CN111533171B - A simple calcination method for preparing porous MnO2 - Google Patents

Abstract

The invention discloses a simple calcination method for preparing porous MnO₂The method comprises the steps of taking potassium permanganate as a manganese source, taking n-butanol as a carbon source, adopting a liquid phase synthesis method to carry out uniform reaction in a water bath, carrying out centrifugal washing and drying on the obtained product, and then carrying out annealing treatment on the dried product to obtain black powder, namely the final product. The invention designs a simple and cheap liquid phase synthesis method, which can realize large-scale preparation of MnO by combining simple calcination in air₂A nano-material. Preparation of MnO₂The nano-sized particles are uniformly distributed, obvious gaps are formed among the particles, the lithium ion diffusion distance is favorably shortened, the contact area with electrolyte is increased, and the MnO can be greatly improved₂Lithium storage properties of the samples.

Description

A simple calcination method for preparing porous MnO2

technical field

The invention belongs to the technical field of nanomaterial preparation, and in particular relates to a method for preparing porous _MnO2 by a simple calcination method.

Background technique

Nanostructured inorganic materials have attracted more and more attention due to their special electrical, optical, mechanical and thermal properties. Manganese oxides are not only rich in resources, low in price, and non-polluting to the environment, but also have variable compositions, complex structures, and peculiar functions. It shows broad application prospects, so the research on its preparation method, structural characterization, reaction mechanism and application has attracted much attention. Among them, _MnO2 , as an important inorganic functional material, has been widely used in the fields of catalysis and electrode materials.

Most of the traditional commercial lithium-ion batteries use graphite as the anode material, but its low specific capacity (372mAh/g) and energy density limit the performance improvement of lithium-ion batteries. In contrast, manganese oxide has many advantages as a negative electrode material for lithium-ion batteries: first, manganese oxide has a higher theoretical specific capacity (700-1200mAh/g), which is more than twice that of graphite material (this is a great advantage for the development of Capacity lithium-ion batteries provide space); secondly, among the currently known transition metal oxides, manganese oxide has the lowest charge-discharge plateau (about 0.4V) (which is beneficial to improve the voltage and power of lithium-ion batteries); Finally, manganese is abundant in the earth's crust, the raw material price of manganese oxide is low, and the pollution to the environment is small, so it has broad commercial prospects. At present, there are many preparation methods for preparing manganese dioxide nanomaterials, mainly including sol-gel method, hydrothermal method, precipitation method, electrospinning method and vapor deposition method. However, these methods all have disadvantages such as complex preparation process, high production cost, low efficiency, high-end equipment required, and inability to achieve large-scale production, which are of little significance for commercial application.

Aiming at this problem, designing a high-performance manganese oxide anode material with a simple method, cheap and easy to mass-produce becomes the key. In this idea, the preparation of porous materials by calcination has become the first choice. As the most commonly used material synthesis method in industry, the calcination method is not only simple in method and simple in steps, but more importantly, the current cathode materials for lithium ion batteries are mainly prepared by the calcination method. Therefore, this method has the advantage of equipment reuse in lithium-ion battery material synthesis enterprises. However, the materials prepared by the industrial calcination method are often on the micrometer scale, with poor morphology uniformity, and their performance is difficult to meet the requirements of lithium-ion battery anodes. To this end, we designed a porous structure to alleviate the problem of uneven morphology in the calcination method.

SUMMARY OF THE INVENTION

In order to solve the problem that the porous manganese oxide material in the current negative electrode material of lithium ion battery is difficult to prepare on a large scale, the present invention provides a method for preparing porous MnO ₂ by a simple calcination method.

To achieve the above object, the present invention is achieved in this way:

A method for preparing porous _MnO2 by a simple calcination method, using potassium permanganate as the manganese source and n-butanol as the carbon source, adopting a liquid phase synthesis method to uniformly react in a water bath, and the obtained product is washed by centrifugation and dried, and then regenerated. The dried product is annealed, and the obtained black powder is the final product.

