Hydrogen‐Bond Reinforced Superstructural Manganese Oxide As the Cathode for Ultra‐Stable Aqueous Zinc Ion Batteries

Layered manganese oxides adopting pre‐accommodated cations have drawn tremendous interest for the application as cathodes in aqueous zinc‐ion batteries (AZIBs) owing to their open 2D channels for fast ion‐diffusion and mild phase transition upon topochemical (de)intercalation processes. However, it is inevitable to see these “pillar” cations leaching from the hosts owing to the loose interaction with negatively charged Helmholtz planes within the hosts and shearing/bulking effects in 2D structures upon guest species (de)intercalation, which implies a limited modulation to prevent them from rapid performance decay. Herein, a new class of layered manganese oxides, Mg0.9Mn3O7·2.7H2O, is proposed for the first time, aims to achieve a robust cathode for high‐performance AZIBs. The cathode can deliver a high capacity of 312 mAh g−1 at 0.2 A g−1 and exceptional cycling stability with 92% capacity retention after 5 000 cycles at 5 A g−1. The comprehensive characterizations elucidate its peculiar motif of pined Mg‐□Mn‐Mg dumbbell configuration along with interstratified hydrogen bond responsible for less Mn migration/dissolution and quasi‐zero‐strain characters. The revealed new structure‐function insights can open up an avenue toward the rational design of superstructural cathodes for reversible AZIBs.


Introduction
In consideration of increasingly severe carbon emission issues and rigid stipulations for low-carbon society objectives, cur-hinder their development prospect. [5] While Prussian blue analogues can deliver the highest working voltage plateau (normally > 1.4 V vs Zn 2+ /Zn) and feasibility of mass production, the poor cycling stability and low theoretical capacity (<200 mAh g −1 at low current densities) are obvious drawbacks compared with others. [6] Therefore, manganese-based cathodes come under the spotlight due to satisfactory voltage plateaus and specific capacity. [7] In particular, δ-MnO 2 materials with layered structures are the most promising hosts compared to other tunneled MnO 2 polymorphs, such as α-MnO 2 (2 × 2 tunnels), β-MnO 2 (1 × 1 tunnels), γ-MnO 2 (1 × 2 and 1 × 1 tunnels), λ-MnO 2 (1 × 3 tunnels) due to a large d-space (≈0.7 nm) between MnO 6 octahedra formed two-dimensional (2D) slabs and relatively less phase transition during charge storage processes. [8] Nevertheless, this type of cathode is still challenged by inferior cycling stability, sluggish reaction kinetics, and fast capacity attenuation because of dramatic volume changes, structural disorder, and Mn 2+ dissolution. [9][10][11] Many attempts of modifying pristine δ-MnO 2 have been proposed including defect engineering, pre-intercalated guest species, and compositing with electrically conductive agents, delivering enhancements on battery performance to some extent. [12] For instance, the pre-intercalation strategy is one of the most acknowledged methods to tune intrinsic properties of δ-MnO 2 hosts via reconstruction of electronic structures and reinforcement of interlayer interactions. [13,14] Meanwhile, bounded water of guest species can provide shielding effects by reducing electrostatic positive charges with apical oxygen within the host frameworks. As a consequence, electrical conductivity, diffusion kinetics, and structural stability can be effectively improved. [15,16] Nevertheless, there are two scientific facts which are easily neglected in previous studies on the pre-intercalation strategy of δ-MnO 2. First, intrinsic ion-exchangeable properties of δ-MnO 2 hosts have been revealed iteratively. [17][18][19][20] In other words, the "pillar" ions can be replaced by various cations due to the feeble electrostatic interactions on Helmholtz planes. [21] It is conceivable that the merits of enhanced electrochemical performances derived from deliberately introduced "pillars" could hardly be stable especially when confronted with rich Zn 2+ and protons. Second, the irreversible phase transformation induced by shearing and buckling effects of transition metal oxide layers is the culprit responsible for the structural failure during (dis)charge processes for AZIBs, but which was not discussed thoroughly. [22,23] Hence, the innovation of crystallographic structures is urgently needed to bring substantial progress for reversible AZIBs cathodes.
