29
overlooked mainly due to their inadequate electrochemical performances [17, 18].
Under a re-evaluation of this Zn//1M ZnSO 4 //MnO 2 aqueous cell system in 2012,
Kang et al. proposed the term “zinc-ion battery” or ZIB and the chemistry behind its
electrochemical reaction. They demonstrated a directly reversible two-electron
transfer via Zn
2+
-intercalation/de-intercalation in the mildly acidic electrolyte
(pH ~ 4.2) for close to 100 cycles, according to the Eqs. (3) and (4) [8]. The ZIB
utilizes Zn as the anode, Zn-intercalating host as the cathode, and a mildly acidic
aqueous electrolyte solution of ZnSO 4 . Compared to the early primary Zn-MnO 2
alkaline system, this aqueous rechargeable ZIBs are promising due to the efficient
reversible Zn-deposition on metal anodes within a broad electrochemical window,
high theoretical capacities, apparently low activation energy for ion diffusion, costeffectiveness, compatibility with aqueous electrolytes, simple fabrication and environmentally safe characteristics.
Cathode Zn
e
MnO
ZnMn O
:
2
2
2 4
2
2
+
−
+
+
↔
(3)
Anode Zn
Zn
e
:
↔
+
+
−
2
2
(4)
Since the discovery of ZIBs, research has been centered upon upgrading the
performance of the aqueous ZIB system to suit practical stationary storage applications. The two-electron transfer involved in this ZIB implies the storage/release of
more amount of charge across a given area and thus reflects as high energy and
power densities. In addition, the high gravimetric capacity of Zn (820 mAh g
−1
)
when Zn metal is used as the anode and the high redox potential of Zn
2+
/Zn (−0.78 V
vs. SHE) will reflect as energy densities that are higher or competitive to those
achieved for Li-ion batteries. Also, the ionic radius of Zn
2+
(74 pm) is comparable
to those of Li
+
(76 pm) and lesser than Na
+
(102 pm) thus raising the scope for
exploration of possible advancement in ZIB technology [19]. Furthermore, the simplicity of fabrication under open-air conditions combined with the abundant and
less toxic Zn element makes aqueous ZIB one of the most economical, safe, and
green energy storage technologies with prospective use for stationary grid storage
applications. Also, ZIBS are very safe for next-generation technologies based on
flexible, rollup, wearable implantable devices the portable electronics market. A
wide range of approaches and materials, namely, cathodes, anodes, and electrolytes
have been investigated for ZIB applications to date. Herein, we review the progresses and major advancements related to both aqueous and non-aqueous ZIBs,
facilitating energy storage/conversion via Zn
2+
(de)intercalation mechanism.
Despite the developments so far, the research on ZIBs are still faced with challenges and limitations. Unlike the case for LIBs, the problems of large-sized divalent ions with more charge density, the related lower diffusion kinetics and low
redox-active reactions in the operating potential region (vs Zn/Zn
2+
) limit the development of ZIB electrodes [20]. Most of the known cathodes supporting reversible
Zn insertion are based on manganese oxides and vanadium oxides. In addition the
problems of active material dissolution and formation of undesirable byproducts
lead to electrode instability during long-term cycling [21]. These detrimental factors
Recent Developments of Zinc-Ion Batteries
overlooked mainly due to their inadequate electrochemical performances [17, 18].
Under a re-evaluation of this Zn//1M ZnSO 4 //MnO 2 aqueous cell system in 2012,
Kang et al. proposed the term “zinc-ion battery” or ZIB and the chemistry behind its
electrochemical reaction. They demonstrated a directly reversible two-electron
transfer via Zn
2+
-intercalation/de-intercalation in the mildly acidic electrolyte
(pH ~ 4.2) for close to 100 cycles, according to the Eqs. (3) and (4) [8]. The ZIB
utilizes Zn as the anode, Zn-intercalating host as the cathode, and a mildly acidic
aqueous electrolyte solution of ZnSO 4 . Compared to the early primary Zn-MnO 2
alkaline system, this aqueous rechargeable ZIBs are promising due to the efficient
reversible Zn-deposition on metal anodes within a broad electrochemical window,
high theoretical capacities, apparently low activation energy for ion diffusion, costeffectiveness, compatibility with aqueous electrolytes, simple fabrication and environmentally safe characteristics.
Cathode Zn
e
MnO
ZnMn O
:
2
2
2 4
2
2
+
−
+
+
↔
(3)
Anode Zn
Zn
e
:
↔
+
+
−
2
2
(4)
Since the discovery of ZIBs, research has been centered upon upgrading the
performance of the aqueous ZIB system to suit practical stationary storage applications. The two-electron transfer involved in this ZIB implies the storage/release of
more amount of charge across a given area and thus reflects as high energy and
power densities. In addition, the high gravimetric capacity of Zn (820 mAh g
−1
)
when Zn metal is used as the anode and the high redox potential of Zn
2+
/Zn (−0.78 V
vs. SHE) will reflect as energy densities that are higher or competitive to those
achieved for Li-ion batteries. Also, the ionic radius of Zn
2+
(74 pm) is comparable
to those of Li
+
(76 pm) and lesser than Na
+
(102 pm) thus raising the scope for
exploration of possible advancement in ZIB technology [19]. Furthermore, the simplicity of fabrication under open-air conditions combined with the abundant and
less toxic Zn element makes aqueous ZIB one of the most economical, safe, and
green energy storage technologies with prospective use for stationary grid storage
applications. Also, ZIBS are very safe for next-generation technologies based on
flexible, rollup, wearable implantable devices the portable electronics market. A
wide range of approaches and materials, namely, cathodes, anodes, and electrolytes
have been investigated for ZIB applications to date. Herein, we review the progresses and major advancements related to both aqueous and non-aqueous ZIBs,
facilitating energy storage/conversion via Zn
2+
(de)intercalation mechanism.
Despite the developments so far, the research on ZIBs are still faced with challenges and limitations. Unlike the case for LIBs, the problems of large-sized divalent ions with more charge density, the related lower diffusion kinetics and low
redox-active reactions in the operating potential region (vs Zn/Zn
2+
) limit the development of ZIB electrodes [20]. Most of the known cathodes supporting reversible
Zn insertion are based on manganese oxides and vanadium oxides. In addition the
problems of active material dissolution and formation of undesirable byproducts
lead to electrode instability during long-term cycling [21]. These detrimental factors
Recent Developments of Zinc-Ion Batteries