Further, in the annealing treatment, the dried product is transferred to a tube furnace, and in an air atmosphere, the temperature is raised to 300-500°C at a rate of 2-5°C/min, maintained for 3-6 hours, and then cooled naturally.

Further, the heating rate of the annealing treatment is 2°C/MIN, the temperature is raised to 400°C, the temperature is maintained for 3H, and then the temperature is naturally lowered.

Further, the consumption ratio of potassium permanganate and n-butanol is 10-13:1mg/mL, and potassium permanganate is placed in the container that n-butanol is housed, and heated in a water bath, heated to 60-80 ℃ to carry out. The reaction time is 6-12h.

Further, the drying of the product obtained after the water bath reaction is carried out in an oven at a temperature of 60-80° C. and a time of 30-60 min.

Compared with the prior art, the present invention has the following beneficial effects:

The present invention designs a simple and cheap liquid phase synthesis method, which can realize large-scale preparation of MnO ₂ nanomaterials in combination with simple calcination in air. The prepared MnO ₂ nano-sized particles are uniformly distributed and have obvious voids between the particles, which is beneficial to shorten the lithium ion diffusion distance and increase the contact area with the electrolyte, which can greatly improve the lithium storage performance of the MnO ₂ sample. At a current density of 1000 mA/g, the initial discharge capacity reached 1366.4 mAh/g, and the discharge capacity remained around 850 mAh/g after 250 cycles, showing excellent cycle stability. The test results of lithium storage performance show that the ideal _MnO2 anode material can be prepared on a large scale by a simple method, which makes it possible to commercialize the preparation of _MnO2 and replace the traditional graphite material, which has a broad market prospect.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings required in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some of the present invention. In the embodiments, for those of ordinary skill in the art, other drawings can also be obtained according to these drawings without any creative effort.

Fig. 1 is the XRD pattern of the sample before and after annealing of the present invention.

FIG. 2 is the SEM images of the samples before and after annealing in the present invention at different magnifications.

Figure 3 is a graph showing the results of the charge-discharge experiment of the MnO2 sample under a certain current density of the present invention, wherein (a) is the charge-discharge capacity and coulomb efficiency curve of the _MnO2 sample at a current density of 500mA/g; (b) is the current of 500mA/g The charge-discharge curve of the MnO ₂ sample under different current densities; (c) is the charge-discharge specific capacity curve of the MnO ₂ sample at different current densities; (d) is the charge-discharge curve corresponding to (c) at different current densities; (e) 1000mA The charge-discharge capacity curve of the _MnO sample under the first 250 cycles at a current density of /g.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention fall within the protection scope of the present invention.

According to an embodiment of the present invention, a method for preparing porous MnO _{2 by a simple calcination method is provided. The method for preparing porous MnO 2 by} a simple calcination method uses potassium permanganate as the manganese source and n-butanol as the carbon source, The liquid-phase synthesis method is used to uniformly react in a water bath, the obtained product is washed and dried by centrifugation, and then the dried product is annealed, and the obtained black powder is the final product.

In the embodiment of the present invention, the annealing treatment is to transfer the dried product to a tube furnace, and in an air atmosphere, raise the temperature to 300-500°C at a rate of 2-5°C/min, hold for 3-6h, and then naturally Cool down. As a preferred solution of the embodiment of the present invention, the heating rate of the annealing treatment is 2°C/min. Specifically, the temperature is raised to 400°C at a rate of 2°C/min, maintained for 3 hours, and then cooled naturally.

Wherein, the dosage ratio of potassium permanganate and n-butanol is 10-13:1 mg/mL, and potassium permanganate is placed in a container containing n-butanol, heated in a water bath, and heated to 60-80 ° C for reaction , the reaction time is 6-12h.

The drying of the product obtained after the water bath reaction is carried out in an oven at a temperature of 60-80° C. and a time of 30-60 minutes.

The following will further describe in detail with specific embodiments.

