A method for in-situ synthesis of graphene by molten salt electrolysis

Abstract

This invention belongs to the field of graphene material preparation technology, specifically relating to a method for in-situ synthesis of graphene via molten salt electrolysis. Specifically, it uses molybdenum metal as the anode, porous graphite as the cathode, and high-alumina ceramic as the diaphragm. The molten salt electrolyte system includes metal chlorides and alkali metal oxides, and electrolysis is performed at a constant voltage under an inert gas protective atmosphere. Utilizing the synergistic effect of the electrochemical intercalation of molten salt cations and the catalytic activation of graphite edges by oxygen ions provided by the oxides, the graphite surface layer is controllably and directionally exfoliated, transforming in-situ into few-layer graphene. The entire process is carried out under the protection of the porous ceramic diaphragm, effectively blocking interference from anode byproducts. This invention has significant advantages such as simple process, environmental friendliness, controllable product quality, and ease of large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of graphene material preparation technology, specifically relating to a method for in-situ synthesis of graphene materials in a three-dimensional graphite framework based on the principle of molten salt electrochemical catalysis. Addressing the technical bottlenecks of existing graphene preparation methods such as chemical vapor deposition (CVD) and redox exfoliation, including complex processes, the need for subsequent transfer, poor integrity of the conductive network of the product, and susceptibility to contamination, this invention provides an in-situ synthesis technique that, through precise control of electrochemical conditions in a molten salt electrolyte, directly uses a three-dimensional porous graphite electrode as a carbon source and precursor to achieve controllable catalytic conversion of its surface into high-quality, few-layer graphene. The graphene obtained by this method can be directly used as a high-performance electrode material in energy storage and conversion devices. Background Technology

[0002] Graphene, as a two-dimensional carbon nanomaterial, exhibits great application potential in energy, electronics, and composite materials due to its excellent electrical, mechanical, and thermal properties. Currently, its mainstream preparation methods mainly include chemical vapor deposition (CVD) and redox exfoliation. CVD typically uses metal foil (such as copper or nickel) as a substrate for catalytic growth of graphene, but the resulting material requires a complex and fragile transfer process before it can be used. Furthermore, the transfer process introduces contamination, wrinkles, and cracks, severely impairing its intrinsic properties and the integrity of its macroscopic conductive network. Redox methods involve oxidizing graphene with strong acids followed by exfoliation and reduction. This process has a long route, causes significant environmental pollution, and the product contains numerous structural defects (sp...). 3 (Hybridized carbon), electrical conductivity and mechanical strength are significantly degraded.

[0003] Electrochemical exfoliation, as a preparation method that balances cost and efficiency, has attracted widespread attention in recent years. Its basic principle is to drive electrolyte ions into the interlayer of graphite using an electric field, causing the graphite electrode to expand and exfoliate, thereby obtaining graphene material. This method has advantages such as precise controllable voltage and current, mild operating conditions, and good process reproducibility. However, the mainstream electrochemical exfoliation method still faces many key technical challenges, restricting its application in the preparation of high-quality graphene.

[0004] For example, aqueous electrolyte systems suffer from low yields, high and uncontrollable product oxidation. Currently, most studies use acidic aqueous solutions such as dilute sulfuric acid as electrolytes. While simple to operate, aqueous electrochemical exfoliation generally suffers from low graphene yields, thick exfoliated sheets, high oxidation levels, and high defect density, typically requiring additional quenching or reduction treatments. Acidic electrolytes and ionic liquid systems pose serious environmental and safety problems. Existing electrochemical exfoliation technologies often use high-concentration sulfuric acid or other strong acids as electrolytes to improve ion migration rates. Rapid electrochemical exfoliation methods use sulfonic acid or a mixture of sulfonic acid and sulfuric acid as electrolytes. These strongly acidic electrolytes are highly corrosive, increasing equipment maintenance costs and posing serious safety hazards and environmental pollution problems. While ionic liquids can avoid the oxygen evolution problem of aqueous electrolytes, they are expensive, mostly toxic, and non-biodegradable, making large-scale application difficult.

