3 nanoparticles loaded on reduced graphene
sheets. (e) CV curve of the Na3V2(PO4)3-Zn cell in 0.5 M CH3COONa +0.5 M Zn(CH3COO)2 at a sweep
rate of 0.1 mV/s. (f) Glavanostatic charge/discharge profiles of Na3V2(PO4)3-Zn cell under different
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current rates. Reprinted with permission from Ref.[137] Copyright 2016 Elsevier B.V.
Prussian blue and its analogies were also reported as promising cathode alternatives for
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aqueous hybrid batteries because of i) the opening framework with the (C=N)- anions is large
enough to ensure fast ionic diffusion with structure disruption,[138, 139] and ii) the easy
synthesis and low cost.[140, 141] Wang et al. reported an aqueous rechargeable Na-Zn hybrid
battery using Prussian blue nanocubes as cathode.[26] The electrochemistry of
Na0.61Fe1.94(CN)6⋅
0.06-Zn
hybrid cell is identical to the hybrid cells discussed above, with
Na+ ions insert/extract from the open framework of NaFe-PB and Zn2+ dissolution/deposition
on metallic Zn anode (Fig. 14a). Fig. 14b shows the cubic framework of [Fe2(CN)6]-, in
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which high-spine FeIII coordinate with N and low-spin FeII contact with C. The 24d site is
more energetically preferable for Na+ occupancy. The as-prepared of Na0.61Fe1.94(CN)6⋅
0.06
are nanocubes with diameter ranging from 200 nm to 700 nm (Fig. 14c). Galvanostatic
charge/discharge profiles of the Na0.61Fe1.94(CN)6⋅
0.06-Zn
hybrid cell are shown in Fig. 14d.
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This hybrid cell delivers an average output voltage of about 1.1 V (vs. Zn2+/Zn) and a
reversible capacity of 73.5 mAh g-1 at a current density of 100 mA g-1. Moreover, the
subsequent charge/discharge profiles (since the 2nd cycles) overlay well with each other,
light on the structural evolution of Na0.61Fe1.94(CN)6⋅
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giving an nearly 100% couloumbic efficiency.[26] Ex-situ XRD measurements shed more
0.06
nanocubes upon cycling (Fig. 14e).
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During Na+ ion extraction, the rhombohedral NaFe-PB changes into cubic phase at the
voltage of 1.1 V (vs. Zn2+/Zn), and this cubic phase keeps until the end of charging (at 1.6 V,
the yellow pattern). Upon Na+ ion extraction, the phase changes back to rhombohedral at the
voltage of 1.2 V, and the structure keeps unchanged until final discharging state (at 0.8 V).
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The whole process is highly reversible, corroborating the good cycling stability and practical
feasibility of NaFe-PB cathode.[26] Hydrated Nickel hexacyanoferrate (NiHCF),
Ni3(Fe(CN)6)2 was also used as cathode for ARBs. It processes a face-centred cubic PB
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crystal structure. Lu et al. investigated its electrochemical performance in various electrolytes
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with different guest cations such as Li+, Na+, K+ and Zn2+.[142] They found that the
electrochemical activity and kinetics of these cations follow in the order of Zn2+ < Li+< K+ <
Na+. Among which, the insertion of Na+ into and extraction from the open framework
structure is the most efficient. Coupling Ni3(Fe(CN)6)2 cathode, porous Zn anode, and 0.5 M
Na2SO4 +0.05 M ZnSO4, the optimized hybrid batteries deliver an energy density as high as
62.9 Wh kg-1 (on the basis of the total weight of both electrode materials.) .[142]
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Figure 14. (a) Schematic illustration of the redox reactions of Na0.61Fe1.94(CN)6⋅
-
0.06-Zn
hybrid cell. (b)
+
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The crystal structural of the [Fe2(CN)6] and the possible Na occupancy sites at face-center (24d) channels.
aqueous hybrid batteries at a sweep rates of 1 mV/s. (c) Typical SEM image of Na0.61Fe1.94(CN)6⋅
0.06
,
showing nanocubes with high crystallinitiy. (d) Galvanostatic discharge/charge profiles of the
Na0.61Fe1.94(CN)6⋅
0.06-Zn
Na0.61Fe1.94(CN)6⋅
0.06 cathode
hybrid cell at a current density of 1000 mA g-1. (e) Ex-situ XRD patterns of
at different charge/discharge stages (the points in charge/discharge profiles).
Reprinted with permission from Ref[26]. Copyright 2017 Elsevier B.V.
4.2 Proton Redox Cathode- Intercalation Anode Electrochemistry
Traditional ARBs generally suffer from limited energy density and severe capacity
degradation of cathode materials upon cycling. Replacing conventional intercalation cathode
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with proton redox cathode, i.e., Ni(OH)2 and NiS, is intrinsically attractive since it can
overcome the aforementioned challenges to some extent. We designed a new type CoxNi2xS2-TiO2
ARHBs by integrating two reversible electrode processes associated with OH- and
Li+ ion insertion/extraction in CoxNi2-xS2 cathode part and TiO2 anode part, respectively (Fig.
