H2O‐Mg2+ Waltz‐Like Shuttle Enables High‐Capacity and Ultralong‐Life Magnesium‐Ion Batteries

Abstract Mg‐ion batteries (MIBs) are promising next‐generation secondary batteries, but suffer from sluggish Mg2+ migration kinetics and structural collapse of the cathode materials. Here, an H2O‐Mg2+ waltz‐like shuttle mechanism in the lamellar cathode, which is realized by the coordination, adaptive rotation and flipping, and co‐migration of lattice H2O molecules with inserted Mg2+, leading to the fast Mg2+ migration kinetics, is reported; after Mg2+ extraction, the lattice H2O molecules rearrange to stabilize the lamellar structure, eliminating structural collapse of the cathode. Consequently, the demo cathode of Mg0.75V10O24·nH2O (MVOH) exhibits a high capacity of 350 mAh g−1 at a current density of 50 mA g−1 and maintains a capacity of 70 mAh g−1 at 4 A g−1. The full aqueous MIB based on MVOH delivers an ultralong lifespan of 5000 cycles The reported waltz‐like shuttle mechanism of lattice H2O provides a novel strategy to develop high‐performance cathodes for MIBs as well as other multivalent‐ion batteries.


Introduction
[3] Despite the advantages of magnesium-lithium hybrid batteries (MLHBs) in terms of energy density, cycle life, charging and discharging speeds, safety, etc., they still have the dendrite problem of lithium batteries. [4,5]Mgion batteries (MIBs) are promising nextgeneration energy storage electrochemical devices due to the abundance of Mg resources and the high safety endowed by the inhibition of sharp dendrites. [6,7]Similar to charge shuttling by Li + ions in LIBs, the energy storage of MIBs relies on the charge shuttling of Mg 2+ ions.Due to the high charge density of Mg 2+ ions, it is usually believed that the Mg 2+cathode interaction is purely ionic and very strong, and Mg 2+ ions must individually diffuse long distances through the cathode materials, leading to the sluggish kinetics of Mg 2+ migration and collapse of cathode crystal structure. [8,9]Hence, MIBs suffer severely from the low capacity and the short cycle life.To solve this challenge, recent excellent work has used carbon nanotube frameworks as electronic transport channels to enhance the Mg 2+ diffusion kinetics, thereby improving the capacity of MIBs, but the long-term cycling performance is still not satisfactory. [10]he Mg 2+ ions, unlike other metal ions including Li + , have strong interactions with H 2 O molecules and the resulting Mg─O bonds can be as short as 2.04 Å, which makes the migration of Mg 2+ distinguished with the presence of lattice H 2 O molecules.The lattice H 2 O molecules were reported to rapidly shuttle H + in their immobilized network via the strong interaction between H + and H 2 O molecules, which suggests a possible similar ability to shuttle Mg 2+ . [11][14] The solvent H 2 O molecules can even be inserted into the narrow spacing of lamellar cathode prior to cations, which impel the following co-(de)insertion of Mg 2+ /H + in aqueous MIBs. [15]However, it remains unanswered whether H 2 O molecules co-migrate with Mg 2+ in the cathode and how to use the Mg 2+ -H 2 O interaction to increase the capacity and cycle life of MIB cathodes.

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In order to provide enough lattice H 2 O molecules in cathode, we design the lamellar Mg 0.75 V 10 O 24 •4H 2 O (denoted as MVOH) cathode with a large spacing of 13.9 Å, which contains abundant lattice H 2 O to fast shuttle Mg 2+ for high capacity and pillar the lamellar structure for long cycle life; meanwhile, the mixed valence of V 4+ and V 5+ can facilitate the 3d electron hopping and thus increase the electronic conductivity of MVOH, further ensuring the high capacity.Our ab initio calculations and molecular dynamics simulations suggest an H 2 O-Mg 2+ waltz-like shuttle mechanism in our MVOH cathode.Unlike immobilization of H 2 O molecules during shuttling H + , the lattice H 2 O molecules coordinate and co-migrate with Mg 2+ during discharging, which realizes the waltz-like shuttle mechanism via adaptively rotating or even flipping to accelerate Mg 2+ migration kinetics; after Mg 2+ extraction during charging, the lattice H 2 O molecules rearrange to pillar the lamellar structure for stability improvement.Results show that the H 2 O-Mg 2+ waltz-like shuttle mechanism excludes the co-(de)insertion of H + with Mg 2+ , which eliminates electrolyte decomposition, further increasing the cycling life of MIBs.Consequently, the MVOH cathode exhibits a remarkably high discharge capacity of 350 mAh g −1 at a current density of 0.05 A g −1 and high-rate capability (70 mAh g −1 at 4 A g −1 ); the aqueous MIB with the MVOH cathode delivers an ultralong cycling life of 5000 cycles and a high energy density of 67 Wh kg −1 .