Example 1

500 mg of KMnO ₄ was placed in 40 mL of n-butanol (CH ₃ (CH ₂ ) ₂ CH ₂ OH), stirred for 10 min, and heated to 80° C. to react for 12 h. After cooling, the product was washed by centrifugation and dried in an oven at 60°C for 30 min. Subsequently, the dried product was transferred to a tube furnace, heated to 400°C at a rate of 2°C/min in an air atmosphere, kept for 3 h, and then cooled naturally to finally obtain black MnO ₂ powder, which is the final product of the present invention. The resulting porous _MnO2 nanomaterials.

Example 2

400 mg of KMnO ₄ was placed in 40 mL of n-butanol (CH ₃ (CH ₂ ) ₂ CH ₂ OH), stirred for 10 min, and heated to 70° C. to react for 12 h. After cooling, the product was washed by centrifugation and dried in an oven at 70° C. for 50 min. Subsequently, the dried product was transferred to a tube furnace, heated to 450°C at a rate of 2°C/min in an air atmosphere, maintained for 3 h, and then cooled down naturally to finally obtain black MnO ₂ powder, which is the final product of the present invention. The resulting porous _MnO2 nanomaterials.

Example 3

450 mg of KMnO ₄ was placed in 40 mL of n-butanol (CH ₃ (CH ₂ ) ₂ CH ₂ OH), stirred for 10 min, and heated to 80° C. to react for 10 h. After cooling, the product was washed by centrifugation and dried in an oven at 80°C for 30 min. Subsequently, the dried product was transferred to a tube furnace, heated to 460°C at a rate of 2°C/min in an air atmosphere, maintained for 3h, and then cooled down naturally to finally obtain black MnO ₂ powder, which is the final product of the present invention. The resulting porous _MnO2 nanomaterials.

Example 4

500 mg of KMnO ₄ was placed in 40 mL of n-butanol (CH ₃ (CH ₂ ) ₂ CH ₂ OH), stirred for 10 min, and heated to 60° C. to react for 12 h. After cooling, the product was washed by centrifugation and dried in an oven at 60°C for 40 min. Subsequently, the dried product was transferred to a tube furnace, heated to 380°C at a rate of 2°C/min in an air atmosphere, kept for 4 h, and then cooled down naturally to finally obtain black MnO ₂ powder, which is the final product of the present invention. The resulting porous _MnO2 nanomaterials.

Example 5

500 mg of KMnO ₄ was placed in 40 mL of n-butanol (CH ₃ (CH ₂ ) ₂ CH ₂ OH), stirred for 10 min, and heated to 80° C. to react for 10 h. After cooling, the product was washed by centrifugation and dried in an oven at 60°C for 40 min. Subsequently, the dried product was transferred to a tube furnace, heated to 420°C at a rate of 2°C/min in an air atmosphere, kept for 4 h, and then cooled down naturally to finally obtain black MnO ₂ powder, which is the final product of the present invention. The resulting porous _MnO2 nanomaterials.

results and analysis

Based on Example 1, taking the samples before and after calcination in Example 1 as the object, in order to study the structure and morphology of the samples before and after calcination, X-ray diffraction and scanning electron microscopy were performed on the samples. Figure 1 shows the XRD patterns of the samples before and after annealing. It can be seen from (a) of Figure 1 that the XRD pattern of the sample before annealing is relatively complex. Among them, the diffraction peaks corresponding to the red lines are in one-to-one correspondence with the monoclinic MnO ₂ standard card (JCPDS Card No. 42-1317), and the five main diffraction peaks are located at 12.5°, 25.3°, 35.3°, 37.3° , 40.0°, corresponding to the (001), (002), (20-1), (11-1), (201) crystal planes, respectively. The diffraction peak corresponding to the blue line corresponds to the tetragonal Mn ₃ O ₄ standard card (JCPDS Card No. 18-0803). Among them, the five main diffraction peaks located at 18.0°, 28.9°, 32.4°, 36.0°, and 60.0° are respectively associated with (102), (112), (103), (211), (224) crystals of Mn ₃ O ₄ . corresponding to the face. In addition, there are some faint impurity peaks corresponding to other phases of _MnO2 . The above results show that the powder sample before annealing is a mixture of MnO ₂ and Mn ₃ O ₄ , which further proves that the product phase of the low-temperature liquid phase synthesis method is complex. Figure 1(b) is the XRD pattern of the sample after annealing, the position of the diffraction peak is the same as that of _MnO2 in Figure 1(a), and there are no other diffraction peaks, indicating that the sample after annealing is pure phase MnO ₂ . At the same time, the MnO ₂ diffraction peak is more obvious and sharper than Figure 1 (a), indicating that the crystallinity of the sample is improved after annealing.