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15a).[28] Fig. 15b shows the typical CV curves of CoxNi2-xS2 cathode, in which a pair of
well-defined redox peaks in the range of 0-0.5 V (vs. SCE) was identified. It corresponds to
the reversible redox reaction with CoxNi2-xS2:
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LF %-1 S- ., 1 ↔ LF %-1 S- ., + 1
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The redox couple in the range of -1.1 V to -0.8 V (vs. SCE) (Fig. 15c) is assigned to the
reversible electrochemical reduction/oxidization of the anatase TiO2 upon Li+ ion
insertion/de-insertion:
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9%.- + $% 0 + 1 ↔ $% 9%.-
The overall electrochemical reaction in CoxNi2-xS2-TiO2 ARHBs (Fig. 15d) is then given as:
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LF %-1 S- + 9%.- + T$%., ↔ LF %-1 S- .,U + $%U 9%.Upon charging, CoxNi2-xS2 is oxidized and TiO2 electrode is reduced, accompanied by the
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OH- insertion and Li+ ion insertion processes at the cathode and anode, respectively. Reverse
reactions take place during discharging. CoxNi2-xS2-TiO2 ARHBs operate with an average
potential of 1.15 V and deliver specific capacities of 73.4 mAh g-1 at 2.6 A/g, 60.7 mAh g-1 at
3.9 A/g (Fig. 15e). Moreover, a capacity retention of 75.2 % is obtained after 1,000
charge/discharge cycles, indicating good cycling performance (Figure. 15f). The hybrid
electrochemistry and 3D electrode structural design ensure the enhanced electrochemical
performance and practical feasibility of CoxNi2-xS2-TiO2 ARHBs. From the perspective of
fundamental basics, the coupling of an alkaline battery active cathode with an aqueous
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rechargeable anode together could lead to the following merits: i) decent energy capacity
because of breaking of the capacity limitation of the conventional intercalation cathodes; ii)
good rate performance because of the large proton diffusion coefficient (10-12 to 10-8)[143,
144]; iii) good cycling stability benefiting from the intrinsic electrochemical stability of both
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NiS[145, 146] and TiO2 [48, 49]in alkaline electrolytes. Structurally, the 3D hybrid electrode
design with nano-structured active materials grown directly on a highly conductive GF/CNTs
electrode support ensures: 1) favourable electrochemical kinetics, good mechanical integrity,
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and large mass loading of the active materials and high density of the packaged cell. This also
eliminates the additives (e.g., carbon black and binder).[28] The unique design in both battery
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and electrode structure makes the CoxNi2−xS2//TiO2 battery a promising energy storage device.
Gao et al. reported a Ni(OH)2-TiO2 hybrid cell using a-phase nickel hydroxides as the
cathode and TiO2 nanotube arrays as the anode.[27] This hybrid cell delivers a much higher
voltage plateau (∼1.74 V) due to the more negative potentials for Li+ ion insertion/extraction
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processes of TiO2 nanotube arrays than that of hydrogen evolution/oxidization in metal
hydride electrode. In addition, a specific capacity of 65 mAh g-1 can still be obtained after
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100 cycles at 2C rate (1C=168 mA g-1), demonstrating good cycling performance.[27]
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Figure 15. (a) Schematics of the CoxNi2−xS2-TiO2 hybrid battery. Cyclic voltammetry curves of
CoxNi2−xS2 cathode (b) and TiO2 anode (c) supported on GF/CNTs hybrid films, and the CoxNi2−xS2-TiO2
hybrid battery (d) at different sweep rates. Galvanostatic discharge curves (e) and Cycling performance (f)
of CoxNi2−xS2-TiO2 hybrid battery. Inset shows the CV curves for the 1st and 1000th cycles. Reprinted
with permission from Ref.[28] Copyright 2015 American Chemical Society.
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Table 2. Electrochemical performance of various aqueous hybrid batteries.
Cell type
Electrolyte
Average
Capacity
Working
Capacity
retention (%)
Ref.
