Preparation of three-dimensional compact graphene / manganese oxide composite material and application of research on electrochemical lithium storage performance thereof

Abstract

The application discloses a preparation method of a graphene and MnO particle composite laminated structure (DG / MnO) composite material as a lithium ion battery negative electrode material. The application aims to improve the conductivity of the manganese oxide composite material and provide a stable conductive network in the cycle process, thereby improving the specific capacity and cycle performance of the electrode material. The application mainly comprises: one, preparation of Na-doped manganese dioxide (MnO2-Na); two, preparation of three-dimensional dense graphene / manganese oxide (DG / MnO); the application has the characteristics of simple synthesis process, high yield and the like. Since the MnO particles can provide a stable migration channel for lithium ion migration, the graphene sheet layer can improve the conductivity of the MnO particles, and the DG / MnO-700 electrode can reach a reversible capacity of 1075 mAh g ‑1 -1 at 0.1 A g ‑1 -1, and has an excellent cycle performance of 101% in 500 cycles of 0.5 A g ‑1 -1.

Description

Technical Field

[0001] The present invention relates to the fields of material preparation and energy storage, and in particular to the preparation of a three-dimensional dense graphene / manganese oxide composite material and research on its electrochemical lithium storage performance. Background Art

[0002] The rapid development of the portable electronic devices and electric vehicle markets has attracted widespread attention from researchers around the world to energy storage devices. Lithium-ion batteries have long been considered a promising electrochemical energy storage device. Transition metal oxides are often widely studied as negative electrode materials for lithium-ion batteries due to their superior capacity performance, safety, and price advantages. Among the various transition metal oxides, MnO materials have a high theoretical specific capacity (756 mAh g -1 ), lower hysteresis voltage (<0.8 V), larger density (5.43 g cm -3 ) and environmentally friendly, it is considered a promising negative electrode material for lithium-ion batteries. However, this material also has disadvantages such as poor conductivity, severe volume expansion during charge and discharge, and poor rate performance and cycle stability.

[0003] To address these issues, an effective strategy is to construct composite materials of manganese monoxide and carbon, utilizing nanocarbon materials as a conductive network and buffering volume expansion. Carbon coating can significantly improve the conductivity and polarization resistance of MnO electrode materials, thereby significantly enhancing the electrode's rate performance. However, during charge-discharge cycling, the significant volume expansion can cause the carbon coating layer within the active material to break down, resulting in carbon coating technology still failing to meet the cycling stability requirements for practical applications. Graphene, a widely studied carbon material, not only improves the conductivity of MnO but also acts as a buffer zone during electrochemical cycling, limiting the volume expansion of MnO particles. Unfortunately, even after simple composite coating, MnO particles exposed on the graphene surface can still fragment and agglomerate during charge-discharge, leading to a decrease in the electrode's electrochemical performance. Therefore, developing superior electrode structures that both improve the conductivity of manganese oxide composites and provide a stable conductive network during cycling has become a research priority.

[0004] In summary, the present invention fabricated a composite laminated structure of graphene and MnO particles (DG / MnO). The MnO particles, acting as a support, are distributed within the wrinkled graphene sheets, providing stable and abundant pathways for lithium ion migration. The graphene sheets significantly improve the conductivity of the MnO particles, ensuring rapid charge transfer. This tightly stacked structure improves electron / ion transport kinetics and enhances structural and interfacial stability during charge and discharge, resulting in excellent electrochemical energy storage performance and cycling stability. This structure holds great promise as a lithium-ion battery anode material. Summary of the Invention

[0005] The purpose of the present invention is to design a method for preparing a negative electrode material for lithium-ion batteries that can improve the conductivity of the manganese oxide composite material and provide a stable conductive network during the cycle, thereby improving the specific capacity and cycle performance of the electrode material.

[0006] The present invention uses Na-doped manganese dioxide and graphene as raw materials to prepare a three-dimensional dense graphene / manganese oxide composite material, and the preparation method for the negative electrode of a lithium-ion battery is carried out according to the following steps:

[0007] 1. Room temperature synthesis of Na-doped manganese dioxide (MnO2-Na)

[0008] Weigh 0-140 g of NaCl and dissolve it in 100 mL of deionized water. After vigorous stirring for 30 minutes, add 0.5 g of KMnO4 to the solution and stir vigorously for 3 hours. Then, measure 75 mL of ethanol and slowly add it dropwise to the solution using a separatory funnel. Continue stirring for 30 minutes. After the reaction is complete, immediately centrifuge and wash until there is no Cl in the system. - (Tested using silver nitrate.) After sonication, freeze-dry the resulting sample, denoted as MnO2-Na-x (where x corresponds to the amount of NaCl added, in g). Prepare the MnO2-Na-x sample into a dispersion for later use.