The SEM images of the sample material before and after annealing at different magnifications were obtained by scanning electron microscopy. Figure 2 (a), (b) and (c) are the SEM pictures of the sample before annealing at different magnifications. It can be clearly seen from Figure 2 (a), (b) and (c) that the sample before annealing The particles are small in size, about 50 nm in size. These small particles agglomerate and agglomerate with each other, forming the lumps shown in the figure. Figure 2 (d), (e) and (f) are the SEM images of the annealed samples at different magnifications. From Figure 2 (d), (e) and (f), it can be seen that the MnO ₂ sample particles formed after annealing The particle size is about 200 nm, which is due to the recrystallization of small particles before annealing. Overall, the annealed _MnO particles were uniformly distributed with obvious voids between the particles. This kind of structure with more voids and uniform distribution plays a buffer role for the volume expansion of MnO ₂ due to participating in the reaction, and at the same time increases the contact area between the active material and the electrolyte, shortens the diffusion length of lithium ions, and can effectively improve the battery. stability.

Based on the porous MnO ₂ nanomaterial prepared in Example 1 of the present invention, the charge-discharge experiment of the MnO ₂ sample under a certain current density was carried out. Figure 3(a) shows the cycle stability curve of the MnO ₂ sample at a current density of 500 mA/g. It can be seen from (a) that the first discharge capacity of the MnO ₂ sample reaches 1548.3 mAh/g, the first charge capacity is only 905.1 mAh/g, and its first charge and discharge Coulomb efficiency is only 58.4%. The irreversible capacity loss as high as 643mAh/g is mainly due to two factors: on the one hand, the formation of an irreversible solid electrolyte interfacial film (SEI) on the electrode surface, resulting in a higher first charge capacity than the theoretical capacity of _MnO2 ; on the other hand, part of the _MnO2 The first irreversible lithium intercalation of the particles results in lower coulombic efficiencies. The second discharge capacity of the _MnO sample reaches 894.4 mAh/g. After 100 cycles, the discharge capacity basically remained at around 880mAh/g, proving the excellent cycle stability of the _MnO2 sample. At the same time, the coulombic efficiency of the battery has been stable at around 99.0% from the second to the hundredth cycle, further proving the excellent cycle stability performance of this sample. (b) of FIG. 3 is the charge-discharge curve corresponding to (a), and the charge-discharge voltage window is 0.02-3.00V. It can be clearly seen from the figure that there are two discharge platforms at 1.2V and 0.4V during the first discharge process. For the reduction process of Mn ⁴⁺ and Mn ²⁺ respectively, the specific reaction equations are as follows:

MnO ₂ +2Li ⁺ +2e ^- →MnO+Li ₂ O

MnO+2Li ⁺ +2e ^- →Mn+Li ₂ O

Similarly, two charging platforms also appeared during the first charging process, located at 1.3V and 1.8V, corresponding to the oxidation process of metal Mn and Mn ²⁺ , respectively. The specific reaction equations are as follows:

Mn+Li ₂ O→MnO+2Li ⁺ +2e ^-

MnO+Li ₂ O→MnO ₂ +2Li ⁺ +2e ^-

After the first charge and discharge, the second and third charge and discharge voltage platforms are the same as the first time, which proves that the _MnO2 material has good reversibility. More importantly, after 50 and 100 cycles, the charge-discharge platform hardly changes, which proves the excellent cycle stability of the _MnO sample from another aspect.

