Metal–Organic Framework‐Derived MnO Nanocrystals Embedded in a Spindle Carbon for Rechargeable Aqueous Zinc Battery with a Molten Hydrate Electrolyte

Rechargeable aqueous zinc batteries (RAZBs) are emerging candidates for large‐scale energy storage. However, the lack of high‐capacity cathodes because of the electrostatic interactions between Zn2+ and cathode and the inferior electronic conductivity restricts their performance. The operating voltage limitation imposed by water is another barrier for RAZBs. Herein, manganese oxide (MnO) nanocrystals embedded in a spindle carbon matrix (MnO@C) synthesized from a metal–organic framework are used as a cathode. The uniform distribution of fine‐sized MnO (≈100 nm) in the carbonized matrix (≈5 μm) and the intimate connection between them not only increase the utilization of electroactive material but also eliminate the use of conductive additive. By utilizing the molten hydrate electrolyte, ZnCl2·2.33H2O, a discharge voltage plateau approaching 1.60 V and a high reversible capacity of 106 mAh g−1 after 200 cycles are achieved. This research proposes an approach for affordable RAZBs to fulfill large‐scale energy storage.

Rechargeable aqueous zinc batteries (RAZBs) are emerging candidates for large-scale energy storage.However, the lack of high-capacity cathodes because of the electrostatic interactions between Zn 2þ and cathode and the inferior electronic conductivity restricts their performance.The operating voltage limitation imposed by water is another barrier for RAZBs.Herein, manganese oxide (MnO) nanocrystals embedded in a spindle carbon matrix (MnO@C) synthesized from a metal-organic framework are used as a cathode.The uniform distribution of fine-sized MnO (%100 nm) in the carbonized matrix (%5 μm) and the intimate connection between them not only increase the utilization of electroactive material but also eliminate the use of conductive additive.By utilizing the molten hydrate electrolyte, ZnCl 2 •2.33H 2 O, a discharge voltage plateau approaching 1.60 V and a high reversible capacity of 106 mAh g À1 after 200 cycles are achieved.This research proposes an approach for affordable RAZBs to fulfill large-scale energy storage.
structural water, and in conjunction with the electrolyte optimization. [9]Although these efforts have contributed to the development of RAZBs, the rational design of both the cathode and the electrolyte materials is still in its infancy, leaving substantial room for further improvement of the energy storage performance.
[20][21][22] Apart from the electrodes, it is widely acknowledged that the electrolyte nature is closely linked to the (de)insertion behavior of Zn 2þ ions at the cathode and the rechargeability of the zinc anode.Increasing the concentration of salt above a certain threshold (>20 M) has emerged as a promising approach for constructing RAZBs of decent performance.However, expensive organic fluorinated salts such as bis(trifluoromethanesulfonyl)amide (e.g., 1 M Zn ) were typically required, whose adoption in large-scale applications remained questionable. [23]Recently, a new class of zinc molten hydrate electrolytes consisting of inorganic ZnCl 2 has been demonstrated, in which all water molecules participate in Zn 2þ ions hydrate shells. [24]It is worth noting that these zinc molten hydrates not only reconcile the high cost and beneficial properties brought by conventional superconcentrated electrolytes (Table S1, Supporting Information) but also hold promise for supporting electrochemical reactions that were otherwise impossible by virtue of their unique solvation structures. [25]ere, we present the MnO@C as a cathode material for RAZBs via one-step calcination of manganese gallate (Mn(II) 3,4,5-trihydroxybenzoate dehydrate, termed as Mn-GLA).We adopt this MOF as the precursor because of its extended inorganic connectivity (infinite chains of trans-corner-sharing MnO 6 octahedral) and compact gallic acid linkers that facilitate the formation of modestly porous and dense electrode materials.Additionally, gallic acid is produced on a massive scale from biomass at a low cost of %10 USD kg À1 . [26]The fine-sized MnO is found to be uniformly distributed in the thermally carbonized matrix after calcination, which eliminates the use of conductive additives and increases the utilization ratio of electroactive material.These characteristics have an important influence on the gravimetric and volumetric energy density of the practical cell, but receive little attention in the previous MOF-derived electrode materials.With respect to the electrolyte, it has been demonstrated that the concentration of zinc salt strongly influences the reaction mechanism of the MnO@C cathode and thus the (de)insertion voltage and cyclability.Using the electrolyte of ZnCl 2 •2.33H 2 O (23.82 M), a flat discharge voltage plateau of 1.60 V and a reversible capacity of 106 mAh g À1 over 200 cycles, against 1.0 V and 24 mAh g À1 in ZnCl 2 •50H 2 O of the resultant cell were achieved.Furthermore, we systematically investigated the correlation between the material characteristics of MnO@C and its electrochemical properties, which provides insights into the development of large-scale affordable zinc-based energy storage systems.The affordability of constitute materials and the good electrochemical properties establish the proposed RAZB as a potential candidate for large-scale energy storage systems.

