Disordered Structure and Reversible Phase Transformation from K‐Birnessite to Zn‐Buserite Enable High‐Performance Aqueous Zinc‐Ion Batteries

The layered δ‐MnO2 (dMO) is an excellent cathode material for rechargeable aqueous zinc‐ion batteries owing to its large interlayer distance (~0.7 nm), high capacity, and low cost; however, such cathodes suffer from structural degradation during the long‐term cycling process, leading to capacity fading. In this study, a Co‐doped dMO composite with reduced graphene oxide (GC‐dMO) is developed using a simple cost‐effective hydrothermal method. The degree of disorderness increases owing to the hetero‐atom doping and graphene oxide composites. It is demonstrated that layered dMO and GC‐dMO undergo a structural transition from K‐birnessite to the Zn‐buserite phase upon the first discharge, which enhances the intercalation of Zn2+ ions, H2O molecules in the layered structure. The GC‐dMO cathode exhibits an excellent capacity of 302 mAh g−1 at a current density of 100 mA g−1 after 100 cycles as compared with the dMO cathode (159 mAh g−1). The excellent electrochemical performance of the GC‐dMO cathode owing to Co‐doping and graphene oxide sheets enhances the interlayer gap and disorderness, and maintains structural stability, which facilitates the easy reverse intercalation and de‐intercalation of Zn2+ ions and H2O molecules. Therefore, GC‐dMO is a promising cathode material for large‐scale aqueous ZIBs.

controversial.For example, Xu et al. [30] reported the structural transformation of tunnel-structured α-MnO 2 to spinel ZnMn 2 O 4 , and Lee et al. and Zhang et al. [31,32] reported the structural transformation of the Zn-buserite phase.Similarly, Jiao et al. reported the structural transformation of δ-MnO 2 into a new Zn 4 SO 4 (OH) 6 .5H 2 O (ZSH) phase. [28]urthermore, Alfaruqi et al. [33] explained the multiple phase transformations of γ-MnO 2 .The loss of Mn ions during the cycling process is resolved by the addition of extra Mn 2+ ions to the aqueous electrolyte.Another challenge is that the electrostatic interaction between Zn 2+ and the host electrode affects electrochemical performance. [34]Doping layered δ-MnO 2 with suitable atoms is beneficial for widening the interlayer gaps, improving structural stability, and enhancing the electrochemical performance. [35]In general, the addition of transition metal atoms and crystal water as pillars between the interlayers stabilizes the structure and fast diffusion of Zn 2+ ions.Further, the δ-MnO 2 cathode suffers from poor electronic conductivity and structural breakdown during the charge-discharge process.To inhibit these occurrences, a combination of high surface area 2D layered graphene oxide with δ-MnO 2 can reduce structural changes and resolve the problem of poor electronic conductivity. [36]he birnessite phase of δ-MnO 2 consists of MnO 6 octahedral layers separated by a monolayer of H 2 O and cations.The interlayer spacing of birnessite-MnO 2 was approximately 7 Å.By contrast, the buserite phase of δ-MnO 2 consists of MnO 6 octahedra layers separated by double layers of H 2 O with an interlayer spacing of approximately 10 Å, depending on the number of water layers separating the MnO 2 octahedral sheets. [37]The buserite phase is classified into stable and unstable phases.Furthermore, the incorporation of transition metals, group I/II metals, and many lanthanides facilitate the formation of a stable buserite phase.The stable buserite phase of MnO 2 has been widely applied in battery electrodes, oxidation-reduction catalysts, and ion-exchange materials.Recently, Zhang et al. reported the phase transition of birnessite δ-MnO 2 to the buserite phase of MnO 2 during the first discharge, which enables the easy intercalation of Zn 2+ cations, leading to highperformance aqueous ZIBs. [32]In this study, we investigated whether 2 wt% Co-doping in the birnessite phase of MnO 2 aids lattice distortion and the formation of a stable buserite phase during the first discharge, which has a positive impact on the electrochemical performance of aqueous ZIBs.The ex situ X-ray diffraction analysis (XRD) results confirm the formation of a stable buserite phase of MnO 2 during the first discharge.Although various MnO 2 /conductive-based materials, such as MnO 2 /carbon nanotube, MnO 2 /polyaniline, MnO 2 /carbon, and MnO 2 /poly (3,4-ethylenedioxythiophene), have been reported as cathodes for aqueous ZIBs, [22,38,39] the continuous phase transition in subsequent cycles causes the collapse of the crystal structure in these cases and plays a critical role in the electrochemical properties.
In this study, we designed a novel GC-dMO cathode for aqueous ZIBs, in which a Co-doped layered dMO electrode was combined with graphene oxide through a simple hydrothermal reaction.We have attempted to explain the mechanism of aqueous ZIBs based on XRD, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), electrochemical measurements, ex situ XRD, and X-ray photoelectron spectroscopy (XPS) analysis.Co-doping enhances the interlayer gap and balances the structural transformation during the charge-discharge process.It further creates lattice defects, which facilitate faster reaction kinetics and Zn 2+ ion diffusion.Furthermore, the addition of graphene oxide to Co-doped dMO drastically improves the electrochemical performance of aqueous ZIBs.The high surface area and porous graphene oxide increase the absorption of intercalated H 2 O molecules, which play a significant role in the diffusion of Zn 2+ ion and enhance electronic conductivity.We investigated the electrochemical performance of full-cell aqueous ZIBs using a GC-dMO cathode, Zn anode, and 2 M Zn (CF 3 SO 3 ) 2 /0.1 M Mn (CF 3 SO 3 ) 2 as the electrolyte.The GC-dMO cathode exhibited an excellent capacity of 302 mAh g À1 at a current density of 100 mA g À1 after 100 cycles, which was much higher than that of the dMO cathode (159 mAh g À1 at a current density of 100 mA g À1 ).The drastic increase in capacity is caused by the enlarged interlayer gap and lattice defects, which enhance the diffusion of Zn 2+ ions and the intercalated crystal water content.The detailed characterization of the prepared materials and the exact electrochemical mechanism of the fabricated aqueous ZIBs are explained through ex situ XRD and XPS analyses.

