34
Zhang et al. when studying a spinel-type ZnMn 2 O 4 cathode in aqueous Zn test cell
[40, 41]. The latter cathode showed a discharge capacity of 120 mAh g
−1
and capacity retention of 94% over 500 cycles at 50 and 500 mA g
−1
, respectively [41]. An
akhtenskite ε-MnO 2 cathode electrodeposited on a carbon fiber paper demonstrated
reversible co-intercalation of Zn
2+
and H
+
ions in the presence of an aqueous 2 M
ZnSO 4 electrolyte solution containing 0.2 M MnSO 4 additive, as confirmed from the
formation of MnOOH and ZnMn 2 O 4 phases after full discharge reaction and the
combined analyses of electroanalytical, microscopic, and structural techniques
[42]. Interestingly a ε-MnO 2 cathode without any special tunnel or layer- type structure delivered a discharge capacity of 290 mAh g
−1
at 90 mA g
−1
while, at 6.5 °C
current drain, stable capacities were retained for 10,000 cycles with less than
0.007% decay per cycle.
Overall, the electrochemical reaction mechanism in MnO 2 is mainly dependent
on many parameters, such as MnO 2 polymorphs and electrolyte pH or additives
[43]. In addition, generally, the dissolution of Mn from the electrode and the low
conductivity of the Mn-based oxides tend to show dramatic capacity fade during
cycling. However, Mn dissolution can be suppressed by the addition of Mn
2+
-ions
(MnSO 4 ) as additives in the aqueous electrolyte (2 M ZnSO 4 ) . The corresponding
Zn-α-MnO 2 cell delivered very high specific capacities and enhanced cycle life
(Fig. 1c). Another group reported 92% capacity retention after 5000 cycles in a
similar Zn-MnO 2 cell [35].
The electrochemical performances of early MnO 2 cathodes prepared by different
syntheses were still limited by their inherent problems of poor electrical conductivity and structural instability related to Jahn–Teller dissolution. These factors negatively affected the long-term electrode cycling stability of MnO 2 . Therefore,
composite formation using carbonaceous materials such as carbon nanotube (CNT)
or graphene, and metal doping were included to improve the conductivity of MnO 2
in 2 M ZnSO 4 electrolyte solution containing 0.2 M MnSO 4 additive salt and thus
lead to enhanced electrode performance. Graphene scroll-coated α-MnO 2 demonstrated long-term cycling stability with 94% capacity retention after 3000 cycles
[44]. Remarkably, high specific energy and specific power (406.6 Wh kg
−1
at
135 W kg
−1
and 109.2 Wh kg
−1
at 9450 W kg
−1
) could be obtained. A V-doped
α-MnO 2 electrode showed 31% higher capacity and superior cycling performance
than that of the bare α-MnO 2 electrode in 1 M ZnSO 4 electrolyte solution [45].
Other Mn-based oxides, such as todorokite MnO 2 , α-Mn 2 O 3 , and Mn 3 O 4 are also
promising to be used as cathodes in ZIBs. The large 3 × 3 tunnel of todorokite structure facilitates fast Zn
2+
ion diffusion into the structure. Compared to α-MnO 2 , the
todorokite cathode exhibited a discharge capacity of 98 mAh g
−1
and cycling stability over 50 cycles in 1 M ZnSO 4 electrolyte [46]. α-Mn 2 O 3 cathode in 2 M ZnSO 4
electrolyte showed a reversible capacity of 148 mAh g
−1
and a long-cycle life
(82.2 mAh g
−1
at 2 A g
−1
over 1000 cycles) [47]. In the same electrolyte medium,
spinel Mn 3 O 4 cathode was able to deliver a discharge capacity of 232 mAh g
−1
at a
current density of 0.2 A g
−1
[48]. The cathode exhibited a discharge capacity of
106.1 mAh g
−1
at 0.5 A g
−1
after 300 cycles. α-Mn 2 O 3 and Mn 3 O 4 cathodes experiJ. Kim et al.
