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[3]"

Fig. 2

(a), (b), (d), (e), (g), (h) Schematic cyclic voltammograms and (c), (f), (i) corresponding galvanostatic discharge curves for various kinds of energy-storage materials[22]"

Fig. 3

(a) Schematic illustrations and corresponding electrochemical responses of intrinsic and extrinsic pseudocapacitors; (b) Extrinsic pseudocapacitance, where high specific capacitance and pseudocapacitive behavior are only observed at the material surface; (c) Intrinsic pseudocapacitance of Nb2O5, where pseudocapacitive behavior is maintained even in thick films due to rapid Li+ transport within the bulk material[23]"

Fig. 4

Multiscale simulation methods, with images integrated from referenced publications [25-31]"

Fig. 5

(a) First-principles modeling schematic of Li+ intercalation into TiO2 (101)[26]; (b) Reorientation of water molecules during geometric optimization, showing initial and final configurations[33]; (c) Pourbaix diagram for the RuO2(110)/water interface; (d) LDOS of Ru and O atoms at br and cus sites[34]; (e) Energy level diagrams of different aromatic isomers[35]"

Fig. 6

(a) Gibbs free energy of Ti3C2T x(T = O, OH) in 1 mol/L H2SO4 electrolyte as a function of H coverage x at different electrode potentials relative to SHE; (b) Faradaic charge (blue, to balance proton transfer), EDL charge (black, due to surface net charge), and total charge (red, net electron transfer number) stored at different electrode potentials[41]; (c) Charge storage per formula unit vs the shift in the potential at the point of zero charge (ΔVPZC) and hydrogen adsorption free energy (ΔGH)[42]; (d) Ion-MXene distance dependence of the experimental specific capacitance[44]; (e) Schematic pictures of the capacitive and pseudocapacitive conditions formed inside the MXene electrode[45]; (f) Pseudocapacitive voltage window size increases with the scaled change in oxidation states during adsorption-induced redox on MXene surfaces[48]"

Fig. 7

(a) Snapshots of the ionic arrangements inside one of the Ti3C2F2 electrode poresatdifferent charged state: negatively charged, neutral, and positively charged, respectively[50]; (b) The RDFs forSO42--H2O and H2O-H2O obtained from MD simulations in 6 mol/L H2SO4[51]; (c) Comparison of probability profiles of dipole orientation of water molecules inside P-MXene and 500-MXene layers[52]"

Fig. 8

(a) Schematic of charge/discharge pseudocapacitive process of MoS2[55]; (b) Schematic illustrating the difference between typical pseudocapacitive intercalation and the desolvation-free intercalation observed in Ti3C2T x in WIS electrolytes[63]; (c) In situ XRD map of vacuum-filtered Ti3C2 in DMSO-based electrolyte; (d) MD simulation result for DMSO-based electrolyte, showing a MXene/Li+/DMSO/fast ion transport tunnel[57]"

Fig. 9

(a) Snapshot of a ReaxFF GCMC simulation of K+ and H2O intercalation into birnessite. The black arrow shows the region of local wrinkling of the layer due to the inhomogeneity of K+ distribution; (b) Snapshot after the interlayer is fully filled, flattening the wrinkling in the simulation domain[27]; (c) The continuous transition from electrostatic double-layer (I), through a transition region (II) to Faradaic intercalation (III) in nanoconfinement that is driven by the extent of ion solvation and the resulting ion–host material interaction[24]"

Fig. 10

(a) Schematic of the simulated one-dimensional hybrid pseudocapacitor cell consisting of a redox-active pseudocapacitive electrode and a carbon electrode with LiClO4 electrolyte in PC. The dashed line encloses the computational domain simulated[29]; (b) Li+ concentration evolution at the electrode/electrolyte interface as a function of applied potential under high scan rate (v = 1 V/s); (c) concentration of intercalated Li+ in the pseudocapacitive electrode at the electrode/electrolyte interface as functions ofcell potential s(t) at scan rate v=1 V/s; (d) Variation of total current density with diffusion coefficient; (e) Variation of total current density with Li+ concentration[65]"