Preparation method of low specific surface area sulfate sodium ion battery positive electrode material

Abstract

The present invention discloses a method for preparing a low-specific-surface-area sulfate-based sodium-ion battery cathode material, comprising the following steps: S1, mixing precursor powders: grinding and mixing an alkali metal source, an anion source, and a transition metal source to form a uniform precursor powder; S2, sintering a pre-crystallized material: pre-sintering the precursor powder obtained in step S1 under protective atmosphere to obtain a pre-crystallized material; S3, preparing a pre-coated precursor: heating, grinding, and mixing the pre-crystallized material obtained in step S2 with a deposition coating material and a structural coating material under protective atmosphere to form a cage-shaped, uniform pre-coated precursor; S4, secondary sintering of a core-shell-coated sulfate material: sintering the pre-coated precursor obtained in step S3 under protective atmosphere to obtain a low-specific-surface-area core-shell-coated sulfate material. The present invention has the characteristics of small specific surface area, excellent electrochemical performance, and good crystallinity.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion battery positive electrode materials, and in particular to a method for preparing a low specific surface area sulfate sodium ion battery positive electrode material. Background Art

[0002] Cathode materials are key to the cost and performance of sodium-ion battery systems. They must possess abundant resources, simple mining processes, low cost, high reversible capacity, high redox potential, stable structure, and good air stability. However, the performance of currently commercially available mass-produced materials varies widely.

[0003] Sulfate materials, a branch of polyanion sodium-ion battery cathode materials, are abundant, simple to prepare, offer a high voltage platform, and possess high system energy density. Currently, they are marketed for low-end power applications. However, sulfate materials have low sintering temperatures and low electronic conductivity, making them unsuitable for traditional carbon source reduction and coating processes, making their mass production more challenging.

[0004] Currently, the main method for synthesizing sulfate materials is solid-phase ball milling: a sodium source, a metal M source, a sulfur source, and a carbon source (such as carbon black, graphene, and carbon nanotubes) are solid-phase ball milled to uniformity, followed by sintering to obtain the final product. However, the materials produced by this process have a large specific surface area, making subsequent processing difficult and hindering mass production. Therefore, finding a new process to reduce the specific surface area of ​​materials produced by solid-phase ball milling is of great significance for the practical application of these materials. Summary of the Invention

[0005] The present invention aims to provide a method for preparing a low specific surface area sulfate sodium ion battery cathode material, which has the characteristics of small specific surface area, excellent electrochemical performance and good crystallinity.

[0006] The present invention can be achieved through the following technical solutions:

[0007] The present invention discloses a method for preparing a low specific surface area sulfate sodium ion battery cathode material, comprising the following steps:

[0008] S1. Mixing precursor powders: grinding and mixing the alkali metal source, the anion source and the transition metal source to form a uniform precursor powder;

[0009] S2. Sintering the pre-crystallized material: pre-sintering the precursor powder obtained in step S1 under protective atmosphere to obtain a pre-crystallized material;

[0010] S3, preparation of a pre-coating precursor: under protective atmosphere conditions, heating, grinding and mixing the pre-crystallized material obtained in step S2 with the deposition coating material and the structural coating material to form a cage-shaped uniform pre-coating precursor;

[0011] S4. Secondary sintering preparation of core-shell coated sulfate material: Under protective atmosphere, the pre-coated precursor obtained in step S3 is sintered to obtain a core-shell coated sulfate material with low specific surface area.

[0012] Furthermore, the melting point of the deposited coating material is asphalt lower than 350° C., and the asphalt is one or more of coal tar asphalt, petroleum asphalt and / or natural asphalt.

[0013] Furthermore, the structural coating material is a one-dimensional or two-dimensional carbon material, and the one-dimensional or two-dimensional carbon material is carbon nanotubes and / or graphene.

[0014] Furthermore, in S3, when asphalt is used together with graphene, the proportion of graphene in the asphalt is 1 to 30%; when asphalt is used together with carbon nanotubes, the proportion of carbon nanotubes in the asphalt is 1 to 20%; when asphalt is used together with graphene and carbon nanotubes, the latter two account for 1 to 25% of the asphalt; when the weight ratio is within this range, graphene or carbon nanotubes can effectively penetrate the asphalt coating, construct an electron transmission network between the material particles, and realize rapid conduction of electrons.

[0015] Furthermore, the sulfate material has the general formula A 2+2x M 2-x (SO4)3、A 2+2x+y M 2-x (SO4) 3-y (PO4) y , wherein A is Na and / or Li; M is Fe, Co and / or Mn; the value range of x is 1.5≤x≤2.0; the value range of y is 0≤y≤0.5.

