Preparation method and application of carbon-coated magnesium-based prussian blue nanosheet

Abstract

The application discloses a preparation method and application of carbon-coated magnesium-based Prussian blue nanosheets, and the method comprises the following steps: dissolving a magnesium source and a potassium source to obtain solution A, dissolving potassium ferrocyanide to obtain solution B, then adding a polyhydric alcohol solvent into the solutions A and B respectively; mixing the two solutions uniformly, and performing a co-precipitation reaction to obtain magnesium-based Prussian blue nanosheets; mixing the magnesium-based Prussian blue nanosheets with an organic carbon source, and calcining, so that the carbon-coated magnesium-based Prussian blue nanosheets are prepared. The method is simple in process, the prepared magnesium-based Prussian blue nanosheets have high heat resistance, and high-temperature carbon coating can be realized. Moreover, the material has excellent electrochemical performance, and is a promising positive electrode material of a potassium ion battery.

Description

Technical Field

[0001] This invention relates to a method for preparing an electrode material and its application, and more particularly to a method for preparing and applying carbon-coated magnesium-based Prussian blue nanosheets. Background Technology

[0002] Lithium-ion batteries, with their advantages of high energy density, long cycle life, and no memory effect, have become the main power source for electric vehicles, portable electronic devices, and even some energy storage systems. However, the scarcity, uneven distribution, and high price of lithium resources hinder their application in large-scale energy storage systems. In recent years, potassium-ion batteries have attracted widespread interest as a promising alternative. Potassium is more abundant in nature than lithium, is cheaper, and its electrochemical mechanism is similar to that of lithium-ion batteries. Furthermore, potassium's redox potential (K / K) is relatively low. + (–2.93V) and lithium (Li / Li + The voltage (-2.71V) is also similar. Furthermore, the radius of solvated potassium ions is smaller, therefore potassium-ion batteries may have higher rate performance and greater potential in practical applications. However, in comparison, K... + radius Compared to Li + The larger the electrode, the greater the impact on the electrode material during charging and discharging.

[0003] The general structural formula of Prussian blue potassium storage cathode materials is K x M[Fe(CN)6] y ·□ 1-y ·zH₂O, where M is a transition metal such as Mn, Fe, Co, and Ni, or a main group metal such as Sn and Ba. It possesses an open three-dimensional framework structure and ion transport channels, high theoretical specific capacity, high operating voltage, and high structural stability, making it an ideal cathode material for potassium-ion batteries. However, Prussian blue-based materials suffer from difficulties in removing [Fe(CN)₆] vacancies and water of crystallization, and this water of crystallization can enter the electrolyte during charge and discharge, causing side reactions and leading to poor cycle stability. Furthermore, Prussian blue-based materials themselves have low electronic conductivity, affecting their rate performance. Traditional transition metal-based Prussian blue-based materials, due to their poor thermal stability (decomposing above 350℃), cannot be removed by high-temperature calcination to remove water of crystallization and carbon coating.

[0004] Traditional co-precipitation methods for synthesizing transition metal-based Prussian blue analogs often result in rapid precipitation rates, leading to numerous [Fe(CN)6] vacancies and water of crystallization, and the resulting particles are typically on the order of tens of nanometers. In recent years, researchers have begun to explore methods such as adding metal complexing agents to reduce the reaction rate and minimize vacancy defects. To improve the conductivity of materials, Prussian blue analogs can be modified by in-situ growth of these analogs on the surfaces of carbon materials such as carbon nanotubes and graphene. However, these preparation processes are complex, involve high raw material costs, and are difficult to scale up for mass production. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a method for preparing carbon-coated magnesium-based Prussian blue nanosheets with excellent cycling performance and rate capability;

[0006] A second objective of this invention is to provide the application of the carbon-coated magnesium-based Prussian blue nanosheets prepared by the above-described method.

[0007] Technical solution: The preparation method of carbon-coated magnesium-based Prussian blue nanosheets of the present invention includes the following steps:

[0008] (1) Solution A is obtained by mixing and dissolving magnesium source and potassium source, and solution B is obtained by dissolving potassium ferrocyanide.

