A method for preparing a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material

Abstract

The invention discloses a preparation method of a sodium-graphitized carbon-coated iron-phosphate cathode material. When biomass carbon is graphitized at high temperature in an inert atmosphere by an iron source, a portion of heteroatoms in the carbon body is retained to form a heteroatom-doped and partially graphitized pre-graphitized carbon material. The method comprises the following steps: heating the material to 800-1100° C. at a heating rate of 1-10° C. / min in a nitrogen atmosphere, keeping the temperature for 3-5 hours, and then naturally cooling the material to obtain an Fe-C composite material, i.e., a pre-graphitized carbon material. The pre-graphitized carbon is then mixed with a sodium source, an iron source, and a phosphorus source for carbon coating. The mixture is spray-dried and then calcined to obtain the sodium-graphitized carbon-coated iron-phosphate cathode material. The sodium-graphitized carbon-coated iron-phosphate cathode material prepared by the invention has a high degree of graphitization and electronic conductivity, stable sodium storage performance, and excellent cycle performance, thereby providing a potential approach for improving the conductivity of the iron-phosphate cathode material.

Description

Technical Field

[0001] The present invention relates to the technical field of preparation of iron-based phosphate positive electrode materials, and more specifically, to a method for preparing a carbon-coated iron-based phosphate positive electrode material. Background Art

[0002] Polyanionic iron phosphate is a common battery cathode material with good ion transport channels and related energy storage properties. It is widely used in various ion battery cathode materials. At present, the cathode materials of this system still have the following problems: First, the specific capacity is relatively low, which makes it difficult to meet the demand for high energy density, which limits its further improvement in battery performance; second, the cathode material will experience problems such as capacity decay and structural changes during long-term cycling, resulting in limited battery cycle life. Therefore, it is necessary to further understand its decay mechanism and develop more stable and durable materials; third, iron-based phosphate cathode materials will experience large resistance increases and diffusion limitations due to structural changes during high-rate charge and discharge, resulting in limited energy density when the device operates at high power density. Therefore, improving the charge and discharge rate is a key challenge, and it is necessary to find a suitable modification method to improve its ion transport rate and electronic conduction performance, especially the electronic conduction part.

[0003] To address these issues, researchers are actively conducting research, including exploring new iron-based phosphate materials, optimizing electrode material design, and improving preparation processes. They are also promoting the application and development of iron-based phosphate cathode materials in the energy storage field. Porous carbon materials, with their high electrical conductivity, large specific surface area, rich pore structure, and tunable three-dimensional ion conduction pathways, are considered to be highly promising electrochemical energy storage electrode materials. Currently, many studies have explored carbon-coated phosphate cathode materials.

[0004] For example, Chinese patent CN101388454A discloses a method for preparing a carbon-coated phosphate cathode material for a lithium-ion battery using a supercritical fluid. The method comprises mixing a phosphate cathode material, an organic polymer, and a supercritical fluid in a stoichiometric manner in a sealed high-pressure stirred reactor, stirring the mixture at a temperature of 0 to 100° C. and a pressure of 1 to 10 MPa for more than 30 minutes, and then rapidly ejecting the mixed slurry through a nozzle into an expansion and separation chamber to form a phosphate cathode material pre-coated with the organic polymer. The pre-coated cathode material is then placed in a high-temperature furnace and calcined at a constant temperature of 500 to 850° C. in a non-oxidizing atmosphere for 3 to 20 hours to decompose the organic polymer material and form a uniform carbon coating layer on the surface of the phosphate cathode material. The mixture is then cooled to room temperature to obtain a uniformly carbon-coated phosphate cathode material.

