A transition layer, its preparation method and use

Abstract

The present application relates to a transition layer and its preparation method and application, and relates to the technical field of battery. The transition layer is a glassy transition layer, and the glassy transition layer comprises at least two of the following raw materials: sodium oxide, lead oxide, magnesium oxide, calcium oxide, zinc oxide, aluminum oxide, tin oxide, or silicon dioxide; and the glassy transition layer is obtained by a temperature rising calcination method. The transition layer can be in close contact with an inorganic solid-state electrolyte through a simple and effective temperature rising calcination method to obtain a solid-state electrolyte containing the transition layer. Meanwhile, the transition layer can be partially reduced with lithium metal in a subsequent section, so that the solid-state electrolyte and the lithium metal are closely combined through a continuous and stable interface, thereby enhancing the contact between the lithium metal and the inorganic solid-state electrolyte and making the two have strong binding.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a transition layer, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, due to their high power density and long cycle life, have been widely used in energy storage systems and dominate the fields of electric vehicles and portable electronic products. Traditional lithium-ion batteries mostly use liquid electrolytes, which have high ionic conductivity and good wettability on electrode surfaces. However, liquid electrolytes often use high-viscosity and flammable organic liquids as solvents, resulting in low ion selectivity, high instability, and relatively low safety, which greatly limits the development of liquid electrolytes. Based on these issues, the strategy of developing solid-state electrolytes has emerged. Using solid-state electrolytes (SSEs) in lithium batteries can not only solve the aforementioned problems of thermal stability and lifespan, but also reduce the need and cost of battery state monitoring, further improving the energy density and power density of the device. Therefore, developing the application of solid-state electrolytes in all-solid-state lithium (ion) batteries has become a research hotspot in recent years.

[0003] However, the reactive chemical properties of lithium metal lead to the use of lithium metal anodes and inorganic solid electrolytes (such as Li) 6.4 La3Zr 1.4 Ta 0.6 O 12 The poor compatibility between lithium metal and inorganic solid-state electrolytes (LLZTO) makes it difficult to directly press lithium sheets / foils onto the surface of the inorganic solid-state electrolyte during the assembly of solid-state batteries based on inorganic solid-state electrolytes. This results in poor contact conditions between the lithium anode and the electrolyte during battery operation, easily leading to large polarization and short circuits. Therefore, improving the contact between lithium metal and inorganic solid-state electrolytes is a technical challenge in this field. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a transition layer that can be brought into close contact with an inorganic solid electrolyte through a simple and effective heating and calcination method to obtain a solid electrolyte containing the transition layer. Simultaneously, in subsequent processes, the transition layer can undergo a partial reduction reaction with lithium metal, thereby enabling the solid electrolyte and lithium metal to bond tightly through a continuous and stable interface, thus enhancing the contact between lithium metal and the inorganic solid electrolyte and generating a strong bond between them.

[0005] To achieve the above objectives, the present invention provides a transition layer, which is a glassy transition layer, comprising at least two of the following raw materials: sodium oxide, lead oxide, magnesium oxide, calcium oxide, zinc oxide, aluminum oxide, tin oxide, or silicon dioxide; the glassy transition layer is obtained by a heating and calcination method.

[0006] Previous studies have shown that one of the main reasons for the poor compatibility between inorganic solid-state electrolytes (such as LLZTO) and lithium metal is the insufficient solid-solid contact between them. This localized contact at the solid-phase interface between the inorganic solid-state electrolyte and lithium metal is detrimental to the electrochemical performance of lithium metal solid-state batteries, even at 0.2 mA / cm². 2 At low surface current densities, a large exchange current still occurs at the two-phase contact point, far exceeding the limiting current density of inorganic solid electrolytes. This leads to structural damage to the solid electrolyte and short circuits caused by lithium dendrite growth. Although sputtering oxides such as alumina, silicon oxide, tin oxide, and zinc oxide onto the LLZTO surface to form a bonding layer can enhance the contact between lithium metal and the LLZTO solid electrolyte, the physical properties of these metal oxides limit the ability to prepare a uniform bonding layer on the surface of inorganic solid electrolytes. Therefore, chemical vapor deposition (CVD) and / or physical vapor deposition (PVD) are necessary. However, the high cost of CVD and PVD deposition processes makes it difficult to commercially apply these oxides on a large scale for surface modification of solid electrolytes.

