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Manganese and Vanadium Oxide Cathodes
for Aqueous Rechargeable Zinc-Ion Batteries:
A Focused View on Performance, Mechanism,
and Developments
Vinod Mathew, Balaji Sambandam, Seokhun Kim, Sungjin Kim, Sohyun Park, Seulgi Lee,
Muhammad Hilmy Alfaruqi, Vaiyapuri Soundharrajan, Saiful Islam, Dimas Yunianto Putro,
Jang-Yeon Hwang, Yang-Kook Sun, and Jaekook Kim*
Cite This: ACS Energy Lett. 2020, 5, 2376−2400
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ABSTRACT: The development of new battery technologies requires
them to be well-established given the competition from lithium ion
batteries (LIBs), a well-commercialized technology, and the merits
should surpass other available technologies’ characteristics for battery
applications. Aqueous rechargeable zinc ion batteries (ARZIBs)
represent a budding technology that can challenge LIBs with respect
to electrochemical features because of the safety, low cost, high energy
density, long cycle life, high-volume density, and stable watercompatible features of the metal zinc anode. Research on ARZIBs
utilizing mild acidic electrolytes is focused on developing cathode
materials with complete utilization of their electro-active materials. This
progress is, however, hindered by persistent issues and consequences of
divergent electrochemical mechanisms, unwanted side reactions, and
unresolved proton insertion phenomena, thereby challenging ARZIB
commercialization for large-scale energy storage applications. Herein, we broadly review two important cathodes, manganese
and vanadium oxides, that are witnessing rapid progress toward developing state-of-the-art ARZIB cathodes.
E
ion batteries (LIBs); among these, LIBs are the most recently
introduced (∼1970s). LIBs have dominated the battery market
share because of their low weight, high energy densities, and
specific power.7,8 Notwithstanding, the approaching energy
density limits, high cost, toxic nature, safety issues, and
concerns of material availability have constrained LIB use in
large-scale applications. Pb−acid and Ni−Cd batteries, both of
which still rule the current stationary storage bracket because
of their low-cost and durability, have drawbacks of relatively
low energy densities (∼30−50 Wh kg−1) and face ecological
risks because of the toxic electrodes.9,10 In contrast,
lectricity derived from the rapidly depleting and
environmentally unsafe fossil fuels currently meet the
majority of global energy demand (>80%). With the
prediction of global population reaching 9.7 billion (from 7
billion) and the energy demand reaching 1000 exajoule (1 EJ =
1018 J) by 2050, fossil fuels are expected to last only for a few
decades because of severe geographical depletion and national
resource politics. In addition, very high levels of CO2 emissions
(74%) has pushed global atmospheric levels to alarming
heights (407.8 ± 0.1 ppm).1−3 Developing efficient energy
storage technologies based on renewable energy sources is
considered the best way forward to mitigate climate change
and replace fossil fuels. Rechargeable batteries are highly
crucial, especially for wind and solar power, which form the
bulk of the renewable sources because of their varying
generation profiles.4−6 At present, four secondary battery
technologies that are relevant in the commercial battery market
are Pb−acid, alkaline Ni/Cd, Ni/metal hydride, and lithium© 2020 American Chemical Society
Received: April 3, 2020
Accepted: June 9, 2020
Published: June 9, 2020
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the parasitic phases came from identifying spinel ZnMn2O4
(ZMO) and zinc basic sulfate (ZBS or Zn4(OH)6SO4·5H2O or
layered zinc hydroxyl sulfate) phases, respectively; the
discharged electrode recovered after initial and continuous
cycling, respectively.33 The Jahn−Teller distortion related to
Mn3+ ions and its contribution to dissolved Mn2+ ions and
hence variant electrolyte pH promoted electrode degradation,
irreversible phase formation, and hence capacity fading upon
continuous cycling. To overcome these issues and ensure
sustained long-term capacity, the pre-inclusion of Mn2+ salt
additive was proposed.20 Despite this, no progress was
reported for almost a decade until a reversible Zn intercalation
in a quasi-solid-state Zn-γ-MnO2 system with a gel polymer
electrolyte (GPE) based on poly(vinylideneflouride) (PVdF)
and zinc trifluoromethanesulfonate (Zn(CF3SO3)2) was
suggested.34 Since then, sporadic efforts on the Zn−MnO2
systems using neutral/slightly acidic electrolyte solutions were
pursued before the clear demonstration of an intercalation-type
ARZIB in 2012.12,35 In a typical ARZIB, an intercalation
compound like MnO2 and zinc foil are used as the cathode and
the anode, respectively. These electrodes are separated by a
Zn-ion conducting, mildly acidic electrolyte (pH ∼4−6) made
from a solution or gel containing a Zn salt like 1 M ZnSO4
(zinc sulfate). The assembled cell with a Zn-free cathode is in
the charged state and thus requires to be discharged. In the
discharge cycle, anodic Zn is dissolved electrochemically as
Zn2+ ions are diffused through the aqueous electrolyte into the
cathode. Simultaneously, the charge-balancing reduction of a
transition metal ion (Mn4+ to Mn3+) occurs at the cathode; this
releases electrons to the external circuit. At the end of the
electrochemical reaction, a Zn-containing product (ZnMn2O4)
is formed at the cathode. Figure 1 presents a typical
intercalation mechanism in Zn ion batteries along with
crystallographic notation of different phases of Mn-based and
V-based metal oxides utilized as cathodes for this technology.
At present, ARZIBs are considered as suitable alternatives
for possible large-scale stationary storage applications, although
there are still challenges to overcome. The greater atomic mass
and positive charge of the Zn2+ ion and their related mobility
and reaction kinetics during intercalation imply that identifying
cathodes for ARZIBs is difficult compared to the case for LIBs.
Further, the suppression of hydrogen and oxygen evolution in
aqueous electrolytes, in general, to widen the operating
potential window for ARZIBs can be challenging. However,
the research on ARZIBs has made continuous progress in the
development of research in all three battery constituentsthe
cathode, anode, and the electrolyte. First, high energy density
cathode materials supporting facile Zn intercalation−deintercalation have been constantly sought after by preparing new
materials or customizing already existent materials. Second,
suitable electrolyte with optimum salt and additive concentration and pH have been explored for unhindered operation
within a wide potential region. In addition, new battery
chemistries and efforts to elucidate the electrode reactions and
their mechanisms have been intensely explored via systematic
experimental and theoretical approaches. Third, studies on the
development of anodes that will permit smooth and
unhindered deposition and stripping of zinc without dendrite
formation during long-term cycling have been pursued to
realize state-of-the-art ARZIB systems. Finally, lab-scale flexible
and wearable ARZIB cell designs of different spatial
dimensions and sizes have been fabricated to realize nextgeneration applications like roll-up displays, textiles, etc.36−43
rechargeable multivalent batteries with aqueous electrolytes
possessing ionic conductivities (∼1 S cm−1) greater than those
of organic electrolytes (∼10−3 S cm−1) can realize higher
energy densities combined with favoring high rate capabilities.11−13 Particularly, ARZIBs satisfy most of the criteria as
alternative green and sustainable stationary storage systems
because of the unique properties of zinc: low cost (∼2 USD
kg−1), abundance (∼75 ppm), high volumetric and gravimetric
capacities (5851 mAh mL−1 and 820 mAh g−1), low redox
potential (−0.76 V against standard hydrogen electrode), and
high stability in water due to the high kinetic overpotential for
hydrogen evolution. In addition, the neutral/mildly acidic
electrolyte used in ARZIBs to facilitate multiple-electron
transfers via Zn2+-intercalation results in higher energy
densities than those in alkaline Zn−MnO2 systems and even
in LIBs or sodium-ion batteries (NIBs).14−20 The early Zn−
MnO2 system introduced in the 1860s utilized alkaline
electrolytes that supported one-electron transfer in MnO2 via
a conversion reaction at high discharge potentials.21−23 More
recently, this alkaline system was investigated to exploit the
reversibility of the two-electron-transfer reaction in MnO2 and
thereby acquire full theoretical capacity even under long-term
cycling.24−26 However, this objective is still faced with the
practically unavoidable challenges of capacity fade associated
with the irreversible byproducts, zinc dendrites, and corrosion
in the alkaline electrolyte.27,28 Compared to this, the neutral/
mildly acidic electrolytes in ARZIBs facilitating multiple
electron transfer enhance cell capacity, stability, and longevity
while lowering operation costs and environmental risks. Also,
besides the energy density advantage, the practical cost of
ARZIBs (∼ <65 USD/kW h−1) can be almost 5-fold times
cheaper than that for LIBs (∼300 USD kW h−1) because of the
low cost for raw materials and cell production.29 In addition,
the similar ionic radii of Zn2+ and Li+ (Li+, 68 pm; Zn2+, 74
pm) and low activation energies for ion diffusion, estimated
from theoretical studies, predict that the approach for
developing Zn intercalation hosts can be the same as that for
lithium, provided that the higher mass and polarity factors of
Zn2+ ions are taken into account. Furthermore, compared to
ARZIBs, research on other multivalent batteries supporting
multiple-electron transfers, such as Mg2+, Ca2+, or Al3+ ions, are
stiffly challenging because of the problems in identifying
appropriate electrolytes that can support reversible metal
plating without corrosion and/or inactive surface layer
formation on the metallic anode.30,31Tunneled and layered
Tunneled and layered structures of
manganese oxides and vanadium oxides, at least for now, form the “holy
grail” in describing the progressive
advancements toward developing
state-of-the-art electrodes for ARZIBs.
structures of manganese oxides and vanadium oxides, at least
for now, form the “holy grail” in describing the progressive
advancements toward developing state-of-the-art electrodes for
ARZIBs.
The history of using sulfate-based electrolytes in the Zn−
MnO2 system dates to the 1980s when the reversibility of the
storage capacity in MnO2 was demonstrated for short-term
cycling (<50 cycles).32 Confirmation of Mn(III) reduction and
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Figure 1. (left) Schematic illustration of a Zn-ion battery. (right) Crystallographic representation of a few Mn-based and V-based metal
oxides used as cathodes for Zn-ion battery.
hamper the facile electrochemical reaction and lead to major
cycling capacity losses. Finally, the low electrical conducting
properties of cathode hosts, specially transition metal oxides,
can slow down the reaction kinetics and be detrimental to
electrode rate capabilities, which is critical for practical
applications. To the above, the genuinely sluggish solid-state
diffusion kinetics of divalent Zn2+ ions within the cathode
lattice is a problem that needs to be overcome. Therefore, we
aim to present the progress made to date in the design and
development of cathodes with stable and long cycle life at even
high current densities by carefully chosen performanceenhancement strategies.
Manganese dioxide (MnO2) cathodes, which have been
extensively studied ever since the introduction of alkaline
batteries, offer unique crystallographic structures, large
theoretical capacity (∼308 mAh g−1 corresponding to one e−
transfer), and high operating potential (∼1.3 V vs Zn/Zn2+) in
addition to their economic and environmental advantages. In
MnO2, every Mn4+ ion occupies the octahedral site of the
MnO6 fundamental units, which are linked together either by
edges or corners depending on the polymorph type. Depending on their crystal structures and their tunnel/channel
dimensions, the polymorphs are categorized as tunnel-type
(α, β, γ, and todorokite),17,55−61 layered (δ; interlayer distance,
7 Å),62−66 and spinel (λ-MnO2 or ZnMn2O4)67,68 phases.
Specifically, α-MnO2, β-MnO2, γ-MnO2, and todorokite-MnO2
are differentiated by their 2 × 2 tunnels (size-width, 4.6 Å), 1
× 2 tunnels (size-width, 1.89 Å), mix of both 1 × 1 and 1 × 2
tunnels, and 3 × 3 tunnels (size-width: 7 Å), respectively.
Besides the polymorphs of MnO2, MnOOH/MnO2, Mn7O13·
2H2O, and α-Mn2O3 cathodes were reported for ARZIBs.69−71
The demonstration of reversible Zn intercalation occurring via
the 2 × 2 tunnels of α-MnO2 to form ZnMn2O4 in a mildly
acidic 1 M ZnSO4 electrolyte was reported in 2012, though the
Zn-insertion possibility was suggested in the past decade.12,35
At 0.5 C discharge−charge rate, a high zinc storage capacity of
210 mAh g−1, which is almost 68% higher than that for alkaline
ZnMnO2 battery, was realized.67 Attempts to improve the
electrical properties of different MnO2 polymorphs in ZnSO4
were pursued by using the strategies of nanostructuring, metalion doping (vanadium-doping), and conductive carbon
coating/composite formation.56,68 Despite this, the problems
of electrode degradation related to manganese dissolution
arising from the Jahn−Teller structural distortion, repeated
structural phase transitions, and irreversible phases formed
Further, within the short period of the rejuvenation of ARZIB
research, few reviews centering on various aspects of its
development have been published.44−54 In light of these
developments, this brief Review focuses on providing a unified
and balanced view on the progressive advancements of
developing manganese and vanadium oxide cathodes and
understanding their electrochemical reaction mechanisms for
zinc-ion battery applications. Further, this Review aims to
highlight the immediate challenges and prospects to be
addressed in furthering the development of the storage
technology applications of ARZIBs.