[0005] For example, Chinese patent CN111342018A discloses a carbon-coated lithium-containing transition metal phosphate cathode material. This invention utilizes a one-step synthesis method to prepare the carbon-coated lithium iron phosphate cathode material: FePO₄ and Li₂CO₃ are mixed in a 1:1 molar ratio, with a selenium-carbon content of 0.5% and a selenium content of 10% in the selenium-carbon coating layer, glucose, and elemental selenium as carbon and selenium sources, respectively. The mixture is then placed in a sand mill and mixed for 6 hours before removal. The mixture is then sintered in high-purity nitrogen at 700°C for 12 hours and cooled to room temperature to obtain the carbon-coated lithium iron phosphate cathode material. This invention incorporates selenium into the carbon coating layer to enhance conductivity, reduce the material's surface potential, and facilitate the influx and outflow of lithium ions. Lithium-ion batteries fabricated with this lithium-containing transition metal phosphate cathode material exhibit higher specific capacity, better rate capability, and long cycle performance.

[0006] For example, Chinese patent CN114709400A discloses a method for preparing a continuously produced carbon-coated nano-cathode material. This method utilizes a combination of sand milling, spray drying, and low-temperature carbonization. A sodium source, a phosphorus source, and an iron source are premixed to form a mixture. A carbon source, a dispersant, a grinding aid, and an organic solvent are then added to form a solution, which is then stirred and pre-dispersed. The pre-dispersed product is then connected to a sand mill and sand-milled to obtain a carbon-coated cathode material precursor slurry. The carbon-coated cathode material precursor slurry is then spray-dried to obtain a carbon-coated nano-cathode material precursor. Finally, the carbon-coated nano-cathode material precursor is pyrolyzed at 200-600°C and cooled naturally to obtain the carbon-coated nano-cathode material. This preparation method allows for continuous production and can effectively improve the electrochemical stability of the material.

[0007] Based on the above existing technologies, it can be seen that the carbon coating layer can better play a conductive role, so that the electrochemical performance of the material is improved, and the strong interaction between the coating layer and the main material enables the material to maintain good contact between the carbon material and the main material under long cycles, so that the cyclability is improved. However, due to the limitation of the pyrolysis temperature of iron phosphate (~500°C), the carbon body coated by the traditional primary pyrolysis temperature is difficult to achieve a high degree of graphitization (>2000°C), and a high degree of graphitization can greatly improve the electronic conductivity of iron-based phosphate positive electrode materials. However, the above existing technologies have failed to achieve a high degree of graphitized carbon coating, so the electronic conductivity of its phosphate positive electrode material has not been substantially improved. At present, researchers have also made some efforts to explore graphitized carbon-coated phosphate positive electrode materials.

[0008] For example, Chinese patent CN109244462A discloses a method for preparing a high-conductivity lithium iron phosphate material, which uses a two-step method to prepare a lithium iron phosphate / carbon composite material. A lithium source, an iron source, and a phosphorus source are stirred and mixed in an organic solvent or a deionized water system, then ground in a sand mill, and then spray-dried to obtain a lithium iron phosphate precursor; the obtained lithium iron phosphate precursor is sintered for the first time in a tube furnace to obtain a single-layer carbon-coated lithium iron phosphate material; the single-layer carbon-coated lithium iron phosphate material is then sintered for a second time to obtain a double-layer carbon-coated lithium iron phosphate material. This inventive method forms a double-layer carbon structure through two sinterings to achieve a high degree of graphitization, thereby preparing a lithium iron phosphate material with high conductivity and low internal resistance. However, in addition to the conventional mixing-sand milling-spray drying process, this method also adds a vapor deposition process and uses a propylene mixed gas reaction gas for calcination. The reaction tail gas has certain pollution properties and may also contain powdery and fragmented substances, which pollute the environment.