-1
(mAh g )
Potential (V)
3M LiCl+4M ZnCl2+0.1M
1.8
KOH
LiNi1/3Co1/3Mn1/3O2
//
0.25M
90% (1000) at
0.32 mA h
[24]
110
[13
4C
Li2SO4+0.125M
Zn
Zn(CH3COO)2
LiFePO4 // Zn
CH3COOLi+Zn(CH3COO)2
1.7
99% (40) at
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LiMn2O4//Zn
0.5C
1.25
> 95% (125)
100
at 1C
0.5 M CH3COONa+0.5M
1.4
Zn(CH3COO)2
0.5 M CH3COONa +0.5M
Zn(CH3COO)2
Na0.44MnO2 // Zn
40
4C
1.42
77% (200) at
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Na3V2(PO4)3-C //Zn
92% (1000) at
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Na0.95MnO2 //Zn
1.5
[13
3]
[13
6]
91
0.5C
1M Na2SO4+0.5M ZnSO4
2]
[13
7]
Above
90%
40
[25]
76.2
[14
(100) at 4C
NiHCF // Zn
0.5 M Na2SO4 + 0.05 M
ZnSO4
1 M Na2SO4
81% (1000) at
-1
500 mA g
1.1
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NaFe-PB // Zn
1.5
80% (1000) at
2]
74.0
[26]
-1
300 mA g
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5. Perspective & Outlook
Shifting the rechargeable batteries from organic systems to aqueous systems, is of
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particular attractive for large-scale energy storage in terms of several advantages: low cost by
using inexpensive and good availability of salts, high power due to higher ionic conductivity
of aqueous electrolytes than nonaqueous ones, more safe for avoiding the usage of flammable
organic electrolytes, and environmental friendliness. Though great progresses have been
made in ARBs until now, there are a series of challenges that retard its widely practical
applications. (i) Limited voltage windows. The stability working window of aqueous
electrolytes is relatively narrow (~1.23 V). Beyond this, aqueous solutions are prone to
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decompose, generating H2/O2 gases. Kinetic limitations of the electrolysis may expand the
stability limit of some aqueous electrolytes to ~2.0 V. However, side reactions may further
block the long term stability. (ii) Side reactions between electrode materials and H2O or
residual O2. The stability of intercalated Li ions in the host materials is an important issue in
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an aqueous electrolyte battery. Side reactions between intercalated Li ions and H2O
molecules are crucial to the cycle stability of aqueous cells. Also, the chemical stability of
active materials will degrade when dissolved O2 is present in electrolyte solutions. (iii)
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Electrode materials dissolution. Materials dissolution in aqueous media at various pH can
occur upon electrochemical operations. Therefore, surface protection by coatings or the
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addition of surface stabilizers should be further explored. (iv) Proton co-insertion effects. In
some of electrode materials, protons will also insert into the crystal structures, which will
block Li diffusion channels. Thus, new development of active materials with minimal proton
co-insertion effects is highly motivated. Though great challenges, some directions are very
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important for the development of ARBs:
First of all, a good fundamental understanding of ARBs and is essential to guide
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optimal electrode design and/or selection. The key challenge lies in correlating electrode
structure/element evolution with electrochemical characteristics, aiming at identifying the
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dominant chemical/physical properties that greatly influence the electrochemical performance.
To achieve these, powerful in-situ characterizations in combination with theoretical
calculations and/or simulation are highly desirable.
Second, ARBs still suffer from the limited operating window (< 2V) because of
electrolysis of water. Recent breakthrough shows that “water-in-salt” electrolytes[147] and
hydrate-melt electrolytes[148] can extend the safety voltage window of battery up to 3-4 V,
overcoming the water electrolysis limitation. These exciting results also open new direction
for the design of high energy density ARBs based on “water-in-salt” electrolytes and/or
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hydrate-melt electrolytes. Another alternative protocol is employing metal anodes in aqueous
systems to broaden the operating voltage of ARBs, which called the good development of
protective techniques for metal anodes.
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Third, the green and sustainable features also make ARBs idea for flexible electronic
applications. Therefore, multifunctional ARBs that are flexible, stretchable and even
lightweight are intriguing. Until now, quite limited flexible ARBs have been reported and
their mechanical and electrochemical properties have no yet optimized. More progresses can
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be made from learning from other flexible battery technology including flexible LIBs, SIBs
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and asymmetric supercapacitors.
Fourth, the development of rechargeable aqueous metal-air batteries will be another
choice for high energy ARBs. The general electrochemical reaction mechanisms of aqueous
metal-air batteries are based on a redox reaction between metal ions and oxygen in aqueous
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solution. For these types of ARBs, though a series of problems blocked the current practical
applications, while, the extremely high energy densities still intrigue people to pay more
attentions. Problems include the sluggish kinetics of oxygen electrochemical reactions, low
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electronic conductivity and corrosion problems of the solid electrolytes, dendrite formation of
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metal anodes during striping/plating, and cell configuration are the most important concerns.
Acknowledgments
This work was supported by the Ministry of Education, Singapore, Tier 2 (MOE2015-T2-1148) and Tier 1 (Grant No. M4011424.110), and National Natural Science Foundation of
China (No. 21503025), Fundamental Research Funds for Central Universities (No.
106112016CDJZR325520), Key Program for International Science and Technology
Cooperation of Ministry of Science and Technology of China (No. 2016YFE0125900), and
Hundred Talents Program at Chongqing University.
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