Research Development in Manganese and Vanadium Oxide
Cathodes for ZIBs. Upon discharging a typical ARZIB, Zn2+
ions from the anode diffuse through the electrolyte and
intercalate the cathode structure, thereby triggering a charge
balancing transition metal-ion reduction in the host and
releasing electrons to the external circuit. The development
and choice of cathodes with appropriate redox potentials that
permit facile diffusion and reversible insertion of Zn2+ are,
therefore, imperative to determine the overall energy density of
the ARZIB system, as in the case of LIBs. In addition, low-cost,
environmental safety, and easy synthesis are three important
criteria that should be satisfied to determine the commercial
scalability and usage of the cathode materials.
The advancements in ARZIB cathode development have
been mainly focused on transition metal oxides. Among these,
promising progress has been witnessed for manganese and
vanadium oxides as they present framework-type crystallographic structures with active 1D/2D/3D intercalation pathways and storage sites to be reversibly accessed by even
divalent charge carriers like Zn2+ ions. In addition, these
transition metal ions tend to exist in high oxidation states and
are appropriate for maintaining charge balance related to
multiple-electron-transfer reactions for ARZIBs. However,
their rapid advancements are persistently faced with a few
scientific and technical challenges. First and foremost, the
accommodation or transport of the heavier and bigger divalent
charge carriers (Zn2+) implies that only selective materials with
open framework structures possessing wide spatial dimension
for ion-diffusion are mostly preferred as the cathode hosts.
Second, some of the available high oxidation transition metal
ions undergo dissolution upon two-electron-transfer reactions,
thereby leading to electrode instability and hence capacity
decline during long-term cycling. Third, the “build-up” of
unwarranted reaction byproducts on the electrode surface can
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upon repeated insertion−deinsertion of divalent Zn2+ in
ZnSO4 electrolytes persisted.57 This affected cycling stabilities
of the electrode polymorphs, especially in γ-MnO2.15,69
After manganese oxide, vanadium oxides were the next to be
explored as cathodes because of their layered and tunnel
polymorphs offering wide spatial dimensions for facile Zn
intercalation−deintercalation. Compared to manganese oxides,
vanadium oxides offer higher specific capacities but lower
operating voltages (∼0.8 V versus zinc) in aqueous electrolytes. Unlike MnO2, most of the vanadium oxides present
layered structures with few exceptions for tunnel-type
structures. The cathodes studied for ARZIBs are mainly
classified as follows: V2O5 and their derivatives;14,72−78
monoclinic layered vanadates (AV 3 O 8 ; A = Li, K,
H2);16,79−81 layered barnesite-type vanadates B2V6O16·nH2O
(B = Na, K);82−84 monoclinic tunnel-type VO2;85−87 mixedvalent V6O13;88,89 Zn-, Cu-, and Fe-based transition metal
vandates;90−92 and finally, NASICON-type compounds.93
However, the different structures offer various active sites for
electrochemical Zn intercalation and thereby tend to influence
the overall battery energy density. Among these, V2O5 offers
the highest theoretical capacity (∼589 mA h g−1) because of
the feasibility of the two-electron transfer reaction related to
the V5+/V3+ redox couple. However, only low percentages of
maximum capacity utilization could be realized for labsynthesized pure V2O5 cathodes in low-cost ZnSO4 (38%,
∼121 mAh g−1 after 400 cycles) and cycling-stable Zn(CF3SO3)2 (50 mAh g−1 after 600 cycles) electrolyte solutions
at 1 A g−1, respectively.74,94 Electrode degradation related to
active material dissolution and inferior electronic conductivity
were cited as the reasons for the poor performance of V2O5.
In view of the inherent mediocre electrical conduction,
structural instability, and capacity limitations in the pristine
transition metal oxide cathodes during initial studies, two
major strategies were pursued to scale-up the electrochemical
characteristics in manganese and vanadium oxide cathodes of
ARZIBs. One such important approach was to develop
nanostructured electrodes and/or their conducting composites
with pre-included additives. The benefits of short or nanoscale
ion-diffusion paths and unique morphologies with porous and
hierarchical architectures tend to promote the facile ion
transport/accommodation in a greater number of redox active
sites. In addition, the formation of composites with electrically
conducting carbonaceous/noncarbonaceous materials is advantageous to not only increase electronic conduction but also
useful to reduce active material dissolution in transition metal
oxides, particularly of those with unstable oxidation states.
More importantly, specific to the use of manganese dioxide
cathodes that are prone to Mn3+ Jahn−Teller instability in
ARZIBs, the strategy of using pre-included additives was
reported to alleviate active material dissolution during repeated
electrochemical reaction. The pre-inclusion of Mn2+ additives,
a strategy introduced decades before in the Zn−MnO2 battery
system, lowered the rate of the disproportionate reaction as
follows:
3+
4+
2+
2Mn(s)
↔ Mn(s)
+ Mn(aq)
Review
benefited manganese oxides more than the vanadium counterparts. MnO2 and their polymorph electrodes exhibited
improved cycling stabilities as sustained capacities were
achieved for even thousands of discharge−charge
cycles.20,59,62,65,66,70,71,95,96 Nanostructures of MnO2 were
mostly prepared as nanowires,95,97 nanorods, nanofibers,20,59
and nanotubes with porous features, generally facilitating more
active sites for electrochemical reaction as well as accommodating discharged products upon repeated cycling, thereby
enabling higher electrochemical reactivity of the cathode. An
α-MnO2 nanofiber cathode prepared by hydrothermal reaction
realizes 161 mAh g−1 specific capacity, of which 92% was
retained for 5000 cycles in an MnSO4-added 2 M ZnSO4
electrolyte solution at 5 C charge−discharge rates. An energy
density of 170 Wh kg−1, which is 5-fold higher than that of
commercial Pb-acid batteries, was estimated at C/3 rate for a
full ARZIB.20 Electrically conductive substances included in
MnO2 were mainly based on carbonaceous sources like
graphene,95 carbon nanotube (CNT),59 carbon nanofoam
(CNF),66 and reduced graphene oxide (rGO).98 Such
inclusion promotes structural stability, improves electrical
conductivity, and hence specific capacity while, at the same
time, relieving electrode dissolution and enhancing the cycling
stability of MnO2. A graphene-scroll coated MnO2 nanowire
cathode prepared by a hydrothermal reaction demonstrated an
exceptional storage capacity of 362.2 mA h g−1 (equivalent to
∼406.6 Wh kg−1 energy density) after 100 cycles, at 0.3 A g−1
in 2 M ZnSO4 electrolyte with 0.2 M MnSO4 additive.95
Further, 94% of the initial specific capacity of 145.3 mA h g−1
at 3 A g−1 was sustained even after 3000 cycles. In the same
electrolyte solution, a porous N-doped carbon-coated MnOx
(MnOx@N−C) composite prepared via calcination of ZIF-8,
an N-containing metal organic framework (MOF), presented a
100 mAh g−1 specific capacity at 2 A g−1 for 1600 cycles.99
Very few noncarbonaceous materials with high electrically
conducting properties like In2O3 have been used to develop
composite cathodes with MnO2 for ARZIBs.100 Surface coating
on MnO2 facilitates rapid charge transfer at the electrode/
electrolyte interface, thereby enhancing the electrical conductivity of MnO2 and ultimately decreasing battery internal
cell resistance. An In2O3-coated MnO2 (α-MnO2@In2O3)
nanotubes cathode obtained via a hydrothermal reaction
delivered a 425 mA h g−1 specific capacity at 0.1 A g−1 after
100 cycles in a 0.1 M MnSO4 additive containing 2 M ZnSO4
electrolyte. In addition, considerable long-term stability at 3 A
g−1 as 75 mA h g−1 was maintained for 3000 cycles without
significant decay.100
Vanadium oxides face relatively fewer dissolution issues
compared to manganese oxides because of their more stable
oxidation state variants and structural stability. Hence, the
approach of pre-including additives for vanadium is scarcely
reported. In one such study, the exact role of the additive in
the enhanced cycling stabilities of layered-type vanadium
cathodes was not discussed. The enhanced cycling stabilities
and electrochemical enactments were attributed to high Zn-ion
diffusion coefficients in the structurally stable layered electrode
with large spatial interlayers.82 However, Wan et al, studying a
similar layered cathode (NaV3O8·1.5H2O) in a equimolar (1
M ZnSO4 + 1 M NaSO4) electrolyte solution, explained that
the additive cation (Na+) not only limited active material
dissolution by reversing the equilibrium reaction for Na
dissolution but also actively participates in a self-healing
electrostatic shield mechanism that suppressed dendrite
(1)
This action reduced Mn2+-dissolution and reaction byproduct
accumulation, thus extending the cycle-life of the Zn−MnO2
battery.20 The strategy of pre-inclusions in the electrolyte
combined with nanostructuring and/or compositing with
electrically conductive materials in electrodes has mostly
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formation at the anode during charging.80 The lower reduction
potential (compared to that of Zn2+) causes the additive cation
(Na+) to be adsorbed onto the uneven surfaces of the dendrite
protrusions to form an electrostatic shield. This shield not only
repels but forces the incoming Zn2+ ions to electroplate on the
smooth areas of the anode, thereby preventing the forefront
formation of sharp dendrite protrusions that tend to short the
cell reaction. Utilizing the prepared cathode with ZnSO4 in a
soft-pack battery configuration, they reported an achievable
capacity of 144 Wh kg−1 (based on the active electrode mass).
In contrast to the case of manganese oxides, there are very few
studies that utilize the electrical network connectivity,
structural stability, and flexible characteristics of (non)carbonaceous materials to form vanadium oxide composites
for ARZIB cathode applications. Carbonaceous materials that
were studied for such cathodes mostly include carbon101 and
rGO/graphene.102−105 Whereas noncarbonaceous introductions included conducting polymers like poly(3,4-ethylenedioxythiophene) (PEDOT) with high structural stability
to form vanadium oxide composite cathodes for ARZIB
applications.106 Two layered-type compounds forming biphasic
composites with enhanced interfacial domains, redox active
sites, and hence increased pseudocapacitive reactions contributing to high electrical conductivity and rate capability were
also designed.107 Among the composites, worth noting are the
electrochemical performances of a select few: The ARZIB
fabricated using a VO2/rGO composite cathode with a unique
architecture of cross-linked 3D porous morphology that
restrains structural collapse related to repeated Zn 2+
insertion−deinsertion demonstrated an energy density of 65
Wh kg−1 even at 7.8 kW kg−1 power density in a 3 M
Zn(CF3SO3)2 electrolyte solution.102 The electrically conductive network and the pseudocapacitive reaction related to
the high surface area of the graphene sheets contributed to
high energy density (168 W h kg−1 at 34 W kg−1 power density
based on the active electrode mass), high rate capability (270
mA h g−1 at 20 C), and cycling stability (>2000 cycles) in
H2V3O8/rGO composites in 3 M Zn(CF3SO3)3 electrolyte
solution.75
Although carbon composites of manganese oxides have been
extensively tested, the progress in developing high-performance vanadium oxide composites is comparatively slow.
Utilizing the high electrical conductivity of CNT/graphene
nanosheets or CNF with large aspect ratios is urgently required
to scale-up the performance in vanadium oxide cathodes. More
importantly, the scope for heteroatom (N, P, S, B, and I)
doped carbon aimed to enhance electrical characteristics of
manganese and vanadium-based cathodes is still open.99 In
addition, the high electrical conductivity and structural stability
advantages in nitrogen/sulfur-containing conducting polymers,
including polyisothianaphthalene and polypyrrole (Ppy), could
be exploited separately or as copolymers with mutually
compensating benefits to develop robust composite cathodes
with manganese or vanadium oxides. The polymer−inorganic
composite can be formed as a polymer surface coating on the
active material, as an active particle-embedment on the
polymer, or as a homogeneous mix of discrete materials.
Irrespective of the method adopted, these polymers can
synergistically interact with inorganic electrodes and facilitate
remarkable enhancements in terms of the structural, thermal,
and electrochemical stability of the composite electrode and
hence their cycling lifetime, potential, and rate performances,
as shown in the case for LIBs.108 In addition, very little about
Review
biphasic composites and their exact influence on the electrode
properties via interface reactions is known; these can be areas
to explore for arriving at promising cathodes for ARZIBs.