[0009] Preparation of two- and three-dimensional dense graphene manganese oxide (DG / MnO)

[0010] 200 mL of GO suspension (0.5 mg / mL) was subjected to intense sonication for 10 minutes and then to sonication with 100 mL of MnO2 dispersion (containing approximately 120 mg to 350 mg of MnO2) until completely dispersed. The two dispersions were placed in a standard ultrasonic bath, and the MnO2 dispersion was added dropwise to the GO solution (approximately 5 minutes) and subjected to intense sonication for 10 minutes. The sonicated liquid was evaporated to dryness in a 45°C water bath until approximately 10 mL of slurry remained. The slurry was then applied to a polyethylene sheet and dried at room temperature to obtain GO / MnO2 flakes, which were then calcined at 600–800°C in a hydrogen atmosphere for 2 hours to obtain the sample.

[0011] Compared with the existing technology, the present invention has the following beneficial effects:

[0012] (1) The present invention combines graphene and MnO particles to form a stacked structure (DG / MnO). The MnO particles serve as a support and are distributed in the middle of the wrinkled graphene sheets, providing stable and abundant migration channels for lithium ion migration. The graphene sheets can greatly improve the conductivity of the MnO particles and ensure rapid charge transfer. This tightly stacked structure is conducive to improving the transport dynamics of electrons and ions, and enhancing the structural and interfacial stability during the charge and discharge process, ensuring good conductivity and cycle stability of the electrode.

[0013] (2) In the graphene and MnO stacked structure (DG / MnO) prepared by the present invention, manganese monoxide particles are evenly and tightly distributed between the graphene sheets. This dense stacked structure can effectively inhibit the structural collapse of the electrode material caused by ion transport and buffer the volume expansion of MnO during the charge and discharge process, which is beneficial to stabilizing the structural stability of the electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a microscopic morphology photo of a DG / MnO composite material prepared using Na-doped manganese dioxide and graphene as raw materials;

[0015] Figure 2 The crystal structure and chemical composition of a DG / MnO composite material prepared using Na-doped manganese dioxide and graphene as raw materials;

[0016] Figure 3 It is a DG / MnO composite material prepared with Na-doped manganese dioxide and graphene as raw materials and has good electrochemical performance;

[0017] Figure 4 This is the morphology of DG / MnO composite material prepared with Na-doped manganese dioxide and graphene as raw materials. DETAILED DESCRIPTION

[0018] Specific embodiment 1: A three-dimensional dense graphene / manganese oxide composite material is prepared using Na-doped manganese dioxide and graphene as raw materials, and is applied to the negative electrode of a lithium-ion battery. The preparation method is carried out according to the following steps:

[0019] 1. Room temperature synthesis of Na-doped manganese dioxide (MnO2-Na)

[0020] Weigh 0-140 g of NaCl and dissolve it in 100 mL of deionized water. After vigorous stirring for 30 minutes, add 0.5 g of KMnO4 to the solution and stir vigorously for 3 hours. Then, measure 75 mL of ethanol and slowly add it dropwise to the solution using a separatory funnel. Continue stirring for 30 minutes. After the reaction is complete, immediately centrifuge and wash until there is no Cl in the system. - (Tested using silver nitrate.) After sonication, freeze-dry the resulting sample, denoted as MnO2-Na-x (where x corresponds to the amount of NaCl added, in g). Prepare the MnO2-Na-x sample into a dispersion for later use.

[0021] Preparation of two- and three-dimensional dense graphene manganese oxide (DG / MnO)

[0022] 200 mL of GO suspension (0.5 mg / mL) was subjected to intense sonication for 10 minutes and then to sonication with 100 mL of MnO2 dispersion (containing approximately 120 mg to 350 mg of MnO2) until completely dispersed. The two dispersions were placed in a standard ultrasonic bath, and the MnO2 dispersion was added dropwise to the GO solution (approximately 5 minutes) and subjected to intense sonication for 10 minutes. The sonicated liquid was evaporated to dryness in a 45°C water bath until approximately 10 mL of slurry remained. The slurry was then applied to a polyethylene sheet and dried at room temperature to obtain GO / MnO2 flakes, which were then calcined at 600–800°C in a hydrogen atmosphere for 2 hours to obtain the sample.