[0007] Therefore, through repeated research and experimentation, comparing various metal oxides, the inventors discovered that a transition layer can be prepared from multiple oxides by calcination. The oxides used for the transition layer have a low lithium potential and can be reduced by lithium metal during subsequent heating processes. This reactivity enhances the wettability of the oxide transition layer to lithium metal. Based on this, the inventors proposed a simple and effective calcination method to prepare the transition layer, which is in close contact with the inorganic solid electrolyte, resulting in a solid electrolyte containing the transition layer. Furthermore, in subsequent processes, the transition layer can undergo a partial reduction reaction with lithium metal, forming a continuous and stable interface between the transition layer and the lithium metal. This allows the solid electrolyte and lithium metal to bond tightly through this continuous and stable interface. Thus, without using costly CVD and / or PVD, the contact between lithium metal and the inorganic solid electrolyte can be enhanced, resulting in a stronger bond between them.

[0008] Furthermore, using the aforementioned raw materials not only forms a glassy transition layer, but its lower melting point also reduces operational complexity and cost. During the heating and calcination process, it can melt and uniformly bond to the surface of the inorganic solid electrolyte. Simultaneously, since both the glassy transition layer and the inorganic solid electrolyte are oxides, the compatibility between the two materials is sufficiently high, enabling good contact and strong bonding. Moreover, this transition layer is stable and possesses high electrical conductivity, allowing it to undergo partial reduction with lithium metal under heating conditions in subsequent processes. This prevents it from being completely reduced by lithium metal due to excessive reactivity, thus preserving its applicability in scenarios requiring tight bonding between lithium metal and solid electrolytes.

[0009] In one embodiment, the glass transition layer comprises silicon dioxide and aluminum oxide, wherein the weight ratio of silicon dioxide to aluminum oxide is 1:(0.8-1.2).

[0010] The present invention also provides a solid electrolyte, comprising an inorganic solid electrolyte and the transition layer, wherein the transition layer is bonded to the surface of the inorganic solid electrolyte.

[0011] The present invention also provides a method for preparing the solid electrolyte, comprising the following steps:

[0012] Preparation of slurry: The raw materials of the transition layer are dispersed in a polymer solution and stirred to obtain a slurry;

[0013] Heating and calcination: The slurry is coated on the surface of the inorganic solid electrolyte, dried, heated to the melting temperature in an oxygen stream, and melted and held at the temperature to obtain a transition layer. The transition layer is in a glassy state and is bonded to the surface of the inorganic solid electrolyte to obtain a solid electrolyte.

[0014] The above preparation method has low process cost and strong operability. The multi-component oxide mixture can effectively improve wettability and has a low melting point, allowing for the preparation of a transition layer via a simple heating and calcination method, which then tightly bonds the transition layer to the surface of the inorganic solid electrolyte. Furthermore, the preparation method uses a mixture of multiple raw materials to prepare the transition layer, increasing the controllability of its physicochemical properties. The properties of the transition layer can be further controlled by optimizing the types and proportions of the components.

[0015] In one embodiment, the polymer solution comprises polyethylene oxide and acetonitrile.

[0016] In one embodiment, the volume ratio of the polyoxyethylene to the acetonitrile is 3:1.

[0017] In one embodiment, the heating rate is 2-5°C / min, the melting temperature is 500-600°C, and the melting and holding time is 20-60min.

[0018] Using the above reaction conditions not only allows for better melting of various oxides, but also enables better bonding between the various oxides and the inorganic solid electrolyte in the molten state, thereby enhancing interfacial compatibility.

[0019] In one embodiment, the heating rate is 5°C / min, the melting temperature is 600°C, and the melting and holding time is 30min.

[0020] The present invention also provides an all-solid-state lithium metal battery, comprising lithium metal and the solid electrolyte, wherein the lithium metal is bonded to the surface of the transition layer through a reduction reaction.