Besides the strategy of nanostructuring and pre-included
additives discussed above, structural and/or defect engineering
were also pursued to improve cathode performance. Most
cathodes with layered-type structures are usually held by weak
van der Waals interactions, and their character of undergoing
various phase transitions during cycling facilitates structural
degradation and hence affects cycling stability at high
discharge−charge rates. To avoid these, the structural
framework of layered-type manganese/vanadium-based oxides
is reinforced by the pre-insertion of conducting polymers or
metal ions. This has been successfully tested mostly in layered
cathodes of vanadium oxides than in manganese dioxide
because of the existence of many layered compounds in the
former. The insertion of polyaniline (PANI) intercalated δMnO2 via an organic−inorganic interface reaction leads to
extended interlayers and renders strength to the layered
structure. This eliminates the H+/Zn2+ insertion-induced phase
transformation and resulting structure collapse, which are vital
for extending the cycle life of the polymer-intercalated MnO2
to almost 5000 cycles with 40% capacity utilization at 2 A
g−1.65 In layered vanadium oxide cathodes, a great deal of work
has been performed to improve the reversible Zn 2+
intercalation upon repeated cycling via different structural
engineering. Structural or crystalline H2O intercalated into the
layers of the V2O5/AV3O8 structure (V2O5·nH2O) by using
appropriate low- or moderate-temperature synthesis benefits
electrochemical performance in layered cathodes because of
their “lubricating” effect. 14,89,92 The H2O-solvated Zn2+
facilitates greatly decreased effective charge and hence minimal
electrostatic interaction within the V2O5 framework, thereby
efficiently acting as a lubricant to facilitate its diffusion. A
V2O5·nH2O (n ≥ 1)/graphene composite as a prototype
cathode delivered a high capacity of ∼390 mAh g−1 at 60 mA
g−1 and excellent cycling stability with 90% capacity retention
above 900 cycles at 6 A g−1 in 3 M Zn(CF3SO3)2 electrolyte.14
Wei et al.109 explained that the Zn(H2O)m2+ ion interactions
in another layered vanadium oxide V10O24·nH2O led to the
bilayered structural vibration in the c-axis direction, thereby
providing plenty of ionic diffusion networks to stabilize the
structure (self-adjustment of interlayers). Chemical intercalation of metal-ions (AI/II) including monovalent (H+, Na+, Li+,
K+, NH4+, and Ag+)75 and bigger divalent (Ca2+, Mg2+, Fe2+,
Ni2+, Mn2+, Cu2+, and Zn2+)110 ions into layered structures like
dehydrated (hydrated) V2O5 (AxV2O5 (·nH2O)) is another
promising strategy to increase the cathode performance. These
intercalated metal ions can serve as “pillars”, thereby providing
enhanced structural stability and promoting greater Zn2+
diffusivity along the interlayer channels and promoting facile
and stable Zn intercalation−deintercalation for long-term
cycling even under high discharge−charge rates. Further,
depending on the specific ion and its concentration, the
electrochemical properties and their reaction mechanisms vary.
As for metal-ion interlayer doping in hydrated V2O5, a calciumstabilized double-layered V 2 O 5 (Ca 0.25 V 2 O 5 ·nH 2 O or
Ca0.24V2O5·0.83H2O) cathode possesses larger CaO7 polyhedra and provides expanded cavities between V4O10 layers.
Calcium stabilization provides superior electrical conductivity
and larger gravimetric and volumetric capacity. Thus, a high
capacity of 340 mAh g−1 at 0.2 C and ultrafast electrochemical
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Figure 2. Long-term cycling performance under high current densities for (a) a layered δ-Na0.44MnO2·H2O (NMOH) cathode prepared via
an oxidation followed by chemical pre-insertion method (inset showing the initial activation of the cathode within 50 cycles) and the same
cathode without structural water after thermal annealing at 500 °C (NMOH-500). Reprinted from ref 112. Copyright 2019 American
Chemical Society. (b) α-MnO2/CNT composite cathode prepared via chemical precipitation and spray granulation (inset showing the
hierarchically assembled microsphere particles with the nanofibrous α-MnO2 and CNTs-based network). Reproduced with permission from
ref 59. Copyright 2019 Elsevier. (c) V2O5 cathode prepared by ball milling commercial V2O5 powder and graphite and polyvinylidene
fluoride, respectively (inset shows electrode activation during initial 20 cycles). Reprinted from ref 113. Copyright 2018 American Chemical
Society. (d) Template-free hydrothermally synthesized VO2 cathode. Reproduced with permission from ref 85. Copyright 2019 Royal
Society of Chemistry.
optimized amount of water for facile and stable Zn
intercalation and the exact role of structural water in affecting
the electrochemical performance of hydrated layer-type
cathodes remain to be addressed. Too much water in the
interlayers can influence the reaction at the electrode/
electrolyte interface and promote the dissolution from the
cathodes based on manganese and vanadium oxides. Therefore, understanding the electrode/electrolyte interface reactions in such hydrated cathodes and the roles of accumulating
discharge byproducts, if any, need to be assessed. Further,
although great electrochemical improvements are reported in
hydrated vanadium oxide cathodes, the exact role of water
during the Zn insertion is still not determined. This can be
obtained by clearly understanding the reaction at the
electrode−electrolyte interface via sophisticated experimental
and theoretical approaches. Similarly, in the metal-ion preinserted layer-type cathodes, the layered framework undergoes
inevitable phase transitions that are reversible or irreversible
depending on various factors, including the amount of preinserted ions, electrolyte pH, and so on, are not well
understood. Hence, more studies on identifying the
transition-inducing reactions and their implications on the
electrochemical reaction are required by systematic investigations on pre-insertion of different charge carrier ions in
such layered-type cathodes. The challenges related to the
studies on oxygen or cation-deficient cathodes lie in the
determination of the exact amount of oxygen deficiency and
kinetics at 80 C with 78% capacity retention after 5000 cycles
in 1 M ZnSO4 electrolyte were registered.110
Defect engineering via creation of oxygen or cation vacancies
benefits electrochemical reactivity in cathodes supporting Zn2+
intercalation. The highly electrochemically active surface
related to the low Zn-adsorption free energy at the proximities
of the oxygen vacancies promote the formation of Zn−O
bonds with fewer electrons than in the vacancy-free
stoichiometric host. Hence, more electrons available for
electronic conduction implies higher specific capacity in the
oxygen-deficient manganese oxide cathode. The oxygendeficient MnO2 nanosheet cathode presented an energy
density of 470 Wh kg−1 while delivering specific capacity of
345 mAh g−1 at 0.2 A g−1, and more than 80% capacity is
retained after 2000 cycles at 5 A g−1.111 The electrochemical
reaction of another oxygen-deficient V2O5·nH2O with zinc
resulted in the formation of a mixed-valent hydrated vanadium
oxide (V10O24·nH2O, V5+/V4+ with molar ratio of 4) which is
also electrochemically active.109 On the other hand, the vacant
(transition metal) cation sites in the host lattice offer lower
electrostatic repulsion and hence enable more diffusion
channels to the incoming (intercalating) Zn2+-ions thereby
enhancing the Zn-ion conduction, charge transfer, and hence
electrochemical property of the cathode.19
Unfortunately, there are still lingering challenges that remain
to be addressed in the effective utilization of the discussed
strategies. For example, issues related to identifying the
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Table 1. Representative Mn-Based Electrodes and Their Electrochemical Performance Developments in ARZIBs from 2012 to
2020
cathode material
electrolyte
electrolyte
additive
α-MnO2
1 M ZnSO4
α-MnO2
1 M ZnSO4
α-MnO2−CNT
2 M ZnSO4
α-MnO2
1 M ZnSO4
α-MnO2
1 M ZnSO4
γ-MnO2
1 M ZnSO4
α-MnO2
1 M ZnSO4
V-doped α-MnO2
1 M ZnSO4
α-MnO2@C
1 M ZnSO4
N-CC@α-MnO2
2 M ZnCl2
β-MnO2
1 M ZnSO4
β-MnO2
α-MnO2@graphene
3M
0.1 M
Zn(CF3SO3)2
Mn(CF3SO3)2
2 M ZnSO4
0.2 M MnSO4
PANI-δ-MnO2
2 M ZnSO4
0.1 M MnSO4
MnO2
1 M ZnSO4
1 M MnSO4
δ-MnO2
1 M ZnSO4
0.2 M MnSO4
β-MnO2
3 M ZnSO4
0.2 M MnSO4
0.5 M MnSO4
0.4 M MnSO4
γ-MnO2
2 M ZnSO4
0.4 M MnSO4
α-MnO2-graphene
2 M ZnSO4
0.1 M MnSO4
α-MnO2@PPy
1 M ZnSO4
0.1 M MnSO4
MnO2−NHCSs (N-doped hollow
carbon sphere)
δ-MnO2
Ca0.28MnO2
3 M ZnSO4
0.15 M MnSO4
2 M ZnSO4
1 M ZnSO4
0.1 M MnSO4
0.1 M MnSO4
voltage
cyclability (mAh g−1)
ref
year
1.0−
1.8
0.7−
2.0
1.0−
1.9
0.7−
2.0
1.0−
1.8
1.0−
1.8
1.0−
1.8
1.0−
1.8
1.0−
1.8
1.0−
1.8
1.0−
1.8
0.8−
1.9
1.0−
1.9
1.0−
1.8
0.8−
2.2
1.0−
1.8
1.0−
1.7
0.8−
1.8
0.9−
1.8
1.0−
1.8
1.0−
1.8
0−2.0
0.4−
1.9
100 after 100 cycles at 6C (nC = a full discharge in 1/n h)
12
2012
140 after 30 cycles at 10.5 mA g−1
61
2014
100 after 500 cycles at 5000 mA g−1
114
2014
130 after 30 cycles at 42 mA g−1
55
2015
147 after 50 cycles at 83 mA g−1
58
2015
158 after 40 cycles at 0.5 mA cm−2
15
2015
104 after 75 cycles at 83 mA g−1
115
2016
131 after 100 cycles at 66 mA g−1
68
2017
189 after 50 cycles at 66 mA g−1
56
2017
262 after 1000 cycles at 1000 mA g−1
116
2017
135 after 200 cycles at 200 mA g−1
57
2017
135 after 2000 cycles at 6.5C (nC = a full discharge of 308
mA g−1 in 1/n h)
145 after 3000 cycles at 3000 mA g−1
60
2017
95
2018
280 after 200 cycles at 200 mA g−1
65
2018
1.67 mAh cm−2 after 1800 cycles at 60 mA cm−2
117
2019
175 after 1000 cycles at 3096 mA g−1
62
2019
71
2019
118
2020
160 after 300 cycles at 720 mA g−1
119
2020
85 after 100 cycles at 100 mA g−1
120
2020
100 after 2000 cycles at 2 A g−1
121
2020
125 after 6000 cycles at 5 mA cm−2
100 after 5000 cycles at 3.5 A g−1
122
123
2020
2020
133 after 1000 cycles at 4C
64.1% capacity retention after 300 cycles at 10 A g
−1
and high energy density, a structurally engineered layer-type δMnO2 with pre-included sodium ions and water molecules
showed prolonged cycling stability of 10 000 cycles with
retained discharge capacity of ∼106 mAh g−1 at 20 C, as
presented in Figure 2a.112 Compared to this, a high tap density
(0.45 g cm−3) α-MnO2/CNT composite formed via anchoring
α-MnO2 nanofibers to a highly conductive CNT showcased
0.98 Wh cm−2 aerial energy density at 0.2 A g−1 with a
remarkable capacity retention of 96% for over 10 000 cycles at
3 A g−1 in a (2 M ZnSO4 + 0.1 M MnSO4) electrolyte solution
(Figure 2b).59 Similarly, for the vanadium case, very recently,
Zhang et al. reported an interesting result using just
commercial V2O5 cathode in 3 M Zn(CF3SO3)2 electrolyte.
In the designed ARZIB, the shielding of Zn2+ ions from the
anion hosts by the co- intercalated H2O and the gradual
transformation of the cathode with solid disc-morphology to
porous ultrathin nanosheets upon cycling led to the effective
utilization of the active materials. A remarkable high energy
their role in obtaining optimized performance. Further, the
subtle differences in the characteristic structural features
between the deficient and stoichiometric compound implies
challenges in following dependable reaction procedures for the
preparation of such cathodes.