To overcome these issues, state-of-the-art cathode materials regarding new configurations of Mn-O motif and enhanced interlayer interactions are promising for less phase changes and inhibited distortion/dissolution during (de)intercalation processes. [24] As a proof of concept, a new superstructural 2D manganese oxide, Mg 0.9 Mn 3 O 7 ·2.7H 2 O, is first-time proposed to gain extraordinary battery performance. More specific, the high specific capacities of 312 mAh g −1 and 132 mAh g −1 at 0.2 A g −1 and 5 A g −1 , respectively, along with a 92% capacity retention after 5000 cycles can be achieved. Meanwhile, through in/ex situ experimental and theoretical comparison of structural/ chemical evolutions and electrochemical properties among conventional layered manganese oxides (δ-Mg 2 Figure 1a. In comparison, δ-Na 0.55 Mn 2 O 4 ·2.4H 2 O and their derivatives exhibit different morphologies of wrinkled layers (Figure 1b and S1, Supporting Information). Meanwhile, energy dispersive X-ray spectroscopy (EDS) mapping images of the derivatives confirmed that no Na residue was left after ion-exchange treatment, implying the fact of inherent properties of δ-MnO 2 hosts as mentioned above. Additionally, the features of surface areas were not only determined by gas adsorption approach on pristine powders, but evaluated by electrochemically active surface area (ECSA) on as-prepared electrodes of δ-Mg 2 Figure 1c. [25] The Rietveld-refinement crystal diffraction profiles further verified that the as-prepared Mg 0.9 Mn 3 O 7 ·2.7H 2 O possess the same space group of R3 with slightly reduced lattice parameters (a = b = 7.534(4) Å, c = 20.767(2) Å, α = β = 90°, γ = 120°) compared with the standard material, especially for the c lattice constant which reflects a decreased d-spacing along [001] direction from 6.93 Å to 6.86 Å. This phenomenon can be interpreted by forming Mg vacancy in the pristine material and will be discussed in detail in the following content. Additionally, the scheme of the conjectural crystallographic structure of Mg 0.9 Mn 3 O 7 ·2.7H 2 O supercell projected from different orientations was demonstrated in Figure S4a, Supporting Information.  [27] Meanwhile, it is seen from the top view that an ordered □-Mn arrangement formed in the layer due to the regularly depleted Mn surrounded by triangular MnO 6 octahedra combinations giving rise to constructing an intrinsic Mn vacancy. Furthermore, Mg 2+ is pinned above and below the vacancy and coordinated with hydrated water molecules. As for δ-Mg 2 Mn 14 O 27 ·nH 2 O, a small amount of pre-intercalated ions and hydrated water molecules form a complex interlayer coordination within MnO 2 slabs. As a result, we speculate that these different superstructural manganese oxides could have significant modulation on their electrochemical properties because of different "pillar" ion stability and allotropic forms of MnO 6 octahedra.