[0021] The present invention also provides a method for preparing the all-solid-state lithium metal battery, comprising the following steps: bonding lithium metal to the transition layer, heating to the reduction temperature in a protective atmosphere, and performing reduction and heat preservation; wherein the lithium metal is bonded to the surface of the transition layer through a reduction reaction.

[0022] The transition layer prepared using the above-mentioned oxides will undergo a partial reduction reaction with lithium metal during heating, forming a continuous and stable interface between the transition layer and lithium metal. This enhances interfacial compatibility and continuity, reduces the interfacial impedance between the inorganic solid electrolyte and lithium metal, and ultimately allows the solid electrolyte containing the transition layer to bond tightly with lithium metal through the continuous and stable interface.

[0023] In one embodiment, the protective atmosphere is argon or nitrogen, the reduction temperature is 180-210°C, and the holding time is 20-60 min.

[0024] The protective atmosphere mentioned above is used because lithium metal is too reactive at high temperatures and easily forms lithium oxide with oxygen. Reacting in a protective atmosphere can avoid operational hazards such as combustion.

[0025] In one embodiment, the argon gas is high-purity argon, and the nitrogen gas is high-purity nitrogen.

[0026] In one embodiment, the inorganic solid electrolyte is LLZTO.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] This invention discloses a transition layer, its preparation method, and its application. This transition layer can be brought into close contact with an inorganic solid electrolyte via a simple and effective calcination method to obtain a solid electrolyte containing the transition layer. Simultaneously, in subsequent processes, the transition layer undergoes a partial reduction reaction with lithium metal, forming a continuous and stable interface between the transition layer and the lithium metal. This allows the solid electrolyte and lithium metal to bond tightly through the continuous and stable interface, thereby enhancing the contact between the lithium metal and the inorganic solid electrolyte and resulting in a strong bonding between them. This transition layer can be used as a surface modifier for the solid electrolyte in lithium metal solid-state batteries, possessing advantages such as high capacity, low interfacial impedance, excellent rate performance, high energy density, stable cycle performance, and excellent safety performance. Attached Figure Description

[0029] Figure 1 This is a schematic diagram illustrating the interface modification between the solid electrolyte and the lithium metal anode in Example 1.

[0030] Figure 2 The image shows a scanning electron microscope image of the interface between the solid electrolyte and the lithium metal anode in Example 1.

[0031] Figure 3 This is a schematic diagram of the assembly of the "lithium|solid electrolyte|lithium" symmetrical battery in Example 1;

[0032] Figure 4 This is a schematic diagram of the assembly of the "lithium|solid electrolyte|lithium" symmetrical battery in Comparative Example 1;

[0033] Figure 5 The charge-discharge curves of the Li / glass-LLZTO-glass / Li symmetric battery in Example 1 of the experimental case are shown.

[0034] Figure 6 The charge-discharge curves of the Li / LLZTO / Li symmetric battery in Comparative Example 1 of the experimental case are shown.

[0035] Figure 7 The graph shows a comparison of the cycle life of the Li / glass-LLZTO-glass / Li symmetric cell prepared in Example 1 and the Li / LLZTO / Li symmetric cell prepared in Comparative Example 1.

[0036] Figure 8 Nyquist plots of the LFP / LLZTO / Li lithium metal battery prepared in Example 1 and the LFP / LLZTO lithium metal battery prepared in Comparative Example 1;

[0037] Figure 9 The charge-discharge curves of the LFP / LLZTO / Li lithium metal battery prepared in Example 1 after 1-100 cycles are shown.

[0038] Figure 10 The charge-discharge curves of the LFP / LLZTO lithium metal battery prepared in Comparative Example 1 for 1-16 cycles are shown.

[0039] Figure 11 The graph shows the cycle stability results of the LFP / LLZTO / Li lithium metal battery prepared in Example 1 and the LFP / LLZTO lithium metal battery prepared in Comparative Example 1.

