Fig 1

A brief development on Mn-based cathodes of ZIBs, including a few representative studies on reaction principles and improvement measures."

Fig 2

Schematic diagrams of the four Zn2+ ion storing mechanisms for manganese-based cathodes of ZIB: (1) Zn2+ insertion/extraction principle; (2) Zn2+ and H+ co-insertion/extraction principle; (3) dissolution-deposition reaction principle; (4) chemical conversion reactions."

Fig 3

(a) The invertible insertion/detachment process of Zn2+ into the tunnel of α-MnO2 cathode. (b) Zn 2p core level spectra of MnO2 cathodes in the pristine, extraction, and inserted condition. (c) X-ray diffraction (XRD) spectrum of α-MnO2 cathodes in the initial, extracted as well as inserted circumstance. Reprinted by permit from the Ref. 58. (d) Formation of Mn defects and invertible Zn2+ interposition/extraction in a MnO skeleton. (e) Electric charge allocation and structural changes of MnO and Mn0.61□0.39O before and after the bivalent Zn2+ ion insertion. Reprinted by permit from Ref. 59. (f) Schematic diagram of the H+ and Zn2+ co-insertion/detachment principle of the cathode in pure water electrolytic solution. (g) Schematic diagram of hydrophilic polyethylene glycol (PEG) electrolyte changing the solvation environment of electrolyte and forming the proton shielding in H2O-50% PEG system. Reprinted by permit from Ref. 60."

Fig 4

(a) Charge-discharge profiles of Zn/MnO2@carbon fiber paper (CFP) battery at various rates in the first circulation. (b) The discharge curves of the cathode using the three-electrode cell in 0.2 mol∙L−1 MnSO4 electrolytic solution with or without Zn2+ at a rate of 0.3C in the range from 0 to 0.8 V (vs. Ag/AgCl). (c) ex situ XRD measurements of the MnO2@CFP cathode at discharge depths of 1.3 and 1.0 V, respectively, keeping the respective discharge depths for 1 h. Reprinted by permit from Ref. 67. (d) Discharge galvanostatic intermittent titration technique (GITT) measurement of the α-MnBOx cathode (at 50 mA∙g−1 for 120 s followed by a 0.5 h rest). (e) Electrochemical impedance spectroscopy (EIS) spectra of cathodes at four different stages of the discharge (marked by four arrows in (d)). (f) Schematic diagram of the H+/Zn2+ insertion/deintercalation principle occurring at the α-MnBOx cathode. Reprinted by permit from Ref. 69 (g) Mechanistic diagram of H+ and Zn2+ insertion in polyaniline (PANI)-intercalated α-MnO2. Reprinted by permit from Ref. 70. (h) Illustration of the joint charge storing principle consisting of nondiffusion controlled Zn2+ ion storage mechanism and diffusion controlled H+ transformation reaction in layered δ-MnO2. Reprinted by permit from Ref. 71."

Fig 5

(a) Schematic diagram of the mass transfer process during the charge-discharge course of the battery. (b) The cyclic voltammetry (CV) curves for the single electrode of MnO2–Zn cell. Reprinted by permit from Ref. 78. (c) The formation of a Zn-Mn flow battery. Reprinted by permit from Ref. 79. (d) Reaction principle of α-MnO2/Zn cell on the basis of Mn solubilization/re-combination in discharging process. (e) Variation of the content of Zn (black regular tetragon) and Mn (blue regular tetragon) at the electrolytic solution in the corresponding discharging-charging curve of the α-MnO2/Zn battery. Reprinted by permit from Ref. 80. (f) Illustration of the dissolution process of MnO2 in acidic electrolytes as well as the corresponding reaction. (g) Schematic illustration of the deposition/solubilization chemistry of MnO2 in prototype Cu//MnO2 batteries. Reprinted by permit from Ref. 81. (h) Charge/discharge curves of the rechargeable Zn/MnO2 battery with the mild electrolysing solution of ZnSO4 in the initial two cycles. Reprinted by permit from Ref. 41."

Fig 6

The challenges and optimization strategies to improve the electrochemical properties of Mn-based cathodes in ZIBs."

Fig 7

(a) Schematic comparison of the adsorption properties of Zn2+ on the facades of ideal δ-MnO2 (001) and δ-MnO2 (001) including oxygen vacancies. The schematic diagram of Zn2+ absorption/desorption processes for (b) ideal MnO2 and (c) MnO2 including oxygen vacancies. Reprinted by permit from Ref. 103. (d) High-resolution XPS spectra of O. (e) CV profiles of 700-Ar, 700-Air, and Mn-BTC electrodes at 0.1 mV∙s−1. (f) EIS profiles of 700-Ar, 700-Air, and Mn-BTC electrodes. Reprinted by permit from Ref. 104. Scanning electron microscopy (SEM) images of the surface morphology of (g) Bir-MnO2 and (h) Odc-MnO2. Reprinted by permit from Ref. 105. (i) X-ray absorption near-edge structure (XAN22ES) spectra of O 1s. Reprinted by permit from Ref. 100."