[0005] High-temperature molten salt electrolysis suffers from bottlenecks such as product powder formation and difficulty in in-situ construction. High-temperature molten salt electrolysis (e.g., chloride molten salt systems like CaCl2-NaCl) offers advantages such as high ionic conductivity and a wide electrochemical window, effectively avoiding oxygen evolution and excessive oxidation problems associated with aqueous electrolytes. However, existing high-temperature molten salt electrolysis methods often employ direct exfoliation of graphene electrodes, resulting in graphene powder that detaches from the electrode and enters the molten salt, requiring subsequent collection, washing, and separation. This powdered product not only suffers from problems such as easy layer recombination and sedimentation, making in-situ growth on a conductive framework difficult, but the exfoliation process is also affected by anolyte byproducts (such as chlorine gas), leading to decreased graphene quality and uneven layer distribution. Summary of the Invention

[0006] The purpose of this invention is to provide a method for in-situ conversion of graphene materials based on the principle of molten salt electrochemical catalytic exfoliation, in order to solve the problems commonly found in existing electrochemical methods for preparing graphene, such as uncontrollable oxidation of aqueous electrolytes, significant pollution of acidic electrolyte environments, and the powdering of products from high-temperature molten salt methods, making it difficult to construct them in-situ on conductive frameworks.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution: A method for in-situ synthesis of graphene via molten salt electrolysis is disclosed. The method employs a diaphragm electrolytic cell system with molybdenum metal as the anode, porous graphite as the cathode, ceramic as the diaphragm, and molten salt electrolyte as the electrolyte. The molten salt electrolyte system includes a metal chloride and an intercalation promoter, wherein the intercalation promoter is selected from at least one metal oxide, and the metal oxide is selected from at least one alkali metal and alkaline earth metal. Electrolysis is performed at constant voltage under an inert gas protective atmosphere, and graphene material is generated in-situ within the pores of the cathode graphite through electrochemical intercalation-catalytic exfoliation.

[0008] By employing the aforementioned technical solution, this invention innovatively uses porous graphite etched and sintered with KOH as the cathode to construct a diaphragm-based molten salt electrolysis system for in-situ graphene exfoliation. This system uses a high-alumina ceramic material resistant to chloride molten salt as the diaphragm of the molten salt electrolysis cell, effectively blocking the interference of chlorine gas evolved from the anode on the cathode reaction interface. The molten salt electrolyte system using alkali metal oxides provides intercalated cations (such as K... + Li + At the same time, through the release of oxygen ions (O) 2- Selective electrochemical activation of the graphite edges at the cathode interface synergistically promotes the exfoliation process. The three-dimensional porous structure of the porous graphite electrode provides a large reaction interface for the "electrochemical intercalation-catalytic exfoliation" process. By applying a precise voltage of 2.1-3 V at 700-1000℃, the aforementioned synergistic reaction is driven, thereby significantly weakening the interlayer van der Waals forces and achieving controllable and directional exfoliation of the graphite surface into few-layer graphene (≤20 layers). Therefore, the molten salt in-situ synthesis technology for graphene materials provided by this invention has advantages such as high reaction efficiency, low energy consumption and raw material costs, and ease of large-scale production, making it a high-performance graphene composite material preparation technology with great industrial application prospects.

[0009] Preferably, the molybdenum anode is at least one of molybdenum wire, molybdenum plate, and molybdenum rod; more preferably, the molybdenum wire is folded into a spiral shape to increase the electrode area.

[0010] Preferably, the porous graphite cathode is a three-dimensional porous graphite with a pore size of 80-500 nm and a porosity of 35-50%. And / or, the porous graphite is a graphite cathode material with a three-dimensional porous structure obtained by etching graphite with potassium hydroxide solution and sintering at high temperature in an inert atmosphere; preferably, the graphite is selected from at least one of graphite rods and graphite plates. And / or, the porous graphite is obtained by immersing a graphite rod in a potassium hydroxide solution with a molar concentration of 10-16 mol / L, etching it at 100-150 °C for 6-24 h, and then sintering the graphite rod at 500-950 °C for 2-8 h in an inert atmosphere at a heating rate of 1-10 °C / min, resulting in a three-dimensional porous structure with a substrate pore size of 80-500 nm and a porosity of 35-50%. More preferably, the size of the graphite rod is 8-15 mm.