[0009] (2) Add polyol solvent to solutions A and B respectively;

[0010] (3) The two solutions obtained in step (2) are mixed and a precipitate is generated by co-precipitation reaction. Then, the solid and liquid are separated, washed and dried to obtain magnesium-based Prussian blue nanosheets.

[0011] (4) Mix magnesium-based Prussian blue nanosheets with an organic carbon source and calcine to obtain carbon-coated magnesium-based Prussian blue nanosheet materials.

[0012] In step (1), the magnesium source is at least one of magnesium acetate, magnesium chloride, magnesium sulfate, and magnesium nitrate, and the potassium source is at least one of potassium acetate, potassium chloride, potassium sulfate, and potassium nitrate.

[0013] In step (1), the concentrations of the magnesium source and potassium ferrocyanide are 0.25–0.75 mol / L, respectively. -1 The concentration of the potassium source is 1.0–3.0 mol / L. -1 The molar ratio of magnesium source to potassium ferrocyanide is 0.9–1.1:1, and the molar ratio of potassium source to potassium ferrocyanide is 2–4:1. If the raw material concentration is too low, magnesium-based Prussian blue precipitate is unlikely to form; if the raw material concentration is too high, it cannot be completely dissolved.

[0014] In step (2), the polyol is at least one of glycerol, ethylene glycol, diethylene glycol or butanetetraol.

[0015] In step (2), the mass ratio of the polyol to the solvent in solutions A and B is 1:2 to 1:1, respectively; the solvent in solutions A and B is preferably water. If the polyol ratio is too low, nanosheet morphology cannot be formed; if it is too high, impurity phases will be generated.

[0016] In step (3), after the solution is mixed evenly, it needs to be allowed to stand for reaction for 6 to 24 hours.

[0017] In step (4), the organic carbon source is at least one of polyvinyl alcohol, polyvinylpyrrolidone, phenolic resin, glucose, or sucrose; the amount of organic carbon source added is 5% to 25% of the mass of magnesium-based Prussian blue nanosheets. If the amount added is too low, the carbon content is low and the conductivity is poor; if the amount added is too high, the carbon content is high, the proportion of active material decreases, and the specific capacity decreases.

[0018] In step (4), the organic carbon source and the precipitate obtained in step (3) are mixed by adding a solvent or by ball milling; when mixing by adding a solvent, the solvent is preferably a volatile organic solvent, and more preferably ethanol.

[0019] In step (4), the calcination temperature is 500–600°C, and the calcination time is 2–6 hours. The calcination is carried out under an inert atmosphere; more specifically, under an argon atmosphere. If the temperature is too low or the time is too short, complete carbonization is difficult, and if the temperature is too high or the time is too long, magnesium-based Prussian blue is prone to decomposition.

[0020] The carbon-coated magnesium-based Prussian blue nanosheets prepared by the above method are used as cathode materials for potassium-ion batteries.

[0021] Invention Principle: Compared with traditional transition metal-based Prussian blue, the magnesium-based Prussian blue synthesized in this invention has a slower precipitation rate and fewer product vacancy defects, thus exhibiting higher thermal stability. Polyols are used as additives in the synthesis process. The high viscosity and strong adsorption of polyols can inhibit the growth of magnesium-based Prussian blue crystals, especially the strong adsorption on the (100) crystal plane, causing it to grow into a nanosheet morphology. By controlling the content and composition of different additives, the morphology of the product can be controlled. Furthermore, high-temperature calcination can remove the water of crystallization in the magnesium-based Prussian blue, and simultaneously, an organic carbon source is added to form a carbon coating layer.