[0009] For example, Chinese patent CN103500833A discloses a method for preparing an in-situ graphitized carbon-coated lithium iron phosphate positive electrode material. The method is to heat and stir polymerize ferrocene, aldehyde, zinc chloride and concentrated hydrochloric acid in a certain proportion, wash, and obtain polyferrocene. A certain proportion of lithium source, iron source, phosphate source, and dopant source are added and mixed in a certain proportion by ball milling for 3 to 9 hours. After being fully vacuum-dried at 80°C, the mixture is heated to 250 to 400°C in a tube furnace, kept warm for 1 to 12 hours, and cooled to room temperature to obtain in-situ graphitized carbon-coated lithium iron phosphate. The invention uses the metal organic polymer polyferrocene as the iron source and carbon source, and prepares graphitized carbon-coated lithium iron phosphate by ball milling and calcination. The lithium iron phosphate powder prepared by this method has a small particle size and excellent rate performance and cycle performance. However, this method requires the preparation of polyferrocene to achieve in-situ catalytic formation of graphitized carbon coating, which increases the process steps and the use of organic reagents.

[0010] For example, Chinese patent CN103682337A discloses a low-content, high-graphitization carbon-coated lithium iron phosphate, the preparation method of which is to mix the iron source, lithium source, and phosphate source according to the molar ratio of Li:Fe:PO4=1:1:1 and fully mix them by ball milling. After the mixed raw materials are dried, they are kept at 250-400°C for 1-12 hours, cooled to room temperature with the furnace, and then ball milled according to the lithium iron phosphate: glucose mass ratio of 1:0.05 and the glucose: graphitization promoter molar ratio of 1:(0.002-0.010). Then, they are kept at 250-400°C for 1-5 hours, and then kept at 650-800°C for 1-10 hours, and cooled to room temperature with the furnace to obtain low-content, high-graphitization carbon-coated lithium iron phosphate. The particle size of the lithium iron phosphate positive electrode material obtained by this invention is small. 50≤100nm, uniform particle size distribution, high purity, and better rate performance and cycle performance, but the invention adds a graphitization promoter, thereby improving the graphitization degree of the material to a certain extent.

[0011] In summary, the researchers prepared carbon-coated graphitized materials through two carbon-coating sintering processes, pre-modified iron source precursors combined with in-situ catalysis, and the addition of graphitization promoters. Graphitization was carried out during the coating process, so that the graphitized carbon-coated phosphate positive electrode materials further had higher electron conductivity, better rate performance and cycle performance. However, there are also some disadvantages, such as the increased use of chemical auxiliary reagents, increased possibility of pollution, or more complicated processes. Therefore, a method is needed with a simple preparation process, cheap and easily available raw materials, which can achieve a high degree of graphitization, improve the electron transmission path of the coated carbon material, enhance the electron conductivity, and can prepare graphitized carbon-coated phosphate positive electrode materials on a large scale. Summary of the Invention

[0012] The present invention provides a method for preparing a sodium-electrographitized carbon-coated iron-phosphate positive electrode material. The preparation process is simple, the raw materials are cheap and easily available, and there is no additional pollution source. The prepared graphitized carbon-coated iron-phosphate positive electrode material has a high degree of graphitization, improved electronic conductivity, stable electrochemical performance, and excellent sodium storage performance.

[0013] The present invention provides a method for preparing a sodium-electrographitized carbon-coated iron-based phosphate cathode material, which is characterized by retaining a portion of heteroatoms in the carbon body when graphitizing biomass carbon at high temperature under an inert atmosphere with an iron source, thereby forming a graphitized carbon material having both graphitized carbon and non-graphitized carbon as well as a portion of heteroatoms, forming pre-graphitized carbon, and then mixing the pre-graphitized carbon with a sodium source, an iron source, and a phosphorus source for carbon coating to obtain a sodium-electrographitized carbon-coated iron-based phosphate cathode material. The specific steps include:

[0014] (1) Weighing appropriate amounts of iron source and carbon source, heating to 800-1100°C at a heating rate of 1-10°C / min under a nitrogen atmosphere, keeping the temperature for 3-5 hours, and then naturally cooling to obtain an Fe-C composite material, i.e., a pre-graphitized carbon material;

[0015] (2) weighing sodium pyrophosphate, iron phosphate, and Fe-C composite material, mixing, stirring, heating, and dispersing in water to obtain a mixed material;

[0016] (3) The mixed material is stirred and dispersed, then sand-milled, spray-dried, and calcined to obtain a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material.

