Coral-like carbon skeleton for aqueous zinc-ion batteries MnO2 cathode material

Rechargeable aqueous zinc-ion batteries (AZIBs) have attracted significant attention in the field of energy storage due to their high theoretical capacity, low toxicity, excellent safety performance, low cost, and affordability. As the cathode material of AZIBs, MnO2 boasts advantages such as cost-effectiveness and low environmental impact. However, it still exhibits certain limitations such as inadequate conductivity and compromised structural stability. In this work, a coral-like carbon skeleton (CCS) was designed to stabilize the electrochemical properties of the MnO2 cathode by growing the MnO2 on the surface of CCS. It is found that CCS@MnO2 exhibits notable advantages in terms of stable cycling performance and high specific capacity. This material exhibits an approximate 100% coulombic efficiency after 500 cycles when subjected to a current density of 1.0 A g−1. The CCS support significantly enhanced the performance of AZIBs and rendering it highly appealing for diverse range of energy applications.


Introduction
In the process of rapid economic development, it is necessary to solve various energy and environmental problems for human production and life, so the development of renewable energy has become a hot topic at present in the coming decades [1,2].In addition, portable mobile electronic devices and electric vehicle devices have entered people's lives, so energy storage devices have higher requirements.In recent years, various rechargeable aqueous batteries have attracted great attention in the application of energy storage [3][4][5][6][7][8][9][10][11][12][13][14][15].Lithium-ion batteries are widely used in various fields due to their advantages such as high energy density and extremely wide voltage window, but also suffer from the low security, high price, limited resources, etc, which limit their development in large-scale energy storage applications [3][4][5][6][7][8].Therefore, there is an urgent need to find an economical, efficient, safe, and environmentally friendly energy storage material.Due to its advantages of high theoretical capacity, low toxicity, good safety, low price and cost, low redox potential, and high ionic conductivity, rechargeable aqueous zinc-ion batteries (AZIBs) gradually come into people's vision [8,16,17].
So far, researchers have been committed to exploring cathode materials with better electrochemical storage performance, such as vanadium oxides, Prussian blue analogues, manganese-based oxides, and so on.Among these candidates, vanadyl oxide has a higher theoretical specific capacity but low operating voltage.Prussian blue analog has a lower capacity.In comparison, MnO 2 has a more appropriate theoretical specific capacity and conforms to the concept of environmentally friendly, safe, and environmental protection; therefore, it has become the best choice at present as the cathode material for AZIBs [4,[8][9][10][11][12][13]15].MnO 2 is characterized by α-MnO 2 , β-MnO 2 , γ-MnO 2 , δ-MnO 2 , etc. Its basic crystal structure mainly consists of an octahedral unit composed of 6 O atoms and 1 Mn atom [15,[18][19][20].δ-MnO 2 is a typical monoclinic crystal system with a typical lamellar structure, which is composed of MnO 6 octahedrons and is a 1×∞ layered structure with large layer spacing.
When δ-MnO 2 is used as the cathode material of AZIBs, it has a larger specific discharge capacity of 256 mAh g −1 compared with other MnO 2 structures.In addition to β-MnO 2 , it is another case in which no hydrogen ion embedment is observed.Due to the diversity of δ-MnO 2 phases, different structural evolution is observed during zinc-ion embedment.In addition, it was found that the capacity of the solvent in the aqueous electrolyte is twice that in the non-aqueous electrolyte, which proves that the solvent has a significant effect on the AZIBs [21,22].In various crystal forms of MnO 2 , δ-MnO 2 has faster ion diffusion ability and better electrochemical energy storage with wider layer spacing，which makes it get high initial capacity.However, due to the partial dissolution of δ-MnO 2 during discharge and re-electrolysis during charging, the reconstruction of this material leads to its inability to maintain its original structure, resulting in a rapid decline in capacity.Therefore, it is necessary to construct a skeleton in the interior of MnO 2 to ensure the stability of its structure as much as possible.
In this work, a coral-like carbon skeleton (CCS) is designed to achieve stable circulation.The physical and chemical characteristics of CCS and its effects on the electrochemical performances of MnO 2 cathode and its mechanism were systematically investigated using different characterization techniques.The results show that as a conductive substrate with continuous skeleton structure, CCS can significantly improve the rate and cycle performance of the AZIBs.