Results and Discussion
2.1.Synthesis and Structural Characterization of the MnO@C Mn-GLA is derived from the carbonization of Mn-GLA that is synthesized by a solvothermal reaction of MnSO 4 •H 2 O and gallic acid (Figure 1a).A gallic acid linker contains five oxygen atoms that are conducive to form the manganese-enriched MOF. [27]The hybrid framework structure is compact rather than porous, which benefits to acquire a dense structure for the formation of compact MnO encapsulated in carbon matrix in the subsequent calcination.The crystal structure of Mn-GLA studied by single-crystal X-ray diffraction (SCXRD) reveals that the Mn(II) center is six-coordinated in a MnO 6 coordination environment.The coordination environment and topological structure are shown in Figure S1, Supporting Information.As expected, the density of Mn-GLA is 1.68 g cm À3 which is obviously higher than that of common MOFs such as MIL-101(Cr) (0.62 g cm À3 ) and ZIF-8 (1.14 g cm À3 ). [28]he powder X-ray diffraction (PXRD) pattern of Mn-GLA matches the simulated pattern from single-crystal data, confirming that the bulk sample is pure (Figure S2, Supporting Information).According to the thermogravimetric analysis (TGA) shown in Figure S3 (Supporting Information), Mn-GLA shows inconsiderable weight change at a temperature greater than 600 °C, indicating the completeness of carbonization and phase transformation from Mn-GLA to the target material.Figure S4 (Supporting Information) shows the PXRD pattern of the obtained samples calcined at 700, 800, and 900 °C (hereafter termed as Mn-GLA-700, Mn-GLA-800, and Mn-GLA-900, respectively).All diffraction peaks of these samples are readily assigned to MnO (PDF 07-0230), except a peak at 26.5°that is exclusively observable for Mn-GLA-900, which is from carbon.The crystallinity of MnO is increased from Mn-GLA-700 to Mn-GLA-900.High-resolution transmission electron microscope (HRTEM) observation and Raman spectra as shown in Figures S5 and S6 (Supporting Information) reveal the coexistence of graphitized and amorphous carbon on the surface of calcined samples.The HRTEM results showed that lattice fringes of carbon were not observed in Mn-GLA-700, indicating a low degree of graphitization.For Mn-GLA-800 and Mn-GLA-900, the lattice fringes of carbon were observed.Raman peaks at around 294, 350, and 670 cm À1 are the characteristic vibration modes associated with Mn-O. [29]These peaks become stronger and sharper as the calcination temperature increases, which indicates significant crystallization of MnO and is in accordance with the PXRD results.Besides, the degree of graphitization, from the intensity ratio of G band to D band of carbon, [30] also increases as the calcination temperature increases.The increased graphitization of carbon enhanced the electronic conductivity and electroactivity of MnO@C materials.