Results and Discussion
Figure 1 depicts the δ-MnO 2 nanomaterials, whose crystallinity and degree of disorder were controlled through hetero-atom doping and graphene oxide composites.In the case of dMO prepared through hydrothermal synthesis, two-dimensional (2D) curved surfaces were aggregated to produce nanoflower-shaped particles with a 3D spherical hierarchical pore structure.In the δ-MnO 2 layer, both K + ions and H 2 O were intercalated to form the birnessite phase.Following Co-doping, the lattice constant of this layer structure changes, causing lattice distortion, which ultimately affects the number of intercalated K+ ions and the crystal structure of δ-MnO 2 .Furthermore, to maintain the hierarchical structure and increase the degree of disorder, Co-doping, and graphene oxide compounding strategies were used together.We attempted to synthesize well-controlled GC-dMO electrodes that can maintain reversible changes in the final phase transformation from birnessite to buserite during the electrochemical intercalation of K + ions and H 2 O into the δ-MnO 2 layer.
The X-ray diffraction patterns of the dMO, C-dMO, and GC-dMO powders are presented in Figure S1, Supporting Information.As Co-doping and graphene oxide compounding proceeded in these δ-MnO 2 samples, it was confirmed that the total peak intensity decreased and the full width at half maximum increased, implying that the degree of disorder increased and the crystallinity decreased.For each sample, Le Bail fitting was performed to investigate the distortion of the crystal structure caused by Co-doping in detail.It was difficult to obtain an accurate lattice parameter for the GC-dMO sample because it had wide peak broadening.The Le Bail fitting and corresponding crystal structures of the dMO and C-dMO samples are depicted in Figure 2.
All the diffraction peaks in the XRD patterns displayed in Figure 2a,b are well matched with the birnessite-type δ-MnO 2 structure (dMO) (JCPDS No. 52-0556).The average crystallite sizes calculated for the dMO, C-dMO, and GC-dMO samples were determined as 21.41, 10.71, and 8.61 nm, respectively, using the Debye-Scherer method.The particle size decreased with 2 wt% Co-doping in the dMO sample, as confirmed by the scanning electron microscopy (SEM) and TEM images (Shown in Figures 3 and 4 suitable for K + , Zn 2+ , and H 2 O intercalation and structural stability.A slight increase in the C value of the C-dMO sample indicated that the interlayer spacing had expanded, but the total lattice volume was reduced owing to Co-doping, indicating that a small amount of K + ions were contained in the K-birnessite phase.Therefore, the C-dMO sample was considered to have a crystal structure and structural stability suitable for Zn 2+ ions and H 2 O insertion with an ion radius smaller than that of K + ions.Figure 2c,d depicts the crystal structures of the dMO and C-dMO samples; δ-MnO 2 has a 1D void created by the edge and corner sharing of MnO 6 units with a C2/m crystal structure.Among the various polymorphs of MnO 2 , the δ-MnO 2 layered structure is suitable for aqueous ZIBs because of its large interlayer gap and structural stability.Co-doping in dMO resulted in an increase in the interlayer gap, lattice distortion, and a decrease in the K + and H 2 O content.Inductively coupled plasma (ICP) analysis and thermogravimetric analysis (TGA) were performed to investigate the amount of intercalated K + ions and H 2 O in the δ-MnO 2 interlayer (Table S1, Supporting Information).The results of the ICP analysis were as follows: the elemental fraction of K of dMO, C-dMO, and GC-dMO were 14.00, 6.01, and 7.40 wt%, respectively.The lattice distortion caused by Co-doping increased the interlayer distance but caused a decrease in the lattice parameter (a) and unit cell volume, which are believed to hinder K + ion insertion during the hydrothermal synthesis.The elemental fractions of Co were 4.77 and 4.39 wt% in C-dMO and GC-dMO, respectively, and indicated a slight decrease owing to the presence of various functional groups on the surface of graphene oxide.TGA was performed to accurately measure the amount of water contained in the cathode material, and the H 2 O adsorbed on the surface and the crystal water in the MnO 6 interlayer were considered separately.In the case of surface H 2 O, it was confirmed that it had a low binding energy and was separated at a temperature of less than 150 °C, and the H 2 O present inside the lattice existed up to approximately 500 °C when the weight of the δ-MnO 2 samples was saturated at higher temperatures (Figure S2, Supporting Information).The crystal water amounts of dMO, C-dMO, and GC-dMO were observed to be 3.The dMO has a 2D lamellar-type structure.Figure 3a-c exhibits the SEM images of the dMO, C-dMO, and GC-dMO powder samples, respectively.