[0040] Figure 12 The graph shows the rate performance results of the LFP / LLZTO / Li lithium metal battery in Example 1;

[0041] Figure 13 The graph shows the rate performance results of the LFP / LLZTO / Li lithium metal battery in Example 1. Detailed Implementation

[0042] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0044] Unless otherwise specified, all reagents, materials, and equipment used in this embodiment are commercially available; unless otherwise specified, all experimental methods are conventional experimental methods in this field.

[0045] Example 1

[0046] A transition layer, its preparation method, and its application.

[0047] Preparation of slurry: Weigh 10 mg each of Al2O3 and SiO2 and disperse them in 10 ml of polyoxyethylene / acetonitrile solution. In this example, the volume ratio of polyoxyethylene to acetonitrile in the polyoxyethylene / acetonitrile solution is 3:1. Stir at room temperature (25-27℃) for 30 min to obtain slurry.

[0048] Heating and calcination: The inorganic solid electrolyte is polished to a mirror finish. In this embodiment, the inorganic solid electrolyte is LLZTO. A slurry is applied to the polished surface of the inorganic solid electrolyte, dried, and then heated to the melting temperature in an oxygen stream at a heating rate of 5°C / min. Melting and holding at this temperature yields a transition layer, which is bonded to the surface of the inorganic solid electrolyte, thus obtaining the solid electrolyte. In this embodiment, the melting temperature is 600°C, and the melting and holding time is 30 minutes. The aforementioned transition layer is in a glassy state, and this glassy transition layer is tightly bonded to the surface of the inorganic solid electrolyte.

[0049] Preparation of surface-modified solid electrolyte: Lithium metal is pressed onto the aforementioned glassy transition layer, allowing the lithium metal to adhere to the glassy transition layer. The mixture is then heated to the reduction temperature in a protective atmosphere to perform a partial reduction reaction. The reduction is maintained at this temperature, forming a continuous and stable interface between the transition layer and the lithium metal. The protective atmosphere is either argon or nitrogen; in this embodiment, high-purity argon is used. The reduction temperature is 190°C, and the reduction holding time is 30 minutes. The schematic diagram of the solid electrolyte and lithium metal anode interface modification is shown below. Figure 1 As shown, the scanning electron microscope image of the interface between the solid electrolyte and the lithium metal anode is as follows: Figure 2 As shown in the figure, it is clear that the lithium metal is in good direct contact with the solid electrolyte, and no pores or gaps were observed.

[0050] Preparation of all-solid-state lithium metal batteries: The surface-modified solid electrolyte with lithium metal incorporated above was assembled into a Li / glass-LLZTO-glass / Li symmetric battery, as shown in the schematic diagram below. Figure 3 As shown.

[0051] Alternatively, using lithium iron phosphate as the positive electrode and employing a surface-modified solid electrolyte incorporating lithium metal in this embodiment, an LFP / LLZTO / Li lithium metal battery is assembled.

[0052] Comparative Example 1

[0053] A Li / LLZTO / Li symmetric cell was assembled using unmodified LLZTO as the substrate. The assembly diagram is shown below. Figure 4 As shown.

[0054] Using unmodified LLZTO as the substrate and lithium iron phosphate as the cathode, an LFP / LLZTO lithium metal battery was assembled.

[0055] Comparative Example 2

[0056] A transition layer and its preparation method.

[0057] The preparation method is basically the same as in Example 1, except that the raw materials for preparing the slurry in Comparative Example 2 are 10 mg each of silicon oxide (SiO2) and iron oxide (Fe2O3). In actual operation, it was found that in the step of preparing the surface-modified solid electrolyte, after pressing lithium metal onto the transition layer and carrying out the reduction reaction in a protective atmosphere, Fe2O3 completely reacts with Li metal to generate elemental iron and loose Li2O. The loose lithium oxide increases the interfacial resistance and ion migration path, and the elemental iron is unfavorable for the diffusion of lithium ions in the bulk phase.

[0058] Therefore, this invention should not use metal oxides with excessively strong oxidizing properties.

[0059] Comparative Example 3

[0060] A transition layer and its preparation method.

[0061] The preparation method is basically the same as that in Example 1, except that the weight ratio of Al2O3 and SiO2 in the raw materials for preparing the slurry in Comparative Example 3 is 5:1.