Additionally, the surface chemical information of as-prepared Mg 0.9 Mn 3 O 7 ·2.7H 2 O and δ-Mg 2 Mn 14 O 27 ·nH 2 O were carried out by X-ray photoelectron spectroscopy (XPS) as shown in Figure 1f and S4c,d, Supporting Information. The deconvolution of core level spectra of Mn 3s clearly identifies the only Mn 4+ species existing in Mg 0.9 Mn 3 O 7 ·2.7H 2 O, while there is partial reduction of valence states of Mn 4+ in δ-Mg 2 Mn 14 O 27 ·7.9H 2 O, reflected by a relatively higher magnitude of peak splitting of 5.1 eV compared with 4.8 eV in Mg 0.9 Mn 3 O 7 ·2.7H 2 O. The phenomena of oxidation states also are in good accordance with previously reported preintercalated δ-MnO 2 . [28,29] Moreover, the core level spectra of Mg and survey spectra further confirm that only Mg 2+ species exist in both Mg pre-intercalated MnO 2 materials. Through XPS and TEM-EDS analysis (Figure 1g,h), it is indicated that the chemical formulas of these two pristine materials are Mg 0.9 Mn 3 O 7 ·2.7H 2 O and δ-Mg 2 Mn 14 O 27 ·7.9H 2 O, which also proves a small amount of Mg vacancy generated in Mg 0.9 Mn 3 O 7 ·2.7H 2 O during the synthesis. The profiles derived from Fourier transform infrared (FTIR) spectroscopy in Figure 1f confirm that both layered materials possess water molecules manifested as distinguished O-H bending and H-O-H stretching vibration modes at ≈1600 cm −1 and 3200-3500 cm −1 , respectively. [30] The thermogravimetric analysis (TGA) was equipped to determine the weight percentage of lattice/absorbed water as shown in Figure 1j. The weight loss of 14% and 21% for Mg 0.9 Mn 3 O 7 ·2.7H 2 O and δ-Mg 2 Mn 14 O 27 ·7.9H 2 O, respectively, can be observed in a temperature range from 100 ≈ 500 °C, and relatively higher dehydration temperature suggest higher thermodynamic stability of the lattice water in Mg 0.9 Mn 3 O 7 ·2.7H 2 O, which could deliver steady charge shielding effect for fast charge diffusion.

Battery Performance Evaluation
To assess the AZIB performance of Mg 0.9 Mn 3 O 7 ·2.7H 2 O and pre-intercalated δ-MnO 2 cathodes, coin-type and Swagelok cells are assembled for the electrochemical performance evaluations. O were also conducted to demonstrate "pillar" effects on the battery performance as shown in Figure S5, Supporting Information. It is seen that all δ-phase MnO 2 cathodes present similar CV curve characters but slightly different response currents of relative magnitude between two pairs of redox-active peaks, implying different charge storage properties related to H + and Zn 2+ co-(de)intercalation reported before. [31] Wang and co-works suggested that the high voltage plateau are dominantly controlled by H + insertion/extrusion due to relatively small hydrated radius, whereas the low voltage plateau are governed by Zn 2+ insertion/ extrusion manifesting a relative sluggish diffusion kinetics. [32] Therefore, δ-Mg 2 Mn 14 O 27 ·7.9H 2 O exhibits the largest magnitude of current responses at low voltage plateau rather than the other two δ-phase MnO 2 , which could be inferred by an enhanced Zn 2+ diffusion kinetics due to the largest interlayer spacing. These results are in good accordance with previous studies on interlayer spacing modulation to improve reaction kinetics. [33,34] Additionally, with pillar effects enhancement, the long cycling performance of δ-Na 0.55 Mn 2 O 4 ·nH 2 O is superior to that of δ-MnO 2 ·nH 2 O reflecting as good capacity retention of 68% against 39%, respectively, after 100 (dis)charge processes at the current density of 0.2 A g −1 , and 72% against 32%, respectively, after 5000 cycling tests at 5 A g −1 . Intriguingly, it is seen that there is remarkable capacity decay of all δ-phase MnO 2 even though some of them delivered "pillar effect" protection from structural collapse. Meanwhile, these unsatisfied cycling stabilities of δ-phase MnO 2 cathodes are also verified by many previous studies (Table S1, Supporting Information), giving rise to concerns about their structural stability.