[0016] Furthermore, in step S3, the heating and grinding temperature is 50-200°C and the time is 1-10 hours. At this temperature, the surface of the asphalt particles melts to generate adhesion, and at the same time, it bonds with graphene and carbon nanotubes to form a composite. Under the action of the linear shear force of grinding, it gradually adheres to the sulfate surface to initially form a pre-coated precursor.

[0017] Furthermore, in step S4, the sintering temperature is 350-450°C and the time is 1-15 hours. At this temperature, the asphalt coating on the surface of the sulfate material further melts, combines with each other and adheres to the surface of the material to form a dense coating containing graphene and carbon nanotube conductive agents, which can ensure the conduction of electrons between and inside the material particles, while avoiding side reactions between the sulfate interface and the electrolyte.

[0018] Furthermore, in step S2, the pre-sintering temperature is 200-300°C. At this temperature, the crystalline water contained in the raw materials can be completely volatilized, and the easily decomposable gaseous products in the raw materials can be completely released. When the temperature exceeds this range, the precursor tends to transition into larger single crystal particles, which are easily destroyed during the subsequent grinding process, affecting the crystallinity of the final product; the sintering time is 1-15H, and the evaluation standard for pre-crystallized materials is to pass the X-ray diffractometer test, and the curve shows obvious diffraction peaks.

[0019] Furthermore, in step S1, the grinding and mixing methods include dry grinding, sand milling and / or ball milling, etc., which use mechanical force or shear force to achieve uniform dispersion of materials. The basis for judging whether the materials are mixed uniformly is the solid particle size Dmax≤0.2um. Below this particle size, during the pre-sintering process, the various material elements can fully diffuse, bond, and crystallize to form a pre-crystallized material.

[0020] Furthermore, in step S1, the alkali metal source is selected from sodium formate, sodium acetate, sodium sulfate, lithium sulfate, and lithium acetate; the anion source is a phosphorus-containing compound and / or a sulfur-containing compound, the phosphorus-containing compound is selected from sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate and / or phosphoric acid, and the sulfur-containing compound is selected from sodium sulfate and / or lithium sulfate; and the transition metal source is selected from ferrous sulfate, manganese sulfate and / or cobalt sulfate.

[0021] Furthermore, the protective atmosphere includes non-oxygen-containing gases such as nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen. The oxygen content in the furnace during the pre-sintering process needs to be ≤20ppm. When the oxygen content is too high, it will cause some transition metal elements to oxidize, affecting the charge ratio between elements and being unfavorable for the crystal growth of the material.

[0022] The low electronic conductivity and synthesis temperature of sulfate materials place high demands on their process routes. The process for preparing sulfate sodium-ion battery positive electrode materials of the present invention reduces the specific surface area of ​​the materials prepared by the solid-phase ball milling method. The process is to grind and mix the alkali metal source, the anion source and the transition metal source to form a uniform precursor; under the protection of an inert atmosphere, the precursor is pre-sintered to remove volatile water and gaseous products to form a pre-crystallized material; then the pre-crystallized material is heated, ground and mixed with asphalt, graphene and / or carbon nanotubes to form a uniform pre-coated precursor; finally, under the protection of an inert atmosphere, the pre-coated precursor is sintered to obtain a low specific surface core-shell coated sulfate material. The material prepared by this process has a large grain size, good crystallinity, high particle integrity, a dense interface coating layer, a low specific surface area, high electronic conductivity and excellent electrochemical performance.

[0023] The present invention provides a method for preparing a low specific surface area sulfate sodium ion battery cathode material, which has the following beneficial effects:

[0024] First, the specific surface area is small. The deposition coating material with a generally low melting point is selected. The melting temperature is lower than the decomposition temperature of the sulfate material. After melting and solidification, the density is high and the specific surface area is low. It is easy to form a dense coating layer on the surface of the sulfate material, effectively reducing the surface porosity of the sulfate material and the contact area between the material and the electrolyte, thereby reducing the occurrence of side reactions.

[0025] Second, the electrochemical performance is excellent. The structural coating material selected has high electronic conductivity and specific surface area. By heating and melting the structural coating material and the performance coating material, the structural coating material will evenly penetrate into the deposition coating material. After it solidifies on the surface of the sulfate material, it penetrates the inside and outside of the interface layer, playing the role of electron transmission. At the same time, during the secondary sintering process, the structural coating material in direct contact with the sulfate will form a bonded CO bond. The existence of this bond reduces the transition energy barrier of electrons at the interface, effectively improving its transmission speed. The traditional one-time physical carbon coating method cannot form an effective bonded interface structure.