[0022] Beneficial effects: Compared with the prior art, the present invention achieves the following significant effects: (1) Carbon-coated magnesium-based Prussian blue nanosheets can be prepared by the method of the present invention. Carbon coating is conducive to rapid electron transport and reduces direct contact with electrolyte, which greatly promotes the utilization of material capacity and improves the cycle stability and rate performance of magnesium-based Prussian blue material as a positive electrode of potassium-ion battery; low vacancy defects and high temperature removal of crystal water also help to improve cycle stability; in addition, the morphology of nanosheets ensures shorter electron and ion transport paths while having a lower specific surface area and higher packing density compared with nanoparticles. (2) The method of the present invention adopts a polyol-assisted co-precipitation method, which has a high initial raw material concentration, simple preparation process, low raw material cost, and is easy to mass-produce; (3) The carbon-coated magnesium-based Prussian blue nanosheets (KMgHCF / C) obtained have excellent electrochemical performance. As a positive electrode material of potassium-ion battery, KMgHCF / C has 84.6 mAh g -1 High reversible specific capacitance and a high average operating voltage of 3.87V; at 5A g -1 It can still maintain a specific capacity of 57.2% at high rate, at 500mA g -1 At high rate, the capacity retention rate after 15,000 cycles is 84.0%, equivalent to a capacity decay of only 0.01‰ per cycle, which is currently the best among Prussian blue-based cathodes in organic electrolyte systems for potassium-ion batteries. Therefore, this carbon-coated magnesium-based Prussian blue nanosheet has great application potential as a high-performance, low-cost cathode material. Attached Figure Description

[0023] Figure 1 Thermogravimetric analysis of KMgHCF in Example 1 and manganese-based Prussian blue KMnHCF in Comparative Example 1 of the present invention;

[0024] Figure 2 This is a schematic diagram of the XRD refinement and crystal structure of the KMgHCF material in Example 1 of the present invention;

[0025] Figure 3 These are XRD comparison images of the KMgHCF / C material before and after carbon coating in Example 1 of this invention;

[0026] Figure 4 This is a SEM image of the KMgHCF / C material from Example 1 of the present invention;

[0027] Figure 5 These are TEM and HRTEM images of the KMgHCF / C material from Example 1 of this invention;

[0028] Figure 6 This is a cyclic voltammetry curve of the KMgHCF / C electrode in Example 1 of the present invention;

[0029] Figure 7The charge / discharge curves of the KMgHCF / C electrode in Example 1 of this invention are shown.

[0030] Figure 8 This is a comparison chart of the rate performance of KMgHCF / C materials in Example 1 and KMnHCF materials in Comparative Example 1 at different current densities.

[0031] Figure 9 The KMgHCF / C materials of Example 1 and the KMnHCF materials of Comparative Example 1 were tested at 10 mA g. -1 Cyclic contrast energy plot at current density;

[0032] Figure 10 KMgHCF / C in Example 1 at 100 and 500 mA g -1 Cyclic performance at current density;

[0033] Figure 11 This is a SEM image of Comparative Example 1 of the present invention;

[0034] Figure 12 This is a SEM image of Comparative Example 2 of the present invention;

[0035] Figure 13 This is a SEM image of Comparative Example 3 of the present invention;

[0036] Figure 14 This is the SEM image of Comparative Example 4 of the present invention;

[0037] Figure 15 This is the XRD pattern of Comparative Example 4 of the present invention;

[0038] Figure 16 This is the XRD pattern of Comparative Example 5 of the present invention. Detailed Implementation

[0039] The present invention will now be described in further detail.

[0040] Example 1

[0041] Preparation of KMgHCF / C materials:

[0042] (1) Solution A: Dissolve 10 mmol magnesium acetate and 40 mmol potassium acetate in 20 mL deionized water; Solution B: Dissolve 10 mmol potassium ferrocyanide in 20 mL deionized water;

[0043] (2) Add 20g of glycerol to solutions A and B obtained in step (1), respectively;

[0044] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, then let stand for 24 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain KMgHCF nanosheets.

[0045] (4) Disperse the KMgHCF nanosheets obtained in step (3) in ethanol, add 15% PVA by mass, stir and evaporate the ethanol.

[0046] (5) Place the mixture obtained in step (4) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 550℃ and calcined for 2 hours. After cooling, the product was obtained.

[0047] Characterization of KMgHCF / C materials:

[0048] Figure 1 Thermogravimetric analysis (TGA) tests were performed on KMgHCF (Example 1) and KMnHCF (Comparative Example 1). The results showed that KMgHCF only began to decompose above 550℃, while KMnHCF in Comparative Example 1 began to decompose at 350℃, indicating that magnesium-based Prussian blue has higher thermal stability. TGA analysis also revealed that the water content in KMnHCF was 7.2%, while that in KMgHCF was only 0.43%. Combined with elemental analysis results, the composition formulas of KMgHCF and KMnHCF are K... 1.96 Mg[Fe(CN)6] 0.99 ·□ 0.01 0.07H2O and K 1.68 Mn[Fe(CN)6] 0.92 ·□ 0.08 ·1.36H2O. KMgHCF has lower vacancy and water of crystallization content.