As shown in Figure 1b, Mn-GLA displays a spindle shape.After calcination, the fine-sized MnO is embedded in the thermally carbonized matrix, which still maintains the same morphology as the precursor (Figure 1c).Moreover, the elemental mapping analysis of Mn-GLA and Mn-GLA-900 displays that C, Mn, and O elements are uniformly distributed throughout the structures, confirming the effectiveness of the approach employing MOFs as a template to derive new materials. [31]The wellcrystallized quality of MnO in Mn-GLA-900 is explicitly reflected by the regularly selected area electron diffraction (SAED) pattern (Figure 1d).The lattice spacing of 0.26 and 0.22 nm matches well with the interplanar distance of the (111) and (200) planes of MnO, respectively (Figure 1e).Simultaneously, the outer carbon shell is intended to function as a support framework for the high volume variation of MnO caused by ions (de)intercalation, as well as facilitating electrical transport. [32]The PXRD pattern of the Mn-GLA-900 indicated the crystalline purity of the obtained sample (Figure 1f ).X-ray photoelectron spectroscopy (XPS) was also performed to study the oxidation state of Mn in Mn-GLA-900 (Figure 1g).The fitting of the two deconvoluted peaks in Mn 2p spectrum suggests the presence of Mn(II).The binding energy located at 641.7 and 653.5 eV is ascribed to Mn 2p 3/2 and Mn 2p 1/2 , respectively. [33]The broad scan survey spectrum of Mn-GLA-900 is shown in Figure S7 (Supporting Information).Mn-GLA-700 did not show significant adsorption behavior, while Mn-GLA-800 and Mn-GLA-900 exhibit similar adsorption behavior.Notably, Mn-GLA-900 has the largest specific surface area of 173 m 2 g À1 and an ordered porosity (Figure S8, Supporting Information).These features should ensure evenly contact between the electroactive material and the electrolyte. [34,35]2.Electrochemical Performance of the Mn-GLA-900 Figure 2a displays the cyclability of the Mn-GLA-900 cathode with the molten hydrate electrolyte of ZnCl 2 •2.33H 2 O at 0.05 A g À1 , using a simple pouch cell with a zinc foil anode.With a stable discharge capacity of 106 mAh g À1 , a satisfactory cycling stability over 200 cycles was achieved.In comparison, Mn-GLA-700 and Mn-GLA-800 can only maintain the capacities of 20 and 44 mAh g À1 after 200 cycles, respectively (Figure S9, Supporting Information), due to the relatively poor crystallinity, low electron conductivity, and disordered structure.Mn-GLA-800 exhibits higher crystallinity and electrical conductivity than Mn-GLA-700, but the (de)insertion of zinc ions may damage the long-range structure, resulting in capacity fluctuations.For Mn-GLA-900, its good crystallinity is beneficial for the structural stability throughout the cycle.The rate performance of Mn-GLA-700, -800, and -900 are shown in Figure S10, Supporting Information.For Mn-GLA-900, when the current density recovers, the capacity also fully recovers.This may be due to the crystallinity of MnO becoming better as the carbonization temperature increases, which is beneficial to the cycling stability upon the charging/discharging process.In addition, as the carbonization temperature increases, the graphitization degree of carbon increases, which significantly improves its electronic conductivity.The electron conductivity orders are: Mn-GLA-900 > Mn-GLA-800 >> Mn-GLA-700 (Figure S11, Supporting Information).The commercial MnO shows a low capacity retention under the same condition (31 mAh g À1 over 100 cycles, Figure S12, Supporting Information), due to its undesirable morphology (Figure S13, Supporting Information).The electrochemical impedance spectroscopy (EIS) of Mn-GLA-900 electrode with the electrolyte of ZnCl 2 •2.33H 2 O was tested, which remained nearly unchanged after the cycling, indicating the electrochemical stability of the active material (Figure S14, Supporting Information).Besides, it is well documented that the reversibility and efficiency of zinc stripping/plating are equally essential to attain good cyclability for RAZBs.[36] To verify the indispensable role of molten hydrate electrolytes, the electrochemical properties of Mn-GLA-900 were measured with an electrolyte of a commonly adopted zinc salt concentration (ZnCl 2 •50H 2 O, 1.11 M).As shown in Figure 2b, although the electrodes have a similar initial capacity of 110 mAh g À1 in the two systems, the cyclability is significantly different.When using ZnCl 2 •2.33H 2 O as the electrolyte, a satisfactory cycling stability after 200 cycles was achieved with a stable discharge capacity of 106 mAh g À1 (95% of its maximum capacity and an almost 100% Coulombic efficiency).In comparison, the Mn-GLA-900 electrode with the ZnCl 2 •50H 2 O electrolyte presents a low capacity of 24 mAh g À1 over 200 cycles, corresponding to 22% of its maximum capacity.An additional interesting feature observed in the case of ZnCl 2 •2.33H 2 O electrolyte is overall uplifted charge/ discharge profiles.It can be seen that nearly 100 mAh g À1 of Mn-GLA-900 is delivered at voltages above 1.5 V, whereas it is merely 20 mAh g À1 as in ZnCl 2 •50H 2 O (Figure 2c).