[0026] Third, the crystallinity is good. The alkali metal source, anion source, and transition metal source can initially form a complete crystal nucleus after solid-phase ball milling and sintering. After further sintering by coating with deposited coating materials, structural coating materials, etc., the ions grow epitaxially along the crystal nucleus into larger single crystal particles. Compared with the traditional one-time sintering process, this two-step sintering process has higher crystal integrity, better crystallinity, and better electrochemical performance. DETAILED DESCRIPTION

[0027] In order to enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention is further described in detail below with reference to embodiments.

[0028] The present invention discloses a method for preparing a low specific surface area sulfate sodium ion battery cathode material, comprising the following steps:

[0029] S1. Mixing precursor powders: grinding and mixing the alkali metal source, the anion source and the transition metal source to form a uniform precursor powder;

[0030] S2. Sintering the pre-crystallized material: pre-sintering the precursor powder obtained in step S1 under protective atmosphere to obtain a pre-crystallized material;

[0031] S3, preparation of a pre-coating precursor: under protective atmosphere conditions, heating, grinding and mixing the pre-crystallized material obtained in step S2 with the deposition coating material and the structural coating material to form a cage-shaped uniform pre-coating precursor;

[0032] S4. Secondary sintering preparation of core-shell coated sulfate material: Under protective atmosphere, the pre-coated precursor obtained in step S3 is sintered to obtain a core-shell coated sulfate material with low specific surface area.

[0033] Furthermore, the melting point of the deposited coating material is asphalt lower than 350° C., and the asphalt is one or more of coal tar asphalt, petroleum asphalt and / or natural asphalt.

[0034] Furthermore, the structural coating material is a one-dimensional or two-dimensional carbon material, and the one-dimensional or two-dimensional carbon material is carbon nanotubes and / or graphene.

[0035] Furthermore, in S3, when asphalt is used together with graphene, the proportion of graphene in the asphalt is 1 to 30%; when asphalt is used together with carbon nanotubes, the proportion of carbon nanotubes in the asphalt is 1 to 20%; when asphalt is used together with graphene and carbon nanotubes, the latter two account for 1 to 25% of the asphalt.

[0036] Furthermore, the sulfate material has the general formula A 2+2x M 2-x (SO4)3、A 2+2x+y M 2-x (SO4) 3-y (PO4) y , wherein A is Na and / or Li; M is Fe, Co and / or Mn; the value range of x is 1.5≤x≤2.0; the value range of y is 0≤y≤0.5.

[0037] Furthermore, in step S3, the heating and grinding temperature is 50-200° C., and the time is 1-10 hours.

[0038] Furthermore, in step S4, the sintering temperature is 350-450° C., and the sintering time is 1-15 hours.

[0039] Furthermore, in step S2, the pre-sintering temperature is 200-300°C; and the sintering time is 1-15 hours.

[0040] Furthermore, in step S1, the grinding and mixing methods include dry grinding, sand milling and / or ball milling.

[0041] Furthermore, in step S1, the alkali metal source is selected from sodium formate, sodium acetate, sodium sulfate, lithium sulfate, and lithium acetate; the anion source is a phosphorus-containing compound and / or a sulfur-containing compound, the phosphorus-containing compound is selected from sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate and / or phosphoric acid, and the sulfur-containing compound is selected from sodium sulfate and / or lithium sulfate; and the transition metal source is selected from ferrous sulfate, manganese sulfate and / or cobalt sulfate.

[0042] Furthermore, the protective atmosphere includes non-oxygen-containing gases such as nitrogen, argon, nitrogen-hydrogen, and argon-hydrogen.

[0043] The preparation method of the present invention effectively overcomes the following defects of the prior art:

[0044] For example, in sulfate materials, metal MO bonds are often separated by SO bonds, resulting in long-distance transmission of electrons along the MOSOM chain, reduced efficiency, slower speed, and increased polarization.

[0045] For example, the SO4 group in sulfate materials has strong electronegativity, which can easily induce the outer electrons of the metal M to deviate from the Fermi level arrangement, resulting in an increase in the structural band gap width, corresponding to a higher electron transition energy barrier, and a significant decrease in conductivity.