Figure 3a displays the nanoflower-shaped morphology with a diameter varying from 1 to 1.4 μm.Individual nanoflowers composed of a large number of nanosheets were uniformly distributed throughout the samples.The nanosheet structure radiated out from the center to form a nanoflower-shaped morphology.Similarly, Co-doped dMO (Figure 3b) exhibits a nanoflower-shaped morphology with a reduced particle size.Furthermore, GC-dMO (Figure 3c) indicated that a large number of nanosheets composed of dMO nanoflowers were uniformly distributed on the rGO sheet.The valence state and elemental compositions of dMO, C-dMO, and GC-dMO powders were evaluated through XPS analysis.Figure 3d-f displays the Mn 2p, O 1s, and Co 2p XPS spectra of the dMO, C-dMO, and GC-dMO powders, respectively.All the XPS curves were deconvoluted using XPSPEAK 4.1 software.The binding energies located at 641 and 652 eV could be attributed to the spin doublets of the Mn 2p 3/2 and Mn 2p 1/2 XPS peaks of the dMO powder (Figure 3d) with a spin-orbital splitting of 11.78 eV.The Mn 2p 3/2 core-level spectra of dMO and C-dMO were deconvoluted into two distinct peaks corresponding to Mn 3+ (640.05eV), and Mn 4+ (641.62 eV). [40]Similarly, the Mn 2p 3/2 peak of the GC-dMO powder (Figure 3d) was deconvoluted into two peaks corresponding to Mn 3+ (641.48 eV) and Mn 4+ (642.90 eV).The Mn 2p 3/2 and Mn 2p 1/2 peaks of the GC-dMO powder were shifted towards higher binding energies, which may be attributed to the surface-absorbed moisture (-OH groups). [40]The O 1s spectra of dMO and C-dMO (Figure 3e) exhibit three distinct peaks at binding energies of 528.69, 529.15, and 530.55 eV, which can be attributed to the Mn-O, Mn-O-Mn, and Mn-O-H bonds, respectively. [28]Similarly, the O 1s peak of GC-dMO powder (Figure 3e) exhibits three distinct XPS peaks at 529.Co 3+ (779.30eV) and Co 2+ (781.35eV). [41]The spin-orbital splitting value of the binding energies of the Co 2p 3/2 and 2p 1/2 peaks is 15.21 eV, which indicates that the Co ions that exist mostly in the Co 3+ state may be attributed to the fact that Co 2+ was oxidized to Co 3+ by MnO 4À .The FTIR spectra of the dMO, C-dMO, and GC-dMO samples are presented in Figure 3g.The wide absorption band at approximately 3284 cm À1 was attributed to the O-H stretching vibration owing to the hydrated nature of the samples, and that in the range of 400-800 cm À1 was attributed to the O-Mn-O stretching vibration of the MnO 6 octahedra. [42,43]The absorption bands in the range 900-1700 cm À1 mostly contain À OH groups, which are generally observed during lowtemperature hydrothermal synthesis. [43]The FTIR spectra of the Codoped δ-MnO 2 samples clearly indicate that they contain major À OH groups, which indicates the intercalation of water molecules in the sample.The Brunauer-Emmett-Teller (BET) surface areas and porosities of the dMO, C-dMO, and GC-dMO powder samples are depicted in Figure 3h,i.The BET surface area of the GC-dMO sample exhibits an excellent surface area of 152.04 m 2 /g.However, the BET surface areas of the dMO and C-dMO samples were 87.57and 35.98 m 2 /g, respectively.The excellent surface area of the GC-dMO sample may be attributed to the composite of rGO sheets and the Co-doped nanoflower-shaped dMO.The average pore diameters of the dMO, C-dMO, and GC-dMO samples (Figure 3i) were found to be 14.08, 11.79, and 16.96 nm, respectively.
Figure 4 displays the TEM and HR-TEM images and elemental mapping of the dMO, C-dMO, and GC-dMO samples.
Figure 4a exhibits the nanoflower-shaped morphology of dMO with a diameter of approximately 100-200 nm.The shape of the MnO 2 nanoparticles depends on the concentration of the precursor KMnO 4 , and the decomposition rate of MnO 4À can affect the nucleation growth of MnO 2 .The crystal growth process has two steps: the initial nucleation stage and the subsequent crystal growth process.At very low Energy Environ.Mater.2024, 7, e12640 concentrations of KMnO 4 , such as 0.1 mmol the decomposition rate is slow.Owing to a thermodynamically preferential growth regime, the growth of crystals in one-dimensionally defined directions, such as nanorods and nanowires, is favored.As the concentration of KMnO 4 increases, the decomposition rate of the K + ions favors crystal growth in three dimensions, leading to the formation of nanoflower-shaped MnO 2 .Furthermore, the possible crystal growth mechanism of MnO 2 nanostructures was well explained by Duan et al. [23] Figure 4b exhibits the nanoflower morphology of C-dMO, with the diameter of the nanoflowers varying within the range of 100-200 nm. Figure 4c exhibits the TEM image of the GC-dMO sample, which clearly displays the distribution of C-dMO nanosheets on the rGO surface with a crumpled morphology.Figure 4d-f displays the HR-TEM images of the dMO, C-dMO, and GC-dMO samples, respectively.The observed interlayer spacing for the dMO sample was 0.705 nm, and that for the C-dMO sample (Figure 4e) was 0.718 nm.As expected, the interlayer spacing increases after 2 wt% Co-doping of dMO, which favors structural stability, crystal defects, and Zn 2+ ion movement during the electrochemical reaction.Similarly, the interlayer distance for the GC-dMO sample is 0.711 nm, which is reduced from that for C-dMO, and the effect of Codoping is slightly reduced by complexing with graphene oxide, but the degree of disorder is expected to increase because smaller crystal grains are evenly distributed on the wide surface of graphene oxide.Figure 4g exhibits the TEM elemental mapping for the dMO sample, which clearly indicates the presence of Mn and O in the sample.Figure 4h,i also indicates that the atomic distributions of the C-dMO and GC-dMO samples clearly exhibit the presence of Mn, O, and Co elements.