Overall, the progress in strategic electrode modification has
clearly shown that tunnel-type manganese oxide composites
with carbonaceous inclusions in MnSO4-contained ZnSO4
electrolyte solutions have led the way to achieving state-ofthe art cathode performance. In the case of vanadium oxides,
the structural engineering of layered materials showed
exceptional electrochemical abilities among vanadium cathodes. However, this perception has been recently challenged
and is changing, as seen by the equally competitive, if not
outperforming, electrochemical properties of the structurally
engineered layered-type manganese oxide and commercial
vanadium oxide (without any structural modifications)
cathode, respectively. From the viewpoint of long cyclability
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Table 2. Representative V-Based Electrodes and Their Electrochemical Performance Developments in ARZIBs from 2016 to
2020
cathode material
Zn0.25V2O5
LiV3O8
V2O5
V10O24
Na0.33V2O5
Na5V12O32
Na0.76V6O15
K2V8O21
Ag0.4V2O5
(NH4)2V10O25
NaV3O8·1.5H2O
KV3O8
Zn2V2O7
Fe5V12O39(OH)
Zn3V2O7(OH)2
V2O5
V5O12
K0.5V2O5
(NH4)2V4O9
Zn0.3V2O5
Mn0.15V2O5
V7O16
K2V3O8
δ-Ni0.25V2O5
LiV3O8
MgV2O4
electrolyte
1
1
3
3
3
2
2
2
3
3
1
2
1
3
1
3
3
1
3
3
1
3
3
3
3
2
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
ZnSO4
ZnSO4
ZnSO4
Zn(CF3SO3)2
Zn(CF3SO3)2
ZnSO4
ZnSO4
ZnSO4
ZnSO4
Zn(CF3SO3)2
ZnSO4 with 1 M Na2SO4 additive
ZnSO4
ZnSO4
Zn(CF3SO3)2
ZnSO4
Zn(CF3SO3)2
Zn(CF3SO3)2
ZnSO4
Zn(CF3SO3)2
Zn(CF3SO3)2
Zn(ClO4)2 in PC
Zn(CF3SO3)2
Zn(CF3SO3)2
ZnSO4
Zn(CF3SO3)2
Zn(TFSI)2
voltage
cyclability (mAh g−1)
ref
year
0.5−1.4
0.6−1.6
0.4−1.4
0.7−1.7
0.2−1.6
0.4−1.4
0.4−1.4
0.4−1.4
0.4−1.4
0.7−1.7
0.3−1.3
0.4−1.4
0.4−1.4
0.4−1.6
0.2−1.8
0.5−1.5
0.2−1.6
0.4−1.4
0.3−1.3
0.3−1.6
0.2−1.7
0.3−1.9
0.3−1.6
0.3−1.7
0.2−1.6
0.2−1.4
200 after 1000 cycles at 2400 mA g−1
∼140 after 65 cycles at 133 mA g−1
182 after 30 cycles at 100 mA g−1
∼135 after 500 cycles at 500 mA g−1
∼250 after 500 cycles at 200 mA g−1
∼200 after 100 cycles at 500 mA g−1
∼110 after 100 cycles at 500 mA g−1
∼220 after 50 cycles at 1000 mA g−1
216 after 2000 cycles at 5000 mA g−1
∼180 after 1000 cycles at 500 mA g−1
221 after 100 cycles at 1000 mA g−1
∼90 after 1000 cycles at 500 mA g−1
197 after 200 cycles at 300 mA g−1
200 after 50 cycles at 1000 mA g−1
101 after 300 cycles at 200 mA g−1
166 after 500 cycles at 588 mA g−1
346 after 100 cycles at 500 mA g−1
150 after 1500 cycles at 8000 mA g−1
328 after 100 cycles at 100 mA g−1
214 after 20000 cycles at 10 A g−1
153 after 8000 cycles at 10 A g−1
175 after 950 cycles at 2400 mA g−1
100 after 3000 cycles at 6 A g−1
100 after 1200 cycles at 5 A g−1
200 after 4000 cycles at 5000 mA g−1
128 after 500 cycles at 4000 mA g−1
124
16
94
109
74
73
77
76
76
125
80
126
92
91
127
128
129
130
131
132
133
134
135
136
137
138
2016
2017
2018
2018
2018
2018
2018
2019
2018
2018
2018
2018
2018
2018
2018
2019
2019
2019
2019
2019
2020
2020
2020
2020
2020
2020
density of 274 Wh kg−1 at 7100 W kg−1 power density
(calculated according to active cathode mass) and cycling
stability (∼400 mAh g−1 with 91.1% capacity retention for over
4000 cycles at 5 A g−1) were achieved, as presented in Figure
2c.113 In comparison, a layered-type hollow sphere VO2(D)
cathode that underwent an irreversible phase transformation to
a water-intercalated (V2O5·xH2O) active material achieved an
energy density of 125.1 Wh kg−1 at an extremely high 12 512.3
W kg−1 power density (calculated based on the cathode active
material mass) and notable cycling sustainability of 0.0023%
fading per cycle for 30 000 discharge−charge cycles in a lowcost, high concentration 3 M ZnSO4 electrolyte solution
(Figure 2d). 85 The unique morphology with hollow
architecture, large specific surface areas, and high porosity
features contributed to the exceptional performance.
In summary, the research on manganese and vanadium oxide
cathodes, since the rejuvenation of ARZIBs in 2012, has
significantly progressed in terms of overcoming the obstacles of
inherent electrical conductivity limitations and structural
instabilities to realize complete active material capacity
utilization. Tables 1 and 2 show the development of different
manganese oxide and vanadium oxide electrodes, respectively,
studied for ARZIBs and their performances over the years. It
can be seen from Table 1 that diverse types of MnO2 including
polymorphs and composites and ZnSO4-based electrolytes
with different salt/additive concentrations were used to
improve the electrode cycling capacity and stability performance from a few tens to hundreds and to some thousands of
discharge−charge cycles. Similarly, Table 2 illustrates the use
of various layered cathodes of vanadium oxides, the different
electrolytes, and their performances in ARZIBs to date.
Although the initial research was based on ZnSO4 electrolytes,
vanadium oxide cathodes were effective in combination with
Zn(CF3SO3)2 to realize long-term cycling capacity and stability
for even a few tens of thousands of discharge−charge cycles.
Although it is a nontrivial task, there is little doubt that the
advancements made thus far for ARZIB electrodes can be
converted into future commercial realities.
Electrochemical Reaction Mechanisms in Manganese and
Vanadium Oxide Cathodes. The electrochemical reaction
mechanism of ARZIBs has been one of the most heated
topics of discussion ever since it was introduced; however, Zn
insertion has been recognized for both manganese and
vanadium oxide cathodes. MnO2 was initially investigated for
ARZIBs, and different reaction mechanisms based on
intercalation were proposed for the different polymorphs. As
the α and δ polymorphs demonstrated high Zn insertion, the
electrochemical reactivity of these polymorphs was mainly
studied in the early stage. Zn intercalation in ARZIBs was first
proposed via the tunnels of α-MnO2 in an aqueous ZnSO4
electrolyte to form spinel ZnMn2O4 (ZMO) as12
Zn 2 + + 2e− + 2α MnO2 ↔ ZnMn2O4
(2)
Later, this reaction was confirmed to occur in a λ (spinel-type)
MnO2 cathode.139 However, subsequent studies revealed that
Zn intercalation in α (tunnel-type)58 and δ (layered-type)63
produced additional Zn-contained parent phases, and the
electrochemical reaction was rewritten as follows:
2Zn 2 + + 4e− + 3α(δ)MnO2
↔ α(δ)ZnMnO2 + ZnMn2O4
(3)
Further computational studies confirmed that Zn intercalation into α-MnO2 was thermodynamically feasible.13
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Unlike the case of other polymorphs, γ-MnO2 demonstrated
reversible Zn intercalation via a complex phase transformation
involving three coexisting phases, namely, ZnxMnO2, spinel
ZnMn2O4, and Znx-γ-MnO2.15
Oh et al. initially reported that Zn intercalation in α-MnO2
caused a reversible transition from tunnel-type to layered-type
Zn-buserite MnO2, an analogue of the chalcophanite,
ZnMn3O7·3H2O (with interlayer spacing of ∼10 Å), by
using real-time synchrotron X-ray diffraction (XRD) measurements during electrochemical cycling.55 They confirmed the
difficulty associated with the exact determination of the
unstable layered phase by using any ex situ measurement
because any drying treatment (≥60 °C) dehydrates the Znbuserite to a Zn-birnessite phase (with a smaller interlayer
width of ∼7 Å).61,140 Further, the in situ study confirmed the
spontaneously reversible hydrolysis reaction related to the
dissolution and precipitation of ZBS arising from the varying
pH of the aqueous solution as follows:
of MnOOH. Further, they concluded that the high
concentration of Zn on the discharged electrode surface
identified by TEM, nuclear magnetic resonance (NMR), and
STEM-EDS studies can be attributed to ZBS. The electrochemical reaction at the cathode can be written as follows:
(6)
MnO2 + H+ + e− ↔ MnOOH
(7)
↔ Zn4(OH)6 (SO4 ) ·x H 2O
(8)
The pre-inclusion of Mn (II) was important to form an
electrochemically active MnOx layer on the electrode surface
via electro-oxidation and thereby compensate the Mn2+
dissolution problem and drastic pH variation, thereby
demonstrating stable long-term cycling capacities (∼92%
capacity retention after ∼5000 cycles). However, the role for
Mn2+ additive ions was limited to sustaining long-term zinc
storage capacities because a small amount of additive was
involved (∼0.1 M MnSO4 additive). Contrary to Oh et al, this
and other studies contended that the contribution of ZBS
formation/dissolution to zinc storage performance is insignificant or attributed as a parasitic reaction.20,141 These studies
thus highlighted that one or more factors in the reaction
environments including electrolyte pH variation,97 metal-ion
dissolution from the cathode, conversion reaction via proton
insertion,20 or Zn2+ insertion motivated the need to clearly
understand the origins and the reactions of ZBS in addition to
identifying the role of Mn2+ ions in the electrochemical
reaction of MnO2.
Irrespective of the electrochemical reaction mechanism, the
gradual accumulation of remnant ZBS during repeated cycling
can hinder Zn2+ insertion, increase cell impedance, and
deteriorate long-term specific capacities in the MnO2 cathode.
However, the origin of the ZBS reaction was debated as Kundu
et al. attributed it to spontaneous parasitic reactions between
dissolved oxygen and the ionic species (Zn2+, OH−, and
SO42−) on the electrode surface and not a pH-dependent
reaction, as suggested earlier by Oh et al. Further, it was
claimed that the large amount of initial H+ insertion leaves a
high OH− ion concentration, thereby triggering the formation
of ZBS.141 Also, the ZBS formed in the initial cycle tends to
react with the dissolved Mn2+ ions from the electrode or the
electrolyte additive and form a Zn-buserite/birnessite type
phase, which was claimed to be both electrochemically active
and inactive, respectively, upon repeated cycling.142 Further,
the role of Mn2+-additive ions in the electrochemical reaction
became a topic of debate in the following studies. While some
researchers assigned a passive role of Mn2+ ions in the
electrolyte, some proposed the active participation of Mn2+additive ions, either as part or the whole of the active material,
in the electrochemical reaction.
In contrast to the existing mechanisms of pure Zn
intercalation or proton insertion, Wei et al. proposed a
combined process of H+ insertion followed by Zn2+ insertion in
an akhtenskite (ε)-MnO2 cathode electrodeposited directly
onto a carbon fiber from the electrolyte solution.143 To
support this, they exploited the ambiguity in explaining the
reaction occurring at upper (until ∼1.3 V) and the lower (until
∼1 V) sloping plateaus in a typical discharge profile of MnO2
(a typical discharge−charge curve is shown in Figure 3a). They
(4)
where the degree of hydrolysis (h) is dependent on the pH.
The higher (than unity) value of h (∼1.9) for a pH of 4
supported that Zn2+ ions in aqueous solutions typically
coordinate with more than one water molecule (such as
[Zn(H2O)6]2+). Using galvanostatic intermittent titration
technique (GITT), it was determined that the chemical
diffusion coefficient during early discharge corresponding to
the sloping profile of a single-phase domain (∼10−16−10−15
cm2 s−1) was greater than the following plateau related to a
two-phase domain (∼10−18−10−17 cm2 s−1). Under prolonged
cycling, the parent cathode was observed to transform to an
amorphous phase, thus leading to reduced electrochemical
reactivity and low zinc storage under long-term cycling. In
contrast to this intercalation-based mechanism, they later
proposed a new conversion-type reaction based on the
reversible precipitation/dissolution of ZBS due to pH variation
during the discharge−charge reaction. A specially fabricated
electrochemical cell set up to monitor real-time pH was
developed, and the assessment of discharged electrolyte
constituents using atomic absorption spectroscopy (AAS)
was performed to confirm this reaction. According to them, the
increased pH due to the high concentration of Mn2+ in the
electrolyte arising from manganese dissolution (Mn3+ →
Mn2+) during discharge cycling facilitated ZBS precipitation.
In reverse, upon charging, the lowering of pH related to the
recombination of oxidized manganese promoted ZBS dissolution. This pH-dependent conversion reaction was claimed
to occur on the surface of any MnO2 polymorph and is shown
below.97
3MnO2 + 8Zn 2 + + 2SO4 2 − + 16H 2O + 6e−
↔ 3Mn 2 + + 2[Zn4(OH)6 (SO4 ) ·5H 2O]
H 2O ↔ H+ + OH−
3Zn 2 + + 6OH− + ZnSO4 + x H 2O
[Zn(H 2O)6 ]2 + + H 2O
↔ [Zn(H 2O)6 (OH)h ](2 − h) + + hH3O+
Review
(5)
Meanwhile, another conversion-type reaction between
MnO2 and MnOOH via reversible proton insertion in the
presence of an Mn2+ salt-contained ZnSO4 electrolyte was
proposed by Pan et al.20 Scanning transmission electron
microscopy combined with energy dispersive X-ray spectroscopy (STEM-EDS) mapped the dominating distribution of Mn
and O elements compared to that of Zn in the bulk of the
discharged product, and their XRD study proved the formation
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spinel-type ZnMn2O4 phase (eq 2). Supporting this theory, a
recent study assigned the consistency of the upper plateau
upon cycling of α-MnO2 at various current densities to the
dominant role of the reversible H+-insertion reaction. Under a
specific cycling condition, the declining trend of the lower
plateau potential was attributed to the reduced contribution of
Zn2+ intercalation (ZnMn2O4) to the electrode storage
capacity.141 However, this study slightly differed with Wei et
al. in establishing the product of H+-insertion. Compared to
the identification of MnOOH as the H-inserted product by
Wei et al., this study could not observe any such phase
formation in their real-time XRD patterns at high discharge
potentials except for the attenuation of the parent MnO2 peaks
and ZBS formation. This study argued that the ZBS formation
was in itself a strong evidence of H+ insertion. Supporting
these findings except for the formation of ZnMn2O4, very
recently, pure H+ insertion via solid-solution type reaction and
causing no structural transition throughout the decreasing
discharge potential was used to explain the reaction
mechanism in MnO2.144 Their operando XRD patterns were
free from new phases (MnOOH or ZnMn2O4) excepting the
parent MnO2 and ZBS phases. Further, ex situ TEM could not
detect the presence of zinc on the discharged electrode, leading
to the conclusion that the reaction mechanism is as follows:
Within a decade of ARZIB development, the highly debated electrochemical mechanisms for manganese
oxide cathodes have evolved from pure
Zn2+ intercalation to H+ conversion
reactions to consecutive/concomitant
Zn2+/H+ co-intercalation to mixed intercalation/conversion reactions to dissolution/deposition mechanisms with
or without structural transitions, respectively, whereas the less debated
storage mechanism for vanadium oxides revolves around Zn2+ intercalation
or Zn2+/H+-co-intercalation reactions
through either a consecutive or concomitant process.
reasoned that the higher diffusion coefficient (obtained by
GITT measurements) at high discharge potentials (∼1.8−1.3
V) corresponds to fast reaction kinetics associated with the
insertion of smaller-sized H+ ions to form MnOOH (eq 7), as
observed from ex situ XRD results. The following lower
discharge potential curve (∼1.3−1.2 V) with low diffusion
coefficient was attributed to the slower reaction kinetics
attributed to the large-sized Zn2+-ion insertion to form a
MnO2 + H+ + e− ↔ HMnO2
(9)
Contrary to these propositions, Mai and co-workers
assumed only a passive role for the Mn2+ additive ion and
pinned the upper and lower discharge potential curves to
highly reversible Zn intercalation occurring at two different
Figure 3. Schematic diagram illustrating the various electrochemical regulations in manganese oxide-based cathodes for ARZIB applications.
