Fig 1

(a) The working voltage and specific capacity of representative cathode materials for zinc battery 20. (b) Schematic diagram of crystal structures of α-MnO2, β-MnO2, γ-MnO2, λ-MnO2, δ-MnO2 and R-MnO2 23."

Table 1

The crystal structure, energy storage mechanism and performance of different crystalline MnO2."

Fig 2

(a) Schematic diagram of the rechargeable Zn//MnO2 battery using CF3SO3?-based electrolyte 45; (b) Schematic diagram of the reaction pathway of Zn-insertion in γ-MnO2 cathode 37; (c) Schematic diagram of co-insertion H+/Zn2+ in layered MnO2 54; (d) Schematic diagram of showing the reactions during the discharge process for Zn//α-MnO2 battery using aqueous ZnSO4 electrolyte 57. (e) Schematic diagram of the redox reactions and crystal structures of related compounds in the Zn|0.2 mol∙L?1 MnSO4 (aq), 1 mol∙L?1 ZnSO4 (aq)| carbon black battery 44."

Fig 3

(a) Schematic diagram of zinc battery based on dissolution-deposition mechanism and (b) its Galvanostatic discharge curves 62. (c) Schematic diagram of zinc battery based on dissolution-deposition mechanism and intercalation mechanism and (d) the role of ZHS in the discharge process 50. (e) Mn fluorescence maps at fully discharged (D) and charged (C) states 63."

Fig 4

(a) Schematic diagram of the synthetic procedure of MOC. (b) The rate performance of zinc ion battery with MOC-5, MOC-15 and MO as positive electrodes and 2 mol∙L?1 ZnSO4 and 0.1 mol∙L?1 MnSO4 as electrolyte at different current densities and (c) the cycle performance at current density of 0.2 A∙g?1 65. (d) HRTEM images of MnO2@PEDOT sample and (e) Cycling performance and Coulombic efficiency collected at 1.86 A∙g?1 for 300 cycles of flexible quasi-solid-state Zn//MnO2@PEDOT battery. (f) Photographs of a neon sign composed of 45 light-emitting diodes powered by three flexible quasi solid Zn//MnO2@PEDOT battery devices and (g) a watch with LED lights powered by three devices 72."

Fig 5

(a) Schematic diagram of crystal structure of β-MnO2 nanorod sample and (b) (101) lattice planes in the β-MnO2 structure. (c) EXAFS spectra of the β-MnO2 nanorod cathode collected after discharging/charging 30. (d) Schematic diagram of the formation of δ-MnO2 nanosheets. (e) Nyquist diagram of δ-MnO2 nano sheet and microsphere electrode (black line is fitting line) 79. (f) Preparation diagrams of α-MnO2 NFs and α-MnO2/CNT HMs at different synthesis stages and SEM images of corresponding products and (g) rate capability comparison at voltage cutoffs of 1.2–1.85 V of ZIBs with α-MnO2 NFs and α-MnO2/CNT HMs 81."

Fig 6

(a) Schematic diagram of PANI-MnO2 nanolayers structure. (b) HR-TEM image of PANI-MnO2 nanolayers structure after removing PANI by heat treatment at 400 ℃ 54. (c) XRD pattern and crystal structure of cw-MnO2. (d) Left: Zn-intercalation structure and corresponding energy; Right: the inner-sphere complexation of zinc leads to the protrusion of the facing Mn, constituting a Zn-Mn dumbbell structure as zoomed (numbers represents the distance between atoms) 83. (e) Schematic diagram of pre-intercalated Na+ and water molecules δ-MnO2 during charging and discharging. (f) ex situ XRD pattern of δ-NMOH electrode 87."

Fig 7

(a) GITT curves and corresponding H+ diffusion coefficients of KMO and α-MnO2 electrodes under different discharge/charge states. (b) Schematic diagram of H+ diffusion in the complete structure and oxygen defect structure of KMO. (c) Long cycle performance of KMO and α-MnO2 electrode and the last ten charge/discharge curves of KMO electrode at 1000 mA∙g?1 92. (d) Crystal structure of β-MnO2 and Ce doping β-MnO2; (e) EIS Nyquist diagram and equivalent circuit of the β-MnO2 and 0.1 mmol Ce doping cathode 85."

Fig 8

(a) Schematic diagram of Zn//MnO2 battery in acetate electrolyte. (b) Atomic structures for the dissolution reaction on MnO2 with a bare surface and with an acetate-rich surface 98. (c) Schematic illustration of Zn//MnO2 in the hybrid electrolytes during charge/discharge and electron density difference between (d) electrooxidized MnO2 (101) and (e) Ni-MnO2 (101) 106. (f) Electrochemical mechanism of Mn(AC)2 (top left corner) and MnSO4 electrolyte (top right corner). (g) The charge/discharge curve of the flow battery with electrolyte of 0.5 mol∙L?1 Mn(AC)2 + 0.5 mol∙L?1 Zn(AC)2 + 2 mol∙L-1 KCl and (h) the cycle performance at current density of 40 mA∙cm?2 109."