[0046] For example, sulfate materials have poor thermodynamic stability. When temperatures exceed 400°C, the SO bond in the SO4 group breaks, generating sulfur-containing SO2 gas, leading to structural decomposition. This is an inherent property of the material. However, the traditional carbothermal reduction process requires temperatures exceeding 500°C, making the present process ineffective for direct interface modification of sulfate materials.

[0047] For example, when sulfate materials are modified solely with conductive agents such as carbon black, carbon nanotubes, and graphene, the large surface areas of the conductive agents and sulfates will result in the final material having an excessively large specific surface area, making the subsequent homogenization process difficult and affecting its practical application.

[0048] Application Example 1Na 2.6 Fe 1.7 Synthesis and electrochemical performance of (SO4)3 / asphalt / graphene

[0049] S1. Mixing of precursor powders: anhydrous sodium sulfate and ferrous sulfate heptahydrate were placed in a ball mill at a molar ratio of 1.3:1.7 and ground evenly. The ball-to-material ratio was 20:1. The grinding time was 10 hours. The raw material particle size Dmax was tested to be ≤0.15 μm. The slurry showed a light green semi-solidified state.

[0050] S2. Sintering of pre-crystallized material: Under nitrogen protection, the semi-solidified precursor slurry is pre-sintered at 280° C. for 7 hours to remove moisture from the slurry and generate a pre-crystallized precursor material.

[0051] S3. Preparation of pre-coated precursor: Under nitrogen protection conditions, the pre-crystallized precursor material, asphalt, and graphene are heated and ground at 150°C for 8 hours in a mass ratio of 0.92:0.068:0.012, so that the asphalt and graphene are interlocked and adhered to the surface of the material. After cooling and solidification, a uniform pre-coated precursor is generated.

[0052] S4. Secondary sintering preparation of core-shell coated sulfate materials: Under nitrogen protection, the pre-coated precursor was sintered at 390 ° C for 10 hours to promote the epitaxial growth of the material along the crystal core into a larger single crystal structure. After natural cooling, a core-shell coated Na 2.6 Fe1.7 (SO4)3 / asphalt / graphene materials.

[0053] The specific surface area of ​​Na 2.6 Fe 1.7 (SO4)3 / asphalt / graphene material test, the results in Table 1 show that the specific surface area of ​​the material is only 5.8m 2 / g, indicating that the asphalt forms a dense coating on the surface of the material, which effectively fills the voids on the surface of the material and the micropores on the surface of graphene, thereby reducing the interface area of ​​the material.

[0054] Will Na 2.6 Fe 1.7 (SO4)3 / asphalt / graphene, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry with a solid content of 60%. The slurry exhibited good fluidity and processing performance, which was related to the material's low specific surface area. This indicated that the asphalt coating of the material effectively reduced the material's contact with the solvent during homogenization, thereby reducing the viscosity of the slurry. The black slurry was then coated on aluminum foil using a 150um four-sided preparation device, and the film was then dried in a vacuum drying oven at 100°C for 2 hours. A sheet puncher was used to punch the electrode film into a disc with a radius of 0.6mm. Sodium metal was used as the counter electrode, 1mol / LNaClO4EC+DEC (1:1vol%) + 5%FEC was used as the electrolyte, and the diaphragm was a PP / PE / PP three-layer diaphragm. CR2016 button batteries were assembled in a glove box.

[0055] The button cell was subjected to constant current charge and discharge tests with a current density of 0.1C (1C = 104 mAh / g). The results in Table 1 show that within the voltage range of 2.0-4.5V, the reversible specific capacity of the electrode is 101 mAh / g, and the capacity utilization rate is close to 96.2%. The high capacity utilization rate is inseparable from the high crystallinity of the material, indicating that the material after pre-crystallization treatment, and then further heating and calcining treatment, is conducive to promoting its epitaxial growth along the pre-crystallized crystal nucleus, and then growing into larger single crystal particles. In addition, the average discharge potential of the electrode is 3.70V, which is consistent with the theoretical sodium deintercalation potential of its structure, indicating that the presence of the asphalt + high-conductivity graphene coating reduces the specific surface area of ​​the material while retaining the high electronic conductivity of the graphene, thereby constructing a conductive surface transmission network for the transfer of charge between material particles. In addition, the results in Table 1 show that within the voltage range of 2.0-4.3V, the electrode has a capacity retention rate of up to 98.5% after 100 cycles at a rate of 1C, indicating that the dense coating layer at the material interface reduces the decomposition of the material interface structure, while reducing the side reactions between the electrolyte and the material, and effectively improving the structural stability of the material during the sodium deintercalation process.