(a) Typical galvanostatic discharge−charge curve and (b) consecutive ion-insertion mechanism. Reproduced with permission from ref 145.
Copyright 2019 Springer Nature. (c) Concomitant ion-insertion mechanism. Reprinted from ref 62. Copyright 2019 American Chemical
Society. (d) Dissolution−deposition mechanism. Reproduced with permission from ref 142. Copyright 2020 Elsevier.
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insertion by Kang and co-workers. They used ex situ XRD and
HR-TEM studies to demonstrate a Zn2+ intercalation, similar
to eq 10, at high discharge potentials (>1.4 V); the only
difference, in this case, is that the reaction occurs via a “nondiffusion controlled” or pseudocapacitive intercalation process.
Further, the reaction at low discharge potentials (<1.4 V) was
ascribed to the usual diffusion-based H+ insertion (eq 7). As a
consequence of this reaction, the formation of MnOOH and
Zn(TFSI)2[Zn(OH)2]3·xH2O, the analogue to ZBS in ZnSO4
electrolytes, was confirmed by ex situ XRD.
Apart from these mechanisms, the H+/Zn2+ co-intercalation
mechanism that occurred concomitantly or simultaneously
throughout the discharge reaction process was also presented.
However, the predominance of H+ insertion and Zn 2+
insertion, corresponding to the upper and lower discharge
potentials, respectively, in the discharge profile of a δ-MnO2
cathode, was ascertained by using ex situ Raman analysis.65 At
decreasing depths of discharge (DOD), Zn−O vibrations were
identified. Also, ex situ STEM-EDS confirmed the predominant
mapping of Mn and Zn on the electrode surface at the upper
and lower discharge potentials, respectively. More recently, the
idea of mixed H+/Zn2+ co-intercalation and the H+/Zn2+ coconversion mechanism in an electrodeposited birnesstie-type
layered MnO2 was proposed by Liu et al.62 They opined that
H+/Zn2+ co-intercalation yielding MnOOH and ZnxMnO2
phases and H+/Zn2+-conversion reaction leading to Mn3O4,
MnO, and layered ZnMn3O7·H2O products could describe the
different reaction kinetics at the upper and lower discharge
plateaus, respectively, of the corresponding electrochemical
profile (Figure 3c). Studies of ex situ XRD patterns at various
DODs for different current densities revealed the overlapping
of diffraction lines pertained to different phases, and the most
possible phases were reasoned out with support from X-ray
fluorescence (XRF), synchrotron X-ray absorption spectroscopy (XAS), and electron microscopy studies. Finally, density
functional theory (DFT) simulations for the calculation of the
potentials related to the two different redox reactions and their
formulations were performed:
H+/Zn2+ intercalation reaction (∼1.4 V)
crystallographic locations: the layered channels of a transformed Zn-buserite (B−Zn x MnO2·nH2O) at the surface
followed by the tunnels of the parent α-MnO2 structure in the
core of the host.95 Later, Kang and co-workers considered an
active role for Mn2+ additive ions and contradicted Wei et al.
by asserting that the high discharge potential domain (>1.4 V)
corresponds to the formation of Zn2+-intercalated parent phase
followed by proton (H+) insertion and active Mn 2+participation (from the electrolyte additive) at low discharge
potentials (<1.4 V). According to them, only slight shifts in the
parent diffraction lines (and no new phases) in the ex situ XRD
at high discharge potential were observed. They excluded the
possibility of water adsorption using a separate experiment of
immersing the electrode host in water. Also, they used
thermodynamic analysis based on density functional theory,
customized electrochemical measurements, and TEM studies
to conclude the reaction at high discharge potentials as
reversible Zn insertion as follows:145
α MnO2 + x Zn 2 + + x e− ↔ α ZnxMnO2 (> 1.4 V)
(10)
The lower discharge potential domain was explained based on
reversible H+ insertion (eq 7), ZBS formation (forward
reaction in eq 8), and an additional reversible conversion
reaction to Mn2O3 as follows (<1.4 V) (Figure 3b):
2MnO2 + 2H+ + 2x e− ↔ Mn2O3
(11)
According to this study, the structure of Mn2O3 was
observed by TEM to be semicoherent with that of α-MnOOH
and can be the reason for the complexity in its identification.
From the first charge cycle onward, the conversion reaction
between ZBS, continuously oxidizing Mn2+, and water yielded
nanoflower-type layered ZnMn3O7·xH2O with a Zn-buserite
structure. Equation 12 was established using a combination of
ex situ XRD, electron microscopy, and comparative electrochemical studies of cells in additive-free and additive-rich
(∼0.5 M MnSO4) electrolytes, respectively.
3(ZnSO4 ·3Zn(OH)2 ·5H 2O) + 3Mn 2 + + 8e−
↔ ZnMn3O7 ·3H 2O + 3ZnSO4 + 18OH− + 8Zn 2 +
+ 12H 2O
84MnO2 + 33Zn + 10ZnSO4 + 100H 2O
(12)
↔ 60MnOOH + 24Zn 0.125MnO2
This finding of a Zn−Mn layered buserite structure was
supported by another study that described a reversible twostep reaction mechanism for ZBS cathode (instead of MnO2)
in an Mn2+-added electrolyte as follows:146
+ 10[Zn4(OH)6 (SO4 ) ·4H 2O]
+
H /Zn
conversion reaction (∼1.26 V)
+ Zn4(OH)6 (SO4 ) ·4H 2O
(13)
+ 4x H
(16)
Contrary to the well-known intercalation or conversion
reactions, a predominant reversible MnOx deposition formed
from the pre-included Mn2+ additive ions on the electrode
surface contributing to active zinc storage was proposed very
recently. This phenomenon supported by apparently lowcapacity contributions from reversible Zn2+ intercalation in the
bulk and Zn2+ insertion in the undissolved surface MnOx layer,
respectively, was demonstrated in a spinel type ZnMn2O4
cathode as follows.148
Step 2: 7ZnMn2O4 + (5 + 2x)H 2O
+ 14e−
(15)
↔ 5Mn3O4 + 3MnO + 2[ZnMn3O7 · 2H 2O]
↔ 3ZnMn2O4 + 4SO4 2 − + 13Zn 2 + + 32H 2O + 6e−
↔ ZnxMn 7O13 + y · 5H 2O + (7 − 2x)Zn
2+
8Zn 0.125MnO2 + 16MnOOH + 4Zn + ZnSO4 + 3H 2O
Step 1: 4Zn4(OH)6 (SO4 ) ·5H 2O + 6Mn 2 +
2+
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+
(14)
The mediocre electrochemical reactivity of Zn-buserite was
found to deteriorate the long-term performance of the ZBS
cathode. However, Jin et al.,147 studying the electrochemical
reaction of δ-MnO2 in an aqueous electrolyte different from
the conventional ZnSO4 -based solutions (zinc di(bis(trifluoromethylsulfonyl)imide) or Zn(TFSI)2) supported the
concept of Zn2+ insertion followed by the consecutive H+
2+
Mn(electrolyte)
+ x H 2O ↔ MnOx(surface) + 2x H+ + 2e−
(17)
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ZnMn2O4(bulk) ↔ Zn(1 − y)Mn2O4 + y Zn 2 + + y 2e−
deintercalation capacity. Negligible capacities of 2.1 and 26.6
mAh g−1 were registered for αMnO2 and δMnO2, respectively.
This observation led to the conclusion that the capacity
contribution to H+/Zn2+-intercalation was negligible. Further,
after acid leaching of the fully discharged electrode to remove
the ZBS, the electrode upon fresh initial charging in ZnSO4
electrolyte revealed negligible capacity due to the absence of
ZBS. Upon repeated cycling, a steady increase in capacity was
observed, as expected. As more ZBS formation consumed
more water molecules, a lack of water molecules to react with
MnO2 occurred. In other words, the generation of ZBS during
discharge cycling suppressed MnO2 dissolution. This work
thus highlighted the importance of ZBS formation in affecting
the reaction kinetics and hence the electrochemical reaction
governing MnO2 cathodes of ARZIBs. Further studies using
(18)
2+
MnOx(surface) + Zn(electrolyte)
+ 2e− ↔ ZnMnOx(surface)
(19)
This reaction mechanism was formulated based on the
evidence gathered from studying the electrochemical profile
curves for the ZnMn2O4 cycled under three different
electrolyte solutions comprising ZnSO4, ZnSO4 + MnSO4,
and MnSO 4 solutions. The electrode in pure MnSO 4
electrolyte demonstrated a very high initial charge−discharge
capacity (396/218 mAh g−1 compared to 168/175 mAh g−1
and 119/124 mAh g−1 in Mn2+-contained ZnSO4 and pure
ZnSO4 electrolytes, respectively) because of the unique
behavior in the corresponding electrochemical curve. An
extended charge plateau between 1.8 and 1.9 V (equivalent to
∼200 mAh g−1) could be attributed to widespread MnOx
deposition from the Mn2+ ions in the electrolyte. However, the
electrode in Mn2+-contained ZnSO4 electrolyte demonstrated
increasingly high reversible capacities (maximum of 280 mAh
g−1), cycling efficiency, and sustained long-term cycling (∼200
cycles) compared to the poor reversibility of the MnOx
deposition leading to severe capacity fade in MnSO 4
electrolyte after the initial several cycles (∼50 cycles). Further,
lower specific capacities and differences in the deposition/
dissolution efficiency for a Super P carbon electrode in the
three different electrolytes was confirmed. This implied that, in
addition to MnOx deposition, reversible Zn intercalation in the
active bulk (ZnMn2O4) and undissolved MnOx on the
electrode surface also contributed to the energy storage
reaction.
Meanwhile, another study highlighted a dissolution−
deposition reaction that dominated over the H+/Zn2+ coinsertion reaction for MnO2 cathodes.142 In the initial
discharge reaction, the disproportionate reaction to Mn2+
occurs.
3MnO2 + 6H 2O + 6e− → 3Mn 2 + + 12OH−
Further studies using innovative or
sophisticated techniques followed by
detailed analysis and discussion are
urgently required to bring clarity to the
complex and divergent theories explaining the mechanism reactions in
ARZIBs.
innovative or sophisticated techniques followed by detailed
analysis and discussion are urgently required to bring clarity to
the complex and divergent theories explaining the mechanism
reactions in ARZIBs.
Zn intercalation in vanadium oxide cathodes, mainly layeredtype V2O5 cathodes, is not as complicated as in manganese
oxides. Conventionally, Li intercalation in crystalline V2O5
occurs in multiple stages representing solid-solution or singlephase reaction and topotactic two-phase reactions, respectively,
and is similar for the case of Zn intercalation. However,
distinctions exist in the limit of intercalation or intermediate
phases formed and their stoichiometry due to varying factors
including the electrode structure, the internal constituents
depending on the presence of interstitial ions or molecules, and
the parameters of the intercalating ion. Zn0.25V2O5·xH2O, a
vanadium oxide cathode, demonstrated a reversible Zn
intercalation reaction as shown below:124
(20)
−
They claimed that the OH released after dissolution reacts
with ZnSO4 to form ZBS (eq 8). This can result in water
deficiency and increase the solution pH, thereby suppressing
further MnO2 dissolution. During initial charging, they
highlighted that the Mn2+ reacts with ZBS (unlike Mn2+ and
H2O proposed in eq 17) and deposits as a birnessite MnO2 on
the cathode (forward reaction in eq 21). Upon further
discharge−charge cycling, the dissolution−deposition reaction
continues between the birnesstie (ZnyMnO2) and Mn2+ and
ZBS, respectively (Figure 3d).