Characterization of 3D MVOH Cathode
Mg 0.75 V 10 O 24 •4H 2 O (denoted as MVOH) was obtained via a simple one-step precipitation method.In the X-ray powder diffraction (XRD) pattern (Figure 1A), the sharp diffraction peak located at 2 = 6.2°relates to the (002) planes, whose corresponding interlayer spacing is as large as 13.9 Å according to the Bragg equation, much larger than that of the reported hydrated Mg x V 5 O 12 (11.9Å). [14] The large interlayer spacing provides potentially abundant lattice H 2 O molecules.The stoichiometric formula of MVOH is determined to be Mg 0.75 V 10 O 24 •4H 2 O by combining the inductively coupled plasma-optical emission spectroscopy (ICP-OES, Table S1, Supporting Information) and thermogravimetric analysis (TGA, Figure 1B).Specifically, the 7.2% weight loss is attributed to the loss of lattice H 2 O, according to which there are 4 lattice H 2 O molecules for each unit cell of Mg 0.75 V 10 O 24 .In the X-ray Photoelectron Spectroscopy (XPS) survey (Figure S1, Supporting Information), the two peaks at 517.4 and 524.9 eV correspond to the V2p 3/2 and V2p 1/2 levels of V 5+ , respectively, while the peaks at 516.1 and 523 eV relate to the V2p 3/2 and V2p 1/2 levels of V 4+ . [16]According to the peak deconvolution, 65% of V exists in the form of V 5+ while 35% of V exists in the form of V 4+ ; the average valence of V is thus calculated to be +4.64,which is consistent with the chemical formula of Mg 0.75 V 10 O 24 •4H 2 O.The existence of V 4+ indicates that Mg 2+ has been pre-inserted into the interlayer of vanadium oxide and the valence of V has thus been partially reduced (Figure S1C, Supporting Information). [17][20] Furthermore, three distinct components can be observed in the O 1s core level, which are attributed to the O atoms in V─O (530.2 eV), Mg-O (531.4 eV), and H 2 O (533.2 eV), respectively (Figure S1D, Supporting Information). [21]The Raman spectrum and Fourier-transformed infrared (FT-IR) spectra of MVOH further verify its lamellar structure constituted by V─O skeleton layers (Figures S4 and S5, Supporting Information).
Scanning electron microscopy (SEM) images of MVOH display a nanoflower morphology, along with homogeneity and high yield, and the petal thickness is ≈60-70 nm (Figure 1C,D).The nanoflower morphology greatly increases the specific surface area of the MVOH and shortens the diffusion path of Mg 2+ from bulk solution to active sites, which thus accelerates the insertion/extraction of Mg 2+ .Transmission electron microscopy (TEM) images further show the nanoflower structure of MVOH (Figure S2, Supporting Information).In the highresolution TEM (HRTEM) images (Figure 1E,F), a lattice spacing of 0.35 nm is observed, which corresponds to the (110) lattice planes of MVOH.According to scanning transmission electron microscopy-Energy-dispersive X-ray spectroscopy (STEM-EDS) mapping (Figure S3, Supporting Information), the Mg, V and O elements distribute uniformly in MVOH nanoflowers, and the atomic ratio of Mg to V is ≈1.0:13.2(Table S2, Supporting Information), which is consistent with the ICP-OES results, further confirming the composition of Mg 0.75 V 10 O 24 •4H 2 O.The N 2 adsorption/desorption isotherm of MVOH exhibits a type-IV isotherm and a distinct H3 hysteresis loop within the relative pressure range of 0.80-0.98,which suggests that the pores in the nanoflowers consist of narrow slits or crack-shaped holes formed by the accumulation of flake particles (Figure S6A, Supporting Information). [22]According to the fitted adsorption/desorption curve in the relative pressure range of 0-0.26 (inset in Figure S6A, Supporting Information), the specific surface area of MVOH nanoflowers is 33.07 m 2 g −1 , which is higher than those of other vanadium oxides. [12,23]The MVOH nanoflowers exhibit an average pore diameter of 19.23 nm with a total pore volume of 0.12 cm 3 g −1 .The pore size distribution of the MVOH nanoflowers (Figure S6B, Supporting Information) centralizes on the macropores ≈52 nm, suggesting the hierarchical pore structure of MVOH nanoflowers, which can enhance the permeability of Mg 2+ between the MVOH cathode and the electrolyte, and thus increase the capacity of MVOH cathode.
To further obtain the MVOH crystalline structure with lattice H 2 O molecules mediating Mg 2+ conduction, density functional theory (DFT) calculations are performed on the basis of structural characterization results including XRD, TGA, and XPS.MVOH has a lamellar structure constituted by VO x polyhedra layers that consist of VO 5 rectangular pyramids and VO 6 octahedra (Figure 1G  to the formation of VO 6 octahedra, denoted as DC-1; and 2) O atoms in VO 5 rectangular pyramids via hydrogen bonds (DC-2).These DC lattice H 2 O molecules combine directly with either the downside surface of upper VO x layers or the upside surface of lower VO x layers in the interlayer space, which connect the adjacent two VO x layers by hydrogen bonds and function as supporting pillars to stabilize the lamellar structure of MVOH, and as targets to facilitate connectivity with Mg 2+ .These DC lattice H 2 O molecules are flexible and abundant to transfer newly inserted Mg 2+ via the waltz-like shuttle mechanism.