[0056] Application Example 2Na 2.6 Fe 1.7 Synthesis and Electrochemical Performance of (SO4)3 / Pitch / CNT

[0057] S1. Mixing of precursor powders: anhydrous sodium sulfate and ferrous sulfate heptahydrate were placed in a ball mill at a molar ratio of 1.3:1.7 and ground evenly. The ball-to-material ratio was 20:1. The grinding time was 10 hours. The raw material particle size Dmax was tested to be ≤0.15 μm. The slurry showed a light green semi-solidified state.

[0058] S2. Sintering of pre-crystallized material: Under nitrogen protection, the semi-solidified precursor slurry is pre-sintered at 250° C. for 10 hours to remove moisture from the slurry and generate a pre-crystallized precursor material.

[0059] S3. Preparation of pre-coated precursor: Under nitrogen protection, the pre-crystallized precursor material, asphalt, and CNT are heated and ground at 180°C for 5 hours in a mass ratio of 0.92:0.064:0.016, so that the asphalt and CNT are interlocked and adhered to the surface of the material. After cooling and solidification, a uniform pre-coated precursor is generated.

[0060] S4. Secondary sintering preparation of core-shell coated sulfate materials: Under nitrogen protection, the pre-coated precursor was sintered at 380 ° C for 7 hours to promote the epitaxial growth of the material along the crystal core into a larger single crystal structure. After natural cooling, a core-shell coated Na 2.6 Fe 1.7 (SO4)3 / asphalt / CNT materials.

[0061] The specific surface area of ​​Na 2.6 Fe 1.7 (SO4)3 / asphalt / CNT material test, the results in Table 1 show that the specific surface area of ​​the material is only 4.2m 2 / g, indicating that the existence of the dense asphalt + CNT coating layer effectively fills the surface, micropores, etc. of the material as well as the CNT interface or internal pores, thereby greatly reducing the specific surface area of ​​the material.

[0062] Will Na 2.6 Fe 1.7(SO4)3 / asphalt / CNT, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry with a solid content of 60%. The slurry had good processability and high slurry fluidity. No scratches or jelly phenomena occurred during the coating process, indicating that the asphalt coating of the material effectively reduced the contact with the solvent during the homogenization process of the material, thereby reducing the viscosity of the slurry. The black slurry was then coated on aluminum foil using a 150um four-sided preparation device, and the film was then dried in a vacuum drying oven at 100°C for 2 hours. A sheet puncher was used to punch the electrode film into a disc with a radius of 0.6mm. Metal sodium was used as the counter electrode, 1mol / LNaClO4EC+DEC (1:1vol%) + 5%FEC was used as the electrolyte, and the diaphragm was a PP / PE / PP three-layer diaphragm. CR2016 button batteries were assembled in a glove box.

[0063] The button cell was subjected to constant current charge and discharge tests with a current density of 0.1C (1C = 104mAh / g). The results in Table 1 show that within the voltage range of 2.0-4.5V, the reversible specific capacity of the electrode is 99mAh / g, and the capacity utilization rate is close to 95.1%. This is related to the high crystallinity of the material, the large secondary sintering grain size, the complete crystallinity, the uniform arrangement of atomic occupancy in the structure, the continuous sodium intercalation and deintercalation channels, and the low deintercalation and deintercalation energy barrier. In addition, the average discharge potential of the electrode is 3.71V, which is consistent with the theoretical sodium intercalation and deintercalation potential of its structure, indicating that the presence of asphalt + high-conductivity CNT coating layer constructs a conductive linear transmission network for the transfer of charge between material particles, effectively reducing the internal resistance between materials. In addition, the results in Table 1 show that within the voltage range of 2.0-4.3V, the electrode has a capacity retention rate of up to 98.0% after 100 cycles at a rate of 1C, indicating that the dense coating layer at the material interface can effectively avoid the lithification of the material interface, the dissolution of elements, the collapse of the structure, and the decomposition of the electrolyte at the interface during the sodium insertion and extraction process, thereby greatly improving its cycle stability.

[0064] Comparative Example 1Na 2.6 Fe 1.7 Synthesis and Electrochemical Performance of (SO4)3 / Pitch

[0065] Step 1: Grind anhydrous sodium sulfate and ferrous sulfate heptahydrate in a ball mill at a molar ratio of 1.3:1.7 evenly, with a ball-to-material ratio of 20:1 and a grinding time of 10 hours. The raw material particle size Dmax is tested to be ≤0.15um, and the slurry is in a light green semi-solidified state.