[0062] In practice, it can be observed that although glassy oxides can be formed, the melting point of the material increases, making it difficult to stably prepare a transition layer. Furthermore, the reactivity between the transition layer and lithium metal decreases, thereby deteriorating the interfacial compatibility. Therefore, even using the same raw materials, different amounts will result in different reactivity between the transition layer and lithium metal or inorganic solid electrolytes.

[0063] Experimental Example

[0064] Electrochemical testing.

[0065] 1. The Li / glass-LLZTO-glass / Li symmetric cell prepared in Example 1 and the Li / LLZTO / Li symmetric cell prepared in Comparative Example 1 were sealed in a Swagelok cell for electrochemical testing. A constant current of 0.2 mA / cm² was used. 2 The stability of the symmetrical battery was tested using a constant current charge-discharge method. The results are as follows: Figure 5 , Figure 6 .

[0066] At 60℃, the current density is 0.2 mA / cm². 2 Under the test conditions, the cycle life of the Li / glass-LLZTO-glass / Li symmetric cell prepared in Example 1 and the Li / LLZTO / Li symmetric cell prepared in Comparative Example 1 were compared, and the results are as follows: Figure 7 As shown.

[0067] The results show that the transition layer is highly effective in reducing charge / discharge voltage and extending cycle stability. The voltage of the Li / LLZTO / Li symmetric cell without the transition layer is approximately 0.08 V, significantly higher than that of the Li / glass-LLZTO-glass / Li symmetric cell with the transition layer (approximately 0.01 V). When the surface of LLZTO is treated with a glassy transition layer, the symmetric cell exhibits a relatively low voltage during cycling, and the charge / discharge curves are flat, indicating limited electrochemical reactions between the electrolyte and the electrode. The only electrochemical step within the LLZTO electrolyte is lithium-ion diffusion.

[0068] 2. Electrochemical performance tests were performed on the LFP / LLZTO / Li lithium metal battery prepared in Example 1 and the LFP / LLZTO lithium metal battery prepared in Comparative Example 1.

[0069] The tests were conducted at 30°C with a cutoff voltage of 2.5-3.8V. Three constant-current charge-discharge cycles were performed at a current rate of 0.1C as the battery activation process; subsequently, cycle stability tests were conducted at a current rate of 1C. The cutoff voltage for rate performance testing was the same as the tests described above, with five cycles each at different current densities: 0.2C, 0.5C, 1C, 2C, and 5C. The results are as follows: Figure 8 , Figure 9 , Figure 10 , Figure 11 , Figure 12 , Figure 13 As shown.

[0070] The results show: Figure 8 Nyquist plots of the LFP / LLZTO / Li lithium metal battery prepared in Example 1 and the LFP / LLZTO lithium metal battery prepared in Comparative Example 1 are shown. The semicircles in the high-frequency range represent the ionic conductivity of the electrolyte. In Comparative Example 1, the internal resistance of the unmodified battery was approximately 3700 Ω, which decreased to approximately 1400 Ω after interface modification. The reduced internal resistance due to the good contact between the solid electrolyte and the lithium anode improves the cycle stability and rate performance of the LFP / LLZTO / Li lithium metal battery.

[0071] Figure 9 , Figure 10 The charging / discharging curves are for LFP / LLZTO / Li lithium metal batteries and LFP / LLZTO lithium metal batteries. Figure 11 The graph shows the cycle stability results for LFP / LLZTO / Li lithium metal batteries and LFP / LLZTO lithium metal batteries. In Example 1 and Comparative Example 1, the loading of the positive electrode active material was approximately 2 mg / cm³. 2The battery was tested at 30°C. The LFP / LLZTO / Li lithium metal battery modified according to this invention exhibited an initial capacity of 144 mAh / g at 1C, with a coulombic efficiency as high as 99.5%. The charging voltage of the first cycle was 3.51V, and the discharging voltage was 3.33V. The charge / discharge plateau remained stable after 100 cycles. A high capacity retention of 96.2% after 100 cycles indicates a stable electrolyte / electrode interface between lithium metal and LLZTO.