[0017] The pre-graphitized carbon material is a carbon material in which a portion of the carbon is graphitized and a portion of the carbon is not graphitized after calcining the carbon source at a temperature of 800 to 1100°C, and some heteroatoms of the carbon source are retained. Because a high temperature of 2000°C is required for complete graphitization of carbon, the present invention introduces an iron source to reduce the activation energy of graphite formation. By calcining at a temperature of 800 to 1100°C, a material having graphitized and non-graphitized carbon can be formed, and the heteroatoms therein are ensured not to be completely volatilized at high temperatures. The heteroatoms of the carbon source are mainly O and N. The heteroatoms are intended to enable the pre-graphitized carbon material to be well combined with sodium pyrophosphate and iron-based phosphate during the subsequent carbon coating process. Without the presence of heteroatoms, it is impossible to connect with sodium pyrophosphate and iron-based phosphate.

[0018] In addition, there are two different states of carbon structures in pre-graphitized carbon materials, which play different roles. The conductive structure of graphitized carbon reinforced materials has a higher degree of graphitization and a more regular sp 2 Conjugated carbon structure, which will be beneficial to the electron transfer of porous carbon, reduce internal resistance, and improve the conductivity of the material, and the conductivity will directly affect the rapid charge and discharge ability of the material; and the ungraphitized carbon forms pores in the material, optimizing the electron transmission path. In the research of carbon materials, micropores contribute the most to the specific surface area, so they can often provide more energy storage active sites for ions. Mesopores are conducive to the transmission of electrolyte ions. The electrolyte carriers in the macropores are similar to free electrolytes, which can easily form a larger ion buffer reservoir than micropores and mesopores, making it easier to shorten the ion diffusion distance, reduce redundant ion transmission paths, and improve carrier transmission efficiency.

[0019] The amount of the carbon source added is 10% to 80% of the total mass of the iron source.

[0020] Because the high-temperature graphitization of the carbon precursor is accompanied by a decrease in the bulk carbon content and the volatilization of internal heteroatoms, the pre-graphitized carbon material needs to have a certain pore channel and graphitized structure. The pore channel directly determines the shuttle speed of the electrolyte carriers in the electrode material, and the graphitized structure directly determines the conductivity of the material. For a specific energy storage system, the two must ensure a structural balance; if the carbon source is too little, that is, less than 10%, the iron content used to catalyze graphitization is relatively higher, which may cause the catalytic graphitization reaction to be too intense during the reaction, and then the activation pore-forming effect occurs. At this time, the carbon body will produce a large amount of gas and unpredictable side reactions, which will greatly reduce the carbon content and destroy the generated graphite stripes, which will in turn reduce the conductivity of the final material. If the carbon source is too much, that is, higher than 80%, it may lead to insufficient graphitization reaction and low degree of graphitization of the product. The carbon material will have more pores and heteroatoms, mostly porous structures, and fewer graphitized structures, which will reduce the conductivity of the material and affect the graphitization effect. Therefore, the carbon source needs to be within a certain rate range of the iron source to truly improve the conductivity and energy storage performance.

[0021] The added amount of sodium pyrophosphate, iron phosphate and Fe-C composite material is such that the total added amount of sodium pyrophosphate, iron phosphate and Fe-C composite material needs to satisfy the phosphorus-iron molar ratio of 2:3 to 5:3.

[0022] Generally speaking, it's difficult to achieve a completely pure, fixed-type iron-based phosphate through synthesis, so these materials are often referred to as "complex phosphates." The iron content significantly influences the properties of the resulting iron-based phosphate. Excessive iron content can cause significant lattice distortion in the iron-based phosphate, reducing its thermal and electrochemical stability, ultimately leading to lattice distortion and weakened electrochemical performance. Too little iron can result in an incomplete lattice structure, which can reduce its electrochemical activity. Therefore, the ratio of the two must be within an appropriate range.