[0011] Preferably, the diaphragm is a porous high-alumina ceramic diaphragm with the chemical composition aAl₂O₃·bCaO·cSiO₂·dMgO, where a = 0.92-0.96, b = 0.01-0.06, c = 0.01-0.06, d = 0.01-0.06, b+c+d = 0.04-0.08, a+b+c+d = 1.0, a porosity of 35-40%, and a pore size of 0.2-2.0 μm. This diaphragm exhibits good blocking effect against anolyte oxygen evolution products.

[0012] Preferably, the metal chloride of the molten salt electrolyte is MClx, where M is selected from at least one metal element selected from Li, Na, K, Mg, and Ca, and x is the valence of the corresponding metal. And / or, the metal chloride composition of the molten salt electrolyte is as follows, by molar percentage: LiCl 0-100%, NaCl 0-100%, KCl 0-100%, MgCl2 0-100%, and CaCl2 0-100%; Preferably, the metal oxide is MxOy, where M is selected from at least one metallic element selected from Li, Na, K, and Ca, and x and y are the valences of the corresponding metals; And / or, the content of metal oxides is 0.5-5.0 wt% of the total mass of the molten salt electrolyte.

[0013] Preferably, the temperature of the molten salt electrolyte is controlled at 720-950 ℃ during constant voltage electrolysis, more preferably 750-950 ℃; And / or, the voltage is adjusted to 1.6-3.5V, more preferably 2.1-3V; the electrolysis time is more preferably 1-24 h; and the voltage is more preferably 2.2-2.6V for 2-4 h.

[0014] Preferably, the electrolysis product is soaked in dilute hydrochloric acid solution, washed with deionized water, and vacuum dried at room temperature to obtain a graphene / graphite composite product; preferably, the product is soaked and washed in dilute hydrochloric acid solution with a molar concentration of 1 mol / L for 5-30 minutes, then washed with deionized water, and vacuum dried at 80°C.

[0015] Preferably, the graphene product has ≤20 graphene layers and a lateral dimension of 1-60 μm. Graphene materials within this layer range exhibit superior electrochemical performance.

[0016] Preferably, the inert atmosphere described in this invention is a gas that does not react with reactants, products, or electrolytes, preferably one or more of helium, nitrogen, and argon, and more preferably high-purity argon.

[0017] The present invention also provides a graphene material prepared by molten salt electrolysis in situ using any of the above-mentioned preparation methods.

[0018] Compared with existing technologies, the advantages of this invention are mainly reflected in the following aspects: Addressing the core bottlenecks commonly encountered in existing electrochemical exfoliation methods for graphene preparation, such as uncontrollable oxidation of aqueous electrolytes, defects in anode / cathode exfoliation, significant pollution from acidic electrolyte environments, and powdering of products in high-temperature molten salt methods due to interference from anode byproducts, this invention innovatively proposes a system for in-situ molten salt electrochemical catalytic exfoliation of graphene materials based on a porous graphite cathode. Three-dimensional porous graphite formed by KOH etching and sintering serves as the cathode and the sole carbon source. Its interconnected three-dimensional pores and high specific surface area provide a large reaction interface and ideal diffusion channels for the "electrochemical intercalation-catalytic exfoliation" process. Under precisely controlled potential and temperature, the surface graphite undergoes directional cleavage and reconstruction through low-energy-barrier pathways to generate high-quality few-layer graphene. The introduced porous ceramic membrane effectively blocks interference from anode byproducts to the cathode reaction interface, ensuring the chemical purity of the exfoliation environment and the integrity of the product structure. In this method, graphene is prepared by one-step electrolysis on pretreated graphite rods to directly produce graphene materials that can be used in high-performance electrodes. It has outstanding advantages such as simple process, environmental friendliness, controllable product quality and structural stability, and has significant potential for industrial application and cost-effectiveness. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of an electrolytic reaction system for the electrochemical in-situ synthesis of graphene materials under an inert gas protective atmosphere, according to the present invention. Figure 1 In the middle, 1-quartz glass reactor, 2-molten salt electrolyte, 3-electrolytic cell (alumina), 4-air inlet, 5-air outlet, 6-spiral metal anode molybdenum, 7-porous graphite cathode, 8-ceramic diaphragm; Figure 2 This is a SEM image of the product from Example 1 at 50,000x magnification; Figure 3 This is a TEM characterization image of the product from Example 1 at 200,000x magnification; Figure 4 This is a cycle performance diagram of the graphene anode material produced in Example 1; Figure 5 This is a rate performance diagram of the graphene anode material produced in Example 1; Figure 6 This is the XRD pattern of the product from Example 1. Detailed Implementation