[0072] In contrast, in Comparative Example 1, the unmodified battery was unstable. Its initial charge capacity was high, approximately 173 mAh / g, almost identical to the theoretical capacity of the LFP. However, this battery only provided a low discharge capacity of about 158 ​​mAh / g, resulting in a low coulombic efficiency of approximately 91%. The low discharge capacity and coulombic efficiency also indicate significant side reactions within the battery. With increasing cycle count, the unmodified battery's capacity rapidly decreased to zero after 20 cycles. The charge-discharge curves show a rapid increase in charging voltage and a rapid decrease in discharging voltage, likely due to the accumulation of internal resistance caused by the deterioration of the interfacial properties between the electrolyte and electrodes.

[0073] Figure 12 , Figure 13 The rate performance of the LFP / LLZTO / Li lithium metal battery in Example 1 shows that the capacity decreases moderately when the rate is below 1C, indicating that it has good rate capacity.

[0074] In summary, this invention is the first to employ a low-melting-point glassy transition layer to coat an inorganic solid electrolyte. Based on the properties of oxides, various low-melting-point oxides melt and coat the surface of the inorganic solid electrolyte, giving it excellent wettability. Lithium metal undergoes a partial reduction reaction with the glassy transition layer and adheres tightly to the surface of the solid electrolyte, achieving good contact. This method is an effective way to reduce the interfacial impedance between the solid electrolyte and the lithium metal anode, providing a new technological breakthrough in the field of interface modification for inorganic solid electrolytes in lithium batteries.

[0075] By using the glassy transition layer of the present invention, the total resistance of the Li / LLZTO / Li symmetric cell is reduced to approximately 175 Ω. The low resistance and high stability of the Li-LLZTO interface result in a high capacity of 144 mAh / g with LFP as the cathode, and a capacity retention of approximately 96% after 100 cycles.

[0076] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0077] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. An all-solid-state lithium metal battery, characterized in that, The invention includes lithium metal and a solid electrolyte. The solid electrolyte is composed of an inorganic solid electrolyte and a transition layer. The transition layer is bonded to the surface of the inorganic solid electrolyte by a heating and calcination method. The transition layer is a glassy transition layer. The raw materials of the glassy transition layer are silicon dioxide and aluminum oxide, and the weight ratio of silicon dioxide to aluminum oxide is 1:(0.8-1.2). The method for preparing the all-solid-state lithium metal battery includes the following steps: bonding lithium metal to the transition layer, heating to the reduction temperature in a protective atmosphere, and performing reduction and heat preservation; the lithium metal is bonded to the surface of the transition layer through a reduction reaction.

2. The method for preparing the all-solid-state lithium metal battery according to claim 1, characterized in that, The process includes the following steps: bonding lithium metal to the transition layer, heating it to the reduction temperature in a protective atmosphere, and then performing reduction and heat preservation. The lithium metal is bonded to the surface of the transition layer through a reduction reaction.

3. The preparation method according to claim 2, characterized in that, The protective atmosphere is argon or nitrogen, the reduction temperature is 180-210℃, and the holding time is 20-60 min.

4. The solid electrolyte in the all-solid-state lithium metal battery of claim 1.

5. The method for preparing the solid electrolyte according to claim 4, characterized in that, Includes the following steps: Preparation of slurry: The raw materials of the glassy transition layer are dispersed in a polymer solution and stirred to obtain a slurry; Heating and calcination: The slurry is coated on the surface of the inorganic solid electrolyte, dried, heated to the melting temperature in an oxygen stream, and melted and held at the temperature to obtain a glassy transition layer. The glassy transition layer is bonded to the surface of the inorganic solid electrolyte to obtain a solid electrolyte.

6. The preparation method according to claim 5, characterized in that, The polymer solution comprises polyethylene oxide and acetonitrile.

7. The preparation method according to claim 5, characterized in that, The heating rate is 2-5℃ / min, the melting temperature is 500-600℃, and the melting and holding time is 20-60min.

8. The preparation method according to claim 7, characterized in that, The heating rate is 5℃ / min, the melting temperature is 600℃, and the melting and holding time is 30min.

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