[0066] Step 2: Under nitrogen protection conditions, the semi-solid precursor slurry is pre-sintered at 280° C. for 7 hours to remove moisture from the slurry and generate a pre-crystallized precursor material.

[0067] Step 3: Under nitrogen protection, the pre-crystallized precursor material and asphalt are heated and ground at 150°C for 8 hours in a mass ratio of 0.92:0.08, so that the asphalt adheres to the surface of the material. After cooling and solidification, a uniform pre-coated precursor is generated.

[0068] Step 4: Under nitrogen protection, the pre-coated precursor was sintered at 390 ° C for 10 hours to promote the epitaxial growth of the material along the crystal core into a larger single crystal structure. After natural cooling, a shell-core coated Na with low specific surface area and high crystallinity was formed. 2.6 Fe 1.7 (SO4)3 / bituminous materials.

[0069] The specific surface area of ​​Na 2.6 Fe 1.7 (SO4)3 / asphalt material test, the results in Table 1 show that the specific surface area of ​​the material is only 3.4m 2 / g, indicating that the asphalt forms a dense coating on the surface of the material, which effectively fills the surface of the material and its micropores, thereby reducing the interface area of ​​the material.

[0070] Will Na 2.6 Fe 1.7 (SO4)3 / asphalt, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry with a solid content of 60%. The slurry exhibited good fluidity and processing performance, which was related to the material's low specific surface area. This indicated that the coating of the material with asphalt effectively reduced the material's contact with the solvent during homogenization, thereby reducing the viscosity of the slurry. The black slurry was then coated on aluminum foil using a 150um four-sided preparation device, and the film was then dried in a vacuum drying oven at 100°C for 2 hours. A sheet puncher was used to punch the electrode film into a disc with a radius of 0.6mm. Sodium metal was used as the counter electrode, 1mol / LNaClO4EC+DEC (1:1vol%) + 5%FEC was used as the electrolyte, and the diaphragm was a PP / PE / PP three-layer diaphragm. CR2016 button batteries were assembled in a glove box.

[0071] The above button cell was subjected to constant current charge and discharge tests with a current density of 0.1C (1C = 104mAh / g). The results in Table 1 show that within the voltage range of 2.0-4.5V, the reversible specific capacity of the electrode is only 45mAh / g, the capacity utilization is only 42.8%, and the average electrode potential is only 3.2V. Compared with Application Example 1, it has a larger voltage polarization, which is related to the low electronic conductivity of asphalt. When the material interface is completely coated with low-conductivity asphalt, the electron transfer rate decreases, the internal resistance of the material increases, the electrode polarization increases, and the capacity utilization rate decreases. Therefore, while a single asphalt coating reduces the specific surface area of ​​the material, it is not conducive to the electrochemical performance of the material. In addition, the results in Table 1 show that within the voltage range of 2.0-4.3V, the electrode has a capacity retention rate of up to 97.8% after 100 cycles at a rate of 1C, indicating that the dense coating layer at the material interface reduces the decomposition of the material interface structure, while reducing the side reactions between the electrolyte and the material, and effectively improving the structural stability of the material during the sodium deintercalation process.

[0072] Comparative Example 2Na 2.6 Fe 1.7 Synthesis and electrochemical performance of (SO4)3 / graphene

[0073] Step 1: Grind anhydrous sodium sulfate and ferrous sulfate heptahydrate in a ball mill at a molar ratio of 1.3:1.7 evenly, with a ball-to-material ratio of 20:1 and a grinding time of 10 hours. The raw material particle size Dmax is tested to be ≤0.15um, and the slurry is in a light green semi-solidified state.

[0074] Step 2: Under nitrogen protection conditions, the semi-solid precursor slurry is pre-sintered at 280° C. for 7 hours to remove moisture from the slurry and generate a pre-crystallized precursor material.

[0075] Step 3: Under nitrogen protection, the pre-crystallized precursor material and graphene are heated and ground at 150°C for 8 hours in a mass ratio of 0.92:0.08 to uniformly mix the graphene and the material.

[0076] Step 4: Under nitrogen protection, the mixed material is sintered at 390 ° C for 10 hours to promote the epitaxial growth of the material along the crystal nucleus into a larger single crystal structure. After natural cooling, Na 2.6 Fe 1.7 (SO4)3 / graphene materials.