Materials and synthesis
Bare MnO 2 .A concentrated 1 ml of H 2 SO 4 was added drop by drop into the 500 ml 0.02 M KMnO 4 solution which then takes 0.015 ml of MnSO 4 dissolved in 500 ml of distilled water and stir well.Under stirring, the dissolved KMnO 4 solution and MnSO 4 solution are poured together into a 1 L beaker to react for 2 h and precipitate for 12 h.CCS.Weigh 5 g of glucose as raw material, add 70 ml of deionized water, stir for 2 h, and then put the sample into the reactor for hydrothermal reaction; the temperature is 260 °C, the time is 3 h, and the self-made coral-like carbon skeleton is obtained.
CCS@MnO 2 .Weigh 0.1 g of the synthesized CCS, add 1.896 g of KMnO 4 , then add a certain amount of water, pour it into the reactor for hydrothermal reaction, for 2 h, and the temperature is 120 °C.

Structural characterization
Scanning electron microscopy (SEM) was used to characterize the morphology and size of the samples.X-ray photoelectron spectroscopy (XPS) was used to analyze the content and valence states of the elements.The samples were characterized by X-ray diffraction (XRD) using Brock D2 PHASER.The specific surface area and mesoporous distribution were determined by Brunauer-Emmet-Teller (BET) method.Thermogravimetric analysis (TGA) was performed by TA TGA 550 analyzer.

Electrochemical tests
The working cathode was obtained by mixing MnO 2 product, self-made carbon, and polyvinylidene fluoride (PVDF) at a mass ratio of 7:2:1.The mixture is then evenly coated on stainless steel foil to prepare the MnO 2 cathode, which is then dried in a vacuum oven at 80 °C for 24 h and pressed under a roller press.The self-made test battery was assembled from the above working cathode, 2 M ZnSO 4 electrolyte, nylon mesh separator, and zinc anode.The electrochemical performance of the battery was tested on the electrochemical workstation (CHI 760E) and the LAND Battery Test System (CT2001A).Discharge and charging performance were tested on the LAND CT2001A instrument.Cyclic voltammetry (CV) tests were performed in the potential range at scan rates of 0.1, 0.2, 0.5, 1.0, and 2.0 mV s −1 .Electrochemical impedance spectroscopy (EIS), rate cycle, and GITT measurements were carried out on the electrochemical workstation.EIS test conditions: CHI 760 E electrochemical workstation of Shanghai Chenhua Technology Co., Ltd.The frequency range is 0.01 Hz to 100 kHz, and the amplitude is 5 mV.All electrochemical tests were carried out at 25 °C.