To disclose the nature of rapid capacity decay of δ-Mg 2 Mn 14 O 27 ·7.9H 2 O cathode, GCD curves by 50 cycles of charge/discharge treatment under a low current density (100 mA g −1 ) were carried out for both Mg 2+ pre-intercalated MnO 2 . Obviously, both cathodes have two distinct discharge plateaus (DPs) which are divided by a tuning point ≈1.35 V referring to different stages of charge storage. Upon the initial activation process of both cathodes, two DPs increase dramatically as shown in the blue-green and yellow-red regions in Figure 2f. With proceeding the cycling measurements, the specific capacity of low DP (<1.35 V) in δ-Mg 2 Mn 14 O 27 ·7.9H 2 O displays a significant drop from the initial 106 mAh g −1 to 47 mAh g −1 compared with that (134 mAh g −1 to 92 mAh g −1 ) of Mg 0.9 Mn 3 O 7 ·2.7H 2 O, while the specific capacity of high DPs in both cathodes are relatively reversible. This phenomenon can be speculated as an irreversible feature of the δ-phase host for Zn 2+ intercalation, which will be proved in structural evolution in the following section.   [35][36][37][38][39][40] V 2 O 5 [41][42][43] and PBAs [44][45][46] based cathodes are summarized in the Ragone plot as shown in Figure 2h, which illustrate outstanding energy/power densities of Mg 0.9 Mn 3 O 7 ·2.7H 2 O were gained as high as 418.4 Wh kg −1 /6.5 kW kg −1 for AZIBs.

Kinetic Behaviors
To reveal underlying mechanisms of promoted electrochemical performances of Mg 0.9 Mn 3 O 7 ·2.7H 2 O compared with δ-Mg 2 Mn 14 O 27 ·7.9H 2 O, the electrochemical reaction kinetics were characterized by CV analysis, galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS). Figure 3a shows the CV profile of Mg 0.9 Mn 3 O 7 ·2.7H 2 O with a 67% capacitive contribution at a sweep rate of 0.5 mV s −1 . Moreover, with the stepwise growth of sweep rates from 0.1 to 1 mV s −1 , it is seen that the fraction of capacitive contribution of Mg 0. 9 (Figure 3c,d). Notably, two pairs of redox peaks (C1 and C2) at various cathodic sweeps of both materials are distinguishable, but the evolution tendency of C1/C2 and A1/A2 ratios is different upon increasing sweep rates due to the limited Zn 2+ transfer kinetics of δ-Mg 2 Mn 14 O 27 ·7.9H 2 O, which is consistent with GCD profiles. Meanwhile, homologous processes of charge storage can be discerned by the linear relationship for log i versus log v using the equation [47] : Where the peak current i and sweep rate v can be readable and a, b are adjustable parameters. The value of b within a range of 0.5 -1 indicates different charge storage behaviors with respect to capacitive (b = 1) and diffusion-controlled (b = 0.5) mechanisms. Therefore, linear fitted b values of both cathodes are conformably fallen into a scope between 0.5 and 0.8 according to well-defined anodic/cathodic peaks from the plots, which implies their electrochemical reaction behaviors are governed by both diffusion-controlled and capacitive processes. The higher b values (0.66, 0.76, 0.55, and 0.78)  Additionally, the solid-state diffusion kinetics of both Mginvolved manganese oxides were evaluated by GITT as shown in Figure 3e, in which the straightaway voltage drop/rise after applied galvanostatic current can be regarded as the uncompensated charge transfer resistance, following with the gradual voltage change during the relaxation period which associates with the ion diffusion process. Hence, there is a remarkable difference in the overpotential values between two DPs which are attributed to the dissimilarity of ion diffusion properties related to proton and Zn 2+ dominated insertion/extrusion reactions, agreeing well with the reported H + and Zn 2+ co-insertion system. Moreover, a relatively higher diffusion coefficient (≈10 −10 to 10 −13 m 2 s −1 ) of Mg 0.9 Mn 3 O 7 ·2.7H 2 O compared with that (≈ 10 −11 to 10 −14 m 2 s −1 ) of δ-Mg 2 Mn 14 O 27 ·7.9H 2 O further prove the improved kinetics in the superstructural manganese oxide. In addition to GITT analysis, EIS characterizations were carried out to determine their charge-transfer features and ion-diffusion kinetics upon initial and fully charged states after cycling tests of two electrodes. Figure 3f presents  O is more favorable for charge storage compared to conventional ribbon-like ordered MnO 6 co-planes along with perishable "pillars" of δ-MnO 2 in AZIBs system.