[0023] Specific embodiment 2: A three-dimensional dense graphene / manganese oxide composite material is prepared using Na-doped manganese dioxide and graphene as raw materials, and is applied to the negative electrode of a lithium-ion battery. The preparation method is carried out according to the following steps:

[0024] The difference between this embodiment and the first embodiment is that the NaCl content in step 1 is 70 g. The rest is the same as the first embodiment.

[0025] Specific embodiment three: A three-dimensional dense graphene / manganese oxide composite material is prepared using Na-doped manganese dioxide and graphene as raw materials, and is applied to the negative electrode of a lithium-ion battery. The preparation method is carried out according to the following steps:

[0026] The difference between this embodiment and the first embodiment is that the content of MnO2 used in step 2 is 175 mg. The rest is the same as the first embodiment.

[0027] Specific embodiment 4: A three-dimensional dense graphene / manganese oxide composite material is prepared using Na-doped manganese dioxide and graphene as raw materials, and is applied to the negative electrode of a lithium-ion battery. The preparation method is carried out according to the following steps:

[0028] This embodiment differs from the first embodiment in that the GO / MnO2 sheet prepared in step 2 is calcined at 700°C in a hydrogen atmosphere for 2 h to obtain DG / MnO-700. Other steps are the same as those in the first embodiment.

[0029] The following test was used to verify the effect of the present invention:

[0030] In this experiment, a three-dimensional dense graphene / manganese oxide composite material was prepared using Na-doped manganese dioxide and graphene as raw materials, and was applied to the negative electrode of lithium-ion batteries. The preparation method was carried out in the following steps:

[0031] 1. Room temperature synthesis of Na-doped manganese dioxide (MnO2-Na)

[0032] Weigh 70 g of NaCl and dissolve it in 100 mL of deionized water. After vigorous stirring for 30 minutes, add 0.5 g of KMnO4 to the solution and stir vigorously for 3 hours. Then, measure 75 mL of ethanol and slowly add it dropwise to the solution using a separatory funnel. Continue stirring for 30 minutes. After the reaction is complete, centrifuge and wash immediately until there is no Cl in the system. - (Silver nitrate was used for testing.) After sonication, the sample was freeze-dried and recorded as MnO2-Na-70. The MnO2-Na-70 sample was prepared into a dispersion for later use.

[0033] Preparation of two- and three-dimensional dense graphene manganese oxide (DG / MnO-700)

[0034] 200 mL of GO suspension (0.5 mg / mL) was subjected to intense sonication for 10 minutes and then to sonication with 100 mL of MnO2 dispersion (containing approximately 175 mg of MnO2) until completely dispersed. The two dispersions were placed in a standard ultrasonic bath, and the MnO2 dispersion was added dropwise to the GO solution (approximately 5 minutes) and subjected to intense sonication for 10 minutes. The sonicated liquid was evaporated to dryness in a 45°C water bath until approximately 10 mL of slurry remained. The slurry was then applied to a polyethylene sheet and dried at room temperature to obtain GO / MnO2 flakes, which were then calcined at 700°C in a hydrogen atmosphere for 2 hours to obtain sample DG / MnO-700.