The cycle life performances of aqueous ZIBs using dMO, C-dMO, and GC-dMO cathodes, Zn anode, and 2 M Zn (CF 3 SO 3 ) 2 + 0.1 M Mn (CF 3 SO 3 ) 2 in deionized water as an electrolyte at a current density of 100 mA g À1 is depicted in Figure 5a.The initially obtained capacities of the dMO, C-dMO, and GC-dMO cathodes were 181, 208, and 312 mAh g À1 , respectively, at a current density of 100 mA g À1 .Furthermore, the GC-dMO cathode exhibited an excellent capacity of 302 mAh g À1 after 100 cycles, which is much higher than those of the dMO and C-dMO cathodes.The discharge capacity of the GC-dMO cathode may be enhanced owing to the effect of Co-doping, which increases the interlayer gap and crystal defects and reduces the concentration of K + ions, leading to the fast diffusion of Zn 2+ ions and water molecules. [27,35]Furthermore, the layered rGO sheets enhanced the surface area and electronic conductivity.Another significant factor contributing to high capacity is the presence of interlayer crystal water, which effectively screens the electrostatic interaction between the Zn 2+ ions and host anion framework, enabling Zn 2+ ions to easily (de) intercalate during the electrochemical process. [34]igure 5b depicts the galvanostatic charge-discharge curves of aqueous ZIBs fabricated using dMO, C-dMO, and GC-dMO cathodes at a current density of 100 mA g À1 .The first discharge capacity reaches 312 mAh g À1 at a current density of 100 mA g À1 for the GC-dMO cathode.The first discharge capacities of the dMO and C-dMO cathodes were 181 and 208 mAh g À1 , respectively.Similarly, the observed discharge capacities after 100 cycles for the dMO, C-dMO, and GC-dMO cathodes were 158, 208, and 303 mAh g À1 , respectively.Typical voltage plateaus were observed during the charging process after 100 cycles for all dMO-based cathodes, indicating steady reaction kinetics.To investigate the electrochemical stability of ZIBs at high current densities, we tested the cycle lives of the dMO, C-dMO, and GC-dMO cathodes at a current density of 1000 mA g À1 , as depicted in Figure S3, Supporting Information.As expected, the GC-dMO cathode exhibited an initial discharge capacity of 206 mAh g À1 and also obtained a decent capacity of 155 mAh g À1 after 500 cycles at a current density of 1000 mA g À1 .This indicated a stable capacity and retained 75% of its initial capacity.Initially, the capacity increased up to 65 cycles owing to the activation of the layered structure cathode, after which the capacity decreased slowly up to 500 cycles with a coulombic efficiency of almost 100%.Aqueous ZIBs fabricated using the GC-dMO cathode exhibited excellent cycle life as compared with other manganese-based cathodes.By contrast, the asprepared dMO cathode indicated initial discharge capacities of 125 and 99 mAh g À1 after 500 cycles.However, Figure S3, Supporting Information, indicates that the C-dMO cathode exhibits a drastic enhancement in capacity owing to the synergistic effect of both the low content of intercalated K + ions and the enlarged interlayer gap, which promotes a large amount of Zn 2+ ion intercalation and deintercalation during the electrochemical process owing to the presence of crystal water in the interlayer, which effectively screens the electrostatic interaction among the Zn 2+ ions.The excellent electrochemical performance of the GC-dMO cathode may be attributed to the combined synergetic effect of the enlarged interlayer gap; rGO sheets provide conductive pathways for faster electron transfer and enhance the mobility of Zn 2+ migration owing to the reduction in the electrostatic screening effect.Figure 5c depicts the rate performance of aqueous ZIBs using the dMO, C-dMO, and GC-dMO cathodes at various current densities from 50 to 5000 mA g À1 and again in the reverse order from 5000 to 50 mA g À1 .The initial discharge capacity of the GC-dMO cathode was 268 mAh g À1 at a current density of 50 mA g À1 ; as the current density increased from 50 to 5000 mA g À1 , the capacity started to decrease.Similarly, the initial discharge capacities of the dMO and C-dMO cathodes were 128 and 131 mAh g À1 , respectively.The observed capacities at a very high current density of 5000 mA g À1 for the dMO, C-dMO, and GC-dMO cathodes were 42, 60, and 64 mAh g À1 , respectively.As the current density switched from 5000 to 100 mA g À1 , the GC-dMO cathode exhibited an excellent capacity retention of 293 mAh g À1 .This indicates the effective reversibility of aqueous ZIBs.By contrast, the observed capacity retentions for the dMO and C-dMO cathodes at a current density of 100 mA g À1 are 245 and 278 mAh g À1 , respectively.The galvanostatic charge-discharge curves of dMO and GC-dMO at varying current density are shown in Figure 5d.The first discharge capacity of GC-dMO shows the high capacity of 267 mAh g À1 at a current density of 50 mA g À1 , while dMO electrode exhibits a capacity of 130 mAh g À1 which is almost 50% lower capacity than GC-dMO electrode.Figure 5d shows at very high current density of 5000 mA g À1 , the observed capacity for dMO and GC-dMO electrodes is 40 and 60 mAh g À1 , respectively.This shows excellent rate performance of GC-dMO electrode at varying current density due to increase in the degree of disorderness, synergistic effect of Co-doping, and rGO composite.The long-term cycle stability of full-cell ZIBs using the dMO, C-dMO, and GC-dMO cathodes at a very high current density of 5 A g À1 is depicted in Figure 5e.The initial discharge capacities of the dMO, C-dMO, and GC-dMO cathodes were 138, 187, and 230 mAh g À1 , respectively.The drastic increase in the initial capacity of the C-dMO (2 wt% Co-doped) electrode as compared with the dMO electrode may be attributed to the increase in the interlayer distance and decrease in the concentration of the K + ions, which facilitates the movement of Zn 2+ ions and water molecules.Furthermore, the GC-dMO electrode displayed an excellent discharge capacity of 230 mAh g À1 owing to the combined effect of Co-doping, high surface area, and electronic conductivity, which maintained its structural stability.The capacity retentions after a long cycle life (1500 cycles) of the dMO, C-dMO, and GC-dMO electrodes were 48, 68, and 99 mAh g À1 with a coulombic efficiency of almost 100% at a very high current density of 5 A g À1 , which is much higher than that of certain previously reported MnO 2 -based cathodes for aqueous ZIBs as listed in Table S2, Supporting Information.Table S2, Supporting Information, shows the comparison of the electrochemical performance of newly prepared novel GC-dMO electrode with reported literature on MnO 2 -based cathodes for aqueous zinc-ion batteries.In the present work, GC-dMO electrode exhibits an excellent capacity of 302 mAh g À1 at 100 mA g À1 (after 100 cycles) and capacity of 99 mAh g À1 at 5 A g À1 (after 1500 cycles), which is better than some literature reported MnO 2 -based cathodes for aqueous ZIBs (See Table S2, Supporting Information).The observed excellent electrochemical performance of GC-dMO electrode may be due to the high degree of disorderness, combined effect of Co-doping, and rGO composite.The GC-dMO cathode can be suitable for excellent cathode for next generation advanced aqueous zinc-ion batteries.The excellent capacity retention of ZIBs fabricated using GC-dMO cathodes is attributed to the simultaneous insertion of Zn 2+ ions and H 2 O molecule and the presence of crystal water between the interlayer plays a significant role in shielding electrostatic interactions between the Zn 2+ ions and host anions.The 2 wt% Co-doping increased the interlayer distance and reduced the concentration of the K + ions, whereas the rGO sheets provided a high surface area and excellent electronic conductivity as compared with the bare dMO cathode.