Morphology and structure
The morphologies of the carbon matrix and C materials were studied by SEM.It is obvious that the morphologies of the two materials are very different.Figure 1a-c present the SEM images of the homemade carbon matrix, and its overall appearance is similar to that of corals formed by spherical aggregation with a smooth surface.The coral-like particles, which are gathered by approximately 200-nm microspheres, may be more conducive to electron transfer and structural stability.The prepared CCS@MnO 2 microsphere has a spherical nucleus of approximately 800 nm, which is similar to the nucleus of the carbon matrix in its overall morphology but at the same time very different.Figure 1d-f show a coral-like structure formed by multiple particles but with a large number of nanosheets on its surface.This is most likely because MnO 2 nanosheets have grown on the surface of the carbon matrix.
Figures 2a and 2b present the TEM images of CCS@ MnO 2 ; the distinct carbon skeleton and spherical local structure with a diameter of about 600 nm of the carbon skeleton can be clearly observed, and the nanosheets are interconnected to form into a lamellar architecture on its surface [3,23].Figure 2b further confirms that δ-MnO 2 particles with the diameter of ~ 150 nm are uniformly complexed with homemade CCS [24].The HRTEM image in Fig. 2c shows that the lattice spacing of 0.718 nm is corresponding to (003) crystal plane of δ-MnO 2 , a typical lattice spacing of 0.359 nm is corresponding to (006) crystal plane of δ-MnO 2 , 0.241 nm is fitting with the (012) crystal plane of δ-MnO 2 , 0.143 nm is corresponding to the (113) crystal plane of δ-MnO 2 , further confirmed that the layered characteristic of the crystal material is indeed the δ-MnO 2 .Figure 2d further investigates that the element distribution reveals the EDS mapping of O, Mn, and C elements; the final results show that the Mn and O elements are covered on the surface of the C, which proves that MnO 2 is indeed uniformly coated on the surface of CCS [25].
The XRD patterns of CCS@MnO 2 and MnO 2 are shown in Figure .3a, both of which are consistent with the δ-MnO 2layered structure.The structure and composition of the final sample have been characterized on planes 001, 002, 111, and 312 of JCPDS 30-0820.It was characterized on the 003 and 006 surfaces of JCPDS 86-0666.CCS@MnO 2 has a peak similar to primitive MnO 2 , but less intense.In addition, no diffraction peak of carbon was observed in MnO 2 [3,17,20].The TG and DTG curves of CCS@MnO 2 in Figure .3b show that the percent weight loss is 12.501%, and the residue percent is 82.900% [26].
Figure 4a displays the Fourier transform infrared (FT-IR) spectra of CCS@MnO 2 .In the picture, several groups can be observed, such as -OH, -C=C, -C-H, and the bands of 1288 and 1614.To characterize the specific surface area of CCS@MnO 2 samples, Bruner-Emmett-Teller (BET) tests were performed.As shown in Figure .4b, absorption and desorption isotherms of MnO 2 and CCS@MnO 2 and the N adsorption-desorption isotherms of both samples belong to type I-IV isotherms with H3 hysteresis rings, indicating the existence of fissure-type pores.Due to the addition of CCS in the synthesis process, the agglomeration tendency of MnO 2 nanosheets was effectively reduced, and the specific surface area of CCS@MnO 2 samples was significantly increased.The pore size distribution was calculated using the Barret-Joyner-Halenda (BJH) method (Figure . 4c).The specific surface area is 59.0203 m 2 g −1 , the average pore diameter is 14.2423 nm, and the pore volume is 0.197977 cm 3 g −1 .It can be seen that the CCS@MnO 2 electrode has a large specific surface area and a large total pore volume, which is conducive to electrolyte penetration, so as to obtain a high electrochemical capacity [3,27,28].
To further study the elemental composition and electronic structure of the CCS@MnO 2 material, it was systematically analyzed by X-ray photoelectron spectroscopy (XPS).Figure 5a shows a high-energy XPS full spectrum measurement spectrum, and the sample shows the presence of C, O, and Mn without other impurities, which is consistent with the XRD result.It is worth noting that Figure .5b shows the high-resolution Mn 2p spectrum in which two typical peaks appear, Mn 2p 3/2 and Mn 2p 1/2, with binding energies of 656.80 eV and 654.27 eV, respectively.As shown in