[0023] The iron source includes a divalent iron source and a trivalent iron source, wherein the divalent iron source is selected from one of FeS, FeBr2, FeCl2, FeO, Fe(NO3)2, FeCO3, FeSO4, Fe3(PO4)2, Fe(HCOO)2, FeHPO4, Fe(SCN)2, and Fe(CH3COO)2, and the trivalent iron source is selected from one of Fe2S3, FeBr3, FeCl3, Fe(NO3)3, Fe2(CO3)3, Fe2(SO4)3, FePO4, Fe(CH3COO)3, and Fe(HCOO)3.

[0024] The carbon source includes carbohydrate biomass and other biomass, wherein the carbohydrate biomass is selected from one of glucose, sucrose, maltose, and chitosan, and the other biomass is selected from one of plants, microorganisms, and waste products thereof. These two types of carbon sources are rich in heteroatoms, which can retain the heteroatoms required for connecting the sodium pyrophosphate salt and the iron phosphate during the high-temperature calcination process.

[0025] Sodium pyrophosphate, iron phosphate and Fe-C composite material are weighed, dispersed in water, heated at a temperature of 25 to 85° C., and stirred for 0.5 to 6 hours.

[0026] The rotation speed of the sand mill is 1000-3000 rpm, and the sand milling time is 0.5-10 hours.

[0027] The inlet air temperature of the spray drying is 100-180°C, and the outlet air temperature is 50-100°C.

[0028] The calcination temperature rise rate is 1-10°C / min, the temperature is 500-550°C, and the time is 1-10h.

[0029] The present invention introduces pre-graphitization into traditional carbon-coated materials, utilizes an iron source to catalyze the graphitization of a carbon source at high temperature, reduces the activation energy of graphite formation, and obtains a specific pre-graphitized carbon material. Then, the pre-graphitized carbon material is used as a carbon source to carbon-coat a basic iron-based phosphate, thereby constructing an "electron bridge" between the iron-based phosphate particles, that is, a graphitized structure that connects the electron pathways between the basic iron-based phosphate particles after the carbon source is catalyzed by the iron source. Therefore, the electron transfer path of the coated carbon material is strengthened, the charge transfer resistance is reduced, the transmission of ions within the material is promoted, and the electron transfer process of the traditional carbon-coated phosphate positive electrode material is improved. At the same time, the electron bridge can also provide an additional conductive path, increasing the channel density of ion transmission.

[0030] In addition, the preparation process of the inventive method is simple, the raw materials are cheap and easily available, and graphitized carbon-coated iron-phosphate positive electrode materials can be prepared on a large scale. The sodium-electrographitized carbon-coated iron-phosphate positive electrode material prepared by the present invention has a high degree of graphitization, higher electronic conductivity, stable electrochemical properties and high energy density, which improves the electron transfer rate and exhibits excellent sodium storage performance. The present invention also provides a potential idea for improving the conductivity of iron-phosphate positive electrode materials, which is also of practical significance in the application of sodium ion batteries and even other ion battery energy storage fields such as lithium ion batteries and potassium ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a process flow chart of Examples 1 to 5.

[0032] Figure 2This is the XRD result of Example 1.

[0033] Figure 3 This is the XRD result of Example 2.

[0034] Figure 4 This is the XRD result of Example 3.

[0035] Figure 5 This is the XRD result of Example 4.

[0036] Figure 6 This is the XRD result of Example 5.

[0037] Figure 7 It is the XRD result of comparative example.

[0038] Figure 8 This is the cycle result of Example 1 with a power reduction of 0.5C.

[0039] Figure 9 This is the cycle result of Example 2 with a power reduction of 0.5C.

[0040] Figure 10 This is the cycle result of Example 3 with a power cut of 0.5C.