Mg 2+ Conduction for Fast Electrochemical Kinetics
In order to evaluate the intrinsic Mg 2+ storage capability of the MVOH nanoflowers as MIB cathode, a three-electrode configuration was assembled in ambient air with a platinum electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode in 2 m Mg(CF 3 SO 3 ) 2 aqueous solution.The initial three cycles of cyclic voltammetry (CV) curves within a potential window of −0.7-1.0V versus SCE at a scanning rate of 1 mV s −1 (Figure S8, Supporting Information).26][27] The CV curves of the initial three cycles differ slightly, suggest-ing excellent cyclic reversibility of the MVOH nanoflowers.At a current density of 0.05 A g −1 , the MVOH cathode exhibits a remarkably high discharge capacity of 350 mAh g −1 (Figure 2A), which surpasses those of reported Li 3 V 2 (PO 4 ) 3 (115.1 mAh g −1 ), MgMn 2 O 4 (166 mAh g −1 ), and -MnO 2 (220 mAh g −1 ) as MIB cathodes (Table S3, Supporting Information).This remarkably high capacity implies fast migration kinetics of Mg 2+ in MVOH nanoflowers with abundant lattice H 2 O.When the current density increases to 1 A g −1 (Figure 2A,B), a capacity of 149 mAh g −1 is obtained, which is about two times higher than that of the reported Mg x V 5 O 12 cathode. [14]With the current density increasing to 4 A g −1 , a desirable capacity of 70 mAh g −1 can be consistently achieved, suggesting a high-rate capability of the MVOH cathode.Upon recovering the current density to 0.05 A g −1 , the specific capacity of MVOH nanoflowers promptly recovers to 350 mAh g −1 , indicating a longer cycling stability of the MVOH cathode.The galvanostatic charge/discharge curves at different current densities present similar charge and discharge voltage plateaus at 0.3 and −0.2 V, respectively (Figure 2B), highlighting the fast Mg 2+ migration kinetics and good rate capability of MVOH nanoflowers cathode.Even after undergoing 100 cycles at a high current density of 3 A g −1 , the discharge capacity still maintains 64% of its initial value (Figure S9, Supporting Information), whose retention rate is high for beaker cells, since slight dissolution of vanadate is commonly observed in the presence of abundant aqueous solution. [28]The high capacity and cycling stability of the MVOH cathode is achieved by the pillaring function of abundant lattice H 2 O molecules.
To quantitatively investigate the Mg 2+ conduction kinetics in the MVOH cathode, CV tests are performed at different scan rates between 0.2 and 1.0 mV s −1 (Figure S10, Supporting Information).The CV curves exhibit similar shapes and redox peaks, indicating fast migration kinetics and highly reversible insertion/extraction of Mg 2+ .The Mg 2+ storage kinetics is evaluated by utilizing the quantitative equation i = a b , which establishes a relation between the peak current (i) and the scan rate (v).Results show that the Mg 2+ storage process is synergistically controlled by both Faradaic intercalation and pseudocapacitance mechanisms (Figure S10A,B, Supporting Information). [29]The pseudocapacitive behavior contributes to ≈62.50-74.31% of the total capacity at a scan rate of 0.2-1.0 mV s −1 (Figure S10C,D, Supporting Information).These results indicate that the high capacity of the MVOH cathode primarily originates from the pseudocapacitive redox reactions on the MVOH surface, which effectively accounts for the exceptional reversibility and rate capability of the MVOH nanoflowers.Electrochemical impedance spectroscopy (EIS) tests are carried out to further investigate the Mg 2+ conduction kinetics in the MVOH cathode (Figure S11, Supporting Information).The diffusion coefficient of Mg 2+ (D Mg ) in the MVOH cathode is calculated to be 1.08 × 10 −9 cm 2 s −1 , which is 1-3 order(s) of magnitude greater than those of conventional cathodes (Table S4, Supporting Information).This high D Mg indicates fast migration kinetics inside the MVOH crystal.The charge transfer resistance (R ct ) at the cathode/electrolyte interface is fitted to be a remarkably low value of 5.29 Ω, indicating fast crossing of Mg 2+ at the solid/liquid interface.The fast bulk migration and interface crossing facilitate the surface pseudocapacitive reactions, leading to the remarkably high capacity and good rate capability of the MVOH cathode.
To investigate the Mg 2+ conduction mechanism of the MVOH cathode, systematic ex situ characterizations are performed at various charged/discharged states in the initial cycle (Figure 2D).In the ex situ XRD patterns (Figure 2E), new diffraction peaks do not appear throughout the continuous Mg 2+ insertion/extraction.Meanwhile, the peak intensities change little.These results sug-gest the absence of phase transformations during the insertion/extraction of Mg 2+ in the MVOH cathode.During the initial discharging (Points A-D), the diffraction peak of the (002) lattice plane shifts from 6.5°to 6.9°with Mg 2+ inserted into the MVOH nanoflowers, signifying shrinkage of the interlayer spacing from 13.6 to 12.8 Å.The decrease in interlayer spacing is attributed to the strong electrostatic interaction between the inserted Mg 2+ ions and the VO x polyhedra layers. [30,31]Due to the "pillar effect" [32] of both lattice H 2 O molecules and preintercalated Mg 2+ ions in the interlayer, the crystal structure of MVOH can be well preserved and the interlayer spacing shrinks by only 7.5%.During the charging process (Points D-H), the (002) peak gradually shifts toward the left to 6.8°, which corresponds to an interlayer spacing of 13.0 Å.This result indicates that part of the inserted Mg 2+ cations are retained in the interlayer between VO x layers.During the second discharging process (Points H-L), the (002) peak shifts toward the right to 7.0°, and shifts back to 6.8°in the second charging process (Points L-P).After the second discharging/charging cycle, the interlayer spacing recovers to 13.0 Å.This indicates that a portion of Mg 2+ is permanently inserted into the interlayer of MVOH after the first discharge, further strengthening the structural stability of the V 10 O 24 skeleton.Ex situ TEM characterizations show that the lattice spacing of the (110) planes in the fully discharged MVOH cathode decreases from 0.35 nm (pristine) to 0.20 nm (Figure S12A,B, Supporting Information).When fully charged, the lattice spacing of the (110) crystal planes increases to 0.33 nm, indicating a limited variation of crystal structure (Figure S12C, Supporting Information).
Ex situ XPS spectra of V 2p, Mg 1s, and O 1s were acquired from the pristine, fully discharged, and fully charged MVOH cathodes, respectively.In Figure 2F, the V 2p core-level spectrum of the pristine MVOH cathode exhibits a small amount of V 4+ , which is attributed to the incorporation of pre-intercalated Mg 2+ cations in MVOH.Notably, the component of V 4+ increases significantly after full discharge, while the proportion of V 5+ decreases.These results indicate that the reduction of V 5+ takes place during the insertion of Mg 2+ into the MVOH cathode, providing a high discharge capacity.When the MVOH cathode gets fully charged, the valence change of V is reversed and V 4+ has been oxidized to V 5+ to maintain a content similar to pristine MVOH.For the Mg 1s core level (Figure 2G), the pristine MVOH cathode shows a signal peak at 1304.7 eV, which is attributed to the pre-intercalated Mg 2+ ions that function as pillars in the interlayer.The fully discharged MVOH cathode exhibits a new signal peak at a higher binding energy of 1305.3 eV, together with the signal at 1304.7 eV, which is related to the insertion of the Mg 2+ cations into the interlayer during discharging.In comparison to the pre-intercalated Mg 2+ , the interaction between the newly inserted Mg 2+ and the VO x polyhedra layers is weaker. [33]he fully charged MVOH cathode exhibits a slightly higher signal at 1304.7 eV than the pristine one, indicating the slight residual of newly inserted Mg 2+ in the interlayer of MVOH and the pillar function of residual Mg 2+ cations for structural stabilization.Figure 2H shows the O 1s core levels of MVOH, in which three distinct O atoms are observed, including the O atoms in the VO x polyhedra layers (V─O, 530.4 eV), the O atoms coordinated with Mg 2+ (Mg─O─V and Mg─OH 2 , 531.7 eV), and the O atoms in DC lattice H2O in the interlayer (H 2 O, 533.1 eV). [21]When fully discharged, the Mg─O signal increases noticeably while the sig- nal of DC lattice H 2 O decreases significantly.This observation indicates coordination between the DC lattice H 2 O and the newly inserted Mg 2+ cations. [34]Particularly, the binding energy of the DC lattice H 2 O is increased in the discharged MVOH cathode in comparison to the pristine cathode.This indicates that the remaining DC lattice H 2 O molecules in the interlayers are closer to a "free" physisorbed state, rather than being chemically bonded with the skeleton, which implies that these H 2 O molecules shuttle Mg 2+ during discharging. [35]At the fully charged state, all O signals in the MVOH cathode return to their initial states as in the pristine cathode, indicating a reversible intercalation mechanism involving the rearrangement of DC lattice H 2 O molecules in the interlayer of MVOH.