[0020] To better clarify and understand the objectives, process solutions, and advantages of this invention, the technical solutions and implementation methods of this invention will be further described clearly, completely, and in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the embodiments described in this invention are implemented under the premise of the technical solutions of this invention, providing detailed implementation methods and specific operating procedures, but are only some embodiments of this invention, not all embodiments. The specific implementation methods described are limited to illustrating and explaining this invention and do not limit this invention. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0021] Unless otherwise specified, the experimental methods and conditions used in the embodiments of this invention are conventional methods and conditions. The materials, reagents, instruments, and equipment used in the embodiments, unless otherwise specified, are all conventional substances or equipment known to those skilled in the art and can be obtained commercially or prepared by conventional methods. The reaction conditions described in the invention's content can all achieve the stated reactions and obtain the desired products. Due to space limitations, some embodiments are listed below to further illustrate the advantages of the technical solution of this invention.

[0022] This invention proposes a method for in-situ synthesis of graphene materials via molten salt electrochemical catalytic exfoliation. A porous ceramic membrane is used to block >99% of anolyte byproducts (Cl2 / O2), ensuring a pure cathode interface. A three-dimensional porous graphite rod, etched and sintered with potassium hydroxide solution, serves as the cathode and the sole carbon source. Electrochemical intercalation-catalytic exfoliation is induced on the graphite surface at 700-1000℃ using a DC voltage of 1.5-3.5 V in a molten salt electrolyte of a specific composition. The molten salt electrolyte comprises LiCl, NaCl, KCl, MgCl2, and CaCl2, with 0.5-5 wt% alkali metal oxides added to promote interlayer diffusion and exfoliation kinetics. During electrolysis, molten salt cations (such as KCl, NaCl, KCl, MgCl2, and CaCl2) are released into the molten salt electrolyte. + Li + The active metal atoms (such as Ca and K) generated by electroreduction act as "transient intercalation agents" and "interfacial catalysts," selectively adsorbing onto the edges and interlayers of graphite. This significantly weakens the interlayer van der Waals forces, driving surface carbon atoms to dissociate from the substrate at low energy barriers and recombine into few-layer graphene (≤20 layers). This graphene material can be used as a self-supporting electrode for high-performance supercapacitors.

[0023] Compared with traditional technologies, this invention has the following outstanding advantages: Through the "molten salt electrochemical catalytic exfoliation" mechanism, in-situ integrated construction of graphene on a three-dimensional porous graphite framework is achieved, avoiding the necessary transfer steps and the resulting damage, pollution, and interfacial resistance problems of chemical vapor deposition (CVD); at the same time, this method is simple and environmentally friendly, and can be completed in a single molten salt electrolysis step, without the use of strong acids or oxidants. By adjusting parameters such as potential and molten salt composition, the number of graphene layers, defect degree, and coverage can be precisely controlled, ensuring excellent crystallization quality of the product.

[0024] Example 1 The porous graphite electrode was obtained by immersing a graphite rod in a 13 mol / L potassium hydroxide solution and etching it at 120 °C for 5 h. After etching, the graphite rod was heated to 900 °C at a rate of 5 °C / min in an argon atmosphere and held at that temperature for 3 h before being cooled to room temperature in the furnace. The pore size was 200 nm and the porosity was 43%.