Figure 2d displays the cyclic voltammetry (CV) profiles of Mn-GLA-900 electrode measured at 0.05 mV s À1 .A reversible redox couple at 1.79 and 1.60 V is found for ZnCl 2 •2.33H 2 O, perfectly matching the charge and discharge behaviors at 0.05 A g À1 .The oxidation peak was located at 1.55 V, and the reduction peaks appeared at 1.38 and 1.24 V for ZnCl 2 •50H 2 O as noticed from the CV profiles obviously, which were consistent with the charge and discharge behaviors.Previous studies have reported that the cathode undergoes intercalation/deintercalation of H þ and Zn 2þ in zinc-diluted electrolytes, [37] resulting in the repeated expansion/contraction and structural damage of the cathode and the evident capacity attenuation.The average discharge voltage plateau of the Mn-GLA-900 electrode with the electrolyte of ZnCl 2 •2.33H 2 O is 1.60 V, which is notably higher than manganese-based and other cathode materials reported so far (Table S2, Supporting Information).As shown in Figure 2e, the working voltage of the two cells connected in series is sufficient to power up a green light-emitting diode (LED) bulb with a 3.0 V onset potential, implying potentially practical applications.In addition, the charge/discharge test of Mn-GLA-900 electrode that started from discharge delivers only limited capacity, indicating its incapability to accommodate guest species without electrochemical oxidation (Figure S15, Supporting Information).

Reaction Mechanism of the Mn-GLA-900
To gain insight into Mn-GLA-900 electrode showing enhanced electrochemical performances with ZnCl 2 •2.33H 2 O compared with that of ZnCl 2 •50H 2 O, the charge storage kinetics of the Mn-GLA-900 cathode were systematically studied by CV, EIS, and galvanostatic intermittent technique (GITT) measurements.The representative CV profiles at variable scan rates are displayed in Figure 3a and S16a (Supporting Information).The dependence of current (i) of an electroactive material on the scan rate (v) abides by the equation of i = av b , where a and b are constants. [38]The electrode kinetic can be evaluated by b value: diffusion-controlled when b = 0.5, while surface-controlled when b = 1.0.Through plotting log(i) versus log(v), b value of each redox peak (A and A 0 ) in ZnCl 2 •2.33H 2 O is calculated to be 0.642 and 0.634, respectively (Figure 3b).These values indicate that the redox reaction involves surface reactions at the Mn-GLA-900-electrolyte interface and the (de)intercalation and diffusion occurs within the bulk Mn-GLA-900.In contrast, the redox reaction of Mn-GLA-900 in ZnCl 2 •50H 2 O is mainly controlled by merely the ion diffusion inside the Mn-GLA-900 with ZnCl 2 •50H 2 O, as b value of each redox peak (B and B 0 ) is evaluated to be 0.502 and 0.513, respectively (Figure S16b, Supporting Information). [38]As shown in Figure 3c, the contribution of pseudocapacitance increased with the increase of scan rate.