H + /Zn 2+ Electrochemical Reaction Mechanism
To gain further mechanistic insight of structural evolution for Mg 0.9 Mn 3 O 7 ·2.7H 2 O and δ-Mg 2 Mn 14 O 27 ·7.9H 2 O, multiple in situ and ex situ characterizations of the electrodes upon varied (dis)charge states. The ex situ SEM images and EDS of both Mg involved layered manganese oxide cathodes were detected to determine elemental ratio variations and morphology changes at multiple stages, which display different results regarding to Mg species content within the electrodes. As for Mg 0.9 Mn 3 O 7 ·2.7H 2 O, it is seen that there are no prominent changes of Mg:Mn ratio (≈0.7:3) upon varied charge/discharge states (Figure 4a,b, and S7a, Supporting Information). In contrast, no trace signals of Mg were found in all states of δ-Mg 2 Mn 14 O 27 ·7.9H 2 O electrode after the cycling test, implying an absence of deliberately introduced "pillar" ion in the framework ( Figure S8a, Supporting Information). Meanwhile, a dramatically increased zinc species during the discharge process along with flakes crystal occurring on the surface of both electrodes indicate the generation of Zn 4 SO 4 (OH) 6 ·5H 2 O (ZHS) especially when the working voltage is close to fully discharged state. Furthermore, this precipitation is reversible and can disappear as the working voltage rise during the charging process, which can be also identified by XRD characterizations in the following content. The phenomena have been extensively characterized in Mn-based cathode for AZIBs owing to the localized pH increase on the neighboring surface of electrodes via the depletion of the proton. [48] Additionally, ex situ XPS results of Mg 0.9 Mn 3 O 7 ·2.7H 2 O electrodes were presented in Figures 4c and S7b,c, Supporting Information, to further validate the change of elemental composition and valence states upon fully charge/discharge processes. The core-level Mn 2p 3/2 spectra clearly reveal a reversible oxidization and reduction of Mn species at charge and discharge states, respectively, manifesting as the reversible shift of the binding energy from 641.9 eV to 641.6 eV agreed with previously reported redox pairs of Mn 3+/4+ species. [49] Moreover, the core-level spectra of Zn 2p and Mg 1s clearly indicate the inserted zinc species vary with signal intensities, and steady Mg 2+ content exists in the (dis)charge electrodes (Figures 4c and S7b,c, Supporting Information). While it is also observed that there is a reversible redox pair of Mn 3+/4+ species along with varied strength of Zn 2p signals upon (dis)charge processes in the electrodes of δ-Mg 2 Mn 14 O 27 ·7.9H 2 O characterized from Mn/Zn 2p core-level and survey spectra, respectively. However, there is still no Mg species appearing in these electrodes. These results further confirm the unstable feature of pillar ions in δ-phase MnO 2 ( Figures S8b and S9a,b, Supporting Information) and which agreed well with the previous observation of Na + leaching from the layered MnO 2 host during electrochemical reactions. [20,50] In addition to elemental characterizations, Figure 4d demonstrates the structural evaluation of Mg 0.9 Mn 3 O 7 ·2.7H 2 O at varied (dis)charge states via ex situ XRD. It is seen that all characteristic peaks corresponding to Mg 0.9 Mn 3 O 7 ·2.7H 2 O can be easily identified from the plots. Meanwhile, newly emerged diffraction peaks are well-indexed with ZHS (JCPDS No. 44-0674) at both discharge states of 0.9 V in 1 st and 20 th cycled electrodes, in line with the results from SEM images. Moreover, there are neglectable shifts (from 5.91° to 5.83°) of the (003) diffraction peak after the battery discharge, and then reversibly return to 5.91° at the 20 th charged state, implying a superior reversibility of crystallographic structures and ≈1% lattice volume changes upon the H + and Zn 2+ (de)intercalation. Whereas the δ-Mg 2 Mn 14 O 27 ·7.9H 2 O electrode experienced more significant changes of d-spacing of (001) plane along caxis reflecting a relatively irreversible shift of peaks from 4.7° to 3.9° (∆d-spacing = 1.8 Å) at the 1 st and 20 th charged states, respectively, accompanying with gradually broader full width at half maximum compared with the peak located at 4.2° as for the pristine electrode ( Figure S9c, Supporting Information), which indicates a relatively large volume changes along with increased chaos of crystal lattices upon cycling treatments. Furthermore, ZHS can be identified at fully discharged states in δ-Mg 2 Mn 14 O 27 ·7.9H 2 O electrodes, and it can vanish upon charge processes. Figure 4e exhibits the HAADF-STEM image referring to the fully