[0041] Figure 11 This is the cycle result of Example 4 with a power cut of 0.5C.

[0042] Figure 12 This is the cycle result of Example 5 with a power cut of 0.5C.

[0043] Figure 13 This is the cycle result of the comparative example with a power deduction of 0.5C.

[0044] Figure 14 The Raman spectra of Examples 1 to 3 and the comparative example are combined with A D / A G Fitting result analysis chart. DETAILED DESCRIPTION

[0045] The embodiments of the present application will be described in more detail below. The present application can be implemented in various forms and should not be construed as being limited to the embodiments set forth herein. Instead, these embodiments are provided for a more thorough and complete understanding of the present application. It should be understood that the embodiments of the present application are intended for exemplary purposes only and are not intended to limit the scope of protection of the present application.

[0046] As used herein, the term "including" and its variations are open-ended, i.e., "including but not limited to." The term "according to" means "at least in part according to." The term "one embodiment" means "at least one embodiment," and the term "another embodiment" means "at least one additional embodiment." Definitions of other terms are provided in the following description.

[0047] The process flow chart of Examples 1 to 5 of the present invention is as shown in the attached Figure 1 As shown, the specific preparation steps are as follows:

[0048] (1) Weighing appropriate amounts of iron source and carbon source, wherein the amount of carbon source added is 10% to 80% of the total mass of the iron source, and then placing them in a porcelain boat. Under a nitrogen atmosphere, heating the temperature at a rate of 1 to 10°C / min to 800 to 1100°C, keeping the temperature for 3 to 5 hours, and then naturally cooling to obtain an Fe-C composite material;

[0049] (2) according to the molar ratio of sodium pyrophosphate and iron phosphate being 1:2, the Fe-C composite material is 25% of the total added molar amount of the three, that is, the total phosphorus and iron molar ratio of the added amount of sodium pyrophosphate, iron phosphate and Fe-C composite material is 1:1, and then 500 mL of water is added, stirred and heated to disperse, the heating temperature is 25-85 ° C, and the stirring time is 0.5-6 h;

[0050] (3) dispersing the product obtained in step (2) in a beaker and placing it in a sand mill, and sand milling it at a speed of 1000-3000 rpm for 0.5-10 h;

[0051] (4) transferring the sand-milled dispersion into a spray dryer and spray drying the dispersion at an inlet air temperature of 100 to 180° C. and an outlet air temperature of 50 to 100° C. to obtain a precursor;

[0052] (5) The precursor was placed in a tubular furnace filled with N2 atmosphere and sintered at 500-550°C for 1-10 h at a heating rate of 1-10°C / min to obtain the final sample powder. The powder was ground and passed through a 400-mesh sieve to obtain a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material.

[0053] In the comparative example, the Fe-C composite material is not obtained by pre-graphitization, but the iron source, sodium source, phosphorus source and carbon source are directly mixed to prepare the sodium-graphitized carbon-coated iron-based phosphate positive electrode material, wherein the phosphorus-iron molar ratio of the sodium pyrophosphate salt and the iron-based phosphate is 1:1, and the carbon source is 25% of the total added molar amount of the three. The other conditions are consistent with steps (2) to (5) of Examples 1 to 5.

[0054] The specific preparation conditions of Examples 1 to 5 and the comparative example are shown in Table 1:

[0055] Table 1 Specific preparation conditions of Examples 1 to 5 and Comparative Examples

[0056]

[0057] The products of Examples 1 to 5 and the comparative example were subjected to XRD test and button battery 0.5C cycle test. The XRD test results are shown in the attached figure. Figures 2 to 7As shown, the 0.5C cycle results of the button battery are shown in the attached Figures 8 to 13 As shown; from the attached Figures 2 to 7 The XRD test results shown in the figure show that the obtained result is iron phosphate. Figures 8 to 13 The 0.5C cycle test results of the button battery shown show that the button battery has excellent sodium storage performance.