Electrochemical performance
Figure 6a shows the first five cycles of the CV curve at 0.1 mV s −1 , which shows the cyclic voltammetry and indicates that the electrochemical cycle maintains a certain structural stability.In the first cycle, only one oxidation peak appeared at 1.61 V when forward scanned starting from the open circuit voltage.When negatively scanned, two reduction peaks appear at 1.35 V and 1.22 V, respectively.In the following cycles, a reduction peak at ~ 1.3 V and an oxidation peak at ~ 1.62 V are observed.Remarkably, the plateau at 1.5 V was observed.To further study the reaction kinetics of the discharge platform.Figure 6b shows CV execution at different scanning rates (0.1, 0.2, 0.5, 1.0, and 2.0 mV s −1 ) in the voltage range of 1.0-1.8V.With a pair of characteristic oxidation/reduction peaks at approximately 1.7/1.3V, the reduction peak at 1.3 V gradually strengthened as the scanning rate increased.The cyclic curve is complete, which shows that the reaction has good reversibility and stability.At the same time, the CV curves remain similar, the peak current increases obviously, and the oxidation/reduction peak is wider than the other peaks.It is obvious that the reduction peak gradually moves to low potential, and the oxidation peak gradually moves to high potential.This is because the diffusion resistance increases as the scanning rate increases.However, it is noteworthy that the peak value is suppressed at high potential sweep rates, such as 2.0 mV s −1 .According to previous studies, there is a relationship between the peak current and sweep rate of ln(i) = ln(a) + b ln(ν).The value range of b is 0.5-1.0.When b is 0.5, the reaction is a diffusion control process and insertion/extraction reaction.When b is 1.0, the reaction is a surface control capacitor response process.
In the formula i = av b , i represents the current (mA), v represents the scan rate (mV s −1 ), a is the parameter, and b is the slope of log(i) − log(v).Through the formula, the corresponding peak current i is processed at different scanning rates v, and the corresponding b values are calculated, which are 0.41 and 0.37, respectively (Figure . 6c).A b value close to 1.0 indicates a capacitive control process, and a b value close to 0.5 indicates a diffusion control process.Here, the b value is somewhere in between, so the positive terminal is controlled by both processes.Using capacitance behavior control (k 1 v) and diffusion process control (k2v 1/2 ), the capacitance contribution of the cathode is further calculated The results show that the contribution of material capacitance increases with the increase of scanning rate, and CCS@MnO 2 electrode has a high capacitance effect.To further understand the effect of internal CCS and its electrochemical impedance in CCS@ MnO 2 , Figure .6b compares the Nyquist plots of CCS@ MnO 2 and MnO 2 .The electrodes were tested using EIS.The Nyquist diagram of the battery assembled with two electrode materials under the same conditions (current density of 0.3 A g −1 , voltage window of 1.0-1.85V) after 500 charge and discharge cycles is drawn in the frequency range of 0.01 Hz to 100 kHz with an amplitude of 5 mV.
The resistance of the sample CCS@MnO 2 decreased during the final scan, indicating that CCS in CCS@MnO 2 can enable rapid charge transfer on the electrode/electrolyte contact surface, thereby reducing the overall resistance inside the zinc-ion battery and improving the conductivity of the electrode material, which is conducive to the diffusion of Zn 2+ into the cathode material.This is consistent with CV results.It is generally believed that the semi-circle in the high-frequency region is mainly related to the interface film of the electrode electrolyte, while in the low-frequency region, the electrochemical impedance spectrum is characterized by large impedance values and slow changes.This is because the electrochemical reaction rate in the lowfrequency region is slow, and the physical and chemical characteristics at the interface between the electrode and the electrolyte change slowly, resulting in a relatively slow change in the impedance value.On the other hand, the electrochemical impedance spectra in the low-frequency region are also affected by the double-layer capacitance and diffusion capacitance at the interface between electrodes and electrolytes.Figure 6c shows the fitting curve of the cathode current peak and anode current peak [3,31,32].Slope b is 0.41 and 0.37, respectively, approximately 0.5, indicating its charge storage behavior related to the diffusion control process; compared to MnO 2 , CCS@MnO 2 has a larger slope value, corresponding to the relationship between the peak current and the square root of the scanning rate that indicates a stronger ion diffusion capability.
Figure 6d shows the constant-current charge-discharge distribution curves at different current densities (0.2-2.0A g −1 ).This result corresponds to the CV redox voltage platform at different scanning rates, but it is only obvious at low current densities and is almost invisible at high current densities, indicating that different redox reactions occur during the whole reaction process.We will make changes to this statement as required in a timely manner.In order to clearly evaluate the ion diffusion rate (D Zn 2+ ) of ions in the CCS@MnO 2 cathode, galvanostatic intermittent titration technique (GITT) was used to determine the ions in the experiment.The measurement procedure was for 20 min 2+ is basically maintained between 10 −13 and 10 −11 in both charging and discharging processes.This is mainly attributed to CCS in CCS@MnO 2 , where the aggregated coral-like particle structure and MnO 2 nanosheets grow uniformly on the surface of the carbon matrix.The unique structural characteristics of CCS@MnO 2 enable it to provide high electrical conductivity of the active material, sufficient ion transport channels, and short ion diffusion paths, thereby promoting the transport and storage of Zn 2+ ions in the electrode.In addition, the good structural stability of the electrode is also an important factor for its good cycle performance and magnification performance [4,[33][34][35][36][37][38][39][40][41].
To analyze the electrochemical impedance of CCS@ MnO 2 , Nyquist diagrams of MnO 2 and CCS@MnO 2 are compared, as shown in Figure .7a.The charge transfer resistance (RCT) in the electrochemical process belongs to the high-frequency region, while the ion diffusion in the electrochemical process belongs to the low-frequency region.By comparing the curve radius of the two samples in the highfrequency curve, it can be seen that the electrode of MnO 2 is lower than that of CCS@MnO 2 and has better performance.As shown in Figure .7b, when the current density is 0.2 mA h g −1 , the specific capacity of CCS@MnO 2 is 392 mA h g −1 , showing a high reversibility.Moreover, it has relatively good rate performance with capacities of 392, 343, 258, 202, 165, and 75 mA h g −1 at 0.2, 0.4, 0.6, 0.8, 1.0, and 2.0 A g −1 , respectively.This indicates that CCS@MnO 2 has better capacity retention than MnO 2 and delivers a large specific capacity and excellent rate performance.Figure 7c shows the sample cycling performance of 500 cycles at a current density of 1.0 A g −1 for the two cathodes.CCS@MnO 2 drops significantly in the first cycle and becomes stable in the later cycle.The battery shows high structural stability, lasting cycling stability, and no obvious capacity attenuation.The Coulombic efficiency is approximately 100%.The specific discharge capacity of MnO 2 is reduced from 82 to 30 mA h g −1 , keeping the capacity at 63%.In contrast, the specific discharge capacity of CCS@MnO 2 dropped from 170 to 70 mA h g −1 , maintaining 58% of the capacity.The main advantages of CCS@MnO 2 electrochemical performance are its high theoretical specific capacity, high specific surface area, and its ability to maintain approximately 100% of the initial capacity [17,20,35,36,42].