To investigate the structural evolution during the electrochemical process and explore the exact energy storage mechanism of aqueous ZIBs using the dMO, C-dMO, and GC-dMO cathodes, we conducted ex situ XRD and XPS measurements.For ex situ measurements, the coin cells were held at specific voltage points (0.8-1.9 V) at which they were disassembled, and the obtained cathodes were cleaned properly for ex situ measurements, as depicted in Figure 6a,b.
Ex-situ XRD of dMO and GC-dMO cathodes at pristine and various voltage states during the first discharge and charge states is shown in Figure 6.The charge-discharge curve during the ex-situ test of both electrodes and the specific potential points at which ex-situ XRD was measured are shown in Figures 6a,b, respectively.The XRD pattern of the pristine dMO cathode displays all the characteristic peaks of the δ-MnO 2 phase (K-birnessite).Figure 6c indicates a few extra peaks arising from Zn (CF 3 SO 3 ) 2 during the first charge (at 1.9 V); the dMO electrode remained the same.On discharging from 1.9 to 0.8 V, the K-birnessite phase gradually weakened, and a new phase emerged.These new peaks occurred at 6.17°, 12.82°, 19.41°, 26.15°, and 32.90°and were attributed to the crystallographic planes of the layered Zn-buserite phase; certain extra peaks were attributed to the electrolyte Zn(CF 3 SO 3 ) 2 phase. [32]Some studies on Zn-buserite have been reported for layered MnO 2 minerals. [37,44,45]This phase consists of H 2 O layers between the MnO 6 octahedra.Furthermore, the present changes in the ex-situ XRD pattern differ from those previously reported for L-ZnxMnO 2 , birnessite, spinel ZnMn 2 O 4 , γ-Zn-MnO 2 , Zn-MnO 2 , and MnOOH. [7,21,31,46]Figure 6c indicates that during the first discharge process, there was a peak shift towards a higher angle for the 002 planes (2θ ∼ 12.12°).The peak shift of the 002 planes towards a higher angle suggests a decrease in the interplanar distance (d-value), which may be caused by the simultaneous reverse intercalation of Zn 2+ ions and the de-intercalation of H 2 O. [28,33,47,48] At the beginning of the second charge from 0.8 to 1.9 V, the Zn-buserite phase gradually weakened and the K-birnessite phase remained the same along with certain additional peaks attributed to the electrolyte Zn (CF 3 SO 3 ) 2 phase.This indicates that the phase transition between the K-birnessite and Zn-buserite phases in the dMO electrode with different d-spacings occurs continuously during the cycling test, which is considered to adversely affect Zn 2+ ions storage.In addition, a peak shift of 002 planes (2θ ∼ 12.12°) was noted towards a lower angle owing to an increase in the interplanar distance (d-value), which may have been caused by the intercalation of H 2 O molecules and the de-intercalation of Zn 2+ ions.The enlarged portion of the peak shifts of the 002 planes during the first cycle is depicted in Figure 6e.Similarly, the ex-situ XRD patterns of Energy Environ.Mater.2024, 7, e12640 ZIBs using the GC-dMO cathode at various voltages are depicted in Figure 6d.The pristine GC-dMO cathode exhibits the characteristic peaks of the K-birnessite phase but has a disordered structure owing to the synergistic effect, which can be attributed to Co-doping and graphene oxide composites.During the first charge, the phase gradually weakened, and an extra peak occurred owing to the electrolyte Zn (CF 3 SO 3 ) 2 phase.In the first discharge process from 1.9 to 0.8 V, a new Zn-buserite phase having a crystal structure, such as K-birnessite, occurs without any other phase.In the second charge process, the Zn-buserite phase gradually weakens after 1.2 V and reversibly forms the original disordered structure.The ex-situ XRD results clearly indicate clear reversible structural changes during the sequential charge-discharge process.Figure 6f depicts the peak shifts of the (002) plane towards higher and lower angles during the first discharge and second charge process.The peak shifts of the (002) plane may be attributed to the reverse intercalation and de-intercalation of Zn 2+ ions and H 2 O molecules during the discharging and charging process. [47]The Zn-buserite phases composed of Zn 2+ cations reside above and below the Mn vacant site and the three O atoms adjacent to the vacancies from interlayer H 2 O, which was further confirmed by the ex-situ XPS analysis, as explained in the following section.