1.1Zn 2 + + 2.2e− + Zn 0.25V2O5 ·x H 2O ↔ Zn1.35V2O5 ·x H 2O
(22)
Quantitative estimation of elemental constituents via ex situ
inductively coupled plasma optical emission spectroscopy
(ICP-OES) and EDS studies after discharge and charge
cycling, respectively, confirmed the stoichiometry of the final
Zn-intercalated product. Operando XRD investigations concluded that the Zn intercalation proceeded via two singlephase or solid-solution domains separated by a two-phase
transition corresponding to Zn0.55V2O5. Further, the sharp
contraction/expansion of the interlayer width for the narrow
zinc composition window (∼Zn0.55V2O5−Zn0.6V2O5) during
the Zn-intercalation/deintercalation process, respectively,
corresponded to the expulsion and uptake of structural water
from the electrode (Figure 4a). Zn intercalation via solid
solution and a two-phase reaction was also confirmed in a new
Ca0.24V2O5 cathode subsequently.110 Another LixV2O5 cathode
prepared by chemical Li-intercalation was observed to
demonstrate multistep Zn insertion via Zn0.25V2O5 and
3Mn 2 + + 12OH− + y Zn 2 + − (6 − 6y)e−
↔ 3Zn yMnO2 + 6H 2O
Review
(21)
This electrochemical reaction mechanism was confirmed by
studying the similar electrochemical behaviors in the respective
electrochemical charge-discharge (ECD) curve, cyclic voltammetry (CV), and GITT analysis of three different cathodes,
αMnO2 , δMnO 2, and ZBS, respectively. Further, this
observation suggested a common reaction mechanism in
Mn(II)-contained electrolytes of ARZIBs. In addition, after the
MnO2 was discharged close to its maximum practical capacity
(∼0.8 V) in MnSO4-added ZnSO4 electrolyte, the electrode
was freshly charged in ZnSO4 solution to eliminate the
dissolution capacity contribution and assess pure Zn+/H+2387
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Review
Figure 4. Schematic diagram illustrating the various electrochemical regulations in vanadium oxide-based cathodes for ARZIB applications.
(a) Exclusive Zn2+ (de)intercalation mechanism. Reproduced with permission from ref 124. Copyright 2016 Springer Nature. (b) Zn2+
(de)intercalation from the transformed product-related mechanism. Reproduced with permission from ref 152. Copyright 2019 Royal
Society of Chemistry (c) Displacement−intercalation mechanism. Reproduced with permission from ref 76. Copyright 2018 Elsevier.
reversible H+ insertion. Concomitantly, the unique and
consistent binding energy peak for inserted Zn2+ in the Zn
XPS spectra confirmed the role of reversible Zn2+ insertion.
Further, upon performing the ICP analysis of the discharge−
charge end products using the basic understanding of
correlating their values with corresponding specific capacities,
the proton contribution (83%) was observed to be higher than
the Zn2+ ion (17%) in the co-intercalation process. The
concomitant H+/Zn2+ intercalation process was supported by
Wang et al.,132 who studied a Zn0.3V2O5·1.5H2O cathode (the
analogue was used to demonstrate exclusive Zn2+ intercalation
initially) in aqueous and nonaqueous Zn(CF3SO3)2 electrolyte
solutions and compared their electrochemical behaviors. In
addition, they confirmed the reversible precipitation of
Zny(CF3SO3)x(OH)2y−x·nH2O or zinc basic triflate (ZBT),
the equivalent of ZBS in ZnSO4 electrolyte solutions. The
formation of ZBT was confirmed by other research groups
studying manganese- and vanadium-based electrode reactions
in Zn(CF3SO3)2 solutions.144,149 It was observed that the
precipitation of ZBT is not predominant as observed for ZBS
because of the low degree of crystallinity observed in the XRD
of the discharged electrode in the former case. Also, both the
ZBS and ZBT precipitates formed during the reaction tend to
act as a buffer and prolong the stabilization of mildly acidic pH
solutions, thereby facilitating greater proton insertion and
Zn0.29V2O5 phases using a combination of ex situ XRD and
TEM investigations.78 Concurrently, the possible existence of
more than one phase transition during the discharge reaction
with zinc for a stoichiometric LiV3O8 cathode was suggested in
another study, citing their analyses of in situ synchrotron XRD
and ex situ XAS patterns.16 Interestingly, the charge
mechanism followed a solid-solution behavior supporting the
trend observed for Li deintercalation from the fully intercalated
cathode (Li4+xV3O8). In a separate study of a V2O5 cathode
coated with a polymer (PEDOT) coating, ex situ XRD
confirmed the formation of Zn0.29V2O5 as the final intercalated
product.106 Although the prospect of a Zn2+ intercalation was
acknowledged in all these studies, the role of protons in the
electrochemical reaction was not investigated or remained
elusive. However, later, Wan et al.80 demonstrated that, in
addition to Zn intercalation, simultaneous proton intercalation
occurred in a NaV3O8·1.5H2O cathode in 1 M ZnSO4 + 1 M
Na2SO4 electrolyte solution. Analyses of ex situ XRD, 1H
NMR, and X-ray photoelectron spectroscopy for various DOD
and states of charge (SOC) confirmed this reaction
mechanism. The reversible formation of ZBS (eq 8) was
acknowledged using ex situ XRD. In particular, the same trend
of the characteristic ZBS diffraction lines with varying
intensities followed for the additional 1H NMR peak at 2.7
ppm during discharge−charge reactions, thereby confirming
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similar mechanism was also observed in V2O5 based on the
referencing of the pyrovanadate by XRD measurements.128 It is
worth noting that this transformed complex oxide phase can be
frequently mismatched with ZBS through X-ray diffraction
analysis, thereby leading to further complications in understanding the electrochemical ingress/egress mechanism. Other
than intercalation with or without structural transformation, a
displacement-cum-intercalation reaction in vanadate cathodes
like Mg0.34V2O5153 and Ag0.4V2O576 was introduced for
ARZIBs. For example, in the latter case, the Ag+ sites in
Ag0.4V2O5 are displaced with Zn2+ to form a new phase of
Zn2(V3O8)2, in addition to the regular intercalation during
discharge reaction, thereby leading to ZnxAg0.4V2O5 as shown
in Figure 4c. The charge process exactly follows the discharge
path, resulting in a reversible electrochemical reaction via a
displacement/intercalation mechanism.
Overall, ever since the reintroduction of the mildly aqueous
Zn−MnO2 system, the research and understanding on the
electrochemical reaction mechanism in ARZIBs have progressed significantly. The energy storage process was, in the
early stages, attributed to mainly Zn-intercalation reaction with
or without structural phase transitions until the introduction of
an Mn2+-additive in the aqueous ZnSO4 electrolyte solution for
ARZIBs. Utilizing this additive-included electrolyte to improve
the electrode cycling performance, the role of a proton
insertion mechanism via conversion reaction was highlighted.
Two types of conversion reactions via H+ insertion or
structural phase transitions also have been described, in
general. Following this, H+/Zn2+-co-intercalation occurring
either concomitantly or consecutively was considered. In
addition, mixed intercalation/conversion reaction possibilities
and the recent deposition/dissolution reactions have been
realized. It is highly likely that these discrepancies depend on
various factors such as cathode crystal structures as well as
their preparation techniques and defects, electrolyte types,
additives, and their concentrations. In contrast, the electrochemical mechanism in vanadium oxide-based cathodes is a
major (de)intercalation process predicted through available
analyses, including operando X-ray analysis. Despite this, there
is still a debate on the intercalation reaction mechanism. More
work is required to clarify the underlying mechanisms to scaleup the performance of vanadium-oxide cathodes. In short,
within a decade of ARZIB development, the highly debated
electrochemical mechanisms for manganese oxide cathodes
have evolved from pure Zn2+ intercalation to H+ conversion
reactions to consecutive/concomitant Zn2+/H+ co-intercalation to mixed intercalation/conversion reactions to dissolution/deposition mechanisms with or without structural
transitions, respectively, whereas the less debated storage
mechanism for vanadium oxides revolves around Zn2+intercalation or Zn2+/H+-co-intercalation reactions through
either a consecutive or concomitant process. Each of these
proposed theories has a unique approach and shares very few
common traits, including the byproducts formed and the cause
of these reactions. Therefore, further studies using innovative
or sophisticated techniques followed by detailed analysis and
discussion are urgently required to bring clarity to the complex
and divergent theories explaining the mechanism reactions in
ARZIBs.
Other Aspects Related to Zn Dendrites and Hydrogen Evolution
on Zn Anode. Aqueous batteries are well-known for being costeffective, safe, and easy to assemble. Nevertheless, aqueous
batteries face major difficulties related to H2/O2 evolution,
hence greater storage capacities. It was also concluded that the
stability of these precipitates is crucial for the rechargeability
and hence the reaction mechanism in manganese/vanadium
oxide cathodes.144 However, Wang et al. combined STEMEDS results and the attained specific capacity (equivalent to
the number of electrons transferred) during the discharge
reaction to estimate that the proton contribution (44%) was
lower than the contribution from Zn2+ (56%); the result was
contrary to the proton-dominant contribution observed for the
NaV3O8·1.5H2O cathode by Wen et al. The electrochemical
reactions describing the co-intercalation process were as
follows:132
0.75Zn 2 + + Zn 0.3V2O5 ·1.5H 2O + 1.17H+ + 2.67e−
↔ H1.17Zn1.05V2O5 ·1.5H 2O
(23)
0.585Zn 2 + + ax /2Zn(CF3SO3)2 + 1.17OH− + an5H 2O
↔ aZn y(CF3SO3)x (OH)2y − x ·nH 2O
Review
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This study strongly opened the path to further understanding the electrochemical reaction mechanism in vanadium
oxides for ARZIBs. Contradicting the concomitant intercalation, Liu et al.150 suggested a consecutive mechanism in which
Zn-ion insertion precedes H+ insertion corresponding to the
upper (∼1.5−0.8 V) and lower (∼0.8−0.5 V) discharge
potential domains for a V10O24·12H2O cathode, as in the case
for manganese oxides. The higher diffusion coefficients
estimated by using GITT studies for the upper potential
regime were attributed to Zn-ion insertion, while the slower
reaction kinetics in the lower potential region was ascribed to
successive insertion of protons. They argued that the low
diffusion kinetics of proton insertion was related to its lower
concentration (≪2 M) compared to that of zinc concentration
(∼2 M). In other words, a slight reduction in proton
concentration during proton insertion can further lower the
reaction kinetics of protons because of the limited availability.
Also, they confirmed proton insertion using electron
microscopy to show the morphological variations (of flakelike particles) related to ZBS/counterpart precipitation in
different electrolyte solutions occurred in the lower discharge
potential region. Another layered polymorph of this vanadium
oxide cathode (V3O7·H2O) was initially reported to show
exclusive Zn2+ ingress/egress, but later, under similar electrochemical conditions, it was corrected by in-depth studies that
oxide cathodes including V3O7·H2O experience predominant
H+ ingress/egress mechanisms.144,151 Electrochemical curves
combined with operando XRD studies and ex situ STEM
imaging with EDS mapping confirmed the early intercalation
of Zn2+ corresponding to Zn0.6V3O7·H2O, which makes for
only 30% of the capacity contribution. However, the later surge
of the ZBS phase during discharge reaction combined with
molecular dynamics simulation suggested the facilitation of
proton insertion via the surface reconstruction by H +
stabilization on the hydroxyl-terminations at the oxide−water
interface. This mechanism supported the consecutive insertion
of Zn2+ and H+, respectively, as proposed earlier by Liu et al.150
Apart from the regular intercalation mechanism in vanadium
oxides, the transformation of a layered NH4V4O10·nH2O
cathode to a zinc pyrovanadate Zn3V2O7(OH)2·2H2O phase
in the initial discharge cycle followed by reversible Zn2+
deintercalation from the transformed product for the following
charge/discharge cycling was introduced (Figure 4b).152 A
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Despite the discussion of HER related to the anode side, the
oxygen evolution reaction (OER) associated with ARZIBs is
less studied. Interestingly, Bischoff et al.,168 on the other hand,
controlled the potential window through visualized pH
variation and hence OER via in situ pH technique by adding
an optimized pH tablet in a 2 M ZnSO4 + 0.1 M MnSO4
electrolyte of the Zn−MnO2 battery within the standard
potential region of 0.9−1.9 V vs Zn2+/Zn. At high working
potentials (>1.7 V vs Zn2+/Zn), a drastic change in local pH
was envisaged by corresponding color change, thereby causing
an oxygen evolution reaction (OER). This observation
indicated that the overall pH of the electrolyte gradually
increased for increasing cycle numbers and suggested that this
variation triggered the formation of ZBS at pH 5.2 in the initial
cycle and the formation of ZnO dendrites on the anode side
whenever the pH exceeds 5.6 under cycling. Furthermore,
employing diluted H2SO4 and surfactant separately in the
electrolyte could be a few other options for controlling the gas
evolution. Controlling the pH by taking high concentration of
Zn and Mn salts in the electrolyte without affecting the pH
significantly is an attractive prospect. In other words,
optimizing the electrolyte concentration without taking the
pH to the more acidic region is recommended.
The conclusions of the research to date regarding these
promising materials are listed below:
(1) The attraction of manganese oxides and vanadium
oxides as cathodes for ARZIBs stems from their different
crystal structures, especially those based on layered and tunnel
frameworks, with spatial geometric dimensions offered for
reversible Zn insertion. Manganese oxide cathodes of ARZIBs
present moderate average potential (>1 V) and high energy
density with less cycling stability. In comparison, vanadium
oxide cathodes have low average working potential (<1 V),
high power density, and high cycling stability. However, in
certain geometries, there were issues of detrimental macroscopic phase transitions to form intermediate products or new
end (by)products, which are dependent on various factors,
including type of crystal structure, synthesis route, particle
morphology, and electrolyte choice.
(2) Various performance-enhancement strategies conducted
on manganese and vanadium oxide cathodes are focused on
utilizing different structural polymorphs, surface modifications,
composite formation, structural engineering via pre-inclusion
of structural water, metal ions/polymers, and defect engineering via composition tuning and studying new materials and
concepts. It is worth noting that while the effects of
nanostructuring and composite formation are well-known,
ambiguities in understanding the effectiveness of structural and
defect engineering of transition metal oxide cathodes need to
be cleared up.