Conclusions
In summary, we successfully prepared CCS@MnO 2 by using a simple hydrothermal reaction, and comprehensively analyzed its electrochemical performance as the cathode material for AZIBs.The XRD patterns of the as-prepared CCS@ MnO 2 indicated the presence of lamellar nanoscale structure of it.SEM investigations of it revealed that CCS@MnO 2 possesses a uniform coralline morphology with a spherical nucleus of about 800 nm.XPS and TEM show that Mn, O, and C are evenly distributed across the surface of the material.The BET measurements revealed that the surface area and the overall pore volume of the CCS@MnO 2 were 59.0203 m 2 g −1 and 0.00633 cm 3 g −1 , respectively.Under prolonged cycling, the Coulombic efficiency of the CCS@ MnO 2 electrodes can be maintained at nearly 100%.A series of electrochemical tests show that different from normal MnO 2 , CCS@MnO 2 has durable cyclic stability and high theoretical specific capacity, which is our further exploration of rechargeable AZIB cathode in order to achieve better electrochemical performance.

Fig. 1 a
Fig. 1 a-c SEM images of the homemade carbon matrix.d-f SEM images of CCS@MnO 2

Fig. 5 a
Fig. 5 a CCS@MnO 2 before and after discharge XPS spectrum, b CCS@MnO 2 after discharge Mn 2p XPS spectrum, c O1s XPS spectrum, and d C1s XPS spectrum

Fig. 6 a
Fig. 6 a First five CV curves at 0.1 mV −1 , b CV curves at different scanning rates, c log j-log ν curve of specific peak current, d constant current charge-discharge distribution under different current densi-

Fig. 7 a
Fig. 7 a Nyquist plots, b rate performance, and c cycle performance at 1.0 A g −1