38
energy density of 504.9 W h kg
−1
(33.95 mW h cm
−3
), a high peak power density of
8.6 kW kg
−1
in addition to retaining more than 77.7% of its initial capacity with
nearly 100% Coulombic efficiencies after 300 cycles [61].
Vanadium-Based Cathodes
The presence of many layered vanadium-based materials that can promote facile
guest-ion insertion/de-insertion has sparked renewed interest in its many electrochemical technologies and aqueous rechargeable ZIBs is not excluded. Vanadium
oxides, in general, have both electronic and magnetic properties; interestingly, V x O y
is known to exist in a wide range of oxidation states from V
2+
to V
5+
, to that of oxygen coordination with basic building motifs of V 2 O 5 , V 4 O 10 , V 3 O 7 , etc. Since most
of the vanadium oxides possess layered or open network structures and hold rich
crystal chemistry due to their multiple oxidation states, coordination polyhedra and
metal–oxygen connectivity, vanadium containing cathodes are good candidates for
aqueous rechargeable batteries. The influence of phase evolutions of the cathodes
during electrochemical reactions are mostly observed through intercalation/deintercalation mechanism for layered and tunnel structures.
The first vanadium-based electrode material for ZIBs was demonstrated by the
Nazar group. The classical properties of a vanadium bronze cathode, i.e.,
Zn 0.25 V 2 O 5 ·nH 2 O, with high specific capacities of 250-300 mAh g
−1
and long
cyclability (>1000 cycles) in 1 M ZnSO 4 at very high current rates of 15C
(1C = 300 mAh g
−1
) was demonstrated [62]. In situ XRD analysis predicts the role
of water molecules, which is crucial for reversibly expanding and contracting the
layered galleries to allow Zn
2+
ingress/egress, leading to good kinetics and high rate
performance. It is evident that Zn
2+
/H 2 O pillared layered V 2 O 5 and the effect of
water molecules (both lattice and/or water molecules from the electrolyte) expand
the layered galleries and buffer the high charge density of the intercalating ions;
thereby electrochemical reactions are greatly enhanced. A schematic representation
of Zn-ions intercalation/de-intercalation is represented in Zn 0.25 V 2 O 5 ·nH 2 O gallery,
as shown in Fig. 3a. The cycling results, in Fig. 3b, indicate that the corresponding
cell can be cycled for over 200 cycles at 1200 mA g
−1
current density. An ease of
overall battery assembly/fabrication with an aqueous electrolyte (1 M ZnSO 4 ), a
metallic Zn negative electrode and scalable processing of the Zn
2+
host material,
help in meeting the requirements essential for large-scale storage applications.
Hence, many layered and tunnel-type vanadium oxides including LiV 3 O 8 [63],
VO 1.52 (OH) 0.77 [64], Zn 3 V 2 O 7 (OH) 2 ·2H 2 O [65], H 2 V 3 O 8 [66], V 2 O 5 ·nH 2 O [67],
Na 0.33 V 2 O 5 [68], Na 1.1 V 3 O 7.9 [69], and Zn 2 V 2 O 7 [70] have been investigated recently
as potential cathode materials for ZIBs.
Similar to the case of manganese-based electrodes, the effect of different metal
ions in the interlayers, which act as pillars to not only holding the backbone and
increasing the stability of the vanadium oxides but also intensively improve the ion
diffusion rate, as well as the electronic conductivity, have been studied for ZIBs.
Recently, Na 0.33 V 2 O 5 nanowire cathode delivered a very high capacity of 367 mAh g
−1
J. Kim et al.
energy density of 504.9 W h kg
−1
(33.95 mW h cm
−3
), a high peak power density of
8.6 kW kg
−1
in addition to retaining more than 77.7% of its initial capacity with
nearly 100% Coulombic efficiencies after 300 cycles [61].
Vanadium-Based Cathodes
The presence of many layered vanadium-based materials that can promote facile
guest-ion insertion/de-insertion has sparked renewed interest in its many electrochemical technologies and aqueous rechargeable ZIBs is not excluded. Vanadium
oxides, in general, have both electronic and magnetic properties; interestingly, V x O y
is known to exist in a wide range of oxidation states from V
2+
to V
5+
, to that of oxygen coordination with basic building motifs of V 2 O 5 , V 4 O 10 , V 3 O 7 , etc. Since most
of the vanadium oxides possess layered or open network structures and hold rich
crystal chemistry due to their multiple oxidation states, coordination polyhedra and
metal–oxygen connectivity, vanadium containing cathodes are good candidates for
aqueous rechargeable batteries. The influence of phase evolutions of the cathodes
during electrochemical reactions are mostly observed through intercalation/deintercalation mechanism for layered and tunnel structures.
The first vanadium-based electrode material for ZIBs was demonstrated by the
Nazar group. The classical properties of a vanadium bronze cathode, i.e.,
Zn 0.25 V 2 O 5 ·nH 2 O, with high specific capacities of 250-300 mAh g
−1
and long
cyclability (>1000 cycles) in 1 M ZnSO 4 at very high current rates of 15C
(1C = 300 mAh g
−1
) was demonstrated [62]. In situ XRD analysis predicts the role
of water molecules, which is crucial for reversibly expanding and contracting the
layered galleries to allow Zn
2+
ingress/egress, leading to good kinetics and high rate
performance. It is evident that Zn
2+
/H 2 O pillared layered V 2 O 5 and the effect of
water molecules (both lattice and/or water molecules from the electrolyte) expand
the layered galleries and buffer the high charge density of the intercalating ions;
thereby electrochemical reactions are greatly enhanced. A schematic representation
of Zn-ions intercalation/de-intercalation is represented in Zn 0.25 V 2 O 5 ·nH 2 O gallery,
as shown in Fig. 3a. The cycling results, in Fig. 3b, indicate that the corresponding
cell can be cycled for over 200 cycles at 1200 mA g
−1
current density. An ease of
overall battery assembly/fabrication with an aqueous electrolyte (1 M ZnSO 4 ), a
metallic Zn negative electrode and scalable processing of the Zn
2+
host material,
help in meeting the requirements essential for large-scale storage applications.
Hence, many layered and tunnel-type vanadium oxides including LiV 3 O 8 [63],
VO 1.52 (OH) 0.77 [64], Zn 3 V 2 O 7 (OH) 2 ·2H 2 O [65], H 2 V 3 O 8 [66], V 2 O 5 ·nH 2 O [67],
Na 0.33 V 2 O 5 [68], Na 1.1 V 3 O 7.9 [69], and Zn 2 V 2 O 7 [70] have been investigated recently
as potential cathode materials for ZIBs.
Similar to the case of manganese-based electrodes, the effect of different metal
ions in the interlayers, which act as pillars to not only holding the backbone and
increasing the stability of the vanadium oxides but also intensively improve the ion
diffusion rate, as well as the electronic conductivity, have been studied for ZIBs.
Recently, Na 0.33 V 2 O 5 nanowire cathode delivered a very high capacity of 367 mAh g
−1
J. Kim et al.