Zhang et al. when studying a spinel-type ZnMn 2 O 4 cathode in aqueous Zn test cell
[40, 41]. The latter cathode showed a discharge capacity of 120 mAh g
−1
and capacity retention of 94% over 500 cycles at 50 and 500 mA g
−1
, respectively [41]. An
akhtenskite ε-MnO 2 cathode electrodeposited on a carbon fiber paper demonstrated
reversible co-intercalation of Zn
2+
and H
+
ions in the presence of an aqueous 2 M
ZnSO 4 electrolyte solution containing 0.2 M MnSO 4 additive, as confirmed from the
formation of MnOOH and ZnMn 2 O 4 phases after full discharge reaction and the
combined analyses of electroanalytical, microscopic, and structural techniques
[42]. Interestingly a ε-MnO 2 cathode without any special tunnel or layer- type structure delivered a discharge capacity of 290 mAh g
−1
at 90 mA g
−1
while, at 6.5 °C
current drain, stable capacities were retained for 10,000 cycles with less than
0.007% decay per cycle.
Overall, the electrochemical reaction mechanism in MnO 2 is mainly dependent
on many parameters, such as MnO 2 polymorphs and electrolyte pH or additives
[43]. In addition, generally, the dissolution of Mn from the electrode and the low
conductivity of the Mn-based oxides tend to show dramatic capacity fade during
cycling. However, Mn dissolution can be suppressed by the addition of Mn
2+
-ions
(MnSO 4 ) as additives in the aqueous electrolyte (2 M ZnSO 4 ) . The corresponding
Zn-α-MnO 2 cell delivered very high specific capacities and enhanced cycle life
(Fig. 1c). Another group reported 92% capacity retention after 5000 cycles in a
similar Zn-MnO 2 cell [35].
The electrochemical performances of early MnO 2 cathodes prepared by different
syntheses were still limited by their inherent problems of poor electrical conductivity and structural instability related to Jahn–Teller dissolution. These factors negatively affected the long-term electrode cycling stability of MnO 2 . Therefore,
composite formation using carbonaceous materials such as carbon nanotube (CNT)
or graphene, and metal doping were included to improve the conductivity of MnO 2
in 2 M ZnSO 4 electrolyte solution containing 0.2 M MnSO 4 additive salt and thus
lead to enhanced electrode performance. Graphene scroll-coated α-MnO 2 demonstrated long-term cycling stability with 94% capacity retention after 3000 cycles
[44]. Remarkably, high specific energy and specific power (406.6 Wh kg
−1
at
135 W kg
−1
and 109.2 Wh kg
−1
at 9450 W kg
−1
) could be obtained. A V-doped
α-MnO 2 electrode showed 31% higher capacity and superior cycling performance
than that of the bare α-MnO 2 electrode in 1 M ZnSO 4 electrolyte solution [45].
Other Mn-based oxides, such as todorokite MnO 2 , α-Mn 2 O 3 , and Mn 3 O 4 are also
promising to be used as cathodes in ZIBs. The large 3 × 3 tunnel of todorokite structure facilitates fast Zn
2+
ion diffusion into the structure. Compared to α-MnO 2 , the
todorokite cathode exhibited a discharge capacity of 98 mAh g
−1
and cycling stability over 50 cycles in 1 M ZnSO 4 electrolyte [46]. α-Mn 2 O 3 cathode in 2 M ZnSO 4
electrolyte showed a reversible capacity of 148 mAh g
−1
and a long-cycle life
(82.2 mAh g
−1
at 2 A g
−1
over 1000 cycles) [47]. In the same electrolyte medium,
spinel Mn 3 O 4 cathode was able to deliver a discharge capacity of 232 mAh g
−1
at a
current density of 0.2 A g
−1
[48]. The cathode exhibited a discharge capacity of
106.1 mAh g
−1
at 0.5 A g
−1
after 300 cycles. α-Mn 2 O 3 and Mn 3 O 4 cathodes experiJ. Kim et al.