discharged state of Mg 0.9 Mn 3 O 7 ·2.7H 2 O, in which the intercalated zinc ions with slightly expanded d-spacing (2.43 Å) of the lattice fringe corresponding to the (211) plane shows the consistent change as observed from XRD characterizations. Also, in situ Raman spectra were carried out to further determine the structural evolution (Figure 4f). Comparably, the vibration modes of manganese oxides have two characteristic bands sensitive to the layered framework. The one (ν 1 ) at ≈570 to 590 cm −1 attributes to Mn-O stretching mode of the basal plane within octahedral MnO 6 layers, whereas the other one (ν 2 ) shows up ≈620-680 cm −1 referring to Mn-O symmetric stretching vibration in octahedral MnO 6 . [51] It is also verified that the distinguishable features with strong intensities exist in Mg 0.9 Mn 3 O 7 ·2.7H 2 O at the positions of 571 and 665 cm −1 , respectively, at fully charged states. During the discharge process, both peaks belonging to the two vibration modes gradually become weaker, especially at the fully discharged state, which is in agreement with previously reported alkali ion inserted δ-MnO 2 , attributing to structural change induced varied phonon properties upon Zn 2+ /H + intercalation. [52] Meanwhile, it is also observed that both vibration modes have slight redshifts to 567 and 662 cm −1 during discharge processes suggesting a reduction of Mn(IV) species in octahedral MnO 6 . Reversibly, the Raman peaks experience blueshift upon following charge processes and finally reach to the same position, demonstrating its superior structural stability. In contrast to the reversible changes of Raman vibration modes in Mg 0.9 Mn 3 O 7 ·2.7H 2 O, the bands at 668 cm −1 and 498 cm −1 standing for the out-plane Mn-O stretching vibration modes in pristine δ-Mg 2 Mn 14 O 27 ·7.9H 2 O present dramatical redshifts at the 10 th fully charged state (660 cm −1 ), and more offset (658 cm −1 ) were characterized at 20 th fully charged state, suggesting gradually softening photon modes presumably derived from irreversible local coordination environment in the lattice and chemical states ( Figure S10a, Supporting Information). Similarly, it is seen that there are more significant red shifts occurring in discharged states of the 10 th and 20 th cycles for δ-Mg 2 Mn 14 O 27 ·7.9H 2 O electrodes, respectively, along with newly emerging broad peaks at 289-345 cm −1 (Zn-O stretching modes). The results indicate an insertion of Zn 2+ /H + , which agree with previous Raman spectroscopic studies on intercalated δ-MnO 2 . [52,53] Additionally, the inductively coupled plasma optical emission spectrometry (ICP-OES) analysis of the Mn species concentration in a cycled 3 M ZnSO 4 electrolyte exhibits that the dissolution of Mn species could be significantly alleviated in Mg 0.9 Mn 3 O 7 ·2.7H 2 O cathode after 10 and 20 cycles, respectively, at fully discharged states ( Figure S10b

DFT Simulations
To gain deep insights into geometric and electronic structure variations of Mg 0.9 Mn 3 O 7 ·2.7H 2 O during the electrochemical process, DFT calculations were conducted. The structure of bulk MgMn 3 O 7 ·3H 2 O are calculated, which is exhibited in Figure 5a. It is further confirmed that the structure consists of two-dimensional MnO 6 octahedral layers where 1/7 of the Mn atoms are removed orderly in the (001) direction (Mn 3 O 7 layers) and a layer of Mg 2+ is located between Mn 3 O 7 layers to form Mg-□Mn-Mg dumbbell superstructural motif. Moreover, Mn, O1, O2, and WO (WO refers to the oxygen from the bounding water) locate on Wyckoff positions 18f, and Mg and O3 are on Wyckoff positions 6c. One Mg 2+ cation is bonded to three equivalent O2 and three equivalent MO atoms that come from the molecular water to form MgO 6 octahedra that share corners with six equivalent MnO 6 octahedra. The electronic structure of MgMn 3 O 7 ·3H 2 O including the partial density of states (PDOS) and band structures are also calculated and projected in Figure 5b,c. For the valence band top, the 2p orbitals of O made the major contribution, while the 3d orbitals of Mn primarily locate above the Fermi level. The electrons of 3p (Mg) and 2p(O) hybridize with the 3d orbitals of Mn 4+ at the valence band top and conduction band bottom. The band gap is indirect from A to H in Figure 5c and (Figure 5d) and the average interlayer spacing is 6.97 Å, further verifying its robust character as quasi-zero strain hosts for zinc storage. This is mainly due to the hydrogen bonds and MgO 6 acting as pillars between Mn 3 O 7 layers, which is beneficial for reversible charge/discharge processes.
