VO
2(B) is suitable for use as the electrode materials of AZIBs due to higher theoretical capacity and the tunnel structure with plus sizes (0.34, 0.82 and 0.5 nm
3 along
a-,
b- and
c-axes, respectively) for the Zn
2+ migration. Ding
et al. [
46] investigated the interaction behavior of Zn
2+ with VO
2(B) in AZIBs assembled with VO
2(B) nanofibers as cathode, Zn foil as anode, and Zn(CF
3SO
3)
2 as electrolyte (
Fig. 3a). VO
2(B) cathode displayed a specific capacity of 357 mAh/g (
Fig. 3b) and superior rate capability (171 mAh/g at 51.2 A/g in
Fig. 3c), ascribing to the intercalation pseudocapacitance behavior and the ultrafast kinetics of Zn
2+ into the tunnel structures of VO
2(B). The lattice parameter variation with the Zn
2+ intercalation into the tunnels (paralleled to the
b- and
c-axes of VO
2(B)) tested by
in-situ XRD diffraction was less, suggesting that these tunnels were stable and large enough to ensure the fast Zn
2+ intercalation/de-intercalation (
Fig. 3d). Therefore, VO
2(B) cathode with tunnel-structure was proved feasible to permit the insertion/extraction of Zn
2+ in laboratory-scale research. Park
et al. [
47] further revealed the electrochemical reaction mechanism of AZIBs using a bond-valence sum energy map and a first-principle calculation. Among the four sites (Zn
c, Zn
A1, Zn
A2, Zn
c') of inserting Zn
2+ into VO
2(B) structure shown in
Fig. 3e, Zn
A2 site was the optimal location of Zn
2+ into VO
2(B) and the predicted redox potential of Zn
xVO
2(B) was ~0.61 V (
Fig. 3f), close to the experimental value of ~0.7 V (VO
2(B) + 0.57Zn
2+ + 1.14e
− ↔ Zn
0.57VO
2(B),
Fig. 3g). The pseudocapacitance contribution of Zn
2+ intercalating into VO
2(B) structure, evaluated using potentiodynamic electrochemical impedance spectroscopy (PDEIS) technique was negligible. But the strong Coulombic ion-lattice interaction between Zn
2+ and VO
2(B) structure slowed down the kinetics and further limited the improvement of electrochemical properties. Oxygen-deficient VO
2 nanostructure could address the above-mentioned problem through providing the jumping charge transport sites
via enlarging the
b tunnels for fast zinc-ion diffusion. VO
2 with high-concentration oxygen vacancies prepared by Li
et al. [
48] displayed a discharge specific capacity of 375 mAh/g at 100 mA/g and long-term cyclic stability with a retained capacity of 175 mAh/g at 5 A/g over 2000 cycles in 3 mol/L Zn(CF
3SO
3)
2. However, in aqueous electrolytes, Zn
2+ often exists in the form of Zn(H
2O)
62+, indicating that larger desolvation energy and insertion energy need to be overcome in the process of Zn
2+ insertion. And the strong electrostatic repulsion between Zn
2+ and host structure also causes poor diffusion kinetics and mild deformation in the host structure. So Li
et al. [
49] announced a H
+ (de)insertion mechanism with less structure distortion of VO
2(B) than the Zn
2+ insertion mechanism. The VO
2(B) cathode exhibited excellent electrochemical performances of the first discharge capacity (353 mAh/g at 1.0 A/g), a 75.5% capacity retention after 945 cycles at 3.0 A/g in 1 mol/L ZnSO
4 (
Fig. 3h). The whole reaction steps were shown in
Fig. 3i. The generated H
+ in the deposition process of Zn
4(OH)
6SO
4·5H
2O could insert into VO
2(B) to contribute most of capacity by the first-principle analysis (
Fig. 3j) and keep long-term cycle stability. Zhang
et al. [
50] further verified the proton insertion mechanism by the bond valence method using VO
2(M) as cathode and ZnSO
4 as electrolyte. While storing Zn
2+ in VO
2(D) phase, H
+ and Zn
2+ could simultaneously insert into and extract from VO
2(D) during the cycling process, resulting in a phase transition from VO
2(D) to V
2O
5·xH
2O [
51]. H
2O molecules with Zn
2+ co-intercalation into host structure facilitated the intercalation of Zn
2+ due to shielding the electrostatic interactions between charge carrier and VO
2(D).