[0049] Figure 2 The image shows the refined XRD structure of KMgHCF and the corresponding crystal structure diagram, indicating that the material is a pure-phase compound. Figure 3 The XRD comparison of KMgHCF and KMgHCF / C before and after carbon coating shows that the high-temperature carbon coating process does not damage the structure of KMgHCF.

[0050] The size, morphology and microstructure of the obtained KMgHCF / C material were analyzed using SEM, TEM and HRTEM images. Figure 4 The image shows a SEM image of the KMgHCF / C material, which reveals that the nanosheets exhibit a square structure with a particle size of 1–2 μm. Figure 5 The KMgHCF / C nanosheets are approximately 100 nm thick, with a uniform carbon coating layer of about 3 nm on the surface.

[0051] Electrochemical performance testing:

[0052] Using 1-methyl-2-pyrrolidone as a solvent, the KMgHCF / C prepared in this example was ground and mixed with conductive carbon black and polyvinylidene fluoride at a mass ratio of 80:10:10 until homogeneous. The resulting uniform slurry was coated onto aluminum foil and vacuum dried at 80°C for 12 hours. Using 3 mol L... -1 A potassium bis(fluorosulfonyl)imide triethyl phosphate solution was used as the electrolyte for the potassium-ion battery, and glass fiber and metallic potassium were used as the separator and counter electrode, respectively. Electrochemical performance was tested using CR2032 batteries. Battery assembly was conducted in a glove box filled with argon atmosphere, with water and oxygen concentrations both less than 0.1 ppm. Constant current charge-discharge tests were performed at room temperature using a Blue Electric CT2001A multichannel battery testing system at 2.0–4.3 V (vs. K). + / K) Performed within a fixed voltage range. See details for specific performance specifications. Figures 6 to 10 .

[0053] Figure 6 The KMgHCF / C electrode is used at 2.0–4.3V (vs. K). + The first three cycles of the cyclic voltammetry curves within the / K voltage range show a pair of redox peaks at 3.98 / 3.80V, and the curves basically overlap, indicating that the material has good reversibility of potassium insertion / extraction. Figure 7 KMgHCF / C at 2.0–4.3V (vs. K + The first ten charge / discharge curves within the / K) voltage range, with a current density of 10 mA g -1 Its reversible specific capacity reaches 84.6 mAh g. -1 It exhibits a voltage plateau at 3.87V. Figure 8 The graphs show the rate performance of KMgHCF / C and KMnHCF in Comparative Example 1 at different current densities. It can be seen that KMgHCF / C has a higher capacity retention, even at 5A g. -1 At high current densities, the capacity retention of KMgHCF / C can still reach 57.2%, while that of KMnHCF in Comparative Example 1 is only 35.1%. Figure 9 The KMgHCF / C and the material of Comparative Example 1 were compared at 10 mA g. -1 Cyclic performance at current density: KMgHCF / C showed almost no degradation after 200 cycles, with a capacity retention of 96.8%, while Comparative Example 1 only had 53.0%. Figure 10 KMgHCF / C at 100 and 500 mA g -1 Cyclic performance at current density: The graph shows the KMgHCF / C at 500 mAg. -1 After 15,000 cycles, the capacity retention rate is still 84.0%.

[0054] Example 2

[0055] (1) Solution A: Dissolve 11 mmol magnesium chloride and 20 mmol potassium chloride in 20 mL deionized water; Solution B: Dissolve 10 mmol potassium ferrocyanide in 20 mL deionized water;

[0056] (2) Add 10g of ethylene glycol to solutions A and B obtained in step (1), respectively;

[0057] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, then let stand for 6 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain KMgHCF nanosheets.

[0058] (4) Disperse the KMgHCF nanosheets obtained in step (3) in ethanol, add 25% PVP by mass, stir and evaporate the ethanol.

[0059] (5) Place the mixture obtained in step (4) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 600℃ and calcined for 2 hours. After cooling, the product was obtained.

[0060] The KMgHCF / C material was subjected to structural characterization and electrochemical performance testing in the same manner as in Example 1. The structural characterization results were basically the same as those in Example 1, and the electrochemical performance test results are shown in Table 1.