[0025] Reference Figure 1 ,like Figure 1 This is a system for in-situ molten salt electrolysis synthesis of graphene materials using a diaphragm method. The anode is a spiral molybdenum electrode 6 with a molybdenum wire diameter of 1.6 mm and a spiral diameter of 50 mm. The cathode is a porous graphite electrode 7. The high-alumina ceramic diaphragm 8 is a tubular porous high-alumina ceramic diaphragm with the following composition: a Al2O3·b CaO·c SiO2·d MgO, where a = 0.95, b = 0.02, c = 0.02, d = 0.01, porosity of 38%, and pore size of 0.2-2.0 μm. The electrode spacing between the cathode and anode is preferably 1-5 cm. The molten salt electrolyte 2 is CaCl2 and alkali metal oxide CaO, with the alkali metal oxide accounting for 2 wt% of the molten salt electrolyte mass. The electrolysis temperature is 850 ℃. Electrolytes CaCl2 and alkali metal oxide CaO were heated to 900 °C at 5 °C / min under a protective atmosphere of flowing argon, held at this temperature for 1 h to ensure complete dissolution of CaCl2, and then cooled to 850 °C at 5 °C / min. A constant voltage of U = 2.4 V was applied between the cathode and anode for electrolysis for t = 4 h. After electrolysis, the product obtained from the porous graphite at the cathode was washed with 1 mol / L dilute hydrochloric acid and deionized water, and then vacuum dried at 80 °C. SEM characterization results are shown below. Figure 2 This indicates that the obtained graphene has 10 layers and a lateral dimension of approximately 52 μm.

[0026] Example 2-21 Following the method of Example 1, the composition of the molten salt electrolyte CaCl2 in Example 1 was adjusted, replacing it with NaCl + CaCl2 (molar ratio 1:2) to lower the melting point of the molten salt and reduce the electrolysis temperature to 750 °C. All other parameters remained the same as in Example 1. After electrolysis, the obtained product was washed and dried. The electrolysis product obtained at the cathode was then subjected to the following process... Figure 3 SEM characterization revealed that the obtained product was graphene with 18 layers and a lateral dimension of approximately 20 μm.

[0027] By adjusting the molten salt composition of the electrolyte in Example 1, the electrolysis temperature was changed accordingly. Other process parameters remained the same as in Example 1, and the results are shown in Table 1.

[0028] Table 1. Effects of molten salt composition on the synthesis of graphene materials.

[0029] As can be seen, adjusting the composition of the molten salt electrolyte can effectively reduce the electrolysis temperature. Since the cation intercalation ability affects the number of graphene layers, such as Li... + Strong intercalation can easily produce thin sheets with fewer layers, but excessively strong intercalation can lead to smaller lateral dimensions of the resulting graphene. Secondly, the oxidant content and temperature work together to control the degree of exfoliation and also affect the quality of the final product. Low concentration (e.g., 2wt% CaO) and moderate temperature are conducive to the formation of large-sized, intact thin sheets, while high concentration or high temperature can easily lead to over-etching and product fragmentation.

[0030] Examples 22-27 Following the method of Example 1, the conditions for preparing the porous graphite cathode in Example 1 were changed. After electrolysis, the obtained product was washed and dried, and the morphology of the product was characterized by SEM. The results are shown in Table 2. The sintering time had little effect on the porous graphite rod, so it is omitted from the table.

[0031] Table 2. Effects of different KOH solution concentrations and sintering temperatures on the synthesis of graphene materials.

[0032] As shown in the table above, obtaining ideal graphene with few layers and large size requires a good match between KOH etching concentration and sintering temperature. With increasing KOH etching concentration, the porosity and pore size of the graphite matrix both increase, and the number of reaction interfaces significantly increases. This provides richer active sites for cation intercalation and oxygen ion catalytic activation during subsequent electrolysis, which is beneficial for the efficient exfoliation of thin-layer graphene. However, excessive etching can disrupt the continuity of the graphite framework, leading to severe fragmentation of the product during electrolytic exfoliation. On the other hand, high-temperature sintering can repair some of the structural damage caused by etching, promote carbon atom rearrangement, and construct a stable porous graphite framework, effectively supporting the formation of large-size graphene sheets during electrolytic exfoliation. Low-temperature sintering, due to insufficient framework strength, is prone to disintegration under electrochemical intercalation stress, resulting in smaller product size and increased fragmentation. Therefore, by precisely matching the KOH etching intensity and the thermally stabilized sintering temperature, the graphene structure can be controllably adjusted from "fragile small sheets" to "complete large sheets."