Mg 2+ Transfer via the Waltz-Like Shuttle Mechanism in MVOH
In order to investigate the microscopic migration mechanism of Mg 2+ in MVOH, ab initio calculations based on DFT are conducted (see computational details in the Experimental Section of the Supporting Information).Based on the determined preintercalated structure of MVOH, we analyzed and identified the energetically favorable sites for Mg 2+ to occupy.Further, we locate the nearest neighboring site for Mg 2+ to hop to based on symmetry analysis and rigorous structural relaxations.According to the relaxed structures, we discover the waltz-like Mg 2+ shuttle mechanism by DC lattice H 2 O molecules in MVOH.stabilize the lamellar structure of MVOH as their initial distribution configuration.
To verify the Mg 2+ shuttle function of lattice H 2 O molecules, in situ Raman characterization of MVOH cathode during discharging/charging has been conducted (Figure 4A).The Raman shift of scattering peak at 871 cm −1 , corresponding to the V─OH 2 bonds, variates at different discharged/charged states.In the discharging process, the Raman shift decreases from 871 to 850 cm −1 , which suggests an increase in the electron cloud density of the V─OH 2 bonds.This highly agrees with the Mg 2+ shuttle function of lattice H 2 O molecules.The DC-2 lattice H 2 O molecules coordinate with Mg 2+ cations to shuttle, generating the MgO 5 rectangular pyramids in the interlayer, while the DC-1 lattice H 2 O molecules maintain in the VO 6 octahedra via the V─OH 2 bonds to stabilize the lamellar structure of MVOH (Figure S13, Supporting Information).However, the coordination of Mg 2+ with O atoms from the VO 6 octahedra causes a more positive charge of V to contribute to the V─OH 2 bonds, leading to the shorter V─OH 2 bond length and thus the higher electron cloud density.According to the ab initio calculation results, the pristine MVOH has a V─OH 2 bond length of 2.58 Å; this bond length in IS-state MVOH decreases to 2.29 Å and decreases further to 2.26 Å in the FS-state MVOH, as labeled in Figure S13 (Supporting Information).In the charging process, the Raman shift of V─OH 2 bonds returns to 871 cm −1 , which implies the bond length recovery of V-OH 2 bonds, indicating the arrange-ment recovery of DC lattice H 2 O molecules in MVOH after Mg 2+ extraction.This is in good accordance with ex situ XPS characterization results in Figure 2H.To further verify the Mg 2+ shuttle function of lattice H 2 O molecules, the time of flight-secondary ion mass spectrometry (TOF-SIMS) is employed to analyze the fully discharged and charged MVOH cathode (Figure 4B-F; Figure S14, Supporting Information).The much higher intensity of MgVO − signal in discharged MVOH than that in charged MVOH suggests successful insertion of Mg 2+ during discharging (Figure 4B,E,F).The MgOH + signal in discharged MVOH is significantly higher than that in charged MVOH, which demonstrates the strong coordination of lattice H 2 O molecules with Mg 2+ inserted during discharging, verifying the H 2 O─Mg 2+ shuttle mechanism (Figure 4C,E,F).Furthermore, the H + signal intensities and distribution in discharged and charged MVOH are almost the same, which indicates that the H 2 O-Mg 2+ waltzlike shuttle mechanism excludes the co-insertion or co-extraction of H + together with Mg 2+ (Figure 4D-F).