In addition, Nam et al. also verified that an interlayer spacing of ≈7 Å could alleviate Mn dissolution in crystal water accommodated δ-MnO 2 since a higher diffusion barrier has to overcome. [54] In contrast, for a larger interlayer spacing (>10 Å), Mn 2+ could migrate into the electrolyte during Zn 2+ insertion as the trend is energetically and kinetically favorable, which takes responsibility for capacity fading in manganese oxides. As the results, ab initio molecular dynamics (AIMD) simulations were adopted for the 111-atom supercell of Zn 1 Mg 5 Mn 18 O 42 ·15H 2 O at 300 K in the NVT ensemble, which indicate that Zn insertion is unable to cause Mn diffusion in the host and significant structural distortion (Figures S11 and S12, Supporting Information), which is varied from conventional layered MnO 2 with respect to significant MnO 6 distortion for forming Zn-Mn "dumbbell" superstructures ( Figure S13, Supporting Information).
The average Mn-O bond changes are also illustrated in Figure 5d, those bond length changes less than 0.1 Å are generally ignored, For Mg 6 1-4) have also been illustrated in Figure 5e. The insertion of Zn indicates a considerable enhancement in the electrical conductivity with the shrinkage of the band gap. There are all Mn 4+ , the dx 2 -y 2 , dz 2 , and dxy are occupied in Mg 6 Mn 18 O 42 ·15H 2 O. With the reduction caused by Zn insertion, the electrons will gradually occupy the dyz and dxz, and the mixture of dyz and dxz peak appears obviously in the PDOS of 2 Zn for the first Zn balance the value state change of the Mg defect. Then, the continuous intercalation of Zn gradually increases the dyz and dxz mixed peak around the Fermi level and greatly improved electronic conductivity by narrowing the band gap.

Conclusion
In summary, this study demonstrates that new superstructural 2D manganese (IV) oxides, Mg 0.9 Mn 3 O 7 ·2.7H 2 O, possess pinned MgO 6 layers and built-in Mn vacancy within its distinguishable layers along with interlaminar hydrogen bond reinforcement. This cathode material can deliver a high specific capacity of 312 mAh g −1 at 0.2 A g −1 and outstanding cycling stability of 92% capacity retention after 5000 cycles at 5 A g −1 . Meanwhile, superior rate capabilities and quasi-zero volumetric change of the lattice structures further verify its feasibility as a promising cathode host in practical applications. Importantly, through a detailed comparison with typical δ-MnO 2 with various pre-intercalated cations, we proposed a solution to meet the unstable nature of pillar cations caused by loose electrostatic interactions and serious manganese dissolution from the structural disorder. Thus, a new paradigm of 2D material is successfully developed in this study, which could offer more inspiration on modulation of geometric orders on both octahedral MnO 6 and pillar ions layers in superstructural materials for high-performance AZIBs application.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.