[0077] The specific surface area of ​​Na 2.6 Fe 1.7 (SO4)3 / graphene material test, the results in Table 1 show that the specific surface area of ​​the material is as high as 13.8m 2 / g, indicating that there are many surfaces and micropores at the interface between the material and graphene, resulting in an excessively large specific surface area.

[0078] Will Na 2.6 Fe 1.7 (SO4)3 / graphene, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry with a solid content of 60%. The slurry exhibited poor fluidity, almost a semi-solid state, high viscosity, and difficulty in processing. This suggests that the material's large specific surface area increased its contact with the solvent, resulting in a decrease in free solvent molecules and an increase in slurry viscosity. The black slurry was then coated onto aluminum foil using a 150μm four-sided preparation device. The coating effect was poor, with uneven thickness, no load, scratches, and black spots. The film was then dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched out into discs with a radius of 0.6mm using a sheet puncher. Sodium metal was used as the counter electrode, 1mol / L NaClO4EC+DEC (1:1 vol%)+5%FEC was used as the electrolyte, and the separator was a PP / PE / PP three-layer membrane. CR2016 button cells were assembled in a glove box.

[0079] The above button cell was subjected to constant current charge and discharge tests with a current density of 0.1C (1C = 104mAh / g). The results in Table 1 show that within the voltage range of 2.0-4.5V, the reversible specific capacity of the electrode is 89mAh / g, the capacity utilization rate is only 85.6%, and the average discharge potential of the electrode is 3.50V. The low capacity utilization rate and discharge potential are closely related to the unevenness in the electrode manufacturing process. When the active material, binder and conductive agent cannot be evenly dispersed in the preparation of the electrode slurry, the conductive agent on the electrode will be too dispersed, the electron transmission will be discontinuous, the voltage polarization will be large, and the capacity utilization rate will decrease. In addition, when the binder cannot adhere the active material and the conductive agent to the aluminum foil, the volume of the material will expand during the charge and discharge process, part of the active material will fall off, and the cycle stability of the electrode will decrease. In addition, the results in Table 1 show that within the voltage range of 2.0-4.3V, the capacity retention of the electrode at a rate of 1C after 100 cycles is only 82.1%. The low capacity retention is mainly caused by the decomposition of the material interface structure, the side reaction between the electrolyte and the material interface, and the shedding of active substances during the sodium deintercalation process.

[0080] Comparative Example 3Na 2.6 Fe 1.7 Synthesis and Electrochemical Performance of (SO4)3 / CNT

[0081] Step 1: Grind anhydrous sodium sulfate and ferrous sulfate heptahydrate in a ball mill at a molar ratio of 1.3:1.7 evenly, with a ball-to-material ratio of 20:1 and a grinding time of 10 hours. The raw material particle size Dmax is tested to be ≤0.15um, and the slurry is in a light green semi-solidified state.

[0082] Step 2: Under nitrogen protection conditions, the semi-solid precursor slurry is pre-sintered at 250° C. for 10 hours to remove moisture from the slurry and generate a pre-crystallized precursor material.

[0083] Step 3: Under nitrogen protection, the pre-crystallized precursor material and CNT are heated and ground at 180°C for 5 hours in a mass ratio of 0.92:0.08 to uniformly mix the CNT and the material.

[0084] Step 4: Under nitrogen protection, the pre-coated precursor was sintered at 380 ° C for 7 hours to promote the material to further grow epitaxially along the crystal core into a larger single crystal structure. After natural cooling, a shell-core coated Na with low specific surface area and high crystallinity was formed. 2.6 Fe 1.7 (SO4)3 / CNT material.

[0085] The specific surface area of ​​Na 2.6 Fe 1.7 (SO4)3 / CNT material test, the results in Table 1 show that the specific surface area of ​​the material is as high as 12.5m 2 / g, indicating that there are many surfaces and micropores at the interface between the material and CNT, resulting in an excessively large specific surface area.

[0086] Will Na 2.6 Fe 1.7 (SO4)3 / CNT, AB, and PVDF were mixed in a mass ratio of 9.2:0.4:0.4 to prepare a slurry with a solid content of 60%. The slurry exhibited poor fluidity, almost a semi-solid state, and high viscosity, making processing difficult. This suggests that the material's large specific surface area increases its contact with the solvent, resulting in a decrease in free solvent molecules and an increase in slurry viscosity. The black slurry was then coated onto aluminum foil using a 150μm four-sided preparation device. The coating effect was poor, with uneven thickness, no load, scratches, and black spots. The film was then dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched out into discs with a radius of 0.6mm using a sheet puncher. Sodium metal was used as the counter electrode, 1 mol / L NaClO4EC+DEC (1:1 vol%)+5%FEC was used as the electrolyte, and a PP / PE / PP three-layer separator was used. CR2016 button cell batteries were assembled in a glove box.