Figure 3(e) is the cycling stability curve of the MnO ₂ sample at a current density of 1000 mA/g. It can be seen from the figure that the first discharge capacity reaches 1366.4 mAh/g, and the second discharge capacity reaches 725.0 mAh/g. During the first 15 cycles, the capacity decreased slightly, and the 15th discharge capacity was 657.2 mAh/g. After this, the capacity starts to slowly ramp up. After 130 cycles, the discharge capacity reaches 891.6 mAh/g. This capacity increase phenomenon, which often occurs on transition metal oxides, is mainly caused by the activation of active materials during cycling. It can be clearly seen from the figure that the capacity of the MnO ₂ sample does not decrease when it is cycled to 250 times, and is basically stable at about 850 mAh/g, which proves the cycling stability of the MnO ₂ sample.

To further verify the lithium storage performance, the rate characteristics of the _MnO samples were tested. Figure 3(c) is a graph of the rate performance of the _MnO2 samples at different current densities in the voltage window of 0.02-3.00V. When the current densities were 500, 1000, 2000, 5000 and 10000 mA/g, the corresponding discharge specific capacities were 889.7, 714.2, 596.0, 316.7 and 122.2 mAh/g, respectively. When the current density recovered to 500 mA/g, the discharge specific capacity rose to 904.5 mAh/g, indicating that the MnO ₂ sample had good cycle stability and rate capability. FIG. 3( d ) is a charge-discharge graph corresponding to FIG. 3( c ). It can be seen from the figure that when the current density is 500, 1000, 2000 and 5000 mA/g, there are two discharge plateaus in the discharge curve, which is consistent with the result of Fig. 3(b). When the current increases to 10000mA/g, the original 0.4V discharge plateau is less than 0V due to the overpotential caused by the large current, which is not within the voltage window and does not participate in the discharge reaction, resulting in the current density in Figure 3(c) at 10000mA/g The discharge specific capacity is too low at g.

To sum up, the present invention designs a simple and cheap liquid phase synthesis method, which can realize large-scale preparation of _MnO2 nanomaterials in combination with simple calcination in air. The prepared MnO ₂ nano-sized particles are uniformly distributed and have obvious voids between the particles, which is beneficial to shorten the lithium ion diffusion distance and increase the contact area with the electrolyte, which can greatly improve the lithium storage performance of the MnO ₂ sample. At a current density of 1000 mA/g, the initial discharge capacity reached 1366.4 mAh/g, and the discharge capacity remained around 850 mAh/g after 250 cycles, showing excellent cycle stability. The test results of lithium storage performance show that the ideal _MnO2 anode material can be prepared on a large scale by a simple method, which makes it possible to commercialize the preparation of _MnO2 and replace the traditional graphite material, which has a broad market prospect.

The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited to this, and any changes or substitutions that are not thought of through creative work should be included within the protection scope of the present invention.

Claims (2)

1. Simple calcination method for preparing porous MnO₂The method is characterized in that potassium permanganate is used as a manganese source, n-butyl alcohol is used as a carbon source, a liquid phase synthesis method is adopted for uniform reaction in a water bath, the obtained product is centrifugally washed and dried, the dried product is annealed, and the obtained black powder is the final product;

the annealing treatment is to transfer the dried product into a tube furnace, heat the dried product to 300-500 ℃ at the speed of 2-5 ℃/min in the air atmosphere, keep the temperature for 3-6h, and then naturally cool the product;

the using amount ratio of potassium permanganate to n-butyl alcohol is 10-13:1mg/mL, potassium permanganate is placed into a container filled with n-butyl alcohol, water bath heating is adopted, heating is carried out until the temperature reaches 60-80 ℃, and reaction is carried out, and the reaction time is 6-12 h;

drying the product obtained after the water bath reaction in an oven at the temperature of 60-80 ℃ for 30-60 min.

2. The method of claim 1, wherein the simple calcination process is used to prepare porous MnO₂The method is characterized in that the temperature rise rate of the annealing treatment is 2 ℃/min, the temperature is raised to 400 ℃, the temperature is kept for 3h, and then the temperature is naturally reduced.

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