[0033] Examples 28-33 Following the method of Example 1, the electrolysis time t and voltage U in Example 1 were changed. After electrolysis, the obtained product was washed and dried. The morphology of the product was characterized by SEM, and the current efficiency and graphene were calculated. The results are shown in Table 3.

[0034] Table 3. Summary of the effects of different electrolysis process conditions on the synthesis of graphene materials.

[0035] Table 3 shows that there exists an optimal "time-voltage" window (approximately 3.5 hours, 2.2-2.6V) that maximizes current efficiency (>90%). Under this window, the exfoliation process is most efficient and causes minimal damage, easily yielding high-quality graphene with the fewest layers and the largest size. Deviating from this window, whether due to insufficient energy or excessive input, will lead to decreased efficiency and deterioration of product quality.

[0036] Examples 34-39 Following the method of Example 1, the composition of the membrane material a Al2O3·b CaO·c SiO2·d MgO in Example 1 was adjusted, while the porosity and pore size remained basically unchanged. After electrolysis according to the method of Example 1, the obtained product was washed and dried, the SEM characterization results were analyzed, and the membrane service life was examined. The results are shown in Table 4.

[0037] Table 4. Summary of the Influence of Different Separating Membrane Materials on Synthetic Graphene Materials

[0038] The results show that graphene materials can be prepared without a separator, but the number of graphene layers increases and the lateral dimensions decrease. The composition of the separator does not affect the growth of graphene, but only its lifespan; it has no effect on the composition of the final graphene product.

[0039] Example 40 The electrochemical performance of graphene materials was tested by assembling CR2032 coin cells using the product obtained in Example 1. Electrochemical performance was also tested by assembling cells using the products obtained in other examples.

[0040] First, electrode sheets were prepared by mixing polyvinyl acid (PAA) and deionized water at a mass ratio of 1:1 and dissolving them at 60 °C until no particles or bubbles remained, thus obtaining a binder. Then, graphene material, conductive agent acetylene black (Super-P), and polyvinyl acid (PAA) binder were mixed at a mass ratio of 6:2:2 and thoroughly stirred in a homogenizer. The resulting viscous slurry was then uniformly coated onto copper sheets (12 mm in diameter and 0.1 mm thick), with approximately 1 mg of slurry on each electrode. The coated sheets were then dried in a vacuum oven at 80 °C for 12 h, compacted under 18 MPa pressure, and returned to the vacuum oven at 80 °C for another 8 h of drying.

[0041] The battery assembly process was completed in an argon-filled glove box (O2 < 0.1 ppm, H2O < 0.1 ppm). The electrode was placed in the middle of the positive electrode shell, and a 20 mm diameter Cellgard 2500 separator was placed on the surface of the electrode. Electrolyte (1 M lithium hexafluorophosphate (LiPF6) dissolved in equal volumes of ethylene carbonate (EC), dimethyl carbonate (DEC), and ethylene carbonate (EMC)) was added. Then, fresh lithium metal sheets were placed on the separator, an appropriate amount of electrolyte was added, and the negative electrode shell was covered and sealed with a sealing machine. The battery was then left at 30 °C for 24 h. The half-cells after being left to stand were tested at 30 ℃ using the CT-3008W Neware battery testing system from Shenzhen Neware Electronics Co., Ltd. Long-cycle performance tests were conducted at a rate of 0.1C, and rate performance tests were performed at 0.5C, 1C, 2C, 5C, and 10C rates, with the voltage range being 0.01-1.5 V.

[0042] Table 5. Summary of the electrochemical performance of graphene materials from some embodiments.