[0087] The button cell was subjected to constant current charge and discharge tests at a current density of 0.1C (1C = 104 mAh / g). The results in Table 1 show that within the voltage range of 2.0-4.5V, the electrode's reversible specific capacity was 85 mAh / g, with a capacity utilization rate of only 81.7%. The average discharge potential of the electrode was 3.55V. These low capacity utilization rates and discharge potentials are likely related to uneven electrode manufacturing.

[0088] In addition, the results in Table 1 show that within the voltage range of 2.0-4.3V, the capacity retention rate of the electrode is only 78.4% after 100 cycles at a rate of 1C. The low capacity retention rate is mainly caused by the dissolution, collapse, and side reactions of the material interface structure.

[0089] Table 1 Performance test results

[0090]

[0091] The above embodiments are merely specific embodiments of the present invention, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make various modifications and improvements without departing from the scope of the present invention, and such obvious alternatives are all within the scope of protection of the present invention.

Claims

1. A method for preparing a low specific surface area sulfate sodium ion battery cathode material, characterized in that The following steps are involved: S1. Mixing precursor powders: grinding and mixing the alkali metal source, the anion source and the transition metal source to form a uniform precursor powder; S2. Sintering of pre-crystallized material: Under protective atmosphere, pre-sinter the precursor powder obtained in step S1 to obtain pre-crystallized material. The pre-sintering temperature is 200-300° C. and the sintering time is 1-15 hours. S3. Preparation of a pre-coated precursor: Under protective atmosphere, the pre-crystallized material obtained in step S2 is heated, ground, and mixed with a deposition coating material and a structural coating material to form a cage-shaped uniform pre-coated precursor; the deposition coating material is asphalt having a melting point below 350° C., and the asphalt is one or more of coal tar asphalt, petroleum asphalt, and / or natural asphalt; the structural coating material is a one-dimensional or two-dimensional carbon material, and the one-dimensional or two-dimensional carbon material is carbon nanotubes and / or graphene; S4. Secondary sintering preparation of core-shell coated sulfate material: Under protective atmosphere, the pre-coated precursor obtained in step S3 is sintered to obtain a core-shell coated sulfate material with low specific surface area.

2. The method for preparing a low specific surface area sulfate sodium ion battery positive electrode material according to claim 1, wherein: In step S3, when asphalt is used together with graphene, the proportion of graphene in the asphalt is 1-30%; when asphalt is used together with carbon nanotubes, the proportion of carbon nanotubes in the asphalt is 1-20%; when asphalt is used together with graphene and carbon nanotubes, the latter two account for 1-25% of the asphalt.

3. The method for preparing a low specific surface area sulfate sodium ion battery positive electrode material according to claim 1, wherein: The sulfate material has the general formula A 2+2x M 2-x (SO4)3、A 2+2x+y M 2-x (SO4) 3-y (PO4) y , wherein A is Na and / or Li; M is Fe, Co and / or Mn; the value range of x is 1.5≤x≤2.0; the value range of y is 0≤y≤0.

5.

4. The method for preparing a low specific surface area sulfate sodium ion battery positive electrode material according to claim 1, wherein: In step S3, the heating and grinding temperature is 50-200° C. and the time is 1-10 h.

5. The method for preparing a low specific surface area sulfate sodium ion battery positive electrode material according to claim 1, wherein: In step S4, the sintering temperature is 350-450° C. and the sintering time is 1-15 hours.

6. The method for preparing a low specific surface area sulfate sodium ion battery positive electrode material according to claim 1, wherein: In step S1 , the grinding and mixing methods include dry grinding, sand milling and / or ball milling.

7. The method for preparing a low specific surface area sulfate sodium ion battery positive electrode material according to claim 1, characterized in that: In step S1, the alkali metal source is selected from sodium formate, sodium acetate, sodium sulfate, lithium sulfate, and lithium acetate; the anion source is a phosphorus-containing compound and / or a sulfur-containing compound, the phosphorus-containing compound is selected from sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate and / or phosphoric acid, and the sulfur-containing compound is selected from sodium sulfate and / or lithium sulfate; and the transition metal source is selected from ferrous sulfate, manganese sulfate and / or cobalt sulfate.

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