[0061] Example 3

[0062] (1) Solution A: Dissolve 4.5 mmol magnesium nitrate and 20 mmol potassium nitrate in 20 mL deionized water; Solution B: Dissolve 5 mmol potassium ferrocyanide in 20 mL deionized water;

[0063] (2) Add 10g of butanethyl alcohol to solutions A and B obtained in step (1), respectively;

[0064] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, then let stand for 24 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain KMgHCF nanosheets.

[0065] (4) The KMgHCF nanosheets obtained in step (3) are ball-milled and mixed with 10% glucose by mass;

[0066] (5) Place the mixture obtained in step (4) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 600℃ and calcined for 2 hours. After cooling, the product was obtained.

[0067] The KMgHCF / C material was subjected to structural characterization and electrochemical performance testing in the same manner as in Example 1. The structural characterization results were basically the same as those in Example 1, and the electrochemical performance test results are shown in Table 1.

[0068] Example 4

[0069] (1) Solution A: Dissolve 5 mmol magnesium sulfate and 10 mmol potassium sulfate in 20 mL deionized water; Solution B: Dissolve 5 mmol potassium ferrocyanide in 20 mL deionized water;

[0070] (2) Add 10g of diethylene glycol to solutions A and B obtained in step (1), respectively;

[0071] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, then let stand for 24 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain KMgHCF nanosheets.

[0072] (4) Disperse the KMgHCF nanosheets obtained in step (3) in ethanol, add 5% by mass of phenolic resin, stir and evaporate the ethanol.

[0073] (5) Place the mixture obtained in step (4) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 500℃ and calcined for 6 hours. After cooling, the product was obtained.

[0074] The KMgHCF / C material was subjected to structural characterization and electrochemical performance testing in the same manner as in Example 1. The structural characterization results were basically the same as those in Example 1, and the electrochemical performance test results are shown in Table 1.

[0075] Example 5

[0076] (1) Solution A: Dissolve 15 mmol magnesium acetate and 60 mmol potassium acetate in 20 mL deionized water; Solution B: Dissolve 15 mmol potassium ferrocyanide in 20 mL deionized water;

[0077] (2) Add 10g of glycerol to solutions A and B obtained in step (1), respectively;

[0078] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, then let stand for 24 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain KMgHCF nanosheets.

[0079] (4) The KMgHCF nanosheets obtained in step (3) are ball-milled and mixed with 10% by mass of sucrose;

[0080] (5) Place the mixture obtained in step (4) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 550℃ and calcined for 2 hours. After cooling, the product was obtained.

[0081] The KMgHCF / C material was subjected to structural characterization and electrochemical performance testing in the same manner as in Example 1. The structural characterization results were basically the same as those in Example 1, and the electrochemical performance test results are shown in Table 1.

[0082] Comparative Example 1

[0083] Traditional manganese-based Prussian blue analogue KMnHCF nanoparticles:

[0084] (1) Solution A: Dissolve 2 mmol manganese acetate and 40 mmol potassium acetate in 40 mL of deionized water; Solution B: Dissolve 2 mmol potassium ferrocyanide in 40 mL of deionized water;

[0085] (2) Add solution A from step (1) to solution B, keep stirring for 4 hours, separate the resulting precipitate by centrifugation, wash with water 3 times, wash with ethanol once, and finally dry under vacuum at 80°C to obtain KMnHCF.

[0086] The prepared KMnHCF material was characterized structurally and its electrochemical performance was tested using the same method as in Example 1. Its morphology is as follows: Figure 11 As shown, KMnHCF consists of fine particles of about 20 nm. Figure 1 The data shows that the KMnHCF decomposes at 350℃, therefore it cannot be further modified with carbon coating. Figure 8 The chart shows a comparison of the rate performance of KMnHCF and KMgHCF / C. The capacity retention of KMgHCF / C is higher than that of KMnHCF. Figure 9 The graph shows a comparison of the cycling performance of KMnHCF and KMgHCF / C. The graph indicates that the cycling stability of KMnHCF is far inferior to that of KMgHCF / C. The above tests show that the potassium storage performance of traditional KMnHCF nanoparticles is far inferior to that of KMgHCF / C nanosheets.

