44
1.73 V [93]. Interestingly, the prepared cathode retained 80% discharge capacity in
the 3 M ZnSO 4 electrolyte at 300 mA g
−1
current density over 200 cycles, inferring
that the electrode possesses excellent stability.
An analogue structure, CuHCF (KCu[Fe(CN) 6 ]), shows a new, safe, and environmentally friendly ZIB with 20 mM ZnSO 4 aqueous solution (pH of 6) as the electrolyte through the effects of Zn
2+
ions intercalation and H 2 evolution [94]. The
battery provided ~96% capacity retention over 100 cycles at 1C with an average
discharge potential of 1.73 V. The high cycling efficiency is attributed to the deliberate choice of electrolyte pH and the low zinc concentration that ultimately leads to
the formation of compact zinc morphology with relatively low surface area at the
anode. Upon repeated cycling, the rough surface of the Zn anode facilitates improved
Zn-deposition and hence effective long-term cycling by actively preventing the dendritic growth formation and suppressing the corrosion-related hydrogen evolution
reaction at the anode. This study proved that by operating the electrode at the appropriate pH, the design of preferentially oriented zinc ZIBs with long-term cycling
stability can be realized [95]. Jia et al. explained the Zn storage mechanism in
Cu-HCF electrode in 1 M ZnSO 4 electrolyte via a Zn
2+
intercalation/de-intercalation
mechanism accompanied by solid phase diffusion kinetics (direct interstitial) [96].
In this reaction, the Zn
2+
-ions intercalate migrate and occupy just the interstitial sites
without altering the overall structure of the active material; however, the electrostatic force of the intercalated Zn
2+
ions tend to change the binding energies of the
ions in its vicinity. In addition, the ions initially occupying their respective interstitial positions present steric hindrance and obstruct the diffusion of the incoming
Zn
2+
ions, thereby the latter ions require more energy for intercalation. This effect is
reflected in the corresponding two discharge plateaus at 0.8–0.5 V and 0.1–0.01 V
domains, respectively. Thus, 56 mA h g
−1
discharge capacity or nearly 65% of the
theoretical capacity is realized. The cell shows discharge capacity of 43 mAh g
−1
at
20 mA g
−1
current density over 20 cycles. Further investigations by Kasiri et al.
revealed that the nature of the anion and the Zn-concentration in the electrolyte
affects the electrode stability [25]. Among the different electrolytes (ZnSO 4 , ZnF 2 ,
Zn(ClO 4 ) 2, and Zn(NO 3 ) 2 ) studied for Cu-HCF, ZnSO 4 showed stable cycle
performance at a given concentration. The (de)intercalation in CuHCF initially follows a single-phase reaction, however, after attaining an equilibrium stage (critical
amount of Zn
2+
ions in the lattice of the cathode), the phase partially converted into
ZnHCF; later insertion follows a two-phase reaction. This phase transition depends
Table 2 (continued)
Cathode morphologies/
preparative methods
Electrolyte +
additive
Potential
Window
Current
density
(mA g
−1
)
Cyclability
(mAh g
−1 )
Ref.
Mn 0.15 V 2 O 5 ·nH 2 O flower-like
/ microwave assisted
hydrothermal
1 M Zn(ClO 4 ) 2
in PC
0.2–1.7
10,000
153 after
8000 cycles
[90]
V 7 O 16 nanotubes /
3 M
Zn(CF 3 SO 3 ) 2
0.3–1.9
2400
175 after
950 cycles
[91]
J. Kim et al.
1.73 V [93]. Interestingly, the prepared cathode retained 80% discharge capacity in
the 3 M ZnSO 4 electrolyte at 300 mA g
−1
current density over 200 cycles, inferring
that the electrode possesses excellent stability.
An analogue structure, CuHCF (KCu[Fe(CN) 6 ]), shows a new, safe, and environmentally friendly ZIB with 20 mM ZnSO 4 aqueous solution (pH of 6) as the electrolyte through the effects of Zn
2+
ions intercalation and H 2 evolution [94]. The
battery provided ~96% capacity retention over 100 cycles at 1C with an average
discharge potential of 1.73 V. The high cycling efficiency is attributed to the deliberate choice of electrolyte pH and the low zinc concentration that ultimately leads to
the formation of compact zinc morphology with relatively low surface area at the
anode. Upon repeated cycling, the rough surface of the Zn anode facilitates improved
Zn-deposition and hence effective long-term cycling by actively preventing the dendritic growth formation and suppressing the corrosion-related hydrogen evolution
reaction at the anode. This study proved that by operating the electrode at the appropriate pH, the design of preferentially oriented zinc ZIBs with long-term cycling
stability can be realized [95]. Jia et al. explained the Zn storage mechanism in
Cu-HCF electrode in 1 M ZnSO 4 electrolyte via a Zn
2+
intercalation/de-intercalation
mechanism accompanied by solid phase diffusion kinetics (direct interstitial) [96].
In this reaction, the Zn
2+
-ions intercalate migrate and occupy just the interstitial sites
without altering the overall structure of the active material; however, the electrostatic force of the intercalated Zn
2+
ions tend to change the binding energies of the
ions in its vicinity. In addition, the ions initially occupying their respective interstitial positions present steric hindrance and obstruct the diffusion of the incoming
Zn
2+
ions, thereby the latter ions require more energy for intercalation. This effect is
reflected in the corresponding two discharge plateaus at 0.8–0.5 V and 0.1–0.01 V
domains, respectively. Thus, 56 mA h g
−1
discharge capacity or nearly 65% of the
theoretical capacity is realized. The cell shows discharge capacity of 43 mAh g
−1
at
20 mA g
−1
current density over 20 cycles. Further investigations by Kasiri et al.
revealed that the nature of the anion and the Zn-concentration in the electrolyte
affects the electrode stability [25]. Among the different electrolytes (ZnSO 4 , ZnF 2 ,
Zn(ClO 4 ) 2, and Zn(NO 3 ) 2 ) studied for Cu-HCF, ZnSO 4 showed stable cycle
performance at a given concentration. The (de)intercalation in CuHCF initially follows a single-phase reaction, however, after attaining an equilibrium stage (critical
amount of Zn
2+
ions in the lattice of the cathode), the phase partially converted into
ZnHCF; later insertion follows a two-phase reaction. This phase transition depends
Table 2 (continued)
Cathode morphologies/
preparative methods
Electrolyte +
additive
Potential
Window
Current
density
(mA g
−1
)
Cyclability
(mAh g
−1 )
Ref.
Mn 0.15 V 2 O 5 ·nH 2 O flower-like
/ microwave assisted
hydrothermal
1 M Zn(ClO 4 ) 2
in PC
0.2–1.7
10,000
153 after
8000 cycles
[90]
V 7 O 16 nanotubes /
3 M
Zn(CF 3 SO 3 ) 2
0.3–1.9
2400
175 after
950 cycles
[91]
J. Kim et al.