[0035] Figure 1a shows the microscopic morphology of DG / MnO-700. It can be seen that the morphology of the DG / MnO-700 sample is a tightly stacked layered structure, and the manganese monoxide particles are evenly and tightly distributed between the graphene sheets. Among them, the graphene sheets can increase the overall conductivity of the electrode material, ensure the rapid migration of electrons in the electrode, and buffer the volume changes of manganese monoxide particles caused by the insertion and deinsertion of lithium ions during the charge and discharge process. Manganese monoxide particles can not only serve as active materials, but also as a supporting structure between sheets to prevent the restacking of graphene sheets during the charge and discharge process, and maintain the structural stability of the electrode and the integrity of the ion transmission channel. Moreover, this "sandwich" structure is conducive to the full contact between the electrolyte ions and the active material, improving the electrode wettability, the diffusion rate of the electrolyte ions and the effective specific surface area of ​​the electrode material ( Figure 1 a). MnO particles are evenly distributed in the DG / MnO-700 material, and manganese monoxide can support the graphene sheet, providing more ion transport channels ( Figure 1 b, 1c). The graphene sheets are tightly connected to the MnO, ensuring the overall conductivity of the material. The wrinkles of the graphene sheets are obvious. This special structure can provide two typical interlayer ion transport channels, namely the ridge channels provided by the graphene wrinkles and the support channels provided by the MnO particles ( Figure 1 c). Transmission electron microscopy was used to further determine the internal microstructure and crystal structure of DG / MnO-700. It can be seen that MnO particles are evenly distributed on the graphene sheets. The particle size and distribution are consistent with those of scanning electron microscopy. Figure 1 The stacking structure of the material is obvious ( Figure 1 d), further confirming the dense stacking structure of DG / MnO-700. Analysis of the lattice fringes revealed a lattice spacing of 0.26 nm, corresponding to the (111) crystal plane of MnO (Figure 1e). The selected area electron diffraction spectrum of DG / MnO-700 shows a clear diffraction pattern consistent with that of manganese monoxide, indicating good crystallinity of MnO (Figure 1f). The element distribution diagram of DG / MnO-700 shows that C is evenly distributed in the material, while Mn and O are concentrated in spherical areas (Figure 1g).

[0036] Figure 2The structure, chemical composition and valence state of DG / MnO-700. In Figure 2a, all samples show obvious characteristic peaks at 34.9˚, 40.5˚, 58.7˚, 70.2˚ and 73.8˚, corresponding to the (111), (200), (220), (311) and (222) crystal planes of cubic MnO, respectively, proving the existence of MnO (JCPDS NO. 07-0230). The broad peak near 26˚ corresponds to the C peak of graphene, indicating the existence of dense layered graphene structure. The characteristic peak around 15.8˚ in the XRD spectrum of MnO-700 sample corresponds to Na 0.7 MnO 2.05 The (002) crystal plane of the MnO sample indicates that the conversion is incomplete and trivalent manganese exists, which is consistent with the results observed by transmission electron microscopy and selected area electron diffraction. The Raman spectrum is shown in Figure 2b. At 602 cm -1 The characteristic peak at 1340 cm corresponds to the vibration of the Mn-O bond in MnO. It is worth noting that the characteristic peak of DG / MnO-700 due to the vibration of the Mn-O bond is significantly blue-shifted compared with MnO, which may indicate that there is a strong electronic coupling between MnO and graphene. This phenomenon often occurs when transition metal oxides are composited with carbon-based materials, confirming that MnO and graphene are effectively composited rather than simply physically mixed. Compared with MnO-700, DG / MnO-700 and G / MnO-700 samples have higher peaks at 1340 and 1596 cm -1The two characteristic peaks appearing at the π-axis correspond to the D and G peaks of the carbon material, confirming the presence of graphene in the material. The ratio of these two peaks (ID / IG) is often used to determine the amount of defects in the carbon material's microstructure. Calculated ID / IG values ​​for DG / MnO-700 and G / MnO-700 are 0.974 and 0.893, respectively, indicating a high proportion of disordered and defective structures in DG / MnO-70, consistent with TEM measurements. Thermogravimetric analysis of the DG / MnO-700 composite under argon is shown in Figure 2c. The decrease in mass of the DG / MnO-700 composite from room temperature to 300°C corresponds to a decrease in the amount of adsorbed and structural water released from the composite. The mass change between 400 and 500°C is attributed to the gradual conversion of MnO to Mn2O3. Calculated graphene content in the DG / MnO-700 composite is 33.8 wt%. XPS analysis further confirmed the elemental composition and manganese valence state of the DG / MnO-700 composite, as shown in Figure 2d. Figure 2d shows the full XPS spectrum of the DG / MnO-700 composite. The XPS spectrum indicates the presence of C, Mn, O, and Na, consistent with the elemental analysis results. The Mn2p spectrum, shown in Figure 2e, reveals two distinct peaks at binding energies of 653.7 and 641.8 eV, corresponding to the Mn2p1 / 2 and Mn2p3 / 2 peaks, respectively. The binding energy difference between the two peaks is 11.6 eV, indicating a Mn(II) valence state, consistent with the XPS spectrum of manganese monoxide and in good agreement with the XRD results. After fitting, Mn 2p3 / 2 consists of three characteristic peaks located at 644.0, 642.1 and 640.9 eV. The former corresponds to Mn (III) and the latter two correspond to Mn (II). The content of Mn (III) in the sample can be estimated by the area of ​​the fitting curve. After calculation, the content of Mn (III) in the sample is 29.7%. The presence of trivalent manganese ions in the sample is due to the presence of Mn 7+ Xiang Mn 2+ The conversion is incomplete. In order to maintain electrical neutrality, cation vacancies appear in the manganese monoxide lattice. The appearance of such ion vacancies can improve the conductivity of the material, provide active sites for electrochemical reactions, and be more conducive to the transmission of electrolyte ions in the electrode material, which is beneficial to the electrochemical performance of the battery. According to the Mn2p spectrum of the MnO-700 sample in Figure 2i, the Mn in the MnO-700 sample is calculated. 3+The content is 45.3%, which is significantly higher than that of DG / MnO-700, indicating that the conversion of manganese monoxide in the MnO-700 sample is incomplete. The presence of carbon in the DG / MnO-700 sample makes it easier to reduce manganese ions to a lower valence state. 3+ In contrast, due to the 2+ It has a relatively lower Gibbs free energy and the most stable electron configuration in the outer orbital, so Mn 2+ It is difficult to spontaneously oxidize to Mn 3+ , MnO exists stably in air. In the C1s spectrum, the peak at 284.8 eV is consistent with the characteristic peak of graphitic carbon, while the peaks at 285.5, 288.1, and 291.5 eV correspond to C-OH, C=O, and OC=O, respectively (Figure 2 f). In the O1s spectrum, the two peaks at 535.7 and 534.1 eV correspond to HOH and Mn-OH, respectively, while the strong fitting peak at the binding energy of 531.4 eV corresponds to Mn-O-Mn (Figure 2 g). In the Na1s spectrum of the DG / MnO-700 sample, a characteristic peak can be clearly observed at 1071.3 eV, indicating that the Na in the manganese dioxide raw material is retained in the DG / MnO-700 sample during the synthesis process (Figure 2 h).