[0087] Comparative Example 2

[0088] Preparation of KMgHCF without adding polyols:

[0089] (1) Solution A: Dissolve 10 mmol magnesium acetate and 40 mmol potassium acetate in 20 mL deionized water; Solution B: Dissolve 10 mmol potassium ferrocyanide in 20 mL deionized water;

[0090] (2) Pour solution A from step (1) into solution B, stir rapidly for 5 seconds, then let stand for 24 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain KMgHCF.

[0091] (3) Disperse the KMgHCF obtained in step (2) in ethanol, add 15% PVA by mass, stir and evaporate the ethanol to dryness.

[0092] (4) Place the mixture obtained in step (3) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 550℃ and calcined for 2 hours. After cooling, the product was obtained.

[0093] The morphology and electrochemical performance of the prepared KMgHCF material were characterized using the same method as in Example 1. Its morphology is as follows: Figure 12 As shown, KMgHCF consists of plate-like particles with a size of 3–6 μm and a thickness of approximately 600 nm. Due to the lack of adsorption by polyols, the KMgHCF grains grow significantly, which is detrimental to subsequent potassium ion transport. The electrochemical performance of the prepared material was tested using the same method as in Example 1, and the results are shown in Table 1. Its specific capacity and rate performance decreased significantly.

[0094] Comparative Example 3

[0095] Preparation of KMgHCF using monohydric alcohols:

[0096] (1) Solution A: Dissolve 10 mmol magnesium acetate and 40 mmol potassium acetate in 20 mL deionized water; Solution B: Dissolve 10 mmol potassium ferrocyanide in 20 mL deionized water;

[0097] (2) Add 20g of ethanol to solutions A and B obtained in step (1) respectively;

[0098] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, then let stand for 24 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain KMgHCF.

[0099] (4) Disperse the KMgHCF obtained in step (3) in ethanol, add 15% PVA by mass, stir and evaporate the ethanol to dryness.

[0100] (5) Place the mixture obtained in step (4) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 550℃ and calcined for 2 hours. After cooling, the product was obtained.

[0101] The morphology and electrochemical performance of the prepared KMgHCF material were characterized using the same method as in Example 1. Its morphology is as follows: Figure 13 As shown, KMgHCF consists of quasi-cubic particles with a size of 1–2 μm. Under the influence of ethanol, the KMgHCF grains do not exhibit oriented growth, which is detrimental to subsequent potassium ion transport. The electrochemical performance of the prepared material was tested using the same method as in Example 1, and the results are shown in Table 1. Its specific capacity and rate performance decreased significantly. Therefore, the use of polyols for preparation is necessary.

[0102] Comparative Example 4

[0103] Preparation of KMgHCF with excessive polyol addition:

[0104] (1) Solution A: Dissolve 10 mmol magnesium acetate and 40 mmol potassium acetate in 20 mL deionized water; Solution B: Dissolve 10 mmol potassium ferrocyanide in 20 mL deionized water;

[0105] (2) Add 30g of glycerol to solutions A and B obtained in step (1), respectively;

[0106] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, then let stand for 24 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain the precipitate.

[0107] (4) Disperse the precipitate obtained in step (3) in ethanol, add 15% PVA by mass, stir and evaporate the ethanol to dryness.

[0108] (5) Place the mixture obtained in step (4) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 550℃ and calcined for 2 hours. After cooling, the product was obtained.

[0109] The morphology and structure of the prepared KMgHCF material were characterized and its electrochemical performance was tested using the same method as in Example 1. Its morphology is as follows: Figure 14 As shown, although KMgHCF has a nanosheet structure, its surface contains many impurities. Its XRD results are as follows... Figure 15 As shown, obvious impurities are present, indicating that excessively high polyol concentrations can lead to impurity formation. The electrochemical performance of the prepared material was tested using the same method as in Example 1, and the results are shown in Table 1. Its specific capacity, cycle life, and rate performance decreased significantly.