Mg 2+ Transfer Path and Energy Barrier
Nudged elastic band (NEB) simulations [36] are performed to derive the minimum energy path for the atomic hop of Mg 2+ between its favorable site and the nearest neighboring site.Figure 5D shows that the double flipping of the H 2 O molecule minimizes the system energy and leads to closer coordination with Mg 2+ since the length of the Mg─OH 2 coordination bond decreases from 2.04 Å (Im-S state) to 2.01 Å (FS state).From the IS to the FS state, the H 2 O molecule coordinates with Mg 2+ more closely and more strongly via its adaptive rotation and flipping, which makes the Mg 2+ cation ready for the axisymmetrical round of H 2 O shuttled zig-zag migration in the right half part of the lattice unit (dashed blue track in Figure 5A).In addition, the energy barrier of the H 2 O shuttled zig-zag migration is as low as 0.54 eV, which indicates the fast migration kinetics of Mg 2+ in MVOH (Table S5, Supporting Information).This result agrees very well with the determined high capacity and good rate capability of the MVOH cathode.Furthermore, the volume change of MVOH before and after Mg 2+ insertion is investigated (Figure S15, Supporting Information).After Mg 2+ insertion, the lattice constant a expands by 0.27% while the lattice constant b expands by 0.35%, with the lattice constant c shrinking by 0.63%.The total volume of one lattice unit shrinks as low as 0.012%, which is negligibly small.These data demonstrate the high structure stability of MVOH during charging/discharging, which well explains the long cycling life of MVOH cathode.Therefore, due to the waltz-like shuttle of coordinated lattice H 2 O molecules, Mg 2+ can migrate in the interlayer of MVOH with fast kinetics and minor lattice change, which endows MVOH to exhibit high capacity, good rate capability, and long cycling life for Mg 2+ storage.