(3) Progressive research thus far has identified that the
strategies of electrode nanostructuring and compositing with
electrically conducting materials combined with pre-included
additives is successful, at least to a certain extent, to overcome
the electrical and stability limitations in manganese oxide and
improve their electrochemical storage abilities and cycling
stabilities. Among the different electrical additives researched,
the usage of CNT and graphene seem to be better, although
various other additives are still being researched.
(4) For the layered materials of more stable vanadium oxide
cathodes, electrode nanostructuring combined with preintercalation of crystalline water and/or metal-ion doping,
especially in layered framework materials, are the most
dendrite formation on anode surface, and other notable side
reactions induced by pH change. Specifically, the accompanying unwanted side reactions are detrimental for long-term
cycling. Hence, these drawbacks need to be addressed for
aqueous batteries, and the ARZIB is not an exception.
Unlike in Li metal, the standard reduction potential of Zn
metal is quite high, and hence, it is relatively stable in both
nonaqueous and aqueous electrolytes. The spontaneous layer
formation (solid electrolyte interphase (SEI)-like) with
electrolyte in Li-based systems is higher because of the
stronger oxidizing behavior of lithium metal than in the case of
Zn-based systems. This added advantage of Zn with high
theoretical capacity (820 mA h g−1, 5854 mA h L−1) in its
metallic state could be useful for high energy/power density
batteries. Reversible Zn stripping−plating progresses well in
mild acidic aqueous electrolyte with pH around 4−6; however,
under long-term cycling, the reversibility is affected because of
the dendrite formation Zn(OH)2 and/or ZnO arising from the
local pH change in the electrolyte.12,154 This implies that the
nature of the electrolyte salt and pH are crucial for reversible
Zn deposition/dissolution. Among the available zinc salts, both
ZnSO4 and Zn(CF3SO3)2 salts facilitate better performance.19
In addition to using appropriate salts, numerous efforts to
protect the Zn metal from dendrites and passivated layer
formation by including electrolyte additives have been
employed. These include the addition of surfactants155 and
bioionic liquid;156 creating a deep eutectic mixture;157 or
amine-, amide-, and phosphate-based additives to the electrolyte.158−160 For example, an electrolyte solution made from the
combination of triethyl phosphate (TEP) and low-concentration zinc salt (0.5 M Zn(CF3SO3)2 in TEP:water (7:3))
facilitates the evolution of a well-connected 3D porous
network Zn anode instead of the commonly observed
dendrites phase under continuous cycling, thereby enhancing
the reversibility and Coulombic efficiency.159 Surface coating is
yet another widely used technique to improve Zn metal
efficiency. The coating, in general, is achieved using carbonaceous materials including graphene oxide; inorganic materials
such as CaCO3,161 TiO2,162 3D-ZnO,163 and Au nanoparticles164 or conducting polymers.165 Atomic layer deposition (ALD) is very effective to form a very thin surface coating
layer of TiO2 that can act as a good passivation layer to avoid
direct contact between bare Zn metal and electrolyte, thereby
greatly suppressing gas evolution at the interface, as evidenced
by the reduced formation of Zn(OH)2 on the anode side due
to the hydrogen evolution reaction (HER).162
Besides dendrite formation, the narrow electrochemical
window (1.23 V) due to the O2/H2 evolution above/below the
limit is another drawback in aqueous batteries including
ARZIBs. To overcome this and ensure electrochemical window
stability, the pH of the electrolyte is crucial. In other words, the
pH−potential (known as Pourbiax) diagram is essential to
regulate the stability window and hence control the H2/O2 gas
evolution. In order to adjust the stability of the water
electrochemical window with controlled H2/O2 evolution
reactions, few studies have been devoted to the utilization of
water-in-salt166 and surfactant-assisted167 electrolytes; both
enhance the potential windows and will be an area of research
in the near future. Similarly, surface protection through coating
with polymer, inorganic metal oxides, and carbonaceous
materials is frequently used to suppress the HER on the
anode side.
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Review
byproduct formation and their accumulation via structural
transitions or parasitic reactions still hinder complete active
material utilization. In addition, the irreversible/reversible
phase transitions during repeated Zn insertion cause drastic
volume changes and promote electrode degradation. These
factors altogether contribute to hinder the full utilization of the
active material.
(7) In addition to the pre-inclusion of electrolyte additives,
especially for manganese oxides, the choice of the electrolyte
salt is one of the important but inevitable criteria to enhance
electrochemical performance parameters. The main choice of
electrolytes for manganese and vanadium oxides are highly
concentrated solutions of low-cost ZnSO4 (2/3 M) and bulky
anion salt (2/3 M) Zn(CF3SO3)2 salts, respectively. The salt
selection is attributed to their good solubility to form mildly
acidic solutions in aqueous media and their capability to
demonstrate electrochemical stability and promote less
dendrite formation and corrosion in ARZIBs. Specifically,
ZnSO4 is low-cost and participates in parasitic byproduct
formation, and their accumulation thereby impedes the
electrochemical reaction and hence electrode cycling stability.
However, Zn(CF3SO3)2 is almost 26 times more expensive but
promotes stable electrochemical reaction within a wide
operating potential window under long-term cycling. Mostly,
high performance in manganese oxide was showcased in
ZnSO4 electrolytes, whereas, in general, high energy density in
vanadium oxide cathodes was realized in Zn(CF3SO3)2
electrolyte solutions, though exceptions also exist. Very
recently, zinc chlorate (Zn(ClO4)2) has been introduced to
facilitate high cycling stability in vanadium-based electrodes.
(8) Real-time monitoring of ARZIBs with manganese and
vanadium oxide cathodes, using high-end instrumentation
techniques, has shed light on their electrochemical reaction
mechanism. The debate in understanding the reaction
mechanism of manganese oxide is more complicated than
that in vanadium oxides. Five different reaction mechanisms
(pure Zn2+ intercalation, H+ conversion reactions, consecutive/concomitant Zn2+/H+ co-intercalation, mixed intercalation/conversion, and dissolution/deposition- with or without
structural transitions) have been proposed for manganese
oxides. Further scrutiny of these concepts and electrode
degradation studies using highly sophisticated or comparative
experiments can surely not only lead to establishing the
electrochemical regulations governing these batteries but also
develop new battery chemistry and concepts.
(9) Compared to the dispute in manganese oxides, the
storage mechanism in vanadium oxide cathodes is less debated
as it is mainly attributed to intercalation. The dispute exists
mainly on the type of ions involved and the mechanism of the
intercalation reaction. Three major intercalation possibilities
were pursued for vanadium oxides, namely, Zn2+-intercalation,
Zn2+/H+ co-intercalation, and displacement/intercalation
reaction with or without structural phase transitions. These
intercalation mechanisms were explained based on two-phase
reactions, mixed two/single-phase or displacement/intercalation reactions, respectively. In addition, the possibilities for a
reversible/irreversible phase evolution of the layered parent
cathode to form different intermediates, end parent-derived
discharge products, or entirely transformed new complex oxide
phases like Zn3V2O7(OH)2·2H2O (ZVO) products are also
proposed. Further, the formation of ZVO and ZBS is
contended because the XRD reflection lines of both these
phases tend to overlap and are difficult to distinguish.
investigated performance-enhancing strategies. These approaches tend to evade active material dissolution and alleviate
structural amendments and low solid-state ion diffusion of
Zn2+ ion, thereby increasing specific capacities and extending
cycling stability under high current densities in the layeredtype modified electrodes.
(5) A direct comparison of energy densities reported by
several groups under different conditions of active material
preparation, electrochemical measurements, cell fabrication,
calculations, and lab-scale methods to that of other aqueous
systems or energy densities of equivalent commercial battery
systems can provide an empirical roadmap to evaluate the
achievements of the present-day ARZIBs. Such comparisons
show that the electrochemical exploits reviewed herein are
significant, if not exceptional, as some of the cathode
performances of strategically designed manganese and
vanadium oxides (∼50−200 Wh kg−1) are more competitive
with those for aqueous LIBs (∼50−100 Wh kg−1) and aqueous
Na-ion batteries (∼30 Wh kg−1), respectively, and much
higher than the energy densities of commercial Pb−acid (∼30
Wh kg−1) and Ni−Cd technologies (∼50 Wh kg−1). The broad
range in the energy densities achieved by ARZIBs using
transition metal oxide cathodes offers solutions for devices that
can act as intermediates between supercapacitors and nonaqueous LIBs. Figure 5 describes the appropriate position of
Figure 5. (top) Electrochemical features of Zn ion batteries
including type of mechanisms and feasible applications. (bottom)
Specific energy/power comparison among the various available
batteries.
ARZIBs in the energy−power density comparison profile along
with the most rejuvenated technologies, including LIBs,
lithium metal battery, lead−acid, Ni−Cd, and Ni-metal
hydride (MH) batteries. From this plot, it can be concluded
that ARZIB is a potential candidate in energy storage
applications. On the basis of the literature survey, the
utilization of ARZIBs can be categorized through type of
materials, devices, electrochemical regulations, and applications, as shown in top of Figure 5.
(6) Despite the major strides in developing high-performance manganese and vanadium oxide cathodes, persistent
issues of mediocre inherent electrical conduction, active
material dissolution, especially in manganese oxides, and
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The electrochemical performance of the manganese and
vanadium oxide cathodes are promising, but the stage is not
yet reached for developing commercial prototypes because
more explorations are crucial. Brief descriptions of what to
expect for future research directions regarding cathodes of
manganese and vanadium oxide are listed below.
(1) As seen from this Review, tunneled and layered
structures of manganese oxides and vanadium oxides, at least
for now, form the “holy grail” in the progressive advancement
toward developing state-of the-art electrodes by tailoring the
pristine structures with performance-enhancement strategies.
Accordingly, the electrochemical reactivity, energy density, and
cycling stability are significantly elevated, mostly under high
current drains. These enhancements must be translated at low
current densities under real-time monitoring of the reaction to
discern the underlying mechanism, which is highly debated, for
the practical realization of these electroactive materials to their
full utilization.
(2) From the viewpoint of developing electrodes for
ARZIBs, early research progress was focused on mostly
tunneled structures for manganese oxides and layered
structures for vanadium oxides. However, lately there has
been a shift in the research as the metal-ion preintercalation
approach (usually pursued for layered vanadium oxide
cathodes) was shown to be successful for layered-type
manganese oxide cathode with high energy density. Similarly,
tunnel-type structures of vanadium oxides are under deep
scrutiny to be exploited for upgrading their electrochemical
enactments further.
(3) Besides manganese and vanadium oxides, Prussian blue
analogues156,169,170 and metal sulfide171,172 electrodes have
been studied for ARZIBs. The low capacity (<100 mAh g−1),
short cycle-life with fading related to unexpected phase
transformations, and low rate capacities are significant
bottleneck limitations for Prussian blue cathodes. Similarly,
the lower electronegativity of sulfides (than oxygen) appears
attractive to stabilize the electrode structure; however,
environmental concerns, structural phase transitions, and
apparently low specific capacities and hence energy densities
associated with their high molecular weight make them less
attractive. In other words, tremendous breakthroughs are
essential before these electrode families can be scaled up for
practical applications.
(4) The study of materials based on amorphous oxides,
phosphates, fluorophosphates, or organic−inorganic hybrid
materials can be the way forward to identify new and
prospective manganese- and vanadium-based future cathodes
for ARZIBs. Amorphous materials can generally avoid the issue
of macroscopic phase transitions during the electrochemical
reaction, thereby alleviating the issue of cycling instability.
Also, they act as a buffer to accommodate structural distortions
during repeated Zn insertion and thereby enhance electrode
stability for long-term cycling. The unique 3D frameworks of
NASICON-type phosphates or fluorophosphates composed of
covalently bonded polyanion (PO4)3− units offer high
structural integrity that can withstand degradation for longterm cycling, even under a long operating potential window.
(5) Despite the adopted techniques for performance
enhancement, well-known strategies, including heteroatom
doping, electrolyte optimization, and polymer/bicomposite
formation, that can further benefit active material performance
remain untested for manganese oxide and vanadium oxide
cathodes. For example, heteroatom doped carbon in the
Review
Despite the adopted techniques for
performance enhancement, wellknown strategies, including heteroatom doping, electrolyte optimization,
and polymer/bicomposite formation,
that can further benefit active material
performance remain untested for
manganese oxide and vanadium oxide
cathodes.
composite cathode, optimization of electrolyte leading to the
extension of operating potential window (>1.5 V), the use of
various highly conducting polymers for composite formation
or surface modifications of the electrode, and forming biphasic
composite cathodes, which are less known for ARZIBs, can be
worth considering for manganese and vanadium oxides.
Further, the recently introduced concept of “water-in-salt”
electrolyte looks attractive as the excessive salt reduces the
influence of water, thereby preventing undesirable reactions,
including hydrogen evolution, and give potential advantages.
However, the excessively high salt concentration (∼21 M) of
the very expensive Zn(CF3SO3)2 salt makes it almost
impractical for even lab-scale purposes.
(6) In the process of adopting various strategies to enhance
the performance of existing manganese and vanadium oxide
cathodes, there are some challenges and undetermined factors
that need to be addressed in a timely manner. Nanostructured
electrode materials, in general, are known to be advantageous
for electrochemical capabilities, but their low tap density in the
present 2D battery structure can be detrimental to efficiency.