EIS tests were also performed to evaluate the pseudocapacitive contribution of Mn-GLA-900 electrode in ZnCl 2 •2.33H 2 O or ZnCl 2 •50H 2 O (Figure S17, Supporting Information).In the high and middle frequencies, it is related to the bulk and chargedischarge resistance (R bulk and R ct ).An increase in the electrolyte concentration causes an increased R bulk , but the charge transfer between the electrolyte and the active component of the electrode decreased significantly, which indicates a more favorable electrochemical reaction in ZnCl 2 •2.33H 2 O.Moreover, the slope in the low frequency of ZnCl 2 •2.33H 2 O is steeper than that of ZnCl 2 •50H 2 O, implying an increased capacitive contribution.Compared with ZnCl 2 •50H 2 O, the Mn-GLA-900 electrode with the electrolyte of ZnCl 2 •2.33H 2 O has a lower charge transfer resistance, indicating a higher interfacial conductivity.Therefore, with the electrolyte of ZnCl 2 •2.33H 2 O, charge transfer is faster, and the redox kinetics behavior of the cathodes is enhanced.Mn-GLA-900 with the electrolyte of ZnCl 2 •2.33H 2 O can speed up the transport of ions, thereby significantly improving the rate capacities. [39]From the GITT result (Figure 3d), the reaction overpotential is %40 mV based on the difference between the equilibrium potential of charging and discharging profiles, even without the use of additional conductive additive such as Super P. Additionally, the discharge platform voltage of the cell remains stable during consecutive cycles.The apparent diffusion coefficient of ions (D) of Mn-GLA-900 electrode in the ZnCl 2 •2.33H 2 O electrolyte at different (de)intercalation states evaluated from GITT is about 10 À10 -10 À8 cm 2 s À1 (Figure 3e), which are close to recently reported works using zincconcentrated electrolytes. [40]In comparison, the D value of commercial MnO estimated by the same method is obviously lower than the Mn-GLA-900 electrode (about 10 À11 cm 2 s À1 , Figure S18, Supporting Information).To understand the reaction mechanism of the Mn-GLA-900 electrode during ions (de)insertion with ZnCl 2 •2.33H 2 O electrolyte, a series of structural analyses by ex situ PXRD, TEM, XPS, and X-ray absorption spectroscopy (XAS) were conducted at full charge and discharge states.As shown in Figure S19 (Supporting Information), a new diffraction peak appeared at approximately 37°when the cathode was charged to 1.9 V, due to the formation of MnO 2 .In addition, energy dispersive spectroscopy (EDS) mapping of the electrode shows that zinc and chlorine appeared when charging to 1.9 V, which originates from the intercalation of anionic species [ZnCl 4 ] 2À in ZnCl 2 •2.33H 2 O into the electrode (Figure S20, Supporting Information). [18]The structural evolution was further studied by HRTEM (Figure S21, Supporting Information).The observed lattice fringes correspond to the (111) plane of MnO 2 .Mn 2þ ions are oxidized into Mn 4þ to balance the charges during the charging process. [41]Furthermore, the MnO 2 phase forms out when [ZnCl 4 ] 2À ions insert.The Mn-GLA-900 structure is recovered upon full discharging to 0.2 V. XPS measurements were performed at full charge and discharge states.Mn 2þ /Mn 4þ oxidation during charging is demonstrated by the Mn 2p spectrum (Figure 4a).A binding energy of 644.6 eV is observed during the charging process, which is the characteristic of Mn 4þ . [42]During the discharging process, the Mn 2p spectrum returned to the same position as pristine.The zinc and chlorine appear at the fully charged status and disappear when discharged to 0.2 V (Figure 4b,c), implying the reversible (de)intercalation of [ZnCl 4 ] 2À within the cathode.