Electrochemical Performance of MIBs with MVOH Cathode
To further explore the excellent performance of the MVOH cathode, we fabricate a full MIB with the MVOH nanoflowers as the cathode, perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) as anode, and 2 m Mg(CF 3 SO 3 ) 2 aqueous solution in PEG/H 2 O volume ratio of 1:1 as electrolyte (Figure S16, Supporting Information).The CV curves in Figure 6A depict the performance of the full cell within the potential range of 0-1.7 V at scan rates of 1-5 mV s −1 .A pair of cathodic/anodic peaks are located at 0.73 and 1.01 V, which correspond to the insertion and extraction of Mg 2+ cations in the MVOH cathode, respectively.In addition, the shape of the CV curves changes little as the scan rate increases, indicating highly reversible Mg insertion/extraction and good rate capability of the full cell.Figure 6B,C shows the rate performance of the PTCDA//MVOH full cell at various current densities, along with the corresponding galvanostatic charge-discharge (GCD) curves.The full cell delivers a high discharge capacity of 133 mAh g −1 at a current density of 0.02 A g −1 .Remarkably, even with a 200-fold increase in current density (4 A g −1 ), the full cell maintains a capacity of 42 mAh g −1 , demonstrating its impressive rate capability.Upon changing back to the low current density of 0.02 A g −1 , the discharge capacity delivered by the full cell returns to 120 mAh g −1 , suggesting its satisfactory cycling stability.All the GCD curves are not straightly linear, which indicates a pseudocapacitive storage mechanism of the electrodes.These results are consistent with the observation in Figure 2. The cycling stability of the PTCDA//MVOH full cell is investigated at a current density of 4 A g −1 .As shown in Figure 6D, the discharge capacity of the full cell decreases from the initial value of 58-36 mAh g −1 after 5000 cycles, with a capacity retention ratio of 62%, which is due to the gradual damage of Mg 2+ insertion/extraction to the lamellar structure.Moreover, the Coulombic efficiency remains consistently ≈100%, demonstrating the high cycling stability of the PTCDA//MVOH full cell.Notably, the ultralong lifespan of 5000 cycles achieved by the PTCDA//MVOH full cell has surpassed those of the previously reported full MIBs (Table S6, Supporting Information).Moreover, the full cell in this study exhibits significantly higher capacity, exceptionally superior cycling life, and more high-rate capability in comparison to other new kinds of metal ion batteries reported so far (Table S7, Supporting Information).The resulting full MIB cell shows a maximal energy density of 67 Wh kg −1 and a maximal power density of 2 kW kg −1 ; this energy density is remarkably high among aqueous full MIBs. [37,38]