To investigate the exact reaction mechanism of GC-dMO based on the disordered structure and electron states of the MnO 6 octahedron, ex situ XPS measurements of dMO and GC-dMO cathodes at the pristine and first charge and discharge states, respectively, are depicted in Figure 6g,h.The pristine dMO electrode is deconvoluted into three major peaks: Mn 2+ (640.73 eV), Mn 3+ (642.15eV), and Mn 4+ (643.91 eV). [40]In the first charge, the intensity of the Mn 2p XPS spectra decreases and shifts towards a lower binding energy, which may be attributed to the de-intercalation of Zn 2+ ions and intercalation of H 2 O, leading to a decrease in the Mn valence state. [28]In the first discharge Figure 6.a, b) Typical charge-discharge curves of the initial two cycles at a current density of 50 mA g À1 in 2 M Zn (CF 3 SO 3 ) 2 + 0.1 M Mn (CF 3 SO 3 ) 2 aqueous electrolyte for dMO and GC-dMO electrodes.c, d) Ex situ XRD patterns of dMO and GC-dMO curves at selected states during the first charge and discharge process.e, f) Ex situ XRD within the scan angle (2θ) of 10-16°of the dMO and GC-dMO electrodes.g, h) Ex situ Mn 2p XPS spectra of pristine OCV; first charge and discharge states for the dMO and GC-dMO electrodes.
Energy Environ.Mater.2024, 7, e12640 state, the Mn 2p XPS spectra shift to higher binding energies and return to their original positions.During the discharge process, the intercalation of Zn 2+ and Mn ions into the dMO electrode and de-intercalation of H 2 O occurs.The deconvoluted ex situ XPS spectra of GC-dMO for the pristine and first charge and discharge curves are presented in Figure 6h.The first charge curve (1.9 V) is shifted towards a lower binding energy owing to a decrease in the valance states of manganese, Zn 2+ ions, and the intercalation of H 2 O molecules.However, in the discharge process, the XPS peak shifts towards a higher binding energy owing to the simultaneous reverse intercalation of Zn 2+ and H 2 O molecules.Furthermore, the aforementioned statements are clearer from the ex situ XPS curves of the O 1 s and Zn 2p spectra.The ex situ O 1s spectra of dMO are depicted in Figure S4a, Supporting Information.The pristine O 1s spectra of the dMO electrode exhibit three major characteristic peaks at 529.4, 530.05, and 531.5 eV, respectively.The binding energy at 529.4 eV is attributed to the O 2À group originating from δ-MnO 2 ; the peak at 530.5 eV is caused by the functional group OH À attached to Mn, and the peak at 531.5 eV is caused by the intercalated H 2 O molecules in the interlayer spacing of the dMO electrode. [28,29,49]n the first charge process, the O 2À group remained the same at 529.5 eV.However, H 2 O molecule peaks shifted towards higher binding energies with an increase in peak intensity owing to the intercalation of H 2 O during the charging process.Furthermore, in the discharge process, the OH À functional group and H 2 O molecule peaks shifted slightly towards lower binding energies owing to the de-intercalation of H 2 O molecules.Similarly, Figure S4c, Supporting Information, depicts the ex situ O 1s spectra of the GC-dMO electrode; here, the same reversible intercalation and de-intercalation of H 2 O molecules occur during the charging and discharging processes.However, the XPS peak intensity of the OH À group and H 2 O molecules for the GC-dMO electrode was much higher than that for the dMO electrode.Co-doping increases the interlayer distance and creates a disordered structure, and high surface area rGO sheets enable the intercalation of a large number of Zn 2+ ions and H 2 O molecules, as well as maintain structural stability for the long-term cycling process.These intercalated H 2 O molecules can effectively screen the electrostatic interactions between the Zn 2+ ions and enhance the mobility of the Zn 2+ ions, which leads to excellent electrochemical performance. [29,34,49,50]The ex situ Zn 2p XPS spectra of the dMO and GC-dMO electrodes are presented in Figure S4b,d, Supporting Information.In the case of the dMO electrode, the peak intensities of Zn 2p 3/2 and Zn 2p 1/2 in the first charge state were much lower than those at the first discharge state.This confirms the intercalation and de-intercalation of Zn 2+ ions during the discharge and first charge process, respectively.Similar phenomena were observed in the ex situ XPS spectra of GC-dMO electrodes.However, the Zn 2p XPS peak intensity at the first charge and discharge states for the GC-dMO electrode was much higher than that of the dMO electrode because of the enlarged interlayer distance and defects created owing to the Co-doping and pre-intercalated K + ions and the high surface area and porous nature of the rGO sheets.Hence, the ex situ XPS spectra of Mn 2p, O 1s, and Zn 2p in the pristine, first charge and discharge states confirmed that simultaneous reverse intercalation and de-intercalation of Zn 2+ ions and H 2 O molecules during the discharging and charging processes is the main reason for the excellent electrochemical performance of aqueous ZIBs.
The superior rate performance and excellent diffusion kinetics of Zn 2+ and H 2 O in the fabricated CR2032 coin-cell aqueous ZIBs using dMO, C-dMO, and GC-dMO cathodes can be further verified through cyclic voltammetry curves at a scan rate of 0.1-1 mV in the potential window of 0.8-1.9V.As depicted in Figure 7a, the cathodic and anodic peaks are located at 1.56 (Peak 1) and 1.32 V (Peak 2) at a scan rate of 0.1 mV s À1 for the dMO cathode.