Fig 8

(a) Schematic diagram of the diffusion pathway of Zn2+ in the ideal ZnMn2O4 spinel framework and in ZnMn2O4 containing Mn vacant sites. Reprinted by permit from Ref. 108. (b) XPS spectra of initial and first totally charged statuses. Calculated density of states (DOS) of (c) partial MnO without Mn defect and (d) partial MnO with Mn defects. Reprinted by permit from Ref. 59. (e) Illustration of relative energy profiles along the reaction pathways (steps Ⅰ to Ⅳ). Reprinted by permit from Ref. 75. (f) Atomic configuration of manganese defects Mn3O4 (MO) (101) and six categories of manganese atoms. (g) A schematic diagram of the most possible structures for MO anchored on carbon nanotubes surface (DMO) and the formation of surface active sites. Reprinted by permit from Ref. 109. (h) Rate the performance of different materials at various current densities. Reprinted by permit from Ref. 113."

Fig 9

(a) Schematic of charge distribution for Mn2O3 and NM20 (denoting the atomic ratio of Ni and Mn as 1 : 20 of the NiMn-layered double hydroxide). (b) The projected density of states (PDOS) of the Mn2O3 and NM20. Reprinted by permit from Ref. 113. (c) Illustration of the prepared cobalt-modified δ-MnO2. Reprinted by permit from Ref. 117. (d) The crystal construction diagram of HxMn2O4 microspheres. Reprinted by permit from Ref. 120. (e) GITT profiles and the related Zn2+ ion diffusion coefficients in the batteries cycling process for MnO2 and S-MnO2 electrodes. Charge density difference distribution diagrams for Zn ion in (f) MnO2 (g) and S-MnO2. Reprinted by permit from Ref. 122. (h) Schematic demonstration of N-MnO2–x@TiC/C arrays. Density of states (DOS) of (i) initial MnO2 and (j) N-MnO2–x. Reprinted with permission from Ref. 100."

Fig 10

(a) The XRD pattern and (inset) crystalline structure of MnO2 containing crystal water (cw-MnO2). (b) The generation of a zinc-manganese dumbbell-shape construction in cw-MnO2. Reprinted by permit from Ref. 127. (c) The crystalline construction diagram of the as-synthesized Na0.55Mn2O4∙0.57H2O (NMOH). (d) SEM image of NMOH. (e) Galvanostatic charging-discharging curves of the Zn/NMOH battery examined from 0.8 to 1.9 V with various current densities. Reprinted by permit from Ref. 128. (f) Structural display of CaMnO. Reprinted with permission from Ref. 129. (g) Structure diagram of PANI-intercalated MnO2 nanoscale layers. Reprinted by permit from Ref. 70."

Fig 11

(a) Field-emission scanning electron microscopy (FE-SEM) image of the carbon-coated MnO2. Reprinted by permit from Ref. 141. (b) The high-resolution transmission electron microscope (TEM) (denoted as HRTEM) of α-MnO2/graphene scrolls. Reprinted by permit from Ref. 142. (c) Energy-dispersive X-ray (EDX) elemental mapping images of MnO2/polypyrrole nanorod. (d) The optimized configuration of the unordered MnO2 surface with polypyrrole covering. Reprinted by permit from Ref. 149. (e) Preparation schematic diagram of MnO2/rGO/PANI. Reprinted by permit from Ref. 137. (f) The model diagram of zinc ion insertion into CaSO4·2H2O (CS). (g) A schematic diagram of the possible migration path of Zn2+ in CaSO4∙2H2O. Reprinted by permit from Ref. 153. (h) Comparing of long-term cyclic performance of MnO@C with some former Mn-based cathodes in the Zn-including electrolytic solution without the addition of Mn2+ for ZIBs. Reprinted by permit from Ref. 154."

Fig 12

(a) SEM image of Mn3O4 deposited on carbon paper. (b) The cyclic performance diagram of the cell at a current density of 1 A∙g−1. (c) The illustration of the rate capability testing of the battery at diverse current densities. Reprinted by permit from Ref. 166. (d) TEM images of the α-MnO2@CNT foams. (e) Illustration of the preparation of flexible quasi-solid-state ZIBs using α-MnO2@CNTs composite films. Reprinted with permission from Ref. 172. (f) Schematic representation illustrating electron/ion transport of the heterofibrous network scaffold (HNS) electrode. (g) Contrast of the MnO2-based gravimetric energy/power densities (represented as Wh∙kg−1/W∙kg−1): HNS electrode vs. formerly investigated MnO2 electrodes. Reprinted by permit from Ref. 173."

Fig 13

(a) SEM image of the as-prepared MnO2 samples. (b) N2 adsorption/desorption isotherms and related pore size distributions of prepared MnO2 samples (inset). (c) Profiles of charging/discharging at a current density of 0.3 A∙g−1. Reprinted by permit from Ref. 184. (d) Schematic illustration of ultrathin layered 2D δ-MnO2 nanosheet formation. Reprinted by permit from Ref. 185."

Fig 14

(a) The schematic diagram of ZIB assembled with MnSO4 additive in ZnSO4 electrolyte. (b) The illustration of the enhanced structural stability of MnO2 cathode due to the presence of Mn2+ additive in the electrolyte. Reprinted by permit from Ref. 194. (c) A schematic diagram of the use of 0.1 mol∙L−1 Mn(CF3SO3)2 additive in ZIB. (d) Low magnification SEM image of MnO2 cathode with a scale bar of 5 μm. (e) High magnification SEM image of MnO2 cathode with a scale bar of 1 μm. Reprinted by permit from Ref. 195."

Fig 15

Schematic diagram of the practical application of excellent-performance ZIBs based on the Mn-based cathode materials. Reproduced by permit from Ref. 174,207-209."