Similarly, composite electrodes can sometimes display
randomly dispersed microstructures that can be detrimental
to electrochemical performance. Hence, the preparation of selfassembled or precisely controlled novel microstructures can be
effective to enhance the electrochemical properties of the
electrodes. In addition, the role and effect of electrolyte
parameters, including the choice and amount of salts and
additives and electrolyte pH, require optimization. For
example, the strategy of using electrolyte additive tends to
alter the pH and have an effect on the electrochemical
reactivity and the product/byproduct formations that are, at
least in some cases, electrochemically reactive or inactive.
Further, ZBS or ZBT precipitates were proposed to be crucial
to sustain the mildly acidic pH to promote greater ion
insertion. This implies that ZBS/ZBT precipitation is critical
for the reaction mechanism and attaining high reversible
storage capacities. These factors need to be fairly explored in
more detail using high-end characterizations to fully understand the corresponding enhancements in the electrochemical
parameters. Further, new electrolytes based on zinc chlorate
salts can be interesting to study for high cycling stability of
ARZIB electrodes. Another issue is that the origin of the
structural transitions occurring in the electrode during the
electrochemical reaction needs to be determined. In the
strategy of structural engineering, the exact amount of water or
metal ions and their exact role to promote facile and stable Zn
insertion in cathodes are yet to be identified. Specifically, the
case of additives used for vanadium oxide cathodes in
equimolar concentrations with salt demonstrated enhanced
cycling stability, and dendrite formation can be controlled. The
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use of such electrolyte solutions can be an interesting method
to improve the electrochemical properties of prospective
cathodes prone to dissolution.
(7) From the authors viewpoint, one of the most important
problems encountered for ARZIBs is the lack of a common
unifying electrochemical mechanism that can explain all
accompanying reactions, including structural phase transitions
and byproduct formation. This remains highly crucial for the
transition of this technology toward future commercialization.
As seen from the Review, the reaction mechanism of ARZIBs
has evolved from Zn intercalation to proton conversion to
sequential or concomitant co-intercalation with or without a
coconversion process to a deposition process. For example, the
paradigm shift from the intercalation/conversion reaction to a
recent deposition process related to the crucial formation of a
stable ZBS, which was even assumed to be just a byproduct
earlier, calls for unambiguity in understanding the electrochemical reaction in the cathodes, especially manganese oxides,
of ARZIBs. Also, each proposition was made using different
parameters for electrolytes (as seen from Tables 1 and 2)
without a complete understanding of the role of especially the
pre-included additive in manganese oxide cathodes. Therefore,
a balanced and rigorous analytical approach of combining
different experimental real-time spectroscopic investigations,
including X-ray, electrochemical, vibrational, and NMR
techniques with theoretical approaches using computational
studies, can aid in understanding the electrode degradation
mechanism, the role of byproducts and Zn-ion intercalation
reactions occurring at the electrode/electrolyte interface, and
the formation/accumulation of solid-electrolyte-interface layer
in detail. Ultimately, these studies can settle the controversies
surrounding the reaction mechanisms of manganese and
vanadium oxides of ARZIBs.
(8) The role of pH, H2/O2 gas evolution, and other side
reactions associated with the cathode−electrolyte interface are
a few important topics that should be urgently addressed in the
near future. Taking a cue from other counterpart technologies,
using a separate host for Zn as an anode instead of commercial
bare Zn foil is attractive for eluding possible parasitic reactions
caused on the surface and hence influencing the cycle life of
the aqueous battery. Thus, with a Zn host anode, the weight/
volume can be minimized compared to using a bare Zn foil,
thereby leading to further gain in the maximum energy
achieved. However, this advantage comes at the cost of not
being able to compensate for the continuous loss of useable Zn
as in commercial bare Zn foil during electrochemical
cycling.154 Further, the utilization of Zn anode with wellconnected 3D network architecture can not only benefit
enhanced deep plating/stripping but also greatly suppress the
unwanted reaction during electrochemical reaction.173 These
factors should be considered in the near future while
developing Zn anode for ARZIBs. Another issue is that
dendrite formation can become significant and challenging
when fabricating large-sized ARZIBs beyond lab-scale limitations. This can be addressed by suppressing hydrogen
evolution in the electrolyte during electrochemical reaction via
the surface modification of the anode via protective coating or
growing an alternative in situ SEI layer on the anode. Utilizing
an equimolar mixture of salt and additive containing dendritehindering cation in the electrolyte solution can also be
beneficial to prevent dendrite issues. The use of solid/gel
electrolytes with high flexibility and mechanical and ductile
strength can prevent dendrite formations. Figure 6 emphasizes
Review
Figure 6. Outlook and future direction for ARZIBs.
the most important challenges in pursuing Mn/V-based
cathodes and possible remedies to be taken in the future
direction for making renewable, safe, and economically feasible
next-generation energy technologies.
(9) Ever since the reconceptualization of ARZIBs, the race
to develop flexible solid-state electrolytes for wearable ARZIBs
of different types, shapes, and designs mostly based on the
environmentally safe and cheap MnO2 cathodes, is gathering
pace, as seen from the literature.174 Although still far from
practical realizations, further performance-improvement via
strategic engineering to match practical energy density can
bring these devices closer to the possible application in lowcost and safe wearable electronics.175
(10) Cheap, green, and sustainable battery systems are the
crucial new order for emerging energy economies. The lowcost and eco-friendly manganese and ZnSO4 electrolytes make
manganese-based electrodes attractive for use in ARZIBs.
However, the vanadium element is faced with environmental
and cost issues. Vanadium is environmentally unsafe, and the
raw material cost has fluctuated widely in recent years. The
cheapest raw material of vanadium, V2O5, was low-priced (∼2
USD/lb) in 2016 before the prices shot up by almost 17 times
(∼33.2 USD/lb) by 2018 and is currently trending at still 3
times higher than the initial price (6.32 USD/lb). More
importantly, vanadium oxide cathodes are compatible with
Zn(CF3SO3)2 electrolyte, which is also much more expensive
than ZnSO4 solution. However, it is possible that the vanadium
price in the market can be cheaper provided environmentally
safe methods are developed urgently to extract vanadium from
the ore and safely produce V2O5 at large scale. Further,
considering that triflic acid and ZnCO3, the precursors used to
produce Zn(CF3SO3)2, are easily available and cheap, the cost
of the triflate salt can be expected to come down.
(11) The study of electrodes, in general, for ARZIBs has
recently generated concepts like the hybrid-ion batteries,
wherein Zn2+ is one of the participant ions. Zn2+ ion can be
matched with monovalent, divalent, or trivalent ions depending on the requirement.176 The properly matched ion (Li+) can
lead to higher operating potential advantage in the hybrid-ion
battery. Recently, the development of new battery concepts of
high potential (∼2.68 V) battery with dual electrolytes, both
mildly acidic and alkaline counterparts, was introduced.177
Similarly, a membrane-free Zn/MnO2 flow battery has also
been introduced through a dissolution−deposition mechanism,
which exhibits high discharge voltage of ∼1.78 V with excellent
cycling stability.178 The stability of the optimized manganese
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and vanadium oxide cathodes reviewed here can be tested in
such battery configurations to exploit their full potential for
useful battery applications. The knowledge gained regarding
manganese and vanadium electrodes can be utilized to
encourage the development of 3D electrode architectures to
overcome tap density issues and thereby realize 3D batteries, a
next-generation concept.
■
Review
Madras Christian College. His research interests include developing
nanostructured electrodes for rechargeable battery systems.
Balaji Sambandam received his Ph.D. degree from Anna University,
Chennai, and M.S. from Bharathidasan University, India. He is
currently a research fellow in the group of Professor Jaekook Kim at
Chonnam National University, South Korea. His research interests
include synthesizing and developing electrode materials for energy
storage applications.
AUTHOR INFORMATION
Seokhun Kim received his Masters’ degree in Materials Science and
Engineering (2016) from the Chonnam National University, and he is
currently a Ph.D. candidate in the group of Professor Jaekook Kim in
the Chonnam National University. He has been working in the field
of next-generation rechargeable battery electrodes.
Corresponding Author
Jaekook Kim − Department of Materials Science and
Engineering, Chonnam National University, Gwangju 500-757,
Republic of Korea; orcid.org/0000-0002-6638-249X;
Email: jaekook@chonnam.ac.kr
Sungjin Kim received his Ph.D. degree from Chonnam National
University, South Korea, under the supervision of Professor Jaekook
Kim. He is currently a research fellow in the group of Professor
Jaekook Kim. His current research interests focus on the synthesis and
characterization of electrode materials for rechargeable batteries.
Authors
Vinod Mathew − Department of Materials Science and
Engineering, Chonnam National University, Gwangju 500-757,
Republic of Korea
Balaji Sambandam − Department of Materials Science and
Engineering, Chonnam National University, Gwangju 500-757,
Republic of Korea
Seokhun Kim − Department of Materials Science and
Engineering, Chonnam National University, Gwangju 500-757,
Republic of Korea
Sungjin Kim − Department of Materials Science and
Engineering, Chonnam National University, Gwangju 500-757,
Republic of Korea
Sohyun Park − Department of Materials Science and
Engineering, Chonnam National University, Gwangju 500-757,
Republic of Korea
Seulgi Lee − Department of Materials Science and Engineering,
Chonnam National University, Gwangju 500-757, Republic of
Korea
Muhammad Hilmy Alfaruqi − Department of Materials
Science and Engineering, Chonnam National University,
Gwangju 500-757, Republic of Korea; orcid.org/00000002-0012-4148
Vaiyapuri Soundharrajan − Department of Materials Science
and Engineering, Chonnam National University, Gwangju 500757, Republic of Korea
Saiful Islam − Department of Materials Science and Engineering,
Chonnam National University, Gwangju 500-757, Republic of
Korea
Dimas Yunianto Putro − Department of Materials Science and
Engineering, Chonnam National University, Gwangju 500-757,
Republic of Korea
Jang-Yeon Hwang − Department of Materials Science and
Engineering, Chonnam National University, Gwangju 500-757,
Republic of Korea; orcid.org/0000-0003-3802-7439
Yang-Kook Sun − Department of Energy Engineering, Hanyang
University, Seoul 133-791, Republic of Korea; orcid.org/
0000-0002-0117-0170
Sohyun Park received her Masters’ degree in Materials Science and
Engineering from Chonnam National University in 2016. She is now a
Ph.D. candidate under the supervision of Prof. Jaekook Kim at
Chonnam National University. Her research mainly focuses on the
preparation and characterization of electrode materials for nextgeneration rechargeable batteries.
Seulgi Lee received his B.S. (2014) and is currently pursuing his M.S
Degree in Materials Science and Engineering at Chonnam National
University (CNU), South Korea. He is a member of Prof. Jaekook
Kim’s group, and his area of interest is the synthesis and
characterization of anode materials for Li-rechargeable batteries.
Muhammad Hilmy Alfaruqi received his Ph.D. degree from
Chonnam National University, South Korea, under the supervision
of Professor Jaekook Kim. He is currently a research fellow in the
group of Professor Jaekook Kim. His current research interests focus
on the synthesis and characterization of electrode materials for energy
storage applications.
Vaiyapuri Soundharrajan received his B.S. degree (2013) from
Central Electrochemical Research Institute, India. He is an integrated
(M.S.-Ph.D) student in Professor Jaekook Kim’s group in Department
of Materials Science and Engineering, Chonnam National University.
His research interests include the syntheses and design of electrode
materials for rechargeable battery applications.
Saiful Islam obtained his B.Sc. and M.Sc. degree from the SUST,
Bangladesh. Now he is a Ph.D. candidate at Chonnam National
University, South Korea, under the supervision of Prof. Kim Jaekook.
His research interests mainly focus on the synthesis and characterization of electrode materials for ZIBs.
Dimas Yunianto Putro is a doctoral candidate of Materials Science
and Engineering at Chonnam National University. He is working on
rechargeable electrodes using liquid and gel in aqueous electrolytes for
Zn-ion batteries.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsenergylett.0c00740
Biographies
Jang-Yeon Hwang received his Ph.D. degree (2018) on cathode
materials for sodium-ion batteries at Hanyang University. He is
currently a Professor in the Department of Materials Science and
Engineering at Chonnam National University, Korea. His research
focuses on the development of novel electrode materials for highenergy alkali-ion and alkali-metal batteries.
Vinod Mathew is a Research Professor in the group of Professor
Jaekook Kim at Chonnam National University. He received his Ph.D.
in Energy from the University of Madras and M.S. in Physics from
Yang-Kook Sun received his Ph.D. degree from Seoul National
University. His contribution as a group leader at Samsung Advanced
Institute of Technology led to the commercialization of the lithium
Notes
The authors declare no competing financial interest.
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polymer battery. As a Professor at Hanyang University since 2000, his
research interests include the syntheses of new rechargeable battery
electrodes.
Jaekook Kim is a Professor in the Department of Materials Science
and Engineering at Chonnam National University since 2003. After
completing his Ph.D at University of Texas (Austin), he worked as a
staff scientist in Argonne National Laboratory. His research deals with
the design and development of next-generation energy storage
materials. Webpage: http://www.nel-chonnam.com/
■
ACKNOWLEDGMENTS
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MIST) (No. 2020R1A2C3012415) and (MIST)
(2017R1D1A3B03034253).
■
Review
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