Conclusion
In this work, we prepared a novel lamellar material of Mg ; Figure S7, Supporting Information).The lattice H 2 O molecules distribute in the interlayer space and combine with the VO x polyhedra layers either indirectly or directly.The indirectly combined (IDC) lattice H 2 O molecules combine with VO x layers via pre-intercalated Mg 2+ cations; two such IDC lattice H 2 O molecules coordinate with one Mg 2+ cation, which simultaneously coordinates with O atoms in VO 5 rectangular pyramids, leading to the formation of MgO 5 hexahedra between VO x layers.The resulting MgO 5 hexahedra also support the lamellar structure of MVOH as pillars, further stabilizing the MVOH crystal structure.The directly combined (DC) lattice H 2 O molecules involve coordination with 1) V 4+ /V 5+ cations, leading

Figure 2 .
Figure 2. Electrochemical performance and electrochemical reaction mechanism of the MVOH cathode: A) Rate capability plots.B) Discharge-charge curves of MVOH at different rates.C) EIS spectrum.D,E) Ex situ XRD patterns at different discharge/charge states of the MVOH cathode.F) Ex situ high-resolution XPS spectra of the MVOH cathode at pristine, fully discharged, and fully charged states with V 2p core level.G) Ex situ high-resolution XPS spectra of the MVOH cathode at pristine, fully discharged, and fully charged states with Mg 1s core level.

Figure 3 .
Figure 3.The structural change of MVOH in the a direction projection during the H 2 O-Mg 2+ waltz-like shuttle: A) Initial state.B) Transition state of 1. C) Intermediate state.D) Transition state of 2. E) Final state.F) Migration of Mg 2+ along the a direction in the interlayer of MVOH.