[0058] By the attached Figures 2 to 7 It can be seen from the XRD results that each sample includes sodium iron pyrophosphate PDF#89-0579, sodium iron phosphate PDF#29-1216, and sodium iron pyrophosphate PDF#80-2409. Since the prepared material is not a single compound, but a composite iron-containing phosphate system, and due to the effect of carbon coating, there is a certain amount of amorphous carbon inside it, so the specific composition and content cannot be determined. Therefore, the present invention prepares sodium-electrographitized carbon-coated iron-based phosphate positive electrode material.

[0059] Attachment Figures 8 to 13 The specific capacity in the 0.5C cycle test results refers to the energy that the battery can store per unit mass or unit volume. The higher the specific capacity, the more energy the battery can store under the same mass or volume. The number of cycles refers to the number of cycles that the battery can complete during the charge and discharge process. The more stable the material structure, the less the specific capacity decay after the cycle. Figure 13 The 0.5C cycle test results of the comparative example show that the initial specific capacity in the 0.5C cycle test results is 40mAh / g. After 400 cycles, the specific capacity decays to 28mAh / g, showing poor sodium storage performance. Figure 8 、 11 1 and 12 are the 0.5C cycle test results of Example 1, Example 4 and Example 5, respectively, wherein the amount of carbon source added is 50% of the mass of the iron source. The initial specific capacity in the 0.5C cycle test results of these three embodiments is 115-125 mAh / g, which is 2.8-3.2 times higher than that of the comparative example. After 400 cycles, the specific capacity decays to 95-105 mAh / g, with a small decay amount and a slow decay in the late cycle, indicating that the material structure of the present invention is stable, the electrochemical performance is stable, and the sodium storage performance is excellent. Figure 9 The 0.5C cycle test results of Example 2 show that the amount of carbon source added is 10% of the mass of the iron source, and the carbon source accounts for a small proportion. The initial specific capacity in the 0.5C cycle test results is 80 mAh / g. After 400 cycles, the specific capacity decays to 70 mAh / g. Figure 10The 0.5C cycle test results of Example 3 show that the amount of carbon source added is 80% of the mass of the iron source, and the carbon source accounts for a large proportion. The initial specific capacity in the 0.5C cycle test results is 65mAh / g. After 400 cycles, the specific capacity decays to 42mAh / g. The sodium storage performance of these two materials is relatively poor, but compared with the comparative example, they still show improved sodium storage performance and electrochemical performance, indicating that selecting a suitable ratio of carbon source to iron source has an important influence on the electrochemical properties of the material; the above proves that the pre-graphitized carbon material has an improving effect on the electrochemical properties of the sodium-electrographitized carbon-coated iron-phosphate positive electrode material, and the surface pre-graphitized carbon material can strengthen the electron transfer path of the carbon-coated positive electrode material, reduce the charge transfer resistance, promote the transmission of electrons inside the material, improve its sodium storage performance, and have stable electrochemical properties.

[0060] Attachment Figure 14 The Raman spectra of Examples 1 to 3 and the comparative example are combined with A D / A G The fitting result analysis diagram shows that the improvement of the electronic conductivity of the material is related to the structure of the material. Usually, the graphitization degree of carbon materials directly determines the electronic conductivity of the material. The graphitization degree of carbon materials is usually determined by combining Raman spectroscopy with A D / A G The fitting results show that the 1200cm -1 The T peak at 1490 cm-1 affected by impurities or heteroatoms -1 The D" peak caused by the carbon layer defect is obtained at 1350cm -1 The real defect peak D peak and the peak at 1580cm -1 The graphitization peak G peak, its A D / A G Represents the area ratio of the peaks of material defects and graphitization degree, so A D / A G The lower the value, the higher the degree of graphitization; therefore, Figure 14 It can be seen that compared with the comparative example, after high temperature pre-graphitization, the A D / A G The degree of graphitization is significantly reduced; Example 1 has a balanced degree of defect and graphitization, and Example 2 has a relatively low iron content, so the catalytic graphitization effect is slightly weaker, so A D / A G The value is slightly higher. Since the iron content in Example 3 is relatively high, the catalytic graphitization effect is stronger. D / A G The value is slightly lower; overall, after pre-graphitization, the materials have a stronger degree of graphitization than those without pre-graphitization, so the electronic conductivity of the materials is greatly improved.