[0043] The results show that in this system for in-situ molten salt electrolysis synthesis of graphene materials using the membrane method, the battery prepared from the product obtained under the conditions used in Example 1 exhibits the best cycle performance and rate performance. Examples 2-18 show the electrochemical performance of batteries prepared from graphene obtained with different molten salt compositions. Compared to Example 1, these molten salt compositions yielded more graphene layers and more graphite fragmentation, resulting in poorer electrochemical performance. Examples 19 and 20 show the electrochemical performance of batteries prepared from graphene materials synthesized at different KOH solution concentrations and sintering temperatures. At excessively low KOH solution concentrations, the number of graphene layers is too high, leading to a decrease in battery capacity. At excessively high KOH solution concentrations, severe graphene fragmentation results in poor electrochemical performance. Examples 21 and 22 show the electrochemical performance of batteries prepared from graphene synthesized under different electrolysis process conditions. Excessively high or low electrolysis time and voltage both lead to a decrease in the quality of the graphene material, thus resulting in poor electrochemical performance. Compared to the product prepared without a separator in Application Example 23, the graphene material obtained using the separator in Application Example 24 has fewer layers and a larger lateral dimension because the separator can prevent anode byproducts and improve the quality of graphene. Application Example 24 only changed the content of the chemical components of the separator, and the resulting graphene material was not significantly different from the sample in Example 39. Therefore, under the same conditions, the electrochemical performance of the battery was also excellent.

[0044] Comparative Example 1 Following the method of Example 1, the porous graphite cathode in Example 1 was replaced with a pure graphite rod cathode, with other conditions remaining unchanged. After electrolysis, the resulting product was washed and dried, and the morphology of the product was characterized by SEM.

[0045] Experimental results: Only slight exfoliation occurred on the surface of the cathode graphite rod, and very few electrolysis products were obtained. SEM characterization showed that the number of obtained graphene sheets was extremely small, with approximately 45 graphene layers and a lateral dimension of approximately 8 μm.

[0046] Comparative Example 1 was assembled into a CR2032 coin cell according to the method of Example 40 to test the electrochemical performance of the material.

[0047] The results showed that the specific capacity after 100 cycles was 245 mAh / g, and the coulombic efficiency in the first cycle was 81%.

[0048] Comparative Example 2 Following the method of Example 1, the molybdenum wire anode in Example 1 was replaced with a pure graphite rod anode, with other conditions remaining unchanged. After electrolysis, the resulting product was washed and dried, and the morphology of the product was characterized by SEM.

[0049] Experimental results: The cathode product has approximately 39 graphene layers and a lateral dimension of approximately 12 μm.

[0050] Comparative Example 2 was assembled into a CR2032 coin cell according to the method of Example 40 to test the electrochemical performance of the material.

[0051] The results showed that the specific capacity after 100 cycles was 212 mAh / g, and the coulombic efficiency in the first cycle was 79%.

[0052] Comparative Example 3 Following the method of Example 1, the molybdenum wire anode in Example 1 was replaced with a platinum sheet anode, with other conditions remaining unchanged. After electrolysis, the resulting product was washed and dried, and the morphology of the product was characterized by SEM.

[0053] Experimental results: The cathode product has approximately 33 graphene layers and a lateral dimension of approximately 11 μm.

[0054] Comparative Example 3 was assembled into a CR2032 coin cell according to the method of Example 40 to test the electrochemical performance of the material.

[0055] The results showed that the specific capacity after 100 cycles was 217 mAh / g, and the coulombic efficiency in the first cycle was 82%.

[0056] Comparative Example 4 Following the method of Example 1, the CaO in the molten salt electrolyte of Example 1 was removed, and only pure CaCl2 was used as the electrolyte, with all other process parameters remaining identical. After electrolysis, the resulting product was washed and dried, and its morphology was characterized using SEM.

[0057] Experimental results: The cathode product contained approximately 32 graphene layers with a lateral dimension of approximately 15 μm. The exfoliation effect was significantly worse than in Example 1, and the graphene yield was significantly reduced.

[0058] Comparative Example 4 was assembled into a CR2032 coin cell according to the method of Example 40 to test the electrochemical performance of the material.

[0059] The results showed that the specific capacity after 100 cycles was 203 mAh / g, and the coulombic efficiency in the first cycle was 77%.