Figure 3A -
E shows the structural change of MVOH (projected along the a direction) during the H 2 O-Mg 2+ waltz-like shuttle.Figure 3F summarizes the waltz-like Mg 2+ shuttle process of DC lattice H 2 O molecules along the a direction in the interlayer of MVOH.In the initial state (IS) of MVOH with Mg 2+ inserted, the Mg 2+ cation is located at the left edge of the lattice unit (dashed rectangle), which coordinates with five O atoms to form MgO 5 hexahedron, including one O atom from a DC-2 lattice H 2 O molecule; such coordination converts this DC-2 lattice H 2 O molecule into an IDC lattice H 2 O molecule.As Mg 2+ migrates along the a direction, a MgO 4 square forms with Mg 2+ in the square center (TS-1).As Mg 2+ migrates further to the mirror plane of the lattice unit (Im-S), Mg 2+ coordinates with the O atom at the right edge of the lattice unit, leading to the formation of MgO 4 tetrahedron.Meanwhile, this IDC H 2 O molecule coordinated with Mg 2+ migrates together with Mg 2+ , as shown from IS to Im-S in Figure 3F.Interestingly, the coordinated IDC lattice H 2 O molecule flips with its red H atom (H1) moving upside down to generate a H-bond with one O atom at the right edge of the lattice unit (TS-2); Mg 2+ moves inside vertically away from the screen along the b direction.Thereafter, this coordinated IDC H 2 O molecule flips again with the pink H atom (H2) moving upside down to generate an H-bond with the O atom at the left edge of the lattice unit, whereas the red H1 atom distributes in the right part of the lattice unit (FS); Mg 2+ moves back closer to the screen but proceeds along the a direction.After flipping this coordinated IDC H 2 O molecule twice, Mg 2+ adjusts its location along the b direction but proceeds further along the a direction.The cooperative rotation and flipping of the new IDC H 2 O molecules coordinated with inserted Mg 2+ cations facilitate the Mg 2+ migration in MVOH, which demonstrates the waltz-like Mg 2+ shuttle mechanism of lattice H 2 O molecules in the MVOH cathode.As mentioned above, the ex situ XPS characterizations (Figure 2H) uncovered a rearrangement of structural H 2 O molecules in the interlayer during charging.Combining these facts, it is deduced that after extraction of Mg 2+ from MVOH, the Mg 2+ ─H 2 O bonds break, and the released H 2 O molecules distribute in a configuration with high energy.In order to minimize the system energy, these released H 2 O molecules rearrange to generate H-bonds with VO x polyhedra layers, which

Figure 4 .
Figure 4. Electrochemical reaction mechanism of the MVOH cathode: A) In situ Raman spectrum at different discharge/charge states of the MVOH cathode.B-D) Depth profiles.E,F) TOF-SIMS images of H + , O + , Mg + , MgOH + , MgVO − , and CF 3 SO 3 − of fully discahged and charged electrode.
Figure 5A-C shows the zig-zag migration path of Mg 2+ in a

Figure 5 .
Figure 5. Migration of Mg 2+ along the a direction in the interlayer of MVOH: A-C) The zig-zag migration path of Mg 2+ in a lattice unit (dashed rectangle) in the different planes of MVOH.D) Minimum energy path of Mg 2+ ion migration in the MVOH crystal.

Figure 6 .
Figure 6.Electrochemical performance of PTCDA//MVOH full cell: A) CV curves at different scan rates.B,C) Rate capability and GCD curves at different current densities.D) Cycling stability at 4 A g −1 .
2+ pre-intercalated hydrated V 10 O 24 •nH 2 O (denoted with MVOH) as MIB cathode.The pre-intercalated Mg 2+ cations and structural H 2 O molecules enlarge the interlayer spacing and stabilize the lamellar structure, which improves the Mg 2+ migration kinetics and structure stability.The waltz-like shuttle mechanism of H 2 O molecules coordinated with Mg 2+ has been disclosed during the charging/discharging of the MVOH cathode: The Mg 2+ ion migrates together with the coordinated H 2 O molecules (from IS to FS), while the H 2 O molecules adjust their orientations by www.advancedscience.comrotating or even flipping to facilitate the zig-zag migration of Mg 2+ , ensuring the stability of each step of the Mg 2+ migration.Similarly, Mg 2+ can also undergo symmetrical migration through the same mechanism (from FS to IS).After extraction of Mg 2+ from MVOH, the Mg 2+ ─H 2 O bond breaks, and the lattice H 2 O molecule creates empty chemical bonds, which allows it to rearrange and form H bonds in the interlayer of MVOH to stabilize the lamellar structure.With the shuttle function of lattice H 2 O molecules, the MVOH cathode exhibits a remarkably high discharge capacity of 350 mAh g −1 at a current density of 0.05 A g −1 and high-rate capability (70 mAh g −1 at 4 A g −1 ).The aqueous full MIB cell based on MVOH cathode delivers an ultralong cycling life of 5000 cycles with a high energy density of 67 Wh kg −1 .This work has revealed the sophisticated shuttle mechanism of lattice H 2 O molecules for Mg 2+ migration, which is realized by the formation and cleavage of H bonds with VO x layers.Due to the common existence of H bonds between H 2 O molecules and transition metal oxides, we anticipate that the waltz-like Mg 2+ shuttle mechanism of lattice H 2 O molecules is universal in transition metal oxide cathodes.Therefore, lattice H 2 O molecule is a favorable component for Mg 2+ migration, which provides promises for advancing the development of MIBs for practical applications.