[0037] Figure 3 shows the electrochemical performance analysis of the three-dimensional dense graphene / manganese oxide composite material. Figure 3.a shows the first three cycles of cyclic voltammetry test curves of DG / MnO-700 electrode material at a scan rate of 0.1 mV s -1 , operating voltage is 0.01~3.0 V (vs. Li / Li +). In the first cycle of the cathodic scan, two reduction peaks can be observed near 0.5 and 1.4 V. This is due to the formation of a solid electrolyte interface film (SEI) on the electrode surface and the decomposition of the electrolyte during the first cycle of discharge. The disappearance of these two platform in the subsequent cyclic voltammetry curve indicates that the SEI film formed on the electrode surface is stable. The presence of two characteristic peaks in the anodic scan of DG / MnO-700 may be due to the fact that graphene improves the conductivity of the electrode material and the refinement of MnO grains in the electrochemical reaction promotes electrochemical kinetics. The fact that the peak current does not decay after cycling indicates that the electrode has good reversibility. The stacking of graphene can protect the MnO particles from being coated with Li2O, thereby improving the conductivity of the electrode material. Figure 3b shows the charge and discharge curves of the DG / MnO-700 sample for the first three cycles at a current density of 100 mA g-1. Two discharge platforms can be observed in the first cycle discharge curve of DG / MnO-700, and two charge platforms can be observed in the charge curve, which is consistent with the CV curve, indicating that the prepared composite material exhibits enhanced kinetics in the reaction. Many previous studies have shown that the fast Li ion reaction kinetics during the charge and discharge process may be the cause of the Mn 2+ In the first discharge cycle, the specific capacity of the DG / MnO-700 electrode was 1645 mAh g -1 , charging capacity is 1039 mAh g -1 The initial coulombic efficiency was 63.14%. In the second and third charge-discharge cycles, the coulombic efficiency increased to 93.5% and 94.3%, respectively. Figure 3c shows the rate performance comparison of the DG / MnO-700 sample. The DG / MnO-700 electrode exhibits good rate performance due to its unique microstructure. At 0.1 A g -1 At a charge and discharge current density of 1.5 GHz, the reversible capacity of the DG / MnO-700 electrode is 1075 mAh g -1 , and at 2 A g -1 At a current density of 1.5 GHz, the specific capacity is 703 mAh g -1 Figure 3 d shows the rate performance of DG / MnO-700 at high current density. -1 The specific capacity can still be maintained at 327.8 mAh g -1 After charging and discharging at different current densities, when the current density suddenly decreases to 0.5 A g -1 When the specific capacity is 879.1 mAh g -1, indicating that the DG / MnO-700 composite material has good electrochemical reversibility and stability. Figure 3e shows the electrochemical impedance spectroscopy of the DG / MnO-700 sample. As can be seen from the figure, the semicircle diameter of DG / MnO-700 is the smallest, indicating that its charge transfer rate is the fastest, reflecting the structural superiority of the DG / MnO-700 composite material. Figure 3f shows the current density of 0.5 A g -1 Cyclic stability test curve of DG / MnO-700 electrode, the test voltage range is 0.01~3 V (vs. Li / Li + ), during the first few cycles, the specific capacity of the electrode material gradually increased, indicating the gradual activation process of the electrode. After 500 cycles, the specific capacity of the DG / MnO-700 electrode material was still 915.3 mAh g -1 The capacity retention rate was 101%, demonstrating excellent cycling stability. Figure 3g shows the electrochemical impedance spectroscopy (EIS) of the DG / MnO-700 electrode material at different cycle numbers. With increasing cycle number, the slope of the low-frequency linear portion of the EIS curve gradually increases, corresponding to a gradual decrease in the lithium ion diffusion resistance in the electrolyte. This indicates that the lithium ion transport channels gradually become unblocked and the migration rate increases during the cycling process, which is consistent with the cycling performance curve. Figure 3 h shows the DG / MnO-700 electrode material at 0.1~1 mV s -1 Cyclic voltammetry curves at scan rates. It can be seen that at 1mV s -1 At a high scan rate, the cyclic voltammetry curve of the DG / MnO-700 sample did not change significantly, with the curve shape and peak position similar to those at a low scan rate. This indicates that the dense, stacked "sandwich" structure of DG / MnO-700 provides stable charge transfer and ion migration pathways for electrochemical reactions, resulting in a relatively complete conductive network and good structural stability. These results demonstrate that the three-dimensional dense graphene / manganese oxide (DG / MnO-700) composite is a promising supercapacitor electrode material.