As the scan rate increases from 0.1 to 1 mV/s, the cathodic peak shifts slightly towards higher voltages and the anodic peak shifts towards lower voltages owing to the fast reaction kinetics.The obtained cathodic and anodic peaks are in good agreement with the MnO 2 -based cathodes reported in the literature. [35,51]Similarly, the observed cathodic and anodic peaks of the C-dMO and GC-dMO cathodes at varying scan rates are depicted in Figure 7b,c.To better understand the excellent rate performance and cycling stability, we quantified the contribution of capacity from the diffusion-and capacitive-controlled mechanisms. [52,53]The CV curves follow a power-law relation with varying scan rates (ν) as per the equation given below: or where a and b are variable parameters.The slope of log(ν) versus log (i) indicates the value of b for both the cathodic and anodic peaks.In general, b lies in the range of 0-1.If b = ∼ 0.5, the process is diffusion-controlled electrochemical, and if b = ∼ 1.0, then it is caused by a capacitive-controlled dominant process. [52]Figure 7h presents the electrochemical impedance spectra (EIS) curves of aqueous ZIBs fabricated using the dMO, C-dMO, and GC-dMO cathodes and Zn metal anodes.The EIS curves displayed a depressed semicircle and a small inclined spike.The semicircles in the high-and mid-frequency regions are attributed to the charge transfer resistance (R ct ), and the inclined spike is attributed to the diffusion of Zn 2+ ions to the electrodes (Warburg impedance W s ).All the obtained EIS curves were fitted using the EC lab software by varying the values of the resistance and capacitance of the equivalent circuit to obtain the approximate fit of the EIS curves.The calculated EIS curves of the full-cell ZIBs were 818, 378, and 31 Ω for the dMO, C-dMO, and GC-dMO cathodes, respectively.We observed that 2 wt% of Co-doping in the dMO matrix drastically reduced the charge transfer resistance.Further, the addition of 10 wt% rGO to the GC-dMO composite significantly decreased the charge transfer resistance up to 31 Ω.The lower charge transfer resistance in the case of the GC-dMO composite cathode may be attributed to the conductive nature of rGO, and Co-doping widens the interlayer gap, which facilitates the easy mobility of Zn 2+ ion intercalation and de-intercalation inside the electrodes.[56] The fabricated coin cells Zn-ion batteries are discharged or charged at 0.05 A g À1 for 10 min and then rested for 30 min in order to make voltage reach steady state.This process is repeated up to a minimum voltage of 0.8 V and maximum voltage of 1.9 V (Figure S5, Supporting Information).The diffusion coefficient D 2þ Zn is calculated from the given formula: where m is the active material on the electrode (g), V is the molar volume (cm 3 mol À1 ), M is the molecular weight (g mol À1 ) Energy Environ.Mater.2024, 7, e12640 9 of 12 of the active material, τ is the duration time (s) of discharging or charging, S is the contact area between the electrode and electrolyte, ΔEs is the variation of initial voltage and final steady-state voltage, ΔEτ is the variation of the voltage during titration.The calculated D 2þ Zn coefficient of dMO, C-dMO, and GC-dMO electrode in region 1 during the discharge state is 9.272 × 10 À8 , 2.670 × 10 À6 , 1.851 × 10 À5 cm 2 s À1 respectively.The value of D 2þ Zn coefficient for GC-dMO electrode is in the range of 10 À5 cm 2 s À1 , which is better than dMO and C-dMO electrode.The comparison of calculated D 2þ Zn coefficient of dMO, C-dMO, and GC-dMO electrode is shown in Table S3, Supporting Information.The Zn-ion diffusion coefficient of GC-dMO electrode exhibits an D 2þ Zn coefficient of 1.851 × 10 À5 cm 2 s À1 , which is three orders higher than dMO electrode (9.272 × 10 À8 cm 2 s À1 ).The synergistic effect of Co-doping and rGO composite enlarges the interlayer gap and enhances the surface area and electrical conductivity, and structural stability enables fast diffusion of Zn 2+ ions during electrochemical reaction.

Conclusion
In summary, we have successfully synthesized a low-cost, high-capacity, and highly stable GC-dMO cathode using a simple cost-effective hydrothermal method.The detailed electrochemical reaction mechanism of the fabricated ZIBs was investigated using electrochemical measurements, ex situ XRD, and XPS analysis.The interlayer gap of the dMO electrode increases upon 2 wt% Co-doping, and graphene oxide sheets further enhance its electronic conductivity and structural stability, addressing the serious problem of the structural collapse of the dMO electrode.We observed that the dMO and GC-dMO electrodes undergo a structural transition to a Zn-buserite phase upon first discharge, which enhanced the intercalation and de-intercalation of Zn 2+ ions and H 2 O molecules.The simultaneous reverse intercalation and de-intercalation of Zn 2+ ions and water molecules exhibit the excellent electrochemical performance of ZIBs using a GC-dMO cathode.In addition, pre-intercalated K + ions and 2 wt% Co-doping created lattice defects and widened the interlayer gap, which improved the structural stability and facilitated the movement of Zn 2+ ions.The intercalated crystal water plays a significant role in inhibiting the electrostatic interaction between Zn 2+ ions and the host for easy movement of Zn 2+ ions.The Mn + ion loss is compensated by a 2 M Zn (CF 3 SO 3 ) 2 + 0.1 M Mn (CF 3 SO 3 ) 2 aqueous electrolyte.The aqueous ZIBs fabricated using the GC-dMO cathode displayed an excellent capacity of 302 mAh g À1 at a current density of 100 mA g À1 after 100 cycles, as compared to the dMO cathode (159 mAh g À1 at a current density of 100 mA g À1 after 100 cycles).In addition, the GC-dMO cathode exhibited a very good rate performance with a current density of 50-5000 mA g À1 and vice versa, indicating clear reversibility in aqueous ZIBs.We demonstrated that the GC-dMO cathode with a long cycle life retained a capacity of 99 mAh g À1 at a very high current density of 5000 mA g À1 after 1500 cycles.Therefore, the present low-cost, high-capacity, safe, and remarkable electrochemical performance of a GC-dMO cathode indicates that it holds promise for potential application in large-scale aqueous ZIBs.

Experimental Section
Material synthesis: dMO nanoflowers were synthesized using a hydrothermal method. [23]In a typical synthetic process, 8 mmol of KMnO 4 (Sigma Aldrich) was dissolved in 60 mL of deionized water.The resulting solution was stirred for 1 h at 22 °C The obtained solution was transferred to a Teflon vessel (autoclave) for the hydrothermal reaction.The temperature of the reaction was maintained at 200 °C for 24 h.The obtained product was centrifuged several times using deionized water and ethanol.The final product was freeze-dried for 48 h.