To further understand the change of the Mn oxidation states in Mn-GLA-900 during the charging/discharging process, the ex situ X-ray absorption near edge structure (XANES) measurement was performed.K-edge XANES profiles of Mn-GLA-900 at full charge and discharge states are shown in Figure 4d.The Mn absorption edge of the pristine sample is basically consistent with that of the standard MnO sample with the valence state of Mn 2þ .The spectrum is moved to high binding energy at the fullcharged state, which indicates the oxidation of Mn.In particular, the full-charged oxidation state is slightly less than that of MnO 2 , indicating that Mn 2þ is partially oxidized to Mn 4þ . [33]The spectrum was recovered at the subsequent full-discharged states, indicating the reversible oxidation state change of Mn in Mn-GLA-900.The ex situ extended X-ray absorption fine structure (EXAFS) spectroscopy at the Mn K-edge was also conducted to more in-depth analyze the electronic structure evolution during cycling (Figure 4e).The strong peak located at 1.7 Å is assigned to the Mn-O.The peaks at 2.7 Å are assigned to Mn-Mn. [43]When the electrode is fully charged, the 2.7 Å peak widens and slightly moves to larger distance, which is associated with the formation of Mn-O-Zn. [44]Furthermore, the 1.7 Å peak slightly shifts to lower distance, implying the appearance of MnO 2 . [45]When the electrode is fully discharged, no significant spectroscopy deviation is observed as compared to that of the pristine sample.These results show that the Mn-GLA-900 cathode can reversibly accommodate the guest species of [ZnCl 4 ] 2À during charging and return to the original status when discharging.Based on the earlier analysis, the reversible Zn storage process can be summarized as the reversible (de)intercalation of the [ZnCl 4 ] 2À species during charging and return to the original status when discharging, simultaneously accompanied by the redox of MnO.

Conclusion
In summary, we employ a MOF with extended inorganic connectivity and tight organic linkers as the precursor to synthesize the cathode material MnO@C for RAZBs.The fine-sized MnO is uniformly generated in the thermally carbonized matrix, which eliminates the need of conductive additives and increases the utilization ratio of electroactive material.With the molten hydrate electrolyte, ZnCl 2 •2.33H 2 O, an acceptably flat discharge voltage plateau of 1.60 V and a stabilized reversible capacity of 106 mAh g À1 over 200 cycles were obtained.The electrochemical reversible process can be attributable to the reversible (de)intercalation of the species of [ZnCl 4 ] 2À during the charging/discharging process, accompanied by the redox of MnO.We anticipate that the MOF-derived MnO@C with both well-controllable morphology and desirable electrochemical performance can provide a new direction for the development of high-performance cathode materials of RAZBs.

Figure 2 .
Figure 2. a) Cyclability of Mn-GLA-900 electrode with the electrolyte of ZnCl 2 •2.33H 2 O at 0.05 A g À1 ; b) Cyclability of Mn-GLA-900 electrode with the electrolyte of ZnCl 2 •50H 2 O at 0.05 A g À1 ; c) Typical galvanostatic charge-discharge profiles of Mn-GLA-900 electrode measured at 0.05 A g À1 with the electrolyte of ZnCl 2 •2.33H 2 O and ZnCl 2 •50H 2 O, respectively; d) CV of Mn-GLA-900 electrode and a zinc anode with the electrolyte of ZnCl 2 •2.33H 2 O and ZnCl 2 •50H 2 O at 0.05 mV s À1 , respectively; e) Photograph of green LED bulbs powered by two cells connected in series.

Figure 3 .
Figure 3. a) CV profiles at various scan rates of Mn-GLA-900 electrodes with ZnCl 2 •2.33H 2 O; b) b value of each redox peak by plotting log(i) versus log(v) with ZnCl 2 •2.33H 2 O; c) Contribution ratios of the capacitive effects of Mn-GLA-900 with ZnCl 2 •2.33H 2 O at different scan rates; d) GITT charge/discharge profiles of Mn-GLA-900 electrode with the electrolyte of ZnCl 2 •2.33H 2 O; e) Diffusion coefficient of Mn-GLA-900 electrode calculated from GITT data.

Figure 4 .
Figure 4. XPS spectra of a) Mn 2p, b) Zn 2p,and c) Cl 2p for the Mn-GLA-900 electrodes at full charge and discharge states; synchrotron spectra of d) Mn K-edge X-ray absorption near edge structure (XANES) and e) extended X-ray absorption fine structure (EXAFS).