33
the Zn metal anode. Furthermore, First Principle Calculation studies by Xu et al.
confirmed the thermodynamic stability of α-MnO 2 to accommodate Zn-ion in the
tunnel structure [31].
On the contrary, Lee et al. claimed a different electrochemical reaction mechanism based on the formation of ZnMn 3 O 7 .3H 2 O (Zn-buserite), and precipitation of
layered zinc hydroxide sulfate (Zn 4 (OH) 6 (SO 4 )·5H 2 O, ZHS) during the discharge
reaction of α-MnO 2 in aqueous Zn test cell [32–34]. In other words, they contended
that the discharge and charge capacities of α-MnO 2 in ZIB originated from the precipitation and dissolution of Zn 4 (OH) 6 (SO 4 )·5H 2 O on the electrode surface, respectively. They also proposed that the condition for the formation of the hydroxy sulfate
precipitate was related to the drastic variation in the electrolyte pH arising from the
disproportionation reaction of Mn (III) and the subsequent dissolution of Mn (II)
into the electrolyte during the electrochemical discharge/Zn-intercalation reaction.
They concluded that this reaction was reversible as the irreversible capacity loss
was minimal upon the consecutive charge process. On the other hand, Pan et al.
observed the formation of MnOOH during the discharge reaction of α-MnO 2 in 2 M
ZnSO 4 of an aqueous Zn test cell and proposed a conversion reaction mechanism
due to proton insertion [35]. They proposed that this electrochemical reaction contributed to the corresponding Zn-α-MnO 2 cell exhibiting high reversible capacities
of 210 and 255 mAh g
−1
at C/5 (1C = 308 mA g
−1
) with an average operating voltage
of 1.44 V. Interestingly, the possible ZHS byproduct formation related to the high
salt concentration (~2M ZnSO 4 ) and the resulting sharp pH varation were not
addressed in this study.
Remarkably, when tunneled γ- and layered δ-MnO 2 polymorphs were used as
cathodes of ZIBs, Alfaruqi et al. observed the reversible formation of spinel
ZnMn 2 O 4 phase and intermediary Mn(II) phases in the presence of mildly acidic
1 M ZnSO 4 electrolytes [36, 37]. In detail, γ-MnO 2 underwent a structural transformation to spinel Mn(III) phase ZnMn 2 O 4 and two new intermediary Mn(II)
Zn-inserted phases, i.e. tunnel-type γ-Zn x MnO 2 and layered-type Zn y MnO 2 . These
electrochemically induced phase transitions caused the γ- and layered δ-MnO 2 electrodes to exhibit high discharge capacities of 285 (at 0.05 mA cm
−2
) and 252 mAh g
−1
(at 83 mA g
−1
), respectively. However, under repeated cycling, these intermediary
phases appeared to collapse. Zang et al. studying aqueous Zn test cells using 3 M
Zn(CF 3 SO 3 ) 2 (Zinc(II) trifluoromethanesulfonate) aqueous electrolyte concluded
that a common electrochemical mechanism exists for all tunnel-type (α-, β-, and γ-)
MnO 2 polymorphs. Upon initial Zn-intercalation into β-MnO 2 , the initial transformation of the tunnel to layered structure (Zn-buserite, ß-Zn x MnO 2 ·nH 2 O) occurs
while, for the subsequent cycles, the buserite structure is retained [38]. These phase
transitions contributed to the high capacity of 225 mAh g
−1
in the ß-MnO 2 cathode.
However, a slight amount of the unreacted ß-MnO 2 phase still existed after cycling.
This suggests the possibility of accommodating Zn-ions in the ß-MnO 2 structure, as
also observed by Islam et al. when they used ß-MnO 2 in 2 M ZnSO 4 aqueous electrolyte and a capacity of 270 mAh g
−1
at 100 mA g
−1
could be achieved [39].
Yuan et al. used λ-MnO 2 with spinel structure as a cathode in ZIB and the structure is retained upon repeated Zn-intercalation/de-intercalation, as observed by
Recent Developments of Zinc-Ion Batteries
the Zn metal anode. Furthermore, First Principle Calculation studies by Xu et al.
confirmed the thermodynamic stability of α-MnO 2 to accommodate Zn-ion in the
tunnel structure [31].