Claims

1. A method for preparing a three-dimensional dense graphene / manganese oxide composite material prepared using Na-doped manganese dioxide and graphene as raw materials, which is applied to the negative electrode of a lithium-ion battery. The preparation method comprises the following steps:

1. Room temperature synthesis of Na-doped manganese dioxide MnO2-Na Weigh 0-140g of NaCl and dissolve it in 100mL of deionized water. After vigorous stirring for 30min, add 0.5g of KMnO4 to the above solution and stir vigorously for 3h. Then, measure 75mL of ethanol and slowly add it dropwise to the solution using a separatory funnel. Continue stirring for 30min. After the reaction is completed, centrifuge and wash immediately until there is no Cl in the system. - ; After ultrasonication, freeze-drying was performed, and the obtained sample was recorded as MnO2-Na-x, where x corresponds to the amount of NaCl added in g. The MnO2-Na-x sample was prepared into a dispersion for later use; Preparation of two- and three-dimensional dense graphene manganese oxide DG / MnO 200 mL of GO suspension was subjected to strong ultrasonication for 10 minutes and ultrasonicated with 100 mL of MnO2 dispersion until completely dispersed; the above two dispersions were placed in an ordinary ultrasonic water tank, and the MnO2 dispersion was added dropwise to the GO solution and strongly ultrasonicated for 10 minutes. The sonicated liquid was evaporated to dryness in a 45°C water bath until 10 mL of slurry remained. The slurry was applied to a polyethylene plate and dried at room temperature to obtain GO / MnO2 flakes, which were calcined at 600-800°C in a hydrogen atmosphere for 2 hours to obtain the sample.

2. The preparation method according to claim 1, characterized in that The NaCl content in step 1 is 30-100 g.

3. The preparation method according to claim 1, characterized in that The amount of MnO2 added in step 2 is 150mg to 350mg.

4. The preparation method according to claim 1, wherein The calcination temperature in step 2 is 650-800°C.

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