Similarly, the GC-dMO nanocomposite was prepared using a hydrothermal process.In the synthesis process, 8 mmol of KMnO 4 (Sigma Aldrich) and 2 wt% of Co (NO 3 ) 2 (Sigma Aldrich) were dissolved in 60 mL of deionized water.Graphene oxide (20 mL) was mixed with deionized water and sonicated for 1 h.The two solutions were mixed together and stirred for 1 h.The obtained solution was transferred to a Teflon vessel (autoclave) for the hydrothermal reaction, and the temperature was maintained at 200 °C for 24 h.Finally, the obtained product was centrifuged three times using deionized water and ethanol, respectively.The final product was freeze-dried for 48 h.
Material characterization: The high-resolution XRD patterns of the synthesized powder samples were obtained using synchrotron XRD at the 9B HRPD beamline (λ = 1.5297Å) at the Pohang Accelerator Laboratory (PAL) in Pohang, South Korea.Data were collected at room temperature with a six-detector system over an angular range of 10°≤ 2θ ≤ 100°at a step width of 0.01°.The ex situ XRD measurements were performed using a BRUKER D8 Advance A25 X-ray diffractometer with Cu Kα radiation at a wavelength of 1.5418 Å.The morphology and elemental analysis of the prepared powder samples were performed using a low accelerating voltage field-emission scanning electron microscope (JEOL JSM-7900F), an energy-dispersive X-ray spectrometer (EDS), and a 300-kV fieldemission transmission electron microscope (TECNAI).Thermo Fisher Scientific K-Alpha+ XPS (X-ray source, 200 m) was used for the X-ray photoelectron spectroscopy (XPS) of the powder samples.The BET surface areas and pore-size distributions of the powder samples were measured using a [D14] surface area and pore-size analyzer.
Electrochemical measurements of fabricated CR2032 coin-cell aqueous ZIBs: Galvanostatic charge-discharge and cyclic voltammetry measurements of aqueous ZIBs were performed using a Zn metal anode and the newly developed dMO, C-dMO, and GC-dMO electrodes as the cathode.The electrode slurry was prepared by mixing the GC-dMO powder, Super P carbon, and PVDF in the ratio of 80:10:10.The required amount of N-methyl pyrrolidone was mixed with this mixture and stirred continuously for 24 h.The as-prepared uniform slurry was coated on carbon paper using the doctor-blade technique.The coated slurry was then dried in a vacuum oven at 80 °C for 12 h.The cathode and anode electrodes were cut into pieces with diameters of 12 and 16 mm, respectively.The thickness of the Zn metal used was 0.25 mm.The weight of the cathode electrode was between 1.0 and 1.5 mg.The aqueous electrolyte was prepared by mixing 2 M zinc trifluoromethanesulfonate (Zn (CF 3 SO 3 ) 2 , Sigma Aldrich) and 0.1 M manganese trifluoromethanesulfonate (Mn (CF 3 SO 3 ) 2 , Sigma Aldrich) in deionized water for use in aqueous ZIBs.A circular Whatman glass microfiber filter paper (18 mm diameter) was used as the separator.The galvanostatic charge-discharge and cyclic voltammograms of the fabricated aqueous ZIBs were measured using a battery-testing instrument (WonATech).Electrochemical impedance spectroscopy measurements of the full-cell aqueous ZIBs were performed using a biological instrument.
).The lattice parameters for dMO calculated from Le Bail fitting were as follows: a = 5.119 Å, b = 2.848 Å, and c = 7.257 Å, which led to a cell volume of 103.5 Å3 .Similarly, the lattice parameters calculated for the C-dMO samples were as follows: a = 4.998 Å, b = 2.842 Å, and c = 7.335 Å with a cell volume of 100.3 Å3 .The C value of the C-dMO sample increased, confirming the expansion of the interlayer gap

Figure 2 .
Figure 2. a, b) Le Bail fitting of XRD patterns of dMO and C-dMO samples.c, d) Crystal structures of dMO and C-dMO samples.
13, 531.26, and 532.91 eV corresponding to the Mn-O-Mn, Mn-O-H, and H-O-H bonds, respectively.The presence of Mn-O-H and H-O-H bonds indicates the presence of water molecules absorbed from the surface of graphene oxide into the sample.The spin doublets of the Co 2p 3/2 and Co 2p 1/2 XPS peaks of the C-dMO and GC-dMO powder samples are depicted in Figure 3f.The XPS peaks of Co 2p 3/2 and Co 2p 1/2 of C-dMO were deconvoluted into two distinct peaks corresponding to

Figure 3 .
Figure 3. a-c) SEM images of dMO, C-dMO, and GC-dMO powder samples.d-f) Mn 2p, O 1s, and Co 2p XPS spectra of dMO, C-dMO, and GC-dMO samples.g) FTIR spectra of dMO, C-dMO, and GC-dMO samples.h, i) BET surface area and porosity of dMO, C-dMO, and GC-dMO samples.

Figure 5 .
Figure 5. a, b) Cycle life at current density 100 mA g À1 and rate performance of ZIBs using dMO, C-dMO, and GC-dMO electrodes.c, d) Galvanostatic charge-discharge curves of dMO, C-dMO, and GC-dMO samples.e) Long-term cycle life at current density 5000 mA g À1 of ZIBs using dMO, C-dMO, and GC-dMO electrodes.

Figure 7 .
Figure 7. CV curves at various scan rates within the range 0.1-1 mV s À1 : a) dMO, b) C-dMO, c) GC-dMO electrodes.d-f) Log(i) vs log(v) curves for Peaks 1 and 2 for ZIBs using the dMO, C-dMO, and GC-dMO electrodes.g) Diffusion contribution and capacitive contribution of the GC-dMO electrode.h) EIS curves of ZIBs using the dMO, C-dMO, and GC-dMO electrodes.