On the contrary, Lee et al. claimed a different electrochemical reaction mechanism based on the formation of ZnMn 3 O 7 .3H 2 O (Zn-buserite), and precipitation of
layered zinc hydroxide sulfate (Zn 4 (OH) 6 (SO 4 )·5H 2 O, ZHS) during the discharge
reaction of α-MnO 2 in aqueous Zn test cell [32–34]. In other words, they contended
that the discharge and charge capacities of α-MnO 2 in ZIB originated from the precipitation and dissolution of Zn 4 (OH) 6 (SO 4 )·5H 2 O on the electrode surface, respectively. They also proposed that the condition for the formation of the hydroxy sulfate
precipitate was related to the drastic variation in the electrolyte pH arising from the
disproportionation reaction of Mn (III) and the subsequent dissolution of Mn (II)
into the electrolyte during the electrochemical discharge/Zn-intercalation reaction.
They concluded that this reaction was reversible as the irreversible capacity loss
was minimal upon the consecutive charge process. On the other hand, Pan et al.
observed the formation of MnOOH during the discharge reaction of α-MnO 2 in 2 M
ZnSO 4 of an aqueous Zn test cell and proposed a conversion reaction mechanism
due to proton insertion [35]. They proposed that this electrochemical reaction contributed to the corresponding Zn-α-MnO 2 cell exhibiting high reversible capacities
of 210 and 255 mAh g
−1
at C/5 (1C = 308 mA g
−1
) with an average operating voltage
of 1.44 V. Interestingly, the possible ZHS byproduct formation related to the high
salt concentration (~2M ZnSO 4 ) and the resulting sharp pH varation were not
addressed in this study.
Remarkably, when tunneled γ- and layered δ-MnO 2 polymorphs were used as
cathodes of ZIBs, Alfaruqi et al. observed the reversible formation of spinel
ZnMn 2 O 4 phase and intermediary Mn(II) phases in the presence of mildly acidic
1 M ZnSO 4 electrolytes [36, 37]. In detail, γ-MnO 2 underwent a structural transformation to spinel Mn(III) phase ZnMn 2 O 4 and two new intermediary Mn(II)
Zn-inserted phases, i.e. tunnel-type γ-Zn x MnO 2 and layered-type Zn y MnO 2 . These
electrochemically induced phase transitions caused the γ- and layered δ-MnO 2 electrodes to exhibit high discharge capacities of 285 (at 0.05 mA cm
−2
) and 252 mAh g
−1
(at 83 mA g
−1
), respectively. However, under repeated cycling, these intermediary
phases appeared to collapse. Zang et al. studying aqueous Zn test cells using 3 M
Zn(CF 3 SO 3 ) 2 (Zinc(II) trifluoromethanesulfonate) aqueous electrolyte concluded
that a common electrochemical mechanism exists for all tunnel-type (α-, β-, and γ-)
MnO 2 polymorphs. Upon initial Zn-intercalation into β-MnO 2 , the initial transformation of the tunnel to layered structure (Zn-buserite, ß-Zn x MnO 2 ·nH 2 O) occurs
while, for the subsequent cycles, the buserite structure is retained [38]. These phase
transitions contributed to the high capacity of 225 mAh g
−1
in the ß-MnO 2 cathode.
However, a slight amount of the unreacted ß-MnO 2 phase still existed after cycling.
This suggests the possibility of accommodating Zn-ions in the ß-MnO 2 structure, as
also observed by Islam et al. when they used ß-MnO 2 in 2 M ZnSO 4 aqueous electrolyte and a capacity of 270 mAh g
−1
at 100 mA g
−1
could be achieved [39].
Yuan et al. used λ-MnO 2 with spinel structure as a cathode in ZIB and the structure is retained upon repeated Zn-intercalation/de-intercalation, as observed by
Recent Developments of Zinc-Ion Batteries