[0060] Comparative Example 5 Following the method of Example 1, the CaO content of the molten salt electrolyte in Example 1 was increased from 2 wt% to 8 wt%, while all other process parameters remained the same. After electrolysis, the resulting product was washed and dried, and its morphology was characterized using SEM.

[0061] Experimental results: The number of graphene layers obtained is extremely uneven. Some areas are over-peeled to form ultrathin fragments of 3-5 layers, while some areas are still thick graphite layers of more than 30 layers. The overall lateral size is only about 6-12 μm, and the product contains a large number of amorphous carbon particles.

[0062] Comparative Example 5 was assembled into a CR2032 coin cell according to the method of Example 40 to test the electrochemical performance of the material.

[0063] The results showed that the specific capacity after 100 cycles was 231 mAh / g, and the coulombic efficiency in the first cycle was 83%.

[0064] Comparative Examples 1-5 show that deviations from the preferred range of this invention in cathode structure, anode material, and oxide content all lead to increased graphene layer thickness, reduced size, or structural degradation. The synergistic matching of the various technical features of this invention enables the efficient preparation of high-quality graphene with fewer layers and larger dimensions.

[0065] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Other variations and modifications may be made without departing from the technical solutions described in the claims.

Claims

1. A method for in-situ synthesis of graphene via molten salt electrolysis, characterized in that, The method employs a diaphragm electrolytic cell with molybdenum as the anode, porous graphite as the cathode, ceramic as the diaphragm, and molten salt electrolyte as the electrolyte. Electrolysis is carried out under constant voltage protection with inert gas, and graphene material is obtained in situ within the pores of the cathode graphite. The molten salt electrolyte includes a metal chloride and an intercalation promoter, wherein the intercalation promoter is selected from at least one metal oxide, and the metal oxide is selected from at least one alkali metal and alkaline earth metal.

2. The method according to claim 1, characterized in that, The molybdenum metal is at least one of molybdenum wire, molybdenum plate, or molybdenum rod.

3. The method according to claim 1, characterized in that, The porous graphite is a three-dimensional porous graphite with a pore size of 80-500 nm and a porosity of 35-50%.

4. The method according to claim 3, characterized in that, The method for preparing the cathode porous graphite includes: immersing a graphite rod in a potassium hydroxide solution with a molar concentration of 10-16 mol / L, etching it at 100-150 ℃ for 6-24 h, and then sintering the graphite rod at 500-950 ℃ in an inert atmosphere to obtain a three-dimensional porous graphite with a pore size of 80-500 nm and a porosity of 35-50%.

5. The method according to claim 1, characterized in that, The diaphragm is a porous ceramic diaphragm with the chemical composition aAl2O3·bCaO·cSiO2·dMgO, where a=0.92-0.96, b+c+d=0.04-0.08, a+b+c+d=1.0, porosity is 35-40%, and pore size is 0.2-2.0 μm.

6. The method according to claim 1, characterized in that, The metal chloride mentioned is MCl. x M is selected from at least one metallic element from Li, Na, K, Mg, and Ca, and x is the valence of the corresponding metal. And / or, the composition of the molten salt electrolyte is as follows by molar percentage: LiCl 0-100%, NaCl 0-100%, KCl 0-100%, MgCl2 0-100%, and CaCl2 0-100%.

7. The method according to claim 1, characterized in that, The metal oxide is M x O y M is selected from at least one metallic element from Li, Na, K, and Ca, and x and y are the valences of the corresponding metals; And / or, the content of the metal oxide is 0.5-5.0 wt% of the total mass of the molten salt electrolyte.

8. The method according to claim 1, characterized in that, During constant voltage electrolysis, the temperature of the molten salt electrolyte is controlled at 720-950 ℃; And / or, adjust the voltage to 1.6-3.5V; And / or, the electrolysis time is 1-24 h.

9. The method according to claim 1, characterized in that, The electrode spacing between the cathode and anode is 1-5 cm.

10. The method according to claim 1, characterized in that, The graphene product has ≤20 graphite layers and a lateral dimension of 1-60 μm.

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