[0110] Comparative Example 5

[0111] Preparation of KMgHCF / C at excessively high calcination temperatures:

[0112] (1) Solution A: Dissolve 10 mmol magnesium acetate and 40 mmol potassium acetate in 20 mL deionized water; Solution B: Dissolve 10 mmol potassium ferrocyanide in 20 mL deionized water;

[0113] (2) Add 20g of glycerol to solutions A and B obtained in step (1), respectively;

[0114] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, then let stand for 24 hours. The resulting precipitate is separated by centrifugation, washed 3 times with water and 1 time with ethanol, and finally dried under vacuum at 80°C to obtain KMgHCF nanosheets.

[0115] (4) Disperse the KMgHCF nanosheets obtained in step (3) in ethanol, add 15% PVA by mass, stir and evaporate the ethanol.

[0116] (5) Place the mixture obtained in step (4) in an argon atmosphere tube furnace and heat it at 5°C for 5 min. -1 The temperature was increased to 650℃ and calcined for 2 hours. After cooling, the product was obtained.

[0117] The KMgHCF / C material was characterized structurally and its electrochemical performance was tested using the same method as in Example 1. The XRD results are as follows: Figure 16 As shown, obvious impurity phases are present, indicating that excessively high calcination temperatures will cause KMgHCF to decompose. The electrochemical performance of the prepared material was tested using the same method as in Example 1, and the results are shown in Table 1. Its specific capacity, cycle life, and rate performance decreased significantly.

[0118] Comparative Example 6

[0119] Preparation of KMgHCF with excessively low raw material concentration:

[0120] (1) Solution A: Dissolve 2 mmol magnesium acetate and 8 mmol potassium acetate in 20 mL deionized water; Solution B: Dissolve 2 mmol potassium ferrocyanide in 20 mL deionized water;

[0121] (2) Add 10g of glycerol to solutions A and B obtained in step (1), respectively;

[0122] (3) Pour solution A from step (2) into solution B, stir rapidly for 5 seconds, and then let stand for 24 hours. No precipitate is formed, so subsequent preparation and characterization cannot be carried out.

[0123] Table 1 Electrochemical performance data

[0124]

Claims

1. A method for preparing carbon-coated magnesium-based Prussian blue nanosheets, characterized in that, Includes the following steps: (1) Solution A is obtained by mixing and dissolving the magnesium source and the potassium source, and solution B is obtained by dissolving the potassium ferrocyanide; the concentrations of the magnesium source and the potassium ferrocyanide are 0.25~0.75 mol L, respectively. -1 The concentration of the potassium source is 1.0~3.0 mol L. -1 ; (2) Add polyol solvent to solutions A and B respectively; the polyol is at least one of glycerol, ethylene glycol, diethylene glycol or butanetetraol; the mass ratio of the polyol to the solvent in solutions A and B is 1:2 to 1:1 respectively; (3) The two solutions obtained in step (2) are mixed and a precipitate is generated by co-precipitation reaction. Then, the precipitate is obtained by solid-liquid separation, washing and drying to obtain magnesium-based Prussian blue nanosheets. (4) Mix magnesium-based Prussian blue nanosheets with an organic carbon source and calcine to obtain carbon-coated magnesium-based Prussian blue nanosheet materials; the calcination temperature is 500~600 ℃ and the calcination time is 2~6 h.

2. The method for preparing carbon-coated magnesium-based Prussian blue nanosheets according to claim 1, characterized in that, In step (1), the molar ratio of magnesium source to potassium ferrocyanide is 0.9 to 1.1, and the molar ratio of potassium source to potassium ferrocyanide is 2 to 4:

1.

3. The method for preparing carbon-coated magnesium-based Prussian blue nanosheets according to claim 1, characterized in that, In step (3), the two solutions are mixed evenly and then allowed to stand for reaction for 6 to 24 hours.

4. The method for preparing carbon-coated magnesium-based Prussian blue nanosheets according to claim 1, characterized in that, In step (4), the amount of organic carbon source added is 5% to 25% of the mass of magnesium-based Prussian blue nanosheets.

5. The method for preparing carbon-coated magnesium-based Prussian blue nanosheets according to claim 1, characterized in that, In step (4), the organic carbon source is at least one of polyvinyl alcohol, polyvinylpyrrolidone, phenolic resin, glucose, or sucrose.

6. The application of carbon-coated magnesium-based Prussian blue nanosheets prepared by the method of any one of claims 1-5 as a cathode material for potassium-ion batteries.

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