Fig. 1. (a) Ragone plot of different energy storage devices; (b) Schematic diagram comparing the fundamental charge storage mechanisms of electrode materials in batteries and supercapacitors [9].

Fig. 2. Graftonite structure type: (a) [100], (b) [010] projection [48].

Fig. 3. 2D layers and 3D framework crystal structures: (a) in Mn3(PO4)2?0.5H2O, (b) in γ-Mn3(PO4)2 [43].

Fig. 4. Schematic diagram of manganese phosphate crystal transformation [43].

Fig. 5. (a) Schematic representation; (b) all morphologies of six different polymorphs of manganese phosphate synthesised by different preparation routes: (i) layered microsheets, (ii) hexagon-like microsheets, (iii) microdendrites, (iv) pentagon-like microsheets, (v) microbelts, (vi) thin microsheets [97].

Fig. 6. Schematic illustration of working principle of symmetric supercapacitor device constructed with AMP-P in aqueous electrolyte of 3 M KOH, the magnified view shown OH- are inserted onto the Mn site [97].

Fig. 7. (a) Typical SEM, (b) HRTEM images and (c) XRD patterns of NCM and MP-NCM [107].

Fig. 8. (a, d, g) Schematic illustration of the synthesis of P-MnO2-x/VMG shell/core arrays; SEM images of arrays under different magnification: (b, c) VMG, (e, f) MnO2/VMG, (h, i) P-MnO2-x/VMG [126].

Fig. 9. (a, d) TEM-HRTEM, (b-f) SAED images of MnO2/VMG and P-MnO2-x/VMG shell/core arrays; (g) EDS elemental mapping images of C, Mn, O, and P of P-MnO2-x/VMG [126].

Fig. 10. Schematic diagram illustrating the synthesis procedure of the PMn nanosheet arrays [128].

Fig. 11. TEM images at different magnifications: (a, b) Mn3O4 nanosheet, (c, d) PMn nanosheet [128].

Fig. 12. Schematic illustration of the exfoliation and assembly process for the assembly films and ASSS [139].

Fig. 13. TEM images: (a) Mn3(PO4)2·3H2O, (b) graphene, (c) assembled nanosheets [139].

Fig. 14. Schematic illustration for 2-propanol-assisted exfoliation process from bulk VOPO4·2H2O to graphene-like VOPO4 nanosheets [142].

Fig. 15. Schematic illustrating the construction of the VOPO4/RGO nanocomposite with vertically porous 3D structures through ice-templated self-assembly [143].

Fig. 16. SEM image of the porous 3D microstructures: (a) VOPO4-RGO nanocomposite developed in nickel foam; (b-d) Top view, cross-sectional side view and tilt view of porous 3D microstructures of the VOPO4-RGO nanocomposite prepared via ice-templated self-assembly; (e, f) High-magnification SEM and TEM images of the porous VOPO4-RGO nanocomposite [143].

Fig. 17. FE-SEM images of the obtained materials: (a) GO, (b) dried sample for delaminated slurry of VOPO4·2H2O, (c) VGO, (d) VRGO, (e) TEM image of VRGO [144].

Fig. 18. Electron density and atomic structure difference after proton intercalation: (a, c) R-MnO2, (b, d) β-MnO2 [162].

Fig. 19. SEM images (including inset): (a) 0.1 mg, (b) 0.2 mg, (c) 1 mg, (d) 5.8 mg, (e) 10.8 mg (insets: the corresponding magnifying SEM images, scale bar, 200 nm); (f) XRD of as-prepared δ-MnO2 carbon cloth after 20,000 cycles GCD [163].

Fig. 20. Schematic for the synthesis of V doped MnO2 nanosheets, and the insert shows the crystal structure of α-MnO2 with (1 × 1) or (2 × 2) tunnels [164].

Fig. 21. HRTEM image: (a) sample 0.5 VMO, (b) SEM image of sample with a V dopant amount at 1 mmol in the reaction solution, the insert shows the EDX [164].

Fig. 22. TEM images of layered δ-MnOx: (a) Li-MnOx, (b) Na-MnOx, (c) K-MnOx; (d) XRD patterns [165].