Table 1

A comparison of monovalent/multivalent metals in the standard potential, theoretical capacity, ionic radius and cost[7]"

Fig. 1

Operation voltages versus specific capacities of representative cathode materials for ZIBs[8]"

Fig. 2

(a) Schematic diagram showing theworking mechanism of the electrolytic Zn||MnO2 battery[19]; (b) Diagram of the electrochemical energy storage mechanism in the Zn||Co-Ni-MnO2 battery[19]"

Fig. 3

Polyhedral representations of manganese oxide crystal structure: (a) β-MnO2; (b) R-MnO2; (c) γ-MnO2; (d) α-MnO2; (e) Romanechite-type MnO2; (f) Todorokite-type MnO2; (g) δ-MnO2; (h) λ-MnO2[7]"

Fig. 4

BET and specific capacity of battery (A) α-MnO2; (B) δ-MnO2; (C) γ-MnO2 and (D) β-MnO2. Using as-prepared MnO2 electrode in 1 mol/L ZnSO4 aqueous electrolyte with current density of 0.1 A/g between 0.9 —1.9 V, the specificcapacity is first discharge capacity[23]"

Fig. 5

SEM images of (a) α-MnO2 nanorods[26]; (b) α-MnO2 nanowires[27]; (c) α-MnO2 nanofibres[10]; (d) α-MnO2 nanotube[28]; (e) δ-MnO2 nanosheets[31]; (f) δ-MnO2 nanoflowers[33]"

Table 2

Structure and electrochemical properties of MnO2 materials modified by various dopings in AZIBs"

Fig. 6

(a) XRD patterns; (b)TG curves; (c) Cycling performance of the P-MnO2, Al-MnO2, Cu-MnO2 and Mg-MnO2[35]; (d) Rate performances; (e) Long-term cycling performances at 1 A/g of the MO and BMO[27]"

Fig. 7

(a) EDS mapping of LCMO sample; (b) Capacities at 0.1 A/g; (c) Cycling performance of the MO,LMO,CMO and LCMO electrodes[44]; (d) Crystallographic structure; (e) Charge-discharge voltage profiles at 0.5 C; (f) Cycling performances at 5 C; (g) Rate capability at various current densities of the β-MnO2 and Ce-doped β-MnO2[45]"

Table 3

Structure and electrochemical properties of MnO2 materials combined with highly conductive materials in AZIBs"

Fig. 8

(a) SEM images; (b) The N2 adsorption/desorption isotherm; (c) Evolution of ex situ X-ray powder diffraction (XRD)patterns during cycling; (d) Schematic of the discharge reaction process; (e) Cycling performance and the corresponding coulombic efficiency at a current density of 300 mA/g of the 0.2 MnO2/C electrode[14]; (f) Schematic illustration of the nanocrystalline MnO2 electrodeposited on CFP process, and the corresponding SEM images of CFP before and after electrodeposition; (g) GITT profiles;(h)Cycling performance and the corresponding coulombic efficiency at rates of 6.5 C&1.3 C[55]"

Fig. 9

(a) TEM images of the MnO2-graphene sample; (b) TG curves for the MnO2 and MnO2-graphene samples; (c) Cycling performance for the MnO2 and MnO2-graphene electrodes and the corresponding Coulombic efficiency[62]; (d) The HRTEM of MGS; (e) Rate performances of MGS and MNW; (f) Long-term cycling performances at 7 and 3 A/g[63]"