[0061] In addition, the sodium-electrographitized carbon-coated iron-based phosphate positive electrode materials prepared under all process conditions in the specific preparation steps of Examples 1 to 5 can achieve the above-mentioned improvement effect relative to the comparative example.

[0062] The above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. For example, any technician familiar with the technical field can easily think of changes or replacements within the technical scope disclosed in this application, which should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for preparing a sodium-graphitized carbon-coated iron-phosphate cathode material, comprising mixing a phosphorus source, a sodium source, an iron source, and a carbon source to obtain a mixture, stirring and dispersing the mixture, sand-milling the mixture, spray drying the mixture, and calcining the mixture, wherein: The step of mixing the phosphorus source, sodium source, iron source and carbon source is carried out in two steps, including: (1) Weighing appropriate amounts of iron source and carbon source, heating to 800-1100°C at a heating rate of 1-10°C / min under a nitrogen atmosphere, keeping the temperature for 3-5 h, and then cooling naturally to obtain an Fe-C composite material, i.e., a pre-graphitized carbon material; (2) Weigh sodium pyrophosphate, iron phosphate and Fe-C composite material, mix, stir, heat and disperse in water to obtain a mixed material.

2. The method for preparing a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material according to claim 1, characterized in that: The amount of the carbon source added is 10% to 80% of the total mass of the iron source.

3. The method for preparing a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material according to claim 1, characterized in that: The added amounts of sodium pyrophosphate, iron phosphate and Fe-C composite material must satisfy the total phosphorus-iron molar ratio of 2:3 to 5:

3.

4. The method for preparing a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material according to claim 1, characterized in that: The iron source includes a divalent iron source and a trivalent iron source, wherein the divalent iron source is selected from one or more of FeS, FeBr2, FeCl2, FeO, Fe(NO3)2, FeCO3, FeSO4, Fe3(PO4)2, Fe(HCOO)2, FeHPO4, Fe(SCN)2, and Fe(CH3COO)2, and the trivalent iron source is selected from one or more of Fe2S3, FeBr3, FeCl3, Fe(NO3)3, Fe2(CO3)3, Fe2(SO4)3, FePO4, Fe(CH3COO)3, and Fe(HCOO)3.

5. The method for preparing a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material according to claim 1, characterized in that: The carbon source includes carbohydrate biomass and other non-carbohydrate biomass, wherein the carbohydrate biomass is selected from one or more of glucose, sucrose, maltose, and chitosan, and the other non-carbohydrate biomass is selected from one or more of plants, microorganisms, and biological waste.

6. The method for preparing a sodium-electrographitized carbon-coated iron-based phosphate cathode material according to claim 1, characterized in that: Sodium pyrophosphate, iron phosphate and Fe-C composite material are weighed, mixed, stirred and heated in water for dispersion at a heating temperature of 25 to 85° C. for a stirring time of 0.5 to 6 h.

7. The method for preparing a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material according to claim 1, characterized in that: The rotation speed of the sanding is 1000 to 3000 rpm, and the sanding time is 0.5 to 10 hours.

8. The method for preparing a sodium-electrographitized carbon-coated iron-based phosphate positive electrode material according to claim 1, characterized in that: The inlet air temperature of spray drying is 100-180℃, and the outlet air temperature is 50-100℃.

9. The method for preparing a sodium-electrographitized carbon-coated iron-based phosphate cathode material according to claim 1, characterized in that: The heating rate of calcination is 1-10°C / min, the temperature is 500-550°C, and the time is 1-10h.

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